Assessing the use of P-enriched steel slag as a sustainable alternative to conventional fertilizers for crop production
Abstract
El fòsfor (P) és un macronutrient crític essencial per al creixement de les plantes, el desenvolupament de les arrels i la producció d'aliments. Juga un paper central en la transferència d'energia, la síntesi d'àcids nucleics i la formació de membranes, fet que el converteix en vital per a la seguretat alimentària mundial. Com a recurs finit obtingut principalment a través de la mineria de roques sedimentàries, el seu esgotament i impacte ambiental han fet insostenibles les pràctiques actuals. La Comissió Europea ha classificat el P com a Matèria Primera Crítica (European Commission, 2020), destacant la necessitat de recuperar i reciclar aquest element. Aquest estudi avalua la viabilitat de les escòries d'acer enriquides amb fòsfor com a alternativa sostenible als fertilitzants convencionals mitjançant un experiment controlat de 8 setmanes amb plantes d’alfàbrega (Ocimum basilicum). Es van provar cinc condicions: un control negatiu (C , sense fertilitzant), un control positiu (C+, fertilitzant estàndard) i tres tractaments amb escòries enriquides amb fòsfor a diferents concentracions (E, E10 i E20). Els resultats van demostrar que els tractaments amb escòries proporcionaven fòsfor biodisponible. El tractament E10, que contenia 10 vegades el contingut de fòsfor de C+, va ser el més efectiu. L’anàlisi ambiental va confirmar una lixiviació mínima de metalls, dins dels llindars internacionals de seguretat. Aquesta investigació destaca el potencial de les escòries d’acer per a una agricultura regenerativa, donant suport al reciclatge de fòsfor i a una gestió sostenible dels nutrients. Es recomanen estudis addicionals per optimitzar els mètodes d’aplicació de les escòries i avaluar els impactes a llarg termini sobre la salut del sòl.
Full text
1 Treball realitzat per: Maria Lluna Fornells Cernadas Dirigit per: Jaume Puigagut Juárez Diego Aponte Hernández Grau en: Enginyeria Ambiental Barcelona, February 2025 Departament d’Enginyeria Civil i Ambiental (DECA) Assessing the use of P-enriched steel slag as a sustainable alternative to conventional fertilizers for crop production TREBALL FINAL DE GRAU
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3 TABLE OF CONTENTS ABSTRACT ............................................................................................................................... 5 RESUM ...................................................................................................................................... 6 RESUMEN ................................................................................................................................ 7 TABLES .................................................................................................................................... 8 FIGURES ................................................................................................................................... 9 ABBREVIATIONS ................................................................................................................. 10 1. Introduction ...................................................................................................................... 11 2. Objectives ........................................................................................................................ 13 3. Literature review and basic concepts ............................................................................... 14 3.1. Phosphorus ............................................................................................................... 14 3.1.1. Phosphorus cycle ............................................................................................. 14 3.1.2. Phosphorus and plant metabolism ................................................................... 15 3.1.3. Sources and depletion ...................................................................................... 16 3.1.4. Phosphorus in an agricultural context .............................................................. 16 3.2. Steel slags................................................................................................................. 17 3.2.1. Origin and characteristics ................................................................................ 17 3.2.2. Slags as fertilizers ............................................................................................ 18 3.3. Microbial activity and regenerative agriculture ....................................................... 19 3.4. Metals and heavy metals of interest ......................................................................... 20 4. Materials and methodology.............................................................................................. 23 4.1. Experimental design................................................................................................. 23 4.1.1. Replicas and amendments ................................................................................ 23 4.1.2. Experimental setup and considerations ............................................................ 24 4.1.3. Materials and proceedings ............................................................................... 25 4.2. Variables considered and monitored ........................................................................ 29 4.2.1. Plant health....................................................................................................... 29
4 4.2.2. Soil health ........................................................................................................ 32 4.2.3. Environmental factors ...................................................................................... 35 5. Results .............................................................................................................................. 38 5.1. Plant health and growth ........................................................................................... 38 5.2. Soil health ................................................................................................................ 43 5.3. Slag’s metals fate: plant and leachate. ..................................................................... 46 6. Conclusions ...................................................................................................................... 50 6.1. Limitations ............................................................................................................... 51 6.2. Future lines of reasearch .......................................................................................... 51 7. Sustainability analysis and ethical implications .............................................................. 53 7.1. Development of the thesis ........................................................................................ 53 7.1.1. Environmental perspective ............................................................................... 53 7.1.2. Economic perspective ...................................................................................... 57 7.1.3. Social perspective ............................................................................................ 59 7.2. Potential execution of the project ............................................................................ 59 7.2.1. Environmental perspective ............................................................................... 59 7.2.2. Economic perspective ...................................................................................... 60 7.2.3. Social perspective ............................................................................................ 60 7.3. Risks and limitations ................................................................................................ 61 7.4. Ethical implications ................................................................................................. 62 7.5. Sustainable Development Goals .............................................................................. 62 REFERENCES ........................................................................................................................ 64 ACKNOWLEDGEMENTS ..................................................................................................... 70 ANNEX I ................................................................................................................................. 71
5 ABSTRACT Phosphorus (P) is a critical macronutrient essential for plant growth, root development, and food production. It plays a central role in energy transfer, nucleic acid synthesis, and membrane formation, making it vital for global food security. As a finite resource primarily obtained from sedimentary rock mining, its depletion and environmental impact have made current practices unsustainable. The European Commission has classified P as a Critical Raw Material (European Commission, 2020), emphasizing the need for recovery and recycling. This study evaluates the feasibility of phosphorus-enriched steel slags as a sustainable alternative to conventional fertilizers through an 8-week controlled agronomical experiment using basil plants (Ocimum basilicum). Five conditions were tested: a negative control (C-, no fertilizer), a positive control (C+, standard fertilizer), and three treatments with P-enriched slag at varying concentrations (E, E10, and E20). Results showed that slag amendments effectively provided bioavailable phosphorus. The E10 treatment, containing 10 times the phosphorus content of C+, was the most effective in terms of biomass production. Environmental analysis confirmed minimal metal leaching, within international safety thresholds. This research highlights the potential of steel slags for regenerative agriculture, supporting phosphorus recycling and sustainable nutrient management. Further studies are recommended to optimize slag application methods and assess long-term soil impacts. Key words: Phosphorus, Steel Slags, Sustainable Agriculture, Regenerative Agriculture, Nutrient Recycling, Microbial Activity, Bioavailable Phosphorus, Circular Economy, Environmental Safety
6 RESUM El fòsfor (P) és un macronutrient crític essencial per al creixement de les plantes, el desenvolupament de les arrels i la producció d'aliments. Juga un paper central en la transferència d'energia, la síntesi d'àcids nucleics i la formació de membranes, fet que el converteix en vital per a la seguretat alimentària mundial. Com a recurs finit obtingut principalment a través de la mineria de roques sedimentàries, el seu esgotament i impacte ambiental han fet insostenibles les pràctiques actuals. La Comissió Europea ha classificat el P com a Matèria Primera Crítica (European Commission, 2020), destacant la necessitat de recuperar i reciclar aquest element. Aquest estudi avalua la viabilitat de les escòries d'acer enriquides amb fòsfor com a alternativa sostenible als fertilitzants convencionals mitjançant un experiment controlat de 8 setmanes amb plantes d’alfàbrega (Ocimum basilicum). Es van provar cinc condicions: un control negatiu (C- , sense fertilitzant), un control positiu (C+, fertilitzant estàndard) i tres tractaments amb escòries enriquides amb fòsfor a diferents concentracions (E, E10 i E20). Els resultats van demostrar que els tractaments amb escòries proporcionaven fòsfor biodisponible. El tractament E10, que contenia 10 vegades el contingut de fòsfor de C+, va ser el més efectiu. L’anàlisi ambiental va confirmar una lixiviació mínima de metalls, dins dels llindars internacionals de seguretat. Aquesta investigació destaca el potencial de les escòries d’acer per a una agricultura regenerativa, donant suport al reciclatge de fòsfor i a una gestió sostenible dels nutrients. Es recomanen estudis addicionals per optimitzar els mètodes d’aplicació de les escòries i avaluar els impactes a llarg termini sobre la salut del sòl. Paraules clau: Fòsfor, Escòries d’Acer, Escòries Negres, Agricultura Sostenible, Agricultura Regenerativa, Reciclatge de Nutrients, Activitat Microbiana, Fòsfor Biodisponible, Economia Circular, Seguretat Ambiental
7 RESUMEN El fósforo (P) es un macronutriente crítico esencial para el crecimiento de las plantas, el desarrollo de las raíces y la producción de alimentos. Desempeña un papel central en la transferencia de energía, la síntesis de ácidos nucleicos y la formación de membranas, lo que lo hace vital para la seguridad alimentaria global. Como recurso finito obtenido principalmente a través de la minería de rocas sedimentarias, su agotamiento y su impacto ambiental han vuelto insostenibles las prácticas actuales. La Comisión Europea ha clasificado el P como Materia Prima Crítica (European Commission, 2020), destacando la necesidad de su recuperación y reciclaje. Este estudio evalúa la viabilidad de las escorias de acero enriquecidas con fósforo como una alternativa sostenible a los fertilizantes convencionales mediante un experimento controlado de 8 semanas con plantas de albahaca (Ocimum basilicum). Se probaron cinco condiciones: un control negativo (C-, sin fertilizante), un control positivo (C+, fertilizante estándar) y tres tratamientos con escorias enriquecidas con fósforo en diferentes concentraciones (E, E10 y E20). Los resultados demostraron que los tratamientos con escorias proporcionaron fósforo biodisponible, mejorando la biomasa vegetal, el desarrollo de las raíces y la actividad microbiana. El tratamiento E10, que contenía 10 veces el contenido de fósforo de C+, fue el más efectivo. El análisis ambiental confirmó una lixiviación mínima de metales, dentro de los límites internacionales de seguridad. Esta investigación destaca el potencial de las escorias de acero para una agricultura regenerativa, apoyando el reciclaje de fósforo y una gestión sostenible de nutrientes. Se recomiendan estudios adicionales para optimizar los métodos de aplicación de las escorias y evaluar los impactos a largo plazo sobre la salud del suelo. Palabras clave: Fósforo, Escorias de Acero, Escorias Negras, Agricultura Sostenible, Agricultura Regenerativa, Reciclaje de Nutrientes, Actividad Microbiana, Fósforo Biodisponible, Economía Circular, Seguridad Ambiental
8 TABLES Table 1. Most common chemical compositions of EAF slag in Spain. Source: Skaf et al., 2017 .................................................................................................................................................. 18 Table 2. Collection of upper limits or recommended intakes of the selected list of metals for water and food consumption. Source: European Commission, 2023; Institute of Medicine, US, 1997-2011, WHO, 2022. Except for: ....................................................................................... 21 Table 3. Composition of the soil in the different amended soils. ........................................... 24 Table 4. Initial nutrient and physical properties of the peat-moss. .......................................... 26 Table 5. Metal in steel slag’s leachate compared to Spanish limits set by the Decreto 32/2009, de 24 de febrero ....................................................................................................................... 27 Table 6. Chemical composition of the adapted Hoagland solution. ........................................ 28 Table 7. Watering amounts during the first 3 weeks. .............................................................. 35 Table 8. Average chlorophyll readings throughout the duration of the experiment per experimental condition and standard deviation. ...................................................................... 39 Table 9. Plant height average initial and final readings and their standard deviation per environmental condition .......................................................................................................... 41 Table 10. Final nutrient content and physical conditions of the soil per each experimental condition. ................................................................................................................................. 44 Table 11. Leachate results in comparison with drinking water guidelines (source in Table 2 of this present document) and slag’s leachate analysis. The results for the tab water used for watering and the Hoagland solution provide a conceptual zero to the results. In bold the maximum values. Highlighted in light red the values that surpass the reference values......... 47 Table 12. Metal concentration within plant tissue compared to maximum and reference intakes for the studied metals (Source in table 2). In bold the maximum values. Highlighted in light red the values that surpass the reference values. ...................................................................... 48 Table 13. Total metal amount per average plant of each of the experimental conditions. In bold the maximum values. ............................................................................................................... 49 Table 14. Carbon footprint calculation including the inventory, the value, the emission factor used and its sources and the total values. ................................................................................. 55 Table 15. Cost per individual item of the project. Total cost calculation. ............................... 58
9 FIGURES Figure 1. Experiment set up. .................................................................................................... 25 Figure 2. Plan view of a group of pots, showing the soil mixture used. .................................. 26 Figure 3. Picture of the slags used as amendments. ................................................................. 28 Figure 4. Pictures during the final harvest. .............................................................................. 29 Figure 5. Chlorophyll meter SPAD plus Konika Minolta. ...................................................... 30 Figure 6. Absorbance peaks of chlorophyll. Source: Spad-plus manual ................................. 30 Figure 7. Correlation between SPAD value and nitrogen concentration for basil plants. Font: (Ruiz-Espinoza et al., 2010) ..................................................................................................... 31 Figure 8. Set up for the analysis of microbial activity using the ATP extraction method. ...... 33 Figure 9. Rhizon samplers for pore water extraction attached to a luer syringe. .................... 34 Figure 10.LED lights used. ...................................................................................................... 36 Figure 11. Light intensity distribution [Klux]. Each cell of the figure represents a pot position within the experimental set-up. ................................................................................................ 37 Figure 12. Distributions used throughout the experiment. The colour refers to the experimental condition: blue (C-), pink (C+), green (Esc), orange (Esc 10) and yellow (Esc20). The numbers refer to the specific replica within the experimental condition (1-4). ...................................... 37 Figure 13. Chlorophyll readings per environmental condition through time. ......................... 39 Figure 14. Plant height average readings per environmental condition in time. ..................... 40 Figure 15. Final plant weight (above and below ground) average and standard deviation and moisture average content per experimental condition. ............................................................ 42 Figure 16. Bellow ground (roots) final weight (average and standard deviation) and length (average) per experimental condition ...................................................................................... 42 Figure 17. Microbial activity average in time per experimental condition. ............................. 43 Figure 18. Microbial activity initial and final values (average and standard deviation) per experimental condition............................................................................................................. 44 Figure 19. Nutrient content (Ca, C and N) in percentage of dry weight and pH of the final soil per experimental condition ...................................................................................................... 45 Figure 20. pH average of leachate samples in time per experimental condition ..................... 46 Figure 21. Pictures of the plants before the harvest 06/06/2024.............................................. 71
16 gravitropic development, and denser and longer root hairs (Poirier et al., 2022). Additionally, plants have evolved sophisticated mechanisms to sense and respond to variations in Pi availability (Khan et al., 2023). 3.1.3. Sources and depletion Approximately 90% of mined P is used in fertilizer production to sustain crop yields (Poirier et al., 2022). There are two types of P mining: surface mining (up to 100m below ground) and underground mining (>100m). Surface mining can affect a wider area than underground leading to erosion of soils. During the extraction process water usage affects the hydrology of the area. Following extraction, the product undergoes the process of beneficiation, where unnecessary minerals are removed generating a wide range of potentially harmful byproducts. Discharges from the industry to water bodies and the soil include acid mine drainage, toxic metals and radioactive elements (Pb, Cd, Hg, Cr, As, U, Th and Ra). These all result in significant effects on ecosystem and human health. Dust, fluoride and radon gas emissions generate air quality problems as well. (Reta et al., 2018) Commercially extracted P primarily comes from sedimentary deposits of apatite formed in marine environments over geological time. By 2022, the largest reserves are concentrated in Morocco (including Western Sahara) and China, which together hold approximately 70% of the world’s P-rich deposits (Poirier et al., 2022). This geographic concentration creates vulnerabilities in the food system, as access to fertilizers is heavily dependent on geopolitical stability. For example, Morocco’s control of Western Sahara’s phosphate resources has sparked international legal and ethical concerns, with claims that the extraction and trade of phosphates from the region violate international law. China has imposed high export tariffs to prioritize domestic supply, further emphasizing the political sensitivity of P distribution (Cordell et al., 2009). Recently, a massive P deposit of 70 billion tons was discovered in Norway, potentially extending the global P supply by 50 years at current usage rates (Redacció 324, 2023). However, while this discovery delays the predicted depletion of P reserves, issues such as uneven global distribution, pollution from excessive P use, the impact of mining and the nonrenewable nature of phosphate rock persist. These challenges underscore the importance of sustainable P management within the framework of a circular economy. 3.1.4. Phosphorus in an agricultural context P is often the limiting nutrient in terrestrial primary production, playing a crucial role in crop yields and agricultural sustainability. Crop available P largely depends on abiotic weathering and biotic processes, such as root exudations, microbial activity, pH, iron and aluminium
17 content (Roberts & Johnston, 2015). This variability affects the efficiency of fertilizers: due to immobilization, only 10-20% of applied P is used by plants the following year after application (Hata et al., 2010). The soil suffers P loss through crop removal and abiotic and biotic weathering. Historically, natural processes have occasionally provided sufficient P inputs to offset losses in available P. These natural processes include annual sedimentation from flooding or volcanic ash rich waters. Traditional agriculture commonly used biological phosphate sources such as animal faeces, crop and organic waste or crushed bones to keep P levels along with erosion control. This organic P transformed to Pi through microbial activity in the soil which was boosted by biological fertilization. By the 19th and early 20th centuries, phosphorus shortages became a recognized limitation to agricultural productivity in parts of Europe, the Americas, Australia, and Southern Africa. This recognition drove the adoption of additional P sources, including guano and rock phosphates (fossil phosphorites). (Reijnders, 2014). Currently, as seen previously, rock phosphates dominate global phosphorus supply. The sustained use of synthetic fertilizers and fossil phosphorites, being a driving agent for mid-19th century “Green revolution”, has played a critical role in supporting modern agricultural productivity and meeting global food demands being and will continue to do so (Ashley et al., 2011). Acknowledging the challenges posed by phosphorus (P) as a finite resource, including concerns over depletion and pollution, significant research efforts over the past 30 years have focused on developing recycling and recovery techniques, particularly in the wastewater treatment industry. These advancements include methods such as calcium phosphate precipitation (Deng & Dhar, 2023), the use of layered double hydroxides coupled with biochar (Keyikoglu et al., 2022), and Fe-based materials (Zhu et al., 2024). Despite these developments, further research is required to advance low-tech solutions, such as constructed wetlands and filter systems, to make phosphorus recovery more accessible and sustainable (Bird & Drizo, 2009). 3.2. Steel slags This section explores the origin, characteristics, and potential applications of steel slags, with a focus on their use as agricultural amendments. It begins with an overview of the types of slags produced in steel manufacturing and their composition, followed by a detailed discussion on their role as fertilizers. Special attention is given to their nutrient profile, environmental impact, and their potential for promoting sustainable agricultural practices. 3.2.1. Origin and characteristics There are commonly three slag types depending on its origins: blast furnace (BF) steel slag, iron melter (IM) slag and the electric arc furnace (EAF) steel slag. (Bird & Drizo, 2009). Off
18 the three, EAF are more efficient in energy use when producing steel. For this experiment, EAF steel slags were chosen since it is the most prevalent steelmaking process in Spain (70% of all steel is produced in EAFs) (Skaf et al., 2017). Electric arc furnace steel slags, known in Catalan as Escòries Negres, originate from the steel production process in electric arc furnaces. The primary raw material used for steelmaking in EAFs is steel scrap. During the melting stage, impurities such as manganese and silicon are removed, resulting in slags that accumulate these impurities. Find the slag’s common chemical composition in table 1. (Escorias Negras | CEDEX, n.d.). Table 1. Most common chemical compositions of EAF slag in Spain. Source: Skaf et al., 2017 This residue is in constant abundant supply: in 2017, a total of 13704000t of ordinary steel was produced in Spain, which generated 1781560t of slags (110-150kg of slag/t of steel) (Escorias Negras | CEDEX, n.d.). They are commonly revalorized within the civil construction sector, used as aggregates in concrete or road construction. The revalorization of these slags is a process overseen by Directive 2008/98/CE of the Europea Parliament on residues transposed in Spain through Ley 22/2011 on residues and contaminated soils. Both rulings incorporate the requisites that certain types of residues need to meet to get out of the residue stage MITECO, 2023). In 2018, in Spain only 4% of these slags ended up in landfills (MITECO, 2023). 3.2.2. Slags as fertilizers Slags have demonstrated significant potential as soil amendments and fertilizers (Das et al., 2019). Their characteristics not only reduce the leachate potential of heavy metals and mitigate greenhouse gas (GHG) emissions but also due to their composition, see table 1, they provide essential nutrients that enhance plant development and microbial activity (Das et al., 2019; O’Connor et al., 2021). The alkaline nature of slags, derived from their calcium and magnesium oxide content, increases soil pH by releasing hydroxide ions (OH⁻) during dissolution, effectively mitigating soil acidification. (Das et al., 2019). These benefits are likely due to shifts in microbial metabolism and modifications in microbial habitats (Das et al., 2019). Although no immediate direct threats to human health from slag application have been identified, potential risks such as heavy metal contamination, leachate contamination, and bioaccumulation of heavy metals in plants remain critical considerations. (O’Connor et al., 2021) Countries such as Japan, Korea, and China have extensively used steel-making and blast furnace slags as raw materials for fertilizers. These fertilizers are categorized into slag silicate
19 fertilizers, lime fertilizers, slag phosphate fertilizers, and iron-based fertilizers (Das et al., 2019). In Europe, approximately 2.7% of slags are currently utilized as fertilizers. (O’Connor et al., 2021) Slags have the capacity to remove P from aqueous solutions through adsorption. Their efficiency in P removal depends on the slag’s particle size, initial P content and ratio of slag mass to aqueous solution volume (Vu et al., 2021). This characteristic has resulted in steel slag filter technology emerging as an effective and low-cost phosphorus-adsorbing agent. It is promising in reducing P concentrations from municipal, domestic, and agricultural wastewater (Bird & Drizo, 2009). 1998 the first agricultural trial was conducted using BF slags amongst other P removal materials and concluded that slags showed great potential as fertilizers. Following up, in 2009 another agricultural assay was conducted by Bird & Drizo in Vancouver. They obtained promising results using P-enriched EAF slags as fertilizers compared to standard chemical fertilizers. They utilized naturally enriched slags, where dairy farm wastewater was used to phosphate the slags instead of relying on chemical phosphorus sources. This resulted in a more realistic approach to P availability within the slags accounting for possible biofilm and interactions that may occur. While this assay provided evidence supporting the availability of phosphorus adsorbed in slags, a more comprehensive approach that incorporates soil and plant quality indicators is needed to advance towards its scaling up in a context of regenerative agriculture. 3.3. Microbial activity and regenerative agriculture Microbial activity in soil encompasses the diverse array of biochemical processes facilitated by soil microorganisms (Collins English Dictionary). These processes are driven by bacteria, fungi, actinomycetes, protozoa, and other microorganisms that interact to decompose organic matter and recycle essential nutrients for plant growth. They participate in nutrient fixation and solubilization, assist in stress management, produce phytohormones, and positively influence plant phenology and crop yield. Additionally, soil microorganisms are instrumental in forming soil aggregates, which improve soil structure, stability, water retention, and air circulation. Furthermore, microorganisms facilitate eco-friendly agricultural practices such as bioremediation and the biocontrol of phytopathogens. The diversity of microorganisms is fundamental to maintaining soil health and quality, as different groups of microorganisms contribute to vital soil processes (Nazir et al., 2024). Factors such as soil acidity and nutrient levels significantly influence the diversity and activity of these microorganisms. (Nadarajah & Abdul Rahman, 2023; Prashar & Shah, 2016) As a result, soil microorganisms have been widely accepted as bioindicators of soil health and activity, providing a tangible measure of the soil’s biological vitality and ecological status. This
20 approach is taken particularly by regenerative agriculture. Which is widely recognized as a collection of nature-friendly farming practices designed to revitalize soil health. These practices include no-till farming, cover cropping, crop rotation, agroforestry, and the use of organic, homeor farm-based inputs. Chemical fertilizers and pesticides are often considered essential for industrial agriculture and global food security. However, they are associated with significant environmental and health impacts, particularly their role in the eutrophication of water bodies. According to the United Nations Environmental Protection, approximately 30–40% of lakes and reservoirs worldwide are affected by eutrophication to varying degrees. This process restricts the use of water for fisheries, recreation, industry, and drinking due to the excessive growth of undesirable algae and aquatic weeds, as well as oxygen depletion caused by their decomposition. Periodic surface blooms of cyanobacteria in drinking water supplies pose serious health risks (M. N. Khan & Mohammad, 2014). Additionally, chemical inputs through fertililzer alter the microbial properties of soil, often reducing microbial diversity and activity. These challenges highlight the urgent need to transition toward sustainable agricultural practices that prioritize soil health and enhance microbial vitality (Prashar & Shah, 2016). 3.4. Metals and heavy metals of interest This section provides an overview of the key metals and heavy metals relevant to this study, focusing on their role in agricultural systems, potential risks, and thresholds for environmental and human health. It discusses essential micronutrients alongside potentially harmful elements. Additionally, the regulatory limits for these metals in soil and water are examined to contextualize their presence in the slag amendments and their implications for sustainability and safety. The list of metals referred to in this report are decided based on the criterium stablished by Directive 2003/33/EC on limit concentrations of leachate on inert materials. These metals are: Zinc (Zn), Barium (Ba), Nickel (Ni), Selenium (Se), Copper (Cu), Molybdenum (Mo), Arsenic (As), Vanadium (V), Chromium (Cr), Cadmium (Cd), Mercury (Hg) and Lead (Pb). This list includes the metals limited in the Real Decreto 1051/2022, 2022 of Spain regarding the maximum content of metals that can be used in agrarian soil. This confirms the relevance of this list within the scope of the study. The list includes essential micronutrients (Zn, Cu, Mo, Ni) and potentially harmful metals (Cd, Pb, As, Hg and Cr). Micronutrients are essential elements that maintain plants’ overall health by being instrumental in their growth, development, and reproduction. They are required in minimal quantities. Their levels need to be monitored to ensure they are within a tolerable and beneficial range. (Ahmed et al., 2024)
21 The contamination of agricultural soils by heavy metals is a major driver of soil degradation (Vácha, 2021). When heavy metals accumulate to toxic levels, these non-biodegradable elements adversely impact crop health and productivity. They can disrupt the normal structure and function of cellular components in plants, interfering with metabolic and developmental processes essential for growth and productivity (Rashid et al., 2023). They also have a significant effect on human health. Heavy metals have the ability to cause membrane and DNA damage, impairing protein production and altering their functions including enzymatic activity. Their effects can include, amongst others, the impairment of kidney function, infertility, disturbance of development, increase the risk of cardiovascular disease.(Witkowska et al., 2021) Table 2. Collection of upper limits or recommended intakes of the selected list of metals for water and food consumption. Source: European Commission, 2023; Institute of Medicine, US, 1997-2011, WHO, 2022. Except for: *For Ba, the value shown is the no observed adverse effect level (NOAEL), established at 0.21 mg/kg body weight for an average bodyweight of 70kg. Source: (Choudhury & Cary, 2001). ** This value was not found on the WHO limits, it was found at (EPA, 2000) Metal Maximum levels in food [mg/kg wet weight] (European Commission, 2023) Dietary Reference Intakes per day [mg/day] (Institute of Medicine, US, 1997-2011) Tolerable Upper Intake [mg/day] (Institute of Medicine, US, 1997-2011) WHO drinking water guidelines [mg/L] (WHO, 2022) Zn - 8 for women / 11 for men 40 No health guideline, >4mg/L result in astringent taste Ba - 1.4 * - 1.3 Ni - 1 0.07 Se - 0.055 0.4 0.04 Cu - 0.9 10 2 Mo - 0.045 2 0.07 As Ranges from 0.1 to 0.3 for cereal based products - - 0.01 V - - 1.8 0.23* * Cr - 0.025 for women/ 0.035 for men - 0.05 Cd 0.2 for fresh herbs - 3 Hg No information on vegetables. Ranges from 0.1-1 on seafood. - - 0.006 Pb 0.3 for leafy brassica - - 0.01
22 The toxicity to crops is influenced by several factors, including crop type, growth conditions, developmental stage, and the specific toxic properties of the metals involved. Additionally, the physical and chemical properties of the soil, the bioavailability of the metal ions in the soil solution, and rhizosphere chemistry play critical roles in determining the extent of their impact. (Rashid et al., 2023) International organizations like the World Health Organization or the European Commission set thresholds and limits on regards to human and environmental health on these elements. In table 2, we collected the limits/recommended intakes of the elements monitored in this study that applied to drinking water and food consumption. These limits will be used as reference values in this study to assess and properly understand the danger associated with slags and their potential leachates. Addressing and monitoring heavy metal contamination is therefore essential for maintaining soil health, supporting sustainable agricultural practices, and protecting environmental and human health.
23 4. Materials and methodology 4.1. Experimental design To evaluate the potential of phosphorus (P)-enriched steel slags as fertilizers, an 8-week experiment was conducted using 20 basil plants (Ocimum basilicum). The experiment spanned a total of 57 days (8.14 weeks), which, based on previous agricultural studies involving basil, is considered sufficient to assess the impact of treatments on plant development and growth (Álvarez-González et al., 2022; Bird & Drizo, 2009). This timeframe allowed for the observation of key growth stages and provided reliable data on the effects of phosphorusenriched slag amendments. Five experimental condition groups were established, as follows: 1. Negative control [C-/1]: Basil growth evaluation under nutrient-deficient soil conditions. Watered with tab water. 2. Positive control [C+/2]: Basil growth evaluation under the application of a standard Hoagland liquid solution. 3. Amended condition 1 [E/3]: Amendment with P-enriched slag equivalent to the amount of P dosed via the Hoagland solution. 4. Amended condition 2 [E10/4]: Amendment with P-enriched slag equivalent to 10 times the amount of P in the Hoagland solution. 5. Amended condition 3 [E20/3]: Amendment with P-enriched slag equivalent to 20 times the amount of P in the Hoagland solution. The positive and negative control groups were necessary to maintain a reference point throughout the experiment. The amended groups at different concentrations of slags were studied to see the potential benefits or dangers of slags at different concentrations and assess the bioavailability of adsorbed P at different concentrations. Each condition consisted of four replicates arranged in a random distribution. The experimental layout was rotated every 2.5 weeks to minimize positional bias. During the experiment, soil microbial activity, plant height, and chlorophyll content of leaves were monitored. At the end of the experiment, above-ground and below-ground biomass were measured. Biweekly water pore water samples were collected to assess potential metal leaching. 4.1.1. Replicas and amendments The amount of slag was determined based on the phosphorus demand of basil plants. Following the recommendations of Álvarez (2022), the basil plant weekly P provided was of 3.09mg. This resulted in a total of 24.78 mg by the end of the experiment (8 weeks). For C+ group, P was
24 weekly provided by the Hoagland solution. For the slag-amended conditions, the total P added at the start was adjusted as follows: • E: Equivalent to the P dosed weekly by the Hoagland solution. • E10: Equivalent to 10 times the weekly P dose. • E20: Equivalent to 20 times the weekly P dose. Using a known concentration of 1.09 mgP/g slag and the total P demand of 24.78 mg, the required amount of slag was calculated and rounded to the nearest multiple of five to account for the limitations of precision balances at this weight range. The volumes of the soil mixture were adjusted to ensure consistent pot volumes of approximately 0.9 L. See Table 3 for the composition of each experimental condition. Table 3. Composition of the soil in the different amended soils. Experimental Condition Substrate (Peatmoss + Perlite) [g] Slag [g] Slag % of total weight C350 0 0% C+ 350 0 0% E 350 25 6.67% E10 310 250 44.64% E20 260 500 65.79% 4.1.2. Experimental setup and considerations The experimental setup consisted of five rows of four pots, each 1 L in volume, placed under four transversal 250V LED lights at 55 cm from the ground, see Figure 1. One seedling was planted in each individual pot. Each pot had a Rhizon sampler inserted for pore water extraction. The arrangement was rotated twice during the experiment to ensure uniform light exposure. In order to get a homogeneous moisture and equal volume of soil into the different experimental conditions, the soil and slags were previously saturated in water.
25 Figure 1. Experiment set up. 4.1.3. Materials and proceedings 4.1.3.1. Basil Culinary herb O. Basilicum is commonly referred to as sweet basil or basil. It is native to Asia and Africa, but is cultivated worldwide. Basil is considered low maintenance growing well indoors and outdoors under sunny and moist conditions (Li & Chang, 2016). The basil plants used in the experiment were as seedlings, in the same stage of development. Three stemmed plants were chosen to ensure maximum homogeneity. 4.1.3.2. Soil The substrate used was a 1:1 mixture of peat moss and perlite, selected for its nutrient-poor characteristics to isolate nutrient intake from fertilizers and slags. An initial analysis of the peat moss confirmed its low nutrient availability, see table 4. Peat moss is an organic material formed from partially decomposed sphagnum moss and other vegetation in waterlogged, anaerobic (low oxygen) conditions over thousands of years. It is primarily harvested from peat bogs in regions such as Canada, Northern Europe, and Russia. Its unique properties make it a widely used substrate in horticulture and agricultural experiments. One of the defining features of peat moss is its water retention capacity, as it can hold up to 20 times its weight in water. It has a significantly low pH, ranging 3-4. This property ensures a
32 The samples were sent dry at the analysing facility Eurofins, Lleida. To obtain enough biomass, all the above and below biomass of the plants within the same experimental conditions were joined together. 4.2.2. Soil health Soil is constituted by solid, liquid and gaseous phases. Water and air quality are determined by their degree of pollution and the resulting impact on health and ecosystems (Bünemann et al., 2018). However, soil quality is defined much more broadly as “the capacity of soil to function as a vital living system, within ecosystem and land-use boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and promote plant and animal health” (Doran & Zeiss, 2000). As such, three variables have been studied in this experiment to give a wholistic approach to assessing soil quality: a biotic indicator (microbial activity), the quality of its liquid phase (assessing pore water metal content) and the evolution of the nutrient composition and physical properties of the soil. 4.2.2.1. Microbial activity Every two weeks, microbial activity was assessed using an ATP (adenosine triphosphate) hydrolysis test. A small soil sample, approximately 0.25 g, was collected from the surface to a depth of around 3 cm and within a radius of 3 cm from the plant stem. The ATP method is a well-established technique for quantifying microbial activity in soil. ATP, the universal energy molecule present in all living cells, serves as an indicator of microbial metabolic activity. The analysis utilized a bioluminescence assay in which ATP, extracted using specific chemical reagents, reacts with the enzyme luciferase to produce light. The intensity of the emitted light is directly proportional to the ATP concentration, providing a precise measurement of microbial activity in the soil samples. In the present experiment, the LuminUltra ATP testing kit, see figure 8, was used. It provided the following protocol for the analysis of solid samples: 1. Sampling and extraction of ATP: • Take 0.25 g of substrate and place in a tube containing UltraLyse ATP extracting reagent. • Shake vigorously during 5 minutes. • After shaking, extract 1 mL of the solution and transfer to a dilution tube. 2. ATP measurement: • Place 100 µL of the diluted solution in a specialized vessel. • Add 100 µL of Luminase to reveal the presence of ATP. • Start the meter and wait for the ATP reading.
33 The blank is made with a reagent-free solution to serve as a result control which is expressed in RLU (Relative Light Units). The calculation of ATP determination, provided by the manufacturer, is as follows: 𝐴TP(pgATP g sample)=RLUsample − BackgroundRLU RLU Luminase ·50000 (pgATP) mass of sample (g) Figure 8. Set up for the analysis of microbial activity using the ATP extraction method. 4.2.2.2. Pore water analysis To extract pore water from the pots for leachate analysis, Rhizon samplers were used. These instruments, provided by Rhizosphere, are designed to extract pore water while filtering it, making it suitable for heavy metal testing. The model used was the MOM sampler, which features a 5 cm membrane with a pore size of 0.15 micrometers for filtration and a total length of 12 cm. The samplers were fully inserted into the pots to their total length. Sampling was performed four times during the experiment, beginning two weeks in and subsequently every two weeks. For sample collection, 10 ml Luer lock syringes were attached to the Rhizon samplers, and negative pressure was applied by pulling the syringe plunger and locking it in place using a pipette tip. This setup, see Figure 9, allowed the pore water to be extracted overnight. Once collected, the leachate from all plants within the same group was pooled to ensure sufficient sample volume for analysis. The samples were then sent to the Department of Materials Science and Engineering at UPC, where they were acidified and stored under refrigeration until they
34 were sent to Technologic and Scientific Centres at Universitat de Barcelona (CCitUB) for the metal testing. Figure 9. Rhizon samplers for pore water extraction attached to a luer syringe. The objective of this was to properly assess whether the slags were leaching and in which concentration. Along with the metal analysis on the plant’s biomass, this test allowed us to get a comprehensive picture of how the slags were affecting the environment. Again, the metal list included the following: Zn, Ba, Ni, Se, Cu, Mo, As, V, Cr, Cd, Hg, Pb. Furthermore, pH and conductivity were also tested. These results compared to the initial leaching analysis of the slags give us information on the impact of such slags in agricultural conditions. 4.2.2.3. Soil composition and properties Initial and final soil composition analyses were conducted to characterize the growing conditions of the experiment focusing on nutrient availability. The parameters studied were: humidity, pH, conductivity, Phosphorus concentration, Potassium, Calcium, Magnesium, elemental Nitrogen and elemental Carbon. Given the alkaline nature of slags and their potential leachate, the monitoring of pH and electric conductivity was deemed important. The initial testing was made solely on the peatmoss to assess the nutrient content. The results pointed to a nutrient-poor soil. This aligned with our objectives since we needed the fertilizer and the slags to be the sole source of nutrients. For the final soil composition analysis, the mixture of peat moss and perlite was sent for testing (at Eurofins, Lleida) after undergoing a sieving process in which all slag content was removed. Since this final analysis also included the peatmoss, the results are not completely comparable with the initial testing but it does allow for a comparison within experimental conditions.
35 4.2.3. Environmental factors 4.2.3.1. Soil moisture Moisture is certainly a critical factor for plant growth. In general, a moisture content in between 20% and 40% is considered ideal for microbial activity and nutrient cycling, as these conditions allow efficient decomposition of organic matter by microbes and increased availability of nutrients to plants (Cherlinka, 2024). In this experiment, a soil moisture level of 26% was intended to be maintained to create a stable environment conducive to healthy plant development and microbial processes. This moisture content represents 80% of the field capacity of the soil studied, which was established at 0.325 m3 water/m3. Corrections had to be made throughought the experiment to keep such moisture and was not always achieved in a constant manner. Field capacity refers to the maximum water retention capacity of a substrate. It represents the upper limit of soil moisture for optimal plant growth, providing sufficient water while ensuring adequate air space for root respiration and microbial activity. At this stage, soil pores are filled to capacity without being saturated: larger pores empty by gravity, while smaller pores retain water by capillary action. This balance enables plants to extract water easily, minimizing the risk of waterlogging that can harm roots and soil microorganisms (ScienceDirect Topics Overview of Field Capacity, n.d.; Zotarelli et al., 2010). This capacity varies according to soil texture and structure. As an example, sandy soils have a lower field capacity (0.15 m3 water/m3 soil) due to their larger pores, which drain quickly, whereas clay soils (0.45 m3 water/m3 soil), with their smaller pores, retain water longer. Plants were watered in a circular motion, to distribute the water evenly. This was done biweekly without exception. In order to keep a rather equalized water supply, initially all watering was homogeneous, see Table 7. This initial constant watering was maintained during the first 3 weeks (6 waterings) to allow for equal conditions amongst all experimental conditions. Table 7. Watering amounts during the first 3 weeks. CC+ E E10 E20 Water 50 mL / / / / P-less Hoagland / / 50 mL 50 mL 50 mL Hoagland / 50 mL / / /
36 As the experiment moved forward, more developed plants required more water than the less developed ones. We considered this a disadvantage that was limiting growth. Therefore, we added an additional water supply in proportion to the soil’s moisture. This soil moisture content was monitored at the time of watering using moisture probes. The moisture generated by the addition of the 50 mL of fertilizer was determined. This proportion of mL added to the increase of moisture levels was measured and calibrated throughout the experiment. This ratio will be referred to as mL:moisture. For all waterings, all pots except those in the Cgroup, received 50 mL of fertilizer plus an additional water supply. In order to calculate in a reasonable manner this additional water supply, the moisture of each pot was measured. Then, using the mL:moisture ratio the amount of extra water to be added was measured: (Wd – Wt) · mL:moisture= Additional water Wd refers to the desired moisture content, 26% Wt refers to the moisture measured at the time of watering mL:moisture refers to the ratio found between mL of water added and moisture increase 4.2.3.2. Light exposure The trial was conducted under artificial lighting with a light:dark cycle of 16h:8h. This proportion is most representative of late spring to early summer in Barcelona though it slightly exaggerates natural summer daylight hours to promote plant growth in controlled experimental settings. 250 V LED lights were used to provide controlled and consistent light exposure and for their energy efficiency and durability, see Figure 10. Light exposure plays a critical role in plant growth and development, serving as the primary energy source for photosynthesis. Photosynthetically Active Radiation (PAR), the range of light wavelengths between 400–700 nm, is vital for this process as it drives the conversion of light energy into chemical energy. (Lee et al., 2023) Figure 10.LED lights used. Light intensity was measured using a photon meter. Initial measurements revealed heterogeneity in light distribution, with plants at the centre of the setup receiving higher light
37 intensities compared to those at the edges, see figure 11. To address this, a rotation schedule was implemented to compensate for uneven light exposure. Plants were rearranged approximately every 2.5 weeks, ensuring that each plant experienced both high and low light conditions over the course of the experiment. The rotation strategy involved a randomized initial distribution followed by systematic reassignments to balance light exposure across all treatments. Figure 11. Light intensity distribution [Klux]. Each cell of the figure represents a pot position within the experimental set-up. Three different light distribution patterns were adopted during the experiment to address the observed heterogeneity (see Figure 12). This approach was critical to minimize variability in light exposure, which could influence photosynthetic efficiency and, consequently, plant growth. Additionally, data collected using the photon meter allowed for precise monitoring and validation of the adjustments made. While it was not feasible to achieve completely uniform light exposure, the compensatory rotations ensured that light availability was not a confounding variable in the comparative analysis of treatments. Future iterations of this experiment could explore further refinements in light setup, such as the use of diffused lighting systems or supplementary side lighting, to reduce light intensity disparities. Figure 12. Distributions used throughout the experiment. The colour refers to the experimental condition: blue (C-), pink (C+), green (Esc), orange (Esc 10) and yellow (Esc20). The numbers refer to the specific replica within the experimental condition (1-4).
38 5. Results To assess the impact of the treatments on basil crops compared to the controls, this section provides an in-depth discussion of the monitored variables throughout the experiment. The analysis is divided into three categories: plant health and growth, soil health, and metal pollution in the plants and leachate. For practicality, the term "positive conditions" will refer to C+, E, E10, and E20. Annex I provides images of the final state of the basil plants to support the findings. 5.1. Plant health and growth Chlorophyll The data represented in figure 13 represents the average chlorophyll content for the different experimental conditions (C-, C+, E, E10, E20). The units used are SPAD meter readings, which would require calibration to quantify total chlorophyll. However, for the purposes of this experiment, relative comparisons are sufficient. As a general trend, all positive conditions had a chlorophyll peak early in the experiment (week 2 and 3) and all except condition E finished the experiment in a negative trend. Since all experimental conditions flowered at around week 7 and 8 a natural decline was expected. Condition Cseems to have a rather constant declining trend while also showing the lowest chlorophyll content. This result is consistent with its experimental condition, in nutrient deficient plants chlorophyll content is negatively affected. The positive control, C+, peaked at week 3 and showed a constant behaviour until the flowering period. All three amended conditions – E, E10 and E20peaked at week 2. E then stabilized for the duration of the experiment. E10 and E20 fluctuated significantly. This behaviour could be coherent with different P-releasing rates of the slags. Since each plant had different watering amounts in proportion to its development this could have resulted in different geochemical conditions in the soil which could produce uneven release of P. It is important to note that C+ received a weekly phosphorus contribution, whereas E, E10, and E20 had their entire phosphorus content applied to the soil at the start. This difference may explain the consistent performance observed in C+. At weeks 6 and 7, before and the initial stages of flowering, there was no significant difference amongst positive conditions: E20 showed the highest results while E showed the lowest. The final measurement showed significant differences amongst positive conditions: E showed the highest chlorophyll content and E10 the lowest. This could be a consequence of different flowering rates or different growing rates that would have resulted in faster nutrient depletion.
39 Figure 13. Chlorophyll readings per environmental condition through time. When factoring in variability, see table 8, we see that the negative control, C-, shows the lowest variability through time. Condition E is the amended condition with the lowest variability while the positive control, C+, shows the highest. All positive conditions have a similar average chlorophyll content, reflecting overall equal health. Table 8. Average chlorophyll readings throughout the duration of the experiment per experimental condition and standard deviation. Average chlorophyll content Standard Deviation C29.1 2.81 C+ 37.3 4.50 E 36.13 3.01 E10 35.05 4.34 E20 36.58 4.24 20 25 30 35 40 45 SPAD reading (relative to chlorophyll content) CC+ E E10 E20 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8
40 Plant height To further investigate plant development, growth was monitored over time, see Figure 14. As previously explained height is not a proper assessment of a plant’s growth it lacks dimensions, so this information will be used to assess growth rates and will be corrected with information on final biomass. As expected, Cexhibited the least growth, consistent with its nutrient-deficient condition. All positive conditions showed significant growth starting from week 2. E10 began to outperform the other groups by week 3 and maintained its lead through the duration of the experiment. The amended condition E, which contained the same amount of phosphorus as C+, and E20 demonstrated similar growth rates to C+. Figure 14. Plant height average readings per environmental condition in time. Table 9 shows the average initial and final plant length values per experimental condition and the standard deviation. For the positive control, C+, the standard deviation is 9.12cm, 17.05% of the final length value. Therefore, this deviation is considered significant and challenges the comparability of the results. This suggests that, under control conditions, individual plants exhibited different growth rates, possibly due to inherent genetic differences or microenvironmental variations within the experimental setup. Experimental condition E10 was not only effective, as seen previously, but also favoured a rather uniform plant growth. The other amended groups, E and E20, showed even more uniformity. Lower variability suggests that these conditions provided a more stable and consistent environment. 0 10 20 30 40 50 60 70 Height [cm] CC+ E E10 E20 Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8
41 Table 9. Plant height average initial and final readings and their standard deviation per environmental condition Initial length (cm) Final length (cm) AVG SD AVG SD C2.68 0.30 9.45 4.19 C+ 2.35 0.24 53.50 9.12 E 2.70 0.10 47.75 2.36 E10 2.80 0.26 58.88 3.33 E20 2.40 0.15 56.75 2.63 Above and below ground biomass Complementing with information with the final biomass is crucial as height is an insufficient indicator of a plant’s vigour. See figure 15 for the average and standard deviation of the final above ground fresh weight for each experimental condition and its moisture content. Consistent with its height, Cexhibited the lowest biomass, reflecting its nutrient-deficient condition. E10 maintained its superior performance, producing 7.66% more biomass than the runner up, C+. However, E displayed proportionally less biomass relative to its height, highlighting the importance of measuring biomass to gain a more comprehensive understanding of plant development. E’s performance indicates that, although the phosphorus provided by the slag supported plant growth, it was significantly less effective than the standard fertilizer alone. Amongst the positive conditions, E10 followed by E20 demonstrated the highest uniformity among individual plants, while E exhibited the lowest. Interestingly, C+ showed greater consistency in biomass compared to its variability in height. Moisture content remained consistent across all positive conditions, averaging approximately 85%.
48 Following up with the plant’s metal content. The analysis gave the results in mg of metal per kg of dry weight, to easily compare with the European Union’s maximum metal content in food and knowing that all experimental groups showed a rather constant moisture content, these results were converted to mg of metal per kg of wet weight. In Table 12 highlighted in light red are the values that surpass the reference values; however, this marking is inflammatory because the stablished serving size of basil plants is at around 5 leaves or 2.5 grams/day, far from 1kg. Taking this into account, all the results are within acceptable values of daily intake and far from the maximum tolerable amount. Table 12. Metal concentration within plant tissue compared to maximum and reference intakes for the studied metals (Source in table 2). In bold the maximum values. Highlighted in light red the values that surpass the reference values. Surprisingly, the most amount of maximum values were found in Cplants. The metal values are expressed in terms of concentration. Cplants were the ones with the least amount of mass, therefore the initial values of essential nutrients have not diluted into a larger biomass as may have happened with the other groups. Table 13 shows the total amount of each metal per group. This has been achieved by multiplying the concentration by the average biomass of the experimental condition. This table reveals that indeed the total amounts of each element are much greater in the positive conditions compared to the negative control.
49 The concentration of Zn follows a decreasing trend when compared to the amount of slags amended. Healthier plants are associated with greater concentration of Zn, deficiency could relate to damage to roots or poor irrigation practices, neither of which have been observed in the present experiment. Further testing would be necessary to determine the reason for this. Ni and Pb are present in all conditions, highly concentrated in C-. This indicates that they were probably present in the soil used. Slag induced metals - Ba, Mo and V - are in higher concentrations in E20, as expected. Disregarding Cconcentrations, these metals showed proportionality with the amount of slag. Comparing with the pore water results we find some differences. Se, As and Cr were not present in the pot’s leachate but were found in plants from all groups. Se is not essential for plant growth but can be beneficial, however, As and Cr are pollutants and should not have been present within the plant. The slag’s leachate could have a small contribution to their presence with a 0.01ppm concentration of As and 0.03ppm for Cr. Their presence is within health limits but their origin is unknown and suspected to be from the own seedling or the soil. This evidence indicates that, although the metals leached from the slags remain within acceptable limits, they are still absorbed by the plants. The presence of metals which were directly attributed to the slag amendments, underscores the need for careful monitoring and management in future applications. While the concentrations observed in this study comply with safety standards, the cumulative effects of long-term slag use and the potential for metal accumulation in soil and crops must be carefully evaluated. Table 13. Total metal amount per average plant of each of the experimental conditions. In bold the maximum values. Element C- [μg] C+ [μg] E [μg] E10 [μg] E20 [μg] Zn 300.42 1247.89 1040.17 1099.24 661.99 Ba 12.97 65.83 63.59 233.85 272.39 Ni 1.05 5.74 4.25 7.91 7.14 Se 0.60 4.62 2.56 5.54 5.19 Cu 11.76 38.96 20.39 38.51 37.32 Mo 1.17 7.92 7.17 14.23 15.58 As 0.36 0.66 2.05 2.37 2.60 V 0.70 3.50 3.18 13.92 17.39 Cr 0.85 7.00 5.33 8.46 6.81 Cd 0.32 0.73 0.72 0.71 0.45 Pb 0.70 5.15 2.66 5.85 4.93
50 6. Conclusions The findings of this study confirm that enriched slags can serve as sustainable, cost-effective, and regenerative fertilizers by providing bioavailable phosphorus. Among the experimental conditions, E10 consistently outperformed the other groups across nearly all monitored variables, including plant height, aboveand below-ground biomass, root length, and microbial activity, while maintaining high uniformity among individual plants. The C+ group generally exceeded the performance of the E group, indicating that not all phosphorus adsorbed by the slag becomes bioavailable. E10, which contained ten times the phosphorus provided to C+, showed slightly better results than C+, indicating that slightly more than 10% of the phosphorus adsorbed by the slags became bioavailable. The results on microbial activity indicate that slags have no negative impact on its development, demonstrating their compatibility with soil ecosystems. Although the high variability in the results limits the ability to make precise comparisons between conditions, E10 exhibited the most favorable results with 26.52% more activity than C+. These findings highlight the ecological compatibility of phosphorus-enriched slags as a potential soil amendment, as they support microbial health while providing bioavailable nutrients. The monitoring of metal leaching from the slags has conclusively demonstrated their safety for both human and ecosystem health. The slag’s leachate did contribute to the presence of certain metals, especially Ba, Mo and V which were also absorbed by the plants. However, the concentrations of these metals within the plants and in the soil’s pore water were consistently well below international reference and safety thresholds, underscoring their environmental compatibility. These findings provide strong evidence that phosphorusenriched slags can be safely utilized as a soil amendment in agricultural practices. However, the cumulative effects of long-term slag use and the potential for metal accumulation in soil and crops must be carefully evaluated. These findings have positive social, economic, and environmental impact within the agricultural sector, these ideas are further developed in the additional chapter 7. Sustainability analysis and ethical implications. Low-cost filtering technologies such as wetlands coupled with the P-removing qualities of slags could be used in agricultural waste water treatment. This affordable technology could bring empowerment to rural areas which would produce effective fertilizer while keeping nutrient pollution controlled and closing the P cycle locally.
51 6.1. Limitations During the experiment and the subsequent analysis, several limitations were identified, which provide opportunities for refinement in future research. This section highlights these challenges and proposes recommendations for further investigation. One key limitation was the watering method used in the experiment. The approach did not account for varying water consumption rates among plants. More developed plants consumed water more quickly than less developed ones, leading to greater variability in soil moisture content. This inconsistency arose because all experimental conditions followed the same watering schedule. While this may have influenced growth rates, the results show that the E10 group established its lead by week three and maintained it throughout the experiment. Therefore, there was probably no impact on the results. However, implementing a more automated watering system, capable of maintaining consistent soil moisture levels, would ensure uniform conditions for all plants and yield more accurate results. Budgetary constraints also restricted the scope of metal testing. Although promising results were obtained, expanding the analysis to include a broader range of metals would offer a more comprehensive understanding of the environmental and agronomic impact. Specifically, future research could examine the following elements: • Iron: A crucial micronutrient for plants that significantly influences microbial activity. • Manganese: Present in slags, it could positively affect enzymatic functions if absorbed by plants. • Magnesium and Calcium: Typically found in slags, these elements improve soil structure and fertility. Additionally, a more detailed initial assessment of the plant’s nutrient content as seedlings could help clarify the concentration of metals at early developmental stages. Similarly, while the initial soil analysis focused on peat moss to understand the nutrient context, including an analysis of the complete soil-perlite mixture would have facilitated a more accurate comparison with the experimental results. 6.2. Future lines of research In light of the results and the identified limitations, future iterations of this study are recommended with changing parameters that could enable more precise measurements and better insights into the efficiency and impact of using steel slags as fertilizers. The following areas of interest have been considered:
52 • Comparison of slag types: investigating the properties of different slag types, such as blast furnace (BF) and induction melting (IM) slags, could reveal variations in their effectiveness as fertilizers. • Use of slags from actual filters: according to Drizo & Bird’s assay in 2009, Pfiltering wetlands often result in slags being covered with biofilms, solids, and other deposits, which may alter their performance. The present study simplified the process by using chemically charged slags, but future experiments should compare these to slags used in real-world filters to better understand how these conditions affect phosphorus bioavailability. • Monitoring development stages: building on the work of Drizo and Bird (2009), future research could examine plant development across different growth stages. Their findings suggested that EAF steel slags perform better in the long term, requiring extended periods to release phosphorus effectively. While this study observed consistent performance throughout the experiment, further comparisons across varying harvest times could clarify these discrepancies. • Effect of slag granulometry: the particle size of slags affects their phosphorusadsorbing properties and may influence phosphorus bioavailability. Testing slags with varying particle sizes, including crushed slags, could identify the most efficient granulometry for use as fertilizers. Pursuing the proposed research directions will provide a deeper understanding of the potential of steel slags as sustainable fertilizers and optimise their efficiency.
53 7. Sustainability analysis and ethical implications In this section the environmental, economic and social implications of the development of the thesis as well as its application will be explored, including a review on potential risks and the identification of the project’s limitations. Understanding the interactions and the characteristics of the three pillars of sustainability is crucial in the development of new technologies and services. This analysis is divided in three subsections for clearer assessment: the development of the thesis, the potential application of the slags and a section dedicated to the risks and limits of this analysis. A section on ethical implications and the alignment with the Sustainable Development Goals is also provided. 7.1. Development of the thesis 7.1.1. Environmental perspective The ultimate goal of this experiment lies within circularity: recycling slag materials as fertilizers to shut down the phosphorus cycle. However, materials, transport and resources were needed to perform the experiment and those have certain impact in terms of emissions. The main preoccupation revolving around the experiment was the presence of heavy metals, which have been thoroughly monitored. No dangerous leaching has been found at the experiment’s conditions. For the purpose of this analysis, and given the fact that the experiment is part of a larger project, boundaries need to be set. The scope of this analysis starts with the slags (not its production process), and ends with the dismantling of the pot experiment. Furthermore, the analysis externalized at outside facilities had an impact beyond our capabilities. Most of the material used was already in possession of the research group or the lab, including pots, moisture meters and LED. In fact, reducing the amount of new material was always a priority while setting up the experiment. Energy consumption was minimized using LED lights, which are energy efficient. Watering was kept to the necessary amount to avoid percolation. Here is a list of potential categories of impact: • Resources: o Water: ▪ For irrigation: 31.06 L ▪ To clean lab equipment. We do not have the means to estimate this value since this was not monitored throughout the experiment.
54 o Energy: ▪ Lighting: LED lighting during 16 hours a day for 57 days, resulting in a total of 912 hours of light from the 4 led transversal lights. LED consumption can range from 10 to 50Wh/hour. To be in the conservative side we will measure an impact of 50Wh/hour light, giving a total of 182.4 kWh • Transportation: o Slag production plant to experimental site (UPC Campus Nord): 46 km o Rhizon samplers were sent from the Netherlands. There was a mistake while ordering so it was returned and sent again using express sending. 1500km per way. 4500km in total. o Leachate samples were sent for testing to the University of Barcelona (CcitUB), which is less than 1km from UPC Campus Nord. Therefore, we will neglect this impact. o Soil and plant analysis were sent to Eurofins, Lleida for testing. 262 km. o Staff transport to the experimental site (Sant Cugat to UPC Campus Nord by train). 13km each way for about 4 months. An approximated 3224km in total. o The basil seedlings were picked up at Jordi Planters Scp at Viladecans, Barcelona. 62km. • Material: o The ATP detection kit required the use of single use plastics. A rough estimate of a total of 800g of single use plastics were used. o The usage of a laptop for the duration of the experiment and the generating of documents (4 months of usage). The previous information is summarized in table 14. Using emission factors extracted from various sources specified in the table, a rough estimate of the total carbon footprint of the project was generated.
55 Table 14. Carbon footprint calculation including the inventory, the value, the emission factor used and its sources and the total values. Inventory Element Corresponding Value Emission Factor (kg CO2e/unit) Total Emissions (kg CO2e) Source Water for irrigation 31.06 L 0.000344 0.01068464 Lifecycle analysis (LCA) studies for water systems (e.g., IWA) Lighting (LED, 4 lights, 912 hours) 182.4 kWh 0.233 42.4992 Spain lectricity gridemission data (Global Carbon Atlas, REE) Transport: Slag production plant to experimental site 46 km 0.12 5.52 IPCC guidelines for diesel vehicle emissions Transport: Rhizon samplers 4500 km 0.12 540 IPCC guidelines for freight trucks (average emission factor) Transport: Staff commuting (FGC) 3224 km 0.03 96.72 FGC Barcelona emission data (official reports) Transport: Soil and plant analysis (Lleida to site, 132 km) 262 km 0.12 31.44 IPCC guidelines for diesel vehicle emissions Transport: Basil plant pick up at Viladecans 62 km 0.12 7.44 IPCC guidelines for diesel vehicle emissions Single-use plastics from the ATP kit 800 g 6 4.8 LCA studies for plastics (EPA, Plastics Europe) Laptop usage 960 hours 0.05 48 Average laptop energy consumption data (IEA, LCA studies) TOTAL 776.43KgCO2e
56 The resulting total impact calculated is 776.43 KG of equivalent CO2 for the development of this experiment. For reference, the average European person generates 7.25 t equivalent CO2 per year (JRC). From looking at the table, we deduce that the weak spot of the project in terms of sustainability was transportation. But aside from the rhizon, which we simply could not find from local suppliers, there was an effort to use local testing facilities to reduce the environmental and economic cost of the whole project. These are the suppliers that mainly participated in the experiment: • Celsa to produce the slags. On their website (https://www.celsagroup.com/) they have a section on their environmental commitment, claiming they are the leading steel company in Europe in terms of circular production and low emissions. They recycle ferrous scraps and turn them into steel. They have the goal to become Net positive in 2050. • AdecGlobal for the slag’s revalorization. They specialize in revalorizing construction residues into products. They are committed to sustainable construction, decarbonization and promoting a circular economy. They comply with the regulations from the Agència de Residus de Catalunya. (Find more information at: https://adecglobal.com/) • Rhizosphere Research Products for the Rhizon samplers. No information was found on the website (https://www.rhizosphere.com/). However, after contacting them, they assured that although they are a small company and do not have the means for extensive documentation, all Rhizon samplers are produced in-house, they minimise waste and prioritize re-use (e.g. packaging material) wherever possible. They produce within Europe, therefore they comply with European regulations. • LuminUltra for the ATP kits. On their website they have a section on environmental compliance (https://www.luminultra.com/export-environmentalcompliance/) where they acknowledge the obligation of minimizing the environmental impact of their products. They facilitate the disposal of their products and use environmentally friendly materials. • Jordi Planters Scp for the basil seedlings. No information regarding the environmental commitment of this vendor could be found. In summary, although no specific environmental reports could be found, all the experiment’s suppliers show a commitment towards sustainability. In the case of the slags’ origins, sustainability and circularity are the backbone of both involved companies.
57 7.1.2. Economic perspective This experiment turned out to be more costly than expected due to the externalization of samples. Throughout the project there was an emphasis on reusing and limiting the purchase of new material. The human cost was minimal since the principal author and executor of this experiment was not remunerated. The same approach will be made to the previous section. We generated the following list for approximated potential costs of the most relevant items. • Rersources: o Water: ▪ For irrigation: 31.06 L. Negligible costs. ▪ To clean lab equipment. We do not have the means to estimate this value since this was not monitored throughout the experiment. o Energy: ▪ Lighting: 182.4 kWh. • Transport: o By car (at current gasoline rate of 1.4 EUR/L at a 6.5 L consumed per 100km): ▪ Slag production plant to experimental site (UPC Campus Nord): 46 km by car. 4.18 EUR. ▪ The basil seedlings were picked up at Jordi Planters Scp at Viladecans, Barcelona. 62km in total. 6.64 EUR. o Soil and plant analysis were sent to Eurofins, Lleida for testing. 262 km by transportation service. Fee of aprox 30 EUR. o Leachate samples were sent to the University of Barcelona (CcitUB) for testing which is less than 1km from UPC Campus Nord, we will then neglect this impact. o Rhizon samplers. 4500 km by transportation service. Billed 68.35 EUR. o Staff transport to the experimental site (Sant Cugat to UPC Campus nord by train). 53 EUR corresponding to the fee of 1.33 T-Jove cards. • Material: o Slags: were provided for free. o Lab equipment (reactives and material): this cost was minimal since the Hoagland solution used up very low concentrations of its components. o Rhizon samplers. 150 EUR. o ATP kit. For the amount used up a rough estimate of 700 EUR.
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70 ACKNOWLEDGEMENTS I would like to express my heartfelt gratitude to my tutors, Jaume Puigagut and Diego Apontes, for always finding solutions to every problem and offering explanations for even the most unexpected results. To Rosa Manyosa, my dearest colleague, for her patience, reliability, unwavering support, and all the coffee breaks that kept me going throughout the experiment—and so much more. To Marta Fernández-Gatell for her kindness, expertise, and valuable guidance, and to Emma Gouérec, who accompanied and supported me through the final stages of the experiment. I am also deeply grateful to the entire GEMMA team for their invaluable advice, willingness to help, and genuine curiosity. I extend my deepest thanks to my parents and Nona, who drove me to the lab when my leg was broken and provided boundless support throughout this journey. Specially my mom who read this thesis more times than I did. Thank you Guru, my best friend through thick and thin, for making going to uni worth it every day, for singlehandedly managing Ramoona’s while I was writing this thesis, for endless laughter, and for even more endless dreams. Finally, I want to thank the basil plants for their excellent behavior and for providing us with such satisfying results.
71 ANNEX I Figure 21. Pictures of the plants before the harvest 06/06/2024.
72