Solar photocatalytic treatment of landfill leachate using a solid mineral by-product as a catalyst

1 Solar photocatalytic treatment of landfill leachate using a solid mineral byproduct as a catalyst Rodrigo Pobletea, Lucía Prieto-Rodríguezb, Isabel Ollerb, Manuel I. Maldonadob, Sixto Malatob, Emilia Otala, Luís F. Vilchesa, Constantino Fernández-Pereiraa,*. a Departamento de Ingeniería Química y Ambiental. Escuela Técnica Superior de Ingeniería, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain. b Plataforma Solar de Almería (CIEMAT), Carretera de Senés, km 4, 04200, Tabernas, Almería, Spain. * Corresponding author: Tel.: +34 954481379; fax: +34 954461775. E-mail address: pereir[email protected] (C. Fernández-Pereira). This is an Accepted Manuscript of an article published by Elsevier in Chemosphere, Vol. 88, August 2012, available at: https://doi.org/10.1016/j.chemosphere.2012.04.044. Copyright. Elsevier 2012. En idUS: Licencia Creative Commons CC BY-NC-ND 2 Highlights -A solid industrial by-product containing Ti and Fe was used as photocatalytic agent. - The by-product studied acts coupling photo-Fenton and TiO2 catalysis reactions. -The TiO2 by-product promoted degradation of MSW using solar energy. - The photocatalytic process developed increased landfill leachate biodegradability. ABSTRACT The treatment of municipal solid waste landfill leachate in a pilot plant made up of solar compound parabolic collectors, using a solid industrial titanium by-product (WTiO2) containing TiO2 and Fe(III) as a photocatalyst, was investigated. In the study evidence was found showing that the degradation performed with WTiO2 was mainly due to the Fe provided by this by-product, instead of TiO2. However, although TiO2 had very little effect by itself, a synergistic effect was observed between Fe and TiO2. The application of WTiO2, which produced coupled photo-Fenton and heterogeneous catalysis reactions, achieved a surprisingly high depuration level (86% of COD removal), higher than that reached by photo-Fenton using commercial FeSO4 (43%) in the same conditions. After the oxidation process the biodegradability and toxicity of the landfill leachate were studied. The results showed that the leachate biodegradability was substantially increased, at least in the first stages of the process, and again that WTiO2 was more efficient than FeSO4 in terms of increasing biodegradability. Keywords: Industrial by-product, Landfill leachate, Photo-Fenton, Solar collector, Titanium dioxide. 1. Introduction Landfill leachate (LL) is generated when municipal solid waste (MSW) is disposed of, essentially due to the percolation of rainwater through the landfill layers and waste 3 decomposition (Renou et al., 2008; Puig et al., 2011). LL may contain large amounts of hardly-biodegradable organic matter, where humic acid-type constituents are an important group, as well as other pollutants. It has been reported that one of the most important aspects related to the planning, design, operation and long-term management of a MSW landfill is the management of leachate (Halim et al., 2010). LL may be characterized as an aqueous solution of four groups of contaminants: dissolved natural organic matter (including recalcitrant compounds), inorganic macrocomponents (common cations and anions including sulfate, chloride, iron, aluminum, zinc and ammonia), heavy metals (Pb, Ni, Cu, Hg), and xenobiotic organic compounds such as halogenated organics, (PCBs, dioxins, etc.) (Vilar et al., 2011a). Therefore, MSW leachate discharge may cause serious pollution problems to groundwater and surface waters. Technologies commonly used for leachate treatment, whether physico-chemical or biological, depend on the age and leachate composition, but none of them alone are efficient enough for suitable depuration of LL (Xu et al., 2010). Consequently, it is still necessary to develop new technology which allows the proper elimination of the pollutants commonly found in LL. For mature LL, due to its high level of recalcitrant organic compounds, treatment options are reduced to physical or chemical processes. Recently, reverse osmosis has been used to treat this kind of leachate, but although this process is very efficient, it leads to high investment costs and energy consumption. Membrane fouling, which requires extensive pre-treatment or chemical cleaning of the membranes, results in a short membrane lifetime and decreases process productivity. Finally, the treatment also results in the generation of large volumes of concentrate, which is normally recycled to landfill (Renou et al., 2008). Another promising option for LL treatment is related to the use of advanced oxidation processes (AOPs), since with the use 4 of these technologies, the organic matter present in water, including recalcitrant compounds can be reduced. This objective is achieved through the hydroxyl radical action, generated by several processes such as Fenton, photo-Fenton (both using Fe and H2O2), O3/UV, and heterogeneous photocatalysis by TiO2, among others (Weichgrebe et al., 1993; Gonze et al., 2003; Kurniawan et al., 2006; Deng and Englehardt, 2007; Primo et al., 2008), which enhance the biodegradability of recalcitrant compounds in the leachate. However, it has been reported that the greatest disadvantage of AOPs is also the high demand for electric power and the consumption of expensive chemicals, which increase the operation costs of the process (Lopez et al., 2004; Matilainen and Sillanpää, 2010). For this reason, there is a growing interest in developing AOP applications using solar energy (e.g. photocatalysis with TiO2 or photo-Fenton) along with low-cost catalysts (Malato et al., 2009; Vilar et al., 2011a,b). According to the literature, LL is more efficiently treated by the photo-Fenton reaction than by TiO2, since the reaction rate is much higher and the Fe concentrations needed is very low (Rocha et al., 2011; Vilar et al., 2011b). Therefore, in order to improve the photocatalytic process, a new TiO2 catalyst doped with Fe has recently been developed to increase the production of H2O2 during the AOP (Araña et al., 2011). In a previous study, an industrial titanium by-product (WTiO2), containing not only TiO2 but also Fe(II) and mainly Fe(III), was used as a catalyst to degrade p-cresol, humic acids and LL using a UV photo-reactor (Poblete et al., 2011). The potential of WTiO2 as a catalytic agent in AOPs was demonstrated since this catalyst showed an enhanced removal percentage of organic matter and therefore much higher activity in the degradation of humic acids or LL under UV light irradiation than commercial TiO2. Soluble Fe(III) compounds may well have had a synergistic effect on the catalytic activity of TiO2 when WTiO2 was used. 5 The aim of the present work was to enhance the efficiency of the conventional UV photo-Fenton treatment of LL by using the same industrial by-product (WTiO2) as a lowcost photocatalyst together with H2O2, applying solar UV irradiation in a pilot plant based in a solar compound-parabolic collector (CPC) photoreactor. The purpose was to produce a combination of solar photo-Fenton (or photo-Fenton like) and heterogeneous photocatalysis, due to the presence in WTiO2 of Fe(II), (or Fe(III)) and TiO2, respectively. In addition, the leachate biodegradability was studied before and after the oxidation process to know the degree of improvement achieved. This work intends to establish a technological basis, to analyze the possible use of this by-product in LL treatment plants, completing the treatment scheme. 2. Experimental 2.1. Landfill leachate and photocatalyst Leachate was collected from a storage unit with a capacity of 12000 m3 situated in the integrated MSW plant Mancomunidad de La Vega, Seville, Spain, which produces a leachate flow rate of 7000 m3 yr-1. The main characteristics of LL are presented in Table 1. The by-product tested as a catalyst (WTiO2) is a solid waste product obtained from the concentrated sulfuric acid digestion of ilmenite at 150–220 ºC, where titanium separates in the form of titanyl sulfate which, after hydrolysis, precipitation, and calcination, is transformed into TiO2. The main components of WTiO2 (wt%) are TiO2 (50.6), Fe2O3 (19.1), SiO2 (11.1), Al2O3 (1.7), CaO (0.28), and MgO (0.13) (Vilches et al., 2003). Fe was found in its trivalent state, mainly in oxy-hydroxide form (FeOOH), and very small changes in the Fe chemical environment were produced after the use of WTiO2 as a photocatalyst (Poblete et al., 2011). Before use, WTiO2 was dried at 105 ºC 6 for 2 d and milled, selecting particle sizes in the range of 63-75 μm. A conventional Fenton photocatalyst was used in the form of FeSO4·7H2O (Panreac) along with H2O2 (30% w/v, Panreac) for photo-Fenton treatment. 2.2. Experimental setup Photocatalytic experiments were carried out in a CPC specifically developed for photo-Fenton applications, installed at the Plataforma Solar de Almeria, using solar energy as a UV light source (Kositzi et al., 2004). The reactor, with a total volume (VT) of 21 L, consists of five borosilicate glass tubes of 50 mm external diameter, 142 cm length and 1.4 mm thickness, with a transmissivity between 0.900 and 0.915 (λ ≥ 350 nm of wavelength). The irradiated volume (Vi) and area were 11 L and 5.1 m2, respectively. The CPC reactor is provided with a tank where some parameters such as pH or temperature were measured and different reagents were added, such as FeSO4, WTiO2, H2O2, H2SO4 or NaOH. Solar UV radiation was measured by a global UV radiometer (KIPP&ZONEN, Model CUV 3) on a platform tilted 37º (the same as the CPCs), which provides data in terms of incident irradiance (W m-2). This gives an idea of the energy reaching any surface in the same position with regard to the sun. Using Eq. (1), a combination of the data from several days’ experiments and a comparison with other photocatalytic experiments was possible (Malato et al., 2003). 11,30,30 , 30   nnn T i nnWnW ttt V V UV ttt (1) where t30W is a normalized illumination time (min), tn is the experimental time for each sample and UV is the average solar ultraviolet radiation measured during Δtn. In this case, time refers to a constant global UV irradiance of 30 W m-2 (typical solar UV 7 irradiance on a perfectly sunny day around noon). The starting point of the process, t30W = 0, was considered when LL was exposed to the sunlight. In order to obtain optimal oxidation conditions for the solar photocatalysis application, the LL was pre-treated. Firstly, its pH was adjusted to 3 using H2SO4 (98%) (Panreac) and NaOH (50%) (J.T. Baker) since the original leachate pH was alkaline (pH = 8.3). A pH value of 3 was chosen because it has been reported that an acid pH favors conventional Fenton leachate treatment (Hermosilla et al., 2009). Moreover, in preliminary tests we did not find a significant difference in COD removal rates in the pH range between 2 and 4. Furthermore, the pH 3 value was very adequate to effectively flocculate organic colloids and improve their filtration. In a second step, the supernatant of the LL pH adjustment was subjected to a coagulation-flocculation process with the addition of FeCl3 (Merck) in order to remove the nonsettleable suspended solids (SS) remaining. To optimize the process, a representative LL sample was tested first, at laboratory scale using jar-test experiments, with the purpose of determining the exact amount of FeCl3 necessary to clarify LL and settle most of the SS. Finally, and prior to the solar photo-oxidation process, the pre-treated effluent was filtered using a 5 μm pore size filter. After the pH adjustment, coagulation-flocculation and filtration steps, 21 L of the pre-treated LL was transferred to the CPC solar pilot plant to perform the corresponding solar photocatalytic treatment. The pilot plant was operated in batch mode with a LL flow rate of 20 L min-1 using a recirculation pump during the experimental time (tn) necessary to obtain the desired mineralization. All the experiments were carried out from October to December 2010, during cloudy and sunny days. 2.3. Biodegradability 8 In order to assess the biocompatibility of LL after the photocatalytic treatment, with the aim of studying the joint treatment of LL with urban wastewater, toxicity and biodegradability tests at different stages of the solar photo-Fenton treatment were carried out. With this purpose, photo-Fenton experiments were performed in a solar simulator (Suntest XLS + photoreactor, Atlas) equipped with a Xe lamp (765 W m-2) and a thermostating unit to keep the temperature at 35 ºC. In these experiments H2O2 was added in several steps to the continuously stirred flask of 2 L of effective volume. This means that when H2O2 was exhausted, samples were taken for analysis, and another dose of H2O2 was added. As a result of this procedure, H2O2 did not affect bioassays. To assess the evolution of the treatment dissolved organic carbon (DOC) and COD were monitored during the process, which allowed direct comparison with the results obtained in the CPC photoreactor, and a subsequent connection of the results of bioassays with the kinetics calculated in the CPC photoreactor. Toxicity evolution during the photocatalytic experiments was analyzed by respirometry using a BMT Respirometer (SURCIS, S.L), provided by an oxygen probe (Hamilton). The respirometer was loaded with the required amount of activated sludge (1 L, according to the manufacturer specifications) taken from a conventional municipal wastewater treatment plant (WWTP). Afterwards, in order to inhibit the nitrification process and measure the sample effect only on the heterotrophic bacteria, 3 mg of N- allylthiourea (Aldrich) g-1 of volatile solids (VS) was added to the activated sludge. For all the runs VS was 2.7 g L-1. Then, continuous aeration to attain air saturation conditions and continuous agitation were applied to the respirometer reactor flask. The temperature was maintained at 20 ºC during the experiment. Toxicity, expressed by means of the inhibition percentage, was measured comparing the activated sludge maximum respiration rate (Rsmax) in the presence of 50 mL of distilled water with 1 g L-1 of sodium 9 acetate (J.T. Baker), added to avoid the absence of substrate, with respect to the activated sludge respiration rate (Rs) obtained in the presence of 50 mL of a partially treated LL sample (after adjusting the pH to 6.5 – 7.5, filtered and aerated) with 1 g L-1 of sodium acetate. The percentage of inhibition, %I, was calculated by Eq. (2). 100) Rs Rs 1(I% max  (2) where Rs and Rsmax are expressed in terms of mg O2 L-1 h-1. For those samples which presented non-significant toxicity values, biodegradability assays based on respirometry were also carried out with the same BM-T respirometer. For the biodegradability tests, the respirometer was also loaded with 1 L of activated sludge taken from a conventional WWTP (VS = 2.7 g L-1). The nitrification process was also inhibited and continuous aeration and agitation were applied to the reactor flask. Finally, 100 mL of the partially treated LL (after adjusting the pH to 6.5 – 7.5, filtered and aerated) was added to the respirometer reactor and the oxygen consumption was monitored automatically during the assay. At the end of the test, the respirometer software gave the readily biodegradable fraction of the filtered COD (rbCOD) according to the total oxygen consumption registered by the equipment. The rbCOD/DCOD ratio shows the sample biodegradability (where DCOD is the dissolved COD of the sample), taking into account that values higher than 0.3 mean that the sample can be classified as highly biodegradable, values higher than 0.1 as slowly biodegradable, while values lower than 0.05 indicate that the sample is not biodegradable (values between 0.05 and 0.1 are considered samples with low biodegradability). 2.4. Analytical determinations 16 Both photo-Fenton photocatalytic processes using FeSO4 or WTiO2 showed an increase of AOS which could be representative of a toxicity decrease and an improvement in biodegradability (Fig. 3). AOS increase was quicker and required a lower H2O2 dose when the catalysis used was WTiO2 (Fig. 3b). Figure 4 shows inhibition and biodegradability percentages achieved in the course of the photo-Fenton with FeSO4 or photocatalysis with WTiO2 treatments of LL. As can be observed, in both cases toxicity measurements achieved 0% of inhibition just after the beginning of the process. However, after a consumption of 5 g L-1 of H2O2 during photo- Fenton treatment, toxicity increased up to 30% and then decreased until 0% at the end of the process. This behavior could be due to the generation of highly toxic intermediates (González et al., 2007; Trovo et al., 2009; Arslan-Alaton et al., 2010) which are later eliminated in the process. Concerning the biodegradability results, raw LL biodegradability was enhanced due to the oxidation process from 3% to 64% or 12-89% by photo-Fenton or photocatalysis with WTiO2, respectively. The enhancement of biodegradability fits well with the observed continuous increase in AOS. 4. Conclusions In this work the removal of organic matter from a MSW landfill leachate was accomplished using the WTiO2 by-product in a CPC reactor at pilot plant scale. One of the main conclusions of the work was that the use of WTiO2 produced a higher organic matter removal rate (86%) than photo-Fenton using commercial FeSO4 (43%). The proposed solar photocatalytic integrated treatment of LL using WTiO2 was more efficient than photo-Fenton using FeSO4 in terms of treatment time, COD removal and H2O2 consumption. The main reason for the high WTiO2 efficiency in LL depuration appeared 17 to be its Fe content, since the use of the iron-free by-product resulted in a DOC removal efficiency lower than 37%. Biomass from activated sludge produced in a municipal WWTP has the ability to utilize the LL studied, without a previous adaptation time, but only to a certain extent, because extensive biodegradation is usually restricted by the high level of recalcitrant compounds in LL. Process integration of Solar photo-catalyst and aerobic degradation seem conceptually advantageous to treat LL, since AOPs can transform the refractory compounds into more readily biodegradable organic matter. The idea has been proved experimentally, demonstrating that LL biodegradability improved after the two AOPs studied. In summary, the results obtained show that the treatment proposed offers many advantages for potential industrial use in MSW landfills such as, low-cost catalysts, energy savings and lower equipment costs, as compared with the UV lamp oxidation process. Acknowledgements The authors acknowledge the financial support for this research by the Spanish Ministry of Science and Innovation (MICINN), under the project FOXMORE (CTM2006-05114), and the MICINN ICTS (“Programa de Acceso y Mejora de Grandes Instalaciones Científicas Españolas”, Plataforma Solar de Almería, GIC-05-17) for its economic support to the development of the Solar Photocatalysis at Plataforma Solar de Almería (Spain). Rodrigo Poblete thanks the Spanish Ministry of Science and Innovation for his Ph.D. research grant. Appendix A. Supplementary material 18 Supplementary data associated with this article can be found, in the online version. References Achilleos, A., Hapeshi, E., Xekoukoulotakis, N.P., Mantzavinos, D., Fatta-Kassinos, D., 2010. Factors affecting diclofenac decomposition in water by UV-A/TiO2 photocatalysis. Chem. Eng. J. 161, 53–59. APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st Edition. (APHA), Washington DC. 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Fig. 1 t30W (min) H2O2consumed (mg L-1) DOC (r) DOC (s) H2O2(s) H2O2(r) b) 050 100 150 200 250 300 350 400 RAD-ON 0 2000 4000 6000 8000 10000 12000 DOC (mg L-1) t30W (min) DOC (p) DOC (q) H2O2consumed (q) H2O2consumed (p) a) 050 100 150 200 250 300 350 400 0 500 1000 1500 2000 2500 RAD-ON 23 Fig 2. Organic matter degradation achieved in LL in a CPC photoreactor using WTiO2 (1 g L-1) without Fe or without a catalyst (blank test). Both reactors used 200 mg L-1 of H2O2. Fig. 2 050 100 150 200 250 300 350 400 450 500 550 0 500 1000 1500 2000 2500 RAD-ON 0 2000 4000 6000 8000 10000 12000 DOC (mg L-1) t30W(min) H2O2consumed (mg L-1) WTiO2without Fe (blank test) H2O2consumed (blank test) H2O2consumed (WTiO2without Fe) 24 Fig. 3. DOC, COD and AOS evolution, and H2O2 consumption for LL in a photo-Fenton process using (a) FeSO4 and (b) WTiO2. Fig. 3 DOC (mg L-1) H2O2consumed (g L-1) DOC COD AOS AOS DOC COD AOS AOS H2O2consumed (g L-1) COD (mg L-1) 0 2 4 6 8 10 12 0 1000 2000 3000 4000 5000 6000 7000 -1,0 -0,5 0,0 0,5 1,0 0 1 2 3 4 50 1000 2000 3000 4000 5000 6000 7000 -1,0 0,0 1,0 2,0 b) a) 25 Fig. 4. Biodegradability (%) and Inhibition (%) achieved at different landfill leachate treatment stages using FeSO4 or WTiO2 as a catalyst. Fig. 4 012345678910 11 12 13 0 20 40 60 80 100 0 10 20 30 40 Biodegradability (%) Inhibition (%) Biodeg. (FeSO4) Biodeg. (WTiO2) Inhib. (FeSO4) Inhib. (WTiO2) H2O2consumed (g L-1)