Impact of the Biogas Impurities on the Quality of the Precipitated Calcium Carbonate in the Regenaration Stage of a Chemical Absorption Biogas Upgrading Unit

Journal of Environmental Chemical Engineering 12 (2024) 113868 Available online 17 August 2024 2213-3437/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/). Impact of the biogas impurities on the quality of the precipitated calcium carbonate in the regenaration stage of a chemical absorption biogas upgrading unit J. Salinero a , Luz M. Gallego Fern´ andez a,* , E. Portillo a , Judith Gonz´ alez-Arias b , Francisco M. Baena-Moreno a , B. Navarrete a , L.F. Vilches a a Departamento de Ingeniería Química y Ambiental/ Escuela T´ ecnica Superior de Ingeniería (Universidad de Sevilla), Avd. Camino de los Descubrimientos s/n, Sevilla 41092, Spain b Departamento de Química Inorg´ anica / Instituto de Ciencia de Materiales de Sevilla (Universidad de Sevilla, Consejo Superior de Investigaciones Científicas), Av. Am´ erico Vespucio, 49, Sevilla 41092, Spain ARTICLE INFO Keywords: Biogas upgrading Carbon capture Chemical absorption CO 2 utilization Energy recovery Aqueous carbonation process ABSTRACT Combining Carbon Capture and Storage (CCS) with producing competitive secondary raw materials is key to decarbonizing industry and reducing resource extraction. Biogas upgrading to biomethane stand out as an alternative, but a significant gap remains in integrating this process within a circular economy framework. This issue has been recently addressed by a process that integrates biogas upgrading via caustic absorption with the production of Precipitated Calcium Carbonate (PCC) and the recovery of sodium hydroxide from waste brine solution using membrane technologies. The profitability of this approach depends on the quality of the PCC, a critical factor that this work addresses. By characterizing PCC is determined whether trace compounds in biogas contaminate the PCC and potentially affect its commercial value. It also examines the CO 2 absorption process and analyzes the aqueous samples from the filtration phase of the PCC slurry. Results confirm the high purity of PCC obtained from biogas treatment using Raman spectroscopy, X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The analyses show that the PCC is pure calcium carbonate, mainly in the stable calcite form, with a typical tetrahedral morphology and no detectable impurities. Characterization of aqueous solutions revealed organic trace compounds from biogas, with TOC concentrations of 9.7 (±6.4) and 16.0 (±8) mg C/l. Silicon measurements showed similar concentrations in the absorbent solution and filtrated PCC slurry. Additionally, ammonia escapes as gas, and hydrogen sulfide in the biogas likely contributed to sulfate salt formation. Analysis of the CO₂ absorption shows a first-order reaction with OH - , where the amount of CO₂ absorbed (46.3–50.0 g) closely matches the theoretical value of 48 g. The study reveals that most of the biogas impurities dissolve into the aqueous solution, being crucial for future studies and downstream membrane treatments, and the PCC is unaffected by these impurities with a purity suitable for commercial applications. 1. Introduction With the increase in greenhouse gas emissions and depletion of natural resources, policies and measures to evolve towards circular economy is required [1,2]. On the one hand, the rise of the global temperature needs a decarbonized industry that, in most proposals, use CO 2 removal by Carbone Capture and Store (CCS) [3,4]. On the other hand, reducing the extraction of natural resources needs the production of competitive second raw materials from an economical and technical point of view [5–7]. Within this circular framework of reducing, reusing, and recycling, the valorization of biomass and waste diminishes dependence on fossil fuels and produce renewable fuels and chemicals [5–7]. Among alternatives, biogas production stands out as a highly promising sustainable solution [5,8–11]. Biogas is a fuel gas derived through anaerobic digestion of biodegradable organic materials like agriculture and forestry biomass, or sewage sludge [12]. It contains methane (40–70 % v ) and carbon dioxide (30–45 % v ) with varying levels of nitrogen (up to 20 % v ) and oxygen (up to 5 % v ), Additionally, it contains trace compounds grouped as siloxanes, halogens, volatiles organic compounds, and ammonia, in * Corresponding author. E-mail address: [email protected] (L.M. Gallego Fern´ andez). Contents lists available at ScienceDirect Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece https://doi.org/10.1016/j.jece.2024.113868 Received 25 April 2024; Received in revised form 29 July 2024; Accepted 16 August 2024 Journal of Environmental Chemical Engineering 12 (2024) 113868 2 concentration ranging from ppb v to ppm v [13–15]. Biogas is used either as a direct energy source for cooking and lighting or, after a cleaning process, utilized for heat and power production [16]. However, it can be also upgraded to biomethane, removing most of the CO 2 , and be used as a substitute of the natural gas in applications such as transport sector or be injected into the natural gas grid [17]. While research on heat and power production using biogas are focused towards optimizing and enhancing its utilization, assessing the economic viability of the processes, and offering practical knowledge for the successful implementation of such systems across diverse contexts [18]; biomethane utilization deals with its integration into the transport sector and injection into the natural grid [19–21]. Biogas upgrading allows to remove CO 2 and other minor contaminants indicated above to obtain a high-purity biomethane. The technologies available for this goal are, among others, chemical scrubbing (both with amine-based solvents and caustic solvents), physical scrubbing, membrane technology, and pressure swing adsorption. The economy of these technologies and the policy implications behind the biogasbiomethane chain have been assessed by several works [22–26]. All these previous studies have one common point: nowadays, biomethane production without incentives is hardly profitable and hence lacks of interest for investors. Therefore, economics of biogas upgrading via other technologies or process approaches is a must. Furthermore, despite the availability of commercial-scale alternatives for biogas upgrading, which primarily focus on enhancing the purity of the biomethane obtained, there remains a lack of integration between these technologies and a circular economy perspective due to the minimal emphasis on producing secondary raw materials [27]. In fact, only a few works addressing this integration have been found in the literature, mainly proposing the co-production of biomethane and urea [28], formic acid [29], methanol [30]and syngas [31]. The mentioned gap has recently addressed by a process that integrates the upgrading of biogas via caustic absorption, with the production of solid calcium carbonate, and recovering the sodium hydroxide from the waste brine solution using membrane technologies [9,27,32–34]. Approach, sketched in Fig. 1, that is tailored for implementation in biogas production plants. This process demonstrates promising scenarios, reporting high precipitation efficiencies of calcium carbonate with commercial-grade purity [32–34]. However, these studies exclude biogas impurities, which precludes an evaluation of their potential impact on purity. This limitation highlights a gap in the literature regarding the influence of trace compounds on the overall quality of the precipitated calcite. Regarding biogas impurities and trace compounds, literature is focused on their removal due to their propensity for causing corrosion, mechanical wear, and undesired emissions [16]. Therefore, this paper aims to determine whether trace compounds in biogas contaminate Precipitated Calcium Carbonate (PCC), obtained within the regeneration process of the sodium hydroxide (see Fig. 1), and affect its commercial value. The motivation is driven by the need to incorporate a circular economy perspective, generating valuable secondary raw materials, such as high-quality PCC. To achieve this, the PCC obtained after CO 2 absorption from a synthetic biogas doped with impurities is characterized, followed by precipitation using calcium chloride. The objective is to gain practical knowledge into the impact of biogas impurities on the aqueous carbonation process by designing experimental conditions that simulate a worst-case scenario. It is assumed that no preliminary cleaning step is undertaken to remove any trace compounds from the biogas, resulting in their complete dissolution into the absorbent solution from the gas phase. The solid characterization of PCC under these conditions is compared with samples obtained using a mixture of CO 2 and N 2 doped with impurities, as well as samples without any trace compounds. This comparison helps evaluate the extent to which biogas impurities affect the quality and commercial viability of PCC. Additionally, the analysis includes discussions and characterizations of the resulting brine solution, variations in CO 2 concentration in the gas outlet during the absorption phase, and changes in pH and electrical conductivity. 2. Theory As is detailed below and proposed by the authors elsewhere [9], the biogas upgrading involves two steps: the CO 2 absorption in an aqueous solution of sodium hydroxide followed by the production of Precipitated Calcium Carbonate (PCC) adding calcium chloride into the aqueous solution from the absorption process. The mechanism of CO 2 absorption in an aqueous solution of sodium hydroxide is well known [35,36]. The process comprises the following individual reactions: NaOH(l)⇄Na+(aq) + OH−(i) CO2(g)⇄CO2(ii) CO2(aq) + OH−(aq)⇄HCO− 3(iii) HCO− 3(aq) + OH−(aq)⇄CO2− 3(iii) Due to the high ionization of the NaOH in water (i), CO 2 is rapidly and physically absorbed as an aqueous compound (ii), reacting with initially present OH- ions to form HCO3- (iii). Immediately, at pH higher than 10.5, it is transformed into CO 3 2- (iv). Within the mentioned pH range, reactions (i-iv) occur at a rapid rate, and CO 2 absorption takes place within the liquid film [35–37]. The net reaction of the CO 2 absorption is summarized in Eq. 1. 2NaOH(aq) + CO2(g)⇄Na2CO3(aq) + H2O(1) where the sodium carbonate exists in ionized form in the aqueous solution because of its high solubility in water [38]. Similarly, the PCC involves the following reactions: CaCl2(s)⇄Ca2+(aq) + 2Cl−(v) Ca2+(aq) + CO2− 3(aq)⇄CaCO3(vi) The high solubility of the CaCl 2 in water involves a rapid incorporation of the Ca 2+ and Cl - ions into the solution [38], and subsequently, a quick nucleation of the CaCO 3 is produced followed by growth and precipitation of the calcium carbonate [39]. For pH higher than 10.5 reaction (vi) is very fast, even elsewhere is considered as instantaneous [36]. Therefore, the overall precipitation process is summarized in Eq. 2. Na2CO3(aq) + CaCl2(s)→CaCO3(s) + 2NaCl (2) The aqueous sodium chloride solution is then subjected to Electrodialysis with Bipolar Membranes (see Fig. 1), recovering of sodium hydroxide while generating hydrochloric acid. Fig. 1. Aqueous carbonation process for the CO2 from biogas within a circular economy framework (adapted from [27]). J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 3 3. Methods & materials 3.1. Experimental set-up Fig. 2 sketches the lab scale installation where the CO 2 absorption is carried out via caustic absorption. Two different mixtures of gases are used: a mixture of N 2 and CO 2 , with a CO 2 concentration between 33 % and 34 % v , and a synthetic biogas whose composition is in Table 1. These gases are bubbled through a cylindrical sparger with a diameter of 1 cm and a length of 2.5 cm with a pores size of a 50 μ m. The required volumetric flows are set by three different calibrated rotameters. The sparger is placed at the center of the bottom part of 1-liter Pyrex cylindrical container with a diameter of 11.5 cm, which is equipped with three inputs: sparger, and sensors of pH and Electrical Conductivity (EC), and one output for the outlet gas. 0.75 liter of aqueous solution of sodium hydroxide at 10 % w is stirred by an electromagnetic magnet at 1000 rpm. The output gas is conducted, through a silica gel trap that removes its moisture content, into a gas analyzer for measuring the CO 2 concentration. Fig. 3 sketches the experimental set-up where the Precipitated Calcium Carbonate (PCC) is carried out. CaCl 2 powder is added into the aqueous brine solution obtained from the previous CO 2 absorption step. The reaction is carried out into another Pyrex reactor. It is 2 liters open vessel to the atmosphere, facilitating the addition of reagents, the tracking of the pH and EC parameters of the aqueous solution, and the stirring by a mechanical agitator at 550 rpm. Moreover, the reactor is equipped with a cooling jacket that control the reaction temperature of the solution by recirculating water from a thermal bath. Once the precipitation step is finished, the slurry of PCC is then filtered, washed with pure water to remove the remaining brine of the aqueous solution, and the solids are dried at 55 ◦C during 48 h. CaCl 2 reagent is added in its powder form to simulate the worst-case scenario for the quality of precipitated calcium carbonate. Upon CO 2 absorption, CO 3 2- ions are dissolved in the solution. When CaCl 2 is added as a powder, it must first dissolve into the solution (according to reaction v, see §2). This can decrease the precipitation kinetics and increase the likelihood of trace compound interferences. 3.2. Trace compounds None of the trace compounds commonly identified in the biogas, except for sulfur-containing ones, are present in the synthetic gas used here (see Table 1) [13,14]. These can be grouped as siloxanes, halogens, volatiles organic compounds, and ammonia with concentrations ranging from ppb v to ppm v [14]. As is remarked above (see §1), a worst-case scenario can be drawn assuming that all the volume of these trace compound would be dissolved into the aqueous solution of sodium hydroxide, which means a null resistance to the gas-liquid mass transfer [40]. Table 2 shows the trace compounds included in this study, along with their respective maximum concentrations found in literature for biogas generated during the anaerobic digestion of diverse organic feedstocks [13]. Considering these concentrations and the duration of the absorption tests, typically 30 min, the mass of each compound that would be bubbled is also in Table 2. Previously to each test, these masses are directly added into the aqueous solution by a 5 μ l syringe from a pure-grade liquid reagent. Based on their solubility, except for the siloxanes, all these trace compounds are dissolved in the aqueous solution of sodium hydroxide. Halogenated compounds are excluded from this study as they are only found in measurable quantities in landfill biogas [14]. 3.3. Process characterization The experimental tests vary depending on the absorption step conducted. Initially, tests are carried out to produce Precipitated Calcium Carbonate (PCC) as a reference blank. Consequently, a mixture of 2 Nl/ min of N 2 and 1 Nl/min of CO 2 is fed into the sodium hydroxide aqueous solution without any trace compounds. To assess the impact of organic compounds from the biogas on PCC, a second set of tests is conducted using an identical N 2 and CO 2 mixture with the organic compounds listed in Table 2 dissolved in the solution. Finally, a third kind of Fig. 2. Experimental set-up for the CO 2 absorption using NaOH aqueous solution: (1) Gas cylinders of N 2 , CO 2 and synthetic biogas, (2) Rotameter, (3) Manometer, (4) Valve, (5) sparger, (6) Magnetic stirrer, (7) Reactor, (8) NaOH solution, (9) pH sensor, (10) EC sensor, (11) Dehumidifier, (12) CO 2 gas analyzer, (13) PC for data acquisition. Table 1 Synthetic biogas composition. CH 4 % v 61.49 CO 2 % v 36 N 2 % v 1.5 O 2 % v 0.5 H 2 % v 0.5 H 2 S % v 0.01 Fig. 3. Experimental set-up for the precipitation of the calcium carbonate: (1) Reactor, (2) Aqueous brine solution from the previous CO 2 absorption step, (3) Mechanical stirrer, (4) cooling system, (5) CaCl 2 powders, (6) EC sensor, (7) Ph sensor, (8) PC for data acquisition. Table 2 Mass of the biogas trace compounds added to the sodium hydroxide solution based on their concentration in the biogas. Concentration, ppm v Mass, mg Ref. Siloxanes Decamethylcyclopentasiloxane 1.5 2.2 [13] Octamethylcyclotetrasiloxane 1.5 1.8 [13] Hexamethyldisiloxane 1.5 1.0 [13] Cyclic organic compound D-Limonene 3 1.6 [13] Oxygenated organic compound Tetrahydrofuram 3 0.9 [13] Aromatic organic compound Toluene 5 1.9 [13] Others volatile organic compound n-Hexane 5 1.7 [13] Ammonia NH 3 70 4.8* [14] * Added as NH4OH from an aqueous solution at 25 %w/w J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 4 absorption tests involves feeding 2.7 Nl/min of the synthetic biogas into the aqueous solution containing all compounds in Table 2. Following the absorption step, the resulting brine solution reacts with CaCl 2 powder to produce calcium carbonate. Table 3 summarizes the methodology for all experimental tests. The feeding of the gases is maintained while the pH is higher than 10.5 since the absorbed CO 2 is as carbonate [36,41]. That pH values are reached around 25 and 30 min for the N 2 and CO 2 mixture and for the synthetic biogas, respectively. The initial concentration of the sodium hydroxide solution is set at 10 % w/w to achieve the PCC concentration reported elsewhere after this step (140 g of Na 2 CO 3 per liter of brine solution) [9,42,43]. For the calcium carbonate formation and precipitation, the CaCl 2 powder is added using a molar ratio of 1.2 regarding the sodium carbonate dissolved. The reaction is stopped after 30 min, and its temperature is kept below 45 ◦C by the cooling system sketches in Fig. 2. Both the molar ratio as the reaction time is recommended by the authors after analyzing the PCC efficiency [9]. Moreover, the range of the temperature of the aqueous solution between room temperature and 70 ◦C is also showed negligible regarding this efficiency [9]. The variation of the pH, EC, and temperature of the aqueous solutions during the CO 2 absorption steps and the precipitation ones are recorded by a portable multi-parameter system ACCSEN PC 7 vio. It adjusts the pH and EC measurements using the temperatures that is also measured by the electrodes to a reference value of 25 ◦C. Moreover, the CO 2 concentration in the outlet gas after the absorption is tracked using Siemens ULTRAMAT 23. Based on these measurements, the amount of CO 2 absorbed is experimentally calculated integrating the area enclose by the outlet of the CO 2 concentration [41]. 3.4. Physicochemical characterization of the precipitated calcium carbonate Precipitated Calcium Carbonate (PCC) undergoes a comprehensive characterization process involving SEM, XRD, and Raman spectroscopy to gain insights into its physical structure and chemical composition. For Raman measurements of the powdered samples, along with a Leica DMLM microscope, a Thermo DXR2 spectrometer is used. The excitation line employed has a wavelength of 532 nm from an ion laser, and a 50x objective with an 8 mm optical configuration is employed to focus the depolarized laser beam onto a spot approximately 3 µm in diameter. X- ray diffraction (XRD) analysis was carried out using an X’Pert Pro PAN analytical instrument. The 2θ angle is systematically incremented by 0.05◦, with each step taking 450 times, covering a range from 10 to 90◦. The diffraction patterns are then recorded at 40 mA and 45 kV, using Cu K α radiation with a wavelength of 0.154 nm. The microstructural characterization is conducted via SEM analysis with a JEOL JSM6400 instrument operating at 20 KV. This instrument is equipped with energy dispersive X-ray spectroscopy (EDX) and wavelength dispersive X-ray spectroscopy (WDS) systems. The characterization of aqueous solutions from filtering the PCC slurry involves measuring various parameters, including alkalinity (mg CaCO 3 /l), ammoniacal nitrogen (mg N/l), calcium (mg Ca 2+ /l), chloride (mg Cl - /l), silicon (mg SiO 2 /l), sodium (mg Na + /l), sulfate (mg SO 4 2- /l), and Total Organic Carbon (TOC) (mg C/l). These measurements adhere to the ALPHA-AWWA-WPCF Standard Methods, except for TOC analysis, which follows the UNE-EN 1484 standard. Silicon and total organic carbon (TOC) are used, respectively, as indicators for the presence of siloxanes and organic compounds from biogas impurities, while ammoniacal nitrogen is indicative of nitrogen-based impurities. 3.5. Materials For the experimental tests are used solid, liquid and gas reagents. PanReac-AppliChem supplies the CaCl 2 powder with a pure-grade or pharma-grade, and the solution of sodium hydroxide at 50 % w used to prepare the aqueous solution of sodium hydroxide at 10 % w . It also needed demineralized water obtained from a Water Purification Systems supplied by Wasserlab. The ammonium hydroxide used is supplied by GlobalChem with a purity of 25 % w , and the gas cylinders are prepared and supplied by Linde. Moreover, the liquid reagents of each trace compounds are supply by Sigma-Aldrich as a pure-grade. 4. Results & discussion 4.1. CO 2 absorption Fig. 4 shows the results of the chemical absorption of the CO 2 in the synthetic biogas into the aqueous solution of sodium hydroxide with initial values of the pH and EC of 12.9 and 163.3 mS/cm, respectively. The variation of the CO 2 concentration (see Fig. 4a) shows a quick reduction by more than 90 % in just few seconds. From this point, and almost until the last quarter of the test, the CO 2 concentration ranges from 2 % v to 4 % v . It starts rising matching up with a significant decrease of the pH of the aqueous solution. Moreover, Fig. 4b shows a linear decrease of the EC of the absorbent until the value of 94.8 mS/cm when the test is stopped since the pH fall below 10.5. The temperature of the aqueous solution during absorption also increases linearly from 32 to 49 ◦C since the exothermic reaction summarized in Eq. 1. These variations of the CO 2 concentration of the outlet gas, and pH and EC of the absorbent solution can be explained considering the absorption mechanism of the CO 2 in a sodium hydroxide solution summarized previously (see §2). On the one hand, as CO 2 is injected and absorbed, the depletion of OH - reduce the pH, and, as the produced Na 2 CO 3 exists as dissociated form in the absorbent, there is an accumulation of CO 3 2- [36]. On the other hand, the linear reduction of the EC of the aqueous solution is because of the equivalent EC of the deleted OH - is larger than that of the generated CO 3 2- , being the Na + is a spectator ion [36]. The variations of the pH and Electrical Conductivity (EC) of the aqueous solutions in all the absorption tests show the same patterns than those in Fig. 5. Results are summarized in Table 4 where, in addition, shows the total amount of absorbed CO 2 experimentally estimated integrating the area enclose by the outlet of the CO 2 concentration as explained above (see §3.3). It is close to the stoichiometric amount that can be absorbed by Eq. 1 considering the total sodium hydroxide in solution. The absorption capacity of the absorbent can be drawn using the cumulative amount of absorbed CO 2 into the solution as the absorption test moves forward. Fig. 4 shows the result from the test of Table 3 Methodology for each test of precipitation of calcium carbonate. CO 2 absorption CaCO 3 precipitation Drying Test 1 Gas: 2 Nl/min N 2 +1 Nl/min CO 2 Solution: 0.75 l NaOH at 10 % w/w Temperature ∈(30 – 55)◦C Time ∈(25 – 30) minutes Powders : CaCl 2 Solution: brine for the CO 2 absorption step Temperature ∈(35 – 40)◦C Time: 30 minutes Oven 10 kw Temperature: 55◦ Time: 48 h Test 2 Gas: 2 Nl/min N 2 +1 Nl/min CO 2 Solution: 0.75 l NaOH at 10 % w/w +Organic trace compounds at Table 2 Temperature ∈(35 – 55)◦C Time ∈(25 – 30) minutes Test 3 Gas: 2.7 Nl/min of synthetic biogas Solution: 0.75 l NaOH at 10 % w/w +Trace compounds at Table 2 Temperature ∈(30 – 55)◦C Time ∈(25 – 30) minutes J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 5 biogas upgrading carried out here (tests 3 in Table 3) and those reported elsewhere where, using aqueous solutions of sodium hydroxide at concentration from 1 % to 5 % w , studies the CO 2 capture in a volumetric flow of 3 l/min at 31.5 % v (with the balance nitrogen). The absorption rate (slope) increases as the NaOH concentration does, which is in line with the largely adopted assumption that Eq. 1 behaves as a first-order reaction dependent on the OH- concentration [36]. Fig. 6 shows the variations of the pH and the electrical conductivity (EC) of the aqueous brine solution of Na 2 CO 3 during the Ca 2 CO 3 formation. 30 seconds after adding CaCl2 powder, the pH decreases from Fig. 4. Variations in (a) CO 2 concentration in the gas outlet during the absorption phase and (b) pH and EC values in the sodium hydroxide absorbent solution. Fig. 5. Cumulative amount of CO 2 absorbed using an aqueous solution of NaOH at 10 % w from a biogas upgrading test (see Table 3) and those, adapted from elsewhere [36], where using aqueous solutions of NaOH at 1, 2, 3, 4, and 5 % w , absorbs the CO 2 in 3 l/min at 31.5 % v (with the balance being nitrogen). Table 4 Summary for the CO 2 absorption tests a . pH, — Electrical Conductivity, mS/cm CO 2 absorbed, g Initial final Initial final Tests 1 b 13.2 (± 0.2) 10.4 (± 0.1) 161.9 (± 2.5) 92.7 (± 5.1) 46.3 (±8.1) Tests 2 b 12.4 (± 0.9) 10.2 (± 0.2) 162.6 (± 13.6) 95 (±1) 47.2 (±1.5) Tests 3 b 12.9 (± 0.6) 10.3 (± 0.2) 173.4 (± 11.9) 1. (±0.3) 50.0 (±1.9) a Average value (±standard deviation) b See description at Table 3 Fig. 6. Variations of the pH and the electrical conductivity (EC) of the aqueous brine solution of Na 2 CO 3 during the Ca 2 CO 3 precipitation. J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 6 10.2 to a stable range of 5.52–5.62 over 3–4 min. During this time, the EC rises from 94.8 to 129.8 mS/cm. The EC continues to increase, reaching 153.50 mS/cm by the end of the 30-minute test. These changes can be explained by the individual reactions summarized above (see §2). These changes are due to the removal of CO 3 2- ions, which lowers the pH to slightly acidic levels, and the introduction of Cl - ions along with excess Ca 2+ ions. Despite the precipitation of calcium carbonate, which removes both Ca 2+ and CO 3 2- from the solution, the overall EC increases because of the introduced Cl - ions and excess Ca 2+ relative to the theoretical dissolved CO 3 2- . Along with these variations, the initial temperature of the aqueous solution is 30◦C. As the pH stabilizes after 3 min, the temperature peaks at 47◦C. Subsequently, it decreases to 26◦C by the end of the test. Table 5 summarizes the final pH and electrical conductivity (EC) values of the aqueous solution after the precipitation tests (initial values are listed in Table 4). It should be noted that, although the analysis of the CO₂ absorption process is not the primary goal of this work, the agreement between our experimental measurements and those reported elsewhere validates the absorption process before precipitating the calcium carbonate. 4.2. Precipitated calcium carbonate production Fig. 7 shows the Raman spectrum of the precipitated calcium carbonate (PCC) from the various tests conducted in this study, as well as the spectrum for a standard and commercial CaCO 3 sample. All spectra display a prominent peak at 1100 cm −1 , which is characteristic of CaCO 3 , consistent with previous studies [9,44,45]. Additionally, a distinct band at 700 cm −1 is observed in both the PCC samples and the standard CaCO 3 sample. The absence of additional bands in the spectra suggests a high purity of the PCC, indicating that no impurities or trace compounds are present in the solid samples. This is important as it confirms that the precipitated material is composed entirely of calcium carbonate without contamination from other substances. This aspect is further substantiated below. Although Raman analysis establishes that the precipitated solids obtained from the three different tests can be considered as pure calcium carbonate, the presence of trace compound or impurities in the aqueous brine solution during the precipitation process could affect somehow the crystal polymorph form in which the solids precipitates. It can exist in three different primary crystal polymorphs: calcite, aragonite, and vaterite [9,44,46]. To investigate the specific crystal polymorphs formed, X-Ray Diffraction (XRD) is used. Fig. 6 collects the XRD patterns of the solid powders of the PCC (same samples used for the Raman study) showing that the polymorph obtained corresponds to calcite, which is the most thermodynamically stable form of CaCO 3 [46,47]. Calcite is characterized by a distinct, sharp diffraction peak at approximately 28 2Ө degrees, along with several weaker reflections at 23, 36, 39, 43, 47, and 48 2Ө degrees [46,47]. Furthermore, Fig. 8 illustrates that the XRD pattern of the obtained solid powders perfectly matches that of the reference calcium carbonate. This alignment signifies that all the samples exhibit the crystalline structure of calcite and are devoid of any impurities. This fact confirms the result obtained by Raman analysis and corroborates that the biogas impurities do not precipitate along the solid targeted. Indeed, from an economic perspective, this is a remarkable result. To gain deeper insights into the physicochemical characteristics of the precipitated carbonates, scanning electron microscopy (SEM) is employed. Fig. 9 shows SEM images of the solid powder samples precipitated in the tests where can be seen the typical tetrahedral morphology characteristic of calcite [9], without any seeable effect of the impurities in the morphology of the final solid samples. This observation aligns with the findings of previous XRD and Raman analyses, reaffirming the purity of the precipitated samples. 4.3. Characterization of the precipitated calcium carbonate slurry Table 6 shows the characterization of the aqueous solutions obtained from filtrating the calcium carbonate slurry in tests involving dissolved trace compounds (see Table 3). Additionally, it shows the characterization of the sodium hydroxide solution used as an absorbent. On the one hand, the elevated sodium concentration primarily originates from the sodium hydroxide solution, as it remains dissolved throughout the process. Concerning the chloride and calcium Table 5 Summary for the CaCO 3 precipitation tests a . pH, — Electrical Conductivity, mS/cm final final Tests 1 b 6.0 (±1.0) 147.7 (±9.5) Tests 2 b 5.9 (±0.4) 147.9 (±7.0) Tests 3 b 5.8 (±0.3) 152.7 (±1.7) a Average value (±standard deviation) b See description at Table 3 Fig. 7. Raman spectra of the solid powders obtained and commercial CaCO 3 (see description of the different tests 1, 2 and 3 in Table 3). Fig. 8. Comparison of XRD diffractogram of the solid powders obtained and commercial CaCO 3 (see description of the different tests 1, 2 and 3 in Table 3). J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 7 concentrations, they are from the calcium chloride reagent. Whereas chloride dissolves, most of the calcium precipitates along with the calcium carbonate. The measured concentration of calcium is attributed to the 20 % excess of calcium chloride added relative to the stoichiometric ratio considering the dissolved sodium carbonate from the absorption phase. On the other hand, as explained above (see §3.3), the measured concentration of silicon is intended to correlate with siloxane compounds. However, the quantity resulting from the dissolution of these compounds is obscured due to the silicon content in the absorption solution. Nevertheless, the detected TOC concentration, which correlates with the organic compounds present in biogas impurities, reveals that these compounds are dissolved in the aqueous solution. Furthermore, the low concentration of ammoniacal nitrogen in the samples from the third test suggests that a significant portion escapes as NH 3 from the solution. Additionally, the sulfate concentration can be attributed to the salt generated from the dissolution of hydrogen sulfide in the biogas into the caustic solution during the absorption step as sulfate salt with high solubility (as is predicted by PHREEQC software [48,49]. The reported and discussed characterization indicate that a significant portion of the trace compounds and impurities of the biogas could dissolve into the aqueous solution emerging from the filtration step. It holds considerable importance, particularly considering the membrane treatment planned downstream in the global process [9,27,32–34]. This treatment aims to recover the sodium hydroxide solution used for CO 2 absorption and chloride acid production. Consequently, tests to evaluate the behavior of the regeneration process should be carried out. 5. Conclusion This work investigates the potential contamination of Precipitated Calcium Carbonate (PCC) by trace compounds in biogas, and its impact on the commercial value of PCC, within a circular process integrating biogas upgrading via caustic absorption and sodium hydroxide recovery through membrane technologies [9,27,32–34]. Lab-scale tests are carried out to physiochemically characterize the obtained PCC. Additionally, both the CO 2 absorption process and the aqueous samples from the filtration phase of the PCC slurry are also characterized and analyzed. This study shows that: (i) the PCC obtained shows a high purity of calcium carbonate suitable for commercial applications, with no discernible impact on the crystal polymorphism related to the presence of trace compounds or biogas impurities in the aqueous brine solution during the precipitation process. Moreover, scanning electron microscopy (SEM) reveals the typical tetrahedral morphology characteristic of calcite. This high level of purity and crystallinity is essential for applications requiring precise material properties, ensuring the PCC produced in this study meets the necessary standards for commercial and industrial use. (ii) The characterization of the aqueous solutions obtained from filtering the PCC slurry indicates the presence of organic trace compounds from the biogas in the aqueous phase. This conclusion is supported by the detected Total Organic Carbon (TOC) concentrations, which were 9.7 (±6.4) mg C/l and 16.0 (±8) mg C/l for the second and third battery tests, respectively. These values correlate well with the organic compounds present as impurities in the biogas. Regarding siloxanes, limited information is available because indirect measurements of silicon are obscured by their presence in the water and/or sodium hydroxide reagents. However, the silicon concentration from the absorbent solution was 6 (±1.8) mg SiO₂/l, which is similar to that measured in the filtrated PCC slurry, with values of 3.3 (±0.9) mg SiO₂/l and 4.9 (±1) mg SiO₂/l. Moreover, the results reveal that a significant portion of ammonia escapes as gas from the solution, and hydrogen sulfide in the biogas could be generating sulfate salts. (iii) The variations in the CO 2 concentration in the outlet gas, as well as the pH and Electrical Conductivity (EC) of the absorbent solution, tracked during the upgrading of the biogas, can be explained by the absorption mechanism of CO 2 in sodium hydroxide solution reported elsewhere [35,41]. The absorption rate correlates with the concentration of sodium hydroxide, supporting the widely accepted assumption that CO 2 absorption is as a first-order reaction dependent on the OH - concentration. The amount of CO₂ absorbed is between 46.3 and 50.0 g, which is close to the theoretical 48 g of CO₂. Experimental results indicate that a significant proportion of biogas impurities dissolve into the aqueous solution during the absorption and precipitation phases. This finding is critical for future research, particularly with the planned downstream membrane treatment in the overall process [9,32–34]. Additionally, scaling up PCC production through tests with varying volumes while maintaining constant power input per Fig. 9. SEM images of the solid powders obtained from the experimental test 1 (A), 2 (B) an 3 (C) (see description of the different tests 1, 2 and 3 in Table 3). Table 6 Analytical characterization of the aqueous solution from filtrating the calcium carbonate slurry a . NaOH solution Tests 2 b Tests 3 b Alkalinity, mg CaCO 3 /l — 241 (± 1110) 359 (±16) Ammoniacal nitrogen mg N/l — — 2.8 (±1.3) Calcium mg Ca 2þ / l <1 7800 (± 1555) 5614 (± 855) chloride mg Cl - /l <50 94800 (± 3422) 98500 (± 3279) Silicon mg SiO 2 /l 6 (±1.8) 3.3 (±0.9) 4.9 (±1) Sodium mg Na þ /l 54000 (± 5292) 51034 (± 1605) 53367 (± 863) Sulfate mg SO 4 2- /l — — <3 Total Organic Carbon mg C/l — 9.7 (±6.4) 16.0 (±8) a Average value (±standard deviation) b See description at Table 3 J. Salinero et al. Journal of Environmental Chemical Engineering 12 (2024) 113868 8 unit volume or tip speed of the stirrer [50], is another crucial aspect currently being addressed. CRediT authorship contribution statement L.F. Vilches: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization. E. Portillo: Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Judith Gonz´ alez-Arias: Writing – review & editing, Writing – original draft, Software, Methodology, Investigation, Formal analysis, Data curation. Francisco M. Baena-Moreno: Writing – review & editing, Writing – original draft, Visualization, Conceptualization. B. Navarrete: Supervision, Project administration, Funding acquisition, Conceptualization. J. Salinero: Writing – review & editing, Writing – original draft, Software, Methodology, Investigation, Formal analysis, Data curation. Luz M. Gallego Fern´ andez: Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data Availability The data that has been used is confidential. 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