DOI: 10.3303/CET2292092 Paper Received: 3 December 2021; Revised: 4 March 2022; Accepted: 28 April 2022 Please cite this article as: Antxustegi M., Corro E., Baloch M., Volpe R., Gonzalez Alriols M., 2022, Production of Activated Bio-chars for Wastewater Treatment: Characterization, Activation and Evaluation of the Adsorption Capacity, Chemical Engineering Transactions, 92, 547- 552 DOI:10.3303/CET2292092 CHEMICAL ENGINEERING TRANSACTIONS VOL. 92, 2022 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti Copyright © 2022, AIDIC Servizi S.r.l. ISBN 978-88-95608-90-7; ISSN 2283-9216 Production of Activated Bio-chars for Wastewater Treatment: Characterization, Activation and Evaluation of the Adsorption Capacity Mirari Antxustegia, Eriz Corroa, Marya Balocha, Rosaria Volpeb, María Gonzalez Alriolsa,* a Biorefinery Processes Research Group (BioRP), Chemical and Environmental Engineering Department, University of the Basque Country (UPV/EHU), Plaza Europa 1, San Sebastian 20018, Spain b Department of Electrical, Electronics and Computer Engineering, University of Catania, viale Andrea Doria 6, 95125 Catania, Italy maria.gonzalez@ehu.eus Bio-char prepared from lignocellulosic wastes is a sustainable and low-cost material with many interesting applications, such as the adsorption of contaminants from wastewaters, among others. It is obtained from the pyrolysis of biomass through a thermochemical process in the absence of oxygen. In this process, biomass hydrocarbons, volatile matter, oxygen and hydrogen are converted into a carbon-enriched material, known as Bio-char (BC). Several parameters will define the structural properties and characteristics of the BC, some of which are related to the used raw material or biomass type and others to the process conditions. In order to evaluate the potential of the BC as adsorbent of toxic compounds from wastewaters, the most relevant properties are related to the active surface area, pore size and distribution, and stability. For the use of BC as adsorbent, in order to increase the adsorption capacity, activation of the BC is typically done. In this work, cork residues from wine bottles were thermo-chemically treated under a pyrolysis process and subsequently activated with KOH. The obtained Activated Bio-Chars (ABC) were characterized in terms of proximate analysis, active surface area and pore size distribution. Furthermore, BC and ABC adsorption capacity was tested. In particular, the adsorption of the dye methylene blue, MB; was studied. The obtained results indicated that the activation process was necessary in order to reach high MB adsorption capacities, over 90%. Because of its characteristics, it is proposed that BC without activation could be used to remove lower molecular weight components, such as gaseous ones. Finally, focusing on the design of a low-impact process and following green chemistry and sustainable practices, the recycling potential of the ABC was studied in terms of number of cycles, used solvent, dye recovery and adsorption performance. Reused ABC presented good performance in terms of adsorption capacity during several batches, similar to the one of raw ABC. The performance of several solvents was studied and acetone was selected as the better one due to good results in terms of ABC regeneration capacity and low boiling point for its subsequent recuperation. 1. Introduction The environmental impact of the industrial activities is being progressively submitted to stricter controls in order to limit their impact in soil and water environments (Elessawy et al., 2020). Many industrial processes, such as textile, pulp and paper, paint and pigments, or pharmaceutics ones, generate huge quantities of effluents and residual streams that can be potentially eco-toxic, and should be treated before their disposal (Hou et al., 2019). In particular, effluents containing dyes, have been proven to have high stability to photochemical, chemical and biological degradation, so they can affect the water ecological balance and present potential danger of bioaccumulation (Hamadi et al., 2021). A wide variety of techniques are typically used for wastewaters remediation, but most of them are ineffective and expensive, due to low selectivity and high associated costs (El Hassani et al., 2019). Adsorption technology has been identified as a convenient technique for the removal of pollutants from wastewaters as a simple, cheap and effective technology to treat effluents with dyes (Zhou et 547 al., 2019). Activated carbon presents many benefits as adsorbent, such as high surface area and reactivity, high porosity given by a porous structure in the micro scale with uniform pore size distribution, which confers high adsorption capacity. The drawback of this material is its high cost, which has pushed the search for alternative options, especially to be used at large scale (Dai et al., 2018). Biomass-based materials have been studied for this purpose, as they can actively retain the toxic compounds in their porous structure while the available chemical groups in its composition offer selective reactivity to several compounds (Dias et al., 2021). Agro- forestry waste is particularly interesting for this use, as its lignocellulosic structure contains carboxyl, hydroxyl and other reactive groups, that will actively contribute in the removal of toxics from aqueous effluents (de Freitas et al., 2019) while giving a use to a residue under the philosophy of “treating waste by waste” (Huang, 2017). Available research works about the biosorption of toxic compounds by agro-forestry wastes support the potential of this feedstock for the removal of dyes, heavy metals or metallic ions, but several drawbacks have been highlighted, particularly related to its low stability and perishability (Zhou et al., 2019). To improve the performance of the adsorbent, lignocellulosic wastes have been submitted to thermochemical activation with very promising results in terms of total removal ratios of the toxic compounds (Ahmed et al., 2019). This study presents the results of the thermochemical activation of cork residues and the use of the generated BC and ABC as adsorbent of methylene blue (MB) from an aqueous stream. MB is a heterocyclic aromatic widely used in chemical textile industries, and medicine and the cationic-type dye that concerns the most researches from over the world (Nayak and Pal, 2017). The prepared ABC have been characterized (proximate analysis, BET) and tested as adsorbents for the removal of MB by UV absorbance. Moreover, the prepared ABCs were submitted to recovery and reuse cycles in order to evaluate the dye recycling potential as well as the ABC reuse capacity. Different solvents were tested (ethanol, methanol and acetone) and the adsorption-desorption behaviour was tested. Good results were achieved, particularly with acetone that allowed to a total recovery of the original dye and reuse of the ABC. 2. Materials and methods Cork residues (CR) provided from local suppliers have been used as raw material in this work. Samples were homogenized to a particle size between 0,5 and 1mm. Proximate and ultimate analysis (TruSpec Micro LECO) were done on the raw and treated samples. Higher Heating Value (HHV) was measured using a IKA C200 Calorimeter. The rest of the values were obtained using the following UNE methods: Volatile matter (UNE- EN 15148-2010), mineral matter (UNE-EN 14775) and moisture (UNE-EN 14774-3). The results of the characterization are included in Table 1. Table 1: Proximate and ultimate analyses, (dry basis, wt. %) and calorific value of cork residues (CR) and the obtained BC from pyrolysis (pCR). Parameter CR pCR Proximate analysis (wt.%) Fixed carbon (%) 14,25 - Volatile matter (%) 84,18 - Inorganic matter (%) 1,57 0,02 Moisture (%) 4,51 - Ultimate analysis (wt.%) C (%) 59,46 87,02 H (%) 8,38 0,24 N (%) 0,90 0,95 S (%) 0,97 - O (%) 28,64 11,59 2.1 Pyrolysis and activation of the BCs Pyrolysis process was conducted in a horizontal tube furnace, under the following conditions: heating rate: 10 K/min up to a temperature of 1173 K, dwelling time: 1 h in a nitrogen atmosphere, after which samples were cooled down to room temperature inside the tube furnace under 40 L/min N2 flow. pCR samples were characterized by the following techniques: porous texture characterization was done by N2 (ASAP2420 Micrometrics equipment at 77K) and CO2 (BELSORP-maxII equipment at 273K) adsorption-desorption analysis. After pyrolysis, samples were chemically activated by mixing mechanically pCR and KOH (1/1 w/w ratio). The resulting mixture was carbonized in a horizontal tube furnace with a heating rate of 20 K/min up to 1173K in a 10 L/min N2 flow. Samples were kept at 1173K for 2 h and then cooled down under N2. Then, samples were thoroughly washed in three steps: with deionized water, with a 3.8 M HNO3 solution and, again with deionized water until neutral pH. Finally, the activated samples (apCR) were dried at 105 ºC for 12 h. 548 2.2 Adsorption experiments Adsorption tests were carried out as batch-type ones, in the following conditions: 0,15 g of the BC and ABC samples were added to 25 mL of a solution with an initial concentration of 35 mg MB/L and submitted to stirring to favour a good adsorbent-solution contact. Various contact times (1, 2, 4, 8 and 24h) were used in order to observe adsorption kinetics of the samples. After 24 h, the mixture was filtered and absorbance measurements of the resulting solution were done in a Jasco UV-VIS Spectrophotometer V-750ST, at a 665 nm wavelength. The absorbance and concentration values of MB were quantified with a calibration curve and results expressed as: 𝑄𝑒 = (𝐶0−𝐶𝑒𝑞) 𝑚 · 𝑉 Adsorption-ratio (mgMB adsorbed/gBC) Eq. 1 𝑅 = (𝐶0−𝐶𝑒𝑞) 𝐶0 · 100 MB removal (%) Eq. 2 where: C0: initial concentration (mgMB/L); Ceq: final or equilibrium concentration (mgMB/L); m: BC mass (g); V: solution bulk (L). 2.3 BC reuse and MB recycle Regeneration of the adsorbent after reaching the saturation point is a crucial step of the process regarding its sustainability. A reuse of the BC in several cycles would help to reduce the accumulation of solid waste from the used adsorbent and would allow the recycle of the dye. Three different solvents, methanol, ethanol and acetone, were tested for the desorption of MB from the BC. An experimental set to study the potential recovery of the BC and its use in several cycles was asset (Figure 1). Figure 1: BC recovery and reuse experimental set. Used adsorbents were separated by filtration and dried out at 60ºC for 24h prior to the recovery experiments. Then, samples were placed in a flask with the selected solvent and stirred continuously for 4h. Afterwards, the adsorbent was filtered, washed in distilled water and dried at 60ºC for 24 h. 3. Results 3.1 BC characterization Porosity of BC samples was studied using adsorption-desorption N2 isotherms at 77K. Figures 2a and 2b shows N2 adsorption - desorption curves at 77K and Figures 3a and b the pore size distribution curves obtained using the NDFT method for pCR (a) and apCR (b). pCR samples showed very low surface areas (9,56 m2·g-1) and extremely low adsorption kinetics. This fact was interpreted as a presence of narrow, slit-like, micro-pores in the BC structures. 0,15 g BC MB adsorption Sampling Filter-wash-dry the solid Dye recovery Filter-wash-dry the solid 0.CYCLE + Filter-wash-dry the solid Filter-wash-dry the solid Filter-wash-dry the solid Filter-wash-dry the solid Dye recovery Dye recovery + Absorbance measurement (UV-VIS) 1.CYCLE 2.CYCLE3.CYCLE 549 Figure 2a: pCR N2 adsorption-desorption isotherms Figure 2b: apCR N2 adsorption-desorption isotherms Figure 3a: pCR pore size distribution curves (NDFT) Figure 3b: apCR pore size distribution curves (NDFT) The obtained curve met the type I adsorption isotherm, according to the IUPAC classification (Muttakin et al., 2018), with a pronounced raise at low relative pressure P/P0, indicating micro-pore filling, followed by a stabilization. Moreover, in the isotherms an appreciable hysteresis loop could be observed, which is usually related to the presence of mesoporous structure. These results were confirmed when pore size distributions were calculated by NDFT method (Figure 3). A broad pore size distribution was found, with the highest percentage of porosity located in the micro pore region (mean pore diameter around 1.5 nm). On the other hand, apCR samples showed a totally different porosity, with a dramatically increased surface area (1227 m2·g-1). The apCR adsorption-desorption isotherms fitted a type I isotherm as well, and a pronounced “knee” form could be observed at low p/p0 values, typical of highly microporous solids. A more defined pore size distribution was found for apCR, with a broader distribution in the higher micro pore and lower mesoporous region (up to 4nm), followed by three small peaks around 5.3, 6.7 and 8.6nm. Because of these results, it is reasonable to assert that activation process has contributed both to the creation of new porosity and the widening of the existing pores that, consequently, has improved the accessibility to these pores, decreasing the diffusion problems encountered previously on the non-activated samples. 3.2 Adsorption results Figure 4 shows the results of the adsorption of MB by apCR samples, which reached a maximum MB removal rate (R) of about 100% in 2.2 h (pCR samples maximum removal rate was 45.8% in 24 h). The available functional groups on carbon surface have been reported to enhance the MB adsorption capacity of MB, via electrostatic interaction (Santoso et al., 2020). This explains the better performance of the activated samples 0 10 20 30 40 50 60 70 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 V a / c m 3 (S T P ) g -1 p/p0 pCR adsorption / desorption isotherm (N2 77,3 K) ADSORPTION pCR DESORPTION pCR 0 50 100 150 200 250 300 350 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 V a / c m 3 (S T P ) g -1 p/p0 apCR Adsorption / desorption isotherm (N2 77,3 K) ADSORPTION apCR DESORPTION apCR 0 0,0002 0,0004 0,0006 0,0008 0,001 0,0012 0,0014 0,000 0,002 0,004 0,006 0,008 0,010 0,012 0,014 0,016 0 5 10 15 20 25 30 35 40 45 50 C u m u la ti v e P o re V o lu m e (c m 3 /g ) pCR Pore Size Distribution (N2 77,3 K) Cumulative pore volume (cm3/g) dV(cm3/nm/g) d V (c m 3 /n m /g ) Pore width (nm) 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0 5 10 15 20 25 30 35 40 45 50 C u m u la ti v e P o r e V o lu m e ( c m 3 /g ) d V ( c m 3 /n m /g ) Pore Width (nm) apCR Pore Size Distribution (N2 77,3 K) dV (cm³/nm/g) Cumulative Pore Volume (cm³/g) 0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,000 0,002 0,004 0,006 0,008 0,010 0 5 10 15 20 25 30 35 40 45 50 C u m u la ti v e P o r e V o lu m e ( c m 3 /g ) d V ( c m 3 /n m /g ) Pore Width (nm) 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0 5 10 15 20 25 30 35 40 45 50 C u m u la ti v e P o re V o lu m e ( c m 3 /g ) d V ( c m 3 /n m /g ) Pore Width (nm) apCR Pore Size Distribution (N2 77,3 K) dV (cm³/nm/g) Cumulative Pore Volume (cm³/g) 0,000 0,002 0,004 0,006 0,008 0,010 0 5 10 15 20 25 30 35 40 45 50 d V ( c m 3 /n m /g ) Pore Width (nm) 550 due to the generated improvement in the BC morphology and the increase in surface functional groups. Different mechanisms of interaction between MB and the available functional groups on carbon surface, especially phenolic groups and carboxylic acid, have been identified. MB molecule has aromatic structures, positive charges, nitrogen and sulphur in its molecule which interact with the carbon surface” (Vargas et al., 2011). Figure 4: MB removal (R) and adsorption ratio (Qe) for apCR samples. 3.3 Adsorbent regeneration and reuse experiments Methanol, ethanol and acetone were the used for the MB recovery experiments. They were selected due to their low boiling temperatures, which would allow an easy recovery of the dye and used solvent by evaporation. These experiments were conducted using raw CR and the MB removal rate of recovered CR samples after 24 h was compared with the removal rate of the raw CR ones and expressed as percentage (Table 2). Table 2: Recovery of MB from CR samples, after 24 h. Recovery solvent Removal ratio at 24h (Recovered CR/ Raw CR) Methanol 62,9% Ethanol 73,3% Acetone 93,2% Acetone offered the best recovery results and, thus, it was selected for the recovery experimental set, in which apCR was submitted to three adsorption-desorption cycles and the adsorption ratio, Qe of each cycle was compared with the one of the fresh apCR (Table 3). Table 3: Desorption of MB from CR samples, after 24 h. Sample Desorption solvent MB removal % Qe (mg adsorbed MB /g apCR) Fresh apCR None 99,89 5,8095 apCR cycle 1 Acetone 99,75 6,0874 apCR cycle 2 Acetone 99,91 6,2721 apCR cycle 3 Acetone 99,42 5,0140 The MB removal ratio was close to 100% in all the performed cycles. The fact of finding a higher MB removal ratio in the second cycle than in the fresh sample is associated with some possible removal of remaining particles that might have been left from the activation phase and could be effectively blocking some of the internal pore structure and became loose during the stirring process. Moreover, changes on the surface polarity of the apCR could improve as well the adsorption capabilities of the resulting adsorbent. The obtained results confirmed a successful recovery and reuse the BC in several cycles and the potential recycle of the MB dye to the original process. 4. Conclusions Residual cork has been succesfully used as precursor for the generation of ABCs with good results. The pyrolysed samples, pCR a porosity type (high percentages of microporosity) not adequate for MB removal from aqueous 0 1 2 3 4 5 6 7 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 14 16 18 20 22 24 M B r e m o v a l (% ) t (h) apCR (R) apCR (Qe) Q e ( m g M B /g a p C R ) 551 effluents, the chemical activation of the BC considerably improved the adsorption capabilities by enhancing the accessibility of the adsorbate to the porous structure of the ABCs. The MB adsorption results of the apCR were very promising, with removal rates close to 100% in about 2h. Finally, it was found that the used ABC could be reused in several cycles with no loss in adsorption capabilities. The performed experimental set, with three different solvents (methanol, ethanol and acetone) indicated that acetone was the most suitable one so that this was the one selected in the fnal design, which would help the minimisation of waste generation related to the treatment as well as allow the recycle of the MB dye to the original industrial process through a simple evaporation process. This finding reinforces the importance of the present research work as it is aligned with the principles of the circular economy and the life cycle engineering enhancing the sustainability of the process by using renewable sources, minimizing the inputs and effluents and allowing the recycle of a valuable compound, MB, back to its original industrial process. Acknowledgments This research has been founded by the Gipuzkoa’s Provincial Government, program “Etorkizuna Eraikiz” 20/21. 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