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Graphical abstract Highlight • Co-disposal of red mud and lignin via the thermo-chemical process. • Sustainable thermo-chemical process using CO 2 as reaction media. • Fabrication of biochar composite with red mud and lignin. • Valorization of biochar composite for using a reduction catalyst. Abstract The massive generation of red mud has been considered as a serious environmental burden because of its toxicity, alkaline nature, and complex compositional matrix. Accordingly, diverse technical approaches for red mud utilization have been extensively developed, but their practical implementation has not been fully established because of technical incompleteness. In these respects, establishing reliable strategies for disposing red mud is of great importance. To enhance the technical viability of red mud valorisation, utilizing an existing disposal platform for solid wastes can be an alternative option. Therefore, co-pyrolysis of red mud and lignin was conducted in this study. Furthermore, the possible utilization of CO 2 during the co-pyrolysis was explored to valorise the end-product (biochar), which enhanced its porosity. In addition to the enhanced porosity, CO 2 utilization during the co-pyrolysis of red mud and lignin led to surplus generation of CO by shifting the carbon distribution from pyrolytic oil to CO. In detail, CO generation in the CO 2 environment was enhanced 24 times more than that in the N 2 environment. Thus, the surplus CO in the CO 2 environment was used to transform iron oxides in the red mud into zero-valent iron. In sum, two functionalities (enhanced porosity and zero-valent iron content) were leveraged by the CO 2 , which synergistically enhanced the reduction capability of the biochar. Reduction of p -nitrophenol and Cr(VI) was successfully completed using biochar, of which removal efficiency by reduction reached up to 99 and 69.7%, respectively. Therefore, the experimental findings provide a breakthrough for valorising two widespread waste materials, red mud and lignin. [ABSTRACT FROM AUTHOR]

Abstract: This study introduced a new approach for simultaneously enhancing Cr(VI) removal performance and mitigating release of dissolved Fe during nanoscale zero-valent iron (nZVI)-mediated reactions. After entrapping nZVI-impregnated biochar (BC) in the matrix of calcium-alginate (CA) bead, the physicochemical characterization of nZVI/BC/CA composites revealed that nZVI/BC particles were embedded inside CA having a spherical shape and several cracks on its outer layer. The multi-functionality of nZVI/BC/CA composites consisting of reductant (nZVI), porous adsorbent (BC), and external screening layer (CA) enhanced the removal of Cr(VI) with the maximum adsorption capacity of 86.4 mg/g (based on the Langmuir isotherm) and little release of dissolved Fe. With the XPS analysis and fitting results of kinetics (pseudo second order) and isotherms (Redlich-Peterson model), plausible removal mechanisms of Cr(VI) were simultaneous adsorption and micro-electrolysis reactions by nZVI/BC/CA composites. The practical applicability of nZVI/BC/CA composites was further demonstrated through the fixed-bed column experiments. These results provide new insights into the design of high-performance engineered biochar for wastewater treatment. Graphical abstract Image 1 Highlights • Biochar and Ca-alginate stabilizes nZVI as a novel multi-layered composite. • Multi-functional biochar composite improves nZVI reactivity and reduces Fe leaching. • Simultaneous adsorption and micro-electrolysis are plausible in biochar composite. • nZVI/BC/CA composite is applicable for high Cr(VI) removal in fixed-bed columns. [ABSTRACT FROM AUTHOR]

Co-impregnated biochars were fabricated by pyrolysis of spent coffee grounds (SCG) or glucose as carbon (C) sources under N 2 and CO 2 . The formed biochar samples were characterized with FE-SEM, TEM/EDS, XRD, TGA, Raman, XPS, and BET, and further used as catalytic medium for the reduction of p -nitrophenol in the presence of NaBH 4 . The physicochemical characteristics of biochar significantly changed with the types of C precursor and flow gas stream. The biochar from pyrolysis of SCG under N 2 gas stream showed good dispersion of Co nanoparticles (10–20 nm) in carbon matrix as compared to that produced under CO 2 stream, but BET surface area was very small (4.5 m 2 g −1 ) due to pore blockage by thermal degradation products. SCG-derived biochar formed in the presence of CO 2 possessed highly porous structure due to CO 2 -assisted C volatilization. In contrast, for glucose-derived biochar, significant reduction in porosity of biochar occurred with excess devolatilization of C under CO 2 conditions, while the pyrolysis under N 2 flow produced porous biochar. These contrasting results demonstrated the significance of structure crystallinity and thermal degradability of C precursors. The catalytic performance of biochar was better correlated with the external surface area rather than microporosity. [ABSTRACT FROM AUTHOR]

A novel nitrogen-doped biochar embedded with cobalt (Co-NB) was fabricated via pyrolysis of glucose pretreated with melamine (N donor) and Co(II). The Co-NB showed high catalytic capability by converting p -nitrophenol (PNP) into p -aminophenol (PAP) by NaBH 4 . The analyses of FE-SEM, TEM, BET, XRD, Raman, and X-ray photoelectron spectroscopy XPS of the Co-NB showed hierarchical porous structure (BET 326.5 m 2 g − 1 and pore volume: 0.2403 cm 3 g − 1 ) with well-dispersed Co nanoparticles (20–60 nm) on the N-doped graphitic biochar surface. The Co-NB showed higher PNP reduction capability compared to the Co-biochar without N-doping, achieving 94.3% removal within 4 min at 0.24 g L − 1 catalyst dose and initial concentration of 0.35 mM PNP. Further conversion experiments under varying environmental conditions ( e . g ., NaBH 4 concentration (7.5–30 mM), biochar dosage (0.12–1.0 g L − 1 ), initial PNP concentration (0.08–0.17 mM)) were conducted in batch mode. The reusability of Co-NB was validated by the repetitive conversion experiments (5 cycles). The overall results demonstrated biochar potential as catalysts for environmental applications if properly designed. [ABSTRACT FROM AUTHOR]

An engineered biochar was fabricated via paper mill sludge pyrolysis under CO 2 atmosphere, and its adsorption capability for As(V) and Cd(II) in aqueous solution was evaluated in a batch mode. The characterization results revealed that the biochar had the structure of complex aggregates containing solid minerals (FeO, Fe 3 O 4 and CaCO 3 ) and graphitic carbon. Adsorption studies were carried out covering various parameters including pH effect, contact time, initial concentrations, competitive ions, and desorption. The adsorption of As(V) and Cd(II) reached apparent equilibrium at 180 min, and followed the pseudo-second-order kinetics. The highest equilibrium uptakes of As(V) and Cd(II) were 22.8 and 41.6 mg g −1 , respectively. The adsorption isotherms were better described by Redlich-Peterson model. The decrease in As(V) adsorption was apparent with the increase in PO 4 3− concentration, and a similar inhibition effect was observed for Cd(II) adsorption with Ni(II) ion. The feasibility of regeneration was demonstrated through desorption by NaOH or HCl. [ABSTRACT FROM AUTHOR]

One of the main challenges of biochar application for environmental cleanup is rise of pH in water or soil due to high ash and alkali metal contents in the biochar. While this intrinsic property of biochar is advantageous in alleviating soil and water acidity, it severely impairs the affinity of biochar toward anionic contaminants such as arsenic. This study explored a technical approach that can reduce the basicity of lignin-based biochar by utilizing FeCl 3 during production of biochar. Three types of biochar were produced by co-pyrolyzing feedstock composed of different combinations of lignin, red mud (RM), and FeCl 3 , and the produced biochar samples were applied to adsorption of As(V). The biochar samples commonly possessed porous carbon structure embedded with magnetite (Fe 3 O 4) particles. The addition of FeCl 3 in the pyrolysis feedstock had a notable effect on reducing basicity of the biochar to yield significantly lower solution pH values than the biochar produced without FeCl 3 addition. The extent of As(V) removal was also closely related to the final solution pH and the greatest As(V) removal (>77.6%) was observed for the biochar produced from co-pyrolysis of lignin, RM, and FeCl 3. The results of adsorption kinetics and isotherm experiments, along with x-ray spectroscopy (XPS), strongly suggested adsorption of As(V) occurred via specific chemical reaction (chemisorption) between As(V) and Fe–O functional groups on magnetite. Thus, the overall results suggest the use of FeCl 3 is a feasible practical approach to control the intrinsic pH of biochar and impart additional functionality that enables effective treatment of As(V). [Display omitted] • Co-pyrolysis of lignin, red mud, and FeCl 3 produced Fe-biochar with porous structure. • Addition of FeCl 3 in pyrolysis feedstock substantially lowered basicity of Fe-biochar. • The low pH condition rendered enhanced As(V) removal. • As(V) biding to Fe-biochar mainly occurred via chemisorption to magnetite. [ABSTRACT FROM AUTHOR]

Here in this study, co-pyrolysis of paper mill sludge (PMS) and spent coffee ground (SCG) was conducted to mainly recover energy and additionally produce engineered biochar as an adsorbent. More specifically, to enhance the generation of syngas (H 2 and CO) through co-pyrolysis of PMS and SCG and modify the physico-chemical properties of biochar, carbon dioxide (CO 2 ) was used as reaction medium. The CO 2 co-feeding impact on co-pyrolysis with various mass ratios of PMS and SCG was also evaluated to explore any catalytic effects arising from CO 2 and Fe/Ca species in PMS. The decrease in pyrolytic oil substances was apparent due to the synergistic effects of CO 2 and Fe/Ca species. Moreover, the generation of CO was catalytically enhanced in the presence of Fe/Ca species in PMS. Co-pyrolysis in CO 2 not only converted the impregnated Fe ions into magnetite (Fe 3 O 4 ), but also created porous biochar. One biochar fabricated at the optimal mass ratio (PMS:SCG = 7:3) exhibited the highest adsorption capability toward As(V) due to adequate balancing between the amount of Fe/Ca solid minerals and biochar porosity. As a result, co-pyrolysis using CO 2 as reaction medium can be a feasible option for the generation of CO along with production of biochar as a byproduct which can be used in environmental application. [ABSTRACT FROM AUTHOR]

This study investigated the preparation of magnetic biochar from N 2 - and CO 2 -assisted pyrolysis of spent coffee ground (SCG) for use as an adsorption medium for As(V), and the effects of FeCl 3 pretreatment of SCG on the material properties and adsorption capability of the produced biochar. Pyrolysis of FeCl 3 -pretreated SCG in CO 2 atmosphere produced highly porous biochar with its surface area ∼70 times greater than that produced in N 2 condition. However, despite the small surface area, biochar produced in N 2 showed greater As(V) adsorption capability. X-ray diffraction and X-ray photoelectron spectrometer analyses identified Fe 3 C and Fe 3 O 4 as dominant mineral phases in N 2 and CO 2 conditions, with the former being much more adsorptive toward As(V). The overall results suggest functional biochar can be facilely fabricated by necessary pretreatment to expand the applicability of biochar for specific purposes. [ABSTRACT FROM AUTHOR]

Pyrolysis of paper mill sludge (PMS) was conducted to produce syngas (H 2 and CO) and fabricate magnetic biochar (PMS biochar) for use as an adsorbent for As(V). The enhanced generation of CO was observed in the CO 2 atmosphere due to reactions triggered by CO 2 . Particularly, the generation of syngas from pyrolysis of PMS in CO 2 (9.6 mol% at 720 °C) was superior to the cases of pyrolysis of acid-washed PMS ( i.e. , minerals-eliminated PMS) in CO 2 (2.9 mol% at 720 °C), which evidences catalytic effects attributed by Fe and Ca species contained in PMS. Based on the results of X-ray diffraction (XRD), pyrolysis of PMS in CO 2 led to the conversion of Fe species into magnetite (Fe 3 O 4 ) solid phase that imparted the magnetic property to the biochar (saturation magnetization: 28.4 emu g −1 ). The PMS biochar exhibited high As(V) adsorption capacity of 34.1 mg g −1 in a given condition, which could be attributed to the great adsorption affinity of Fe 3 O 4 /CaCO 3 mixture toward As(V). The integrated approach for the utilization of PMS could satisfy growing demand for renewable energy and environmental sustainability. [ABSTRACT FROM AUTHOR]

Nano Fe(III) oxide (FO) was used as an amendment material in CO 2 -assisted pyrolysis of spent coffee grounds (SCG) and its impacts on the syngas (H 2 & CO) generation and biochar adsorptive properties were investigated. Amendment of FO led to 153 and 682% increase of H 2 and CO in pyrolytic process of SCG, respectively, which is deemed to arise from enhanced thermal cracking of hydrocarbons and oxygen transfer reaction mediated by FO. Incorporation of FO successfully created porous structure in the produced biochar. The adsorption tests revealed that the biochar exhibited bi-functional capability to remove both positively charged Cd(II) and Ni(II), and negatively charged Sb(V). The adsorption of Cd(II) and Ni(II) was hardly deteriorated in the multiple adsorption cycles, and the adsorption of Sb(V) was further enhanced through formation of surface ternary complexes. The overall results demonstrated nano Fe(III) oxide is a promising amendment material in CO 2 -assisted pyrolysis of lignocellulosic biomass for enhancing syngas generation and producing functional biochar. [Display omitted] • Co-pyrolysis using nano Fe(III) oxide and CO 2 for yield of syngas and metal biochar. • Enhancement of syngas production in the presence of nano Fe(III) oxide. • Creation of porous structure and Fe 3 O 4 phase in metal biochar after co-pyrolysis. • Ability of metal biochar that can simultaneously adsorb Sb(V), Cd(II) and Ni(II). • More Sb(V) removal by metal biochar due to formation of surface ternary complexes. [ABSTRACT FROM AUTHOR]

A new-fashioned fabrication recipe for a magnetic carbon nanocomposite (Fe 3 O 4 @C) via pyrolysis of sugar with magnetite (Fe 3 O 4 ) nanoparticles was developed for the practical environmental application as an adsorbent. In order to synthesize Fe 3 O 4 @C, the thermal degradation of sugar was firstly investigated via thermo-gravimetric analysis (TGA) to explore the optimal pyrolytic conditions for fabricating Fe 3 O 4 @C. This study laid a great emphasis on the physicochemical characterization of pyrogenic Fe 3 O 4 @C through various analytical techniques, which experimentally validated that Fe 3 O 4 @C retained thin graphitic carbon layers containing carboxyl groups on the surface with the point of zero charge (pH pzc ) of 7.5. Based on adsorption tests of methylene blue (MB), it was found the optimal mass ratio of sugar to Fe 3 O 4 was 0.15 with pyrolysis temperature of 500 °C. The adsorption capacity of Fe 3 O 4 @C for MB was 52.6 mg g −1 and MB adsorption showed a strong pH dependence, which implies an active role of electrostatic interactions in the adsorption process. In regeneration experiments, Fe 3 O 4 @C retained 84% of its initial adsorption capacity after completing four consecutive adsorption cycles. [ABSTRACT FROM AUTHOR]

This study mechanistically investigated the influences of CO 2 on syngas (H 2 and CO) production during thermo-chemical conversion of red seaweed, and further explored the possible utility of the produced biochar as a medium for adsorption of inorganic/organic contaminants in aqueous phase. In order to elucidate the key roles of CO 2 in the thermo-chemical process, the composition analysis of syngas and the qualitative analysis of pyrolytic oil were conducted and compared with those in pyrolysis in N 2 condition. Pyrolysis of red seaweed in the presence of CO 2 led to the enhanced generation of syngas at the entire experimental temperatures. For example, the ratio of CO to H 2 in the presence of CO 2 at 620 °C was enhanced by ∼400%, as compared to the case in N 2 . This enhanced generation of syngas resulted in significant pyrolytic oil reduction by ∼70% at 620 °C via the unknown reactions between VOCs and CO 2 . In addition, biochar generated in the CO 2 environment exhibited comparatively higher surface area (61 m 2 g −1 ) and more porous structure. The morphological modification induced by CO 2 provided the favorable condition for removal of methylene blue from the aqueous phase. Thus, this study experimentally demonstrated that exploiting CO 2 as a reaction medium would provide an attractive option for the enhanced generation of syngas and the tuned adsorption capability of biochar. [ABSTRACT FROM AUTHOR]

[Display omitted] • Application of Fe-impregnated bentonite as a catalyst in pyrolysis of grass cut. • CO 2 gas was employed as a pyrolysis gas medium. • Fe-impregnated bentonite enhanced syngas production. • Fe-impregnated bentonite induced formation of methyl 1,1-methyl-dodecanoate (C 14 H 28 O 2). • Obtained biochar had good As(V) adsorption capability. Fe-impregnated bentonite was employed in CO 2 -assisted pyrolysis of grass cut as a catalyst to improve syngas production, bio-oil quality, and biochar sorptive property. The gas analysis revealed that H 2 and CO production was enhanced by 332 and 7,110%, respectively, by applying Fe-impregnated bentonite in pyrolysis of grass cut while untreated bentonite had a limited influence on syngas production. Bio-oil produced from Fe-impregnated bentonite and grass cut (Fe-BG) had more homogenized chemical species in bio-oil as compared to that produced from pyrolysis of grass cut, and contained methyl 1,1-methyl-dodecanoate (C 14 H 28 O 2), one of the major components in biodiesel. The produced biochar had high surface area (69.2 m2 g−1) and possessed Fe0 and graphitic carbon structure, which was deemed to arise from reduction of Fe-oxides by syngas. Biochar also showed high As(V) adsorption capacity. The overall results of this work demonstrated the beneficial utility of Fe-impregnated bentonite in pyrolysis of biomass to improve syngas production as well as the quality of pyrogenic products. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synergistic effects of blending chitin and oyster shell as co-pyrolysis feedstock. • CO 2 -assisted co-pyrolysis increased syngas production and enhanced biochar property. • Biochar produced in CO 2 condition showed strong pH buffering capacity. • Cu(II) adsorption capacity was 2.5 time greater for CO 2 biochar than N 2 biochar. Common seafood wastes, chitin and oyster shell, were co-pyrolyzed in different atmospheric conditions of N 2 and CO 2 to investigate the effects of blending the two wastes on syngas production and adsorption capability of the produced biochar in Cu(II) adsorption. The gas analyses indicated that amendment of oyster shell to chitin pyrolysis substantially enhanced production of H 2 and CO as compared to using chitin as a sole feedstock. The enhanced syngas production was deemed to arise from the catalytic effect of calcite (CaCO 3) contained in oyster shell that expedited thermal decomposition of chitin. The syngas production was more pronounced in N 2 condition than in CO 2 condition. The biochar samples produced from co-pyrolysis in both atmospheric conditions had different mineralogical composition and surface characteristics. The biochar produced under CO 2 condition showed strong pH buffering capacity to maintain the pH increase within neutral range. The maximum adsorption capacity of CO 2 biochar was 2.5 times (∼1,000 mg g−1) higher than N 2 biochar (∼400 mg g−1), which was attributed to the abundance of calcite particles that served as major reactive sites of ion exchange reaction with Cu(II). In overall, this study offers an effective strategic way to maximize syngas production and produce biochar with specific functionality. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fabrication of metal-biochar by co-pyrolysis of microalgae and bauxite tailings. • Use of CO 2 as a reaction medium substantially enhance pyrogenic gas products. • Utilization of fabricated metal-biochar as a persulfate activator. • Degradation of methyl orange was driven by 1O 2 formed from persulfate activation. This study explored the feasibility of simultaneously producing synthetic gases and metal-biochar catalyst from co-pyrolysis of microalgae (chlorella vulgaris , CV) and industrial waste (bauxite tailings, BT). Co-pyrolysis was conducted in two different atmospheric conditions of N 2 and CO 2. Real-time syngas monitoring revealed the use of CO 2 substantially enhanced CO production by expediting CO 2 -medicated thermal cracking of CV and its impact was further pronounced when BT was incorporated in the pyrolytic process. Characterization of produced metal-biochar revealed that metal-biochar have porous structure, Fe 3 O 4 phase, and graphitic carbon layers with defective sites. The metal-biochar removed > 72% of 5 mg L−1 methyl orange within 60 min in the presence of 2 mM peroxydisulfate at 0.1 g L−1 biochar dose. Quenching test revealed the removal of methyl orange was mainly driven by singlet oxygen (1O 2) generated by persulfate activation by metal-biochar. The reusability test indicated metal-biochar maintained>80% of its catalytic capability up to five repetitive reaction cycles of methyl orange removal. Collectively, co-pyrolysis of microalgae and industrial waste containing transition metals in CO 2 condition can be a viable option to harvest energy resources from biomass wastes and to produce catalytic medium applicable to remove a wide range of redox active contaminants. [ABSTRACT FROM AUTHOR]

• Zirconia (ZrO 2)-assisted pyrolysis of coffee waste using CO 2 as feeding gas • Enhancement of CO production in the presence of ZrO 2 • Highly graphitic- and porous carbon of composite covered by ZrO 2 nanoclusters • Good As(V) adsorption ability of reusability of ZrO 2 -incorporated composite This work newly employed monoclinic zirconia (ZrO 2) as a promoter to improve CO 2 pyrolysis of coffee waste (CW). The CO 2 pyrolysis of CW presented the high level of CO production (14.3 mol%) during two stages of non-isothermal (280 to 700 °C) and isothermal pyrolysis (kept at 700 °C). At the same condition, the incorporation of ZrO 2 improved the CO generation up to about twice that of CW (29.5 mol%) by possibly inducing more conversion of pyrolytic oil into gas. The characterization results exhibited that ZrO 2 -impregnated biochar (ZrB) possessed the distinctive surface morphology that highly graphitic- and porous carbon layers were covered by ZrO 2 nanoparticle clusters. In a series of adsorption experiments, ZrB composite showed pH-dependent As(V) adsorption and pH neutralization ability. The adsorption proceeded relatively rapid with 95% removal during 120 min in the early stage, followed by 5% removal in the remaining 240 min. The maximum adsorption capacity was found to be 25.2 mg g-1 at final pH 8. The reusability and stability of ZrB were demonstrated in the 6 consecutive cycles of adsorption/desorption. As a result, ZrO 2 -assisted CO 2 pyrolysis can potentially produce fuel gas with high CO fraction and composite adsorbent suitable for As(V) removal in acidic wastewater. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Valorization of red mud and lipid waste via the thermo-chemical process. • Enhanced energy recovery in the thermo-chemical process using CO 2. • Fabrication of biochar composite from red mud and lipid waste. • Biodiesel synthesis using biochar composite. Abstract Co-pyrolysis of lipid waste and red mud was investigated to achieve valorization of red mud by fabricating biochar composite. For the further sustainable approach, this study intentionally employed carbon dioxide (CO 2) as reaction medium in the co-pyrolysis process. The use of CO 2 on co-pyrolysis of lipid waste and red mud enabled manipulation of the carbon distributions between pyrogenic products. CO 2 expedited the thermal cracking of lipid waste and further reacted with lipid waste during the thermolysis. These mechanistic roles of CO 2 were catalytically enhanced by the presence of mineral phases (Fe 2 O 3) in red mud, thereby resulting in the enhanced formation of CO (40 times more at 550 °C). However, CO 2 suppressed dehydrogenation of lipid waste (∼ 50%), which resulted in the different pathway for reducing iron oxide in red mud. Moreover, as an aspect of valorization of red mud, catalytic capability of biochar composite was evaluated. As a case study, biodiesel (FAMEs) were synthesized, and all experimental findings suggested that biochar composite could be an effective catalyst for biodiesel synthesis. As compare to biodiesel synthesis using silica (92% yield at 360 °C), the equivalent biodiesel yield was achieved with the biochar at much lower temperature (130 °C). [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Thorn-like Fe-Mn-S@biochar composite was newly fabricated. • Engineered biochar composite consists of FeS 2 , FeMnO 3 , and Fe 2 O 3. • Adsorption capacity of Pb by Fe-Mn-S@biochar composite reached 182 mg g−1 at pH 7. • Synergistic removal by complexation, reduction, and precipitation was demonstrated. Abstract Hierarchical porous biochar derived from corn straw containing ternary needle-like iron-manganese-sulphur composites (Fe-Mn-S@HCS) are fabricated, and their physicochemical characteristics and performance for Pb removal were examined in detail. Introduction of Mn (transition metal) into Fe-biochar composites can effectively alter the chemical state of Fe; simultaneous doping with S can enhance cation exchange for Pb removal. High uptake of Pb by Fe-Mn-S@HCS in a short time period was observed with the adsorption capacity of 181.5 mg g−1 and the pseudo-second-order rate constant of 0.075 g mg-1 h−1. Complexation, reduction, and precipitation were found to be involved in the Pb removal by Fe-Mn-S@HCS based on the results of HRTEM, XPS, and XRD analyses. This study demonstrated the feasibility of Fe-Mn-S biochar composites for high-efficiency Pb removal from aqueous solution. [ABSTRACT FROM AUTHOR]

• Drinking water treatment sludge (PAC sludge) was valorized by pyrolysis. • PAC sludge-valorized biochars exhibited excellent adsorption performance for As species. • Biogeochemical behavior of DMA was identified in a wetland microcosm. • Biochars were practically applied to the mitigation of DMA in wetlands. This study aims to mitigate As pollution in wetlands by using biochar composites, which are a byproduct of valorizing drinking water treatment sludge (i.e. , polyaluminum chloride (PAC) sludge). Biochar composites were fabricated under N 2 and CO 2 environments, systematically characterized by X-ray diffraction, thermogravimetric, and Brunauer–Emmett–Teller/Barrett–Joyner–Halenda analyses, and tested for the adsorption of As species. Both biochar composites exhibited excellent adsorption performance for both inorganic As (As(III) and As(V)) and organic As (dimethylarsinic acid, DMA). A detailed study of the adsorption behavior of DMA revealed that the adsorption reaction complies with the pseudo-second-order kinetics and Freundlich isotherm models. A laboratory-scale microcosm test showed that ∼ 30 % of spiked DMA was removed by biochar and that the total As fixed in the sediment decreased by ∼ 20 %. In addition, the As speciation results for the sediment and biochar revealed demethylation of the DMA and reduction of As(V) to As(III) by microorganisms, which was confirmed by a microbial growth batch test. Finally, a large-scale field experiment carried out in an artificial ecological wetland ensured that the addition of biochar could reduce the total amount of As to be immobilized in wetland sediment by 19 %. In addition, the presence of biochar could alter the migration trend of As species in plants by reducing the amount of organic As to be fixed in the sediment. The aforementioned results demonstrate the practical feasibility of using PAC sludge-derived biochar as an adsorbent for As species. [ABSTRACT FROM AUTHOR]

[Display omitted] • Efficient algae harvesting using magnetic iron oxides after CO 2 purging. • CO 2 -supported co-pyrolysis of magnetically harvested algae. • Increased syngas generation through phase transition of iron oxides. • Thermochemical conversion of CO 2 into CO by oxygen-deficient iron oxides. • Low level of toxin by addition of metal-biochar into CO 2 -purged algal solution. With rising of harmful algae blooming and toxin exposure, practical utilization of harmful algae has been developed. This work aimed to magnetically harvest Microcystis aeruginosa (MA) using iron oxides and investigate the feasibility of algae/iron oxides mixture as feedstock in pyrolytic platform to produce syngas and metal biochar. Carbon dioxide (CO 2) was used as a feeding gas to enhance the production efficiency of syngas and also functioned pH controller for better MA harvesting and toxin removal. CO 2 support brought multiple benefits: magnetite (Fe 3 O 4) and maghemite (γ-Fe 2 O 3) recovered MA in a relatively short period of time (∼1 min), the recovered biomass generated 34-fold increased carbon monoxide, and metal biochar adsorbed higher amount of toxin from MA (2.8-fold). Pyrolytic utilization of harmful algae supported by CO 2 and iron oxides could be one of promising techniques for evolution of metal biochar to remove toxin, while efficiently recover biomass and enhance syngas production. [ABSTRACT FROM AUTHOR]