Fabrication of PAC sludge-valorized biochars and their practical application to the remediation of methyl arsenic 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). PAC sludge produced when PAC is used as a coagulant contains various aluminum and iron components because of the high affinity of PAC for heavy metals. Therefore, the valorization of PAC sludge by pyrolysis can be a strategic method to secure environmental safety for sludge utilization by destroying the organic pollutants and immobilizing heavy metals while simultaneously producing biochar composites that can be used as an environmental adsorbent. Biochar composites were fabricated under N₂ and CO₂ 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 lab-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.