Wastewater post-coagulation sludge recycled as a multifunctional adsorbent via pyrolysis enhanced in carbon dioxide (CO2)

Abstract Massive wastewater post-coagulation sludge (WPCS) generated from the tertiary treatment facilities has been regarded as an environmentally burdensome waste. Herein, to take advantage of the abundant amounts of Al/Fe (hydr)oxides, the WPCS was converted into functional char via pyrolysis under CO2 and N2 atmosphere. The higher organic matter content and porous structure of WPCS than drinking water treatment sludge made it a more suitable precursor for biochar and adsorbent production. CO2 expedited the thermolysis of the organics in WPCS and the Fe (hydr)oxides in WPCS further decreased the temperature of CO2-mediated reaction. Therefore, the corresponding products outcompeted the chars in N2, achieving ∼37% higher specific surface area, stronger aromaticity and more amorphous Al and Fe contents of 201.19 ± 2.25 and 27.03 ± 0.56 mg g−1, accompanied by more loss of surface functional groups like carboxyl and hydroxyl. Accordingly, WPCS chars under CO2 showed superior performance for removing phosphate (15.58 ± 0.19 mg g−1), along with the adsorption of heavy metal (37.17 ± 1.25 mg g−1 of Pb (II)) and dye (14.45 ± 0.11 mg g−1 of methylene blue). In sum, this study proposes a win–win strategy to convert coagulation sludges into resources and a new candidate for multifunctional adsorbent production.