Functionally-designed metal salt and ball milling co-modified sludge biochar for adsorptive removal of trace level sulfamethoxazole: Behavior, characterization, mechanism and DFT study

Sulfamethoxazole (SMX) is a representative sulfonamides that has been widely detected in various waters, and posing adverse effects on ecological safety and human health even at trace level. Various metal salts of ZnCl2, MnCl2 and MgCl2 were applied to activate the special biochar derived from sludge (SBC), and ZnCl2 functionalized SBC (ZnSBC) exhibited the optimal removal capacity for SMX. Then, ball milling coupled with ZnCl2 was used to co-functionalize SBC to produce BZnSBC for further improving its adsorption ability. The maximum adsorption ability of BZnSBC (6.05×104 μg/g) for SMX was 8.27 times that of SBC, and it exhibited the optimum adsorption performance in the pH rang of 3–5. The outstanding elimination effect of SMX by BZnSBC (its elimination rates ranged from 90.7 % to 98.1 % in diverse real waters) was attributed to its high anti-interference to a wide range of pH, salts and humic acid concentrations. Characterization, adsorption experiments, models fitting together with DFT calculation demonstrated that π-π conjugation, pore filling, H-bonding and Zn-O complexation were the primary driving forces for SMX binding to BZnSBC. NaOH could efficiently regenerate the used BZnSBC and its adsorption capacity was 99.8 % of the original one in the fifth cyclic experiment. Also, BZnSBC performed favorable safety in view of the low leaching risk of Zn despite of pH, salts and humic acid concentrations range.