Mechanistic insights into dissolved organic matter removal by magnesium-doped activated carbon: From model humic acid to real wastewater.

Dissolved organic matter (DOM) poses a major challenge for efficient water treatment. While traditional adsorbents are often developed using model compounds like humic acid (HA), their efficacy against complex DOM in real wastewater remains uncertain. This study aims to elucidate the mechanistic basis of this performance discrepancy by evaluating the adsorption behaviors of HA, salicylic acid (SA), and real wastewater DOM on magnesium-doped activated carbon. The results show that increasing magnesium doping enhanced HA removal, achieving up to 98% removal, whereas higher magnesium loading reduced adsorption capacity for real DOM. In contrast, unmodified activated carbon achieved only a 12% removal rate for HA but exhibited an 80% removal rate for SA, and demonstrated a higher capacity for adsorbing real DOM (with a maximum adsorption capacity of 54.77 mg C/g) and broad-spectrum DOM removal (achieving 95-97% removal for C1-C3 components identified by PARAFAC analysis). Mechanistic analysis revealed that magnesium doping enhances HA removal primarily through metal-organic complexation, as evidenced by zeta potential measurements showing neutralization of HA's negative surface charge from -44.5 mV to -10.7 mV, a 3.2-fold increase in the particle size of HA-Mg complexes, and potentiometric titration indicating that 94% of HA's carboxyl groups (which account for 89.2% of its acidic sites) were involved in magnesium binding. This study provides quantitative mechanistic evidence that the effectiveness of Mg-based strategies depends primarily on the carboxyl functional groups of DOM, highlighting that adsorbent development needs to move beyond HA benchmarks and consider the actual compositional features of real wastewater DOM.