Enhanced humic acid removal by sludge activated carbon/Fe²⁺ composite in peroxymonosulfate activation: mechanism and performance

In subsurface water, humic acid (HA) can react with active chlorine to form carcinogenic compounds, posing ecological issues and health risks. This study aims to create sludge activated carbon (SAC), combine it with Fe²⁺, and activate peroxosulfate (PMS) to remove HA from water. To verify the successful modification of SAC, the physicochemical properties were characterized using various methods such as scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Brunauer Emmett Teller (BET) analysis, and X-ray photoelectron spectroscopy (XPS). Moreover, in the Fe²⁺/SAC/PMS system, the removal rate of HA rose significantly, peaking at 96%. The removal of HA conformed to the first-order reaction kinetic equation, with a reaction rate constant of 0.048 min-1, showing that activated PMS generates potent oxidative radicals or non-radicals, boosting reaction efficiency. After four repeated cycles of use, the removal rate of HA remained above 90%, demonstrating its excellent stability and reusability. The results of three-dimensional fluorescence spectroscopy (3D-EEM) indicated that the Fe²⁺/SAC/PMS system could effectively degrade dissolved organic matter (DOM) in water. Quenching experiments and electron paramagnetic resonance (EPR) analysis further confirmed that singlet oxygen (¹O₂) is the primary oxidative species for degrading HA. Therefore, the Fe²⁺/SAC/PMS reaction system presented promising application prospects to remove HA from water.