Sorptive stabilization and fractionation of dissolved organic matter by Al and Fe oxyhydroxide-rich water treatment residuals

Sorption of dissolved organic carbon (DOC) by mineral oxides is a key process for preserving organic matter (OM) against rapid microbial degradation and leaching. While this mechanism is well studied in natural soils, the use of water treatment residuals (WTRs) rich in metal oxyhydroxides remains underexplored. This study investigated the potential of aluminum (Al) and iron (Fe) WTRs to stabilize DOC in batch sorption experiments. DOC, derived from anaerobically digested sludge, was added at concentrations of 0 – 120 ppm. Sorption capacity was assessed using distribution coefficient (Kd) derived from the Initial Mass (IM) isotherm model, with Fe-WTR showing a higher Kd value (14.2, R2 = 0.9619) than Al-WTR (6.1, R2 = 0.9753). Post-sorption fractionation using Liquid Chromatography-Organic Carbon Detection-Organic Nitrogen Detection (LC-OCD-OND) and fluorescence spectroscopy showed that both WTRs decreased hydrophobic DOC, biopolymers, humic substances and low molecular substances. Parallel factor analysis indicated greater removal of humic-like (19.2%) and tryptophan-like proteins (61.1%) by Al-WTR. Despite Fe-WTR's higher predicted sorption, the SUVA25, model's “b" value and desorption data indicated weaker sorption and unstable Fe complexes. Al-WTR appeared as a more effective adsorbent for enhancing DOC stabilization. These results suggest that Al-WTR is a more effective material for DOC stabilization. Its sorptive potential could enhance carbon storage thus increasing the potential for carbon sequestration when used in natural systems.