Functional group-directed electron shuttling by adsorbed humic acid exciting nontarget reactions and their enhancement non-radical oxidation in H2O2-based aquifer remediation.

Nontarget reactions (NTRs) have long been considered a major barrier to the efficiency and sustainability of in situ chemical oxidation (ISCO) in contaminated aquifers. Here, we report a paradigm-shifting discovery on the dual role of ubiquitous subsurface components, clay minerals and humic acid (HA), in H2O2-based remediation. We demonstrate through controlled experiments that HA adsorbed onto montmorillonite (MT) enhances dimethyl phthalate (DMP) removal by 18.3-125.6 % compared to systems containing only clay or HA. Contrary to the conventional view of NTRs as parasitic scavengers of reactive oxygen species, they also promote the generation of target hydroxyl radicals (∙OH), as directly verified by quenching experiments and EPR spectroscopy. Electrochemical analyses confirm that HA mediates direct electron transfer (DET) between MT and H2O2, leading to interfacial ∙OH formation. The electron-shuttling efficiency of HA is governed by its molecular structure, with electron-donating groups (e.g., para-OH, enediols) maximizing DET efficacy, as quantified by Hammett correlations (R2>0.90). The improved interfacial reaction diffusion model further reveals that ∙OH-mediated reactions are free from diffusion limitations (η=0.999) and confined to the solid-liquid interface (<200 nm), where non-radical pathways dominate. Our findings redefine NTRs as tunable interfacial processes and establish a mechanistic basis for leveraging their beneficial role in next-generation sustainable aquifer remediation.