Role of surface-associated humic acids in promoting reduction of sorbed nitroaromatic compounds by Fe(II)-bearing clay mineral

Structural Fe(II) within clay minerals is an important electron source for the reductive transformation of nitroaromatic compounds (NACs) in anoxic subsurface environments. However， the impact of mineral-associated organic matter on NAC reduction remains unclear， despite its widespread presence in soils and sediments. This study investigated the reduction of NACs to their corresponding anilines in suspensions containing reduced nontronite (rNAu-2， 10 mM Fe(II)) associated with two types of humic acids (HA， 20 mg C/L). The reduction of 50 μmol/L non-sorbing NACs by rNAu-2 was inhibited by mineral-associated redox-active HA or redox-inert organic matter. Electrochemical measurements indicated that the surface coverage of rNAu-2 limited electron transfer from structural Fe(II) to aqueous solutes through the basal siloxane planes. In contrast， rNAu-2-associated HA accelerated the reduction rates of sorbing NACs by factors of 2.5 to 2.8， increasing its reduction extent by 215% to 230%. Further evidence suggests that the electron-accepting moieties of mineral-associated HA effectively withdraw electrons from structural Fe(II)， shifting electron donation from structural Fe(II) in octahedral sheet to the basal siloxane surfaces. The enrichment of reduced mineral-surface HA as electron donors and adsorbed NACs as electron acceptors on the basal siloxane plane promotes electron transfer reactions between them. The role of mineral-associated HA in mediating electron transfer outweighs the inhibitory effect of surface coverage， leading to enhanced reduction of adsorbed NACs. Our findings reveal an overlooked pathway for the reductive transformation of sorbing NACs in subsurface environments enriched with clay minerals and organic matter.