Humic acid modulates Fe(II) mediated chromium reduction in simulated marine-terrestrial interlaced zones: A...

Groundwater systems in the Marine-Terrestrial Interlaced Zone (MTIZ) represent critically vulnerable environments, where the persistence of chromium (Cr) poses significant environmental risks. Between the two predominant oxidation states of Cr, Cr(VI) exhibits markedly higher solubility and toxicity than Cr(III). Understanding the reduction pathways of Cr(VI) to Cr(III) and identifying key influencing factors in the MTIZ is therefore critical. In this study, a dual-chamber simulation MTIZ system was established to investigate the cooperative mechanism of ferrous iron (Fe(II)) and humic acid (HA)-two prevalent groundwater constituents-on Cr(VI) immobilization. Our results demonstrate that Cr(VI) can be reduced rapidly by Fe(II) in MTIZ system under controlled hydrogeochemical conditions (e.g., pH and dissolved oxygen). Although HA initially inhibited reaction kinetics by impeding diffusion and redox processes, it ultimately enhanced the total Cr(VI) removal by approximately 15 % through multiple mechanisms over an extended period. Notably, reactive oxygen species (·OH, ·O₂⁻, and ¹O₂) were detected in the Cr+Fe+HA system. Quenching experiments further clarified their distinct roles: scavenging ·OH increased Cr(VI) reduction efficiency from 78 % to 82 %, indicating its inhibitory effect, while removing ·O₂⁻ sharply decreased efficiency to 8 %, demonstrating its essential role in driving the reduction. Most of the Cr(VI) was converted into stable Cr(OH)₃ precipitates and HA-Fe-Cr complexes, minimizing soluble residuals. Concurrent Fe(II) oxidation produced FeOOH precipitation, which further aids in Cr sequestration through both reduction and adsorption. These findings provide novel insights into Cr behavior in coastal groundwater and establish an experimental framework for assessing contaminant transformation and environmental risks in the MTIZ.