Impact mechanisms of humic acid on the behavior of CMC-stabilized nZVI in groundwater: Implications for long-term chlorinated contaminant remediation.

Stabilized nanoscale zero valent iron (nZVI) is commonly used as a reductant during groundwater remediation of halogenated hydrocarbons. However, humic acid (HA) in groundwater can affect the environmental behavior of nZVI during contaminant degradation. This study investigated the effects of HA on carboxymethyl cellulose-stabilized nZVI (CMC-nZVI) by studying hydrogen evolution, Fe0 corrosion and carbon tetrachloride (CT) degradation kinetics. The results show that, during the early stage (≤ 24 h), low-concentration HA (HA/CMC-nZVI: 2-8) enhanced the hydrogen evolution reaction (HER) of CMC-nZVI up to 1.36 fold by forming Fe2+-HA complexes that prevented passivation. In contrast, high-concentration HA (mass ratio: 24) inhibited HER during the entire reaction period, as HA rapidly and directly consumed Fe0 from the start of the reaction (final Fe0 consumption by HA: 62.0 %). In this study we also developed a modified differential HER approach to quantitatively determine HA-induced Fe0 consumption. This confirmed that quinone moieties in HA compete for electrons and are reduced to hydroquinones, thereby consuming Fe0. In CT degradation, HA accelerated the initial reaction (rate constant kCT increased from 0.010 min-1 to 0.038 min-1) but significantly reduced electron efficiency (from 6.4 % to 1.3 %) and material longevity at HA/CMC-nZVI ratio of 2-24. Due to HA-mediated depletion of CMC-nZVI, the residual Fe0 had limited capacity for CT degradation after 24 h. The column experiments further confirmed this adverse effect of HA. However, the coexisting ions in actual groundwater were found to mitigate the Fe0 consumption induced by HA. This study reveals HA's dual role in CT remediation: initially HA enhances ZVI reactivity, but subsequently HA reduces ZVI reactivity due to electron competition. This provides novel insights for the optimization of ZVI dosing in groundwater remediation.