Chromium(VI) Reduction Kinetics by Zero-Valent Iron in Moderately Hard Water with Humic Acid: Iron Dissolution and Humic Acid Adsorption

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Abstract
In zerovalent iron treatment systems, the presence of multiple solution components may impose combined effects that differ from corresponding individual effects. The copresence of humic acid and hardness (Ca2+/Mg2+) was found to influence Cr(VI) reduction by Fe0 and iron dissolution in a way different from their respective presence in batch kinetics experiments with synthetic groundwater at initial pH 6 and 9.5. Cr(VI) reduction rate constants (kobs) were slightly inhibited by humic acid adsorption on iron filings (decreases of 7–9% and 10–12% in the presence of humic acid alone and together with hardness, respectively). The total amount of dissolved Fe steadily increased to 25 mg L−1 in the presence of humic acid alone because the formation of soluble Fe−humate complexes appeared to suppress iron precipitation. Substantial amounts of soluble and colloidal Fe−humate complexes in groundwater may arouse aesthetic and safety concerns in groundwater use. In contrast, the coexistence of humic acid and Ca2+/Mg2+ significantly promoted aggregation of humic acid and metal hydrolyzed species, as indicated by XPS and TEM analyses, which remained nondissolved (>0.45 µm) in solution. These metal−humate aggregates may impose long-term impacts on PRBs in subsurface settings.

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