Biogeochemical Controls on Hexavalent Chromium Formation in Estuarine Sediments

Predicting the aquatic and human health impacts of chromium (Cr) necessitates one to determine its speciation as either relatively nontoxic Cr^III or toxic Cr^VI and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as Cr^III associated with sediments. While reduction of Cr^VI to Cr^III is dominant in these anoxic sediments, the potential of Cr^III oxidation and Cr^VI reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous Cr^VI spiked into continuously mixed anoxic suspensions was reduced to product Cr^III under anaerobic conditions. No Cr^VI reoccurrence was observed when conditions remained anaerobic. Aeration caused Cr^VI reoccurrence from the abiotic oxidation of product Cr^III. Rates of aeration-driven Cr^VI reoccurrence increased with pH, and Cr^VI reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥ 7. Aeration-driven oxidation of Mn^II to Mn^III,IV(hydr)­oxides was the underlying mechanism causing product Cr^III oxidation. Cr^VI reoccurrence decreased with sediment loading and negatively correlated with the acid volatile sulfide (AVS) concentration. Although sediment resuspension and oxygenation may create temporary conditions conducive to Cr^VI formation, long-term Cr^VI persistence is unlikely in the presence of sediment reductants. While such natural attenuation in reducing environments mitigates the risk associated with Cr toxicity, this risk may still persist in Mn-rich and reductant-deficient environments