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 CrIII or toxic CrVI and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as CrIII associated with sediments. While reduction of CrVI to CrIII is dominant in these anoxic sediments, the potential of CrIII oxidation and CrVI reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous CrVI spiked into continuously mixed anoxic suspensions was reduced to product CrIII under anaerobic conditions. No CrVI reoccurrence was observed when conditions remained anaerobic. Aeration caused CrVI reoccurrence from the abiotic oxidation of product CrIII. Rates of aeration-driven CrVI reoccurrence increased with pH, and CrVI reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥...