CHROMIUM FATE AND TRANSPORT IN ESTUARINE SEDIMENTS: ROLE OF GEOCHEMICAL PROCESSES IN SUPPORTING MNR
Laboratory-scale batch experiments were performed with diluted suspensions of grab sediment samples wherein the anoxic suspensions were spiked with aqueous CrVI, which reduced to product CrIII under anaerobic conditions. While conditions remained anaerobic no CrVI reoccurrence was observed. However, aeration caused CrVI reoccurrence from the abiotic oxidation of product CrIII. Between 1 and 15 % of the product CrIII was oxidized for the various sampling locations. During aeration, the oxidation of reduced iron and sulfur species lowered the sediment reductant capacity while the oxidation of dissolved manganese (MnII) resulted in the formation of MnIII,IV(hydr)oxides that in turn caused product CrIII oxidation to CrVI. Aeration-driven CrVI reoccurrence decreased with sediment loading and a negative correlation existed between CrVI formation and the acid volatile sulfide (AVS) concentration. The study concluded that even though sediment resuspension and oxygenation may create temporary conditions conducive to CrVI formation in surficial sediments, the abundance of reductant capacity is expected to support monitored natural recovery (MNR) as a remedy for managing Cr contamination in reducing environments.