Chromium fate and speciation in anoxic estuarine sediments: The role of reduced iron-sulfur minerals

Assessing the risk posed by Cr-contaminated sediments to human health and aquatic ecosystems requires determination of the distribution of Cr between its toxic Cr(VI) and nontoxic Cr(III) forms. Predicting environmental exposures also necessitates quantification of rates of oxidation/reduction reactions that control Cr speciation, mobility, and toxicity. In this dissertation, controls on Cr fate and speciation in anoxic estuarine sediments were investigated, with emphasis on quantifying Cr(VI) reduction rates by iron-sulfur mineral reductants prevalent in reducing sediments.;A survey of Cr speciation in sediments and porewaters was conducted for the Baltimore Harbor, a subestuary of the Chesapeake Bay with significant Cr contamination. Concentrations of Cr(VI) in sediments were three to four orders of magnitude lower than total Cr concentrations, indicating significant in-situ Cr(VI) reduction. Titrations of sediments with Cr(VI) revealed complete Cr(VI) reduction so long as added Cr(VI) did not exceed sedimentary sulfide measured as acid volatile sulfide (AVS).;Kinetics of Cr(VI) reduction in Baltimore Harbor sediments were investigated in batch experiments with dilute sediment suspensions. Cr(VI) reduction rates were extremely rapid (half-life of minutes for 1.0 g/L sediment suspensions) and correlated with AVS concentrations of the sediment suspensions. AVS-normalized Cr(VI) reduction rates decreased approximately two orders of magnitude as pH was increased from 5.0 to 8.2. A rate law for Cr(VI) reduction in AVS-containing sediments was developed and used to predict Cr(VI) reduction rates in field sediments. Half-lives for Cr(VI) in sediments containing tens of mucool/g AVS were predicted to be on the order of seconds.;Kinetics and stoichiometry of Cr(VI) reduction by pyrite, the most abundant sedimentary iron-sulfur mineral, were also investigated. Cr(VI) was found to oxidatively dissolve the pyrite surface, releasing Fe(II) and sulfate as reaction products. Biphasic Cr(VI) reduction kinetics were observed, attributable to early, fast Cr(VI) reduction under an adsorption- or electron transfer-limited regime and later, slow reduction under a product desorption/surface site regeneration-limited regime. Kinetic phenomena were well described by a non-steady-state surface complexation model that explicitly tracked time-dependent surface speciation. Reactions of Cr(VI) with pyrite were slower than reactions with sedimentary AVS; however, pyrite may contribute significantly to Cr(VI) reduction in low-AVS sediments.