Influence of Biogeochemical Conditions on the Stability and Fate of Cr(III) Species in Contaminated Harbor Sediments

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Bouwer, Edward CBET-0967236 The proposed research focuses on determining the fate of trivalent chromium, Cr(III), in contaminated harbor sediments. Years of navigational, maritime, and industrial activities have left a legacy of sediment contamination in the coastal bays and harbors across the U.S. The complex mixture of contaminants found in sediments, of which metals are a major component, causes toxicity to benthic organisms, aquatic life, and eventually to humans by bioaccumulation in the food web. Cr is one such metal which commonly occurs in contaminated sediments. The trivalent form of Cr, Cr(III), is relatively less toxic, and less mobile than the hexavalent form Cr(VI), which is a potent human carcinogen. Under reducing environmental conditions Cr(VI) is reduced to Cr(III), whereas under oxidizing conditions Cr(III) is oxidized to toxic Cr(VI) by naturally occurring hydrous manganese oxides, Mn(III,IV) (hydr)oxides. The PIs proposed research will fill important knowledge gaps regarding Cr speciation and behavior of Cr(III) in contaminated sediments by conducting controlled laboratory studies using contaminated sediments from various locations around the Baltimore Harbor, the field site for their research efforts. The goal of their research is to obtain a systematic understanding of the biogeochemical processes influencing Cr speciation and fate in contaminated sediments. Their preliminary results reveal that Cr in Baltimore Harbor sediments is predominantly precipitated Cr(III) Cr(OH)3(s) and CrxFe1-x(OH)3(s), and Mn is predominantly as reducible Mn(III,IV) hydroxides. These results along with the ones from our preliminary spike experiments support their hypothesis that resuspension of sediments through bioturbation, flood events, dredging, and anthropogenic activities could make conditions conducive to Cr(VI) formation via Cr(III) oxidation by Mn(III,IV) (hydr)oxides. The proposed experimental work will take advantage of their HPLC-ICP-MS analytical technique developed to achieve direct speciation of Cr at trace environmental concentrations in bulk sediments. The proposed work involves the collection and characterization of sediment cores to obtain chronological and depth information of metal accumulation and redox conditions. Using surficial sediments, the first set of batch experiments will examine the Cr(III) oxidizing capacity of the Mn(III,IV) (hydr)oxides present in sediments by spiking sediment suspensions with aqueous Cr(III) under aerobic and anaerobic conditions. Included here are the reaction conditions of varying aqueous Cr(III) concentrations, sediment loading, and pH. Since Cr(III) solids limit its solubility under the near-neutral pH conditions of the sediments, the second set of experiments will study the reaction chemistry of Cr(OH)3(s) and CrxFe1-x(OH)3(s) spiked into the sediments under aerobic and anaerobic conditions. A third set of experiments will examine the reactive state of Cr(III) present in the sediments by spiking the aerobic and anaerobic sediment suspensions with laboratory synthesized Mn(III,IV) (hydr)oxides containing varying Mn(III) content birnessite, manganite, and pyrolusite under varying conditions of Mn(III,IV) (hydr)oxide surface area loading, and sediment loading. This set of experiments will give additional insights into the importance of Mn(III) or Mn(IV) species in Cr(III) oxidation. Microbes influence the bioavailability of metals and hence, a fourth set of experiments will examine the influence of Mn-oxidizing microbes in mediating Cr(III) oxidation in sediments by producing biogenic Mn(III,IV) (hydr)oxides. The experimental results from each research task will be synthesized through modeling efforts, and these models will serve as predictive tools to assess risks and provide guidance for remedial options on metal-contaminated sediment sites. Results from this proposed work will aid the Maryland Department of the Environment(MDE) and other government agencies in establishing TMDLs for Cr and similarly other metals in contaminated sediments. The PI has continued his collaboration with the Baltimore Polytechnic Institute High School Mentoring Program, which brings underprivileged and/or minority high school students to explore scientific research at Johns Hopkins University (JHU). Additionally, the PI continually serves as a faculty mentor in the Women in Science and Engineering (WISE) program, which is designed to foster representation of women in science and engineering fields by providing research experience, guidance, and mentoring to female high school juniors. The proposed project will also help continue the PIs active role in the Research Experience for Teachers (RET) program at JHU, in which teachers from Baltimore City public high schools pursue research projects over the summer under the PIs supervision. Such collaborations will enable the Ph.D. and undergraduate students involved in this project to engage in educational outreach activities, gain valuable experiences in mentoring and teaching, and instill a greater sense of social responsibility.