Ferrate (Fe(VI))-Coated Sand Media for Simultaneous Oxidation of Organic Contaminants and Adsorption of Trace Metals in Water

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Chemical oxidants such as chlorine and ozone are widely utilized as disinfectants and reactants to degrade harmful trace organic compounds in conventional water treatment processes. However, chlorination and ozonation can also lead to the formation of toxic byproducts during water treatment. Ferrate, Fe(VI) iron oxide, has emerged as a promising multifunctional reagent for water treatment due to its high redox potential, benign nature and demonstrated ability to serve as an oxidant, disinfectant, coagulant, and sorbent. However, ferrate rapidly decomposes in aqueous solutions at environmentally relevant pHs, which limits its reactivity and effectiveness as oxidant and functional reagent for water treatment. Previous studies have shown that silica gels can delay the decomposition of ferrate and catalyze the production of highly reactive ferrate iron oxyanion intermediate species. Building upon these promising findings, the Principal Investigator (PI) of this project proposes to test the hypothesis that sand, which is primarily composed of silica, can stabilize ferrate decomposition in aqueous solutions at environmentally relevant pH and enhance its reactivity and deployment as multifunctional reagent for water treatment. To test this hypothesis, the PI and project research team propose to 1) design, synthesize, and characterize ferrate-coated sand composite materials and 2) evaluate their performance as chemical oxidants and sorbents for non-selective water treatment. The successful completion of this project will benefit society through the generation of fundamental knowledge to advance the development and deployment of efficient and low-cost ferrate-based multifunctional materials for water treatment. Additional benefits to society will be achieved through student education and training including the mentoring of one graduate student at the University of Washington. Ferrate [Fe(VI)], an environmentally benign iron oxyanion, can oxidize organic pollutants in aqueous solutions while its reduced, non-toxic ferric [Fe(III)] phase can sorb heavy metals in water. However, at environmentally relevant pHs, ferrate rapidly decays thus limiting its treatment effectiveness. Previous studies have demonstrated that silica (SiO2) gels can slow down the charge transfer kinetics of ferrate to enhance its reactivity and efficiency as oxidant. However, these previous studies relied on the non-covalent coating and physical mixing of silica gels with ferrate. To harness the dual benefits of SiO2-stabilized ferrate for oxidation and sorption, the Principal Investigator (PI) and project research team will explore the covalent attachment of ferrate onto sand, a widely used silica-based water filtration media, with the goal of creating novel and low-cost multifunctional materials for water treatment. The specific objectives of the research are to 1) synthesize and characterize covalent composite ferrate-sand media; 2) probe the media stability in aqueous solutions and investigate iron (Fe) speciation during media reactions with organic and inorganic contaminants; 3) investigate and elucidate the interactions between the composite ferrate-sand media with relevant environmental compounds and species (e.g., dissolved organic matter and inorganic ions) that are known to impact redox reactions and sorption processes in aqueous solutions; and 4) investigate opportunities for in-situ regeneration and reuse of the composite ferrate-sand media. The successful completion of this project has the potential for transformative impact through the generation of new fundamental knowledge and materials to guide the design and deployment of more efficient and cost-effective multifunctional media for water treatment. To implement the education, training, and outreach goals of the project, the PI proposes to leverage existing programs at the University of Washington (UW) such as Engineering Days to engage 4th -8th students from the greater Seattle metropolitan area to perform lab demonstration experiments with the goal of exciting them about opportunities in STEM careers. In addition, the PI plans to lead an educational outreach activity in partnership with the “Hip Hop is Green” nonprofit organization to recruit and mentor high school students to gain research experience while working on ferrate treatment media in the PI’s lab at UW.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.