Development of Ferrate (Fe(VI))-Coated Sand Composites for Stabilized Reactivity and Remediation of (In)Organic Contaminants in Synthetic Wastewater Effluent

Abstract

Efficient treatment technologies are needed to address the increased presence of contaminants in wastewater effluents and to protect ecosystems in receiving water bodies. Ferrate (Fe(VI)) iron oxide is a high valence (+6), environmentally friendly chemical widely investigated for water treatment applications because of its demonstrated capacity for simultaneous oxidation, disinfection and coagulation. Furthermore, reduction of Fe(VI) produces non-toxic Fe(III) hydroxide phases that are commonly used as coagulants in water treatment. However, Fe(VI) redox potential, speciation and reactivity are pH dependent. Under alkaline pH conditions, Fe(VI) is chemically stable but possesses a lower redox potential (E0 = 0.72 V) which impedes Fe(VI) reactivity under environmentally relevant pH conditions. Previous studies have demonstrated that silica (SiO2) gels can stabilize Fe(VI) and catalyze oxidation of organic compounds by Fe(VI). However, the use of SiO2 gels during water treatment can result in additional solids that will need to be disposed of after Fe(VI) treatment. Additionally, in treatment systems with infiltration processes, the use of SiO2 gels can lead to frequent clogging. Therefore, to stabilize Fe(VI) reactivity and facilitate its deployment for wastewater effluent treatment application, this research seeks to develop, characterize and apply Fe(VI) coatings onto sand—a commonly used water filtration media with primary constituent (>80%) SiO2. Coating Fe(VI) onto sand substrates can facilitate multiple contaminant treatment pathways: (1) removal in the aqueous phase by aqueous Fe3+ ions; (2) sorption/coagulation with precipitated Fe(III) solids formed form Fe(VI) reduction; and (3) removal on the sand surface via sorption or oxidation by Fe(VI). To generate the composite, sand was first mixed with a silica precursor, tetraethyl orthosilicate, to improve binding affinity to Fe(VI). Then, the modified sand was added to a slurry of potassium ferrate to synthesize a stable Fe(VI)-coated sand composite media. Surface analyses techniques coupled with colorimetric methods confirmed the coating of Fe(VI) onto the sand surface. The aqueous stability of the media was driven by water chemistries (e.g., pH, buffering ions). Batch studies conducted to assess the Fe(VI)-coated sand reactivity revealed removal of phenol—a representative and commonly occurring trace organic compound and moiety in wastewater—was achieved at a faster rate by the composite than by application of aqueous K2FeO4 powder (51% removed after 5 min compared to 37%). Batch studies to evaluate the effects of pH and wastewater effluent ions on the media capacity for treatment of multiple contaminants (i.e., trace metals and trace organics) demonstrated that organic compound oxidation is favored at lower pH whereas metals sorption was enhanced at higher pH. Furthermore, the presence of divalent cations improved metals sorption due to the rapid production of Fe(III). The presence of effluent organic matter promoted formation of the more reactive Fe(V) and Fe(IV) species which led to an increased removal of organic compounds in a synthetic wastewater effluent matrix. This research highlights a novel media for advanced treatment of wastewater treatment and presents an opportunity for more effective deployment of Fe(VI) in water treatment applications, especially for filtration processes.