Ferrate (Fe(VI)) is a strong, environmentally friendly oxidant capable of simultaneous oxidation of organic compounds and adsorption of heavy metals in water. Moreover, the degradation products of Fe(VI) are non-toxic Fe(III) species, which makes Fe(VI) an attractive technology for water treatment. However, Fe(VI) reacts quickly with water molecules and decays rapidly which hinders Fe(VI) reactivity toward organic contaminants. Previous studies revealed that Fe(VI) oxidative capacity toward organic compounds was increased in the presence of silica (SiO2) gel; however, the high-water retention of SiO2 gel makes it impractical for large-scale urban stormwater infiltration systems where high hydraulic conductivity is needed. In this work, we leveraged the property of SiO2 Fe(VI) stabilization to develop a Fe(VI)-coated sand media for urban stormwater treatment infiltration applications. The sand is first mixed with a tetraethyl orthosilicate for activation which acts as a Fe(VI) binding agent. The activated sand is then added to a slurry of potassium ferrate produced using the wet chemical synthesis method to form a stable Fe(VI)-coated sand composite media. X-ray photoelectron and Fourier transform infrared spectroscopy were used to characterize the Fe oxidation state and chemical binding of the Fe(VI)-coated sand, and colorimetric methods were used to determine the rate and speciation of Fe leached from the media surface. Pentachlorophenol and copper served as representative urban stormwater contaminants to probe degradation and complexation kinetics and capacity of the Fe(VI)-coated sand. Our results show that the ferrate-coated sand can degrade pentachlorophenol and copper in aqueous solutions to levels safe for urban stormwater discharge. Reduction in copper concentrations was attributed to adsorption by Fe(VI) species and the Fe(III) oxides formed in solution. The results of this study are promising for urban stormwater infiltration systems where the Fe(VI)-coated sand can be applied to treat multiple types of pollutants.

Synthesis of ferrate, Fe(VI)-coated sand for oxidation and complexation of organic and inorganic contaminants in urban stormwater

ACS Fall 2021

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