There is renewed interest in the tetra-oxy compound of +6 oxidation states of iron, ferrate(VI) (Fe(VI)O4(2-)), commonly called ferrate. Ferrate has the potential in cleaner ("greener") technologies for water treatment and remediation processes, as it produces potentially less toxic byproducts than other treatment chemicals (e.g., chlorine). Ferrate has strong potential to oxidize a number of contaminants, including sulfur- and nitrogen-containing compounds, estrogens, and antibiotics. This oxidation capability of ferrate combines with its efficient disinfection and coagulation properties as a multi-purpose treatment chemical in a single dose. This review focuses on the engineering aspects of ferrate use at the pilot scale to remove contaminants in and enhance physical treatment of water and wastewater. In most of the pilot-scale studies, in-line and on-line electrochemical ferrate syntheses have been applied. In this ferrate synthesis, ferrate was directly prepared in solution from an iron anode, followed by direct addition to the contaminant stream. Some older studies applied ferrate as a solid. This review presents examples of removing a range of contaminants by adding ferrate solution to the stream. Results showed that ferrate alone and in combination with additional coagulants can reduce total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD), and organic matter. Ferrate also oxidizes cyanide, sulfide, arsenic, phenols, anilines, and dyes and disinfects a variety of viruses and bacteria. Limitations and drawbacks of the application of ferrate in treating contaminated water on the pilot scale are also presented.
Phytoremediation has the potential for implementation at mercury- (Hg) and methylHg (MeHg)-contaminated sites. Water hyacinths (Eichhornia crassipes) were investigated for their ability to assimilate Hg and MeHg into plant biomass, in both aquatic and sediment-associated forms, over a 68-day hydroponic study. The suitability of E. crassipes to assimilate both Hg and MeHg was evaluated under differing phosphate (PO4) concentrations, light intensities, and sediment:aqueous phase contamination ratios. Because aquatic rhizospheres have the ability to enhance MeHg formation, the level of MeHg in water, sediment, and water hyacinth was also measured. Hg and MeHg were found to concentrate preferentially in the roots of E. crassipes with little translocation to the shoots or leaves of the plant, a result consistent with studies from similar macrophytes. Sediments were found to be the major sink for Hg as they were able to sequester Hg, making it non-bioavailable for water hyacinth uptake. An optimum PO4 concentration was observed for Hg and MeHg uptake. Increasing light intensity served to enhance the translocation of both Hg and MeHg from roots to shoots. Assimilation of Hg and MeHg into the biomass of water hyacinths represents a potential means for sustainable remediation of contaminated waters and sediments under the appropriate conditions.
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