Degradability of hexachlorocyclohexanes in water using ferrate (VI)

Abstract

Regarding environmental pollution, the greatest public and scientific concern is aimed at the pollutants listed under the Stockholm Convention. These pollutants are not only persistent but also highly toxic with a high bioaccumulation potential. One of these pollutants, γ-hexachlorocyclohexane (γ-HCH), has been widely used in agriculture, which has resulted in wide dispersion in the environment. Remediation of this persistent and hazardous pollutant is difficult and remains unresolved. Of the many different approaches tested, to date, none has used ferrates. This is unexpected as ferrates are generally believed to be an ideal chemical reagent for water treatment due to their strong oxidation potential and the absence of harmful by-products. In this paper, the degradation/transformation of HCHs by ferrates under laboratory conditions was studied. HCH was degraded during this reaction, producing trichlorobenzenes and pentachlorocyclohexenes as by-products. A detailed investigation of pH conditions during Fe(VI) application identified pH as the main factor affecting degradation. We conclude that ferrate itself is unreactive with HCH and that high pH values, produced by K₂O impurity and the reaction of ferrate with water, are responsible for HCH transformation. Finally, a comparison of Fe(VI) with Fe(0) is provided in order to suggest their environmental applicability for HCH degradation.

The adoption and mechanism of KIO4 for redox-equilibrated stabilization of FeO4 2− as an equalizer in water

“…3. It is observed that FeO 4 2− ion in aqueous solution without adding KIO 4 was rapidly decomposed in each temperature ( Fig. 1).…”

“…A set of aqueous K 2 FeO 4 solutions with an initial concentration of 0.55 mmol/L was freshly prepared with/without coexisting KIO 4 of 0.5 mg/ml at 2.57 mol/L NaOH media (the molar ratio of ferrates (VI) vs. KIO 4 is 1:4). The temporal change in FeO 4 2− concentration at different temperatures was measured by UV/vis spectroscopy. The experimental results are plotted in Fig.…”

“…In this study, it was found that FeO 4 2− in water was stabilized by the addition of KIO 4 as coexisting oxidized ions and buffers. The experimental results show that the added KIO 4 greatly increased lifetime of FeO 4 2− in water by orders of magnitude. This finding opens up possibilities for stabilization of solid ferrate compounds, for example, for use in the superiron battery.…”

“…Based on ferrate (VI) compounds, we introduced a super-iron battery with high energy and environmental advantages in 1999 [2]. Due to highly oxidized valence, multiple electron transfer, and high intrinsic energy, the interest in ferrate (VI) compounds is extensive, as shown by the number of publication on subjects of water treatment [3][4][5][6][7][8][9], super-iron battery [10][11][12], organic synthesis [13], and photocatalytic oxidation [14].…”

“…The inherent thermodynamic instability of FeO 4 2− in water has restrictions for its wide applications. The decomposition rate of ferrates depends strongly on the initial ferrate concentration, co-existing ions, pH, and temperature of the solution [2,7,15].…”