Effect of Anions on Electrochemical Degradation of Perchlorate in water Using Zero-valent Titanium
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This study investigated the effect of co-anions on perchlorate degradation using zero-valent titanium (ZVT) anode. Results showed that the rate of perchlorate reduction was not affected or only slightly affected by the presence of nitrate, sulfate, and phosphate. Nitrate was chemically reduced to nitrite and ammonium and some of it may have formed gaseous compounds such as elemental nitrogen, nitric oxide and nitrous oxide. Some decrease in concentrations of sulfate and phosphate was observed and probably was caused by adsorption onto precipitates formed during the reaction. Perchlorate reduction was inhibited by the presences of some aggressive anions such as chloride and bromide, but not by others such as iodide and fluoride. The reason why iodide and fluoride did not inhibit rates of perchlorate reduction is related to both their pitting potentials and tendencies to adsorb onto the oxide film on the surface of the titanium metal. The pitting potentials of the anions investigated are in the order: SO4 2 -> F- > PO4 3- > NO3 - > ClO4 - > Cl- >I- > Br-. Based on these potential values, the anions that have lower pitting potentials than perchlorate could be expected to inhibit perchlorate reduction; however, iodide did not do so. This could be the result of iodide tending to react less with dissolved Ti(II) than do chloride and bromide or the result of iodide tending to adsorb less to the oxide film on the titanium surface.
