Tetrachloroethene degradation by reducing-agent enhanced Fe(II)/Fe(III) catalyzed percarbonate

Abstract:

This project investigated the effect of reducing agents on the degradation of tetrachloroethene(PCE) by Fe(II)/Fe(III) catalyzed sodium percarbonate (SPC). SPC possesses similar function as liquid H₂O₂, such that free H₂O₂ is released into solution when percarbonate is mixed with water. The addition of reducing agents, including hydroxylamine hydrochloride, sodium sulfite, ascorbic acid and sodium ascorbate, accelerated the Fe(III)/Fe(II) redoxcycle, leading to a relatively steady Fe(II) concentration and higher production of free radicals. This, in turn, resulted in enhanced PCE oxidation by SPC, with almost complete PCE removal obtained for appropriate Fe and SPC concentrations.The results of chemical probe tests, using nitrobenzene and carbon tetrachloride, demonstrated that HO^● was the predominant radical in the system and that O₂^●−played a minor role. This was further confirmed by the results of electron paramagnetic resonance measurements and salicylic acid hydroxylationanalysis by high performance liquid chromatography(HPLC). PCE degradation decreased significantly with the addition of isopropanol, a strong HO^● scavenger, supporting the hypothesis that HO^● was primarily responsible for PCE degradation. It should be noted that the release of Cl^- was slightly delayed in the first 20 mins, indicating that intermediate products were produced. However, gas chromatography mass spectrometry (GC/MS) analysis did not detect any chlorinated organic compound except PCE, indicating these intermediates were quickly degraded, which resulted in the complete conversion of PCE to CO₂. In conclusion, the use of reducing agents to enhance Fe(II)/Fe(III) catalyzed SPC oxidation appears to be a promising approach for the rapid degradation of organic contaminants in groundwater.