Environ. Sci. Technol. 2004, 38, 2242-2247
Oxidative Degradation of the Carbothioate Herbicide, Molinate, Using Nanoscale Zero-Valent Iron SUNG HEE JOO, ANDREW J. FEITZ, AND T. DAVID WAITE* School of Civil and Environmental Engineering, The University of New South Wales, Sydney New South Wales 2052, Australia
(C8H11Cl3NO3PS) and molinate (C9H17NOS), respectively, an insecticide and herbicide widely used in rice production in Australia, also showed higher rates of degradation in the presence as compared to the absence of air. Parathion degradation has been reported using ZVI in the presence of UV light and H2O2, although no difference was observed under oxygenated and deoxygenated conditions, suggesting that the primary source of oxidation was from UV photolysis of H2O2 (6). These various results have stimulated further investigation as reported here of the processes operating in systems in which ZVI-induced degradation of trace organic compounds is occurring under oxidizing conditions.
Experimental Section Degradation of the carbothiolate herbicide, molinate, has been investigated in oxic solutions containing nanoscale zerovalent iron particles and found to be effectively degraded by an oxidative pathway. Both ferrous iron and superoxide (or, at pH O2 Fe0 + >O2 f >Fe2+ + 2>e>O2 + >e- f >O2•>Fe2+ f Fe2+ >O2•- f O2•O2•- + O2•- + 2H+ f H2O2 + O2 Fe2+ + O2 + H2O f FeOH2+ + O2•- + H+ Fe2+ + H2O2 f FeOH2+ + OH• Fe2+ + OH• f FeOH2+ FeOH2+ + OH- f Fe(OH)3(s) molinate + OH• f mol• + H2O mol• +O2 f keto-molinate isomers a Where “>” represents surface species and assuming pH >4.8 such that the superoxide radical is deprotonated.
with the ferrous iron conjointly formed on ZVI oxidation to produce highly reactive hydroxyl radicals. Support for the role of hydroxyl radicals is provided by the effect of the competitive scavenger, 1-butanol. The effect of catalase is also consistent with this proposed mechanism since destruction of H2O2 will halt the production of hydroxyl radicals (though, as mentioned earlier, it is also possible that catalase, at the concentrations used in these studies, may also act as an effective hydroxyl radical scavenger). The likely steps in the oxidative degradation of molinate that are consistent with the observed keto-molinate product are shown in Figure 9, and a summary of the proposed reaction pathway leading to molinate degradation is shown in Table 2. The proposed pathway would appear to qualitatively account for the observed results. For example, the slow decrease in ferrous iron concentrations in pH0 4 solutions (Figure 7) most likely represents a balance between Fe2+ release from the oxidizing Fe0 surface and subsequent oxidation of this ferrous iron by hydrogen peroxide to ferric species. Hydrogen peroxide concentrations reach a steady state at pH0 4 but only after initial rapid reaction with released ferrous iron. It is presumably because of this initial consumption of hydrogen peroxide by ferrous iron that we observe a lag in accumulation of H2O2, particularly at the higher ZVI loadings. At higher pH0 (8.1), oxidation of Fe2+ by H2O2 (and oxygen) will proceed more rapidly resulting in very little accumulation of Fe2+ in solution and more rapid consumption of ZVIgenerated H2O2. As noted earlier, the tendency for Fe2+ to remain adsorbed to the surface of either Fe0 or to iron oxyhydroxide particles (formed on hydrolysis of ferric species present as a result of Fe2+ oxidation) would also be expected to lower the measurable concentration of Fe2+ in solution. It is also possible that siderite (FeCO3(s)) could initially form (thereby lowering the ferrous iron activity) in these relatively high carbonate content solutions, but its occurrence would only be transitory in view of the continual oxidation of Fe(II) to Fe(III) species. The formation of iron oxyhydroxide particles at alkaline pH might also be expected to lower the reactivity of the ZVI as a result of surface accumulation however relatively similar rates of molinate degradation are observed at pH0 4 and pH0 8.1, at least over the first hour or so. While the eventual reduction in extent of molinate degradation at higher pH may reflect the effect of surface coatings, it is intriguing that we do observe such a high degree of
degradation as most Fenton reagent-mediated processes are relatively ineffective at higher pH. The difference in this case is the continuing generation of fresh reactants (Fe2+ and H2O2) with the effectiveness of the process at high pH limited by Fe0 availability. The continued degradation of molinate for 2 h or more at pH0 8.1 is also confirmation of the continuing release of ferrous iron, the formation of reduced oxygen species, and the subsequent generation of strongly oxidizing substrates (such as the hydroxyl radical, or possibly ferryl species). Any ferrous iron initially present in the reaction medium (derived from the original ZVI stock) would be expected to be oxidized in a few minutes with the resultant cessation of oxidant generation. The slow release/formation of the key reactants and their continuing effectiveness at degrading the contaminant of interest even at high pH suggests a range of possible applications for ZVI-mediated oxidative processes including the in-situ degradation of contaminants in oxic groundwaters.
Acknowledgments Financial support from the Australian Government funded Cooperative Research Centre for Waste Management and Pollution Control (Project 03-6039) is gratefully acknowledged, and S.H.J. thanks the UNSW Faculty of Engineering for scholarship support. The insightful comments provided by Fulbright Professorial Fellow David Sedlak are also greatly appreciated.
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Received for review October 17, 2003. Revised manuscript received December 30, 2003. Accepted January 14, 2004. ES035157G
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