Degradation of organic and inorganic contaminants by zero valent iron

Abstract

Reduction of trichloroethylene (TCE), chromium (VI), and 2,4 dinitrotoluene (2,4-DNT) by zero valent iron and palladized iron under anaerobic conditions was investigated. Reduction experiments of the contaminants were carried out individually and in combination. All three target contaminants were effectively reduced by both iron (Feo) and palladized iron (Pd/Fe'). However, the rate of reduction by Pd/Fe' was found to be much faster than that by Feo. The reduction of all the contaminants in mixed waste was found to be slower than in the individual experiments, but the difference was most significant in the 2,4-DNT reduction. This observation indicates that there may be a possibility of competition for reactive sites among the contaminants and precipitation resulting from CR(VI) reduction may coat iron surfaces, which may ultimately slow the whole zero valent metals (ZVMS) treatment process in remediating mixed waste sites. The 20 mg/L of CR(VI) was reduced below detection limits in 10 hours by Fe' and in 1.5 hours by the same amount of Pd/Fe' in individual experiment. An initial concentration of 20 mg/L of TCE was reduced below detection limits in 72 hours by Pd/Fe' whereas only 62% of TCE was reduced by the same amount of Fe' in 144 hours in individual experiment. The reaction orders of 1.84 and 2.04 for total TCE loss alone and in mixed waste by Fe' indicates that the reaction mechanisms are complex. The reduction of 72 mg/L of 2,4-DNT proceeded to below detection limits within 3 hours by both Fe' and Pd/Feo. The only product observed in the reduction of 2,4-DNT was 2,4-diaminotoluene (2,4-DAT). The 2,4-DAT produced accounted for 83-100% and only 42-54% of the initial mass of 2@4.DNT under anaerobic and aerobic conditions respectively. Since no degradation of 2,4-DAT alone occurred, these results indicate the possibility of other intermediates or products formation under aerobic conditions. Overall, the results demonstrated the potential application of ZVMs in reducing mixed wastes containing both inorganic and organic contaminant interactions before implementing a ZVMs treatment system, which may help in designing a proper remedial system.