(477i) Mass Transfer and Reaction Kinetics Limitations in Hazardous Waste (Organic and Metal) Treatment

Improper storage and disposal practices have rendered hundreds of sites contaminated with hazardous substances. Some of the commonly found compounds include chlorinated organics such as trichloroethylene (TCE) and metals such as hexavalent chromium, Cr(VI), which occur simultaneously.  This paper presents the feasibility of simultaneously treating TCE and Cr (VI), while addressing the problems related to (i) mass transfer limitations in chemical oxidation of TCE in soils, (ii) Effect of TCE oxidation on desired reduction of Cr (VI) to Cr (III), and (iii) removal of Cr(VI) using selected chelating agents.

Results of using alcohol cosolvents and household detergents for enhancing TCE solubility in water, thereby reducing mass transfer resistance in aqueous and soil systems will be presented. Redox reactions were studied using oxidizing agents, e.g. permanganate (MnO₄^-) for the remediation of TCE in the presence of Cr (VI) and using ferrous ions, Fe (II) for reduction of Cr (VI) to Cr(III) in the presence of TCE. In the redox reactions, TCE is oxidized to chloride, hydrogen, and carbon dioxide while permanganate is reduced to manganese dioxide (MnO₂). Similarly, Cr(VI) is reduced to Cr(III) and Fe(II) is oxidized to Fe(III). Batch experiments were performed to determine the reaction kinetics and extent of redox reactions of TCE and permanganate in the presence and absence of hexavalent chromium in aqueous systems. The reaction was determined to be second order overall, first order with respect to both TCE and MnO₄^- in the aqueous system. The TCE-Cr(VI) reaction was determined to be second-order overall, first order with respect to both Cr(VI) and TCE. The TCE-KMnO₄-Cr(VI) reaction at stoichiometric molar ratios (1:2:1), based on TCE reactions yielded 100% TCE oxidation. Cr(VI) was found to have a significant positive effect on the oxidation of TCE as it acted as an additional oxidant.

In the chelation study, batch experiments were performed to determine the degree and rate of chelation of Cr(VI) for three chelating agents- disodium dihydrogen ethylenediamine tetraacetate (Na₂H₂EDTA),   ethylenediamine tetraacetic acid (H₄EDTA) and Nitrilotriacetic acid (NTA) Kinetic analysis indicated that the order of reaction was 1.32 with respect to Cr(VI) for Na₂H₂EDTA, 1.17 in the case of H₄EDTA and 2.38 for NTA. It was also found that the rate constants for the individual complexation reactions were 6.89 x 10^-2[mg Cr(VI)/g]^1-nhr^-1, 4.98 x 10^-2 [mg Cr(VI)/g]^1-nhr^-1 and 7.25 x 10^-2 [mg Cr(VI)/g]^1-nhr^-1 respectively.