Using Iron Minerals Catalyzed Hydrogen Peroxide-sodium Persulfate To Remove Trichloroethylene In Groundwater

Trichloroethylene (TCE) has a large number of applications in the dry cleaning of clothing, off oil for electronic components and metal parts, solvent extraction for purifing pharmaceutical and other fields. TCE has the potential carcinogenic, teratogenic, mutagenic toxicity. It migrated to the aquifer from the surface, usually exists as dense non-aqueous phase liquid (DNAPL), slow continuously dissolves in the groundwater, threatening the safety of drinking water of groundwater.In situ chemical oxidation (In situ chemical oxidation, ISCO) is a technology that removing pollutants by pouring oxidants into the contaminated soil and groundwater. Hydrogen peroxide mixed with ferrous ion constructing of Fenton's reagent generates hydroxyl radicals (HO) with very strong oxidation, is one oxidantion reagent commonly used in ISCO remediation. But the Fenton reaction proceeds rapidly, of which hydrogen peroxide decompose fastly, releasing a lot of heat and gas, threatening the stability of soil structure and the safety of indigenous biology in groundwater. Sodium persulfate is another commonly used oxidant in ISCO remediation. It is relatively stable in the environment, but need heat, transition metals, UV and etc to activate and produce sulfate radicals (SO42-) with strong oxidation, and then can remove the refractory pollutants. If combine these two oxidants, they are expected to possiblely have better oxidation effects. In this paper, three kinds of naturally occurring iron minerals:siderite, magnetite, hematite were used to catalyze hydrogen peroxide, sodium persulfate, forming single- and double-oxidation systems to removing the TCE target pollutants. Research the removal efficiency and mechanism of degradation for TCE in the single-oxidant systems and the double-oxidant systems and decomposition kinetics of the oxidants in these systems. Main achievements were obtained as follows:The removal efficiency of TCE in these oxidation system were in the order: siderite-double-oxidant system (9.65 mmol/L/87%)> magnetite-double-oxidant system (6.66 mmol/L or 60%)> siderite-single-oxidant system (4.2 mmol/L/38%)> hematite-single-oxidant system (4.10 mmol/L/37%)> magnetite-single-oxidant system (3.85 mmol/L/34%)> hematite-single-oxidant system (3.19 mmol/L/29%). The amount of chloride ions released from TCE in the three siderite-, magnetite-, hematite-single-oxidant systems were 7.65,6.33,3.98 mmol/L, respectively; the amount of chloride ions released in the siderite-, magnetite-, hematite-double oxidation system were 20.35,11.45,8.75 mmol/L.The strongest catalytic role on the decomposition of hydrogen peroxide in the three kinds of iron mineral was paid by using siderite, and then the magnetite, hematite the weakest. This is result of bivalent iron content in the iron minerals and the specific surface area, but more important, is the amount of dissolved ferrous irons in solution. The higher amount of divalent iron ions of iron minerals dissove, the stronger catalytic abilities pay on the decompositon of hydrogen peroxide.The mechanism of degradating TCE in these systems can be summarized as follows: hydrogen peroxide catalyzed by ferrous iron. When hydrogen peroxide catalyzed by trivalent iron produces hydroxyl radicals (HO2·), perhydroxyl radicals, superoxide radical anion (O2·) and single oxygen radicals anion (O·). The Hydroxyl radicals having strong oxidation addition reacted with the double bond of TCE. Superoxide radicals and single oxygen free radicals with reduction replaced chlorine of TCE molecule nucleophilic by substitution reaction. Finally, TCE changed into carbon dioxide and water, and releasing chloride ions.Applications of hydrogen peroxide, sodium persulfate to remove TCE in natural aqueous medium contained trace iron. It was found that the trace iron in the medium catalyzed hydrogen peroxide or sodium persulfate or mixture of them degradated limited TCE. When extra siderite added, degradation of TCE was significantly improved by using the siderite-double-oxidant system. In this system, increased the concentration of sodium persulfate, had no effect on increasing removal TCE, but increased the concentration of hydrogen peroxide, could significantly increase the amount of removal TCE. When the concentration of hydrogen peroxide increased from 1%to 10%, TCE in the system decreased to 0.62 mmol/L (initiate concentration 11.15 mmol/L), obtained the removal rate of 94%for TCE. Here, although the concentration of hydrogen peroxide was enhanced by an order of magnitude, the temperature of the system did not exceed 2 degrees higher than the compared sample, pH values changed little, indicating that change of temperature and pH value in the system has relatively mild impact on biotechnology in groundwater. Therefore, siderite-double-oxidation system has shown a great potential for in-situ remediation of TCE contamination in groundwater.