Evaluating in situ redox manipulation for remediating pesticide- and explosive-contaminated groundwater.

摘要

The prevalent use of chloroacetanilide herbicides has resulted in contamination of some ground and surface waters in the U.S. When natural attenuation is not sufficient, remediation may be required. Dithionite is a strong chemical reductant with potential use for remediating contaminated water, sediment, and soil. Used alone, dithionite rapidly dechlorinated chloroacetanilide herbicides in water. Treating aquifer sediment with dithionite reduces native Fe III, creating a redox barrier of FeII-bearing minerals and surface-bound FeII. Exposing alachlor to dithionite-reduced aquifer sediment in citrate-bicarbonate (C-B) buffer produced the thiosufate derivative of alachlor as the major product. Washing the reduced sediment (in C-B buffer) removed FeII and excess dithionite and no alachlor transformation was observed. Exposing alachlor to reduced sediment in K 2CO3 buffer produced dechlorinated alachlor as the major product. Dechlorination also occurred when washed, reduced sediment (in C-B buffer) was amended with FeII (pH = 8.5) and continued as long as additional FeII was provided. Along with pesticide contamination, soil and water contaminated with munitions is also a serious problem. Major high explosive contaminants include RDX, HMX, and TNT. The transformation rates of all three explosives were a function of pH and they were rapidly degraded by buffered, reduced sediment. RDX transformation increased with the amount of reduced sediment or dithionite concentration. Exposing RDX to reduced, washed sediment (in C-B buffer) slowed transformation but the rate was increased by adding FeII. Permeable reactive barriers of zerovalent iron and dithionite-reduced sediment leave behind dissolved, structural, and adsorbed FeII. Aqueous FeII and its associated mineral precipitates degraded RDX, HMX, and TNT. Reaction rates increased with increasing FeII concentration and RDX degradation was greatest at pH > 6.85. This pH is readily obtainable in aquifers. After adding FeII to alkaline aquifer sediment, RDX was degraded without adjusting pH. Considering that iron corrosion increases the pH of the surrounding pore water to >9 and oxidation of FeII produces significant quantities of FeIII, the effect of FeIII on RDX transformation at alkaline pH was investigated. RDX was hydrolyzed at pH 10 under anaerobic conditions and was enhanced significantly when Fe III was present. The results of this research demonstrate that dithionite is a remediation option in natural environments where iron-bearing minerals are abundant and illustrate the important role of FeII and its freshly precipitated minerals in the degradation of contaminants.