(698g) Extent and Kinetics of Hexavalent Chromium Removal by Selected Chelating Agents

Improper storage and disposal practices have rendered hundreds of sites contaminated with hazardous substances.  Hexavalent chromium (Cr(VI)) which is toxic and probable carcinogenic is listed as one of the common contaminants. In this study, three chelating agents -  disodium dihydrogen ethylenediaminetetraacetate (Na₂H₂EDTA), ethylenediaminetetraacetic acid (H₄EDTA) and Nitrilotriacetic acid (NTA) were used to remove hexavalent chromium from soil obtained from a non-chromium contaminated hazardous waste site in Chelsea, Michigan. Batch experiments were performed to determine the degree and rate of chelation of Cr(VI) in Chelsea soil. In the chelation reaction, the oxidation state of Cr(VI) does not change, rather it is rendered non-toxic by forming an inactive complex with the chelating agent, which claws the metal ion within its structure.  Average chelation complex (chelating agent-Cr (VI) ion) was 23% for Na₂H₂EDTA, 25% for H₄EDTA and 30% for NTA, over a 36 hour-time period (equilibrium) at room temperature. All experiments were conducted at pH of 5, which yielded the highest rate, and is feasible for field operation. There was an increase of 20-50% in the degree of chelation with rising temperature from 15C to 65C. Kinetic analysis using differential method indicated that the reaction rate was first order with respect to Cr(VI) for Na₂H₂EDTA and second order in the case of H₄EDTA and NTA.