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The U.S. Environmental Protection Agency (EPA) reduced the maximum contaminant level (MCL) of arsenic for drinking water from 50 to 10 μg/L on October 31, 2001. Since coprecipitation processes with ferric coagulants are commonly used for removing arsenic in source water, large amounts of iron sludge with elevated arsenic content are generated. Accurate determination of the leachability of arsenic in the water treatment sludge is crucial for both economic concerns and the evaluation of environmental impacts.

Thermodynamic modeling and batch leaching tests, such as toxicity characteristic leaching procedure (TCLP), California wastes extraction test (WET), availability test (NEN 7341) and sequential extraction test, were performed to investigate arsenic leachability in anaerobic and aerobic sludge from two water treatment plants. The anaerobic sludge samples were obtained from a dewatering pond in surface water treatment plant in Los Angeles (LA) with high arsenite As(III) content and the aerobic sludge samples were obtained from the groundwater remediation treatment plant in Washington State (WA) with elevated arsenate As(V) content.

The results showed that arsenic leachability was affected by its oxidation state. Arsenic leachability determined by TCLP was 283 μg/L for anaerobic LA sludge and 21 μg/L for aerobic WA sludge, although total arsenic content was 935 and 8881 mg/kg for LA and WA sludge, respectively. When LA sludge was pretreated by H2O2, the arsenic leachate concentration was reduced dramatically to 30 μg/L. The presence of dissolved oxygen substantially decreased the WET leachability of arsenic in anaerobic LA sludge, while had little effect on the aerobic WA sludge. The availability of arsenic determined by NEN 7341 was increased when the sludge samples were stabilized with lime and cement, while the arsenic leaching behavior was determined by the final pH of the leachate.

Stabilization with cement changed the arsenic association with different phases, that is, exchangeable, bound to carbonate, iron, organic, and residual phase. The molecular structure of As in WA sludge determined by Extended X-ray absorption fine-structure spectroscopy (EXAFS) showed that arsenic formed inner sphere surface complex with iron. The arsenic-iron interatomic distance was 2.89 Å. When the sludge was stabilized with cement, arsenic was associated with calcium and the distance of As-Ca was 3.65 Å.

This research work improved the understanding of arsenic leachability in water treatment sludge. It has implications on the safely disposal and treatment of the arsenic-laden sludge.

Document Type: Research Article


Publication date: 2003-01-01

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