| Laboratory experiments and modeling studies were performed to investigate the arsenic mobility in water treatment sludge and its stabilization mechanisms. When the backwash suspension samples collected from a California water treatment plant in Los Angeles were aged in closed containers for a few weeks, soluble arsenic increased from less than 5 μg/L to as high as 700 μg/L and then decreased dramatically due to biotic reduction of arsenate [As(V)], ferric oxyhydroxide. The experimental results and the thermodynamic models showed that arsenic mobility can be divided into three redox zones: (a) an adsorption zone at pe > 0, which is characterized by strong adsorption of As(V) on ferric oxyhydroxide; (b) a mobilization (transition) zone at −4.0 < pe < 0, where arsenic is released due to reduction of ferric oxyhydroxide to ferrous iron and As(V) to arsenite [As(III)]; and (c) a reductive fixation zone at pe < −4.0, where arsenic is immobilized by pyrite and other reduced solid phases. The U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP) substantially underestimated the leachability of arsenic in the anoxic sludge collected from the dewatering ponds due to the oxidation of Fc(II) and As(III) by oxygen. The leaching test should be performed in zero-headspace vessels or under nitrogen to minimize the transformations of the redox-sensitive chemical species.; The sludge from a groundwater remediation Superfund site in Tacoma, Washington was used to investigate arsenic immobilization mechanisms in iron hydroxide sludge stabilized with cement. The sludge was mixed and immobilized with premixed cement to reach cement to sludge ratio of 2.5, 3.3, 5, 10 and 20 (wt. pre-mixed cement/wt. dry sludge). TCLP, extraction procedure toxicity test (EP Tox), multiple extraction procedure (MEP), deionized water extraction and extended TCLP tests were performed to evaluate the arsenic leachability under different leaching conditions. The experimental results and the thermodynamic model showed that the leaching potential of arsenic was determined by the final pH of the leachate. When pH was less than approximately 9, arsenic leachability was controlled by the adsorption on iron hydroxide. Cement treatment significantly reduced arsenic mobility in a high pH range (i.e., pH > 9.5) because of the formation of the sparingly soluble calcium arsenate. Carbonation of calcium arsenate in the high pH range had an adverse effect on the immobilization of arsenic in cement treated material. |
