Redox transformations of arsenic and iron in water treatment sludge duringaging and TCLP extraction

Laboratory experiments and modeling studies were performed to investigate t he redox transformations of arsenic and iron in water treatment sludge duri ng aging and to evaluate the impact of those transformations on the leachab ility of arsenic determined with the U.S. EPA toxicity characteristic leach ing procedure (TCLP). When the backwash suspension samples collected from a California surface water treatment plant were aged in closed containers fo r a few weeks, soluble arsenic increased from less than 5 mug/L to as high as 700 mug/L and then decreased dramatically because of biotic reduction of arsenate [As(V)], ferric oxyhydroxide, and sulfate. The experimental resul ts and the thermodynamic models showed that arsenic mobility can be divided into three redox zones: (a) an adsorption zone at pe > 0, which is charact erized by strong adsorption of As(V) on ferric oxyhydroxide; (b) a mobiliza tion (transition) zone at -4.0 < pe < 0, where arsenic is released because of 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 i mmobilized by pyrite and other reduced solid phases. The TCLP substantially underestimated the leachability of arsenic in the anoxic sludge collected from sludge ponds because of the oxidation of Fe(II) and As(III) by oxygen. The leaching test should be performed in zero-headspace vessels or under n itrogen to minimize the transformations of the redox-sensitive chemical spe cies.