Modeling arsenic and selenium leaching from acidic fly ash by sorption on iron (hydr)oxide in the fly ash matrix

The sorption of As and Se on iron (hydr)oxide in acidic fly ash is studied as a possible process controlling the leaching of these contaminants. Following controlled leaching experiments with an acidic fly ash, sorption of As and Se is investigated both on crystalline and amorphous iron (hydr)oxide, using partial chemical extractions and isotopic exchange experiments. Virtu ally all As and Se at the fly ash surface, which is potentially available for leaching, is shown to be associated with hydroxylamine extractable ''amorphous'' iron (hydr)oxides in the fly ash matrix. Isotopic exchange experiments suggest that at pH < 10 the oxyanions are partly coprecipitated with secondary-formed amorphous iron (hydr)oxide, a process reducing their availability. The leaching of As and Se from acidic fly ash is described by a simplified model of surface complexation on amorphous iron (hydr)oxide, taking the limited availability of iron-associated As and Se into account. This approach is shown to constitute a useful tool to model sorption control of contaminant leaching from waste materials.