Speciation of Cr in leachates of a MSWI bottom ash landfill

Cr concentrations and speciation were determined in leachate from a municipal solid waste incinerator bottom ash landfill both experimentally and by thermodynamic model calculations. Total dissolved Cr concentrations of 0.2 mmol L-1 were determined by GFAAS. Two orders of magnitude lower values were determined upon preconcentration by an in-situ solid-phase extraction technique based on the 8-HQ cation exchanger that is specific for Cr(III) but unspecific for Cr(VI). This suggests that chromate dominates the dissolved Cr concentrations in the leachates but was up to 5 orders of magnitude undersaturated with respect to the solubility of CaCrO4 or BaCrO4. Chromate adsorption by oxyhydroxides is less efficient in the highly alkaline environment, but coprecipitation and solid-solution formation with BaSO4 can explain the low chromate concentrations in the leachates. This model assumption was verified by EPMA/WDX measurement of Cr in secondary barite precipitates found in aged bottom ash. Scavenging by this secondary weathering product in landfilled MSWI ash can thus cause an efficient immobilization of the toxic chromate.