Factors Limiting Arsenic Mobility in Coal Combustion Product Leachate

Effective management and beneficial use of coal combustion products (CCPs) requires a thorough understanding of their potential environmental impact, especially the tendency of contaminants such as arsenic to leach from the CCPs.  The most valuable insight regarding mechanisms controlling contaminant leaching from CCPs is obtained from examining leachate data generated in field settings where CCPs are managed.  Interpretations of field-generated data can then be corroborated with previous laboratory leaching studies.

This research presents an interpretation of data obtained as part of a previous Electric Power Research Institute (EPRI) field leachate study, with a focus on arsenic leaching behavior.  This study is unique in that it is the first to evaluate a large set of field-generated data from CCP management facilities using geochemical equilibrium speciation modeling.  Based on the evaluation, two models are presented describing the natural weathering progression of CCPs with initially low pH (<7) and those with initially high pH (>9).  For those with initially low pH (<7), scorodite solubility and adsorption limit arsenic concentrations up to pH ~7, the pH at which adsorption plays a key role, depending upon the solubility of the co-precipitated ferric oxide phase.  Between pH 7 to 9, scorodite either dissolves incongruently to form ferric oxides that adsorb arsenate, or ferric iron is reduced to ferrous iron, producing symplesite as the arsenic-limiting phase.  For CCPs exhibiting initially high pH (>9), calcium arsenate solubility limits arsenic concentrations at pH > ~10.  Carbonation reactions favor the precipitation of calcite, and calcium arsenate is unstable below pH ~10, or log pCO₂ of 10^-6.5 bar.  Arsenic released by calcium arsenate is either adsorbed to existing metal oxides or clays, or, in the presence of elevated ferrous iron, may be precipitated as symplesite.