Determining Controls On Element Concentrations in Fly Ash Leachate

Abstract

It is possible, by conducting fly ash leaching tests at two different water:ash ratios, to determine whether or not the concentration of an element in the leachate is controlled by mineral solubility. If a mineral solubility control exists, the element's concentration can be readily predicted with chemical equilibria models. On the other hand, if the concentration of an element is controlled by the rate of release from ash particles, more sophisticated hydrochemical transport models are required. The conduct of leach tests on waste materials at two different water:solid ratios is recommended as a general procedure in waste management. In this study, the concentration of Na, K, Cl, B, and Cr(VI) in Lakeview fly ash leachate were found not to be controlled by mineral solubility. Therefore, the solution concentrations of these elements will be influenced by the relative amounts of water to ash in a mix, and the flux of these elements from hydrating primary ash particles with time. Evidence of solubility control was found for Ca, Sr, SO₄ , Al, Si, As(V), and Se in leachate solutions. Calcium and SO ₄ concentrations were controlled by gypsum solubility; Sr, probably by a (Ba,Sr)SO₄ solid solution; Al and Si, by a hydrous aluminosilicate, probably allophane; Mg, by a hydrous magnesium silicate like sepiolite; and As, by either a ferric metal arsenate or strongly associated with a ferric oxide phase. The probable control on Se concentrations is through coprecipitation in secondary sulphate mineral precipitates.