Preferential Acidic, Alkaline and Neutral Solubility of Metallic Elements In Fly Ash

In the US, over 100 million tons of coal utilization by-products (CUB) are generated annually. To determine if exposure of these materials to aqueous fluids poses an environmental threat, researchers at the National Energy Technology Laboratory (NETL) have conducted extensive leaching tests. Five 1 kg samples of 35 PC fly ashes have been leached with acid, neutral and alkaline solutions at an approximate rate of 130 mL/d for 1 to 3 months. The leachates are analyzed for pH and 19 cations. The data from this leaching experiment has shown that the release of metal ions from fly ash is related to the alkalinity of the ash and the mineral matrix of the element. However, comparing the maximum concentration in a leachate sample to its pH indicates that the dominant influence on solubility of metallic elements is pH. The pH of the leachate is determined by the soluble alkalinity of the ash and the H/OH concentration of the leachant. Those elements that form oxyanions are apparently most soluble at high pH. Heavy metals are more soluble at low pH. Only Ca, Na, and K indicate significant solubility in the circumneutral pH range. For all elements except Ca, solubility is relatively low at all pH’s; generally less than 2 % of the amount present in the ash can be extracted even by long term leaching.