Leachability of elements in alkaline and acidic coal fly ash samples during batch and column leaching tests

Abstract

Three acidic (HA, HB, and MA) and one alkaline (PD) fly ash samples collected from four different power plants were subjected to batch and column leaching tests to evaluate the leachability of several elements. Jar leaching tests (up to 120h), serial batch leaching tests (35days), and long-term (up to 70weeks) batch leaching tests were performed using doubly deionized (DDI) water, and column leaching tests (131–150 pore volumes) were conducted using a synthetic acid rain (SAR) solution. The results of batch and column leaching tests showed that different proportions of As, Cr, Mo, Sb, Se, and V were leached by DDI water and SAR from the four fly ash samples. The increasing leachate concentrations of As, Cr, Mo, Sb, Se (except PD fly ash), and V with time indicated that the long-term release of these elements was controlled by the dissolution of glassy fly ash particles. Mobilization of As from HA fly ash, Cr from PD fly ash, and Se from both HA and PD fly ash also showed a bimodal leaching pattern during column leaching tests, potentially indicating the association of these elements with different fly ash fractions. In general, Mo, Sb, and Se showed high mobilities, while only small amounts of Cr were released from the fly ash samples. Boron, Ca, K, Mg, Mn, and Na were released very rapidly during the initial phases of both serial and column leaching tests. These elements were likely enriched in surface-bound, highly soluble loose particles. During long-term batch leaching tests, HB and MA fly ash leachate concentrations of Ba, Co, Mn, Ni, and Zn decreased with time, indicating possible adsorption or co-precipitation with secondary minerals. Because many of the older fly ash disposal facilities are unlined, these sites are potential sources of hazardous elements to the environment. Therefore, development of effective and economic methods to contain trace elements within the disposal facility site is desirable to prevent contamination of groundwater and surface water resources.