SPECIATION AND MINERAL ASSOCIATIONS OF TRACE ELEMENTS IN COAL COMBUSTION BY-PRODUCT WASTES

Coal combustion by-product (CCB) materials are produced primarily by the electrical power industry. Although there is some reuse of this waste material, the majority is deposited in ash impoundments (lagoons or landfills) where meteoric water may interact chemically with the waste. During and after disposal, soluble salts deposited on ash particles and metastable high-temperature phases formed during the combustion process will convert to thermodynamically stable phases over time in a weathering environment. The geochemical reactions causing this transformation also release trace metals to the pore fluids. Understanding the speciation and mineral associations of trace metals in CCB wastes is critical to predicting the potential impact of ash impoundment leachates on the environment.

A total of 115 ash samples from 4 different coal combustion by-product disposal facilities (CCBDFs) located in the southeastern US have been examined by XRD and SEM with EDX spectroscopy to determine major, minor and trace mineral constituents. Secondary phases formed by chemical weathering include gypsum, calcite, amorphous Fe-oxyhydroxide, and allophane. The ash samples were also subjected to acid microwave-assisted partial digestion to evaluate the concentrations of potentially mobile major, minor and trace element constituents of the ash samples. Although acid-extractable trace metal concentrations varied by as much as an order of magnitude between sites and also varied with depth for individual cores, several element associations and trends were identified. The strongest element correlations were between Al, Ca, Mg and the trace metals Cr, Ni, Pb, Ti and Zn.

Trace element speciation and possible mineral associations were evaluated indirectly through geochemical modeling of CCBDF pore fluids and directly using synchrotron radiation analytical techniques. Collectively, these data show that toxic trace metals are predominantly associated with amorphous aluminosilicate and iron oxyhydroxide solid phases in weathered ash samples. Element maps collected using m-XRF show trace metals correlating with Fe (associated with iron (hydr)oxide phases) or K (inferred association with clay-like phases). Micro-XANES spectra show As in the weathered ash samples to be present as As(V), which has lower mobility than As(III).