Strontium Isotope Tracking of Coal Utilization by-Product (CUB) Interactions with Environmental Waters: Results from Column Leaching Experiments

Solid coal utilization by-products (CUB) from coal-fired power plants, including fly ash and bottom ash, must be recycled or safely disposed of in settling ponds.  Therefore, it is important to determine the extent to which CUB retains potentially toxic elements such as As, Se, B, and heavy metals during interactions with aqueous solutions.  We report initial results of a strontium (Sr) isotope investigation of column leaching experiments performed on four different class F fly ash samples using acetic acid, sodium carbonate, and nitric acid to simulate different environmental fluids. The overall objectives of this investigation are (1) to establish Sr isotopes as a sensitive tracer for interaction of process water, surface water, and groundwater with fossil fuel byproducts; (2) to help understand sources and sinks for metals in CUB subject to aqueous environments; and (3) to develop a forensic tool for demonstrating metal retention by CUB in field applications.

The solutions were pumped through up-flow PVC columns containing 1 kg of CUB, with a total flow through of ~20 L (after 3 months). Leachates collected from different CUB samples after 0.2, 0.6, and 1.3 liters of flow through vary widely in their ⁸⁷Sr/⁸⁶Sr ratios, ranging from ~0.7120 to 0.7108.  While all leachates for a given sample yield the same ratio at the first collection point, divergence in ⁸⁷Sr/⁸⁶Sr (by as much as 0.0004) by the second collection time (0.6 L) was observed among the different leaching solutions, and with increasing liquid/solid ratios for individual solutions.  This indicates that fly ash is not isotopically homogenized during burning, and that isotopically and chemically distinct phases are selectively leached by different solutions.  These preliminary data are consistent with batch leaching experiments at much higher liquid/solid ratios, and they suggest that ⁸⁷Sr/⁸⁶Sr can be used to identify mobile phases in coal fly ash.