Significant Shifts in Boron Isotope Ratios during Column Leaching of Coal Fly Ash

Boron isotopes could be an ideal tracer for interaction of natural waters with coal utilization byproducts (CUB) due to (1) large observed differences in ¹¹B/¹⁰B ratio between coal products and most ground and surface waters, (2) frequently high (and sometimes toxic) concentrations of B in coal and its byproducts, and (3) the potential for B isotope fractionation under alkaline conditions typical of fly ash. We present preliminary data from an up-flow column experiment in which 1 kg of fly ash was leached with weak acetic acid over three months (20 L total flow-through). Boron concentrations in the leachate started out high (3x10⁴ ppm after 0.2 L) but dropped to less than 500 ppm after 1.3 L passed through. Boron isotope ratios were measured by NTIMS, and are reported as δ¹¹B, permil deviation of the ¹¹B/¹⁰B ratio from the SRM951 standard. Measured ratios show an initial rapid change, from a δ¹¹B value of -7 for the first sample, down to -30 after 1.3 L. The measured δ¹¹B remained around -30 at the halfway point of the column experiments. These negative δ¹¹B values are consistent with those measured in coal (Williams and Hervig, 2004, App. Geochem. 19, 1625). The low pH (below 5) of collected solutions argues against mass fractionation during leaching as the cause of the initial shift in δ¹¹B. The data are consistent with the formation of soluble surface minerals and residual refractory phases during coal combustion. The soluble boron could be derived from coal organic matter and adsorbed B with a wide range of δ¹¹B values. The more refractory phase could be associated with clay minerals, typically enriched in ¹⁰B. Continued work will focus on different leaching fluids and additional CUB samples to determine the conditions under which B is released or retained during fly ash disposal.