Mobilization of Iron from Coal Fly Ash Was Dependent upon the Particle Size and Source of Coal:  Analysis of Rates and Mechanisms

The observed iron mobilization rate from size-fractionated coal fly ash is consistent with the model predictions for a limiting case of mass transfer where the dominant resistance is diffusion through a layer of depleted solid between the surface of spherical particles and a shrinking core of unreacted material. The rate of mobilization of iron from coal fly ash under physiologically relevant conditions in vitro was previously shown to depend on the size of the ash particles and on the source of the coal, and these in vitro measurements have been shown to correlate with indirect measurements of excess iron in cultured cells. Existing iron mobilization data were compared to mathematical models for mass transfer and chemical reaction in solid−liquid heterogeneous systems. Liquid-phase diffusion resistance can be ruled out as the rate-limiting mechanism for iron mobilization as the model predictions for this case are clearly inconsistent with the measurements. Other plausible hypotheses, such as a rate limited by a heterogeneous surface reaction, cannot be conclusively ruled out by the available data. These mathematical analysis methods are applicable to the design of future experiments to determine the rate-limiting mechanism for the mobilization of iron and of other transition metals from both ambient air samples and surrogates for major sources of particulate air pollution.