Despite the proposed use of ashes as cement stabilizer or additive in fertilizers, combustion particles produced from anthropogenic activities, such as power plants ashes and kiln dust, are byproducts with limited use and value. These combustion particles accumulate in large deposits and ultimately become materials of environmental concern. In fact, recent studies suggest that a significant fraction of metals in the environment come from the processing of combustion particles, with nearly 5% of Fe2+ in the ocean originating from anthropogenic combustion particles. Herein, we carry out a comparative study of iron mobility and speciation from two different combustion particles: bottom ash from a coal-fired power plant (BA) and lime kiln dust (LKD), both samples originating from Lithuania. The iron leach from these materials was investigated in aqueous suspensions, under environmentally relevant acidic conditions: from pH 1 and 2, simulating the deliquescent layer of typical atmospheric processing, to pH 4, simulating the processing of particles in acidic effluents of ash ponds. For the two combustion particles examined here, the mobility of bioavailable Fe2+ is significantly lower than that of Fe3+, as the combustion process tends to increase the fraction of the more oxidized Fe3+. Yet, the low solubility of Fe3+ at pH higher than 3.5 have a significant impact in its proton-promoted solubility, with a sharp decrease in Fe3+ concentrations at pH 4. On the other hand, the solubility Fe2+, a known environmental micronutrient, from the both BA and LKD was found to be relatively constant through the range of pHs investigated. While LKD was found to mobilize nearly 20% more iron than that of BA, both samples were found to leach similar amounts of iron with respect to their surface areas, suggesting a surface driven solubility process. Overall, this work provides insight in the ability of combustion powders to provide iron micronutrients under different environmentally relevant acidic conditions.

Effect of Ph in iron mobility from aqueous suspensions of kiln dust and bottom ash

ACS Spring 2022

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