Effect of accelerated carbonation on the chemical properties and leaching behaviour of Australian coal fly ash, to improve its use as a compost amendment

Direct agricultural use of most Australian coal fly ashes is limited by their (1) extreme alkalinity, (2) extreme salinity and (3) trace metal leachability. Therefore, in order to use fly ash as a fertiliser source, the bio-availability of its nutrients needs to be improved by reducing its toxic elements and salinity. Application of fly ash as an amendment to carbonaceous compost has the potential to dilute the toxic effects of fly ash while enhancing the release of more nutrients to the soil through the chemical reaction between compost and fly ash. However, the elevation of pH and salinity levels in compost by the addition of fly ash may reduce the availability of some indigenous micro-organisms in the compost, soil respiration, and nitrogen release and may cause toxic heavy metal intrusion into crops, creating health issues. Therefore, the present study examined the applicability of coal fly ash as an amendment to compost for agricultural use, after neutralization by mineral carbonation, taking into consideration the changes which occur in the geo-chemical properties of fly ash, after the mineral carbonation reactions for CO₂ sequestration. The changes in the alkalinity (pH), salinity/electrical conductivity and trace metal concentration of the leachates from column leaching tests were measured and compared before and after the carbonation of three types of Victorian brown coal fly ashes. The results showed a significant drop of pH of the coal fly ash samples from 11–12 to 7.5–8.5 after mineral carbonation, and the salinity reduced from 6–7 to 3.2–3.8 dS/m. Along with the pH reduction, the leachable concentrations of a number of heavy metals, including B, Cd, Cu, Pb, Hg, Zn, As and Se, reduced in the carbonated ash. These favourable changes indicate the enhanced capacity of Victorian brown coal fly ashes as a compost amendment after mineral carbonation reactions.