Fly ash-mediated simultaneous pH regulation and heavy metal elimination for application of low-intensity acid mine drainage in active coal mines: a comparative study of fly ash from four coal fired power plants

Active coal mines that release acid mine drainage (AMD) are usually characterized by low-intensity acidity and metal loads, however due to large-volume, continuous discharges, they can cause devastating ecological impacts. Although fly ash (FA) could offer an alternative remediation that could be sustainable and economy-friendly, its special contribution to low-intensity AMD is not well-investigated yet. This study examined the pH control and heavy metal (Fe, Mn) removing abilities of FA obtained in four different coal-fired power plants (three in Guizhou, one in Yunnan, China). Systematic physicochemical characterization of the FA samples was conducted, followed by batch treatment assays where varying dosages of FA were applied to simulated AMD solutions to evaluate their acidity, Fe and Mn removal efficacies with different FA dosage respectively. The findings showed that the XLT FA was superior to the others, achieving a significant pH increase (from 3.0 to 6.2) and a 94.1% Fe removal (from 21.6 mg/L to 1.3 mg/L) at a dosage of 0.5 g/L. The basis of satisfactory performance has been its relatively high content of active alkaline components (7.55% CaO and 3.67% MgO) and an irregular, highly mesoporous morphology providing more adsorption sites. Fe elimination by FA was an effective synergy of physical adsorption on the surface under highly acidic conditions, and precipitation of Fe(OH) 3 with increasing pH. On the other hand, the removal of Mn in all samples did not exceed 50% because the pH at which the Mn could be precipitated was not achieved; consequently, the Mn elimination could only be removed through physical adsorption. In effect, this work determines the effective alkaline content and mesoporous structure as the conclusive predictive indicators of FA performance for acidity, Fe and Mn, which confirms the specific waste-treats-waste strategy as a possible effective treatment method for low intensity AMD generated by active coal mines.