Environmental Safety Aspects of Solid Residues Resulting from Acid Mine Drainage Neutralization with Fresh and Aged Red Mud

Acid mine drainage (AMD) remediation by red mud (RM) can benefit process sustainability and waste valorization. Though the AMD properties upon using RM have been broadly investigated, the key features and the environmental stability of spent RM are poorly defined. The current study aimed to compare the AMD neutralization and metal removal potential of the fresh (FRM) and aged (ARM) Bayer red mud and evaluate the textural, mineralogical, chemical, radiological properties of solid residues, and the stability of toxic elements. The FRM had larger particles and exhibited higher pH, iron oxide content, and radioactivity than ARM, which could be driven by ARM leaching and settling inside the tailings pond. FRM displayed marginally higher neutralization and metal removal capacity than ARM. Reaction with AMD led to partial dissolution of cancrinite minerals without significant changes in the particle size distribution. The leachability of investigated toxic elements (As, Ba, Cd, Cr, Cu, Ni, Pb, Se, Zn) from neutralization residues complied with the limits set for inert waste by the EN 12457–2 test. Owing to the RM neutralization, the leachability of Al, As, Cr, and V decreased with respect to pristine samples. While Zn and Mn sorbed from AMD were stable following SPLP and EN 12457–2 tests, the sequential extraction revealed increased environmental availability, posing a potential risk during disposal or application if accompanied by a decline in pH. The slight increase in radioactivity upon RM neutralization indicates that changes in radionuclide inventory should be monitored when using RM in neutralization.