Release and redistribution of rare earth elements from coal ash in simulated landfill systems

Municipal landfills have historically co-disposed coal ash (CA) and other solid wastes containing rare earth elements and yttrium (REY). However, REY mobility and their dynamics between residues and leachate during waste stabilization remain poorly understood. This study constructed bench-scale landfill reactors amended with CA and organic waste (OW) to resolve REY leaching and quantify their interactions with dissolved organic matter (DOM) in leachate. During OW biodegradation, accumulation of volatile fatty acids coincided with a decrease in pH and an increase in REY concentration, particularly from exchangeable, acid-soluble, and reducible solid fractions. With pH recovery to neutral-alkaline conditions, DOM became the dominant control on dissolved REY through complexation, with binding contributions ranked as fulvic-like > tyrosine-like > tryptophan-like components. Complexation was driven primarily by carboxylic, polysaccharide C-O, amide carbonyl, and phenolic hydroxyl groups. As OW decomposed, newly formed organo-mineral interfaces developed on CA surfaces. Sorption on organo-mineral surfaces increased REY retention during leachate recirculation. These results explain the partitioning of REY among landfill stabilization products (leachate and humidified fraction) and provide a mechanistic foundation for assessing the potential recovery of REY in the context of landfill mining.