Abstract: This study investigates the leaching of rare earth elements (REEs) from incineration ashes produced at urban wastewater treatment plants (UWWTPs) using environmentally friendly organic acids (malic, citric and oxalic) as leaching agents. Ashes, sourced from the Galindo UWWTP in Spain, were found to contain all 15 REEs, with a total REE concentration of 106.93 mg kg-¹, and lanthanum, cerium, neodymium, praseodymium, and gadolinium as the most abundant elements. Central Composite Design (CCD) was employed to optimise the leaching process by evaluating the effects of acid concentration, temperature, liquid-to-solid ratio and H22 concentration on leaching efficiency and REE concentration in the leachate. Among the acids studied, citric acid exhibited the most effective leaching performance. Under optimal conditions of 53.20°C, an acid concentration of 2.48 M, a liquid-to-solid ratio of 21.07 mL of acid per gram of ashes, and an H2O2 concentration of 0.63%, it achieved leaching yields ranging from 59% to 100% for the target elements of the study: Pr, Nd, Gd, Tb, and Dy. The corresponding concentrations in the leachate were 0.12 mg L-1 (Pr), 0.51 mg L-¹ (Nd), 0.14 mg L-¹ (Gd), 0.05 mg L-¹ (Tb) and 0.09 mg L-¹ (Dy). A significant co-dissolution of matrix elements, particularly alkali and alkaline earth metals, was also observed, requiring consideration in downstream processing. This study confirms the viability of using UWWTP incineration ash as a secondary source of REEs and highlights the potential of citric acid as a sustainable alternative to conventional inorganic leaching agents, contributing to the circular economy and urban mining initiatives.

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