Semi-quantitative assessment of room-temperature acid leaching as a partially matrix-preserving strategy for rare earth element recovery from municipal digestate biosolids.

Authors

John Rey A Romal, Hannah Tysse

Published in

Journal of environmental management. Volume 413. Pages 130441. Jul 07, 2026. Epub Jul 07, 2026.

Abstract

Modern municipal wastewater treatment plants (WWTPs) effectively reduce contaminants of emerging concern in treated effluents, including rare earth elements (REEs). However, REEs preferentially accumulate in sludge, raising concerns regarding downstream sludge processing and biosolids utilization while presenting opportunities for resource recovery. This study presents a proof-of-concept for REE decontamination of biosolids while partially preserving the biosolid matrix through room-temperature acid leaching. Dewatered digestate biosolids collected from WWTP in southern Wisconsin were subjected to acid leaching under various operating conditions to evaluate REE extraction and matrix preservation. The optimal protocol consisted of leaching with 2 M hydrochloric acid at a biosolid-to-leachant ratio of 0.2 kg L^-1 for a single 0.5 h cycle at room temperature in batch operation. Under these conditions, 144 mg kg^-1 of total REEs were recovered, corresponding to 80 ± 4% recovery of the pseudo-total REEs, while resulting in a biosolid mass loss of 37 ± 4%. The protocol maintained comparable performance following a 50-fold scale-up and when applied to biosolids collected from WWTP in northern Illinois, demonstrating process adaptability. Fourier transform infrared spectroscopy (FTIR) and elemental analysis indicated that key polysaccharide components of the biosolid matrix were preserved following acid treatment, although partial denaturation and solubilization of protein and nucleic acid fractions likely occurred. Energy-dispersive X-ray spectroscopy (EDX) showed the persistence of phosphorus, sulfur, magnesium, calcium, manganese, iron, and other micronutrients in the residual biosolids. These findings demonstrate an ambient-temperature, infrastructure-compatible strategy that simultaneously enables REE removal, resource recovery, and partial preservation of a potentially-reusable biosolid matrix, providing a foundation for future circular-economy approaches to municipal biosolids management.

PMID:
42413159
Bibliographic data and abstract were imported from PubMed on 08 Jul 2026.

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