Effect of carbon source on the migration and transformation of sulfate and rare earth elements in wastewater by sulfate-reducing anaerobic digestion

The rare earth extraction wastewater containing trace amounts of rare earth elements (REE³⁺), posed significant environmental risks and complicated recovery efforts. Sulfate-reduced anaerobic digestion (SRAD) offered promising treatment for the wastewater. However, the influence of carbon source on the collaborative transformation of sulfate and REE³⁺ within SRAD remained inadequately explored. Thus, this study explored the transformation of sulfate and REE³⁺ under different carbon source conditions. The results demonstrated that, despite the advantage of ethanol in electron donation, it was less effective than a mixed carbon source in removing REE³⁺. This was attributed to several factors: (1) Mixed carbon sources not only enhances the production of S²⁻ but also promotes the stable forms of REE³⁺ through preferential formation of organically bound fraction/residual fraction coordination complexes; (2) Microorganisms cultivated with mixed carbon sources secreted more tryptophan-like proteins capable of adsorbing REE³⁺; and (3) CO-SRB (complete oxidizers–sulfate-reducing bacteria) enriched by mixed carbon sources exhibited stronger adaptability to REE³⁺ and more robust interspecies interactions. Moreover, the COD/SO₄²⁻ ratio of 1.5 was found to be optimal, achieving approximately 90 % removal of REE³⁺ and 69.38 ± 4.63 % removal of SO₄²⁻. This study provides theoretical guidance for carbon source dosing strategies aimed at the simultaneous removal of sulfate and REE³⁺ from rare earth wastewater.