CdS-mediated electron dynamics in light-assisted anaerobic digestion: Simultaneous cadmium sequestration and methane enrichment by alkali pretreatment.

The pronounced enrichment of cadmium (Cd) in waste activated sludge (WAS) presents dual environmental challenges through its elevated leaching risks and inhibitory effects on subsequent anaerobic digestion (AD) processes, thereby hindering the safe and sustainable management of sludge. While cadmium sulfide (CdS) as a semiconductor has evidenced photostimulated AD enhancement capabilities, critical challenges persist in simultaneously stabilizing Cd2 + into CdS and improving resource recovery within complex sludge matrices. This study introduces a novel strategy for managing Cd-containing waste activated sludge (WAS) by employing a light-assisted AD process following alkali pretreatment. This approach achieved a methane yield of 111.0 ± 2.2 L CH4/kg VSS, significantly outperforming conventional (dark) AD (27.1 ± 1.2 L CH4/kg VSS) and AD with alkali pretreatment only (46.4 ± 1.6 CH4/kg VSS). The alkali pretreatment disrupted the extracellular polymeric substances in the sludge, promoting direct contact between CdS particles and microorganisms, thereby strengthening methanogenic electron capture. Electrochemical analysis corroborated the enhanced electron transfer, further validated by metatranscriptomic evidence that revealed upregulated expression of electron transfer pathways in the microbial community. This study introduces an innovative framework for optimizing AD performance in Cd-containing WAS, offering valuable insights into the efficient treatment of heavy metal-laden sludge and advancing the sustainable utilization of sludge-derived resources.