Resource Recovery Efficiency and Microbial Community Response in Anaerobic Chain Elongation of Discharging Wastewater from Spent Lithium-Ion Batteries
The discharging wastewater from spent lithium-ion batteries is characterized by complex composition, high salinity, and substantial organic load, making its efficient treatment and resource recovery a critical challenge in the lithium battery recycling chain. This study investigated the feasibility of applying anaerobic chain elongation technology for resource recovery from such wastewater. The results showed that the reactor could tolerate up to 40% discharge wastewater in the feed, with caproate production reaching 6.38 g/L. However, when the wastewater proportion increased to 60%, system performance declined sharply, and the synthesis of butyrate and caproate ceased. Batch screening experiments ruled out the influence of high salinity (TDS ≈ 12 g/L) and metal ions such as Li+, Ni2+, Co2+, and Mn2+, identifying fluoride (F-) as the dominant inhibitory factor leading to functional failure. Concentration gradient experiments further quantified the inhibitory effect of F-. At concentrations below 600 mg/L, butyrate production remained largely unaffected; at 900 mg/L, substrate metabolism was severely inhibited, with only slight recovery observed at the final stage; and at 1200 mg/L, chain elongation metabolism was completely blocked. Microbial community analysis revealed that the chain elongation function was undertaken by different taxonomic groups at different stages. Initially, Clostridium kluyveri dominated, followed by a shift to Caproicibacterium and Thermocaproicibacter during the mid-phase. In the recovery phase, a synergistic consortium of Oscillibacter valericigenes and Caproicibacterium sp. emerged. Furthermore, after introducing actual discharging wastewater, microbial groups such as Brevundimonas diminuta and Clostridium ljungdahlii, which are likely involved in degrading complex organics, gradually became enriched, providing the substrate foundation for chain elongation. This study offers a feasible strategy and mechanistic insights for the high-value bioconversion of wastewater from lithium battery recycling.