Application of sodium alginate/iron sulfide/biochar beads as electron donors to enhance nitrate removal from carbon-limited wastewater

[Display omitted] •SA/FeS/BC donated multiple electrons for NO3-N removal from carbon-limited wastewater.•SA/FeS/BC achieved 98.48% NO3-N removal efficiency without NO2-N accumulation.•Dominant genera were changed from Flavobacterium to Thiobacillus by FeS and BC.•Genes related to electron shuttle, FeS oxidation, and denitrification were enriched.•Revealed the NO3-N removal mechanism based on electron consumption and transfer. Efficient nitrate-nitrogen (NO3-N) removal through solid organic carbon-based heterotrophic denitrification is challenging because of the unstable organic carbon release and low electron utilization rate. A functional biocarrier (alginate/iron sulfide/biochar beads (SA/FeS/BC)), integrating heterotrophic and autotrophic denitrification, was prepared using the cross-linking method. SA/FeS/BC achieved high stability and efficiency in reducing NO3-N (98.48 ± 0.56 %) and maintained an average chemical oxygen demand (COD) and a sulfate (SO42–) concentration of 50.39 ± 36.30 and 146.45 ± 1.09 mg/L, respectively, meeting the Chinese Quality Standard for Ground Water (GB/T 14848–2017). Microbial community analysis indicated that FeS and BC induced a change in the dominant genera from Flavobacterium to Thiobacillus, and enriched genes involved in electron transfer, FeS oxidation, and denitrification. This indicated that SA/FeS/BC strengthened autotrophic denitrification and enhanced electron transport activity for NO3-N removal. Furthermore, this study provided new insights into the strengthening mechanisms for NO3-N removal in the SA/FeS/BC-based mixotrophic denitrification system. The findings of this study indicated that SA/FeS/BC with integrated electron donors could enhance NO3-N removal efficiency and electron utilization rate, offering a potential application for effective carbon-limited wastewater treatment.