Mixotrophic denitrification processes in basalt fiber bio-carriers drive effective treatment of low carbon/nitrogen lithium slurry wastewater

•Efficacy of R-BF was studied for the biological treatment of lithium slurry wastewater.•TN removal rate was 4.462 kg/m3/d in R-BF while qmax in Monod was 0.32284 mg-N/mgVSS/d.•R-BF could regulate diverse nitrogen metabolic pathways to adapt to low C/N ratios.•HN-ADs, AOB, NOB and DGAOs were the main functional groups in R-BF.•(Aerobic)chemoheterotrophy, nitrogen respiration, and nitrate reduction were dominant processes.