AbstractThe effect of FeS on reactive brilliant red X-3B (RBRX3) removal performance and on the microbial community was investigated in a homemade constructed wetland–microbial fuel cell (CW-MFC) coupled system. Under the test conditions (RBRX3 concentration=100 mg/L; influent glucose concentration ranged from 0 to 300 mg/L), the decolorization rate of RBRX3 and the chemical oxygen demand (COD) in the FeS group were 95.14%–98.86% and 30.53%–86.65%, respectively; these figures were 13.83%–55.52% and 2.57%–19.9% higher, respectively, than those of the gravel group. The output voltage and maximum power density of the FeS group increased by 0.098–0.101 V and 0.078–0.420 W/m3, respectively, compared to the gravel group. The differences between the two groups mainly occurred in the bottom and anode regions; the FeS filling in these regions played an important role. FeS had an obvious effect on the microbial community structure; Firmicutes and Clostridium in the bottom and anode regions became the dominant species. The conversion of iron and sulfur in FeS between different valence states was achieved under the synergistic action of microorganisms such as iron-reducing bacteria (IRB), sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB), which promoted electron transfer and improved the decolorization and degradation effect of azo dyes and the system’s electricity production performance. FeS added to the CW-MFC system can provide electrons for azo dye wastewater treatment, thereby reducing the addition of organic carbon sources.