Effect of ferric iron (Fe(Ш)) on heterotrophic solid-phase denitrification: Denitrification performance and metabolic pathway

[Display omitted] •Fe(Ш) (10–50 mg/L) inhibited NO3– removal and caused NO2– accumulation of SPD.•Continuous Fe(Ш) addition changed the microbial community structure.•Fe(Ш) had obvious inhibition to aerobic denitrifying genes (e.g. napA/B).•Fe(Ш) inhibited the genes related to electron carrier generation in C metabolism. The effect of ferric iron (Fe(Ш)) on the performance of heterotrophic solid-phase denitrification (SPD) using biodegradable polymer composite as the electron donor was investigated. The results of continuous batch experiments showed that the addition of over 10 mg/L Fe(Ш) significantly inhibited nitrate removal and led to the accumulation of nitrite. The addition of Fe(Ш) reduced the microbial community diversity and shifted the community dominated by complete denitrifiers (e.g. Thauera) to that dominated by incomplete denitrifiers (e.g. Thermomonas, Stenotrophomonas and Sphingomonas). The predicted analysis of microbial function by PICRUSt2 indicated that the relative abundance of denitrifying genes, including napA/B, nirS and nosZ, were remarkably reduced in the Fe(Ш) groups comparing with the control group. In addition, Fe(Ш) inhibited the genes related to the generation of electron carriers, NADH and FADH2, in TCA cycle and glycolysis processes, which could result in a lower carbon utilization efficiency for microbial denitrification.