Novel characteristics on micro-electrolysis-mediated Fe(0)-oxidizing autotrophic denitrification with oxygen level: Efficiency, iron-compounds transformation, N2O and NO2− accumulation, and microbiological characterization

Abstract This study revealed novel characteristics of the Fe(0)-oxidizing autotrophic denitrification [Fe(0)-ADN] process with aeration, including nitrogen removal efficiency, electron donor transformation, N2O/NO2− accumulation, and variation of microbial communities and functional genes/enzymes. Iron-scraps (ISs) was utilized as Fe(0) source under the mediation of micro-electrolysis for ISs-ADN process establishment. The anaerobic ISs-ADN process achieved an initial denitrification load of 0.12 ± 0.02 kgN/(m3·d), and it subsequently shifted to 0.34 ± 0.01 kgN/(m3·d) when dissolved oxygen (DO) was increased from 0.1 ± 0.1 mg/L to 1.6 ± 0.1 mg/L. Significant accumulations of N2O (0.9 ± 0.2 mgN2O-N/L) and NO2− (5.5 ± 0.7 mgNO2−-N/L) were observed in the anaerobic ISs-ADN system, they were reduced to 0.07 ± 0.02 mgN/L and 0.2 ± 0.2 mgN/L, respectively, at DO of 1.6 ± 0.1 mg/L. X-ray diffraction spectra showed that oxygen could enhance bioconversion of Fe3O4 into FeO(OH) and reduce the Fe(0)-coating by Fe3O4, and hence enhance the ME process for H2/Fe2+ production. High-throughput sequencing results showed that iron-compound transformation bacteria (e.g. Sphaerotillus) and facultative autotrophic denitrificans (e.g. Hydrogenophaga) were enriched at higher DO of 1.6 ± 0.1 mg/L. Genes napA (encoding NAP), nirK (encoding NIR) and nosZ (encoding NosZ) exhibited higher oxygen tolerance. Enzymes NAP, NIR and NosZ maintained the aerobic denitrification, and NIR and NosZ inhibited the accumulation of N2O and NO2−, respectively. Exorbitant aeration (DO ≥ 2.2 ± 0.2 mg/L) accelerated iron oxidation to Fe3O4 on the ISs surface and restrained ISs-ADN. This study provides an in-depth understanding on the impact of oxygen on the Fe(0)-ADN. The micro-electrolysis mediated ISs-ADN is competitive in both efficiency and cost with the utilization of ISs.