Influence characteristics and mechanism of organic carbon on denitrification, N2O emission and NO2− accumulation in the iron [Fe(0)]-oxidizing supported autotrophic denitrification process

Abstract Iron [Fe(0)]-oxidizing supported autotrophic denitrification [Fe(0)-AD] is a promising process for low-carbon water treatment. Organic-carbon (OC) has been a common pollutant associated with nitrogen pollution. However, its interaction with Fe(0)-AD remains unclear. In this study, influence characteristics and mechanism of OC on Fe(0)-AD were investigated. Fe(0)-AD was established based on iron-scraps (ISs) and iron-carbon micro-electrolysis (ME) (ME-ISs-AD). The results showed that the OC-free ME-ISs-AD obtained denitrification load of 0.191±0.028 kgN/(m3·d), which shifted to 0.445±0.011 kgN/(m3·d) at OC-dosage of 1.0 mgCOD/mgTN. N2O (1.52±0.16 mgN/L) and NO2– (6.86±1.11 mgN/L) accumulated markedly in Fe(0)-AD and were reduced to 0.03±0.01 mgN/L and 0.31±0.20 mgN/L at OC-dosage of 1.0 mgCOD/mgTN, respectively. XRD analysis demonstrated that OC could reduce the Fe(0)-passivation, accelerating biological transformation and storage of iron-compounds, enhancing H2 (from 0.06±0.01 to 0.22±0.04 mg/L) and Fe2+ (from 7.4±1.0 to 13.9±1.5 mg/L) production. High-throughput sequencing revealed the enrichment of iron-oxidizing bacteria (e.g. Sphaerotilus and Ferriphaselus) and autotrophic denitrifiers (e.g. Hydrogenophaga and Hyphomicrobium). Real-time qPCR and enzyme activity analysis indicated that the nosZ-encoded NosZ and nirK-encoded NIR controlled N2O and NO2− accumulation in OC-dosed Fe(0)-AD, respectively. Overdose of OC (≥1.5 mgCOD/mgTN) accelerated the bio-organics-Fe complexes (BioFe) production and blocked the H2/Fe2+ transfer. NaAC was more efficient and cost-effective than Glucose and EtOH for Fe(0)-AD enhancement. The utilization of ISs under the enhancement of ME reduced the cost of Fe(0)-AD by 3.87-times comparing to the in-situ ZVI-AD process, and the dose of OC further improved the denitrification load by 1.33-times. This study provided an in-depth understanding of the influence of OC on Fe(0)-AD and could contribute to the application, optimization and secondary pollutants control of Fe(0)-AD process in low-carbon water treatment.