High-efficient nitrogen and phosphorus removal and its mechanism in a partially unsaturated constructed wetland with Fe-C micro-electrolysis substrate

Abstract

• Fe addition in partially unsaturated CW could achieve high TN/TP removal (＞75%). • Extra biochar addition decreased Fe dosage but achieved the equivalent TN/TP removal. • Electron generation and transport by Fe-C enhanced denitrification to above 95%. • Fe-metabolic bacteria and electroactive bacteria were enriched by Fe-C addition. • Metallic bonded P and organic P dominated TP removal in Fe-C added CW. Insufficient dissolved oxygen (DO) for ammonia oxidation and limited electron donor for denitrification inhibit the nitrogen removal performance of constructed wetlands (CWs). To solve the problems, partially unsaturated CWs filled with biochar (C) and/or iron shavings (Fe) as functional substrate were proposed in this study and its mechanism on pollutant removal was thoroughly analyzed from the aspects of micro-environment property surrounding substrate, microbial function, plant toxicity, among others. Results showed that partially unsaturated section significantly improved the removal of NH 4 + -N from 25.1 ± 0.3 mg/L to <10 mg/L. The addition of Fe and Fe-C combination resulted in a better total nitrogen removal of 76.1 ± 0.6% and 86.5 ± 1.7% respectively compared with traditional CWs. For Fe-C combination, Fe, rather than C, was supposed to be the electron donor for nitrate (NO 3 – -N) removal. Moreover, the lower oxidation–reduction potential (ORP) in the micro-environment surrounding Fe-C combination than that surronding Fe proved that biochar accelerated the transfer process once Fe released electron, which guaranteed the nitrogen removal with less Fe dosage (only 60% v/v). Massive Fe 2+ and Fe 3+ were produced by Fe, and biochar provided large surface for the bacterial adhesion and co-precipitation of Fe cations and phosphate (P). The enhanced formation of Fe,Ca,Al-P and P org after Fe-C addition led to a high total phosphorus (TP) removal of 98%. The import of biochar reduced the dosage of Fe and its toxicity to Iris wilsoni . Therefore, compared with Fe, Fe-C combination is a high-efficiency and environmental-friendly functional substrate in CWs.