Simultaneous removal of nitrate, phosphate and Cr(VI) in groundwater by iron‑carbon-based constructed wetlands: Performance intensification by organic solid amendments

The effective remediation of groundwater contaminated by multiple pollutants via cost-effective and sustainable approaches is a major challenge. In this study, constructed wetland (CW) microcosms filled with a mixture of iron scraps (IS) and biochar were employed to remediate groundwater containing nitrate, phosphate and Cr(VI) (2.0 and 10.0 mg L−1 Cr in operation phases I and II, respectively), and a walnut shell or pyrite substrate was added to enhance remediation performance. The results showed that the iron‑carbon (FeC)-filled CWs performed notably better in terms of simultaneous nitrate (30.3–46.0 %), phosphate (58.9–59.5 %) and Cr(VI) removal (88.6–90.6 %) than the control CWs filled with only biochar (8.4–13.0 % for nitrate, 22.6–23.8 % for TP and 32.7–44.9 % for Cr(VI)) did. This was because FeC microelectrolysis effectively drove chemical and microbial nitrate and Cr(VI) reduction and bound phosphate and the production of Cr(III) as precipitates inside the CWs. The enhancing effect of walnut shells on remediation performance was significant because they drove mixotrophic denitrification and Cr(VI) reduction, achieving removal efficiencies of 57.0–67.3 % for nitrate, 65.4–69.7 % for TP and 92.8–95.3 % for Cr(VI). Nitrogen removal in the CWs was adversely impacted by the increase in the influent Cr concentration. The characterization of deposits on the IS surface verified the essential role of ISs and supplementary electron donors in pollutant removal. IS amendment caused a decrease in microbial diversity, while some genera related to nitrate and Cr(VI) reduction were enriched. The proportions of Simplicispira and Vibrio were enhanced by the addition of walnut shells or pyrite, which might be crucial for pollutant removal in the intensified CWs. The developed novel FeC-filled CWs with organic solid amendments present highly promising application prospects for the remediation of groundwater contaminated by multiple pollutants.