Effects of chromate on nitrogen removal and microbial community in two-stage vertical-flow constructed wetlands

Nitrogen and chromium (Cr(VI)) pollution in waterbodies pose great threats to human health, and a cost-effective alternative with Cr(VI) and nitrogen simultaneous removal is still needed. This study investigated the influence of Cr(VI) on nitrogen removal in the two-stage vertical-flow constructed wetlands (TS-VFCWs) along with iron ore and woodchip, and explored relationship between Cr(VI) and nitrogen removal. The results showed that efficient Cr(VI) and nitrogen removal were simultaneously achieved in TS-VFCWs together with iron-ore and woodchip under 2 mg/L-Cr(VI), whereas 10 mg/L-Cr(VI) gave significant and recoverable inhibition of nitrogen removal. Cr(VI) supplementation promoted the beneficiation of Cr(VI)-reducing/resistant bacteria IMCC26207 and Bryobacter on iron-ore. Woodchip enriched Cr(VI)-reducing bacteria Streptomyces and Thiobacillus. XRD and XPS showed that abundant bound-Cr existed in the surface of iron ore and woodchip, and Cr(III) precipitation/oxide was the major product. High abundances of nitrifying and autotrophic/heterotrophic denitrifying bacteria ensured good nitrogen removal at Cr(VI) stress.