Lignite-steel slag constructed wetland with multi-functionality and effluent reuse

Constructed wetlands (CWs) are widely used to treat wastewater, while innovative studies are needed to support resource conservation, enhance multi-functionality, and improve the effectiveness of effluent usage. Here, we combined industrial waste (steel slag) and low-rank coal (lignite) in different configurations as CW substrate to evaluate its potential for multiple functions. Based on scanning electron microscope, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and metagenomic sequencing, we found that the experimental treatment with lignite and steel slag mixtures had the highest multi-functionality, including efficient nutrient removal and carbon sequestration, as well as hydroponic crop production. Lignite and steel slag were mixed to form lignite-steel slag particle clusters, where Ca2+ dissolved on the surface of steel slag combined with PO43- in wastewater to form Ca3(PO4)2 precipitation for phosphorus removal. A biofilm grew on the surface of lignite in this cluster, and OH- released from steel slag promoted lignite to release fulvic acid, which provided carbon source for heterotrophic microorganisms and promoted denitrification. Moreover, fulvic acid enhanced carbon sequestration in CWs by increasing the biomass of Phragmites australis. The effluent from lignite-steel slag constructed wetland increased cherry tomato yield and quality while saving N and P applications. These results provide new ideas for “green” and economic development of CW technology.