Atypical removals of nitrogen and phosphorus with biochar-pyrite vertical flow constructed wetlands treating wastewater at low C/N ratio

This study constructed five laboratory-scale vertical flow constructed wetlands (VFCWs) using different ratios of biochar and pyrite as substrates to treat wastewater with a low carbon-to-nitrogen (C/N) ratio. Biochar can release organic carbon to enhance heterotrophic denitrification and serve as an electron shuttle for denitrification, but it can also release phosphate, negatively impacting total phosphorus removal efficiency. Pyrite releases Fe2+ and S2- to promote autotrophic denitrification, with Fe3+ reacting with phosphate to form FePO4 precipitates that deposit on and passivate the pyrite's surface. At a biochar-to-pyrite volume ratio of 1:1, total nitrogen removal efficiency peaked at 86.0 +/- 2.5 %. However, due to the complex interactions between biochar, pyrite, and functional cells in the VFCWs, no optimal ratio for total phosphorus removal was identified. The atypical removal characteristics of TN and TP in the mixed biochar-pyrite VFCWs suggest the potential for manipulating TN and TP removals at low C/N ratios.