Stimulation on microbial nitrogen metabolism by iron under PFAS exposure drives effective nitrogen removal in constructed wetland

Constructed wetlands (CWs) have been employed for treatment of perfluoroalkyl substances containing wastewater. However, suppression on operation performances of CWs was inevitable, particularly on nitrogen removal. In this study, 10–30 mg/L nano zero-valent iron (nZVI) was introduced into CWs because of its potential for driving effective nitrogen removal via promoting functional microbes. Perfluorooctanoic acid (PFOA) exposure led to reduction of ammonia (NH4+-N) transformation, but stimulated nitrate (NO3−-N) removal by up to 83.12 %. With PFOA exposure, 10 mg/L nZVI caused the most significant improvement on NH4+-N removal, with maximum increase of 3.65 %. By contrast, 30 mg/L nZVI was prone to alleviate NO3−-N accumulation by 14.19–24.22 % compared to control group. Lower dosage of nZVI (10–20 mg/L) increased activity of five nitrogen cycling enzymes by 31.42–315.18 % in maximum. Whereas, 30 mg/L nZVI led to limited promotion or even inhibition on these enzymes. Analogously, stimulation on nitrifying, anammox, and denitrifying bacteria, as well as functional key genes were more obvious by nZVI at lower concentration. Besides, nZVI reduced functional gene of dissimilatory nitrate reduction to ammonium and related bacteria, increasing NH4+-N removal indirectly. The most promotion on nitrate dependent Fe oxidation bacteria like Paracoccus could explain improvement on NO3−-N removal by 30 mg/L nZVI.