Will iron-promotion on ecological factors continue with its dosage increasing over time under long-term perfluorooctanoic acid exposure?

Abstract

Considering the potential threat of perfluorooctanoic acid (PFOA) on constructed wetlands (CWs), nano zero valent iron (nZVI) has been employed to promote ecological factors in CWs to enhance the operation performance. Nevertheless, effectiveness of iron-promotion on carbon and phosphorous metabolism with nZVI at various dosage should be investigated. In this study, the most significant enhancement on TP removal was found by 30 mg/L nZVI, with removal efficiency 1.46–12.89% higher than control level. It was consistent to the highest phosphatase activity on this condition. Whereas, lower abundance of key phyla involved in phosphorous metabolism like Proteobacteria, Actinobacteria and Chloroflexi with 30 mg/L nZVI indicated that microbial stimulation was not the dominant manner for strengthening phosphorous removal. In contrast to phosphorous metabolism, the highest increase of PFOA removal by 11.56% was uncovered with 10 mg/L nZVI, and such promotion was reduced by 2.76% since nZVI dosing rising to 30 mg/L. Analogously, inhibition on COD removal was even observed due to higher nZVI concentration. Lower dosage of nZVI generally contributed to more reinforcement on secreting polysaccharide and humus. As for plants, the most obvious improvement of chlorophyll (increased by 33.34–36.85%) and the highest reduction on catalase and malondialdehyde (declined by 221.16% and 45.24% in maximum) were all discovered with 30 mg/L nZVI introduction. It revealed the more potential of nZVI at higher concentration to restore plants under PFOA stress. Information in this study enriched investigation on iron-promoted CWs for treating wastewater containing per- and perfluoroalkyl substances.