Iron-carbon micro-electrolysis simultaneously enhanced nutrient and heavy metal removal in constructed wetlands for purifying polluted groundwater with variable hydraulic loadings

[Display omitted] •Simultaneous removal of nitrate, phosphate, Cr and Pb was achieved in Fe-C CWs with variable hydraulic loadings.•High removal of heavy metals (95%) was maintained by micro-electrolysis process when hydraulic loadings were elevated.•NO3–-N removal rate and N2O emission flux decreased with increasing heavy metal loadings. Heavy metal and nitrate pollution, as a primary water quality issue in shallow groundwater, is posing a severe threat to public health from drinking groundwater. In this study, iron-carbon (Fe-C) fillers were applied in constructed wetlands (CWs) to enhance the nutrient and heavy metal removal simultaneously, while the interaction of effective nitrate reduction and heavy metals mitigation under variable influent hydraulic loadings was also investigated. Groundwater is not conducive to biological heterotrophic denitrification due to its low dissolved organic matter content. Fe-C fillers can form plentiful microscopic galvanic cells and act as inorganic electron donors to promote the autotrophic denitrification process. The results showed that Fe-C micro-electrolytic CWs showed excellent simultaneous removal of nutrients and heavy metals. The nitrate nitrogen (NO3–-N) removal rate was as high as 75.77% due to the increased autotrophic denitrification process, while the removal of total nitrogen (TN) and total phosphorus (TP) could reach 73.72 % and 89.04% with a low nitrous oxide (N2O) emission of 12.12 μg m-2h−1. Besides, the removal rates of chromium (Cr) and lead (Pb) reached 98.96% and 97.09%, respectively. However, the presence of heavy metals might inhibit denitrification by competing with nitrite for electrons and generating free radicals, which decreased the denitrification by 10.02–39.05% particularly with the increasing of the hydraulic loading. On the other hand, heavy metals promoted TP removal by forming metal phosphate precipitates and complexations with phosphorus. This study provides comprehensive insights into processes and mechanism of simultaneous nutrient and heavy metal removal in CW.