Making waves: Does the polyvalent iron cycle truly exist in constructed wetlands during wastewater treatment?

Different iron (Fe) ores have been extensively utilized in constructed wetlands (CWs) for advanced wastewater treatment. Electron regulation through the Fe cycle between ferrous (Fe²⁺) and ferric (Fe³⁺) iron is commonly recognized as the pathway for enhanced nitrogen removal. This study aimed to verify whether a complete Fe cycle could occur in subsurface flow CWs (SSF-CWs) and identify the limiting factors: 1) Under the popular continuous flow mode, the spatial distribution of redox conditions in CWs was not variable to support the in-situ Fe cycle at a certain zone. 2) Although the opposing reactions of Fe²⁺ oxidation and Fe³⁺ reduction could occur in the aerobic and anaerobic zones of CWs, respectively, the unidirectional water flow hindered the shuttling of Fe³⁺/Fe²⁺ for reactant replenishment at different zones to form the ex-situ Fe cycle. 3) Fe³⁺ could precipitate with phosphate or run off from CWs along with effluents, becoming unavailable for subsequent biochemical cycling. Besides, statistical analysis revealed that Fe⁰ or Fe²⁺-based substrates significantly enhanced unidirectional NO₃^- reduction but the transformed Fe³⁺ failed to support equivalent NH₄⁺ oxidation. All the findings pointed out that the reduction of Fe³⁺ to Fe²⁺ might be the rate-limiting step in the Fe cycle, due to the prevailing redox conditions and associated pollutant removal kinetics in CWs. To address these limitations, a strategy of siphon design in CWs along with microorganism/substrate modification was proposed to introduce favorable conditions (e.g., fluctuating water levels and periodical redox conditions) for the inter-conversion between Fe³⁺ and Fe²⁺.