Making waves: Microbial-nitrate-zero valent iron/manganese synergy suppresses arsenic mobilization and greenhouse gas emissions in constructed wetlands.

Constructed wetlands (CWs) face dual challenges of arsenic contamination and greenhouse gas (GHG) emissions, particularly concerning the competing processes of As(III) immobilization and methane-dependent As(V) reduction (AOM-AsR). To address this dilemma, we developed a novel microbial-nitrate-zero valent iron/manganese synergy (MNZS) system that establishes dynamic redox gradients through Fe/Mn-mediated electron flux regulation. The MNZS mechanism leverages zero valent iron/manganese (ZVI/ZVM) oxidation to create oxygen-depleted microzones, generating bioavailable Fe(II)/Mn(II) species while initiating microbial nitrate-reducing-coupled Fe(II)/Mn(II) oxidation (NRFO/NRMO). Thermodynamic analysis reveals this configuration achieves three critical functions: (1) ZVI/ZVM-driven denitrification preferentially immobilizes As(III) through Fe/Mn oxide coprecipitation, (2) suppression of AOM-AsR prevents As(III) leaching, and (3) directs electron flow toward complete denitrification (N2O to N2) and methane oxidation (CH4 to CO2). Experimental validation demonstrated MNZS system received desirable As(III) retention efficiency with concurrent mitigations of CH4/N2O/CO2 emissions in horizontal subsurface flow constructed wetlands (HSSFCWs) compared to the conventional HSSFCWs. Overall, the MNZS system resolves the single-pollutant limitation of traditional remediation through three synergistic mechanisms: ZVI/ZVM redox buffering, microbial nitrate shunt activation, and thermodynamic pathway optimization. This work provides an engineered solution achieving simultaneous As immobilization and GHG mitigation in wetland ecosystems.