Nitrate and nitrite reduction by adsorbed Fe(II) generated from ligand-promoted dissolution of biogenic iron minerals in groundwater

Chemical denitrification by redox-active Fe(II) species is pivotal in the coupled iron and nitrogen cycles. The reductive dissolution of ferric minerals by ligand can generate Fe(II)-ligand complexes, but their reducing capability for electrophilic pollutants like nitrate and nitrite remains uncertain. Here, biogenic secondary iron minerals (SIM) after dissimilatory iron reduction were reductively dissolved by oxalate and the siderophore desferrioxamine B, and subsequently the partially-dissolved SIM (SIMD) effectively removed NO₂⁻ from groundwater via reduction, while exhibiting much lower reactivity towards NO₃⁻. The dissolution and removal processes were well-fitted with the Kabai model and the pseudo-second-order adsorption model, respectively. The equilibrium NO₂⁻ removal capacity (q_e) of SIMD reached 0.146–0.223 mmol/g, accompanied with the rate constants as 0.433–0.810 g/(mmol·h). The emission of N₂O and NO verified the occurrence of chemical denitrification during NO₂⁻ removal by SIMD. From the perspective of Fe(II) reactivity, SIMD exhibited higher densities of surface Fe(II) and more negative Eh values than SIM, and these two indicators showed linear correlations with the removal rates. Combined with microscopic, electrochemical and spectral analysis, our results indicated the redox reaction of adsorbed Fe(II)-complexes with NO₂⁻ on SIMD surface. The concurrent substance biochar was also considered, as it indirectly influenced dissolution and pollutant removal by shifting the iron mineral phase in SIM from magnetite to goethite. These findings highlight the significant role of reductive dissolution of iron mineral in N transformation, expand the electron pool available to support chemical denitrification, and have implications for Fe and N cycling coupling with pollutant reduction.