Simultaneous decomplexation of metal complexes and long-term metal immobilization by N, S-co-doped microscale...

Eliminating highly toxic and extremely stable heavy metal complexes (HMCs) has been a top priority in wastewater treatment. Conventional microscale zero-valent iron (mZVI) can remove HMCs, but is constrained by limited decomplexation reactivity, re-mobilization of liberated metals, and re-complexation from residual ligands. Here we modulate the lattice strain and interfacial properties of mZVI by N, S-co-doping to form N-S-mZVI, which synergistically accelerates mass transport and electron transfer, thereby efficiently removing organic chelated heavy metals (Cd, Pb, Zn, Cu). N-S-mZVI achieves a 165 times faster removal rate for Cd(II)-EDTA than unmodified mZVI together with a 65-fold higher removal capacity, due to intraparticle encapsulation of released Cd(II). Compared with mZVI, this co-modification accelerates dissolution of lattice iron (Fe), producing Fe ions for replacement decomplexation, and the resulting lattice vacancies facilitate inward diffusion and immobilization of heavy metals. The presence of low-resistance FeS and Fe_xN at the interface forms electron transfer channels, directing electron flow toward O₂. Surface Lewis base sites (pyridinic-N) enhance O₂ adsorption, generating reactive oxygen species that oxidatively degrade ligands, and prevent re-complexation. N-S-mZVI effectively treats HMCs involving various ligands (EDTA, oxalate, tartrate and humate) and performs well in practical wastewater treatment. Our study provides insight into the synergistic regulatory mechanisms of N, S-co-doping in enhancing the mZVI reactivity and the stability of removed HMCs in environmental applications.