Sulfamic acid modification promotes electron transfer and surface activation of zero-valent iron: mechanistic insights into efficient Cr(VI) reduction

SA-mZVI bm showed 22-fold faster Cr(VI) removal and 17-fold higher capacity. • Sustained electron transfer and inhibited passivation boost ES by 2-fold. • SA-mZVI bm maintained high performance in real waters, showing practical potential. Zero-valent iron (ZVI) is broadly applied in the remediation of toxic, carcinogenic, and highly mobile Cr(VI), but its inherent reactivity is drastically diminished due to rapid surface passivation during the reaction process. Herein, microscale ZVI (mZVI) was modified with inorganic acid sulfamic acid (SA) via ball-milling to fabricate SA-mZVI bm , and its Cr(VI) removal performance, reaction mechanism, and practical applicability were systematically investigated. Results showed that SA modification reshaped the surface oxide layer of mZVI, facilitating the formation of conductive Fe 3 O 4 and introducing sulfonic acid groups to enhance surface complexation capability. At an initial pH 5 and a dosage of 0.5 g/L, SA-mZVI bm achieved a Cr(VI) removal rate 22 times that of unmodified mZVI bm within the first 5 min, with a surface area-normalized removal capacity approximately 17 times greater over 5 h. Mechanistic studies revealed that SA corrosion restructured the oxide layer to boost Fe(II) generation. DFT calculations suggested that the SA-modified ZVI surface provided more accessible sites for Cr(VI) adsorption in a bidentate binuclear configuration (adsorption energy: −5.9 eV), shortening the Fe-Cr bond distance to 3.6 Å and accelerating electron transfer. Additionally, SA modification significantly improved the electron selectivity (88.4% vs 45.7%) and ZVI utilization efficiency (5.8% vs 0.2%), effectively alleviating surface passivation, while 94.6% of Fe 0 remained unused. SA-mZVI bm also demonstrated superior Cr(VI) removal performance in real water matrices and electroplating wastewater. This study provides new insights and theoretical guidance for developing efficient and stable inorganic acid-modified ZVI materials. Download: Download high-res image (190KB) Download: Download full-size image