Relationships of ternary activities for the enhanced Cr(VI) removal by coupling nanoscale zerovalent iron with sulfidation and carboxymethyl cellulose

Nanoscale zerovalent iron (nZVI) has been extensively applied in water pollution control. However, the reactivity of nZVI toward contaminants is mainly limited by its corrosion and agglomeration. In this study, the nZVI modified by sulfidation coupled with carboxymethyl cellulose (CMC) (C-S-nZVI) was synthesized and characterized by TEM and electrochemical techniques. Taking Cr(VI) as the contaminant, it was found that the sulfidation could couple with CMC modification to not only enhance the reactivity of nZVI toward Cr(VI), but also regulate the sedimentation activity and corrosion activity of nZVI in water. Particularly, the optimal k_obs (0.0816 min⁻¹) obtained by the C-S-nZVI^one-0.16 (i.e., one-step sulfidation and its S/Fe molar ratio was 0.16) was approximately 27.2 times higher than that by the nZVI (0.0030 min⁻¹). Moreover, based on the correlation analysis of the ternary activities, this study confirmed that the reactivity of C-S-nZVI toward Cr(VI) was negatively correlated with its sedimentation activity (slope = −0.7623, R = 0.59) and corrosion activity (slope = −0.0171, R = 0.56), respectively. XPS and TEM results further revealed that CMC could couple with iron sulfides (FeS_x) to enhance the mass transfer of Cr(VI) toward nZVI and subsequent electron transfer from Fe⁰ core to out, ultimately improving the reduction of Cr(VI) by nZVI. Overall, this study introduced a new evaluation method based on the ternary activity of nZVI, providing theoretical support for the practical application of nZVI-based technology.