Sulfidation of Nanoscale Zero-Valent Iron by Sulfide: The Dynamic Process, Mechanism, and Role of Ferrous Iron

Sulfidation of nanoscale zerovalent iron (nZVI) can enhance particle performance. However, the underlying mechanisms of nZVI sulfidation are poorly known. We studied the effects of Fe²⁺ on 24-h dynamics of nZVI sulfidation by HS^- using a dosed S to Fe molar ratio of 0.2. This shows that in the absence of Fe²⁺, HS^- rapidly adsorbed onto nZVI particles and reacted with surface iron oxide to form mackinawite and greigite (<0.5 h). As nZVI corrosion progressed, amorphous FeS in solution deposited on nZVI, forming S-nZVI (0.5-24 h). However, in the initial presence of Fe²⁺, the rapid reaction between HS^- and Fe²⁺ produced amorphous FeS, which deposited on the nZVI and corroded the surface iron oxide layer (<0.25 h). This was followed by redeposition of colloidal iron (hydr)oxide on the particle surface (0.25-8 h) and deposition of residual FeS (8-24 h) on S-nZVI. S loading on S-nZVI was 1 order of magnitude higher when Fe²⁺ was present. Surface characterization of the sulfidated particles by TEM-SAED, XPS, and XAFS verified the solution dynamics and demonstrated that S^2- and S₂^2-/S_n^2- were the principal reduced S species on S-nZVI. This study provides a methodology to tune sulfur loading and S speciation on S-nZVI to suit remediation needs.