Sulfidation\nof Nanoscale\nZero-Valent Iron by Sulfide:\nThe Dynamic Process, Mechanism, and Role of Ferrous Iron

Sulfidation of nanoscale zerovalent\niron (nZVI) can enhance\nparticle\nperformance. However, the underlying mechanisms of nZVI sulfidation\nare poorly known. We studied the effects of Fe<sup>2+</sup> on 24-h\ndynamics of nZVI sulfidation by HS<sup>–</sup> using a dosed\nS to Fe molar ratio of 0.2. This shows that in the absence of Fe<sup>2+</sup>, HS<sup>–</sup> rapidly adsorbed onto nZVI particles\nand reacted with surface iron oxide to form mackinawite and greigite\n(<0.5 h). As nZVI corrosion progressed, amorphous FeS<sub><i>x</i></sub> in solution deposited on nZVI, forming S-nZVI (0.5–24\nh). However, in the initial presence of Fe<sup>2+</sup>, the rapid\nreaction between HS<sup>–</sup> and Fe<sup>2+</sup> produced\namorphous FeS<sub><i>x</i></sub>, which deposited on the\nnZVI and corroded the surface iron oxide layer (<0.25 h). This\nwas followed by redeposition of colloidal iron (hydr)oxide on the\nparticle surface (0.25–8 h) and deposition of residual FeS<sub><i>x</i></sub> (8–24 h) on S-nZVI. S loading on\nS-nZVI was 1 order of magnitude higher when Fe<sup>2+</sup> was present.\nSurface characterization of the sulfidated particles by TEM-SAED,\nXPS, and XAFS verified the solution dynamics and demonstrated that\nS<sup>2–</sup> and S<sub>2</sub><sup>2–</sup>/S<sub>n</sub><sup>2–</sup> were the principal reduced S species\non S-nZVI. This study provides a methodology to tune sulfur loading\nand S speciation on S-nZVI to suit remediation needs.