A comparison of the effects of natural organic matter on sulfidated and nonsulfidated nanoscale zerovalent iron colloidal stability, toxicity, and reactivity to trichloroethylene.

Sulfidated nanoscale zerovalent iron (S-NZVI) is a new remediation material with higher reactivity and greater selectivity for chlorinated organic contaminants such as trichloroethene (TCE) than NZVI. The properties of S-NZVI and the effects of groundwater constituents like natural organic matter (NOM) on its reactivity are less well-characterized than for NZVI. In this study, S-NZVI (Fe/S mole ratio = 15) was synthesized by sonicating NZVI in a Na2S solution, yielding particles with greater surface charge, less aggregation, and higher reactivity with TCE compared to NZVI. The cytotoxicity of S-NZVI was not mitigated effectively due to the smaller size. The addition of Suwannee River humic acid (SRHA) increased the negative surface charge magnitude and dispersion stability and reduced the toxicity of both NZVI and S-NZVI significantly, but also enhanced the corrosion of particles and the formation of non-conductive film. The degradation rate constant (ksa) of both NZVI and S-NZVI was thus reduced with the increasing concentration of SRHA, which decreased by 78% and 60% to be 0.0004 and 0.0053 L m-2 h-1, respectively, with 200 mg C/L SRHA. Additionally, the performance of S-NZVI in field was evaluated to be depressed in simulated groundwater and the negative effect was exacerbated with increased concentration of SRHA. Hydro-chemical conditions like dissolved oxygen (DO), pH, and temperature also influenced the reactivity of S-NZVI. Hence, S-NZVI was a preferred candidate for in-situ remediation of TCE than NZVI. Nevertheless, the integrity of the FeS shell on S-NZVI influenced by NOM need to be considered during the long-term use of S-NZVI in groundwater remediation.