Sulfidated nanoscale zerovalent iron (S-nZVI) has the potential to be a cost-effective remediation agent for a wide range of environmental pollutants, including chlorinated solvents. Various synthesis approaches have yielded S-nZVI consisting of a Fe (or Fe/S) core and FeS shell, which are significantly more reactive to trichloroethene (TCE) than nZVI. However, their reactivity is not as high as palladium-doped nZVI (Pd-nZVI). We synthesized S-nZVI by the co-precipitation of FeS and Fe by using NaS during the borohydride reduction of FeSO (S-nZVI). This resulted in FeS structures bridging the nZVI core and the surface, as confirmed by electron microscopy and X-ray analyses. The TCE degradation capacity of up to 0.46 mol TCE/mol Fe was obtained for S-nZVI at a high S loading and was comparable to Pd-nZVI but 60% higher than the currently most reactive S-nZVI, in which FeS only coats the nZVI (S-nZVI). The high TCE degradation was due to complete utilization of Fe (2 e/mol Fe) toward the formation of acetylene. Although Pd-nZVI yielded 3 e/mol Fe, TCE degradation was comparable because it reduced acetylene further to ethene and ethane. Under Fe-limited conditions, the S-nZVI TCE degradation rate was 16 times higher than that of Pd-nZVI (0.5 wt % Pd) and 90 times higher than that of S-nZVI.

Nanoscale zerovalent iron (NZVI) particles are often coated with polymeric surface modifiers for improved colloidal stability and transport during remediation of contaminated aquifers. Doping the NZVI surface with palladium (Pd-NZVI) increases its reactivity to pollutants such as trichloroethylene (TCE). In this study, we investigate the effects of coating Pd-NZVI with two surface modifiers of very different molecular size: rhamnolipid (RL, anionic biosurfactant, M.W. 600 g mol(-1)) and carboxymethylcellulose (CMC, anionic polyelectrolyte, M.W. 700 000 g mol(-1)) on TCE degradation. RL loadings of 13-133 mg TOC/g NZVI inhibited deposition of Pd in a concentration-dependent manner, thus limiting the number of available Pd sites and decreasing the TCE degradation reaction rate constant from 0.191 h(-1) to 0.027 h(-1). Furthermore, the presence of RL in solution had an additional inhibitory effect on the reactivity of Pd-NZVI by interacting with the exposed Pd deposits after they were formed. In contrast, CMC had no effect on reactivity at loadings up to 167 mg TOC/g NZVI. There was a lack of correlation between Pd-NZVI aggregate sizes and TCE reaction rates, and is explained by cryo-transmission electron microscopy images that show open, porous aggregate structures where TCE would be able to easily access Pd sites.

Sulfidation of nanoscale zerovalent iron (NZVI) enhances its reactivity to chlorinated organic contaminants such as trichloroethene (TCE).

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