A quantitative study of the effects of particle' properties and environmental conditions on the electron efficiency of Pd and sulfidated nanoscale zero-valent irons

Electron efficiency (or electron selectivity, ee) is an important quantitative criterion for zero-valent iron treatment of organohalide contaminated groundwater. The aim of this quantitative study was the systematic exploration and com-parison of the effects of the Pd/Fe and S/Fe molar ratios (i.e., [Pd/Fe] and [S/Fe]), trichloroethylene (TCE) concentra-tions ([TCE]), pH solution, aging time, and water matrices on the ee of Pd-nZVI and S-nZVI. To this end, we used TCE as a probe contaminant. The ee of Pd-nZVI increased and then decreased with [Pd/Fe], while that of S-nZVI increased with [S/Fe], as more hydrophobic FeS2 was formed on S-nZVI at higher [S/Fe]. The ee of S-nZVI and Pd-nZVI increased with increasing [TCE]. Specifically, the ee of S-nZVI and Pd-nZVI at [TCE] of 200 ppm increased by 24.9 % and 79.3%, respectively, compared with that at [TCE] of 10 ppm. As the H2 evolution reaction (HER) was more sensitive to surface passivation than TCE dechlorination, the ee of S-nZVI and Pd-nZVI under alkaline conditions was higher than that under basic conditions, and increased by 11.7 % and 37.8 %, respectively, at pH 10 relative to that at pH 6. The ee also increased with the aging time of the S-nZVI and Pd-nZVI particles; the increase was by 27.2 % and 59.6 %, respec-tively, at aging time of 30 d compared with that of the fresh ones. The ee of both particles were higher in artificial groundwater (AGW) than in real groundwater (RGW). For all batch experiments, the ee of S-nZVI increased over the reaction time and tended to outperform that of Pd-nZVI, even though the ee of Pd-nZVI was higher than that of S-nZVI at the initial stage of TCE dechlorination, thereby justifying the longevity of S-nZVI.

Automated summary

Electron efficiency (ee) of Pd-nZVI and S-nZVI was systematically explored and compared in this study. It was found that the ee of Pd-nZVI increased and then decreased with [Pd/Fe], while that of S-nZVI increased with [S/Fe]. The variables of TCE concentrations, pH solution, aging time, and water matrices also affected the ee of both materials.