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, ɛ) 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 comparison of the effects of the Pd/Fe and S/Fe molar ratios (i.e., [Pd/Fe] and [S/Fe]), trichloroethylene (TCE) concentrations ([TCE]), pH solution, aging time, and water matrices on the ɛ of Pd-nZVI and S-nZVI. To this end, we used TCE as a probe contaminant. The ɛ of Pd-nZVI increased and then decreased with [Pd/Fe], while that of S-nZVI increased with [S/Fe], as more hydrophobic FeS was formed on S-nZVI at higher [S/Fe]. The ε of S-nZVI and Pd-nZVI increased with increasing [TCE]. Specifically, the ε 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 H evolution reaction (HER) was more sensitive to surface passivation than TCE dechlorination, the ε 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 ε also increased with the aging time of the S-nZVI and Pd-nZVI particles; the increase was by 27.2 % and 59.6 %, respectively, at aging time of 30 d compared with that of the fresh ones. The ɛ of both particles were higher in artificial groundwater (AGW) than in real groundwater (RGW). For all batch experiments, the ε of S-nZVI increased over the reaction time and tended to outperform that of Pd-nZVI, even though the ε of Pd-nZVI was higher than that of S-nZVI at the initial stage of TCE dechlorination, thereby justifying the longevity of S-nZVI.