Transport of nanoscale zero-valent iron in saturated porous media: Effects of grain size, surface metal oxides, and sulfidation.

Knowledge of the fate and transport of nanoscale zero-valent iron (nZVI) in saturated porous media is crucial to the development of in situ remediation technologies. This work systematically compared the retention and transport of carboxymethyl cellulose (CMC) modified nZVI (CMC-nZVI) and sulfidated nZVI (CMC-S-nZVI) particles in saturated columns packed with quartz sand of various grain sizes and different surface metal oxide coatings. Grain size reduction had an inhibitory effect on the transport of CMC-S-nZVI and CMC-nZVI due to increasing immobile zone deposition and straining in the columns. Metal oxide coatings had minor effect on the transport of CMC-S-nZVI and CMC-nZVI because the sand surface was coated by the free CMC in the suspensions, reducing the electrostatic attraction between the nZVI and surface metal oxides. CMC-S-nZVI displayed greater breakthrough (C/C0 = 0.82-0.90) and higher mass recovery (84.9%-89.3%) than CMC-nZVI (C/C0 = 0.70-0.80 and mass recovery = 70.9%-79.6%, respectively) under the same experimental conditions. A mathematical model based on the advection-dispersion equation simulated the experimental data of nZVI breakthrough curves very well. Findings of this study suggest sulfidation could enhance the transport of CMC-nZVI in saturated porous media with grain and surface heterogeneities, promoting its application in situ remediation.

Mechanistic role of nitrate anion in TCE dechlorination by ball milled ZVI and sulfidated ZVI: Experimental investigation and theoretical analysis.

Mechanistic role of NO3- in trichloroethylene (TCE) dechlorination by ball milled, micro-scale sulfidated and unsulfidated ZVI (e.g., S-mZVIbm and mZVIbm) was explored through experiments and density functional theory (DFT) calculations. Sulfidation inhibited NO3- reduction by mZVIbm as S weakened its interaction with NO3-. mZVIbm reduced NO3- within 2 h. This just resulted in a short-term electron competition during the dechlorination process by mZVIbm and hardly affected its sluggish dechlorination kinetics (complete TCE dechlorination in 11 d). On the contrary, NO3- suppressed TCE dechlorination by S-mZVIbm. This was attributed to that inhibited NO3- reduction by S-mZVIbm (40 % reduction in 6 h) induced continuous electron competition with TCE during the time span of its dechlorination by S-mZVIbm. NO3- reduction was also observed to facilitate formation/crystallization of Fe3O4 on both ZVI particles, promoting dechlorination by mZVIbm after 4 d while not taking effect to the S-mZVIbm/TCE system, as its dechlorination time was too short for the surface of S-mZVIbm to transform. This observation has important implication on groundwater remediation by ZVI or sulfidated ZVI PRBs under a scenario of upgradient anthropogenic release of NO3-.

Effects of non-reducible dissolved solutes on reductive dechlorination of trichloroethylene by ball milled zero valent irons.

Non-reducible solution anions have been well recognized to affect reactivity of ZVI in dechlorinating chlorinated hydrocarbons. However, their effects and corresponding functional mechanisms on electron efficiency (εe) of ZVI remain unclear. In this study, mechanochemically modified microscale sulfidated and unsulfidated ZVI particles (i.e., S-mZVIbm and mZVIbm) and trichloroethylene (TCE) were used as model particles and contaminant to explore such effects. PO43- as a corrosion promoter enhanced initial dechlorination rate by both particles. However, its passivating role as a surface complex agent became significant at the later stage of dechlorination by mZVIbm, while sulfidation alleviated this effect without inhibition of dechlorination. Compared with enhancing dechlorination, PO43- promoted hydrogen evolution reaction (HER) to a higher extent, decreasing εe for both particles by 17-73 %. HCO3- negligibly affected dechlorination by both particles, while elevated HER. Thus, HCO3- [5 mM] decreased εe for S-mZVIbm and mZVIbm by 1.9 % and 22 %. Different from PO43- and HCO3-, Cl- and SO42- showed no significant effects on dechlorination, HER, and therefore εe for both particles. These results imply that even though some co-existing anions (i.e., PO43- and HCO3-) acting as corrosion promoters could improve the dechlorination by ZVIs, they would lead to decreased εe and shortened particle reactive lifetime.

Sulfidation enhances stability and mobility of carboxymethyl cellulose stabilized nanoscale zero-valent iron in saturated porous media.

Sulfidation can enhance the reactivity and longevity of nanoscale zero-valent iron (nZVI), but little is known about its effect on the fate and transport of nZVI in saturated porous media. This work compared the stability and mobility of carboxymethyl cellulose (CMC) stabilized nZVI (CMC-nZVI) and sulfidated nZVI (CMC-S-nZVI) particles in saturated porous media. After sulfidation, the hydrodynamic size of CMC-S-nZVI was 100-150 nm larger than CMC-nZVI due to enhanced adsorption of CMC onto the S-nZVI surface, which was facilitated by the bidentate bridging interaction between CMC and the FeSx phase on S-nZVI. Of note is that they had a similar core size and zeta potential. In comparison to CMC-nZVI, CMC-S-nZVI exhibited less physical settling (0-5% vs. 5-73%) and chemical dissolution (2-10% vs. 3-27%) within 55 min under the same ionic conditions (Na+, K+ < 200 mM; Al3+ < 0.75 mM). Column breakthrough experiments showed that both CMC-S-nZVI and CMC-nZVI had relatively high mobility in saturated porous media. However, CMC-S-nZVI exhibited greater breakthrough (C/C0 = 0.57-1.0) and corresponding greater mass recovery rates than the corresponding CMC-nZVI (C/C0 = 0.44-1.0) under most of the experimental conditions (e.g., different ion type and concentration, flow rate, and input concentration). The fitted colloid filtration theory model was in good agreement with experiments. This work suggests that in addition to the significant reactivity and longevity improvements demonstrated in other studies, CMC-S-nZVI is also more mobile than CMC-nZVI suggesting that CMC-S-nZVI has many of the characteristics favorable for field application.