Modeling of Fe-bearing mineral surfaces towards understanding the reactivity of chemically modified nanozerovalent iron particles

Chlorinated hydrocarbons (CHCs) such as PCE (perchloroethylene), TCE (trichloroethylene), and DCE (dichloroethylene) represent a serious contamination problem for water resources. Therefore, there is an urgent need to develop new effective in situ remediation technologies for removing CHCs from soil and groundwater.

In the past, laboratory and field studies showed nanoscale zerovalent iron (nZVI) particles as suitable material for removing CHCs. However, the application of this technology is still limited due to rapid particle corrosion and short longevity. There is an intensive effort to suppress nZVI corrosion and increase its longevity without negative effects on particle reactivity. Chemical modification such as sulfidation and recently also nitriding has been recognized as promising treatments to increase the particle longevity. Studies showed that FeSx and FexN mineral phases are formed during the chemical modification.

To further optimize chemical modification of nZVI, it is important to understand processes at surfaces at molecular scale. We built models of typical crystal surfaces of FeSx and FexN minerals detected in modified nanoparticles. They were used in modeling of interactions of CHCs molecules with these surfaces by using density functional theory method. In calculations, solvent effect was also included by using polarizable continuum model. Further, reaction paths of TCE dechlorination was explored by performing transition state calculations and molecular dynamics. We showed that the cleavage of the first C–Cl bond was the rate-limiting step for the dechlorination of CHCs at the γ′–Fe4N(001) surface, with the reaction barriers of 27.0, 29.9, and 40.8 kJ mol–1 for TCE, PCE, and cis‑DCE, respectively.