Application of Zero-valent Iron Nanoparticles for Environmental Clean Up - Advanced Materials for Agriculture, Food and Environmental Safety - page 404

14.3 Reduction Mechanisms and Pathways

In the case of reductive dechlorination of organochlorine compounds by
nFe 0 , there are three possible mechanisms which are as follows:

i. Direct reduction at the metal surface by electron transfer
from nFe 0 to adsorbed contaminant molecules.
ii. Indirect reduction by ferrous ions (Fe 2+ ).
iii. Hydrogen produced as a result of iron corrosion is also capa-
ble of reductive dechlorination in the presence of a catalyst.

The direct reduction pathway mainly includes five steps: mass trans-
fer of target molecule from solution to the surface of nFe 0 , adsorption of
target molecule on the surface of nFe 0 via formation of organic chemi-
sorptions complex, reaction between the adsorbed target molecule and
nFe 0 , desorption of products and diffusion of products into solution.
The main reactions of direct reduction pathway include reductive α / β
elimination (dihalo-elimination), hydrogenolysis (hydrogen substitu-
tion of halogen), or hydrogenation (multiple bonds cleavage). Indirect
reduction pathway includes reduction by ferrous ions and/or hydro-
gen. Although Fe 2+ is a good reducing agent, its reaction is generally
quite slow and is probably not of great consequence. Hydrogen gas is an
excellent reductant for dechlorination reactions, but is insufficient for
reduction of a chlorinated organic compound in absence of a catalytic
surface.
In bimetallic systems, the second metal (i.e., Pd, Cu, Ni, etc.) acts as
a catalyst to generate active atomic hydrogen while Fe 0 acts as an elec-
tron donor. h e coexistence of both e - donor and catalyst leads to higher
dechlorination ei ciency. h e catalytic reductive dechlorination of an
organochlorine compound by a bimetallic iron nanoparticle follows three
steps (second metal is denoted as M):

i. h e corrosion of iron leads to production of hydrogen.
ii. M and hydrogen are combined to form a transitional com-
pound M.H 2 with H 2 embedded in the crystal lattice.
iii. Transitional compound dechlorinates organochlorine
compound.

All these steps are depicted in Eqs. 14.13-14.18. Figure 14.1 presents a
schematic diagram of the dechlorination mechanism occurring in nFe 0 and