Environmental Engineering Reference
In-Depth Information
The dechlorination pathways of chlorinated hydrocarbons by ZVI are fairly well under-
stood. The chlorinated hydrocarbons are usually reduced by ZVI and release chloride ion,
whereas ZVI is oxidized and supplies electrons [33]. The proposed reaction mechanisms
include β-elimination, hydrogenolysis, and hydrogenation. β-Elimination is the primary
reaction pathway for the dechlorination of chlorinated ethylene, while hydrogenolysis and
hydrogenation are the minor reactions [33,96]. These mechanisms can explain the forma-
tion of ethane and ethylene as the main products, with minor amounts of chlorinated
intermediate and acetylene when TCE or PCE is dechlorinated by ZVI.
The β-elimination pathway involves a two-electron transfer to produce chloroacety-
lene from chlorinated hydrocarbons. Chloroacetylene then undergoes hydrogenolysis
to form acetylene, which further undergoes a two-electron transfer to form ethane and
ethene. Arnold and Roberts [96] have proposed that chlorinated ethylene could form the
di-σ-bonded intermediate on the iron surface irst and further reduced to less chlorinated
hydrocarbons. The irst step is the formation of a π-bonded surface species. In this adsorp-
tion step, the alkynes and alkenes serve as Lewis bases, while partially or fully oxidized
metal ions can be represented as Lewis acids [103]. In the second step, the π-bonded inter-
mediate forms a di-σ-bonded surface-adsorbed species, and subsequently undergoes
two successive fast halide ion elimination steps to form a mono-σ-bonded vinyl surface-
adsorbed species and then to acetylene.
The reaction of chlorinated hydrocarbons by nZVI is slightly different from that by
microscale ZVI. The reaction mechanism and product distribution are dependent on the H
2
evolution rate and the degree of crystallinity of the nZVI. Lowry and co-workers [33,34,104]
have investigated the dechlorination rate and pathway of TCE by different properties of
nZVI (Fe
BH
and RNIP). Under both iron-limited and excess iron conditions, Fe
BH
mainly
transformed TCE to ethane, C3-C6 coupling products, and some unsaturated hydrocar-
bons [33]. No acetylene was detected, while chlorinated by-products were detected as
reactive intermediates at very low levels and disappeared quickly. For RNIP, TCE was
dechlorinated primarily into acetylene and ethane under iron-limited conditions. On the
contrary, ethane and ethene were produced when excess RNIP was used to dechlorinate
TCE. A further study [104] has found that the highly disordered nature of nanoscale Fe
BH
and partially oxidized Fe
BH
provides the ability to activate and use hydrogen gas, which
increases the rate and extent of hydrogenation and yields more saturated reaction products
compared with crystalline Fe
BH
, RNIP, and iron ilings. However, the reaction orders for H
2
evolution and TCE dechlorination with respect to nZVI content were different, which may
be attributed to the difference in the rate-controlling steps for each reaction [34]. For H
2
evolution, the formation of adsorbed H species is the rate-limiting step, while TCE reduc-
tion requires that TCE adsorbs to the iron surface, and that TCE reduction occurs via direct
electron transfer or via adsorbed H species.
It has been shown that many chlorinated hydrocarbons can be effectively dechlorinated
indirectly by atomic hydrogen produced from the reduction of water by the corrosion of
ZVI [34,68,105]. This reaction needs a catalyst, and chlorinated hydrocarbons react with
adsorbed hydrogen involving the formation of hydride complexes on the iron surface [68].
Under anaerobic conditions, hydrogen is produced after the corrosion of iron. Although
the produced gases would hinder the mass transfer of the pollutants to the reactive site
on the iron surface [52], the feasibility of rapid dechlorination by hydrogen is still reli-
able in the presence of effective catalysts such as palladium (Pd), platinum (Pt), ruthenium
(Ru), copper (Cu), and nickel (Ni) [12,21,26,37,106]. As ZVI corrodes, protons from water are
reduced to adsorbed H atoms and to H
2
gas at the surface of the catalytic second metal.
Chlorinated hydrocarbons are then adsorbed onto the surface of bimetallic nanoparticles
