Environmental Engineering Reference
In-Depth Information
rate of CT dechlorination. Cu(II) could be an effective metal ion to synergistically enhance
the dechlorination eficiency and rate of CT by iron-bearing minerals.
4.4.4 Effect of Natural Organic Matters
The transformation process of chlorinated hydrocarbon by nZVI is a surface reaction that
requires close contact of the reactive iron surface and target compounds [67-69]. Surface
redox reactions comprise a series of physical and chemical processes, including (i) mass
transfer of the dissolved target compounds from the aqueous phase to the iron surface,
(ii) adsorption of the compounds to the iron surface, (iii) electron transfer from the iron
surface to the target compounds, and (iv) desorption of the by-products from the surface
[70]. Therefore, any nonreactive adsorbate that outcompetes the reactive surface sites with
contaminants would result in the decrease in degradation rate [71-76].
In subsurface environments, natural organic matters (NOMs) are abundant and play
important roles in both electron transfer and adsorption processes. The inhibition of the
dechlorination rate of chlorinated hydrocarbons by ZVMs in the presence of organic mat-
ters was reported. Muftikian et al. [77] reported that the reactivity of Pd/Fe decreased with
time because of the formation of hydroxylated iron oxide ilms on the surface. Tratnyek et al.
[75] found that the reduction rate of TCE was inhibited by natural organic matter (NOM) in
the ZVI system owing to the competitive sorption onto the surface of ZVI. The competitive
reaction between organic compounds for a limited number of reactive sites on the surface of
ZVI was also observed. Cho and Park [78] depicted that the rate constant for TCE reduction
by ZVI decreased 1.5-5× when the solutions contained reducible co-contaminants such as
CT, nitrate, and chromate. Loraine [73] also pointed out that some alcohols, such as etha-
nol and propanol, could inhibit the reduction of TCE by ZVI. Xie and Shang [79] depicted
that the reactivity of ZVI toward bromate reduction declined by a factor of 1.3-2.0× when
5-35 mg of dissolved organic carbon (DOC) per liter of humic acid was added. However,
the reduced functional groups present in humic acid would regenerate Fe(II), resulting in
the maintenance of iron surface activation for bromate reduction in the long run.
Similar results were also observed by Doong and Lai [80]. In their study, humic acids
were found to serve as inhibitors to compete for the reactive sites on the palladized iron
with PCE. After 24 h of equilibrium of humic acid with ZVI, the adsorbed humic acids
served as electron shuttles to effectively accelerate the dechlorination eficiency and rate of
PCE by the palladized iron. The scanning electron microscopic (SEM) images showed that
small protrusions appeared when 0.1 mM Pd(II) was amended into the solution, depict-
ing that Pd was deposited on the surface of ZVI (Figure 4.2a). However, a mucous layer
adhered onto the surface of palladized irons in the presence of humic acid, relecting the
possibility of lowering the dechlorination eficiency of Pd/Fe in the presence of NOM
(Figure 4.2b). However, the quinone moiety in humic acid has different effects on PCE
dechlorination by Pd/Fe. Although the addition of benzoquinone lowered the reduction
rates of PCE, the values of
k
SA
for the respiked PCE after 24 h in lawsone- and hydroqui-
none-amended systems were 1.24× and 1.39×, respectively, higher than that in the absence
of quinones (Figure 4.2c). On the contrary, a rapid and complete dechlorination of PCE
by Pd/Fe was observed within 5 min in the presence of anthraquinone-2,6-disulfonate
(AQDS). Respiking of 1 mg L
−1
PCE into the batches also showed good eficiency of PCE
dechlorination (Figure 4.2d). These results demonstrate that the quinone moiety in humic
acid plays a pivotal role in enhancing the dechlorination eficiency of PCE by Pd/Fe.
