Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 104

Solution pH. The TCE dechlorination rate constant was doubled as pH
decreased from 8.9 to 6.5, and there appeared to be a linear correlation between the
logarithm of the reaction rate constant and pH. TCE reduction occurred with absorbed H
atoms whereas the availability of protons is a function of pH. When pH decreases more
H atoms would be available to be absorbed for TCE reduction, resulting in enhanced
dechlorination.
TCE Concentration. TCE concentration is another important factor in
dechlorination by NZVI. When the TCE concentration increased from 0.027 to 1.3 mM
the TCE degradation rate constant was decreased by almost 2-fold. This is likely
because at a higher TCE concentration, the deactivation effect become apparent because
of the growth of the passive iron oxide layer on NZVI. The NZVI particle reactive
lifetime is therefore affected by TCE concentrations. When TCE concentrations
increased from 0.46 mM to 8.4 mM, the particle reactive lifetime decreased from 60
days to 10 days (Liu et al., 2007). At low TCE concentrations, there is still NZVI not
oxidized by TCE that is available for H 2 evolution and continuous TCE dechlorination
while at high TCE concentrations all available Fe 0 content is quickly exhausted and H 2
evolution could be ceased.
Anions. Nitrate can compete for reactive sites with TCE at higher
concentrations and inhibit TCE dechlorination. At the concentration above 1 mM, nitrate
has a noticeable inhibition effect on TCE degradation and the inhibition increases as
nitrate concentration increases. At 3 and 5 mM NO 3 - TCE degradation decreased by 3-
fold and 7-fold respectively. Since in most case the nitrate concentrations in
contaminated groundwater are less than 1 mM, the effect of nitrate inhibition may not be
observed. Other anions were observed to decrease NZVI reactivity with TCE in the
order of Cl - < SO 4 2- < HCO 3 - < HPO 4 2- at concentrations above 5mM because the
formation of Fe-anion complexes covered on the iron surface inhibit dechlorination (Liu
et al., 2007).
4.4.3 Application of NZVI to Remove Perchlorate ClO 4 -
In the presence of iron, perchlorate was sorbed to iron surface and reduced to
chloride. The removal rate of perchlorate depended on the amount of iron in the batch
experiments, with up to 66% of total ClO 4 - removed in 336 hours. Furthermore, the most
significant perchlorate removal appeared to occur in solutions with slightly acidic or
near-neutral initial pH values (Moore et al., 2003).
The possible mechanism of perchlorate reduction on iron filings surface was
proposed (Huang and Serial 2007). Surface iron atoms may associate with oxygen atoms
in perchlorate molecule to distort the tetrahedron ClO 4 - . Such distortion can eventually