Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 103

products by different NZVI particles demonstrated the importance of nanoparticle
morphology in degradation of chlorinated organic compounds.
Other results also have demonstrated that NZVI particles having their amorphous
properties can increase TCE dechlorination by using the produced H 2 as a reductant (Liu
et al., 2005b). Several commercially available products, however, may or may not
increase TCE dechlorination because the iron oxide shell thickness and particle
composition difference could significantly affect TCE dechlorination as the thicker shell
structure, and more crystalline form of iron nanoparticles often decrease the rate of TCE
dechlorination (Liu et al., 2005b).
The rates of catalytic hydrodechlorination of TCE by NZVI were evaluated
recently in another study (Song and Carraway 2008). Because of the increased hydrogen
concentrations of using NZVI particles during the reductive reaction, the pseudo first-
order reaction constant of TCE dechlorination by NZVI increased by a factor of 4
compared to that of dechlorination by regular ZVI. The reaction rate constant of TCE
reduction had a strong correlation with the initial hydrogen concentration. The rate
constants of subsequent 1,1-DCE reduction, however, were not dependent on the
hydrogen concentration. These observations suggest a possible mechanism of direct
electron transfer from NZVI particle surfaces to the reactants that are physically
adsorbed to nanoparticles. TCE dechlorination by NZVI particles, therefore, may be
achieved through a reactive catalytic pathway with hydrogen evolution.
4.4.2.2 Factors Affecting TCE Degradation by NZVI
There are many factors affecting TCE degradation by NZVI. In one study, the
effects of different Fe 0 contents of NZVI particles and solution pH on TCE
dechlorination were evaluated (Liu and Lowry 2006). The effects of TCE concentration
and major groundwater anions on the TCE reduction by NZVI were also investigated
(Liu et al., 2007). Researchers in both studies used commercially available reactive
nanoscale iron particles (RNIP) for TCE degradation.
Fe 0 Content . In the batch TCE reduction experiments over a long period (230
days), RNIP consisting of 48% of Fe(0) and 52% of Fe 3 O 4 , were applied. TCE
degradation by RNIP remained constant or increased slightly while Fe 0 content of RNIP
decreased from initial 48% to 28%. The slight increase of the dechlorination rate
constant was attributed to the pH decrease in the batch reactors, because when batch
reactors had similar solution pH the rate constants for RNIP with different Fe 0 contents
were almost the same. These results suggest that the reaction of TCE dechlorination
follows zero-order with respect to the Fe 0 content of RNIP.