Application of Zerovalent Iron Particles on the Removal of

D-O4
Application of Zerovalent Iron Particles on the Removal of Recalcitrant
Aromatic Contaminants and their Mechanisms
Yuh-fan Su, Chih-ping Tso, Yu-huei Peng, Meng-yi Chen, Yu-tsung Tai, Chung-yu
Hsu, Yao-cyong Chen, Hsi-ling Chou, and Yang-hsin Shih*
Department of Agricultural Chemistry, National Taiwan University, Taipei, Taiwan
Abstract
Zero-valent iron (ZVI) has been used in the remediation of soil and groundwater
contamination sites. Recently, nanoscale zero-valent iron (NZVI) has been
investigated as a new tool for the treatment of contaminated sites [1, 2]. For persistent
organic pollutants, the feasibility of NZVI treatment, the removal mechanisms, and
the combination of other technologies should be evaluated. We used NZVI particles
synthesized in the laboratory to investigate the effect of material properties and
environmental factors on the reduction kinetics and degradation mechanisms of
recalcitrant aromatic contaminants such as Congo red (CR) and some halogenated
aromatic compounds including decabrominated diphenyl ether (DBDE),
hexachlorobenzene (HCB), and pentachlorophenol (PCP) [3-6]. We also used Pd/Fe
bimetallic particles to study the effect of material properties and environmental
factors on the reduction kinetics and degradation mechanisms of HCB and PCP [7]. In
general, the average particle size of these two nanoparticle grains was around 90 nm.
The effect of the combination of microorganisms or sequential Fenton treatments on
the mineralization of some contaminants was also evaluated.
Polybrominated diphenyl ethers recognized as a new class of environmental
persistent toxic contaminants have been distributed widely in the world [8]. Within
40 min 90% of DBDE was rapidly removed by NZVI as compared to around 40 d
needed for 24-fold weight of microscale ZVI (MZVI). The removal by NZVI is much
faster than that by MZVI due to its high surface area and reactivity. At a different pH,
the pseudo-first-order removal rate constants of DBDE linearly increased from 0.016
to 0.024 min−1 with the decreasing of aqueous initial pH values from 10 to 5. The
degradation of DBDE with NZVI is favorable in an acid condition. The
debromination pathways of DBDE with NZVI were proposed on the basis of the
identified reaction intermediates ranging from nona- to mono-brominated diphenyl
ethers (BDEs) for an acid condition and from nona- to penta-BDEs for an alkaline
condition (Fig. 1). The debromination of PBDEs from para positions is more difficult
than that from meta or ortho positions. Adsorption on NZVI also plays a role on the
removal of DBDE [3].
The removal kinetics of DBDE and monobromodiphenyl ether (BDE-3) with
MZVI indicated two-step kinetics: a fast removal step at the beginning of the reaction
and a follow-up slow removal step [9]. The content of brominated compounds on the
surface of MZVI was measured. About 10-20% of DBDE and 15-30% of BDE-3 were
adsorbed on MZVI. The adsorption of DBDE and BDE-3 on MZVI was confirmed
through the Fourier transform infrared spectroscopy. Surface adsorption of PBDEs on
MZVI dominates the removal mechanism in the beginning and further debromination
with MZVI was found. Finally, about 70% of DBDE and 60% of BDE-3 was
degraded by MZVI within about one month [9]. The great longevity of MZVI on the
PBDE degradation can facilitate the remediation design.
HCB is one of the twelve persistent organic pollutants. The rapid degradation of
HCB by NZVI follows pseudo-first-order kinetics. Increasing the dose of NZVI
particles enhanced the dechlorination rates of HCB. With an increase in temperature,
the degradation rate increases. The activation energy was determined to be 16.6
1
kJ/mol. The values of pH rapidly increased and ORP rapidly decreased during the
experiments. The dechlorination rate constants of HCB linearly increased from 0.052
to 0.12 h-1 with decreasing aqueous pH values from 9.2 to 3.2. The dehalogenation of
HCB with NZVI is also favorable under acid conditions. Furthermore, the degradation
kinetics and efficiency increased with increasing water content in solutions, indicating
that hydrogen ion was also one of the driving forces of reaction. The degradation
kinetics was not influenced by the HCO3−, Mg2+, and Na+ ions. It was enhanced in the
presence of the Cl− and SO42− ions due to their corrosion promotion. The NO3−
competes with HCB so it inhibits the degradation reaction. The Fe2+ ions would
inhibit the degradation reaction due to passivation layer formed, while it was
enhanced in the presence of Cu2+ ions resulted from the reduced form of copper on
NZVI surfaces [4, 6]. The dechlorination pathway was illustrated by the chlorinated
intermediates and products of reducing HCB by nanoscale iron contained
pentachlorobenzene (PCB), two tetrachlorobenzene (TeCB) isomers, and one
trichlorobenzene (TCB). The chlorinated intermediates and products by nanoscale
Pd/Fe bimetallic metal contained PCB, three TeCB isomers, two TCB isomers, and
one dichlorobenzene.
PCP is a generally ionized persistent organic pollutant in neutral conditions. The
total removal of PCP was attributed to both chemical reduction and adsorption
processes on our synthesized Pd/Fe nanoparticle surface. The optimal Pd content of
the bimetallic NPs was around 0.54 mg/g Fe. Three selected cations, Cu2+, Ni2+, and
Fe3+, normally co-present in soil and groundwater contamination sites could facilitate
the degradation kinetics and efficiencies of PCP by Pd/Fe nanoparticles. XANES
absorption spectra were performed to characterize their valences. The enhancement
effect of Cu2+ and Ni2+ ions result from the presence of reduced forms of copper and
nickel on Pd/Fe surfaces (Fig. 2) [7]. The presence of reduced forms of copper and
nickel on Pd/Fe nanoparticles were confirmed by ICP-MS analysis. The addition of
Fe3+ ions caused a decrease in pH and can reasonably account for the enhancement
seen in the PCP degradation process [7].
To integrate the microbial treatments into ZVI technology, the combined effects
of MZVI and anaerobic sludge in DBDE degradation were investigated. The
co-incubation resulted in 63% and 29% enhancement of removal ability when
compared to the single component conditions. By-products generated during the
entire process followed a stepwise sequence with non-uniform accumulation rates.
Microbes hindered the accessibility of MZVI to DBDE and reduced the removal
ability in the initial stage (<12 h). According to the analysis of the microbial
community change, co-incubation with MZVI leads to the enrichment of
heterotrophic microbial populations bearing nitrate- or iron-reducing activities. The
interaction between MZVI and microbes contributed to the synergistic effect (Fig. 3)
[10, 11].
After the treatment of NZVI, degraded byproducts are not completely mineralized
and sometimes more toxic products generated. Since the oxidation of NZVI and
ferrous ions produced, the further oxidation process of byproducts such as Fenton
reactions can be performed to mineralize contaminants. For example, azo-dye
compounds are widely used in the textile industry and are mostly discharged in
industrial wastewater [12]. Rapid decolorization behaviors of CR with NZVI was
observed and follow pseudo-first-order kinetics. Intermediate analysis suggests the
presence of broken azo bonds in CR with the NZVI surface. However, the
mineralization of CR with NZVI is very low. Sequential NZVI/H2O2 processes can
effectively decompose CR due to the Fenton reaction (Fig. 4) [5]. Combination of the
2
reduction power of NZVI and its oxidized ions with H2O2 can be considered an
alternative for treating azo-dye wastewater [5].
In summary, NZVI is one of the most rapidly growing sectors of environmental
remediation nanotechnology. Fast removal of these recalcitrant aromatic compounds
with NZVIs was demonstrated. Furthermore, adsorption process plays an important
role on the removal mechanism on these halogenated aromatic compounds with
zerovalent iron nanoparticles. On the other hand, to combine other treatment
processes, the application of NZVI can be more attractive in pollutant removal. Our
experimental results contribute to a better understanding of aromatic contaminant
degradation and serve as a useful reference for remediation design and prediction of
treatment efficiency of aromatic contaminants with zerovalent iron nanoparticles [13].
Keywords:
Nanoscale zero-valent iron (NZVI), halogenated aromatic compounds, adsorption,
dehalogenation, oxidation, synergistic effect.
(a)
(b)
Figure 3. Possible mechanisms of the
integration of anaerobic sludge and
MZVI for DBDE degradation [11].
Figure 1. Proposed debromination
pathways of DBDE by NZVI at (a) an
acid condition and (b) an alkaline
condition [3].
Figure 4. Total organic carbon (TOC)
removal in the sequential Fenton process
after NZVI reduction of Congo red [5].
Figure 2. Schematic diagram
illustrating the effect of cations on
degradation of PCP by Pd/Fe
nanoparticles [7].
Acknowledgements
This research was supported by NSC grants NSC (97-2313-B-002-048-MY3), NSC
(100-2313-B-002-008), and NSC (101-2313-B-002-035-MY3).
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