Effect of dissolved oxygen concentration on iron efficiency: Removal of three chloroacetic acids

Article abstract of DOI:10.1016/j.watres.2015.01.027

The monochloroacetic, dichloroacetic and trichloroacetic acid (MCAA, DCAA and TCAA) removed by metallic iron under controlled dissolved oxygen conditions (0, 0.75, 1.52, 2.59, 3.47 or 7.09mg/L DO) was investigated in well-mixed batch systems. The removal of CAAs increased first and then decreased with increasing DO concentration. Compared with anoxic condition, the reduction of MCAA and DCAA was substantially enhanced in the presence of O₂, while TCAA reduction was significantly inhibited above 2.59mg/L. The 1.52mg/L DO was optimum for the formation of final product, acetic acid. Chlorine mass balances were 69-102%, and carbon mass balances were 92-105%. With sufficient mass transfer from bulk to the particle surface, the degradation of CAAs was limited by their reduction or migration rate within iron particles, which were dependent on the change of reducing agents and corrosion coatings. Under anoxic conditions, the reduction of CAAs was mainly inhibited by the available reducing agents in the conductive layer. Under low oxic conditions, the increasing reducing agents and thin lepidocrocite layer were favorable for CAA dechlorination. Under high oxic conditions, the redundant oxygen competing for reducing agents and significant lepidocrocite growth became the major restricting factors. Various CAA removal mechanisms could be potentially applied to explaining the effect of DO concentration on iron efficiency for contaminant reduction in water and wastewater treatment.

Full text of DOI:10.1016/j.watres.2015.01.027

Accepted Manuscript
Effect of dissolved oxygen concentration on iron efficiency: Removal of three
chloroacetic acids
Shun Tang, Xiao-mao Wang, Yu-qin Mao, Yu Zhao, Hong-wei Yang, Yuefeng F. Xie
PII:
S0043-1354(15)00055-X
10.1016/j.watres.2015.01.027
WR 11119
DOI:
Reference:
To appear in: Water Research
Received Date: 4 November 2014
Revised Date: 11 January 2015
Accepted Date: 20 January 2015
Please cite this article as: Tang, S., Wang, X.-m., Mao, Y.-q., Zhao, Y., Yang, H.-w., Xie, Y.F., Effect of
dissolved oxygen concentration on iron efficiency: Removal of three chloroacetic acids, Water Research
(2015), doi: 10.1016/j.watres.2015.01.027.
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MCAA
DCAA
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Effect of dissolved oxygen concentration on iron efficiency: Removal of three
chloroacetic acids
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Shun Tang , Xiao-mao Wang , Yu-qin Mao , Yu Zhao , Hong-wei Yang *, Yuefeng F.
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Xie
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State Key Joint Laboratory of Environmental Simulation and Pollution Control,
School of Environment, Tsinghua University, Beijing 100084, P.R. China
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Environmental Programs, The Pennsylvania State University, 777 West Harrisburg
Pike, Middletown, PA17057, USA
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Re-submitted to
Water Research
October, 2014
*Corresponding author.
Address: School of Environment, Tsinghua University, Beijing 100084, People’s
Republic of China.
Tel.: +86-10-6277 2987
Email: yanghw@tsinghua.edu.cn (H.W. Yang)
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Abstract
The monochloroacetic, dichloroacetic and trichloroacetic acid (MCAA, DCAA
and TCAA) removed by metallic iron under controlled dissolved oxygen conditions
(0, 0.75, 1.52, 2.59, 3.47 or 7.09 mg/L DO) was investigated in well-mixed batch
systems. The removal of CAAs increased first and then decreased with increasing DO
concentration. Compared with anoxic condition, the reduction of MCAA and DCAA
was substantially enhanced in the presence of O , while TCAA reduction was
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significantly inhibited above 2.59 mg/L. The 1.52 mg/L DO was optimum for the
formation of final product, acetic acid. Chlorine mass balances were 69-102%, and
carbon mass balances were 92-105%. With sufficient mass transfer from bulk to the
particle surface, the degradation of CAAs was limited by their reduction or migration
rate within iron particles, which were dependent on the change of reducing agents and
corrosion coatings. Under anoxic conditions, the reduction of CAAs was mainly
inhibited by the available reducing agents in the conductive layer. Under low oxic
conditions, the increasing reducing agents and thin lepidocrocite layer were favorable
for CAA dechlorination. Under high oxic conditions, the redundant oxygen competing
for reducing agents and significant lepidocrocite growth became the major restricting
factors. Various CAA removal mechanisms could be potentially applied to explaining
the effect of DO concentration on iron efficiency for contaminant reduction in water
and wastewater treatment.
Keywords: Dissolved oxygen (DO); Iron; Corrosion products; Removal mechanism;
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Chloroacetic acids
1. Introduction
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In recent years, zero valent iron (Fe ) and Fe -doped particles have been shown
very efficient for the aqueous removal of various inorganic and organic contaminants
(Cundy et al., 2008; Fu et al., 2014; Gunawardana et al., 2011; Henderson et al., 2007;
Li et al., 2006; Noubactep, 2008; Scott et al., 2011). The potential effect of the
coexisting oxidants on iron corrosion, such as oxygen (O ) or chlorine, has attracted
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considerable attention (Jung et al., 2011; Rahman and Gagnon, 2014; Sarin et al.,
2004a, 2004b; Stratmann and Müller, 1994; Wang et al., 2012; Zhang and Huang,
2006). However, limited investigation have been completed examining the influence
of O on the removal of contaminants by iron (Ghauch et al., 2010, 2011; Huang and
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Zhang, 2005; Wang et al., 2010). The rate constant for carbon tetrachloride reacting
with iron under an oxic condition was significantly lower than that under an anoxic
condition (Helland et al., 1995). The presence of O in the iron-water system
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decreased the removal efficiency of nitrate (Westerhoff and James, 2003) and bromate
(Xie and Shang, 2007), and slowed the reduction of bromoacetic acid (Zhang et al.,
2004), trichloropropanone (Lee et al., 2007) and diclofenac (Ghauch et al., 2011). The
rapid removal of carbon tetrachloride and trichloroethylene using iron and
palladized-iron cathodes was not interfered even when purging the raw water with air
(Li and Farrell, 2000). The nitrate reduction could maintain stable under various oxic
conditions if amending iron with the aqueous ferrous ion (Huang and Zhang, 2005).
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The existing O failed to inhibit with the quick degradation of tribromoacetic acid,
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trichloronitromethane and trichloracetonitrile, which were mass transfer limited
species (Lee et al., 2007; Zhang et al., 2004). Some attractive results have been
reported on the enhancing effect of O on iron efficiency for the removal of dye
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(Wang et al., 2010) and diclofenac (Ghauch et al., 2010) in a batch system, and the
reduction of nitrate (Westerhoff and James, 2003) and chromium (Yoon et al., 2011)
in a packed column.
A potential mechanism for different O roles was involved the formation of
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various iron corrosion products, which served as a physical barrier, a semiconductor
or a coordinating surface in the core-shell structure of iron corrosion (Scherer et al.,
1998; Sarin et al., 2004a; Yan et al., 2010). Uludag-Demirer and Bowers (2003)
reported O acted as an irreversible inhibitor of the trichloroethylene reduction based
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on the assumption of magnetite and maghemite formed on the iron surface. Huang
and Zhang (2005) reported maghemite or lepidocrocite produced under oxic
conditions would decrease the removal efficiency of iron, while magnetite even in a
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substantial thickness might not impede the nitrate reduction. In these researches, Fe
was widely regarded as an effective reductant (direct reduction), which was
responsible for the decrease of contaminants (Gillham and O'Hannesin, 1994; Li et al.,
2006; Matheson and Tratnyek, 1994). Due to ubiquitous oxide films decreasing the
accessibility of iron surface, the observed removal of contaminants was
predominantly mediated by aqueous and/or solid corrosion products through
adsorption, co-precipitation and subsequent indirect reduction by other reducing
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agents except for Fe (Noubactep, 2008, 2010, 2011, 2014; Noubactep et al., 2010).
The reducibility of aqueous and solid iron corrosion was usually ignored, although the
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aqueous, absorbed or structural divalent iron (Fe ) was thermodynamically capable of
reducing some pollutants (Chun et al., 2005, 2007; Pecher et al., 2002; Schlautman
and Han, 2001; White and Peterson, 1996). The enhancement on removal efficiency
of iron was reported with coexisting iron minerals, such as magnetite (Coelho et al.,
2008; Huang and Zhang, 2006; Mak et al., 2011), green rust (Cho et al., 2010), pyrite
(Kim et al., 2013) and ferric hydroxide (Song et al., 2013). The availability of
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aqueous or solid Fe mentioned above was almost conducted under anoxic condition
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to restrain the rapid oxidation of Fe by O . Hence, the evolution of reducing ability
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or agents for contaminants in iron/water system under various oxic conditions was
necessary to investigate in order to promote its application in the treatment of surface
water and wastewater.
As important byproducts of the chlorination of water and wastewater, the
chloroacetic acids (CAAs) including monochloroacetic acid (MCAA), dichloroacetic
acid (DCAA) and trichloroacetic acid (TCAA) were selected as the model compounds.
The batch experiments with a controlled DO concentration ranged from 0 to 7.09
mg/L were analyzed over a 2-h reaction period, during which the liquid and solid
were periodically sampled to determine the evolution of CAA degradation processes
and iron corrosion products. The removal mechanism of CAAs affected by various
DO concentrations was also explored in the iron/water system.
2. Materials and methods
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2.1. Chemicals
The analytically pure MCAA, DCAA and TCAA were bought from the Tianjin
Fuchen Chemical Regents and Sinopharm Chemical Reagent (China), respectively.
The standard solution for CAAs (2000 mg/L each), formic acid, ammonium formate
and sodium carbonate (analytical reagent) were purchased from Sigma-Aldrich (USA).
The chromatographically pure methanol and acetonitrile were obtained from Fisher
Chemicals (USA). The analytically pure sodium chloride, hydrochloric acid, sodium
hydroxide, ferrous chloride and magnetite were bought from Beijing Chemical Works
(China). Akaganeite substituted for lepidocrocite was synthesized for convenience
(Chitrakar et al., 2006).
2.2. Iron pretreatment
The raw iron particles were collected from a local manufacturing factory, which
had some grease on the surface. The particles were first passed through two sieves
(0.2 and 1.0 mm), then soaked in a detergent solution for 24 h for oil removal, then
rinsed in sequence with tap water, deionized water and ultrapure water for residual
detergent removal, and finally stored in ultrapure water. The final iron particles
contained black coatings and the specific surface area measured via BET was 1.50
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m /g. Fresh iron without oxide films was prepared with 1 M hydrochloric acid lasting
for 10 min, then rinsed with deoxygenated water until no chlorine ion detected in
water.
2.3. Dissolved oxygen control
Refreshing the headspace at a 10-min interval was an effective way to maintain a
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stable DO concentration in the reactor. This operation consisted of the following steps.
(1) A reactor was taken down from the rotator. (2) Two needles were inserted through
the silicone stopper of the reactor and one of them was connected with high-purity
nitrogen gas (N ) lasting for 20 s. (3) A specific amount of O was injected into the
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headspace. (4) Two needles were palled out after waiting for 3-5 s, ensuring that the
gas pressure in the reactor was the same as ambient pressure. The volume of adopted
O included 0, 0.5, 2, 5 and 8 mL and a stream of O was used to provide the highest
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DO concentration.
2.4. Experimental System
The initial solution containing approximately 25 µM CAA was prepared using
ultrapure water. After adjusting pH to 7.0 with NaOH and HCl, CAA solution was
purged with N for 1 h. Deoxygenated CAA solutions (55 mL) and iron particles (0.55
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g) were transferred into a 65-mL brown bottle. The bottle was sealed a screw thread
cap with PTFE-faced silicone septa, then supplied with O , and finally loaded onto a
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rotator at 40 rpm. The samples were collected at 0, 5, 10, 20, 30, 45, 60, 75, 90 and
120 min. Approximately 2 mL samples were preferentially withdrawn from the sealed
reactor for prompt DO measurement. A certain amount of solutions, after passing
through a 0.2 µm pore size syringe-mounted filter, were collected for the chemical
concentration analyses. The suspended precipitates in water and corrosion coatings on
iron surface were collected to analyze their physical properties. All batch experiments
were performed at room temperature (25 ± 1 °C). In this paper, data represented the
mean and standard deviation of duplicates.
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2.5. Analytical methods
MCAA, DCAA and TCAA were determined by LCMS without pretreatment. A
high-performance liquid chromatography (Agilent 1290, USA) was equipped with an
Agilent Eclipse Plus C8 RRHD (1.8 ꢀm × 2.1 mm × 50 mm). The two mobile phases
were pure methanol and ultrapure water containing 0.1% formic acid and 0.125 mM
ammonium formate. A triple-quadrupole mass spectrometer (Agilent 6460, USA) was
equipped with an electrospray ionization source operated in the negative ion mode.
The optimized mass spectrometry parameters were summarized as follows: gas
temperature of 300 °C, gas flow of 7 L/min, nebulizer pressure of 35 psi, sheath gas
heater temperature of 325 °C, sheath gas flow of 11 L/min; capillary voltage of 0 V.
The precursor and product ion for MCAA were m/z 93 and m/z 35.1, and those for
DCAA were m/z 127 and m/z 83 and those for TCAA were m/z 116.9 and m/z 35.
AA and chloride ion were analyzed using an IC (Metrohm 761, Switzerland)
coupled with a conductivity detector and a Metrosep A Supp 7 column together with a
background electrolyte containing 5% acetonitrile and 3.6 mM sodium carbonate. DO
was measured using an oxygen electrode (INESA JPST-605F, China) calibrated with
deoxygenated water and air-saturated water at room temperature. BET surface area
was observed by a porosimetry analysis (Micromeritics Tristar 3020 II, USA). To
detach corrosion coatings, iron particles were conducted in an ultrasonic cleaner (Kun
Shan KQ-500DB, China) at 200 W power lasting for 10 min (Zhuang and Huang,
2006). After filtration, both suspended precipitates and surface coatings were prepared
for X-ray diffraction (XRD) (Rigaku Dmax 2500, Japan) and scanning electron
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microscope (SEM) (Carl Zeiss Merlin, Germany).
2.6. Experimental result expression
The pseudo-first order model (1) was applied to describe the removal of CAA in
reaction system since iron particles used was kept in excess (10 g/L) toward each
CAA (25 µM).
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Where C was the CAA concentration (µM) at any reaction time t (min), C was
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The observed pseudo-first-order rate constants were determined from the slope of the
linear regressions obtained by plotting ln(C/C ) versus reaction time,
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3. Results
3.1. Dissolved oxygen concentration
When the reactor headspace was periodically refreshed using above-mentioned
procedures, the DO concentration was kept stable at 0, 0.75 ± 0.14, 1.52 ± 0.17, 2.59
± 0.23, 3.47 ± 0.25, and 7.09 ± 0.83 mg/L (Fig. 1). The negligible DO concentration
detected in the anoxic system could be attributed to the O dissolution during
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determination.
3.2. CAA removal and reaction kinetics
The effect of DO concentration on the reduction of CAAs by iron was shown in Fig.
2. When the DO concentration was at 0, 0.75, 1.52, 2.59 3.47 and 7.09 mg/L, the final
removal efficiency of MCAA was 1.25%, 19.6%, 27.5%, 21.3%, 19.9% and 14.4%,
and that of DCAA was 26.1%, 64.2%, 68.1%, 49.4%, 43.9% and 22.4%, and that of
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TCAA was 77.2%, 80.5%, 75.3%, 54.4%, 42.0% and 26.0%, respectively. It was
found that gradually increasing DO concentration in the reactor, the CAA removal
efficiency increased first and then decreased. The optimum DO concentration for the
MCAA and DCAA reduction was 1.52 mg/L and that for the TCAA reduction was
0.75 mg/L.
As shown in Table 1, owing to the O supplement, the rate constant for MCAA
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anoxic condition. MCAA removal was insensitive to the change in DO concentration
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The rate constants for DCAA detected at 0 and 7.09 mg/L were obviously lower than
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TCAA was quickly removed at DO concentrations below 2.59 mg/L. The removal of
TCAA was significantly inhibited by further increasing DO concentration, for its rate
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10.22 × 10 min in the absence of O . The susceptibility of CAAs to iron at all DO
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concentrations strictly followed the order of TCAA, DCAA, and lastly MCAA,
probably influenced by the number of substituent chlorine (Li et al., 2012; Zhang et
al., 2004). However, the susceptibility of CAA reduction improved by O followed the
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order of MCAA, DCAA, and lastly TCAA. The differences in observed rate constants
for CAAs gradually decreased with the increase of DO concentration, especially for
approximate rate constants for DCAA and TCAA at the same DO above 2.59 mg/L.
The degradation process of CAAs was composed of their mass transfer from
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bulk water to particle surface, migration across the corrosion coatings and subsequent
redox reaction. The overall mass transfer coefficient for MCAA, DCAA and TCAA
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was calculated at 0.202, 0.184 and 0.170 min , respectively. Details of the
computations were supplied in the appendix A. Since the ratio of observed rate
constant to overall mass transfer coefficient was 0.07 at most, the three CAAs were
not belonged to mass transfer limited species. Therefore, the migration or reduction of
CAAs would become the limiting step in their degradation processes, which might
vary with the DO concentration.
3.3. CAA degradation processes
TCAA degradation processes influenced by various DO concentrations were
shown in Fig. 3. The removal processes of both MCAA and DCAA were also
provided in the appendix B. At DO concentrations less than 2.59 mg/L, the amount of
TCAA reduction was similar and ranged from 19.97 to 21.49 µM. However, there was
obvious difference in its dechlorination products. TCAA was transformed into 16.92
µM DCAA, 3.66 µM MCAA and 0.46 µM AA without O . The accumulation of
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DCAA was related to its low rate constants under the anoxic condition. TCAA was
dechlorinated into 5.31 µM DCAA, 13.20 µM MCAA and 3.50 µM AA at 0.75 mg/L
DO, and 3.81 µM DCAA, 11.12 µM MCAA and 4.98 µM AA at 1.52 mg/L DO.
Under these low oxic conditions, MCAA became the main TCAA degradation product,
companying with a notable increase in the AA formation. This phenomenon might
result from the enhanced rate constants for various CAAs. The reduction of TCAA
and generation of corresponding products were impeded under high oxic conditions,
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while MCAA was maintained as the major product. During a 2-h reaction period, the
amount of AA formation gradually exceeded the DCAA concentration in the DO
range 1.52-3.47 mg/L. Owing to the different rate constants for CAAs affected by O2,
the distribution law of TCAA degradation products varied at various DO
concentrations.
In this study, the mass balance of total carbon (i.e. the sum of AA and all CAAs)
was 92-105% in all anoxic and oxic systems. Therefore, CAAs were mainly removed
by the chemical degradation within the system. The total chlorine consisted of the
chloride ion measured by IC and combined chlorine calculated from CAAs. The mass
balance of total chlorine was only detected in the anoxic systems, 69-95% for other
oxic systems. Based on the carbon mass balance, the combined chlorine content in
CAAs was stable, so the total chlorine loss resulted from a decrease in the chloride
ion, probably involved in the dissolution of corrosion products (Lytle et al., 2005;
Moore and Yong, 2005) or trapped within the lattice structure of oxides (Gilberg and
Seeley, 1981).
The oral LD (lethal dose causing 50% mortality) in rats for TCAA, DCAA,
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MCAA and AA is 3320, 2820, 55 and 3310 mg/kg, respectively, which indicated that
the formation of MCAA or DCAA might increase the biological toxicity of raw water.
Combined to their biodegradability (Tang et al., 2013), AA was the most desirable
product from the degradation of CAAs by iron. As shown in Fig. 4, the formation of
AA during CAA degradation was enhanced first and then partly hindered with an
increase of DO concentration, and the optimum DO concentration was 1.52 mg/L.
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3.4. Iron and corrosion products
As shown in Fig. 5, the raw iron was mainly composed of zero-valent iron based
on the XRD analysis. After hermetically storing in water for a period, the iron
particles used had been covered with a significant amount of magnetite, presenting a
dense surface with cracks from the SEM image. Under the anoxic condition, both
suspended precipitates and surface coatings were identified as magnetite (Fe O ),
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which was showed as black partly rounded crystals. The detected sheet or platy
structure might result from part oxidation during determination. Under the oxic
conditions, lepidocrocite (γ-FeOOH) and magnetite were simultaneously determined
in suspended precipitates while magnetite was the sole mineral in surface coatings,
probably for the insufficient amounts of lepidocrocite on the surface of iron particles.
The weak adherence of lepidocrocite might related to its irregular plates morphology.
Lepidocrocite as a visible brownish-yellowish precipitate was only observed in the
presence of O2.
The change of CAA concentration was observed in solution containing specific
corrosion products. Details of preliminary experiment processes and results were
supplied in the appendix C. Negligible loss of CAAs under neutral condition were
observed in the presence of synthetic or mixed corrosion products with/without
ferrous iron, which was accordant with other report (Chun et al., 2005). The amount
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of Fe detached from iron core was supposed to zero for its rapid oxidation by
aqueous O at pH above 7 (Morgan and Lahav, 2007). Combined with carbon
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recovery mentioned above, the adsorption or reduction of CAAs by suspended
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corrosion products was limited in this study.
The removal of CAAs by iron with acid pretreatment was carried out to
investigate the role of corrosion coatings as shown in Fig. 6. The decreasing order of
iron efficiency over 2 h of reaction toward three CAAs was: iron with oxic corrosion
coatings > iron with anoxic corrosion coatings > fresh iron. TCAA and DCAA could
be partly removed by green rust as oxic corrosion rather than by magnetite (Chun et
al., 2005, 2007), which might prove powerful reductants formed in oxic systems. At
end of removal experiment, there was no substantial decrease of DCAA in the fresh
iron system and almost no loss of MCAA in the absence of O , while considerable AA
2
was formed in the presence of O . This comparison suggested that corrosion coatings
2
0
on the surface of Fe core were responsible for the evolution of CAA removal
efficiency within various iron/water systems.
4. Discussions
Due to the mechanical abrasion in well-mixed batch systems, a thin layer of
0
corrosion coatings was expected on the core of Fe (Zhang and Huang, 2006). The
corrosion product in the ferrous state (FeO) was expected to be closest to the core
(Noubactep, 2008; Sarin et al., 2004b). The black green solution of iron corrosion at
the high stirring speed and loss of chlorine mass balance under oxic conditions could
be a sign of the presence of green rust, which was common intermediate species of
chemical oxidation of ferrous to ferric iron by O and ultimately transformed into
2
stable lepidocrocite (Morgan and Lahav, 2007; Westerhoff and James, 2003).
Combined with the observation of XRD and SEM, two structure of corrosion coatings
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were proposed to explain the CAA removal mechanism under the anoxic and oxic
conditions in Fig. 7.
As a possible limiting step, the migration of CAAs across the corrosion coatings
primarily depended on film permeability and affinity between contaminants and
corrosion oxides (Nesic, 2007; Noubactep, 2008; Noubactep, 2012). The dissolution
constant (pKa) of CAAs was in the range of 0.51-2.87 and the point of zero charge
(pHpzc) of magnetite and lepidocrocite were 6.4-6.9 and 7.3 (Pecher et al., 2002). At
pH range of 7-9 in this study, both CAAs and iron corrosion product were negatively
charged. Hence, the weak affinity was helpful to the migration of CAAs and their
products in corrosion coatings. The porosity (or permeability) against density
increased from metal towards the outer surface of oxide film, and the density of oxide
decreased with increasing oxygen content in its chemical structural. Higher O2
availability would lead a larger volumetric expansion and longer migration path
-12
-16
2
(Domga et al., 2015). The calculated diffusion coefficients of 10 -10 cm /s were
compatible with ion diffusion through a semi-porous substrate (White et al., 1994). It
seemed to be very important for subsequent electrochemical or chemical reduction.
As the other possible limiting step, the reduction of CAAs was related to
available reducing agents, which originated from iron corrosion affected by the
electronic and ionic properties of surface oxides (Sato, 2001). The quantitative
0
contaminant directly removed by Fe core was nonrealistic due to dense corrosion
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coatings decreasing the accessibility of Fe surface (Noubactep, 2008). However,
contaminant removal through electrons was feasible since the FeO, Fe O and/or
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green rust layer were electronic conductive (O’Carroll et al., 2013; Yan et al., 2010),
while few electrons could penetrate the outer lepidocrocite layer (the band gap of 2.3
II
eV). In addition to structural Fe in iron (oxy)hydroxides, the electronegative oxides
were easy to adsorb aqueous cations and then to form indirect reducing agents that are
0
II
more powerful than Fe , such as atom hydrogen or absorbed Fe , with -0.65~-0.34 V
II
0
for Fe compared to -0.44 V for Fe (White and Peterson, 1996; Noubactep, 2012).
II
Surface complex attached Fe might be helpful to the adsorption of CAA ion from the
aqueous solution by the electromotive force. The formation of oxidized layers (such
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as γ-FeOOH) on structural or absorded Fe oxides diminished the rates of aqueous
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metal reduction due to the required transfer of both electrons and Fe from the
underlying unoxidized mineral to the aqueous interface (White and Peterson, 1996),
which leaded to no significant loss of CAAs by suspended corrosion products. Hence,
II
only oxides with continuous replenishment of electron and Fe could rapidly reduce
0
CAAs, as the conductive layer coated on the surface of Fe core. Certainly, it was
necessary for the reduction of contaminants to penetrate the outer nonconductive layer.
The diffusion or migration of CAAs within the conductive layers as reactive
interfaces was not considered in this study.
The reduction of CAAs by iron was shown in the equation (2), and their removal
rates were dependent on the available reductants and CAAs on the reactive surface
(Noubactep et al., 2010).
-
Reductants +CAAs →CAAorAA+Cl
≡
≡
3
3
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(2)
The reducing agents for CAAs under anoxic and oxic conditions was shown in
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the following equations (3). In addition to forming solid corrosion products, soluble
ferrous iron was released into bulk water by penetrating the little cracks in the oxide
layer, and possibly through atom exchange with the ferrous in lattice (Cwiertny et al.,
2008; Pedersen et al., 2005). This diffusion rate decreased with increasing
concentration of aqueous ferrous iron, eventually slowing the rate of iron corrosion
and electron release.
0
2+
Anoxic
II
Oxic
II
Fe →Fe +e    →Fe ,e,H
  →Fe ,e,H
≡FeO,Fe O ,Greenrust
≡
FeO,Fe O4
3
3 4
3
3
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(3)
A relationship of CAA concentration on the conductive surface and in bulk
solution was roughly functioned as the thickness (d) of outer lepidocrocite layer
attached, as depicted in the equation (4). When the thickness was equal to 0, the value
of function was 1. The function value gradually dropped to zero with the continuous
growth of lepidocrocite coating.
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3
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CAAs = CAAs f (dγ −FeOOH
)
≡
bulk
3
3
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(4)
II
The addition of reducing agents such as Fe or H absorbed on various
oxides(Ghauch et al., 2010; Gheju and Balcu, 2011), enhanced dramatically the
removal of CAAs, for decreasing order of iron efficiency: iron with oxic corrosion
coatings > iron with anoxic corrosion coatings > fresh iron. Under the anoxic
condition, CAAs on the magnetite surface were the same as bulk concentrations due
to the absence of lepidocrocite. Because the accumulation of aqueous ferrous iron,
approximate 7.28 µM detected in this study, gave a negative feedback to corrosion of
iron, the degradation of CAAs was limited by available reductants. Under the low
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oxic condition, the reduction of O by ferrous iron was dominating, with the
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formation of powerful green rust (Stratmann and Müller, 1994). The aqueous ferrous
iron could barely be detected above 1.52 mg/L DO, which would lead to additional
electron release. Li and Farrell (2000) reported the cell current was increased from 11
to 18 mA, while the DO concentration was rising from 0 to 9 mg/L. The adverse
effect of lepidocrocite could be postponed by transforming into magnetite (Kuch,
1988; Ritter et al., 2003; Tamaura et al., 1983). These advantages were remarkable in
the low oxic system, resulting in a notable increase in rate constants for CAAs. Under
the high oxic condition, the high reactivity of redundant O with reductants and
2
significant growth of lepidocrocite coating became the restricting factors of CAA
removal. This phenomenon was confirmed by similar degradation rate constants for
DCAA and TCAA. These findings have great potential in analyzing the effect of DO
concentration on iron reactivity towards the removal of other contaminants in
drinking water, domestic or industrial wastewater.
Conclusions
The removal of CAAs by iron increased first and then decreased with increasing
the DO concentration from 0 to 7.09 mg/L. The susceptibility of CAA reduction
improved by O followed the order of MCAA, DCAA, and lastly TCAA. As a
2
desirable CAA degradation product, AA was optimally transformed at 1.52 mg/L DO.
The variation of available reducing agents and corrosion coatings was responsible for
CAA removals at various DO concentrations. Various CAA removal mechanisms
facilitate the preliminary prediction on the effect of DO concentration on iron
efficiency towards the treatment of contaminants in surface water and wastewater.
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Acknowledgement
The authors wish to acknowledge the financial support from National Natural
Science Foundation of China (No. 51290284, 51278269). The authors are also very
thankful to the anonymous reviewers who really helped in improving the quality of
this manuscript.
Appendices: Supplementary data
Supplementary data associated with this article can be found in the online
version.
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ACCEPTED MANUSCRIPT
Table 1. The observed rate constants for CAAs at various DO concentrations.
-
3
-1
a
DO
mg/L)
0
Rate constants (×10 min )
(
MCAA
DCAA
TCAA
0.003 ± 0.04
1.82 ± 0.05
2.73 ± 0.04
2.19 ± 0.10
2.07 ± 0.08
1.35 ± 0.05
1.95 ± 0.17
7.35 ± 0.46
8.85 ± 0.28
5.91 ± 0.11
5.02 ± 0.09
2.53 ± 0.15
10.22 ± 0.75
12.11 ± 0.56
11.13 ± 0.24
7.19 ± 0.22
4.91 ± 0.11
2.95 ± 0.19
0.75
1.52
2.59
3.47
7.09
a
:
Errors represent 95% confidence limits.