Treatment Degradation Polycyclic Aromatic Hydrocarbons (PAHs)
Contamination In Groundwater By Application Sodium Percarbonate
Oxidative
Zhang Yue-hua1,2; Guo Chang-Hong*1; Yue Li-Hong2; Guo Yong2; Do Chung-Mei2
1Key
Laboratory of Molecular Cytogenetic and Genetic Breeding of Heilongjiang Province
College of Life Science and Technology, Harbin Normal University
Harbin, 150025, PR China
2Institute of Applied Microbiology of College of Life Science, Jiamusi University
Jiamusi city, Heilongjiang Province, 154007, PR China
Abstract: Sodium percarbonate has been dictates to oxidize a wide range variety to organic and inorganic
compounds in water under multiple conditions of environment of contamination in groundwater. Sodium percarbonate is effective showed over a wide pH ranges. Even though the theoretical dosage of
[PAHs]/[2Na2CO3·3H2O2] even down to 0.4 for complete degradation of PAHs, it still needed, at least, 3
moles of [2Na2CO3·3H2O2] to degradation this chemical dosage. This indicates that oxidation intermediates
competed sodium percarbonate molecules with the parent PAHs. The primary pH has no obviously efficacy
on PAHs decompose, for hydrogen-ions were conformation during the period of PAHs degradation.
Keywords: Sodium Percarbonate; polycyclic aromatic hydrocarbons; PAHs; Contamination
1. Introduction
Polycyclic aromatic hydrocarbons (PAHs) are widespread contaminants of soils and sediments. Clean-up
processes for soils contaminated with PAHs have so far
been limited to high-cost on-site stabilisation or excavation followed by off-site incineration. In situ remediation
of contaminated soils is considered more cost-efficient
than on-site and off-site treatment [1], but it depends on
the quantity and location of the soil to be treated [2].
Advanced oxidation processes (e.g. modified Fenton
reactions) and biodegradation are promising in situ remediation techniques. In situ biodegradation of highmolecular-weight PAHs is slow [3] and bioavailability of
PAHs in contaminated soil is limited [4, 5]. Therefore
these compounds have serious attention to regulatory
levels. PAHs usually occur in the subsurface as local
zones of residual zones Concentration or occasionally
exist as Monomer products [6]. Because of PAHs in nature of low solubility in fluid, and exist in dense nonaqueous phase liquids persist in aquifers, especially provide a stable supply source of contaminants [7]. Chemical oxidation of PAH employed in studies included
ozone (O3), In Fenton process, hydroxyl radicals are
generated by the reaction of hydrogen peroxide and ferrous ions to oxidize organic compounds. When using the
Fenton’s reagent for the treatment of contaminated soil
and groundwater, reaction pH needs to be controlled between from 2 to 4. Therefore, this technology is difficult
to apply in the field of in-situ pre-treatment [8, 9].1 The
chemical oxidation of toxic and hazardous organic pollutants such as PAHs is often carried out by using single
oxidants such as chlorine, ozone. However, sometimes
this decomposition may be difficult if these pollutants
are present at high concentrations, or if they are especially refractory to the oxidants [10].
Sodium Percarbonate has an active available oxygen
content which is equivalent to 27.5% H2O2. It breaks
down to oxygen, water and sodium carbonate upon decomposition. Sodium Percarbonate offers many of the
same functional benefits as liquid hydrogen peroxide. It
dissolves into water to release oxygen. In comparison to
Fenton’s
reagent,
Sodium
Percarbonate
(2Na2CO3·3H2O2) can effectively oxidize many water
impurities, and is also effective over a wide pH range.
Reaction of 2Na2CO3·3H2O2 with organic compounds
produces Carbon dioxide, water and a small amount of
sodium carbonate, which is naturally present in soils.
Therefore, the production of Carbon dioxide, water and a
small amount of sodium carbonate is not of environmental concern. In addition, 2Na2CO3·3H2O2 is non-toxic to
microbes, making it compatible with bioremediation, so
no hazards environmental due to it breaks down to oxygen, water and sodium carbonate (soda ash) in environmental[6,10]. There are so many studies focusing on
PAHs oxidation, but no literature is available reporting
on the optimum dosage of 2Na2CO3·3H2O2. Therefore,
this study was to investigate he effects of the molar ratio
Aid program by the Education Office of Heilongjiang Province science and
1*Author for correspondence.
technology research project funding, project number: 12521573.
and pH on PAHs oxidation in aqueous phase. In addition,
the extent of dechlorination is also reported.
2. Analysis
2.1. Analysis of samples
Aqueous solution of PAHs (7.0 mL) was extracted with
chloroform twice (30mL each times), and the above layers were collected, dried with Na2SO4, and evaporated to
1.0 mL for the determination of the remains of PAHs
with a flame ionization detector (FID) and HP-6890 column, was used to determine PAH concentrations. For the
analysis of PAHs in the extracts, a HP 6890 gas chromatograph equipped with a flame ionization detector and
a capillary column. The carrier gas flow rate in the GCECD was 1.5mL min-1. The dried temperature was about
200℃. The injection volume of extract was 2µL. Chloride was analyzed by using a DIONEX ion chromatograph (IC), equipped with a 4-mm The IonPac AS14A
anion-exchange column.
2.2. Recovery of PCBs
Sampled 7.0 mL PCBs solution (200µg·L-1) were pretreated by the above described method, then detected by
GC-FID and calculated the recovery of PAHs. Results
showed that the recovery of PAHs was stabilized at the
level of 90%-95%.
2.3. Methods
In headspace GC analysis, 25.0 mL of reaction mixture
and 1.0 mL of PeCl (as internal standard for CK) saturated solution were filled into a 43.0 mL vial, which was
then immediately sealed with a Thermo Scientific
Mininert push-button valve. In turn, the sample filled
vials were put into a water-bath with the temperature
controlled at 70℃ for 30 mins, allowing for complete
volatilization of PAHs and PeCl. Following this, a volume of 500 µL of head-space gas mixture was withdrawn for GC analysis. The chloride was measured by IC
analysis, and samples were filtered through 0.45µm glass
fiber filters to remove the sodium carbonate micro particles [6, 8].
3. Results and Discussion
In general, effect of pH Peroxide can oxidize PAHs under acidic and alkaline conditions, but the oxidation efficiency is higher at lower pH value. Fig.1 depicts the effect of initial pH on the oxidation of PAH.
Under the experimental conditions of [PAHs] =1×103M, [2Na2CO3·3H2O2]=2×10-3 M and initial pH = 5.0,
6.0, 7.0, 8.0 and 9.0, there was no significant difference
for the oxidation ratios of PAHs. Yan et.al [8] have reported that pH did not affect PAHs oxidation by Peroxide, but change the species and concentration of oxidation intermediates. They also found that 90% of carbon
dioxide was produced at acidic pH, and only about 60%
was detected under alkaline conditions. In this study,
although the initial pHs were 5.0, 6.0, 7.0, 8.0 and 9.0,
after 60 mins of reaction time, the final pH of the reaction mixtures were below 2.8 (Table 1). There is no significant difference in PAHs oxidation at different initial
pH value.
Table 1-Variation of pH with time of PAH oxidation range.
Time(mins)
5
10
20
30
60
5.00
8.8
8.7
8.7
8.7
8.7
6.0
8.8
8.8
8.7
8.7
8.7
pH
7.00
8.8
8.8
8.7
8.7
8.7
8.0
8.8
8.8
8.8
8.7
8.7
9.00
8.8
8.8
8.8
8.7
8.7
3.1 Effect of [PAHs]/[2Na2CO3·3H2O2] reaction ratio
In these experiments, the initial PAHs concentration was
kept
at
1.0×10-3M.
The
ratios
of
[PAHs]/[2Na2CO3·3H2O2] were varied by changing the
concentration of Peroxide. Fig.2 shows the effect of
[PAHs]/[2Na2CO3·3H2O2] ratio on the oxidation of
PAHs. When the ratios of [PAHs]/[2Na2CO3·3H2O2]
were 2.0, 1.0, 0.5 and 0.25, then 70%, 45 %, 20% and
5%PAHs, respectively, were remaining in the reaction
mixtures.
1.
2
1
Co1e2tion C
pH=
6
pH=
8
pH=1
0
0.
8
0.
9
0.
6
0.
3
0
0
2
0
30
4
0
5
0
6
0
7
0
8
0
Fig. 1 Effect of initial pH on the oxidation of PAH.
[PAHs]= 1×10-3M，2Na2CO3·3H2O2] =2×10-3 M).
Fig.2 shows the results with 6×10-4M initial PAH concentration, and the ratios of [PAH]／ [2Na2CO3·3H2O2]
were also 2.0, 1.0, 0.5 and 0.25. The remaining percentages of PAHs after 60 mins of reaction were 70 %, 50 %,
20 % and 5 %. By comparing Fig. 2 with Fig. 3, it can be
illustrate that the oxidation of PAH depended on the ratio
of [PAHs]/[2Na2CO3·3H2O2] under the experimental
conditions. The complete oxidation of PAHs occurred at
ratios of [PAH]／[2Na2CO3·3H2O2] greater than 0.25. In
other words, the concentration of sodium percarbonate
has to be four times higher than that of PAHs. According
to one mole of PAH needs 2 moles of sodium percarbonate is complete oxidation. However, from the results
obtained in Figs. 2 and 3, only 50-60 % of PAHs was
oxidized, as shown in Eq-(1). It is assumed that the oxidation intermediates competed with PAHs in capturing
1.
5
1
Sodium Percarbonate
the sodium percarbonate molecules, leading to the inhibition of PAHs oxidation. The removal ratio of PAHs was
about 95%, if the reaction was extended to 60 mins. In
this way, most of the PAHs molecules were decomposed
initially. With increasing reaction time, the concentration
of oxidation intermediates increased gradually, which
may compete with PAHs in capturing 2Na2CO3·3H2O2.
CO2 is generated from the reaction of PAHs oxidation,
and H+ is then produced, leading to the reduction of pH.
Peroxide can also react with water molecules to release
OH-, but this is very slow, compared with the amount of
H+ produced in PAHs oxidation. That is why the initial
pH has no significant effect on TCE oxidation in this
study.
—◆— 4.0×10-4M
-3
—■— 2.0×10
M
—▲— 3.0×10-3M
—○— 1.0×10-3M
0.6
0
0.4
5
0.3
0
0.1
5
0
0
1
0
2
0
Time
(mins)
3
0
4
0
5
0
Fig.2 Effect of [2Na2CO3·3H2O2] on the oxidation of PAHs
at [PAHs].
TABLE 2 – The Relationship between initial [2Na2CO3·3H2O2] and PAH oxidation rate.
[PAH]
(×10-4M)
10
10
10
10
8.0
8.0
8.0
8.0
[2Na2CO3·3H2O2]
(×10-4M)
40
20
10
5.0
32
16
8.0
4.0
Initial rate
(×10-5 M/ min)
14.3
10.5
7.22
3.62
11.4
7.33
4.64
1.22
[PAH]
[2Na2CO3·3H2O2]
Initial rate
(×10-4M)
(×10-4M)
(×10-5M/ min)
6.0
24
8.05
6.0
12
4.37
6.0
6.0
3.37
6.0
3.0
1.85
4.0
16
4.18
4.0
8.0
2.30
4.0
4.0
2.34
4.0
2.0
1.94
released into the reaction mixture; 20 % of chlorine still
Effect of sodium percarbonate on the initial oxidation
remained in the organics. Yan et.al[8]. reported on comrate In Table 2, the initial rates of PAHs oxidation at
plete dechlorination of PAH at a [PAH] ／
different molar ratios of [PAHs]/[2Na2CO3·3H2O2],
[2Na2CO3·3H2O2] ratio of 0.06. However, only 0.25 was
namely 2.0, 1.0, 0.5 and 0.25, are listed. In order to exapplied in the present study. Therefore, it is necessary to
plore the effect of [2Na2CO3·3H2O2] on the reaction rate,
add more sodium percarbonate for intermediate oxidathese initial rates of PAHs oxidation were calculated.
tion.
Fig.4 illustrates the relationship of initial oxidation rate
CONCLUSION
and initial [2Na2CO3·3H2O2]. Results show that the iniExperiments of PAHs oxidation by Sodium Percarbonate
tial rate increased with increasing [2Na2CO3·3H2O2] and
were carried out in this study. The initial rate of PAHs
[PAHs]. As shown in Fig. 2, 70 % of PAHs was removed
oxidation almost increased linearly with increasing the
in the first 5 mins of reaction time, when the concentrainitial concentration of PAHs. The factors, pH and ratio
tion of peroxide was 4.0×10-3 M. The removal ratio of
PAHs was about 95%, if the reaction was extended to 60
of [PAHs]/[2Na2CO3·3H2O2], affecting the reaction have
mins. In this way, most of the PAHs molecules were
been explored. Even the theoretical dosage of
decomposed initially. With increasing reaction time, the
[PAH]/[2Na2CO3·3H2O2] is 0.5 for complete degradation
concentration of oxidation intermediates increased gradof PAHs, it still needed, at least, 4.0 moles of
ually, which may compete with PAHs in capturing
2Na2CO3·3H2O2 to decompose this chemical. This indi2Na2CO3·3H2O2. De-chlorination of PAHs In general,
the toxicity of organic chemicals increases with their
cates that oxidation intermediates and the parent PAHs
number of chlorine atoms. Therefore, removing the chlocompeted with Sodium percarbonate molecules. Initial
rine from the chemical structure can initiate the reduction
pH has no significant effect on the [PAHs] oxidation,
of toxicity. In this study, chloride ions were detected
because hydrogen ions were released during the period
during the oxidation reaction. Theoretically, 1.0M of
PAHs, at most, releases three moles of chloride ions. In
of oxidation reaction.
this case, the ratio of [Cl-]/3[PAHs] was applied to eval4. Acknowledgment
uate the dechlorination reaction. When 95 % of PAHs
was decomposed, only 80 % of chloride ions were
This work has been supported by the Education Office of
Heilongjiang Province science and technology research
project funding, project number: 12521573.
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