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Journal of Hazardous Materials 164 (2009) 900–903
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Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat

Radiation removals of low-concentration halomethanes in drinking water
Zhaobing Guo a,∗, Zheng Zheng b, Chunhui Gu b, Dengyong Tang a
a School of Environmental Science & Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, PR China b State Key Laboratory of Pollution Control and Resource Reuse, School of Environment, Nanjing University, Nanjing 210093, PR China

article info
Article history: Received 26 March 2008 Received in revised form 24 August 2008 Accepted 26 August 2008 Available online 30 August 2008
Keywords: Gamma radiation Halomethanes Drinking water

abstract
Gamma radiation induced removals of four halomethanes, with low initial concentrations in drinking water were investigated. The results show that absorbed dose and solution pH are important factors in affecting halomethanes removals. High-absorbed dose and solution pH drive halomethanes removals. The reactions of halomethanes with e−aq play a crucial role in their removal processes. Halomethanes removal during the radiation follow a pseudo-first-order kinetics model. Gamma radiation results in a slight decrease in pH and TOC values of drinking water.
© 2008 Elsevier B.V. All rights reserved.

1. Introduction
Chlorine is widely applied to disinfection in drinking water, which inevitably produces several disinfection by-products (DBPs), such as halomethanes and haloalkenes [1]. Consumption of such drinking water is associated with increasing cancer risk of urinary and gastrointestinal tracts [2]. Many countries have formulated a strict DBPs standard in drinking water [3,4].
Some efforts have been devoted to DBPs removals from aqueous solution. It is inefficient for most DBPs removals using conventionally biological techniques. While air stripping and activated carbon adsorption may efficiently remove DBPs from aqueous solution, this is only a pollutant transfer process without ultimately solving DBPs pollution problem. Ozone oxidation of DBPs in drinking water is environmentally sound, but money consuming [5]. Recently, ultrasound has been proposed to decompose halomethane mixtures in drinking water, but their removal efficiencies need to be further improved [6]. Therefore, an alternative technique is required for treating low-concentration halomethanes in drinking water.
There have been a few studies on radiolytic decomposition of organic pollutants in aqueous solution [7–10]. In particular, some investigations are involved in radiolytic destruction of THMs from water [11–17]. Unfortunately, the majorities of these studies are not of representative significance because they
∗ Corresponding author. Tel.: +86 25 58731090; fax: +86 25 58731090. E-mail address: guozbnuist@163.com (Z. Guo).
0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.08.082

failed to mirror actual compositions of chlorinated drinking water, which typically consists of DBPs mixtures with a low concentration. In this study, therefore, gamma radiation induced degradation of low-concentration halomethanes in drinking water (pH = 7.13, NO3− = 1.3 mg/L, Cl− = 36.1 mg/L) was carried out. The target compounds are involved in carbon tetrachloride (CCl4), chloroform (CHCl3), bromodichloromethane (CHBrCl2) and dibromochloromethane (CHBr2Cl) with an initial concentration of 10.4, 15.8, 3.2 and 4.7 ␮g/L, respectively. We evaluate the possibility of gamma radiation in drinking water treatment under different absorbed doses and solution pH values.
2. Materials and methods
2.1. Materials
Drinking water was collected from Science Building in Nanjing University. Chromatographic grade hexane is a MEDIA product and standard sample of Halogenated Volatiles Mix 551A was purchased from Supelco. pH value of drinking water was adjusted using diluted HCl and NaOH solution.
2.2. Radiation process
Radiation was conducted by a 60Co source (1.85 × 1016 Bq). Samples (25 mL each) were conserved in 50 mL airtight glass vessels, which were placed in the radiation field to a specific distance from the source to achieve the desired series of absorbed doses.

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Absorbed doses were determined by silver dichromate dosimeter [18].
2.3. Analysis
Four halomethanes in drinking water were extracted with hexane. Concentrations of halomethanes were determined using gas chromatograph (GC, Agilent 6890) equipped with electron capture detector (ECD). Carrier gas: helium; HP-5 capillary column: 28.5 m × 0.45 mm × 0.42 ␮m; GC oven temperature is held constant at 35 ◦C for the first 4 min, then increases up to 220 ◦C with a rate of 5 ◦C/min [6].
Digital pH monitor (JENCO Medel6171) and TOC measuring instrument (Shimadzu-TOC 5000) were used to analyze water qualities before and after gamma radiation.
All the experiments in this study were carried out in duplicate; the averages were calculated to describe the removals of four halomethanes in drinking water.
3. Results and discussion
3.1. Radiation removal of four halomethanes
3.1.1. Removal efficiency Gamma radiation of four halomethanes in drinking water was
conducted at absorbed doses of 0.5, 1.0,1.5, 2.0 and 3.0 kGy, respectively. The concentration variations of four halomethanes with increasing absorbed doses at pH 7.13 are compared in Fig. 1. It can be observed that the initial concentrations of CHCl3, CCl4, CHBrCl2, and CHBr2Cl in drinking water are 15.8 10.4, 3.2 and 4.7 ␮g/L, respectively. At absorbed dose of 3.0 kGy, the residual concentrations of CHCl3, CCl4, CHBrCl2 and CHBr2Cl decreased to 7.3, 0.8, 0.1, and 0.15 ␮g/L, about 53.8% CHCl3, 92.3% CCl4, 96.9% CHBrCl2 and 96.8% CHBr2Cl were removed, respectively. It is indicative of an effective method to remove low-concentration halomethane mixtures from drinking water by using gamma radiation. Besides, we studied the radiation removals of four halomethanes with the same initial concentrations (20 ␮g/L) in an aqueous solution, and found that the removal percentages of CHCl3, CCl4, CHBrCl2 and CHBr2Cl at 3.0 kGy were 51.3%, 89.7%, 92.6% and 92.9%, respectively. This increasing order is generally in agreement with that of four halomethanes in drinking water, although the increase in initial concentrations of four halomethanes decreases their removal percentages to some degree.

Table 1 G values of four halomethanes removals under different absorbed doses (×10−5 molecules/(100 eV))

Absorbed dose (kGy)
0.5 1.0 1.5 2.0 3.0

CHCl3
38.8 35.5 28.0 26.2 22.9

CCl4
42.6 39.5 29.7 25.1 20

CHCl2 Br
15.3 14.1 10.6
8.8 6.1

CHClBr2
11.1 10.6 10.5
9.9 7.0

In diluted solution, gamma radiation of water produces some active species, such as solvated electrons e−aq, hydrogen atoms H· and hydroxyl radicals ·OH, which can be described in formula (1)
(numbers in the brackets present the amount of produced radi-
cals/100 eV energy) [19]. According to reaction rate constants of
halomethanes with the three active species, it can be speculated that reactions between individual halomethanes and e−aq play a significant role in halomethanes removals [20].

H2O → ea−q(2.6) + H(0.55) + HO(2.7)

+H2(0.45) + H2O2(0.71) + H3O+(2.6)

(1)

It is noteworthy from Fig. 1 that residual concentrations of four halomethanes decrease with increasing absorbed doses. A measurement of single halomethane removal efficiency during gamma radiation is generally described by G value. G value can be calculated by the following equation [21]:

G

=

( R)(NA) (D)(6.24 × 1019)

(2)

where R is the amount of reduced pollutants (mol/L); NA is Avogadro constant, 6.02 × 1023 (molecules/mol); D is radiation dose (10−2 kGy); 6.24 × 1019 is conversion constant from kGy to 100 eV/L (100 eV/(L kGy)); G is specific removal efficiency (molecules/(100 eV)).
G values of four halomethanes at different absorbed doses are shown in Table 1. It is found that removal efficiencies (G values) of four halomethanes decrease with increasing absorbed doses, which is in agreement with those reported by Mak et al. [14] and Basfar et al. [16]. The decrease in G value possibly results from intermediate reaction by-products and their competitions with parent compounds for the active species at high-absorbed dose.

3.1.2. Removal kinetics Based on the plots of ln(RD/R0) versus absorbed dose compiled in
Fig. 2, it can be inferred that removals of four halomethanes follow

Fig. 1. Removals of four halomethanes under different absorbed doses. pH 7.13; ( ) CHCl3; ( ) CCl4; ( ) CHBr2Cl; and (♦) CHBrCl2.

Fig. 2. Determination of degradation kinetics of four halomethanes during gamma irradiation. pH 7.13; ( ) CHCl3; ( ) CCl4; ( ) CHBr2Cl; and (♦) CHBrCl2.

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Table 2 G values of four halomethanes removals under different solution pH (×10−5 molecules/(100 eV))

pH
4.61 6.55 7.13 8.32 9.69

CHCl3
17.2 20.7 22.9 24.2 25.6

CCl4
15.7 18.9 20 20.3 20.5

CHCl2 Br
5.5 5.9 6.1 6.1 6.2

CHClBr2
6.3 6.9 7.0 7.1 7.1

Fig. 3. Removals of four halomethanes under different solution pH values. absorbed dose: 3.0 kGy; ( ) CHCl3; ( ) CCl4; ( ) CHBr2Cl; and (♦) CHBrCl2.

a pseudo-first-order kinetic model with respect to absorbed dose, which can be described by the following equation [22]:

RD = R0e−kD

(3)

where RD is residual concentration of halomethanes at different absorbed doses (␮g/L); D is absorbed dose (kGy); R0 is initial concentration of halomethanes (␮g/L); and k is rate constant (1/kGy).
Rate constants (k) of CHCl3, CCl4, CHBrCl2 and CHBr2Cl removals are 0.25, 0.82, 1.21 and 1.22 kGy−1, respectively, and their corre-
sponding correlation coefficients are above 0.97. CHCl3 shows the slowest removal rate constant, followed by an increasing order
of CCl4, CHBrCl2 and CHBr2Cl. Removal rate constant of CHBrCl2 is comparable to that of CHBr2Cl in drinking water. This order is exactly consistent with that of reaction rate constants between single halomethane and e−aq, demonstrating the importance of e−aq in removing low-concentration halomethane mixtures during gamma
radiation. Besides, removal rate constants of single halomethane
are also in dependence on bonds stability. Based on the fact that
bond dissociation energy between carbon and bromine atoms
(293 kJ/mol) is much smaller compared to those between carbon
and chlorine atoms (351 kJ/mol) and carbon and hydrogen atoms
(413 kJ/mol), rate constants of four halomethanes removal should
follow an increasing order of CHCl3 < CCl4 < CHBrCl2 < CHBr2Cl. Our experimental result is nicely in agreement with this
order.

3.2. Effect of pH on removal of four halomethanes
As Fig. 3 shows, four halomethanes removals are associated with solution pH during gamma radiation. Removals of four halomethanes increase with increasing pH values at absorbed dose of 3.0 kGy. However, a distinct increase in CHBr2Cl and CHBrCl2 removals is not observed, which is probably attributed to their inclusive C–Br bonds and high removal percentages under different pH values. Similar to removal percentages, G values and pseudofirst-order rate constants of four halomethanes become higher at high pH compared to those at low pH (Tables 2 and 3).
Removal efficiencies of pollutants during gamma radiation are in dependence on the kind of active species [23]. The predominant species from water radiolysis vary with solution pH. In acidic solution, e−aq is likely to react with H3O+ to generate H· (k = 2.3 × 1010 L/(mol s)), thereby decreasing the concentration of e−aq to react with halomethanes. In alkaline solution, HO· easily reacts with HO− (k = 1.3 × 1010 L/(mol s)), thereby decreasing the

Table 3 Rate constants (k) of four halomethanes removals under different solution pH (1/kGy)

pH
4.61 6.55 7.13 8.32 9.69

CHCl3
0.18 0.22 0.25 0.27 0.30

CCl4
0.45 0.64 0.82 0.85 0.95

CHCl2 Br
0.73 1.07 1.21 1.24 1.31

CHClBr2
0.71 1.11 1.22 1.28 1.33

recombination probability between HO· and e−aq. The decreased removal percentage at low pH and the increased removal percentage at high pH of four halomethanes further demonstrate the importance of e−aq in their degradation during gamma radiation.
3.3. Variation of solution pH and TOC during the radiation
Fig. 4 describes pH values of drinking water before and after gamma radiation. It is noteworthy that solution pH decreases during gamma radiation. Higher absorbed dose results in a more distinct decrease in pH value. pH values decrease from 7.13 to 7.02, 6.91, 6.88, 6.82 and 6.70 at absorbed doses of 0.5, 1.0, 1.5, 2.0 and 3.0 kGy. The decrease of solution pH values is most possibly the result of formation of organic acids during solution radiolysis. pH values of drinking water after gamma radiation are still at the permitted range of drinking water (6.5–8.5) stipulated by China. This indicates that it is feasible to apply gamma radiation to remove halomethanes in drinking water in our study.
TOC values of drinking water are approximate two orders of magnitude larger than carbon concentrations from four halomethanes (Fig. 5), which indicates the existence of other organic matters in drinking water. TOC value in drinking water gradually decreases during gamma radiation, suggesting that gamma radiation leads to both removals of halomethanes and partial mineralization of other organic matters in drinking water.

Fig. 4. pH value variations of drinking water before and after gamma irradiation.

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project on Natural Science Foundation Research of Jiangsu Colleges (No. 07KJD610135). We also thank the editor and the anonymous reviewers for improvements to original manuscript.

Fig. 5. TOC value cariations of drinking water before and after gamma irradiation, pH 7.13.
3.4. Comparison of halomethanes removals using gamma ray and ultrasonic wave
Based on our investigations on the removals of four halomethanes in drinking water using gamma radiation and ultrasonic irradiation [6], we find that these two techniques are feasible for removing low-concentration halomethanes in drinking water. Gamma radiation induced removals of halomethanes in drinking water seem to be more effective than ultrasonic irradiation in our studies. This provides an insight into the removals of trace organic pollutants in aqueous solution using gamma radiation. The removals of halomethanes in drinking water by gamma ray and ultrasonic wave are both associated with molecular polarities. In addition, gamma radiation removals of halomethanes are mainly originated from the reductive reactions of e−aq. However, ultrasonic degradation of halomethanes in drinking water is related to the pyrolysis and ·OH radical oxidation.
4. Conclusion
Gamma radiation is proved to be an effective method to remove low-concentration halomethanes from drinking water. Gamma radiation induced reactions of four halomethanes are chiefly controlled by e−aq and follow a pseudo-first-order kinetic model. A slight decrease in pH value of drinking water demonstrates that gamma radiation is achievable in the treatment of halomethanes from chlorinated drinking water.
Acknowledgements
We gratefully acknowledge financial supports from State Key Laboratory of Pollution Control and Resource Reuse (PCRRF07010), Jiangsu Provincial Key Laboratory of Coastal Wetland Bioresources and Environmental Protection (JLCBE07004), research fund of Nanjing University of Information Science & Technology (QD54) and

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