Decomposition of brominated organic compounds in environment water by y - Ray irradiation

648 Journal of Chemistry, Vol. 44 (5), P. 648 - 653, 2006 DECOMPOSITION OF BROMINATED ORGANIC COMPOUNDS IN EnVIRONMENT WATER BY
-RAY IRRADIATION Received 4 October 2005 Hoang Hoa Mai1, M. Taguchi2 and T. Kojima2 1Institute for Nuclear Science Technique -Vietnam Atomic Energy Commission 2Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Summary In this work decomposition of the toxic brominated compound, namely 2,4,6-tribromophenol (TBP) in water, using
-irradiation method was investigated. The TBP in aqueous solution was effectively decomposed by
-rays. Irradiation up to a dose of 300 Gy of
-rays can absolutely decompose a concentration of 25 µM of TBP in water. OH radicals and hydrated electrons produced by radiolysis of water are main reactive species taking part in the decomposition of the TBP. The chemical-radiation yield of decomposition depends on the concentration of TBP in solution. The presence of acetone or alcohol in solution plays as scavengers of OH radicals and solvated-electrons, resulting in decrease of the decomposition yield of TBP. The present of oxygen is not much effect to the efficiency of decomposition of the parent chemical. Decomposition yield of TBP depends on the concentration of TBP and pH of water. However, in the range of pH from 5 - 9 the decomposition yield get the maximum value and it decreased when pH less than 4 or bigger than 9. Keywords: gamma radiation, radiation decomposition, dose, toxic brominated organic compounds, endocrine instructor, 2,4,6-treibromophenol, OH radical, solvated electron. I - Introduction Brominated compounds have received much attention recently because their toxic effects, including teratogenicity, carcinogenicity and neutrotoxicity, have been observed, in particular the brominated diphenyl ethers (Darnerud et al., 2001; Linda S. et al. 2003; Peter Alexander et al. 2003, Juliette Legler et al. 2003). The environmental behavior and fate of many brominated compounds are thought to be similar to those of chlorinated pollutants in the environment. These chemicals are stable in the environment and easy to be accumulated in humans and wildlife. The evidence shows that brominated diphenyl ethers are widely found in birds and bird’s eggs, fishes, and the marine mammals such as whales and dolphins, frogs, and mussels. The increased levels of polybrominated compounds have also been found in human samples including breast milk and blood. This evidence indicates that brominated congeners have a potential as endocrine disruptors similar to their chlorinated relatives. The existing methods for water treatment are not sufficient to remove the activity of a trace amount of these chemicals. Irradiation with high energy radiation such as
-rays, X rays or electron beam have been introduced as an alternative method for effective decomposition of the chemicals in water. This work aims to study on the use of
-irradiation to decompose the 2,4,6-tribromophenol in water 649 and find the possibility of application radiation for treatment of brominated compounds in environmental water. The efficiency of decomposition of the parent chemical in aqueous solutions saturated with different gas or oxygen were investigated. The effects of the radical scavengers such as acetone or alcohol on the decomposition yield of the parent chemical have been studied. The dependence of chemical- radiation yield of decomposition upon the pH of environment also estimated. II - Experimental 1. Sample preparation and irradiation 2,4,6-tribromphenol (TBP) with high purity (99.99%) supplied by Tokyo Chemical Industry (TCI) were used without any further treatment for preparation of model pollutant samples released into water. The molecular structures of these chemicals are shown in the figure 1. The sample solutions containing TBP were prepared using ultra-pure water supplied from Milli-pore Milli-Q system. The solubility of parent chemical in water was determined approximate to 25 µM of TBP. High purity chemicals (99.99%) such as 2-bromophenol, 4- bromophenol, 2,4-dibromophenol and 2,6- dibromophenol and 4-phenoxyphenol supplied by TCI, diphenyl ether, 4,4’-dihydroxybiphenyl supplied by Wako Pure Chemical Industries were used as reference to identify the transient products in the decomposition of the parent chemicals. Methanol and acetonitrile with analysis purity were used as eluent of organic solvents for HPLC measurements. Solutions were irradiated by
-rays using 60Co-source facility of which the radioactivity of the sources to be 4 TBq. Dose rate at irradiation positions is estimated to be 150 Gy/h. Solution samples of TBP were irradiated to different doses in the range of 0 Gy to 300 Gy. 2. Analysis - HPLC (Agilent 1100 series) with a reverse phase column (RS pak-DE 613 Shodex) was used with a temperature of column oven set at 40oC. The flow rate of eluent was 1.000 mL/min and injection volume of sample solutions was selected to be 200 µL. The UV-VIS absorbance detectors (Waters 2487 Dual  absorbance detectors) were used for measurement of concentration of the chemicals extracted from the column. Eluent compositions were set as 80% methanol + 20% H2O. - Bromide ions released from radiation induced decomposition of parent brominated compounds in sample solutions were determined using the ion chromatograph system (Metrohm 761, Compact – IC) with an anion suppressor. Sample solutions were separated through a column (IC SI-90 4E Shodex). III - Results and Discussion 1. Decomposition of TBP in water by
-rays irradiation Figure 1 shows the HPLC chromatogram of unirradiated and irradiated by
-rays irradiation of TBP aqueous samples. The peaks of the parent chemical were observed at retention time of 24 min. Area of peaks decreased upon the dose. Some new peaks of transient products were observed but they quickly disappeared at higher doses. The main transient products produced from radiation-induced decomposition of TBP were identified as 2,4-dibromophenol, 2,6-dibromophenol and 4-bromophenol. From the decomposition of TBP, 2,6-dibromophenol and 2,4-dibromophenol appeared as the peaks at retention time of 16.41 min and 17.907 min, respectively. The transient products formed from the decomposition of parent TBP are also degraded by the radiation-induced reactive species in water. Figure 2 shows the dependence of concentration of TBP in solutions upon absorbed dose of
-rays. TBP was effectively decomposed by
-rays irradiation resulting in the exponential decrease of concentration with absorbed dose. Bromide ion was found as one of the final stable products of radiation-induced decomposition of TBP in water. The ion chromatogram presented figure 3a for irradiated sample. Bromide ion develops a peak at retention time of 5.94 min. Figure 3b shows the dependence of concentration of bromide ions released in solution upon the dose. 650 0 1 2 3 4 5 6 0 20 40 60 80 100 120 140 160 180 Dose, Gy C on ce nt ra ti on of T B P, uM Figure 1: Decomposition of TBP in water under gamma irradiation viewed through the HPLC –chromatogram Figure 2: Concentration of TBP in solution decreases upon absorbed dose of
-rays The amount of bromide ions in solution sample increased linearly with absorbed dose in the beginning of irradiation and subsequently slowly increased and became sublinear at the higher dose before getting saturated at the final stage of irradiation. The saturated value of bromide ion concentration was decided by initial concentration of parent brominated compounds in solution. When absorbed dose reaches to a high value at which almost parent molecules were completely degraded and all bromide ions were released from the parent molecules so that the concentration of bromide ions in solution reached to the maximum value and saturated. 2. Role of hydroxyl radicals (OH·) and hydrated electrons (e-aq) in decomposition of TBP Water molecules are degradated under irradiation and formed reactive species including OH radical, hydrogen atom, hydrated electron, proton and hydroxide ions: H2O  OH·, H, e-aq, H3O+, OH-, H2O2, H2 (1) Among these species OH radicals and hydrated electrons are the most reactive species and play an important role in decomposition of organic chemicals in water. In case of TBP both OH radical and e-aq should take part in decomposition reactions. The contribution of these species to the decomposition efficiency of TBP under
-ray irradiation was studied by using acetone and alcohols as scavengers of OH radical and hydrated electron respectively. Figures 4a and 4b present the dependence of decomposition yields of TBP upon the concentration of ethanol and acetone added into solutions. The radiation decomposition yields of Vo lts -0.008 -0.006 -0.004 -0.002 0.000 0.002 T B P 2,4B Ph 2,6B Ph 0 Gy 12.5 Gy 25.0 Gy 37.5 Gy 50.0 Gy 75.0 Gy 100.0 Gy 125.0 Gy 150.0 Gy 225.0 Gy 300.0 Gy 651 TBP decreased with the amount of acetone in solutions. The suppression effect of acetone on the decomposition of TBP in irradiated aqueous solution is described as scavenging reaction of acetone with hydrated electrons as follows: CH3COCH3 + eaq -  (CH3)2GCO- (2) This reaction occurs with a high rate (k = 7.7x109 Lmol-1s-1, Buxton et al. 1988) and suppresses the reaction of hydrated electrons with parent brominated compounds in solution, accordingly, the radiation yield of decomposition of TBP decreases. 0 5 10 15 20 25 0 50 100 150 200 250 300 350 Dose, Gy C on cn tr at io n of B ro m id e, 10 -6 M 3a 3b Figure 3a: Ion - Chromatogram shows present of bromide ions released in solution and 3b: Amount of bromide ions released in solution increases upon the dose 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 1000 2000 3000 4000 Concentration of acetone, uM G [- T B P] ,m ol ec ul e/ 10 0e V 4a 4b Figure 4: Both acetone and ethanol cause decrease of G-value of decomposed TRB The similar results have been also found with methanol or propanol added into solution instead of ethanol. The mechanism of reaction between OH radicals and methanol is described as follows: OH + CH3OH  CH2OH + H2O (3) (k = 9.7x108 Lmol-1s-1, Buxton et al., 1988) Scavenging reaction and ethanol can be described as follows: C2H5OH + OH  CH3GCHOH + H2O (4) (k = 1.9x109 Lmol-1s-1, Buxton et al., 1988) The rate of reaction between methanol and OH radical is much higher than that of reaction between methanol and brominated compound in solution. In such a case alcohol molecules act as scavengers and suppress the reaction of OH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 min 0.02 0.04 0.06 0.08 0.10 0.12 0.14 uS/cm Cond 3. 25 3. 60 4. 31 5. 94 7. 94 10 .8 1 12 .4 2 Br- 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 50 100 150 200 250 Concentration of ethanol, uM G [- T B P] ,m ol ec ul /1 00 eV 652 radicals with TBP, which result in the decrease of decomposition of the parent compound. 3. The effect of oxygen and different gases on the decomposition yield of TBP in water The decomposition yields of the parent brominated compound under different gas saturation conditions are listed in table 1. The highest decomposition yield was obtained in the case of the solution saturated with nitrogen oxide. Table 1: Decomposition yield of sample solutions saturated with different gases Decomposition yield (molecule/100eV) Brominated compound Air Oxygen N2O Nitrogen Helium TBP 0.32 0.29 0.59 0.45 0.36 High decomposition yield found in the case of the solution saturated with N2O can be interpreted based on the mechanism of the reaction, in which N2O reacts with hydrated electrons and produces OH radicals as described in the equation (5), in turn the OH radicals take part in the decomposition reaction with parent brominated compounds in solution. In such a case the present of nitrogen oxide in solution increases the total G-value of decomposition of the parent brominated compounds: N2O + e - aq  OH + N2 + OH- (5) with rate constant k = 9.1x109 Lmol-1s-1. In the case of oxygen saturation, oxygen may react with hydrated electrons and produces dioxide radicals O-2, that do not take part in the decomposition of parent compounds as the equation (6). These results in a part of hydrated electrons are spent for reactions that not cause any decomposition of brominated parent compounds: O2 + e - aq  O-2 (6) This argument may also explain the reason why the radiation induced decomposition yield of TBP has a smaller value in the case of sample saturated with oxygen than that in the case of solutions saturated with air, nitrogen or helium. 4. Effect of initial concentration of TBP on the radiation induced decomposition yield Aerated sample solutions with different concentrations in the range of solubility of each parent brominated compound were prepared and irradiated with
-rays to doses from 0 to 37.5 Gy for TBP. Figure 5 presents the dependence of decomposition yield of TBP. Decomposition efficiencies increase with initial concentration of parent brominated compounds in solution. The higher concentration of solutes in solution getting higher probability of OH radicals to react with the parent brominated compounds may result in increasing the decomposition yield of the compounds. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 2 4 6 8 10 12 Initial concentration, uM G [- T B P] ,M ol ec ul e/ 10 0e V Figure 5: The dependence of radiation yield, G[-TBP] upon the initial concentration of TBP 5. Effect of pH on the decomposition of TBP in aqueous solution Sample solution was prepared with concentrations of 5 µM for TBP. Hydrochloric acid (HCl) or potassium hydroxyl (NaOH) were added into sample solutions to control pH at different values in the range of pH = 2 to pH = 12. The sample solutions were irradiated with
- rays to doses from 0 to 37.5 Gy. The decomposition efficiency of TBP was estimated at each value of pH. Fig. 6 presents the dependence of decomposition yield of TBP upon 653 pH. Radiation decomposition yield of TBP keeps constant in the range of pH between 4 to 9 but decreases in the range of pH less than 3 and pH higher than 10. 0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 12 14 pH G -V al ue ,m ol ec ul e/ 10 0e V Figure 6: G-value of decomposed TBP by gamma depends on the pH of medium VI - Conclusions TBP were effectively destructed by
-rays irradiation. OH radicals and hydrated electrons produced by radiolysis of water are main reactive species taking part in the decomposition process of TBP. The radiation yields of decomposition of TBP are not much affected by concentration of oxygen dissolved in solution but depends on the initial concentration of parent compound. The main transient products produced by the decomposition of TBP were identified as 2,4- dibromophenol, 2,6-dibromophenol and 2- bromophenol. All parent molecules and transient products were completely destructed into the final simple inactivated products when radiation dose reaches adequate value. With an initial concentration of 8 µM for the TBP, the decomposition process was completed approximately at a dose of 300 Gy. In order to remove a trace of the brominated compound TBP contaminated in environment water, however, practically required a dose of several tens Gy. This study demonstrates the possibility for application of
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