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55Section 6. High energy chemistry
Section 6. High energy chemistry
Iskenderova Zenfira Iskender, E-mail: [email protected] Guliyeva Ulviyye Aydin, Mammadov Sahib Giyas, Gurbanov Muslum Ahmed, E-mail: [email protected] Abdullayev Elshad Tofiq, ANAS, Institute of Radiation Problems, Azerbaijan
Radiolysis of aerated formic acid solution
Abstract: Co 60 gamma ray radiolysis of aqueous formic acid solution (1x10-2M) was investigated at the dose up to 80 kGy. It has been established that, the value of pH goes up from 2.5 to 4.0 and approximately 90% of formic acid and its derivatives are degraded at the dose of 80kGy. Dissolved O2 significantly affects the formation of CO2 Formation of CO2 occurs at a higher rate at the initial stage until the dissolved O2 is fully consumed. The kinetic model of formic acid degradation in aqueous solution under the gamma irradiation was tested. The suggested mechanism complies with the experimental data both of our own and of that reported earlier.
Keywords: Formic acid, y- irradiation, kinetic model, radiation-chemical yield.
Introduction
Formic acid is one of the proposed intermediate products in the multistage phenol degradation into CO2 under the ionizing irradiation of aqueous solution [1]. Radiolysis of aqueous solutions of formic acid has been previously investigated [2, 3], but they cover either the initial or late stages of formic acid degradation. However, practically, there is no systematic investigation covering wide range of absorbed irradiation dose, which makes it difficult for proper understanding the mechanism of irradiation induced degradation of aqueous formic acid solution and consequently the mechanism of complete phenol degradation into CO2 and H2O.
In the current study the radiolysis of aerated aqueous solution offormic acid has been investigated at the wide range absorbed gamma irradiation dose covering its initial conversion as well as the final stage of deep oxidation.
Experimental
Aerated ([O2]=2.7x10-4M) aqueous formic acid solution (1x10-2 M) was irradiated by Co60 gamma rays at the static condition in sealed glass vials at room temperature. The dose rate was 0.22Gy/sec which was determined by ferrous sulfate dosimeter, pH with pH meter PHS-25, potassium permanganate (KMnO4) was used for measuring chemical oxygen demand (COD), hydrogen peroxide by method [4], ultraviolet spectrum of products was taken by spectrophotometer Varian-Carry-50 at the wavelength (\) range of 200-800nm. CO2 and H2 were analyzed by a gas chromatography.
Results and discussion
The optical absorption spectra of aqueous formic acid solution both before and after the irradiation with different dose are presented in Figure 1.
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Radiolysis of aerated formic acid solution
Fig.1. The optical absorption spectra of aqueous formic acid solution (1x10-2M) both before and after the irradiation with different dose: 0-initial; 1-2kGy; 2-6kGy; 3-13kGy; 4-27kGy, 5-40kGy; 6-80kGy.
According to Fig.1 optical absorbance increases within the dose range 2 to 27kGy and then decreases at the higher doses. It is suggested that the increase of optical absorption is due to the generation of oxalic acid which has higher extinction coefficient rather
than formic acid. At the higher irradiation doses the degradation of oxalic acid takes place as well. Apart from that the beginning of the absorption band shifts toward the low wave length (from 238nm to 226nm) as the irradiation dose increases (Table 1).
Table 1. - The shift of the beginning of the absorption as a function of irradiation dose
D (xGy) 2 6 13 27 40 80
\(nm) 238 236 232 230 228 226
The irradiation induced degradation of aerated well as increases pH and decreases COD. The results formic acid solution leads to formation of CO2 as of pH changes are shown in Figure 2.
Fig. 2. Variation of pH with the irradiation dose in the radiolysis of aerated formic acid solution (1x10-2M)
As seen from the fig.2 with the increasing of irradiation dose the value of pH goes up from 2.5 to 4.0. The value of pH=4, most probably, is due to significant degradation of formic acid and its derivatives and also formation of carbonic acid. It is reported that formation of carbonic acid increases
at the pH 4.0 and higher [8].
Figure 3 presents the formation of H2O2 in the radiolysis of aerated formic acid solution. Accordingly, the concentration of H2O2 goes up to the maximum value of 2x10-4 M at 1kGy with the irradiation dose, then goes down.
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Section 6. High energy chemistry
Fig. 3. Formation of H2O2 with the irradiation dose in the radiolysis of aerated formic acid solution (1x10-2M)
The radiation yield ofH2O2 calculated at the initial stage of its formation is equal to 3,1 molec/100ev and in line with the previous reported value of 3,3molec/100ev [0]. This value is higher than the yield of H2O2 in the radiolysis of pure water (0.7-
0.8), indicating that there is an additional channel for the formation of H2O2 in the radiolysis of aerated formic acid solution.
Chemical oxygen demand (COD) was measured at the different irradiation doses which shows the decrease of the COD from ~164 to 14 (mg O/L) at 80 kGy. Accordingly, irradiation of aerated formic acid solution leads to decreasing of total organic acids (formic acid and its derivatives). COD was recalculated into the unit of molar concentration using the following formula:
COD= (C/Fw)x(RWO)x(32) where C is a concentration of oxydizable compound in the sample; Fw — formula weight of the oxydizable compound in the sample; and RWO is a ratio of the mole numbers of oxygen to
number of moles oxydizable compound in their reaction to CO2 and H2O. Results are shown in the Fig.4. Approximately 90% of formic acid and its derivatives were degraded at the dose of 80kGy.
The kinetics of CO2 formation is
characterized induction period of increase up to dose of approximately 10 kGy and the observed (experimentally detected) amount of CO2 at the same dose i. e. 80kGy by two times less than required by stoichiometric equation. (Fig.5a)
In order to create the model of formic acid radiolysis we applied the schema of the reactions fully describing the radiolysis of water [0]. The radiation chemical yield of water radiolysis products at pH=1-4 and their apparent rate constants were listed at the table 2. Apparent rate constants of radiolysis products Kp were calculated as Kp = G (p)xJx10-2, where G (p) is a radiation chemical yield of relevant product, J is a dose rate which is equal to 0.22kGy/sec or 2.34x10-6 ev*M*sec-1.
Table 2. - The radiation chemical yield of water radiolysis products and apparent rate constants
Water radiolysis products G, molec./100ev Apparent rate constant, sec 1
H+ 3.45 8.06x10-8
OH 2.9 6.77x10-8
e aq 3.05 7.2x10-8
H 0.6 1.4x10-8
H2 0.425 9.93x10-9
H2O2 0.8 1.87x10-8
OH- 0.4 9.3x10-9
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_______________________________________________Radiolysis of aerated formic acid solution
Table 3. - The list of reactions and rate constants of water radiolysis products [6]
Reactions K, L/M xsec
1. H+ + OH- = H2O + H2O 1.4X1011
2. H2O = H+ + OH- 2.52x10-5
3. OH + H2 = H2O + H 3.74X107
4. OH + OH = H2O2 5.3X109
5. OH + HO2 = O2 + H2O 1.4X1010
6. OH + h2o2 = HO2 + h2o 3.82X107
7. OH + HO2- = HO2 + OH- 5x109
8. OH + O2- = O2 + OH- 9.96 x109
9. H + OH = H2O 7 x109
10. OH + e = OH- 3 x1010
11 H + O2 = HO2 2 x1010
12. H + O2- = HO2- 2x1010
13. H + HO2 = H2O2 2 x1010
14. H + H = H2 7.9 x109
15. H + H2O2 = OH + H2O 3.44X107
16. e + O2 = O2- 1.94 x1010
17. e + O2- = HO2- + OH- 1.3 x1010
18. e + HO2 = HO2- 2x1010
19. e + H = H2 + OH- 2.5 x1010
20. e + H2O2 = OH + OH- 1.14 x1010
21. e + H+ = H 2.3 x1010
22. e + e = H2 + OH- + OH- 5.6 x109
23. HO2 + O2- = O2 + HO2- 9.5X107
24 HO2 + HO2 = O2 + H2O2 8.1X105
25. HO2 + H2O2 = O2 + OH + H2O 3.7
26. HO2 = H+ + O2- 7x105
27. H+ + O2- = HO2 4.5 x1010
28 H2O2 = H+ + HO2- 0.0356
29 H+ + HO2- = H2O2 2 x1010
30. H + OH- = e + H2O 3 x1010
31. O2- + O2- = O2 + HO2- + OH- 0.3
32. O2- + H2O2 = O2 + OH- + OH 16
The following preconditions were taken into account at the computer modelling of radiolysis applied to the aerated aqueous formic acid solution: Formic acid concentration=1x10-2M and concentration of dissolved O2 =2.7x10-4M.
As the radiolysis of formic acid and its derivatives were investigated widely, there are different models
to describe the full picture of radiolysis processes leading to the end products of CO2 and H2O. But two of them are significantly different from each other. One of them suggested in [5], where the reaction of oxalic acid, the derivative of formic acid radiolysis with the OH radicals leads to radical products as follows:
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Section 6. High energy chemistry
HOOCCOOH+OH=HOOCCOO +H2O (64) The other approach was in [6], where as a product for the same reaction the bond breakage of C-C bond was proposed with the direct formation of CO2 through reactions (56), (57) and (58).
For computing the model we took both options into consideration. Due to the best correlation between experiments and computed data, we took reactions
(56), (57) and (58) as a base for the modelling rather than (64). Electrolytic dissociation of both formic and oxalic acids have also been included into the model as the rate constants of OH radicals with their molecular and ionic forms are different. The following list of reactions (see: Table 4) was selected as the most appropriate to describe the experimental data both of our own and of that reported earlier.
Table 4. - The list of reactions considered at the modelling of aqueous formic acid solution radiolysis
Reactions K, L/M *sec Ref.
33.HCOOH = H+ + HCOO- 00 X 0 0\ [6]
34.H+ + HCOO- = HCOOH X o_ C [6]
35. *COOH = *COO- + H+ 00 0 г-H X ^1- [6]
36. *COO- + H+ = *COOH X o_ C [6]
37. OH + HCOOH = H2O + COOH 1.3 x108 [7]
38. OH + HCOO- = *COO- + H2O V X 0 or. [7]
39. e + HCOOH = H + HCOO- 1.4x108 [7]
40. H + HCOOH = H2 + COOH, 4.4x105 [7]
41. 2 *COO- = -OOCCOO- X 0 or [6]
42. O2 + *COOH = HO2 + CO2 9 2x109 [6]
43. H2O2 + *COOH = OH + H2O + CO2 5x10 [6]
44. 2*COOH = HOOCCOOH 4 x108 [6]
45. 2*COOH = HCOOH + CO2, X О or [6]
42a. O2 + *COO- = O2 + CO2, 9 2 x109 [6]
46. HOOCCOOH= H++ HOOCCOO- 56x10 [6]
47. H++ HOOCCOO- = HOOCCOOH X o_ C [6]
48. HOOCCOO-= H++ -OOCCOO- 5,4x105 [6]
49. H++ -OOCCOO- = HOOCCOO- X ©_ C [6]
50. e + HOOCCOOH = (HO)2CCOOH + OH- 2.5x1010 [7]
51. e + HOOCCOO- = (HO)2CCOOH + 2OH- 9 3,2x10 [7]
52. e + -OOCCOO- = (HO)2C2COOH + 3OH- 4,6x107 [7]
53. H + HOOCCOOH = (HO)2CCOOH 3.3 x105 [7]
54. H + HOOCCOO- = (HO)2CCOOH +OH- 4 1,6x10 [7]
55. H + -OOCCOO- = (HO)2C2COOH +2OH- 0 H X H [7]
56. OH + HOOCCOOH= H2O+CO2+*COO- +H+ 1.4x106 [6]
57. OH + HOOCCOO-= H2O+CO2+*COO- 1,9x10' [6]
58. OH + -OOCCOO-= OH-+CO2+*COO- 9,7x105 [6]
59. 2 (HO)2CCOOH = GA + HOOCCOOH X 0 00 [6]
glyoxylic acid GA= OCHCOOH;
60. 2 (HO)2CCOOH = DA X 0 00 [6]
dihydroxycitartaric acid DA= (C (OH)2COOH)2
61. 2 (HO)2CCOOH = HOOCCOOH! + 2HCOOH X 0 [6]
62. (HO)2C2COOH + COOH = HA > 0 H X H [6]
63. H2O2 + (HO)2CCOOH = OH + H2O + HOOCCOOH X 0 cx> [6]
Dihydroxymalonic acid=HA
Degradation of formic acid and its derivatives.
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Radiolysis of aerated formic acid solution
In the current study we have investigated COD and H2O2 in the liquid phase and CO2 in the gas phase as molecular products. Fig. 4 represents the computed values offormic, oxalic, dihydroxymalonic, dihydroxytartaric and glyoxalic acids and also the
sum of those acids (line 3) as a computed total concentration of above mentioned acids. Computed total value was compared with the experimentally defined TOC which complies with each other and with the previously reported data [3].
Fig. 4. Kinetics of degradation formic acid and its derivatives by gamma irradiation of aerated formic acid solution (1x10-2M): lines are computed (1)-formic acid; (2)-oxalic acid; (4)-HA; (5)-GA; (6)-DA; (3)-is a sum of lines 1,2,4,5,6 and represents total computed organic carbon; (^-represents experimental TOC
Variation of pH. The considered model enables to compute the concentration of H+ ions i. e. the pH of the solution against to irradiation dose. Computed value of pH was compared with the experimental data in Fig.2 and they are in a good agreement with each other.
Formation of COn. Formation of CO starts
J 2 2
from the beginning of the irradiation. It goes up after experiencing some induction period with the irradiation dose indicating that formation of CO2 is due to both the decomposition of formic acid as well as its derivatives (Fig 5a). But the observed G (CO2) =0,6 molec/100ev is much less than computed for the initial stage of radiolysis. Fig. 5b represents the computed values for the formation of CO2 at the initial stage which is not covered in the current study. Formation of CO2 may be divided into three stages: a) at the presence of dissolved O2; b) direct formation from the formic acid and accumulation of oxalic acid; c) formation of CO2 from the derivatives of formic acid degradation.
The first stage implies when H and e reacts mostly with the O2 and the reaction OH radicals with formic acid responsible for the formation of CO2 Fig.5a represents that stage i. e. G (CO2)=G (OH)=2,8.
At the second stage recombination of *COO radicals leads to accumulation of oxalic acid and significantly reduces the formation of CO2. Formation of *COO radicals goes through reactions of (56), (57) and (58) that contributes to formation of oxalic acid as well.
Third stage includes degradation of oxalic acid and other derivatives of formic acid radiolysis. The formation of glyoxalic, dihydroxytartaric and dihydroxymuconic acids in the radiolysis of aerated formic acid solution has been proposed in [6] but they have not been identified in this study due to their very low concentration. It has also been proposed that they are oxidized by OH radicals into CO2 and H2O. Computed apparent rate constants for the oxidization of intermediate organic acids by OH radicals were estimated as 2x107 L x M-1x sec-1.
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Section 6. High energy chemistry_________________
Experimental and computed values for the CO2 are in a significant disagreement at the higher dose of 60 to 80 kGy. Observed CO2 amount is much less than computed as well as required by the stoichiometric number of moles of CO2 in formic acid reaction to CO2 and H2O. This disagreement
100
90
may be explained by the equilibrium concentration of CO2 at the gas phase. It was reported that at the pH=4 CO2 is partly absorbed by water due to shifting equilibrium to the right for the reaction of CO2 ^ CO2 t [7].
2gas 2water L J
Fig. 5. a. Formation of CO2 by gamma irradiation of aerated formic acid solution (1x10-2M); b- initial stage (computed)
Conclusions
Approximately 90% formic acid and its derivatives were degraded at the dose of 80kGy.
Formation of CO2 may be divided into three stages: a) at the presence of dissolved O2 that ends approximately at 1,5kGy; b) direct formation from the formic acid and accumulation of oxalic acid; c) formation of CO2 from the derivatives of formic acid degradation that starts approximately at 25 kGy.
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