ISSN 1063455X, Journal of Water Chemistry and Technology, 2014, Vol. 36, No. 5, pp. 211–216. © Allerton Press, Inc., 2014. Original Russian Text © E.D. Pershina, K.A. Kazdobin, 2014, published in Khimiya i Tekhnologiya Vody, 2014, Vol. 36, No. 5, pp. 393–404.
PHYSICAL CHEMISTRY OF WATER TREATMENT PROCESSES
On the Transformation of Trichloroacetic Acid in Aqueous Media E. D. Pershinaa and K. A. Kazdobinb, * a
Vernadsky Taurida National University, Simferopol, Ukraine bVernadsky Institute of General and Inorganic Chemistry, National Academy of Sciences of Ukraine, Kiev *email:
[email protected] Received October 12, 2012
Abstract—The influence of the chemical composition of water on the rate of volatilization and degrada tion of trichloroacetic acid (TCA) has been investigated. The schemes of possible secondary pollution of territories adjacent to the polluted water bodies were discussed on the basis of measuring the surface ten sion, TCA distribution in solution, and the analysis of kinetic parameters of its volatilization and oxida tion. DOI: 10.3103/S1063455X14050026 Keywords: secondary pollution, oxidation kinetics, surface tension, natural water, distribution in solution, trichloroacetic acid, volatilization.
INTRODUCTION At present the chemical crop protection products (CCPP, pesticides) occupy the leading place in the inte grated system of controlling pests, plant diseases, and weeds. Due to their migration together with air masses and in water, and also due to their enhanced resistance to chemical degradation a real threat of global poison ing of our planet with toxic chemicals has appeared [1]. The most tangible consequences for ecological equi librium in nature include the pollution of water bodies. Besides the negative impact on organoleptic properties of water, many pesticides possess toxic properties. The stability and toxicity of pesticides directly depend on their chemical structure. The pesticides well soluble in aqueous medium migrate predominantly with aqueous phase and possess the tendency toward accumulation. The cumulative activity of pesticides is affected not only by the adsorptive capacity of soils and bottom deposits, but also the fugacity of pesticides proper and also their ability to degradation in the presence of active forms of oxygen. Many volatile and soluble toxicants in the existing environment, including trichloroacetic acid (TCA), applied as selective action herbicides are formed in the atmosphere as a result of reactions with participation of certain organochlorine compounds and solvents that are used in industry for cleaning and degreasing of materials. An enhanced TCA concentration in the air is hazardous for both plants and animals. This is partic ularly noticeable in regions with unfavorable climatic conditions: in steppes, semideserts, northern and high mountain territories [1]. The purpose of this study is to determine the rate of volatilization and chemical degradation of trichloro acetic acid in distilled and lowmineralized waters. EXPERIMENTAL The kinetics of distribution and oxidation of TCA based on the distilled water and natural water from the Simferopol reservoir was investigated. At the first stage physicochemical parameters characterizing the possibility of interphase transfer of TCA at the air/water boundary were determined. This experiment was performed in two directions: – control of the TCA distribution in the bulk and on the surface of clean water that is determined by the presence of temperature stratification in any body of water and the different structure and composition of water in bulk and on the liquid – gas interphase. During the test TCA solution was added to a vessel containing 2.5 dm3 of distilled water for reaching the concentration of 0.01 M. Three days later water samples were taken from different water layers. A similar test was conducted with natural water; – taking into account the temperature stratification influencing the rate of TCA volatilization from the sur face of water body. The rate of TCA volatilization was determined at 15 and 25°C with TCA initial concentra 211
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tion of 10–2 M. The height of water column amounted to 50 cm. The sampling was performed on a daily basis during 36 days at distances 0.1, 5, 25, 40, and 50 cm from the surface. At the second stage the rate of TCA degradation was estimated in the presence of active forms of oxygen. For eliminating the effect of volatilization the test was conducted in tightly sealed glass vessels having the vol ume of 10 cm3, where 0.3 cm3 of 1 M aqueous solution of TCA (with TCA concentration of 0.03 M) was intro duced. After addition of TCA, 4.0 cm3 of 0.05 M (10–5 M) aqueous solution of KMnO4 were added to water. The results of test were compared with a sample containing the distilled water, in place of natural one, with addition of hydrogen peroxide (H2O2 concentration was 10–5 M). Samples were taken once in four days. No reagents, except TCA, were added to the control samples with distilled and natural waters; they were stored in tightly sealed vessels of dark glass. The TCA concentration was determined on a KFK4 photocolorimeter using blue filter. This method is based on the reaction of alkaliinduced decomposition of TCA during heating: (1) CCl3COOH + 2NaOH = CHCl3 + Na2CO3 + H2O. Next chloroform was extracted by pyridine with formation of colored complex. The color intensity of com plex quantitatively depended on the TCA concentration. The calibration graph was built in the range of TCA concentrations in water 5–0.05 mg/dm3. The waterpyridine mixture with ratio 1:3 was used as a comparison solution. The concentration was calculated with due regard for the results of blank test (water with zero con tent of TCA) [2]. The surface tension was measured by the droplet counting method using a Traube stalagometer calibrated with an accuracy of up to 0.05 drop. This method is based on the Tata Law [2] implying that a droplet formed on a horizontal circular surface detaches when its weight becomes equal to the product of surface tension by the droplet base. Provided the one and the same area of droplet detachment is applied, the droplet weight is proportional to the surface tension. Therefore, as the surface tension of liquid decreases, the quantity of drop lets flowing out from a specific tube volume increases and vice versa. Distilled water was used as a reference solution. The surface tension (STCA) of solution was calculated by formula [3]: S TCA solution = S H2 O ⋅ n H2 O ⁄ n TCA ,
(2)
where S H2 O is the surface tension of distilled water; nTCA is the number of droplets of TCA solution; n H2 O is the number of water droplets. The surface tension of natural water was calculated as follows: S H2 O natur = S H2 O ⋅ n H2 O ⁄ n H2 O natur ,
(3)
S TCA solution = S H2 O natur ⋅ n H2 O ⁄ n TCA natur .
(4)
Concentration of salts was determined on an IRF454 refractometer using a calibration graph. Solar sea salt (Standard TU U 14.42492620078003 : 2008) and distilled water (Standard DSTU ISO 3696 : 2003) were used for preparing standard solutions. The salt concentration was varied from 0.001 to 10 g/dm3 [4]. The COD value was measured in accordance with Standard GOST 527082007. RESULTS AND DISCUSSION The comparison of TCA distribution depending on the place of sampling revealed the differences in TCA content with the increasing distance from the surface and also in distilled and lowmineralized waters (table, Fig. 1). This is explained by the difference of physicochemical and electrochemical properties of the bulk (vol ume) and the surface of solution [5–10] that are directly related to the structural peculiarities of water as a sol vent [11, 12]. The surface water—oxygen structures (as functions of surface tension forces) are not capable of generating electrons in the absence of external actions. In this case the double electric layer at the water/air interface is formed at the expense of rigorous orientation of polar molecules of water at the interphase boundary. Water molecules are oriented by the oxygen and hydrogen atoms into gaseous and liquid phases, respectively. At such orientation for the majority of aqueous systems the interphase electrical potential has a negative value. The presence of labile structures in water [11, 12] makes it possible to damp the electronic density just into the area of boundary layer. The experimental confirmation of such arrangement of water molecules was obtained by measuring the interphase electrical potential [13, 14]. The observed increase of the interphase potential is related to the enhanced structuring of the bulk solution at the expense of formation of mixed hydrogen bonds JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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between molecules of water and acid, and also to the symbate variation of the bulk (volume) and surface entropy that results in the rise of the number of oriented water dipoles in the surface layer. Distribution of trichloroacetic acid in systems under analysis (averaged values) Area of sampling, CTCA in distilled cm water, mol/dm3 On the surface: 10.5 × 10–3 ± 0.1–0.2 2.7 × 10–5 In the bulk: 26.5 × 10–3 ± 5.4 × 10–4 5–25 Bottom area: 25.00 × 10–3 ± 1.8 × 10–5 40–50
Csalts, g/dm3 0.01
Dissociation degree, %
CTCA in natural water, M
97
0.01
87
0.01
88
18.8 × 10–3 ± 6.3 × 10–5 16.2 × 10–3 ± 2.4 × 10–5 15.94 × 10–3 ± 3.2 × 10–5
Csalts, g/dm3 2.54
Dissociation degree, %
2.54
94
2.54
94
90.5
C TCA , M 2.4 1 2.0 2 1.6
1.2
0.8
0.4 0
2
4
6
ΔL, cm
Fig. 1. Distribution of trichloroacetic acid in the bulk of water and on the water surface: natural (1), distilled (2). ΔL is the distance from the water surface.
Thus, the presence of electrolytes stabilizes the interface structure, while the excess concentration of pro tons at the water/air interphase boundary complicates the TCA diffusion toward the surface and changes its dissociation degree. In this case the ionic strength of the surface layer is larger than in the neutral medium that results in enhancing the TCA dissociation degree and its chemical activity (see the table). The rise of miner alization increases the ionic strength of water that reduces the effect of selective distribution and equalizes the values of dissociation degree, and also results in the reduction of TCA dissociation degree of the surface layer with respect to the bulk (see the table). This is the main reason of different TCA distribution in the natural and distilled waters (see Fig. 1). The presence of surfactants and petroleum products at concentrations of up to 0.0035 g/dm3 in natural water [15] changes the structure of water/air interphase boundary that affects the experimental values of sur face tension (Fig. 2). It was established that TCA in distilled water does not display the surface activity. How ever its presence in natural water reduces the surface activity of petroleum products and surfactants. This involves the symbate reduction of the dissociation degree of proper TCA (see the table), i.e., we observe mutual effects of leveling the activity. On the other hand, the display of TCA surface activity in natural water can be enhanced by its mineralization. In this case the mobile trichloroacetateanions with low degree of hydration are formed that can partici pate in formation of the double electric layer at the liquid/gas interface. Concentration of the specified anions on the surface leads to the reduction of surface tension of solution at the expense of the electrolyte saturation of surface microlayer of natural water and the emergence of excess of charged ions that are pushed to the sur face by the electric field. Consequently, the negative ions in the liquid of water/air layer “drop” part of the JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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excess charge transforming in the ultimate case into neutral particles. The electron density in the specified layer decreases and spreads among ions (largely anions), while its value acquires a fractional value. Such dis tribution of charge is typical of unstable molecular complexes [16] and confirms the kinetic calculations of TCA volatilization from distilled and natural waters (Figs. 3 and 4). –3
σ × 10 , N/m
73 1
71
69
2 67 0
0.04
0.08
0.12
CTCA, M
Fig. 2. Relationship of the surface tension of trichloroacetic acid solutions as a function of the acid concentration: distilled water (1) and natural water (2).
ln V 1
n = 1.33
0
n = 1.48
n = 0.36
–1 2
n = 0.18 1
–2
1.21 1.23 1.25 ln C Fig. 3. Kinetic curves and orders of reactions of trichloroacetic acid volatilization of the surface of distilled water at 15°C (1) and 25°C (2).
At the initial stage the order of reaction at two temperatures is more than unity. This indicates the presence of monomolecular decomposition (dissociation of TCA) and the possibility of partial bimolecular interaction (in this case the most probable interaction is hydrolytic one). A sharp drop of the order of reaction to values below unity confirms the formation of molecular complex [17]. Thus, TCA volatilization has a chemical nature and heavily depends on the chemical composition of water and temperature regime, i.e., on factors forming the power characteristics of the surface. With reduction of the surface tension the rate of volatilization will rise [16]. In natural conditions this may enhance the risk of secondary pollution of territories adjacent to the water body by products of TCA interac tion with organic compounds [17]. The risk of secondary pollution increases at temperatures ≥ 25°C since at JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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such temperatures the kinetic relationships of the volatilization have a more complex pattern featuring the for mation of intermediate complex with orders of reactions close to 0.5 (see Fig. 4). At 15°C the rate of volatil ization tends to zero and judging by the kinetic curve the reduction of the order of the temporal reaction does not occur. It consistently corresponds to unity that in the absence of chemical transformations characterizes the dissociation of acid. ln V 1 2 1 0 n = 1.48
n = 0.7
–1
n = 0.47 n = 1.0
–2 1.20
1.22
1.24
ln C
Fig. 4. Kinetic curves and orders of reactions of trichloroacetic acid volatilization from the surface of natural water at 15°C (1) and 25°C (2).
The comparative analysis of kinetic parameters of TCA oxidation (Fig. 5) in the presence of active oxidizers (KMnO4 and H2O2) in distilled and natural waters reveals the distinctions in implementation of TCA oxida tive reactions. In natural water we can observe a substantial induction period (more than 10 days) in the oxi dation reaction, while in distilled water such period actually is not present (oxidation reaction starts from the first day after addition of reagents) (see Fig. 5). The presence of such significant delay can be explained by the presence in natural water of organic compounds (in particular, surfactants and petroleum products) capable of implementing the competitive reactions of oxidation. 3
CTCA, 10 M 6
1 4
2
2
0
0
10
20
30
days
Fig. 5. Kinetics of oxidation of trichloroacetic acid: natural water (1), distilled water (2).
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The order of reaction is close to three at the initial stage in natural water indicating the running of several parallel reactions. For checking this point we carried out the measurement of kinetics of the COD decline in model system and in natural water. The resultant kinetic relationships of COD decline are practically identical and differ only by their absolute values. This was the proof of the fact that the substantial induction period of TCA oxidation in natural water was related to the presence therein of easily oxidizable organic components which consumed the active oxygen. However, after 10 days the TCA oxidation kinetics in natural water starts to coincide with the model system indicating the sufficient amount of active forms of oxygen that are capable of participating in reactions. Moreover, the logarithmic relationships of the variation of the rate of reaction as a function of the TCA concentration and the order of reactions are similar, close in terms of values and have a section (with the order less than unity) responsible for formation of molecular complex. The presence of such complex can be related to the implementation of homogeneous catalytic reaction of formation of active oxidizers with participation of manganese ions making it possible to support high oxidative activity of water. In the absence of manganese ions variations of TCA concentrations were not observed either in model system or in natural water. This was a confirmation of the low oxidative activity of the specified com pound in water. The comparison of obtained data makes it possible to suggest the active participation of organic pollutants (petroleum products and surfactants) in reactions of TCA degradation that may lead to the emergence of other toxic substances resistant to the action of active oxygen. After 35 days the stationary mode of TCA oxidation rate was established in both systems (see Fig. 5) that was registered by the COD variation. Such behavior of systems under investigation indicates the high resistance to TCA oxidation in aqueous media. CONCLUSIONS It has been established by experiment that TCA volatilization has the chemical nature and depends on the chemical composition of water, temperature regime, and factors forming the power characteristics of the water—air interface. In natural conditions this may enhance the risk of secondary pollution of territories adja cent to the water body by products of TCA interaction with organic compounds found in water. The presence of active oxidizers in water containing TCA and other organic pollutants (petroleum products and surfactants) increases the probability of formation of new organic substances that are derivatives of trichloroacetic acid. REFERENCES 1. McCulloch, A., Trichloroacetic Acid in the Environment, EURO—CHLOR Representing the Chlor—Alkali Indus try. Science Dossier, 2002, Marbury Technical Consulting. 2. Novikov, Yu.V., Lastochkina, K.O., and Boldina, Z.N., Metody opredeleniya vrednykh veshchestv v vode (Methods of Determination of Harmful Substances in Water), 1981, Moscow: Khimiya. 3. Voyutskii, S.S., Kurs kolloidnoi khimii (A Course on Colloid Chemistry), 1975, Moscow: Khimiya. 4. Ioffe, B.V., Refraktometricheskie metody khimii (Refractometric Chemistry Methods), 1974, Leningrad: Khimiya. 5. Deryagin, B.V. and Dukhin, S.S., Elektroforez (Electrophoresis), 1976, Moscow: Nauka. 6. Adamson, A., Fizicheskaya khimiya poverkhnostei (Physical Chemistry of Surfaces), 1976, Moscow: Mir. 7. Jaycock, M. and Parfitt, G., Khimiya poverkhnostei razdela faz (Chemistry of Interfaces), 1984, Moscow: Mir. 8. Yukhnovskii, I.R. and Koprylyak, I.I., Elektrolity (Electrolytes), 1988, Kiev: Nauk. Dumka. 9. Khan, V.A., Vlasov, V.A., Myshkin, V.F., et al., [Electronic resource], Nauch. Zhurn. KubGAU, 2012, no. 81, Access mode to journal: http://ej.kubagro.ru/2012/07/pdf/50.pdf. 10. Parfenyuk, V.I., Kolloid. Zhurn., 2002, vol. 64, no. 5, pp. 651–659. 11. Zatsepina, G.N., Fizicheskie svoistva i struktura vody (Physical Properties and Structure of Water), 1998, Moscow: Izdvo Mosk. Unta. 12. Shadrin, G.N., Tarimov, O.E., Khentov, V.Ya., Groshenko, N.A., Pershina, E.D., and Krymova, V.V., Ukr. khim. zhurn., 1996, vol. 62, no. 10, pp. 85–87. 13. Shadrin, G.N., Groshenko, N.A., Pershina, E.D., and Khentov, V.Ya., Ibid., 1996, vol. 62, no. 5, pp. 42–44. 14. Aleksashkin, I.V., Dyachenko, E.A., and Filimonova, E.Yu., Probl. material. kul’tury, Ser. geograf. nauki, 2004, pp. 1–4. 15. Khentov, V.Ya., Fizikokhimiya kapelnogo unosa (PhysicoChemistry of Droplet Entrainment), 1979, Rostovon Don: Izdvo RGU. 16. Emanuel, N.M. and Knorre, D.G., Kurs khimicheskoi kinetiki (A Course on Chemical Kinetics), 1984, Moscow: Vyssh. Shkola. 17. Rozentsvet, V.A., Kozlov, V.G., Ziganshina, E.F., and Boreiko, N.P., Vysokomol. soed., Ser. A, 2008, vol. 50, no. 10, pp. 1770–1776. Translated by A. Zheldak JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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