CC BY
4UDK 541.15:539
RADIATION METHODS OF WATER PURIFICATION
JABBAROVA L.YU., HAMIDOVA A.S. ,RZAYEV R.S
Institute of Radiation Problems, MSE AR
Abstract. The problem of cleaning waste water from oil and oil products is ofgreat importance both for the protection of water resources from pollution and for the extraction of additional oil resources from oil-containing waters. The dependences of the radiation-chemical yield of heptane decomposition on the initial ratio of components at three significantly different temperatures - I000C, 3000C and 4000C are given. It is evident that with a decrease in the concentration of heptane, the radiation-chemical yields of its decomposition decrease sharply.
Key words: radiation-chemical yields, cleaning waste water, heptane.
Only on the Absheron Peninsula, as a result of the exploitation of oil fields for 150 years, more than 200 artificial lakes with oil pollution have formed, the concentration of which sometimes exceeds 25 mg/l. With the increase in oil production, the likelihood of its release into the environment as a result of its extraction, preparation, transportation, processing, storage and, finally, use also increases. Currently, there is an intensive search for effective methods of cleaning water from oil pollution. At high oil content, this process is carried out using physical-mechanical, chemical, biological and thermal methods. At oil content in water below 0.5%, it is necessary to use more subtle and selective chemical methods. Among them, radiation-chemical methods are more promising. At present, of all existing methods of wastewater disinfection, the radiation method, namely electron beam treatment, is recognized as the best. In comparison with other types of ionizing radiation sources, a pulsed electron accelerator is cheaper, does not require the disposal of hazardous waste, has good controllability, low radiation hazard, and the ability to "switch off" at any time. The advantage of using an electron beam to solve this problem is the high efficiency of converting electrical energy into beam energy, while the electron beam has an effect on both microorganisms and chemical pollutants.
Another advantage of this method is that it delivers a portion of energy (absorbed dose) in the form of ionizing radiation to the volume of an aqueous solution. This method is virtually reagent-free - it does not change the chemical composition of water, and is much less energy-intensive compared to thermal methods of disinfection - the water is heated by several degrees. At present, chemical technology, which involves either chlorination or ozonation, is widely used to disinfect natural waters. However, chlorination leads to the formation of toxic organochlorine compounds in the disinfected water, and ozonation leads to the formation of both biodegradable organic matter and complex organic compounds. It should also be noted that in recent decades, numerous outbreaks of diseases have been noted in various regions of the planet, associated with the presence of pathogens of parasitic diseases in drinking water, primarily giardiasis and cryptosporidiosis. The destruction of microorganisms using radiation technology is the most effective of all existing technologies, this fact is due to the universality of the damaging effect on any biological objects.
The disinfection effect is achieved without any chemical reagents, and the death of microorganisms occurs during the action of the electron beam, i.e., there is no need to make a delay before discharging wastewater. The advantage of the radiation method of water purification lies, first of all, in the complex action of radiation. Simultaneously with the decomposition of the main contaminant, radiolysis of all accompanying compounds occurs, coagulation and sedimentation are accelerated, color and odor are eliminated, the values of chemical oxygen demand (COD) and biological oxygen demand (BOD) are reduced, and water is disinfected. The final products of decomposition of contaminants are CO2, H2O, N2 and other simple environmentally friendly compounds. After water treatment with electron radiation, there is no induced radioactivity in the water, since the radiation energy used (0.2 - 3 MeV) is significantly lower than the energy required
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for nuclear reactions. Moreover, if the contamination is not radiation-resistant and its initial concentration does not exceed 100 mg/l, and the required degree of purification is < 90%, then the radiation doses required for purification are small and the radiation method is economically advantageous 16 compared to other existing and used methods in production [1-7].Three different advanced oxidation processes (ozonation at pH 7.5, electron beam irradiation and a combination ozonation/electron beam irradiation) had been applied to study decomposition of aqueous naphthalene-1,5-disulfonic acid (1,5-NDSA) with regard to mineralization and formation of biodegradable intermediates. Formation of biodegradable intermediates could not be indicated for any of the processes used, single electron beam irradiation treatment was the most efficient process for mineralization of organic carbon contained in aqueous 1,5-NDSA. Applied to a real wastewater effluent from a mixed municipal/industrial, wastewater electron beam irradiation with a radiation dose of 2 kGy was sufficient to reduce the concentrations of all naphthalene sulfonic acids and some of the alkylphenol ethoxylates by about two orders of magnitude. Moreover, high energy electrons effectively inactivate indicator bacteria in effluents from municipal wastewater treatment plants and eliminate simultaneously any estrogenic activity originating from natural and synthetic hormones also 5 contained in the wastewater effluents. Inactivation of bacteria and bacterial spores by electron beam irradiation was found to be practically unaffected by the water matrix and suspended solids. There is a strong indication from literature data that these findings are also relevant to viruses of concern (like poliovirus) in water hygiene. Cost analysis of the irradiation process based on actual numbers from the first full scale wastewater treatment plant in the Republic of Korea indicated a total cost of about 0.2 US$/m3 treated water for 2 kGy irradiation dose.
A careful optimization of measuring conditions a Poland allowed the use of HPLC as a convenient method for monitoring the effectiveness of processes for the radiolyticdegradation of organic pollutants and formation of products from these processes. The complete radiolytic degradation of dicamba at concentrations of 110 ppm in aqueous solution required irradiation doses of about 5 kGy. The efficiency of degradation, in contrast to 2,4-D was not affected by the presence of 50 ppm nitrate; however, the concentration of the main toxic decomposition products phenol and 2-chlorophenol was affected by the presence of nitrate. It was found that in numerous systems, that addition of ozone or hydrogen peroxide to irradiated solutions may improve the effectiveness of radiolytic degradation of organic pollutants, including pesticides and phenols. The presence of inorganic scavengers, commonly occurring in natural waters and wastes, affects the consumption of oxygen during the irradiation process, essential for its effectiveness. In the presence of nitrate and hydrogen carbonate, oxygen is consumed at higher irradiation doses than in pure solutions in distilled water. The results obtained indicate that in numerous cases radiolytic degradation may be a suitable and effective method for treatment of industrial wastes. The results obtained in these studies for pesticides 2,4-D, MCPA and carbendazim indicate that radiolytic degradation may be a suitable and effective method for treatment of industrial wastes. In many cases the experimental data on effectiveness of radiolytic decomposition of target compounds were compared with results of kinetic modeling based on rate constants of radical reactions involved with good or satisfactory agreement [8-10 ].The aim of the work is to study the patterns of decomposition of hydrocarbon contaminants in an aquatic environment under the simultaneous action of y-radiation and heat.
Methodology
At room temperature, the yield of hydrogen from water is G(H2)=0.45 molecules/100 eV, and the yield of hydrogen during hydrocarbon radiolysis is approximately 10 times greater and is 4-5 molecules/100 eV. With an increase in temperature to 4000C, the maximum radiation-chemical yield of hydrogen from water increases to 7.5 molecules/100 eV, and this agrees well with the literature data. The temperature dependence of the hydrogen yield from heptane is an exponential function, and at 4000C this value reaches 169 molecules/100 eV. This value of the radiation-chemical yields of hydrocarbon decomposition is typical for such systems.
Comparison of these results shows that the radiation resistance of hydrocarbon systems is much lower than that of water. Note that the primary yields of active particles such as are comparable in
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both objects and do not exceed 7-8 particles per 100 eV. The paths of stabilization of these particles in the form of hydrogen and hydrocarbon gases, as well as reverse recombinations with restoration of the original object, have distinctive reaction rate constants, and therefore a significant difference in the yields of stable products is observed. It is known that in the liquid phase the solubility of hydrocarbons in water is very low (0.014 ml/100 g H2O at 200C), therefore these components create separate layers in the liquid phase. Irradiation of such mixtures at room temperature leads to yields of hydrogen and hydrocarbon gases close to the yields of radiolysis of individual components. Under these conditions, the radiation-chemical yield of hydrogen depends on the composition of the mixture and varies from 0.5 to 4.5 molecules/100 eV, which is very close to the additive yield of hydrogen.At temperatures above 2000C, a significant increase in the yield of products is observed. The formation of products of the interaction of these components occurs under the influence of radiation. At such temperatures and pressures close to atmospheric, both components of this system are in the vapor phase. In the vapor phase state, additivity in the yield of hydrogen is not preserved, i.e., interaction of two components occurs. In pure heptane and binary systems, the observed values of the yield of hydrogen and methane indicate the occurrence of short-chain reactions.
Table 1
Temperature dependence of the radiation-chemical yield of hydrogen during radiolysis of the
Т , 0С h eptane w ater heptane-water = 1/1
settle ment experimental
2 0 4, 2 0, 45 2,35 1,95
2 00 4, 5 0, 8 2,65 2,9
3 00 6 2,9 3, 5 33,2 44,2
4 00 1 69 6, 7 87,8 102,5
The presented data prove that in the vapor phase there is an effective interaction of active particles of different origin. Under these conditions, heptane decomposition occurs almost 100 times faster than in water. This means that during vapor-phase radiolysis of such mixtures, it is possible to observe the transfer of absorbed energy from water to hydrocarbon, which can lead to effective decomposition of the latter. This principle can be used to develop water vapor-sensitized decomposition of hydrocarbons. This process can be observed more effectively for relatively low concentrations (<0.5%) of hydrocarbon, which seems problematic in the practice of protecting water systems from oil pollution.
CP
n о
8 i
О
120 100 80 60 40 20 0
0C
— 400 0C
— 100 0C
10
20
30
40
—■
50
[C7H16]/[H2O] 10 , %
0
Fig. 1. Effect of the concentration ratio [C7H16]/[H2O] on the radiation-chemical yield of heptane decomposition in an aqueous medium P=3.6 kGy/hour.
Fig. 1 shows the dependences of the radiation-chemical yield of heptane decomposition on the initial ratio of components at three significantly different temperatures - 1000C, 3000C and 4000C. It is evident that with a decrease in the concentration of heptane, the radiation-chemical yields of its decomposition decrease sharply. At heptane concentrations in water equal to <10-2%, absorbed doses of 3.6-30 kGy and a temperature of 4000C, the radiation-chemical yields of the process decrease from 35 molecules/100 eV to 7-8 molecules/100 eV. This indicates that at low heptane concentrations, "proper" radiolysis of water occurs.
Under these conditions, the rates of the processes of deactivation of water molecules become comparable with the processes of recharging and energy transfer from water to heptane, i.e. the rates of processes (5) and (6) become comparable: H2O*+ + C7H16 ^ H2O + C7H* (5) H2O'+ + H 2O(e) ——^ H2O (6)
Under such conditions, the observed radiation-chemical yields of the products are very close to the yields in the "proper" radiolysis of individual substances.
Fig. 2. Kinetics of gas formation during radiation-thermal decomposition of heptane in an aqueous medium at a mixture ratio of C7H16]/[H20 =0.02; T=4000C; P=3.6 kGy/hour
As can be seen from Fig. 2, above a dose of 7 kGy (>2 h of irradiation), a tendency towards saturation is observed. Analysis shows that these are not the results of secondary processes occurring in the gas phase, but the results of a decrease in the concentration of heptane in the mixture during radiolysis. Since the overall goal of this work is to purify water from hydrocarbons, it is important to know what part of the decomposed heptane is in the gaseous state. Studies show that the ratio of the amounts of products in the liquid and gas phases depends on the temperature (Table 2).
Table 2
T, 0С Gra3. Gжид. Grаз./Gжид.
100 3,0 43,2 0,07
200 20,2 38,7 0,52
300 125,3 24,1 5,2
400 356,5 22,5 15,8
As can be seen from Table 2, with an increase in temperature from 100 to 4000C, the ratio of the amounts of gas (<C5) and liquid (>C5) products increases from 0.07 to 15.8.
The temperature dependence of the gas yield in Arrhenius coordinates has two linear sections corresponding to the temperature ranges of 40-2500C and 250-4000C. In the low-temperature region, the activation energy value lies within the range of El = 7.7V14.0 kJ/mol, and in the high-temperature region E2 = 28.8□ 51.8 kJ/mol. The activation energy values in the low-temperature region correspond to diffusion processes of radiation-generated particles, and in the high-temperature region - to radical abstraction reactions.
At temperatures above 3000C, even in thermal processes, heptane decomposes at a low rate, i.e. in the range of 300-400°C in radiation-thermal processes there is a certain contribution of purely thermal processes.Table 3 shows the ratios of the rates of radiation-thermal and thermal processes in the temperature range of 3 00-4000C.
Table 3
The influence of temperature on the ratio of the rates of radiation-t
T, 0С 300 350 375 400
Wpt/Wt 101,2 55,2 26,1 1,9
îermal and thermal processes
As can be seen from Table 3, as a result of increasing the temperature in this region, the ratio Wpt/Wt decreases from 101.2 to 1.9, i.e. the role of thermal processes increases with increasing temperature.It follows from these data that after the separation of the thermal contribution, the radiation-chemical yield of heptane decomposition at 4000C is 60-70 molecules/100 eV, and this gives grounds to assert that a chain reaction is taking place.
CONCLUSIONS
1. During thermoradiolysis of water containing 0.5-1.0% heptane, at a temperature of 4000C, a radiation dose of 5-6 kGy, as a result of chain transformation, the concentration of the hydrocarbon decreases to 10-3%, and then a plateau is observed in the kinetic curve, which is associated with a decrease in the probability of interaction of active radiolytic particles of water with heptane.
2. Addition of oxygen to the radiolyzed mixture [C7H16]/[H20]=0.02 at concentration ratios [02]/[C7H16]=0.1-1.0 leads to an increase in the rate of heptane decomposition and a decrease in the yield of hydrocarbons, which is associated with the initiating effect of oxygen in the radiation-thermal decomposition of heptane with the predominant formation of oxygen-containing organic compounds.
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