AbstractâThe activated carbon (AC) bioregeneration was shown to proceed more efficiently during the filtration of preozonized tap water as compared with ...
ISSN 1063455X, Journal of Water Chemistry and Technology, 2009, Vol. 31, No. 4, pp. 220–226. © Allerton Press, Inc., 2009. Original Russian Text © N.A. Klimenko, L.A. Savchina, I.P. Kozyatnik, V.V. Goncharuk, A.O. SamsoniTodorov, 2009, published in Khimiya i Tekhnologiya Vody, 2009, Vol. 31, No. 4, pp. 387–398.
PHYSICAL CHEMISTRY OF WATER TREATMENT PROCESSES
The Effect of Preliminary Ozonization on the Bioregeneration of Activated Carbon During Its LongTerm Service N. A. Klimenko, L. A. Savchina, I. P. Kozyatnik, V. V. Goncharuk, and A. O. SamsoniTodorov Dumanskii Institute of Colloid and Water Chemistry, National Academy of Sciences of Ukraine, Kiev, Ukraine Received October 15, 2007
Abstract—The activated carbon (AC) bioregeneration was shown to proceed more efficiently during the filtration of preozonized tap water as compared with bioregeneration under the similar conditions during the filtration of nonozonized water. The enhanced efficiency of AC bioregeneration after the filtration of preozonized water is determined by transformation of the total organic carbon into a better biodegrading form, increased microbiological activity in the depth of AC bed, and enhanced hydrophilicity of the AC surface area due to its chemical interaction with dissolved ozone. DOI: 10.3103/S1063455X09040031
Natural organic compounds (NOC) are the main source of organic carbon in drinking water that in a great measure determines its quality. The incomplete extraction of the total organic carbon (TOC) during the drink ing water conditioning results in formation therein of undesired secondary products with carcinogenic and mutagenic properties that are produced during disinfection. Thus, after the water conditioning at central water–supply stations its quality does not always conform to safety requirements and sometimes to effective standards. The drinking water after its conditioning may contain residues of NOC and products of water dis infection. It is well known that ozonization may transform biologically stable organic compounds into such com pounds that are easily decomposed [1, 2]. This occurs due to the destruction of NOC structure and transfor mation of macromolecular compounds into products having a low molecular weight [3, 4]. On the other hand, the final products of postozonization having a higher polarity and solubility may reduce the adsorption capacity of activated carbon (AC) [5]. However, the transformation of NOC into easilybiode gradable compounds (biologically accessible organic carbon (BAOC)) is a wellknown technique of increasing the service time of adsorption filters due to the fact that BAOC is biochemically destructable on activated car bon with attached microorganisms resulting in regeneration of AC [6]. Note that the adsorption and biodeg radation proceed simultaneously during the water filtration through AC. In addition to the transformation of TOC into a more biodegradable form, the preliminary ozone treatment increases the microbiological activity in filters with AC. As was shown in paper [7], the preliminary ozone treatment produced a significant effect on the concentration of biomass immobilized on AC. As was established by using the method of estimating the immobilized biomass concentration in terms of the amount of adenosine triphosphate (ATP), this amount varies in a range from 25 to 5000 ng of ATP per 1 cm3 of AC. The highest biomass concentration is observed during the extended operation time of the filter with AC and during preliminary ozonization. Apart from increasing the microbiological activity in the filter bed with AC, ozonization causes the chem ical adsorption of dissolved ozone on AC. The mechanism of AC–O3 interaction was investigated by many researchers [8–11]. The presence of surface groups in AC was shown to determine the ozone decomposition by its transformation into OH• radicals. This occurs mostly in the volume of solution due to the radical chain reaction initiated by OH– and HO2– ions. Hydroperoxide ions appear due to formation of hydrogen peroxide on active surface sites of AC and its subsequent dissociation [12]. In the case of repeated use of AC the subse quent contact with ozone leads to oxidation of the basic and hydroxyl groups resulting in formation of car boxyl, carbonyl, and lactone groups. Next, the AC capacity of enhancing the ozone transformation into hydroxyl radicals declines. However, the decomposition of dissolved ozone on the AC surface that involves the formation of OH• radicals results in the efficient oxidation of water impurities. This effect is used in the so called Carbazone process [13]. 220
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The AC ability to transform ozone into OH• radicals is determined primarily by the value of AC surface area, pore volume, and the AC basicity [14]. It was also established that the natural organic substances using OH• radicals do not produce a negative effect on the ability of transforming ozone into free radicals. Some researchers consider the dissolved ozone–AC system as a catalytic one [15]. However, the AC exposure to high dosages of ozone causes changes in both the activated carbon structure and the chemistry of its surface area [14]. This implies that AC cannot be used as a catalyst in reactions with ozone, but can only be an initiator. As was shown in paper [16], the decomposition of ozone in water in the presence of AC is significantly affected by the value of pH. At pH 2–6 the ozone reaction on the AC surface is not radical, while the molecular oxygen is one of the products of O3 decomposition. At the same time the generation of OH• radicals occurs at pH > 6 in addition to the direct reaction between the molecular ozone and OHcontaining active sites of AC. As was suggested in paper [17], active sites of AC determining the ozone decomposition are, primarily, graphite layers with unpaired πelectrons and functional groups of basic origin that act as Lewis bases reducing ozone to OH– and H2O2, respectively. These active sites initiate the radical–chain reactions in the aqueous medium in accordance with the mechanism described in paper [18]. The interaction of AC with ozone results in formation of hydroxyl, carbonyl, and carboxyl surface groups [10, 19]. The residual content of the dissolved ozone in the water subjected to ozone treatment may reach 3.2– 3.5 mg/dm3 after a 30minute contact with ozone–air mixture [20, 21]. There is no doubt that the ozone dis solved in water after preliminary ozonization of the solution before its filtration having concentrations of 2.0– 3.5 mg/dm3 can hardly change significantly the AC structure, however it may affect the formation of surface oxygencontaining groups and contribute to catalytic oxidation of dissolved organic impurities. All above mentioned factors (the increased bioaccessibility of dissolved organic substances, increased microbiological activity of AC and variation of the AC surface chemistry) may affect the efficiency of natural bioregeneration of AC during the extended operation of the adsorption filter. Bioregeneration of the activated carbon is a critically important phenomenon in applying AC for the drinking water conditioning or waste water treatment. Due to bioregeneration the AC adsorption capacity is restored at the expense of the vital activity of microorganisms, and AC can serve much longer than it is determined by purely physical adsorption. Biore generation can be optimized by varying the nature of microorganisms, conditions of the process behavior, AC loading, variation of the adsorbate chemical structure. In our earlier paper [22] we investigated the efficiency of AC bioregeneration during the extended filtration of the tap water without its preliminary ozone treatment. As was shown, the degree of AC natural bioregener ation varied from 84.7 in the top (with respect to direction of filtering) part of the adsorption filter to 82.7% – in the lower part of the filter. The purpose of this study was to establish the degree of AC natural bioregeneration in a filter having the bed length of 1 m during its longterm operation for filtering the ozonized tap water. EXPERIMENTAL The activated carbon of grade KAU1 was selected as an object of investigation and KAU is used in filters designed for the treatment of drinking and waste waters. It features the following structure–adsorption char acteristics: adsorptive pore volume Va = 0.39 cm3/g, micropore volume Vmi = 0.12 cm3/g, and the effective spe cific surface area Sef = 630 m2/g. The carbon KAU1 through which was filtered the ozonized tap water was designated as KAU1O, while the carbon through which was passed the nonozonized water was designated as KAU1N. The Al operating conditions in a filter for addon treatment of drinking water are as follows: Activated carbon KAU1 Height of the bed layer, cm 100 Weight of carbon in filter, kg 32 Water filtration rate, m/h 3–4 Service life of the filter, year 3.5 In the course of operation tap water having in 2004–2006 the following averaged characteristics was passed through the filters: JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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Turbidity (in terms of kaolin), mg/dm3 0.04–0.49 Color, degr 3–20 pH 6.5–7.6 Permanganate oxidizability (PO), mgO/dm3 3.0–5.0 Ammonia (in terms of nitrogen), mg/dm3 0.05–0.39 3 Hardness, mg–eq/dm 3.3–6.2 3 Iron (total), mg/dm 0.05–0.3 Total organic carbon (TOC), mgC/dm3 6.4–8.0 Chloroform, mg/dm3 0.020–0.028 3 Trichloroethylene, mg/dm 0.013–0.024 3 Carbon tetrachloride, mg/dm < 0.001 Is was shown that the degree of natural bioregeneration under conditions of the incomplete restoration of AC properties could be enhanced by washing AC with water, alkali liquor or chloroform [23]. In the present study the carbon samples selected from the filter were airdried at 20°C to the constant weight, then pored with the required solution, and put into a shaking machine. The following kinds of treatment were carried out: 1) water flushing: a carbon sample (~ 1.0 g) was subjected to triple washing with distilled water of 20cm3 with simultaneous shaking during three hours; 2) washing with 0.1 M sodium hydroxide solution: 10 cm3 of alkali liquor were added to a carbon sample (~ 1.0 g) and kept on water bath (60°C) during four hours; 3) washing with chloroform: 10 cm3 of chloroform were added to a carbon sample (~ 1.0 g) and the mixture was shaken for 36 h. All the carbon samples subjected to such treatment were airdried. The estimation of the structure–adsorption characteristics of AC after the natural bioregeneration, addi tional washing with distilled water, sodium hydroxide solution, and chloroform was performed by measuring the adsorption of nchloroaniline on samples of carbons that were selected in the top part of the adsorption filter at the height of 1 cm and in the lower part at the height of about 100 cm. The method of measuring the adsorption from the aqueous solution of the standard adsorbate of nchloroaniline [24] allows us to sufficiently accurately estimate the AC specific surface area that coincides with the specific surface area determined by the adsorption of nhexane vapors and by the thermal desorption of argon. The structure–adsorption character istics of AC samples were determined by the tmethod also using nchloroaniline. Application of this method is justified on condition that the adsorption isotherms on the investigated and standard sorbents are similar. This is ensured under the condition of identical chemical nature of the surface of both sorbents. This method is based on the idea that the average statistical depths of the adsorption layer on the standard and investigated adsorbents are the same at equal relative equilibrium concentrations Ceq/Cs (where Ceq is the equilibrium con centration of adsorbate, Cs is its water solubility). In this case the relationship of the adsorbed substance vol ume as a function of the relative concentration is common for all sorbents of the same chemical nature. The quantity of surface groups of activated carbon was determined by using an abridged version of the Boehm titration method (without determination of carbonyl groups) [25]. Three samples of dried activated carbon (~ 1.0 g, each) were put into three flasks, next 25 cm3 of 0.1 normal solutions of NaOH, NaCO3 and NaHCO3 were pored into these three flasks, respectively. The samples were held during 24 hours with contin uous shaking. Then 10 cm3 of each filtrate were titrated with 0.1 normal solution of HCl with a methyl red indicator. It is assumed that sodium hydroxide neutralizes carboxyl, phenol, and lactone groups; sodium car bonate neutralizes carboxyl and phenol groups; sodium hydrocarbonate neutralizes only carboxyl groups. The total basicity of sorbent was determined during the interaction of the sample with 0.1 normal solution of HCl by titrating the filtrate with 0.1 normal solution of NaOH. The content of total organic carbon was determined by the catalytic combustion method. Measurements of TOC were performed on a Shimadzu TOCV CSN instrument. RESULTS AND DISCUSSION The estimation of adsorption capacity of the used carbons involved the measurement of adsorption iso therm of the main component of organic compounds in water (fulvic acids) in TOC units. This isotherm is presented in the figure. JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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THE EFFECT OF PRELIMINARY OZONIZATION a, mg C/g 40 30 20 10 0 0
2
4
223
6 8 3 TOC, mg/dm
Adsorption isotherm of FA contained in the tap water on carbon KAY1N.
Using data in the figure and the Langmuir model we determined that the ultimate value of specific adsorp tion of fulvic acids on KAU1N carbon amounted to 32 mg/g. At the equilibrium TOC concentration of 3 mg/dm3 the value of equilibrium specific adsorption amounted to about 15 mg/dm3, i.e. the adsorption space of KAU1 carbon accessible for fulvic acids was occupied by about 47%, while about 53% of this space remained free. This value has to be taken into account below in estimating the degree of AC bioregeneration. It should be noted that the equilibrium conditions are not reached during the solution filtration. The example of carbon KAU1 reveals that at the filtration rate of 3–4 m/h the contact time of AC with the solution amounts to about 20 minutes, while the settling time of adsorption equilibrium during the adsorption of fulvic acids is as high as 14 days. The approximate amount of adsorbed organic matter after the flow of 200 m3 of water through KAU1 carbon amounts to about 1100 g. In accordance with the adsorption isotherm the equi librium quantity of organic matter that can be adsorbed amounts to 900 g. Comparison of these values indi cates that biodestruction processes occur on AC, especially if we take into account that the equilibrium state during the filtration is not reached. The date on the operation efficiency of the filter with AC are given in Table 1. Table 1. Efficiency of the tap water afterpurification using the filter with AC after subjecting this water to preliminary ozone treatment in combination with ultraviolet irradiation Indicator 3
Chemical oxygen demand (COD), mgO/dm Permanganate oxidizability (PO), mgO/dm3
Before the filter with AC
After the filter with AC
20.0–35.0
7.0–8.0
8.0
0.8
As can be seen from Table 1, the filter with AC operated quite efficiently during the whole period of its operation. For estimating the degree of natural bioregeneration of AC during the longterm service we determined the values of the effective specific surface area of AC samples (Sef), adsorption pore volume (Va), micropore vol ume (Vmi), and the values of ultimate specific adsorption (a∞) using the Langmuir and DubininRadush kevich models. The data obtained are presented in Table 2. Table 2. The effect of water ozonization on the variation of AC natural bioregeneration Indicator a, mmol/g: according to the Langmuir model according to the Dubinin Radushkevich model Sef, m2/g Va, cm3/g Vmi, cm3/g
Initial carbon (A1)
KAU1O (A2)
A2 to A1 ratio, %
KAU1N (A3)
A3 to A1 ratio, %
3.67 3.61
3.29 3.22
10.4 10.8
2.20 2.11
40.1 41.6
630 0.39 0.12
627 0.35 0.11
0.5 10.2 8.3
533 0.23 0.02
15.4 41.0 84.3
As can be seen from Table 2, the natural bioregeneration almost completely restored the structure–adsorp tion characteristics of AC with the discrepancy margin of ~10% in the case of filtering the ozonized water through the activated carbon. In contrast, the natural bioregeneration after filtering the nonozonized water in the first AC sample along the water flow was in the range of 58–60% of the specifications of the initial AC. JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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This is determined by the factors discussed above that ensure the more efficient functioning of AC during fil tering of the ozonized solutions. As was shown earlier, the degree of natural bioregeneration under the incomplete restoration of AC prop erties could be increased by washing AC with water, alkali liquor, or chloroform [23]. It can be expected that in the case of nearly complete restoration of the surface area and the adsorption pore volume of AC, additional washing of the carbon with different solvents may lead to a negative effect due to structural ruptures of the bio film formed on AC with products of its vital activity. The biofilm proper (or inoculated microorganisms on the AC surface) occupy only 0.2–0.5 m2/g and therefore do not affect the value of estimate Sef within the limits of experimental error. Table 3 presents data for comparing the structureadsorption characteristics of AC, which were used for longterm filtering of the ozonized and nonozonized tap water, after their washing with different solvents. Table 3. Recovery of the structure–adsorption characteristics of AC after their washing with different solvents Carbon
Indicator
KAU1O
a, mmol/g: according to the Lanmuir model according to the Dubinin Radushkevich model Sef, m2/g Va, cm3/g Vmi, cm3/g a, mmol/g: according to the Lanmuir model according to the Dubinin Radushkevich model Sef, m2/g Va, cm3/g Vmi, cm3/g
KAU1N
water
Value after washing alkali liquor
chloroform
2.92 2.97
3.15 3.11
2.34 2.28
524 0.32 0.12
632 0.34 0.10
545 0.25 0.10
1.42 1.49
0.69 0.68
1.67 1.59
375 0.16 0.02
377 0.07 0
522 0.17 0
The data comparison in Table 2 and Table 3 indicates that the disturbance of “AC surface–inoculated microorganisms” balance results in reduction of Sef, Va, and Vmi in all cases. As the first layers of AC come into contact with water containing the dissolved ozone, it may lead to reac tions of the chemical interaction of the ozone with the carbon surface area. Numerous investigations were devoted to the oxygen and ozone chemosorption by activated carbons from the gaseous phase [26–31]. It was established that both gasses react with AC forming the surface oxides and due to gasification of carbon pro ducing carbon monoxide or carbon dioxide. As was established in paper [32], reactions leading to formation of surface oxides proceed primarily at low temperatures, while the gasification reactions proceed at tempera tures above +80°C. The interaction of ozone with the AC surface in aqueous solution was studied in papers [33–38]. It should be noted that practically all quoted papers analyzed the processes at ozone concentrations close to its water solubility at the given temperature and pressure. The influence of trace concentrations of ozone on processes occurring in aqueous solutions with AC participation has not been actually considered in the literature. How ever, all papers note that oxygen is produced during the ozone decomposition on AC, and even the atomic oxy gen is formed under specific conditions [34]. Investigation of the interaction of trace amounts of ozone with AC surface that has been first carried out in the present paper revealed, as can be seen from data in Table 4, that variation of the content of functional groups on the AC surface varied in the case of contact between the AC surface and the ozonecontaining aqueous solution. As can be seen from Table 4, the chemical interaction between the dissolved ozone and AC surface occurs at concentrations of the dissolved ozone in water amounting to 2–3 mg/dm3 that results in formation of car boxyl and phenol functional groups. This leads to an enhanced degree of the surface hydrophilicity and, cor respondingly, to reduced value of the Gibbs free energy of adsorption of organic matter. In earlier paper [39] JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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we showed that the reduction of the Gibbs free energy of adsorption positively influenced the efficiency of bio filtration of organic matter solutions through AC, i.e., the bioregeneration of AC. Table 4. Variation of the content of functional groups on the AC surface due to its contact with ozonecontaining aqueous solution Carbon KAU1 KAU1O
Static exchange capacity, mgeq/g cationic anionic 0.75 0.64 1.15 0.60
Quantity of functional groups, mgeq/g carboxyl lactone phenol 0.15 0.45 0.15 0.25 0.45 0.45
CONCLUSIONS Thus, the presented data regarding the impact of preliminary ozone treatment of water on the efficiency of bioregeneration of AC make clear the following points. AC bioregeneration during filtering of the preozonized water proceeds more efficiently as compared with the bioregeneration under the similar conditions during fil tering of the nonozonized tap water. The enhanced efficiency of AC bioregeneration after the filtration of preozonized water is determined by the transformation of TOC into a better biodegrading form, increased microbiological activity in the depth of carbon bed, and higher degree of the hydrophilicity of the AC surface due to its chemical interaction with dissolved ozone. REFERENCES 1. Siddiqui, M.S., Amy, G.L., and Murphy, B.D., Water Res., 1997, vol. 31, pp. 3098–3106. 2. Winn–Jung Huang, Guor–Cheng Fang, and Chun–Chen Wang, Science of the Total Environment, 2005, vol. 345, pp. 261–272. 3. Leenheer, J.A., Rostad, C.E., Gates, P.M., Furlong, E.T., and Ferrer, I., Anal.Chem., 2001, vol. 73, no. 1, pp. 1461– 1471. 4. Win, Y.Y., Kumke, M.U., Specht, C.H., Schindelin, A.J., Kolliopouls, G., Ohlenbusch, G., Kleiser, G., Hesse, S., and Frimmel, F.H., Water Res., 2000, vol. 34, pp. 2098–2104. 5. Chiang, P.C., Chang, E.E., and Liang, C.H., Chemosphere, 2002, vol. 46, pp. 929–936. 6. Kim, W.H., Nishijima, W., Shoto, E., and Okada, M., Water Sci. Technol., 1997, vol. 35, pp. 147–153. 7. Magic Knezev, A. and Van der Kooij, D., Water Res., 2004, vol. 38, pp. 3971–3979. 8. GomezSerrano, V., Alverer, P.M., Jaramillo, J., and Beltran F.J., Carbon, 2002, vol. 40, pp. 523–529. 9. Valdés, H., SanchezPolo, M., RiveraUtrilla, J., and Zaror, C., Langmuir, 2002, vol. 18, pp. 2111–2116. 10. Mawhinney, D.B. and Yates, J.T., Carbon, 2001, vol. 39, pp. 1167–1173. 11. Metts, T.A. and Batterman, S.A., Effect of VOC Loading on the Ozone Removal Efficiency of Activated Carbon Filters, Chemosphere, 2006, vol. 62, pp. 34–44. 12. Alvarez, P.M., Garcia–Araya, J.F., Beltran, F.J., Masa, F.J., and Medina, F., J. Colloid and Interface Sci., 2005, vol. 283, pp. 503–512. 13. Lans, U. and Hoigne, J., Ozone Sci. Eng., 1998, vol. 20, no. 1, pp. 67–90. 14. Sanchez–Polo, M., von Gunten, U., and Rivera–Utrilla, J., Water Res., 2005, vol. 39, pp. 189–198. 15. Guiza, M., Ouederni, A., and Ratel, A., Ozone Sci. Eng., 2004, vol. 26, no. 3, pp. 299–307. 16. Beltran, F.J., Rivas, J., Alvarez, P., and MonterodeEspinosa, R., Ibid., 2002, vol. 24, no. 4, pp. 227–237. 17. SanchezPolo, A., RiveraUtrilla, J., Carbon, 2003, vol. 41, pp. 303–307. 18. Staehelin, J. and Hoigne, J., Environ. Sci. Technol., 1982, vol. 16, no. 10, pp. 676–681. 19. Chiang, H.L., Chiang, P., and Huang, C., Chemosphere, 2002, vol. 47, pp. 267–275. 20. Mishchuk, N.A., Goncharuk, V.V., and Vakulenko, V.F., Khimiya i Tekhnologiya Vody, 2003 vol. 25, no. 1, pp. 3–29. 21. Goncharuk, V.V., Vakulenko, V.F., Sova, A.N., Oleinik, L.M., and Shvadchina, Yu.Yu., Ibid., 2003, vol. 25, no. 3, pp. 407–427. 22. Goncharuk, V.V., Kozyatnik, I.P., Klimenko, N.A., and Savchina, L.A., Khimiya i Tekhnologiya Vody, 2007 vol. 29, no. 6, pp. 546–559. 23. Klimenko, N.A., Savchina, L.A., Sidorenko, Yu.V., and Vrubel’, T.L., Ibid., 2005, vol. 27, no. 5, pp. 479–495. 24. Koganovskii, A.M., Klimenko, N.A., Levchenko, T.M., and Roda, I.G., Adsorbtsiya organicheskikh veshchestv iz vody (Adsorption of Organic Matter from Water), Leningrad: Khimiya, 1990. 25. Boehm, H.P., Carbon, 2002, vol. 40, no. 1, pp. 145–149. JOURNAL OF WATER CHEMISTRY AND TECHNOLOGY
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