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OZONATION OF ORGANIC REFRACTORY COMPOUNDS IN IN COMBINATION WITH UV RADIATION

KlYOSHi IKEMIZU, Masafumi ORITA, Masahiko SAGIIKE, Shigeharu MOROOKAand Yasuo KATO Department of Applied Chemistry, Kyushu University, Fukuoka 812

Key Words: Environment, , , Ultraviolet Decomposition, Ozonation, Water Treatment, Pollutant

The ozonation rates of organic refractory compoundssuch as aliphatic carboxylic and were determined in water at 293K. The experiment was carried out by recirculating a solution between an ozone absorption column and a rectangular ozone/UV reactor, and the time-dependent changes in of organic substances were directly measured. In the presence of UVradiation of about 30W m~2, the initial ozonation rate of organic substances was increased by 10-104 times. The total organic carbon was effectively decreased in the presence of UVradiation. This was explained by the destruction of acetic and oxalic by HO' radicals which were produced in the ozone/UV system. The ozone/UV oxidation rate of acetic and was roughly proportional to the degree of .

Introduction that UVradiation enhanced the oxidation rate by 102 to 104 times. Glaze et al.4) destroyed trihalomethane Recently, ozonation is attracting muchattention as precursor by a ozone/UVprocess. Even refractory an advanced water treatment process. Ozone can precursors which were produced by primary oxi- easily oxidize olefins, heterocyclics and nucleo- dation of carbonaceous compounds could be de- philes.6'8'12'13'16) However, refractory compounds stroyed. Underthe experimental conditions of Glaze such as saturated alcohols and carboxylic acids are et al.4\ however, the concentration of ozone in water built up in the system during the ozonation. In was virtually zero in the presence of UVradiation, particular, and oxalic acid are major end and no quantitative information was given on the products after the ozonation of organic compounds roles of dissolved ozone concentration and UV and are very resistant to further ozonation.11} These irradiance. refractory compounds must be oxidized by HO" Most previous investigators measured the con- radicals.1'2'5' sumption rate of ozone to describe the destruction The generation of hydroxyl radicals can be en- rate of organic substances.19) However, radiation of hanced by raising the alkalinity of solution,3'17) but UV light fruitlessly decomposes ozone in the gas this method is not suitable for advanced water treat- phase before it is transferred into the solution ment because it needs the addition of other chemicals. phase.14) In this study, therefore, we directly de- Thus ozonation combined with UV radiation be- termined the change in concentration of aliphatic comes very promising. The UVradiation dissociates compounds. The concentration of dissolved ozone ozone into an and an oxygen atom and the UV irradiance were monitored, and the in the XD state. The latter reacts with water to destruction rate of refractory compounds was de- produce HO ' radicals.10'14) termined as a function of their concentration, pH value and UVirradiance. The oxidation mechanism O3 -O2+OCD) (1) was also studied. OOD) + H2O >2HO (2) 1. Experimental The decomposition rate of ozone in the presence of Figure 1 shows a schematic diagram of the experi- UVradiation was investigated by Ikemizu et al.9) mental apparatus. A mixture of ozone and oxygen Prengle et al.15) applied ozone/UV treatment to was bubbled into an absorption column of 4cm i.d. remove refractory compounds in water and found and 25 cm height. The ozone-saturated solution was Received December 1, 1986. Correspondence concerning this article should be continuously pumped into the photochemical re- addressed to S. Morooka. M. Sagiike is now with Nippon Denso Corp., Aichi 448. actor. The concentration of ozone in the inlet and

VOL. 20 NO. 4 1987 369 rectangular reactor is calculated from the mass bal- ance between the inlet and the outlet of the reactor which is considered to be differential. ~ (Ce- Ca)Qx = VrPk[O3rC« (4) The value of Cr in the rectangular reactor is virtually equal to Ca in the absorption column. To simplify the calculation, the ozone concentration in the ozone/UV reactor is expressed by the average value as described in the previous paper.9) The ozonation rate without UV radiation is proportional to the ozone con- Fig. 1. Schematic diagram of experimental apparatus. centration,^ while the self-decomposition rate of ozone is proportional to [O3]7.9) So we can assume outlet stream of the reactor was frequently measured p=m=m/=1.0 (5) with a spectrophotometer (Shimadzu UV-180) equipped with a micro flow cell. The reaction order of organic substances is dependent The flow to the reactor or to the spectropho- on substance concentration and pHvalue. tometer was changed by switching a three-way valve. From Eqs. (3), (4) and (5), we obtain The concentration of ozone was also determined by iodometry when organic species showed strong absor- -Va^~=VrkT[OslC:+Vak'[_O3\Cna (6) ption spectra near 260 nm. Organic compoundswere determined by liquid chromatography. First, we measurethe reaction rate without UV The photochemical reactor was a horizontal radiation, and then determine the reaction rate in the rectangular channel 10mmwide, 4mmthick and presence of UV radiation knowing the time- 190mmlong, and was sandwiched with transparent dependent change of Ca and using Eq. (6). quartz plates 3 mmthick. Details of the reactor were shown in the previous paper.9) A low- mer- 2. Results and Discussion cury lamp (20W, 30cm long) was the source of UV Figure 2 shows changes in the of radiation, principally at 253.7nm. The mean UV aliphatic alcohols and carboxylic acids in the ozone irradiance in the reactor was measured by actino- treatment. Acetic, oxalic, propionic and succinic acids metry using potassium ferric oxalate.9) are very resistant to ozonation. In particular, acetic Deionized and distilled water was used in the acid is virtually unreactive with ozone alone. experiments. All chemicals were grade and However, these refractory compoundsare oxidized at were used without further purification. The pH value appreciable rates in the presence of UVradiation, as of the solution was controlled by injecting aqueous shown in Fig. 3. NaOH and H3PO4 solutions. The timing and the Figure 4 illustrates the effect ofUVradiation on the injection amount were controlled by a microcom- initial ozonation rate of each substance. (- AC/At)r is puter. The rectangular reactor was immersed in a the initial UV ozonation rate, kI[O3]rC^0, and constant- bath. The liquid temperature (-AC/At)a indicates the initial ozonation rate, was kept at 293 K. The dissolved ozone concentration was 0.12-0.42mol-m"3, and the mean UVirradiance As shown in Fig. 4, there are no large differences in was-24-36W-m~-2.the ozone/UVoxidation rates except for oxalic acid. The ozonation of organic components occurred However, Anbar et al.1} have reported that the reac- both in the ozone absorber and the rectangular tivity of acetic acid toward HO"radical is an order ozone/UVreactor. The solution volumeof the former of magnitude smaller than that of most aliphatic was about 200cm3 and that of the latter was 7.6cm3. compounds. This discrepancy can be explained by The mass balance of an organic compoundin the the difference in the methods of evaluating the reac- absorption column is given by Eq. (3) if the liquid in tion rate. Anbar et al.1] measured the consumption the absorption column is mixed perfectly. rate of HO' radicals by competition reaction with bromide or /?-nitrosodimethylaniline, and con- -va^f=-(Ce~CM+Kk'lO3i:C»a (3) sidered it as the reaction rate of the organic com- pound although the ozonation consisted of sequen- The second term on the right-hand side of Eq. (3) tial reactions initiated by the electrophilic attack meansthe reaction of the organic substance in the of HO' radicals. In the present study, on the other absorption column. hand, the time-dependent change in concentration The conversion of the organic substance in the of organic solutes was directly measured. 370 JOURNAL OF CHEMICAL ENGINEERING OF Fig. 2. Ozonation of aliphatic compounds without UV radiation. [O3]fl=0.33-0.42molà"m~3; Ca0=3.6-5.2molà" Fig. 5. Ozonation of malonic acid with and without UV radiation. [O3]fl=0.38-0.42mol-m~3; [O]r=0.27-0.34mol- m~3; pH=5. m~3; pH=5.

much increased by UVradiation. This is why oxalic acid in the presence of UVradiation is destroyed quickly after the disappearance of malonic acid. The product patterns in the ozonation of typical aliphatic compoundsconfirm that hydrogenatom on the a-carbon is first subtracted by HO' radicals.1}

Fig. 3. Ozonation of aliphatic compounds in the presence of UV radiation. [O3]a=0.28-0.37mol-m"3; [O3]r=0.24- 0.34mol-m~3; Ca0=3.9-5.1 mol-m~3; pH=5. Keys are the same as in Fig. 2.

No accumulation of is observed because its ozonation rate is rapid compared to that of other species. peroxide produced during the ozonation may participate in the reaction chain.18) Figure 6 shows the ozonation of acetic acid at pH=5 in the presence of UVradiation. The major product was oxalic acid in the pH range of 5-8. ,yO HO ,O COCT Fig. 4. Ozonation rate of aliphatic compounds. pH=5. CH3-C^ > CH2-C^ > | (8) Keys are the same as in Fig. 2. The total concentration of acetic acid and oxalic acid was nearly equal to half the TOCvalue. The absorption coefficient at 253.7nm for oxalic Figures 7 and 8 show the initial ozonation rates of acid is about one order of magnitude larger than that acetic acid and oxalic acid respectively. When the for other aliphatic compounds. This mayincrease the initial ozonation rate is very low, the reaction is nearly destruction rate of oxalic acid in ozone/UVtreatment. proportional to the acid concentration for both cases. Figure 5 shows the ozonation of malonic acid with Figure 9 illustrates the effect of pHvalue on the and without UV radiation.Oxidized product is mostly initial ozonation rate of acetic and oxalic acid in the oxalic acid. The results in Fig. 4 indicate that the ratio presence of UYradiation. The initial ozonation rate is of the ozonation rate of oxalic acid to malonic acid is larger at higher pHvalues, and is dependent on the

VOL 20 NO. 4 1987 371 Fig. 6. Ozonation of acetic acid in the presence of UV radiation. [03]r=0.ll-0.29mol-m"3; pH=5.

Fig. 9. Effect of pH on ozonation rate of acetic acid and oxalic acid in the presence of UV radiation. Ca0= 20mol-m~3, 7=28.2-36.1 W-m~2. Other conditions are the same as in Fig. 7.

where Ka is the . In the case of oxalic acid, the second-stage dissociation is con- sidered. The ozonation rate of oxalic acid is well correlated with the degree of dissociation as shown in Fig. 10. This means only oxalate ions dissociated to the second-stage are reactive toward HO" radicals. COO" HO COO Fig. 7. Effect of concentration on ozonation rate of acetic | > \ >CO2 +CO2 >2CO2 acid. [O3]a=0.12-0.33mol-m-3; [O],=0.09-0.31 mol-m'3. COO" COO" (10)

Undissociated carboxyl group attracts electrons of the carboxylate group and decreases the nucleo- philicity of the reactive site.1>7'19) cocr COOH Another explanation is intramolecular hydrogen bonding, which reduces the reactivity of partially dissociated oxalic acid. O O n n C-- C

The difference in ozonation rate for undissociated acetic acid and is also explained by the Fig. 8. Effect of concentration on ozonation rate of oxalic electron-attracting effect of the carboxyl group. acid. [O3L=0.13-0.37mol-m-3; [O3]r=0.09-0.25mol-m"3. Figure ll shows the effect of coexisting salts on the oxidation rate of acetic acid. Whenthe concentration of carbonate ions is 20 times that of acetate ions, the degree of dissociation. oxidation of acetate ions is virtually terminated. ,_ [B~] _ 1 (9) Hydroxyl radicals are scavenged by carbonate ions as [HB]+[B-] l+[H+]/Ka CO^+HO >CO3-+OH~ (ll)

372 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN Nomenclature [B~] = concentration of dissociated acid [mol -m~3] Ca = concentration of organic substance in ozone absorption column [mol à"m"3] Ca0 = concentration of initial value of Ca [mol - m~3] Ce = concentration of organic substance at outlet of ozone/UVreactor or at inlet of ozone absorption column [mol à"m"3] [H+] = concentration of H+ ion [mol-m~3] [HB] = concentration ofundissociated acid [mol-m~3] / = mean UVirradiance in ozone/UV reactor [W-m-2] Ka = dissociation constant [mol -m~3] k' = reaction rate coefficient in ozone absorption Fig. 10. Effect of dissociation on ozonation rate of acetic column, defined by Eq. (3) acid and oxalic acid in the presence of UV radiation. k = reaction rate coefficient in rectangular Experimental conditions are the same as in Fig. 9. Numerals ozone/UV reactor, defined by Eq. (4) in the figure are pH values. m, m' = exponent for concentration of dissolved ozone [-] n, n' = exponent for concentration of organic substance [-] [O3L = concentration of dissolved ozone in ozone absorption column [mol à" m"3] [03]r = concentration of dissolved ozone averaged in ozone/UV reactor [mol - m~3] p = exponent for UV irradiation [-] Qx = liquid flow rate [m^s"1] t = time [s] Va = liquid volume in ozone absorption reactor [m3] Vr = liquid volume in ozone/UV reactor [m3] a = degree of dissociation [-]

Fig. ll. Effect of radical scavengers on ozonation rate of Literature Cited acetic acid in the presence ofUV radiation. /= 30-34 Wà"m~2; [O3]a=0.13-0.2mol-m-3; [O3]r=0.ll-0.13mol-m"3; pH= 1) Anbar, M., D. Meyerstein and P. Neta: /. Chem. Soc. (B), Phys. Org., 1966, 742. 2) Anbar, M. and P. Neta: Int. J. Applied Radiation andIsotopes, 18, 493 (1967). 3) Buhler, R. E., J. Staehelin and J. Hoigne: J. Phys. Chem., 88, The effect of phosphate ions is not appreciable. 2560 (1984). Conclusion 4) Glaze, W. H., G. R. Peyton, S. Lin, R. Y. Huang and J. L. Burleson: Environ. Sci. Technol, 16, 454 (1982). Typical aliphatic compounds were ozonized with 5) Hoigne, J.: "Handbook of Ozone Technology and and without UVradiation. The oxidation rate was Applications," Vol. 1, p. 341, R. G. and A. Neter (eds.), determined by the direct measurement of time- Ann Arbor Science, Ann Arbor (1982). dependent change in substance concentration. The 6) Hoigne, J. and H. Bader: Water Res., 17, 173 (1983). 7) Hoigne, J. and H. Bader: Water Res., 17, 185 (1983). contribution of UVradiation to the oxidation rate 8) Ikemizu, K., S. Morooka and Y. Kato: Chem. Eng. Commun., was separated from that of molecular ozone. The 34, 77 (1985). initial ozonation rate of acetic and oxalic acid in the 9) Ikemizu, K., S. Morooka and Y. Kato: J. Chem. Eng. Japan, presence of UVradiation was found to be controlled 20, 77 (1987). by the formation rate of HO' radicals when the 10) Jones, I. T. N. and R. P. Wayne: Proc. Roy. Soc. London, Ser. A, 319, 273 (1970). substance concentration was high. ll) Krasnov, B. P., D. L. Pakul and T. V. Kirillova: Int. Chem. The reactivity of carboxylate ion toward HO' Eng., 14, 747 (1974). radical was higher than that of undissociated car- 12) Leitis, E.: U. S. Department of Commerce PB80-174349 Final boxylic acid. Carbonate ions acted as scavengers of Report. HO' radicals, but the role of phosphate ions was 13) Morooka, S., K. Ikemizu, H. Kamano and Y. Kato: /. Chem. not appreciable. Eng. Japan, 19, 294 (1986). 14) Norrish, R. G. W. and R. P. Wayne: Proc. Roy. Soc. London, Ser. A, 288, 361 (1965). Acknowledgment 15) Prengle,H.W. Jr.,C. E. Mauk, R.W. LeganandC. G. Hews The authors are grateful to Prof. Makoto Takagi, Dept. of III: Hydrocarbon Process., Oct., 82 (1975). , Kyushu Univ. for useful discussion. 16) Rice, R. G. and M. E. Browning: "Ozone Treatment of

VOL. 20 NO. 4 1987 373 Industrial Wastewater," p. 288, Noyes Data Corp., Park 18) Staehelin, J. and J. Hoigne: Environ. Sci. TechnoL, 16, 676 Ridge (1981). (1982). 17) Staehelin, J., R. E. Buhler and J. Hoigne: /. Phys. Chem., 88, 19) Staehelin, J. and J. Hoigne: Environ. Sci. TechnoL, 19, 1206 5999 (1984). (1985).

REACTIVE CRYSTALLIZATION OF BY LIQUID-LIQUID REACTION

Hideki TSUGE, Yasushi KOTAKI and Shin-ichi HIBINO Department of Applied Chemistry, Keio University, Yokohama 223

Key Words: Reactive Crystallization, Crystallization Kinetics, Crystal Size Distribution, Alkalinity, Calcium Carbonate Crystallization experiments with calcium carbonate by three liquid-liquid reaction systems from a MSMPR crystallizer were conducted to makeclear the characteristics of reactive crystallization kinetics. As it was confirmed that the crystal growth obeys the ALlaw from the CSDobtained, the nucleation rate and growth rate wereobtained. The effects of the operating factors and the reaction systems on the crystallization kinetics were studied. It was clarified that the nucleation rate and the growth rate are correlated by the power law model, and that the kinetic orders in the power law model are correlated with carbonate alkah'nity irrespective of reaction system.

Introduction the effect of three liquid-liquid reaction systems on the crystallization kinetics of calcium carbonate in the The characteristics of crystallization of sparingly range of relatively dense suspension compared soluble salts by chemical reactions are important in with that of Stevens et al.,5J) to correlate the kinetics achieving better design and more efficient operation. for three liquid-liquid reaction systems with the power Many studies of crystallization kinetics, that is, of law modeland to makeclear the effect of the alka- nucleation and growth rates, have been madefor linity conditions on their kinetic orders in the power continuous mixed-suspension mixed-product removal law model. (MSMPR) crystallizers and are correlated by the 1. Experimental following power law model: B°ozGi (1) 1.1 Experimental apparatus and procedure Figure 1 shows a schematic diagram of the experi- Published crystallization kinetics from MSMPRcrys- mental apparatus. Thecrystallizer was a continuous tallizers were reviewed by Garside et al.2) stirred tank reactor madeof acrylic resin, which is Calcium carbonate, a well-known sparingly soluble considered to be a continuous MSMPRreactor. The , is produced industrially by gas-liquid reactive reactor was 0.1 m in diameter and the liquid height crystallization, while calcium carbonate produced by was 0.14m. The impeller used was a 4-blade turbine liquid-liquid reaction is used for special needs, where type and was operated at 357 rpm to ensure complete crystallization kinetics and crystal systems depend on mixing. Feed solutions were pumpedinto the crystal- liquid-liquid reaction systems. The crystallization ki- lizer continuously to produce calcium carbonate. The netics of calcium carbonate by liquid-liquid reaction reaction temperature was maintained at 293 K. The has been studied mainly for the lime-soda water EDTAtitration method was used to determine cal- softening process by Stevens et al.5J) They described cium ion concentration. its kinetics by the power law model, while their Figure 2 shows the relation between the pH value of suspension density was rather low. reactants and reaction time with 0 as a parameter. The objectives of the present paper are to discuss The pH values became constant after five residence Received December 5, 1986. Correspondence concerning this article should be times, when steady state was ascertained by little iddressed to H. Tsuge.

374 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN