Kinetics of the Self-Decomposition of Ozone and the Ozonation of Cyanide Ion and Dyes in Aqueous Solutions

KINETICS OF THE SELF-DECOMPOSITION OF OZONE AND THE OZONATION OF CYANIDE ION AND DYES IN AQUEOUS SOLUTIONS

Masaaki TERAMOTO,Seiichiro IMAMURA,Naoyuki YATAGAI, Yoshio NISHIKAWA and Hiroshi TERANISHI Department of Industrial Chemistry, Kyoto Institute of Technology, Matsugasaki, Kyoto 606

Kinetic studies on the self-decomposition of ozone in water, the decomposition of CN~and CNO~,and the decolorization of several dyes by ozone were performed over a wide range of pHusing a stopped-flow apparatus under homogeneousconditions at 298 K. The rate of self- decomposition of ozone was measured in the pH range from 1 to 13.5, and it was found that in the pH range above 8 the rate was expressed by -dlOs]/dt=374 [O3] [OH-]0-88 while in the acidic region the concentration dependence of the rate was complicated, and a simple rate expression could not be obtained. The rate of decomposition of CN~by ozone was expressed by -d[CN-]/dt=310 [O3]° 8 [CN-]° 55 (9.4methylene blue followed second-order kinetics. Rate constants are presented.

The purpose of this paper is to obtain the intrinsic Intr oduction reaction kinetics of the ozonation of several typical Ozone is a powerful oxidizing agent, and it has contaminants such as cyanide and cyanate ions and been recognized as very effective for disinfection, dyes in water using a stopped-flow technique under decolorization, deodorization and the decomposition homogeneous conditions. The rate of self-decom- of toxic chemicals in wastewater treatment. Thead- position of ozone in water, which is closely related to vantage of ozone treatment over the widespread use the degree of ozone utilization, was also investigated of chlorine is the absence of lasting toxic residuals over a wide range of pH. like chlorinated organic compounds after treatment. However, application of ozone is limited because of 1. Experimental higher installation and running costs compared with Kinetic measurementswere performed by meansof those of chlorination. Therefore, in order to utilize a spectrophotometric method.Therates of reactions ozone effectively it is necessary to obtain a full under- other than the self-decomposition of ozone at low pH standing of the behavior of ozone and of its interaction were quite fast, and were measured by a stopped-flow with various contaminants in water. apparatus (Union Giken RA-110). Water containing The kinetics of the reactions of ozone with contami- a known amount of ozone, prepared by bubbling ozone nants is important basic information for designing through deionized water, was mixed with the same ozone contactors of high performance. However, volume of a buffer solution adjusted to the desired while a numberof kinetic studies have been made values of pHand of the concentration of a substrate on the reaction of ozone with organic compoundsin in a cylindrical cell having 2 mmor 10 mmlight-path organic media, very few papers have been presented length, and the time-course of the absorbance of ozone on the kinetics in aqueous media. Furthermore, most or the reactant was followed. The self-decomposition of them were performed using gas-liquid heterogeneous of ozone at pH below 9 was followed by an ordinary contactors. The results thus obtained are the overall spectrophotometer equipped with a glass-stoppered kinetics disguised by the effect of mass transfer between cell of 10 mmpath length. Calibration charts of the gas and liquid phases. relation between absorbance and ozone concentration Received January 9, 1981. Correspondence concerning this article should be addressed to M. Teramoto. N. Yatagai is now with Kobe Steel Co., were prepared on the basis of the usual analytical Kobe 657. Y. Nishikawa is now with Japan I.B.M., Osaka 550. method employing potassium iodide. The absorption

VOL. 14 NO. 5 1981 383 Table 1 Summary of the kinetics of self-decomposition of ozone Investigators Year pH Temp. [K] m n ' Ar 'Kmol//)1""-" ^ - 1] Rothmund et al.2Z) 1913 2-4 273 2 ~0.37 ~80 la, 2a Sennewald24) 1933 5. 3-8 273 2 -0. 37 ^ 100-400 la, 2a Weiss29) 1935 2-8 273 1.5 <0.5 a) Alder et alP 1950 0.7-2.8 273, 300 1 0.5 2.24x1013exp (-58600/i?r)b> la, lb, 2a Stumm26) 1954 7.6-10.4 274-293 1 0.75 3.4x 1021exp(-111800/i?r) lc, 2a Kilpatrick etal.12) 1956 0.017-6.8 298 1.5 la, lb, Id, 2a 8-10 298 2 0.63 l.OxlO4 la, lb, Id, 2a Raukas et a/.19> 1962 5.4-8.5 278-298 1.5 Czapski et «/.6) 1968 10-13 298 1 1 700 lb, 2a, SF Rogozhkin22) 1970 9. 6-ll.9 298 1 0. 17 2. 5 2b, BC Merkulova et a/.15) 1971 0.22-1.9 278-313 1 or 2 la,2a, £=59.4kJ/mol Hewes ^ a/.8) 1971 2 313, 323 2 6.8 x l012exp(-81250/i?r) la, 2a, TR 4 303-333 2 2.5 x 1015exp(-96740/7?T) la, 2a, TR 6 283-323 1.5-2 3.8 X 1016exp(-96240/i?r)c) la, 2a, TR 8 283-293 1 1.23 x 1010exp(-70530/i?r) la, 2a, TR Horie ^ a/.10> 1973 5-10 293 1. 5 lb, 2a Rizzuti et al^ 1976 12-13.5 291-300 1 1 8.6 x l019exp(-96270/i?r) 2b, PC, d) Morooka et al.17> 1978 2-9 276-308 Eqs. (4) and (5) la, 2a la: iodometry; lb : spectrophotometry; lc: manganese oxidation followed by 77-toludine method; Id: manometric method; 2a : homogeneous ; 2b : gas-liquid heterogeneous ; BC: bubble column; PC : packed column; SF : stopped-flow method; TR: tubular reactor; a) analysis of data obtained by other investigators23>24) ; b) based on method lb. This is about 13 times that obtained by la; c) second-order rate constant ; d) analysis based on the theory of gas absorption with chemical reaction.

ozone in water, and it was found that the rate is strongly dependent on pH of the solution. However, one of the characteristics of such studies is that considerable disagreement was recognized among them concerning the reaction order with respect to ozone and OH~ in the pH range below 8, while in the pH range above 8 or 9 consistent results were obtained; the reaction is first order with respect to ozone. Furthermore, none of the earlier works alone covered a wide range of pH. For this reason, a series of Fig. 1 Self-decomposition of ozone at high pH experiments was carried out in the wide pH range from 1 to 13.5. Typical decay curves of ozone in the high pH region are shown in Fig. 1. It can be seen that the decomposition is markedly accelerated as pH increases, which suggests that hydroxide ion catalyzes the reaction. To analyze the kinetics, the integral method was used. If the rate of decomposition of ozone is given by -dC/dt=kdCá" (1) then the following equations are obtained. Fig. 2 Plot of ln(C0/C) vs. t for self-decom- In(C0/C)=kdt (m=l) (2a) position of ozone l-(C/Co)1-"=fcd(l-/w)Cr1^ (m*l) (2b) bands used were 255 nm for ozone, 425 nm for naphthol yellow and 665 nm for methylene blue. The solu- It was found that the plot of In(Co/C) vs. t gave straight lines as shown in Fig. 2, indicating that m, tions were buffered by use of the reagents shown in the reaction order with respect to ozone, is unity. Fig. 5. The ionic strength of the solution was not precisely adjusted to a constant value. The tempera- All data obtained at pH above 8 followed first-order ture was 298 K. kinetics. The pHdependence ofkd is shown in Fig. 3 together with the data obtained by other investigators. 2. Results and Discussion The agreement among these data is fairly good, and 2. 1 Self-decomposition of ozone the present data was correlated by As summarized in Table 1, a number of studies have /td=374 [OH-]0-88 (pH>8) (3) been madeof the kinetics of self-decomposition of The reaction order with respect to OH~obtained in

384 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN earlier studies ranged from 0.7526) to I6'21), which is in agreement with that obtained in this work (0.88). The reaction order m at pH below 8 was determined so that the best straight line might be obtained in the plots based on Eq. (2). The value ofm changed with pH in a complicated manner; m=l (pH<3 or pH>5) and m=1.5 (3life period of ozone is plotted against pH in Fig. 5, where calculated values based on other works are also shown. This figure clearly shows that the Fig. 4 m vs. pH for self-decomposition of ozone rate changes drastically with pH, the rate at pH 13 being 107 times that at pH 1, and that the pH dependence at lower pH is not so remarkable compared to that at higher pH. It is also seen that considerable scatter is observed in the acidic region. Morooka et al.17) proposed the following empirical rate equation for the self-decomposition of ozone in the pHrange

from-^=fc°a[OH-]°'28C1-B+ikS[OH1C2 to 9.(4)

fco =4.6X 1013 exp (-77480/7*7) | £2=1.8x 1018 exp (-S6190/RT) J From Eq. (4), the half-life period is derived. Fig. 5 Half-life period vs. pH for self-decom- 2 kllV>aVCo~KVo.5(i+WVCo) +

VOL. 14 NO. 5 1981 385 The experiments were performed with CNin a large excess over ozone and the ozone concentration during the decomposition was followed. Based on Eq. (8b) the experimental result is plotted as 1- (C/Coy~m vs. t. As shown in Fig. 6, ifm is assumed to be 0.8, this plot gives straight lines. Half-life periods range from 4 to 12ms, which are about 120 times shorter than those of self-decomposition at the same pH. The values ofkB$ calculated from the slopes of these plots are plotted against Bo in Fig. 7. The slope is 0.55. Thus the following rate expression is obtained. -d[O5]/dt=-d[CN-]/dt=3l0 [O3]°-8[CN-]0-55 (9)

Fig. 6 Ozonation of potassium cyanide The above equation was confirmed to hold in the pH range from 9.4 to ll.6, indicating that hydroxide ion does not catalyze this reaction under the conditions studied. Ikehata et al.ll) measured the rate of this reaction by the same method (stopped-flow method) and presented the rate expression -d[Os]/dt= -d[CN-]/dt=kf /[O3]°[CN-]0 -5 (10) (6

Fig. 7 Determination of n in ozonation of It is seen from comparison between Eqs. (9) and (10) potassium cyanide that while the values of n are approximately equal, a large discrepancy exists betweenthe values of m. This is probably due to the difference in the pH range studied; they observed that the rate depended on ozone concentration if pHwas higher than 8 in accordance with the present result. 2) Ozonation of cyanate ion Excess potassium cyanate was used over ozone. As shown in Fig. 8, plots of the kinetic data based on Eq. (8b) give straight lines ifm is taken as 1.3. The half-life period of ozone ranges from 100 to 450 ms, which is considerably longer than that for the ozon- Fig. 8 Ozonation of potassium cyanate ation of cyanide. By the same procedure, the follow- CN- +O3->CNCr +O2 (i) ing equation was obtained*. 2CNO- +3O3+H2O-^2HCO3- +N2+2O2 (ii) -d[O3]/dt=560 [O^^CNCT]0-55 (1 1) As shown later, reaction (i) is much faster than (ii). In this case, also, pHhad no effect on the rate constant 1) Ozonation of cyanide ion in the pH range from 8.2 to 10.4. It was found that the ozonation rate of CN~obtain- The rate equations(9) and (ll) suggest that the ed with gas-liquid contactors was strongly influenced reaction mechanism is very complicated, although a by mass transfer resistance because of its very rapid detailed discussion cannot be made here. reaction rate28). However, in most of the kinetic 2. 3 Decolorization of dyes studies made under heterogeneous conditions this Decolorizations of naphthol yellow (NY), methylene effect was not taken into account, and the results are blue (MB), indigo carmine blue and methanyl yellow inconsistent13' 14). wereinvestigated. The structures of these dyes are If the rate is expressed by shown in Fig. 9. Excess of ozone over dyes was used - dC/dt= - dB/dt=kCmBn (7) and the reaction was followed by the decrease in the absorbance of dyes. The rates of decolorization of then the following equations are obtained under the condition of BoyCo. * The rate expression for the decomposition of CNO~ In (C0/C)=kBZt (m=l) (8a) could not be obtained because the stoichiometric coefficient of reaction (ii) was not determined due to inaccuracy in the analysis l-(CICQy-"=k(l-m)BSCr1t (/w=£l) (8b) of CNCT.

386 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN indigo carmine blue and of methanyl yellow were too fast to be followed by the stopped-flow technique; the reactions were completed in less than several ms. The stoichiometry of the decolorization of NYand MBwas determined as follows. 200 ml of water containing a knownamount of ozone was poured into dye solution of a known concentration. These concen- trations were adjusted so that the amountof dye was in a small excess over that of ozone. After ozone was completely consumed, the quantity of the consumed dye was determined photometrically. The molar ratios of ozone consumed to dye decolorized were 3.2 for NYand 6.5 for MB, respectively. Integration of Eq. (7) under the condition of Co>i?o gives In (B0/B)=kC?t (n=l) (12) Decolorization of NYwas studied in the range of pH from 3.17 to 9.12. Plot of In([NY]0/[NY]) vs. t gives straight lines as shown in Fig. 10. This suggests that n is unity. From the slope of these lines, which Fig. 9 Structure of dyes employed corresponds to A:[O3]m, mis also given as unity. The rate expression is given by -d\NY]/dt=2.S X 104[O3] [NY] (13) The rate was almost independent of the pHemployed. The half-life period was in the range from 100 to 600 ms. A similarplotis showninFig. ll for MB.Therate was very fast and the half-life period was from 2 to 6ms. By the same procedure the rate equation was given by -d\MB]/dt=l.S x 106[O3] [MB] (45) , excludes the possibility of the attack of ozone on the benzene ring with OHand NO2groups on account of the inductive and/or resonance effect of these groups. Therefore, the reaction seems to occur at 1- or 2-position, which Fig. ll Decolorization of methylene blue has some isolated double bond character2)27), or at 3-position enriched with electron density due to the position of NY,resulting in the faster decolorozation electron-donating action of SO3 group (see Fig. 9). ofMB. In the case of MB, 1- and 2-positions, which con- Reactivity of carbon-carbon double bond toward stitute a butadiene structure, seem to be reactive. It ozone has been studied extensively and is known to be is also reported that carbon-nitrogen double bond is very large7} in accordance with the very fast rate for easily cleaved by ozone3'16>20) (3-position). Their re- indigo carmine blue. In the ozonation of monoazo- activity seems to be larger than that of 1-, 2- or 3- type acidic dyes, which are similar to methyanyl

VOL. 14 NO. 5 1981 387 yellow in structure, the cleavage of N-Cbond was T = temperature presumed by Onari18). He also obtained the rate t = time constant with a bubble column; the value is about t1/2 = half-life period 1000mol-/"1^"1. This small rate constant is probably due to the participation of diffusional effect on 0 = initial the overall rate. It has been often observed that the ozonation of Literature Cited 1) Alder, M. G. and G. R. Hill: /. Am. Chem. Soc, 72, 1884 contaminants in water is remarkably accelerated by (1950). raising the pHof the solution7>25). This seems to be 2) Bailey, P. S., S. S. Bath, F. Dobinson, F. J. Garcia-Sharp due to the production of active species (e.g., hydroxyl and C. D. Johnson: /. Org. Chem., 29, 697 (1964). radical9}) by the hydroxide ion-catalyzed decom- 3) Bailey, P. S., A. M. Reader, P. Kolsaker, H. M. White and position of ozone. In this work, however, no effect J. C. Barborak: ibid., 30, 3042 (1965). of pH on the rates of oxidation of CN", CNO~and 4) Bailey, P.S. and A.G.Lange: /. Am. Chem. Soc, 89, some dyes was found. This is because the rate of 4473 (1967). 5) Bailey, P.S., J.W.Ward and R.E.Hornish: ibid., 93, decomposition of these compoundsby ozone is much 3552 (1971). faster than that of the decomposition of ozone by 6) Czapski, G., A. Samuni and R.Yelin: Isr. J. Chem., 6, hydroxide ion. 969 (1968). 7) Evans, III, F. L.: "Ozone in Wastewater Treatment", Conclusion Ann Arbor Science, Ann Arbor (1972). The rate of self-decomposition of ozone was meas- 8) Hewes, C. G. andR. R. Davison: AIChEJ., 17, 141 (1971). 9) Hoigne, J. and H. Bader: Water Res., 10, 337 (1976). ured over a wide range of pH. It was found that in 10) Horie, M., K. Shizuno and H. Takeuchi: Yosui to Haisui, the range of pH higher than 8, the rate equation was 15, 345 (1973). expressed by Eqs. (1) and (3), and the present result ll) Ikehata, A., K. Ishizuka, N. Tabata and R. Hashimoto: agreed fairly well with those obtained by other investi- Anzenkogaku, ll, 74 (1972). 12) Kilpatrick, M. L., C. C. Herrick and M. Kilpatrick: /. Am. gators. On the other hand, in the acidic region, con- Chem. Soc, 78, 1784 (1956). siderable scatter of the data was generally observed. 13) Kwandelwal, K. K., A. J. Barduhn and C. S. Grove, Jr.: The rates of decomposition of CN~, CNO~and some Adv. Chem. Ser., 21, 78 (1959). dyes by ozone are also presented. The present results, 14) Matsuda, Y., T. Fujisawa, S. Fujikawa, Y. Takasu, Y. Tanaka and H. Imagawa: Nippon Kagaku Kaishi, 602 obtained under homogeneousconditions, are consider- (1975). ed more reliable than those obtained under heterogeneous conditions. 15) Merkulova, V. P., V. S. Lovchikov and M. D. Ivanovskii: Izv. Vyssh. Ucheb. Zaved., Khim. Khim. Technol, 14, 818 (1971). Acknowledgment 16) Miller, R. E.: /. Org. Chem., 26, 2327 (1961). The authors express their deep gratitude to Prof. Wataru 17) Morooka, S., K. Ikemizu and Y. Kato: Kagaku Kogaku Eguchi, Kyoto University, for making the stopped-flow spectro- Ronbunshu, 4, 377 (1978). photometer available for their present work. This research was 18) Onari, Y.: Nippon Kagaku Kaishi, 1570 (1978). supported by Scientific Research Grant of the Ministry of 19) Raukas, M. M., E.K.Siirode and S.R.Kyulm: Trudy Education, Science and Culture, Japan, Grant No. 021 909 (1 975). Tallinskoga Politekniich. Inst. Ser., A198, 219 (1962). 20) Reader, A.M., P.S.Bailey and H.M.White: /. Org. Chem., 30, 784 (1965).

NomenclB ature = concentration of substrate (CN~, CNO" 21) Rizzuti, L., V. Augugliaro and G. Marrucci: Chem. Eng. ScL, 31, 877 (1976). c or dye) [mol//] 22) Rogozhkin, G. I.: Tr. Vses. Nauch.-Issled. Inst. Vodo- E = concentration of ozone [mol// ]

k = activation energy [J/mol] snabzh. , Kanaliz. , Godrotekh. Sooruzhenii, Inzh. Gidrogeol. , = reaction rate constant [(mol//)1-á""71 ^"1] No. 25, 76 (1970). kf = reaction rate constant for self-decomposition 23) Rothmund, V. and A. Burgstaller: Monat. Chem., 34, 665 of ozone [(mol//)1""1-"' à" s"1] (1913). kd = reaction rate constant for self-decomposition Sennewald, K. : Z. Phys. Chem., A164, 305 (1933). of ozone defined by Eq. (1) [(mol//)1"á"à" s"1] Singer, P. C: Water Res., 9, 127 (1975). k« = reaction rate constant for self-decomposition Stumm, W.: Helv. Chim. Acta, 37, 773 (1954). w of ozone defined by Eq. (4) [(mol//)"0-78 ^-1] Sturrock, M.G., B.J.Cravy and V.A.Wing: Can. J. = reaction rate constant for self-decomposition Chem., 49, 3047 (1971). of ozone defined by Eq. (4) [(mol//)"1 à" s"1] m 28) Teramoto, M., Y. Sugimoto, Y. Fukui and H. Teranishi:

n = reaction order with respect to ozone /. Chem. Eng. Japan, 14, 111 (1981). = reaction order with respect to substrate 29) Weiss, J.: Trans. Faraday Soc, 31, 668 (1935). n' = reaction order with respect to OH~for self-decomposition of ozone (Presented at the 1 1th Autumn Meeting of The Soc. ofChem. R =gasconstant [J/mol - K] Engrs., Japan, at Tokyo, Oct. 5, 1977.)

388 JOURNAL OF CHEMICAL ENGINEERING OF JAPAN