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 (9.4<pH<11.6) and the rate of decomposition of CNO~was found to be much slower than that of CN~. The decolorization rates of naphthol yellow and methylene 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 con- siderable 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 (3<pH<5). Figure4 shows the value of magainst pH obtained in this and other works. This figure indicates that the results are inconsistent in the acidic region, i.e., m equals 1, 1.5 or 2. Sennewald24) and Rothmund et al2Z) estimated m as 2, but Weiss29) stated that their data could be interpreted more exactly Fig. 3 kd vs. pH (Keys are the same as shown by assuming m as 1.5. Hewesand his coworker's in Fig. 5) result8} that m equals 2 was obtained only at pH of 2 and 4. Merkulova et al.15) reported that m was 1 or 2, depending on the reaction conditions. From these facts, it is reasonable to assume that mequals 1 or 1.5, although which value is correct has not yet been clarified.

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