Environ. Sci. Technol. 1986, 20,50-55
Aieta, E. M. Ph.D. Dissertation, Stanford University, Stanford, CA, 1984. Aieta, E. M.; Hernandez, M.; Roberts, P. V. J.-Am. Water Works Assoc. 1984, 76,64-70. Sokal, R. R.; Rohlf, F. J. “Biometry”;W. H. Freeman: San Francisco, CA, 1969. Connick, R. E.; Chia, Y.-T. J . Am. Chem. SOC.1959,81, 1280-1284.
Brian, P. L. T.; Vivian, J. E.; Piazza, C. Chem. Eng. Sci. 1966,21, 551-558.
Gilliland, E. R.; Baddour, R. F.; Brian, P. L. T. AIChE J . 1958,4 (2), 223-230.
(24) Perlmutter-Hayman,B.; Wieder, H.; Wolff, M. H. Isr. J . Chem. 1973,ll ( l ) , 27-36. (25) Himmelblau, D. M. Chem. Rev. 1964,64, 527-550.
Received for review June 11, 1984. Revised manuscript received April 26,1985. Accepted August 13,1985. This work was funded by the U.S. Environmental Protection Agency under Research Grant R-808686. It has not been subjected to the Agency’s required peer and adminstrative review and therefore does not necessarily reflect the views of the Agency; no official endorsement should be inferred.
Kinetics of the Reaction between Molecular Chlorine and Chlorite in Aqueous Solution E. Marco Aletat and Paul V. Roberts” Department of Civil Engineering, Stanford University, Stanford, California 94305
The kinetics of the aqueous phase reaction between molecular chlorine and chlorite ion was studied experimentally by measuring the enhancement of chlorine transfer from the gas phase. The results were interpreted by using the penetration theory of mass transfer, in conjunction with the assumption that the reaction is second order overall, being first order with respect to both chlorine and chlorite. The second-order rate constant was determined as a function of temperature and solution ionic strength. These experiments indicated that the reaction rate constant a t 293 K and zero ionic strength is 1.62 X lo4 M-%-l. The reaction rate constant increased with increasing temperature, having an activation energy of 39.9 kJ-mol-l, and decreased with increasing ionic strength. A collision theory interpretation of the observed rate indicates that the steric factor is on the order of unity, suggesting that nearly every collision of sufficient energy results in reaction, regardless of the relative orientation.
Introduction The reaction between dissolved molecular chlorine and aqueous chlorite represents a potential route for generating chlorine dioxide, a promising alternative to chlorine for disinfection in water and wastewater treatment (1-6). Chlorine dioxide commonly is generated by acid activation of aqueous chlorite or by contacting aqueous chlorite with aqueous chlorine under conditions such that the predominant reaction is between chlorite ion and hypochlorous acid (1-4). The latter method, which is most common in U.S.practice, affords an acceptably high yield on the basis of the chlorite reactant, but, as usually practiced, requires an excess of chlorine (1) and can result in the formation of substantial amounts of byproduct chlorate (4). There is some evidence that the yield and purity of the chlorine dioxide product can be enhanced by contacting chlorine gas directly with concentrated chlorite solution (7), thus minimizing the amounts of unconverted chlorine and chlorite and of byproduct chlorate. Important benefits could be gained from choosing conditions to maximize the yield and purity of the chlorine dioxide generated for water treatment. Improving the yield, especially that based on chlorite, decreases the cost of treatment. Decreasing the amount of unreacted chlorine present in the product reduces the extent of reactions between chlorine and organic ‘Present address: Rio Linda Chemical 95814. 50
Co., Sacramento, CA
Environ. Sci. Technol., Vol. 20, No. 1, 1986
constituents in the water being treated, thus minimizing the potential formation of halogenated organic byproducts. Reducing the amounts of chlorite and chlorate helps alleviate concern regarding possible adverse health effects of chlorine dioxide use in water treatment. To realize these potential benefits of improved chlorine dioxide generation, the kinetics of the reaction between molecular chlorine and aqueous chlorite, as well as the chemistry and kinetics of competing reactions, must be understood. This paper is intended to elucidate and quantify the kinetics of the reaction between dissolved molecular chlorine and chlorite ion in aqueous solution, as pertains to chlorine dioxide generation for water treatment.
Chlorine-Chlorite Reaction and Mechanism Stoichiometry. In the oxidation of aqueous chlorite by chlorine or hypochlorous acid, both chlorine dioxide and chlorate appear as products. In acid solution, where the chlorine is present mainly as dissolved molecular chlorine gas, the stoichiometries of the two reactions are Clz + 2c102- = 2c102 + 2c1(1) and C12
+ C102- + H 2 0 = C103- + 2C1- + 2H+
(2)
In solutions near neutral pH, where chlorine is present largely as hypochlorous acid, the stoichiometries are HOCl + 2C102- = 2C102 + C1- + OH(3) and HOCl
+ ClOz- + OH- = C103- + Cl- + HzO
(4)
In alkaline solutions, where the chlorine is present as hypochlorite ion, the reaction is very slow and the only product formed is chlorate ion: OC1- CIOz- = C103- + C1(5)
+
Mechanism. Taube and Dodgen (8), using radioactive chlorine, provided significant insight into the reaction mechanisms of the oxidation of chlorite by chlorine (or hypochlorous acid), the reduction of chlorate by chloride ion (and the reverse reaction), and the disproportionation of chlorous acid. The observations of Taube and Dodgen led them to postulate an unsymmetrical intermediate that was common to all three reaction mechanisms. Subsequent work by Emmenegger and Gordon (9) has amplified and
0013-936X/86/0920-0050$0 1.50/0
0 1985 American Chemical Society
refined the mechanism proposed by Taube and Dodgen (8), but their original findings remain as the foundation for many of the subsequent studies of the kinetics and mechanisms of aqueous chlorine chemistry. From the dependence of the product ratio of chlorine dioxide to chlorate on experimental conditions, the following consensus has emerged (8, 9): (1)The reaction between chlorite ion (or chlorous acid) and chlorine (in acid or neutral solution) is second order overall, being first order in both chlorine and chlorite; (2) Acidic conditions favor the formation of chlorine dioxide, whereas under neutral and akaline conditions, chlorate is the primary reaction product; (3) For a given pH value, an increase in chlorite concentration results in relatively more chlorine dioxide production; (4) Higher chloride ion concentrations favor the formation of chlorine dioxide relative to chlorate, especially at acidic pH; (5) Proportional increases in all reactant concentrations favor the formation of chlorine dioxide relative to chlorate. The mechanism for the chlorine-chlorite reaction proposed by Taube and Dodgen (8) as consistent with experimental observations will be presented. The chlorine atoms with asterisks trace the fate of the chlorine originally present as molecular chlorine or hypochlorous acid. Hypothetical, metastable intermediate species are enclosed in brackets. For the reaction of chlorite with molecular chlorine *a2
+
c10,-
+
= (%-c< l ;)
"CI-
(6)
and with hypochlorous acid Ht
+
HO*CI
+
CI0,-
=
'3
(
C ' I-CI,
+
H20
(7)
The metastable intermediate can decompose to form either chlorine dioxide z(*CI-cI
