Environ. Sci. Technol. 1903, 17, 743-747
Experimental Rate Constants and Reactor Considerations for the Destruction of Micropollutants and Trihalomethane Precursors by Ozone with Ultraviolet Radiation H. William Prengie, Jr. Chemical Engineering Department, University of Houston, Houston. Texas 77004
rn Ozone/UV photooxidation is used for the destruction of refractory and toxic industrial wastewater components. This paper presents its application to removal of micropollutants and trihalomethane precursors in raw water. By use of recently published data, rate constants were determined and correlated by a mathematical model, containing two terms, one for O3 oxidation and the other for 03/UV photooxidation. The data indicate the dominant effect of the latter reaction process. The most significant data produced were for micropollutants and trihalomethane precursors (THMP) in lake water, where the simultaneous oxidation of total organic carbon (TOC) is a very important factor. Organic carbon destruction is relatively unaffected by the presence of micropollutants. Reactor design to obtain high rate mass transfer and ozone utilization is essential to the successful cost effective application of the process. Introduction The ozone/UV process for photooxidation of refractory and toxic inorganic and organic species was developed in the early 1970s. The first quantitative reaction rate data were presented in 1973 ( I ) , and followed by extensive data on oxidation of organic compounds in 1975 (2) and 1976 (3, 4). And subsequent to that several papers (5-8) on process applications were published, as well as three patents (9-11). Three commercial scale installations on industrial type wastewater are now in operation. A substantial body of research has been completed on a broad range of compounds: metal-cyanide complexes; organic and amino acids; alcohols; insecticides; organic nitrogen, phosphorus, and sulfur compounds; chlorinated organics; inorganic anions and cations; numerous composite industrial wastewater streams. Three major areas of application exist for treatment of (a) industrial wastewater, (b) municipal wastewater, and (c) raw source water. And now detailed research, presented by Glaze and coworkers (12, 23), has been carried out on the removal of micropollutants and trihalomethane precursors from raw water by using ozone/UV. The objective of this paper is to present an engineering analysis of the data obtained by Glaze and co-workers, arriving at rate constants applicable to reactor design. The work reported herein was carried out as part of the same project (14, 151, and subsequently reevaluated by this author. Chemistry: Mechanisms The ozone/UV process has been compared to conventional ozone chemical oxidation, but there is a substantial difference in the reaction mechanisms involved and the results achieved. The rates for photooxidation are much faster and oxidation is more complete, compared to oxidation by 03,H20z,and CIOz. The advantages of 03/UV photooxidation, compared to classical ozonation as described by Criegee (16) and Bailey 0013-936X/83/0917-0743$01.50/0
(17), can be understood by comparing the oxidation paths for an organic compound. Briefly, ozonation and ozonolysis in dilute solution lead to reaction with 03,hydroxy, and hydroperoxide species. Ozone bridges carbon-carbon double bonds to form unstable ozonide intermediates, which are cleaved into smaller oxidation species with eventual C 0 2 and H 2 0 formation, as well as some stable organic acids such as formic, acetic, and oxalic acids. The latter may cause the oxidation to plateau at some equilibrium level short of complete oxidation. Peleg (18), Hoigne and co-workers (19),and Kuo (20) present mechanisms for the formation of free radicals and ions, OH-, -H02,OH-, 0-, and H202 which accomplish the oxidation of the parent reactant. Qualitatively, the overall ozonation and ozonolysis reactions of the parent molecule occurs roughly in three steps: (1)partial oxidation of the initial parent species to form intermediates plus some small fragments; (2) oxidation of the intermediates to form secondary intermediates plus fragments; (3) further oxidation to form small and stable organic acid species, causing the disappearance curve to plateau, short of complete oxidation. On the other hand, ozonation with UV radiation involves an additional factor: high energy input to the reaction system. UV photons at 180-490 nm provide 72-155 kcal/mol energy, ample for producing more and other oxidizing free radicals from 03,as well as excited-state species and free radicals from the parent reactant and subsequsnt intermediates not produced by ozonation and ozonolysis. Concerning the latter UV-absorption spectra of parent molecules is evidence of this aspect of the mechanism. At present there has not been a detailed investigation of the mechanisms involved with ozonation in the presence of UV radiation; therefore, the proposed mechanism is speculative. The overall mechanism of 03/UV photooxidation of M species containing sulfur, phosphorus, and halogen in aqueous solution occurs by a combination of mechanisms and can be represented in a simplified way by
O3 + hv
-
0. + H2O
O3 + -OH
+
(la)
20H.
(1b)
.HOz + O2
(IC)
02* 0.
+
'
M+hv-M*
(Id)
(14
.R, H.
M*
+ (hv,-0,.OH, .HO2]
overall: M, M*, R., I
-+
R., I, He
-
(If)
+ (hv, -0,.OH, eo*,.H02)
COz, H20, S042-,Pod2-,C1- (lg) As a result of the use of UV the overall oxidation rate is enhanced, plateauing short of complete oxidation is prevented, and parent molecules are more completely oxidized to lowest energy oxidation state products: C02, H20, S042-, Po43-, C1-, ....
@ 1983 American Chemical Society
Environ. Sci. Technol., Vol. 17, No. 12, 1983
743
The first assumption essentially eliminates the heterogeneous nature of the process. The second eliminates the necessity of determining the concentration of O3plus other oxidizing species in the liquid phase, a formidable task. The third eliminates the necessity to track the individual species along their reaction pathways. The fourth recognizes the two most important reaction contributions. For a batch continuously sparged reactor the differential rate equation for the disappearance of a given parent reactant contains two terms as suggested above. The first term is proportional to the ozone supply rate, by assumption 2 above, and concentration of the parent species, giving in the usual way
Table I. Characteristics of the Lake Water characteristic
Lewisville Lake ( A )
Cross Lake (B)
8.1 < 10 130 10.0 4.3
7.6
