Destruction of Mixtures of Pollutants by UV-Catalyzed Oxidation with

Chapter 5

Downloaded by UNIV OF MICHIGAN ANN ARBOR on February 18, 2015 | http://pubs.acs.org Publication Date: March 8, 1990 | doi: 10.1021/bk-1990-0422.ch005

D e s t r u c t i o n of M i x t u r e s of P o l l u t a n t s by U V - C a t a l y z e d O x i d a t i o n with Hydrogen Peroxide D. W. Sundstrom, B. A. Weir, and K. A. Redig Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269

Hazardous organic compounds present in many water supplies and industrial wastes must often be reduced to very low concentration levels. This project investigated the destruction of mixtures of organic compounds by ultraviolet catalyzed oxidation with hydrogen peroxide as the oxidizing agent. Benzene and trichloroethylene were used as the components because they are common priority pollutants and differ greatly in structure. The combination of ultraviolet light and hydrogen peroxide was effective in decomposing both components individually and in mixtures. The rate of reaction of trichloroethylene was much lower in a mixture with benzene than as a single component. The strong interaction between components demonstrates the need to study actual mixtures instead of attempting to predict mixture behavior from pure component data. Many water supplies and industrial wastes contain hazardous organic chemicals that must be removed prior to use. These compounds are often present at low concentration levels and require high degrees of removal. Packed bed aeration and activated carbon adsorption are currently the most widely applied technologies for removing residual organic compounds. These methods alone are nondestructive and simply transfer the compounds between phases. In recent years, advanced oxidation processes involving combinations of ozone, hydrogen peroxide and ultraviolet light have received widespread attention (1-6). The photochemical action of UV light on ozone or hydrogen peroxide produces hydroxyl radicals and other reactive species that attack the organic molecules. The combination of UV light and an oxidant can give fast reaction rates and high degrees of removal. Under suitable operating conditions, the final products are mainly carbon dioxide and water. Although ozone is usually a stronger oxidizing agent than hydrogen peroxide, it has several process disadvantages. Ozone is 0097-6156/90/0422-0067$06.00/0 © 1990 American Chemical Society In Emerging Technologies in Hazardous Waste Management; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT

an unstable gas that must be generated on s i t e and t r a n s f e r r e d into the l i q u i d phase. The ozone containing bubbles can also s t r i p v o l a t i l e components into the a i r . Hydrogen peroxide solutions are r e a d i l y stored for mixing with the contaminated water according to process demand. Several companies have r e c e n t l y s t a r t e d marketing the UV/peroxide technology for removing t o x i c organics from water supplies. Sundstrom et al. (7-9) have studied the d e s t r u c t i o n of i n d i v i d u a l a l i p h a t i c and aromatic compounds by UV l i g h t catalyzed oxidation with hydrogen peroxide. The r e s u l t s demonstrated that the combination of UV l i g h t and peroxide i s e f f e c t i v e i n destroying a wide v a r i e t y of hazardous compounds present i n water at low l e v e l s . For the c h l o r i n a t e d a l i p h a t i c s , the reacted c h l o r i n e was converted q u a n t i t a t i v e l y to c h l o r i d e i o n , i n d i c a t i n g that the c h l o r i n a t e d structures were e f f e c t i v e l y destroyed. In the case of the aromatics, many intermediates were formed which could be eliminated by extending the treatment time. Most studies on UV catalyzed oxidation processes have emphasized pure compounds or complex n a t u r a l mixtures, such as humic matter. In p r a c t i c e , mixtures of known chemical compounds are often found i n contaminated water. A knowledge of the i n t e r a c t i o n s between r e a c t i n g components i s necessary for the design of oxidative treatment systems. The purpose of t h i s research was to investigate the d e s t r u c t i o n of mixtures of benzene and t r i c h l o r o e t h y l e n e by the UV/peroxide process. Experimental Methods A l l experiments were conducted i n a batch photochemical reactor with an i n s i d e diameter of 7.5 cm, a length of 30 cm and a t o t a l volume of about 1.5 l i t e r (7). The lower two-thirds of the reactor was jacketed to permit temperature c o n t r o l by a c i r c u l a t i n g l i q u i d . An u l t r a v i o l e t lamp entered through a c e n t r a l opening at the top, and other openings were used to add l i q u i d s , withdraw samples and measure temperature. The UV source was a low pressure mercury vapor lamp from Ace Glass with an outside diameter of 0.9 cm and a length of 25 cm. The output of the lamp was about 2 watts at the 254 nm resonance l i n e . The contents of the reactor were agitated continuously by a magnetic s t i r r e r . The reactor was i n i t i a l l y charged with 1 l i t e r of d i s t i l l e d water containing a phosphate buffer and d i s s o l v e d organic compounds. The hydrogen peroxide was added to the reactor and the lamp was turned on. Samples were analyzed for benzene and t r i c h l o r o e t h y l e n e by a Perkin-Elmer gas chromatograph equipped with flame i o n i z a t i o n detectors. Hydrogen peroxide concentrations i n the samples were determined by a glucose oxidase-peroxidase method ( 2 ) · Results Choice of Compounds. Benzene and t r i c h l o r o e t h y l e n e (TCE) were s e l e c t e d as the organic compounds for t h i s study since they are common p r i o r i t y p o l l u t a n t s found i n contaminated water and d i f f e r g r e a t l y i n structure and

In Emerging Technologies in Hazardous Waste Management; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


5. SUNDSTROM ET AL.

69 UV-Catalyzed Oxidation with Hydrogen Peroxide

p r o p e r t i e s . For example, the absorption c o e f f i c i e n t f o r benzene i s 16 times l a r g e r than that f o r t r i c h l o r o e t h y l e n e . These compounds were p r e v i o u s l y investigated separately and found to have s i m i l a r rates of r e a c t i o n (7,8). The concentrations of benzene and t r i c h l o r o e t h y l e n e were chosen to provide the same range of organic carbon concentration f o r each component. The benzene concentrations v a r i e d from 0.05 to 0.2 mM (3.9 to 15.6 ppm) and TCE concentrations from 0.15 to 0.6 mM (19.7 to 78.8 ppm). The hydrogen peroxide concentrations were s e l e c t e d to give 0.75 to 3 moles of peroxide per mole of organic carbon i n solution. Single Components. The e f f e c t s of p o l l u t a n t and hydrogen peroxide concentrations on the rates of d e s t r u c t i o n of the pure compounds are i l l u s t r a t e d i n Figures 1 and 2. For both components, the rates of r e a c t i o n increased with increasing hydrogen peroxide concentration and decreased with increasing p o l l u t a n t concentration. For a given peroxide and organic carbon concentration, the r e a c t i o n rates were s i m i l a r , with t r i c h l o r o e t h y l e n e having a s l i g h t l y f a s t e r rate than benzene. The semi-log p l o t s of f r a c t i o n of p o l l u t a n t remaining versus time are nearly l i n e a r , suggesting f i r s t order r e a c t i o n s . The k i n e t i c s are not true f i r s t order as evidenced by the decrease i n r e a c t i o n rate with increasing p o l l u t a n t concentration. However, pseudo f i r s t order rate constants were c a l c u l a t e d f o r each run to f a c i l i t a t e comparison of r e a c t i o n rates between runs. The r e a c t i o n mechanisms f o r the oxidative d e s t r u c t i o n of the p o l l u t a n t s are complex and involve hydroxyl r a d i c a l s as the major r e a c t i v e species. The dependence of rate constants on concentration probably r e s u l t s from the r o l e of hydroxyl r a d i c a l s i n the chemical reactions. The rate of generation of hydroxyl r a d i c a l s depends mainly upon the UV l i g h t i n t e n s i t y and i t s absorption by hydrogen peroxide i n the s o l u t i o n . As p o l l u t a n t concentration i s decreased at constant peroxide concentration, the number of hydroxyl r a d i c a l s a v a i l a b l e to r e a c t with each p o l l u t a n t molecule i s increased. Mixtures of Components. The r e a c t i o n rates of benzene and t r i c h l o r o e t h y l e n e i n mixtures increased with increasing hydrogen peroxide concentration, as i l l u s t r a t e d i n Figures 3 and 4 f o r a mixture containing 0.1 mM benzene and 0.3 mM t r i c h l o r o e t h y l e n e (each 0.6 mM i n organic carbon). Both components i n the mixture e x h i b i t e d nearly l i n e a r behavior on semi-log p l o t s of p o l l u t a n t concentration versus time. Pseudo f i r s t order rate constants were obtained from the slopes and p l o t t e d versus hydrogen peroxide concentration i n Figure 5. As peroxide concentration was increased from 0.9 to 3.6 mM, the rate constants increased from 0.0185 to 0.046 min" f o r benzene and from 0.0167 to 0.0313 min" f o r t r i c h l o r o e t h y l e n e . The rate increases probably r e s u l t e d from a larger supply of hydroxyl r a d i c a l s at higher peroxide concentrations. The r e a c t i o n rates f o r two binary mixtures containing the same 1

1



70


0.3 mM TCE 0.9 mM H 0 2

2

0.05

Figure 1. E f f e c t o f i n i t i a l t r i c h l o r o e t h y l e n e and hydrogen peroxide concentrations on the decomposition o f t r i c h l o r o e t h y l e n e at 25°C and pH 6.8. 1.0

0.5

Ε

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0.2

ο

0.1 mM Benzene 0.9 mM H 0 2

0.1

2

0.05 mM Benzene 0.9 mM H 0 2

2

0.1 mM Benzene H 0 l.8mM HoOo 2

0.05

2

20 TIME

40

60

M i nu tes

Figure 2. E f f e c t of i n i t i a l benzene and hydrogen peroxide concentrations on the decomposition o f benzene a t 25°C and pH 6.8.



5. SUNDSTROMETAL.

UV-Catalyzed Oxidation with Hydrogen Peroxide

ο

0.05 I 0

1

ι 40

I

TIME

I 80

1

1 120

, Minutes

Figure 3. E f f e c t of i n i t i a l hydrogen peroxide concentration on the rate of d e s t r u c t i o n of t r i c h l o r o e t h y l e n e i n a mixture i n i t i a l l y containing 0.1 mM benzene and 0.3 mM t r i c h l o r o e t h y l e n e at 25°C and pH 6.8.

0.05 I 0

1

ι

40 TIME

ι

I

80

1

1

120

, Minu tes

Figure 4. E f f e c t of i n i t i a l hydrogen peroxide concentration on the rate of destruction of benzene i n a mixture i n i t i a l l y containing 0.1 mM benzene and 0.3 mM t r i c h l o r o e t h y l e n e at 25°C and pH 6.8.


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t o t a l organic carbon and peroxide concentrations are shown i n Figure 6. One mixture i n i t i a l l y contained 0.05 mM benzene and 0.3 mM TCE and the other mixture contained 0.1 mM benzene and 0.15 mM TCE. The rate of benzene disappearance was nearly the same i n both mixtures even though the i n i t i a l concentrations d i f f e r e d by a f a c t o r of 2. In contrast to the single component r e s u l t s , the r e a c t i o n rate for t r i c h l o r o e t h y l e n e increased as i t s concentration i n the mixture was increased. Since an increase i n t r i c h l o r o e t h y l e n e concentration was associated with a decrease i n benzene, these r e s u l t s suggest strong i n t e r a c t i o n s between the r e a c t i n g components. The e f f e c t s of adding the second component to solutions containing e i t h e r 0.1 mM benzene or 0.3 mM t r i c h l o r o e t h y l e n e are shown i n Figures 7 and 8. In both cases the rate constant for benzene or t r i c h l o r o e t h y l e n e decreased s i g n i f i c a n t l y as the other component was added. The change i n r e a c t i o n rates was more pronounced for t r i c h l o r o e t h y l e n e where the rate constants decreased by a f a c t o r of 3 when 0.1 mM benzene was added to 0.3 mM trichloroethylene. A doubling of the hydrogen peroxide concentration moderated the decrease i n the benzene rate constant but had l i t t l e e f f e c t on the decrease i n the t r i c h l o r o e t h y l e n e rate constant. In Figures 7 and 8, organic carbon concentrations increased as the second component was added to the s o l u t i o n s . Thus, part of the decreases i n rate constants could be a t t r i b u t e d to higher p o l l u t a n t concentrations. A f a i r e r method of comparison would be to r e l a t e rate constants for pure components and mixtures at the same t o t a l organic carbon concentration. These comparisons are made i n Figures 9 and 10 for solutions containing 1.5 moles of hydrogen peroxide per mole of organic carbon. In both cases, the rate constants for benzene alone were s i m i l a r i n magnitude to those for benzene i n a mixture. The rate constants for t r i c h l o r o e t h y l e n e i n the mixtures, however, were about one-half of t h e i r values as pure components. These r e s u l t s show that the aromatic component, benzene, has a strong adverse e f f e c t on the rate of d e s t r u c t i o n of the a l i p h a t i c component, t r i c h l o r o e t h y l e n e . This e f f e c t may be due not only to the benzene i t s e l f , but also to the r e a c t i o n intermediates formed as benzene i s o x i d i z e d . Hydroxyl r a d i c a l attack of benzene y i e l d s several aromatic intermediates, i n c l u d i n g the hydroxycyclohexadienyl r a d i c a l (10), phenol (8), and the three dihydroxybenzene isomers (8). The presence of benzene and i t s aromatic o x i d a t i o n products may i n h i b i t the destruction of t r i c h l o r o e t h y l e n e through competition for a v a i l a b l e UV photons and hydroxyl r a d i c a l s . Conclusions The strong i n t e r a c t i o n between components demonstrates the need to study a c t u a l mixtures instead of attempting to p r e d i c t t h e i r behavior from pure component data. As shown for t r i c h l o r o e t h y l e n e i n t h i s study, the rate of d e s t r u c t i o n of a compound may be s u b s t a n t i a l l y lower as a component i n a mixture than as a pure component. The volume of a reactor system may be s e r i o u s l y underestimated i f the design ignores p o t e n t i a l i n t e r a c t i o n s between r e a c t i n g components.


5.

SUNDSTROMETAK

UV-Catalyzed Oxidation with Hydrogen Peroxide

0.05 ρ


c

0.03

ο Ο

σ

0.01

H

2°2

3.6

2.4

1.8

0.9

mM

Figure 5. E f f e c t of hydrogen peroxide concentration on apparent f i r s t order rate constants f o r a mixture i n i t i a l l y containing 0.1 mM benzene and 0.3 mM t r i c h l o r o e t h y l e n e at 25°C and pH 6.8.

0.05 « 0

1

! 40 TIME

I ,

1 80

1

1

120

Minutes

Figure 6. E f f e c t of mixture composition on the rate of d e s t r u c t i o n of benzene and t r i c h l o r o e t h y l e n e at 25°C, pH 6.8 and 0.9 mM i n i t i a l hydrogen peroxide concentration.


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Destruction of Mixtures of Pollutants by UV-Catalyzed Oxidation with

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