Emerging Technologies in Hazardous Waste ... - ACS Publications

Chapter 2

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: March 8, 1990 | doi: 10.1021/bk-1990-0422.ch002

M e c h a n i s t i c Aspects of the Photocatalytic O x i d a t i o n of P h e n o l i n Aqueous Solutions 1

Jesseming Tseng and C. P. Huang

Environmental Engineering Program, Department of Civil Engineering, University of Delaware, Newark, DE 19716

A photo-catalytic oxidation process using photocatalyst TiO and ultra violet light to decompose phenol in aqueous solution was studied. The photocatalyst was thoroughly characterized for specific surface area (BET), surface charge (zeta potential), surface morphology (scanning electron microscopy, SEM) and surface composition (x-ray energy dispersion analysis, EDAX). Parameters such as oxygen, temperature, pH, concentration of photocatalyst, and phenol concentration that may affect the oxidation reaction were thoroughly examined. Results show that oxygen plays one of the most important roles in phenol photo-oxidation. Trace amounts of chloride can inhibit the phenol decomposition reaction while aluminum impurity enhances it. At high phenol concentrations, e.g. ca > 5 X10 M, hydroquinone and a biphenyl dimer are the major intermediates of the oxidation reaction. Free radicals such as hydroxyl and super oxide are keys to photo-oxidation of phenol. 2

-2

Pollution by hazardous organic waste has become an important environmental issue due to their high toxicity potential. The U. S. Environmental Protection Agency (EPA) has identified over one hundred organic chemicals as priority pollutants(1). The adverse effects of these hazardous chemicals are well documented. Phenol and its related compounds are commonly found in various industrial effluents and have been reported in hazardous waste sites around the country(2). According to Jones (3) phenol can be found at concentrations ranging from 10 to 100 mg/L in the industrial wastewater of thermal and catalytic cracking of petroleum products. 1

Address correspondence to this author. 0097-6156/90/0422-0012$08.00/0 © 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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TSENG & HUANG

Photocatalytic Oxidation of Phenol in Aqueous Solutions

Photocatalytic oxidation process involving the use of semiconductors has gained recent attention i n the f i e l d of innovative treatment of hazardous organic wastes. Upon irradiation, a semiconductor generates electron/hole pairs with free electrons produced i n the nearly empty conduction band (cb) and p o s i t i v e holes remaining i n the valence band (vb)(4.5). The holes, acting as strong o x i d i z i n g agents, migrate to the semiconductor surface and react with organic compounds. Figure 1 shows the t y p i c a l reaction scheme of a n-type semiconductor such as T i 0 . Depending on the ambient conditions, the l i f e t i m e of an electron/hole separation process can be from a few nano-seconds to a few hours(6). The recombination of electron/hole pairs can take place either between energy bands or on the surface. As a r e s u l t the photocatalytic e f f i c i e n c y i s reduced. To impede the recombination process, conducting materials such as noble metals, can be incorporated into the semiconductor to f a c i l i t a t e electron transfer and prolong the l i f e t i m e of the electron/hole separation process(7.8). Very recently, researchers have investigated the production of f u e l such as hydrogen from water using semiconductor and l i g h t . This type of work includes photocatalytic d i s s o c i a t i o n of l i q u i d or vapor water on the surface of powdered semiconductors and single crystals(9_,_10) . Izumi et a l (11.12) have used p l a t i n i z e d (10% by weight) T i 0 to decompose benzoic acid and adipic acid. They have proposed a photo-Kolbe type reaction mechanism for the oxidation process. Kawai and Sakata (13-16) have studied the photocatalytic oxidation of various substrates such as chlorine and nitrogen compounds using T i 0 deposited with 5% platinum and reported that organic decomposition and hydrogen production occurs simultaneously and that thermal e f f e c t contributes l i t t l e to the oxidation. Kawai and Sakada (13.15) have also studied the photocatalytic oxidation of natural materials such as glucose, ethanol, c e l l u l o s e and l i g n i n ; food s t u f f s such as potato, f a t t y o i l , and herbs; wood such as cherry wood, white dutch clover and water hyacinth; green algae, dead animals and excrement using T i 0 under a xenon lamp. They have also found that nitrogen and chlorine are converted to NH and HC1, respectively without producing any residual organic chemicals.


2

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F u j i h i r a et al.(17-19). however, have reported an incomplete oxidation of benzene and toluene. Neither Kawai and Sakada nor F u j i h i r a have provided any d e t a i l s of the k i n e t i c s and the nature of the reaction. A l l experiments were conducted i n neutral or i n NaOH (5 M) solution using T i 0 catalyst and a 500 watt xenon lamp. 2

Kruatler and Bard (20) have reported the production of alkane, carbon dioxide and hydrogen from mixtures of organic acids. Pavlik and Tatayanon (21) have demonstrated that lactams can be oxidized to imides following steps s i m i l a r to those of the Fenton solution. Barbeni et a l . (22.23) have studied the degradation of chlorinated hydrocarbons, 2,4,5-trichlorophenoxyacetate and 2,4,5trichlorophenol, using T i 0 as the photocatalyst and proposed the formation of -OH free r a d i c a l s from reactions between the water molecules and the p o s i t i v e holes. The *0H free r a d i c a l s can subsequently p a r t i c i p a t e i n a series of reactions with the chlorinated organic. 2

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

13

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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT


hv

adsorption Figure 1. Schematic Redox Reactions of an η-type Semiconductor.

011is(24.25) has examined the photo-oxidation of chlorinated hydrocarbons with T i 0 and reported that the extent of conversion increases i n the order: chloroolefins > chloroparaffins > chloroacetic acids. Matthews(26-28) and Gratzel (29) have used thin films of T i 0 to study the oxidation of some organic impurities i n water and reported that the k i n e t i c s of photo-oxidation reactions can be described by a Langmuir type adsorption equation. 2

2

While there i s general agreement among researchers that photocatalyst T1O2 i s promising i n decomposing organic substances, a systematic study on the various parameters a f f e c t i n g the photooxidation reaction i s lacking. The major objective of this study was therefore to investigate the decomposition of organic matter exemplified by phenol using titanium dioxide as the photocatalyst and u l t r a v i o l e t as the l i g h t source f o r illumination. Various factors such as pH, oxygen content, phenol concentration, l i g h t intensity and concentration of photo-catalyst were studied. A mechanism for phenol decomposition i s proposed.


2.

TSENG & HUANG

Photocatalytic Oxidation ofPhenol in Aqueous Solutions

Methods and Materials Preparation of Photocatalyst Titanium dioxide, commercial grade, provided by the Du Pont Company (Wilmington, Delaware) was used. The oxide was pretreated by r i n s i n g 15 g of the sample with 1 l i t e r of HC10 (1 M) s o l u t i o n followed by washing with d i s t i l l e d water several times u n t i l the conductivity of the supernatant was ca 10 /xmho/cm. Samples were centrifuged at 10,000 rpm for 30 minutes, using a centrifuge (model RC5 Sorval-Du Pont Company). The titanium dioxide was then dried overnight at 105 °C and ground to fine powder p r i o r to any experiments. During the early phase of this study, the untreated T i 0 was used. The r e s u l t s show that the phenol decomposition with untreated T i 0 was extremely slow. Results from EDAX analysis indicated that untreated sample contains 1.145 % (by weight) chloride. Bickley has reported that the photoadsorption a c t i v i t y of oxygen on the surface of T i 0 i s remarkably decreased i n presence of F" or P0 ~ ions(30). In order to improve the oxidation e f f i c i e n c y , the T1O2 was treated for chloride removal. The treated T i 0 was thus used throughout the remaining study.


A

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Surface Characterization of Photocatalyst The surface charge of the titanium dioxide was characterized by zeta p o t e n t i a l measurements using a Laser-zee Meter (Pen-ken Inc.). The pH i s 9.0 and 9.3 i n NaC10 solutions for the treated and the untreated T i 0 samples, respectively. The s p e c i f i c surface area of T i 0 was determined by the BET method (Quantasorb-Model QS-7, Quanta Chrome Co). The impurities of both the treated and the untreated Ti0 were determined by scanning electron microscopy (SEM) and energy dispersive x-ray (EDAX). The p a r t i c l e s i z e and shape were determined by transmission electron microscopy (TEM). Figure 2 shows the size and shape of the treated T i 0 under 60,000x and 22,000x magnifications. Table I summarizes the major properties of the treated and the untreated T i 0 . z p c

4

2

2

2

2

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Table I. Major Properties of Titanium

Properties Structure PH F"zpc Surface area (nr/g) Band gap energy (eV) Impurities (% by wt.) Z

Size (μπι)

Untreated Rutile 9.3 6.6 3.2 0.729 (Al) 1.145 (Cl)

Dioxide

Treated Rutile 9.0 6.1 3.2 0.708 (Al) #

10.60 4.72

(a)* (b)*

* a represents the length of the p a r t i c l e s and b denotes the diameter of the p a r t i c l e s # Ref (29)


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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEM ΕΝΊ

Figure 2. Transmission Electron Micrograph of the T i 0 P a r t i c l e s After Pretreatment. (a): 60,000x Magnification; (b): 22,OOOx Magnification. 2


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TSENG & HUANG


Light Source The l i g h t source was a 1,600 watt medium pressure mercury vapor discharge lamp (American U l t r a v i o l e t Co.). The spectral irradiance for the UV lamp (260 watts/m ) ranges from 228 and 420 nm at a distance of the 1 meter from the l i g h t source according to infort i o n provided by the maker(31). A t y p i c a l spectrum f o r a mercury lamp i s shown i n Figure 3(32). The l i g h t i n t e n s i t y was recorded with a radiometer (Model 65A, YSI- Kettering Co.).


2

Test Tube Reactors Pyrex glass tubes were used. The transmittance c a p a b i l i t y of pyrex glass tubes was scanned from 200 through 800 nm wavelength. The results show that any l i g h t having a wavelength longer than 325 nm can completely pass through the pyrex glass. A l l photo-catalytic oxidation experiments were performed i n pyrex tubes containing 15 mL of 10" M phenol solution and 0.15 g of T i 0 . In order to study the e f f e c t of oxygen, d i f f e r e n t atmospheres were created by introducing pure nitrogen or oxygen into the system. An Ascarite II trap was used to remove carbon dioxide. A pyrogallol solution was used to remove oxygen p r i o r to nitrogen bubbling. Dissolved oxygen was monitored by a dissolved oxygen (DO) meter c a l i b r a t e d by the modified Winkler method (33). Generally, 0.3 ppm oxygen was found i n solution under nitrogen bubbling. In the pure oxygen atmosphere, 31.5 ppm dissolved oxygen can be obtained. Samples were shaken over a reciprocal shaker (American Optical Co.) at 180 strokes per minute to insure complete mixing. The temperature of the sample was 3

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controlled by a thermostat pump i n conjunction with a cooler. At the end of a given reaction time, samples were f i l t e r e d with 0.45 /xm m i c r o f l i t e r s (Gelman, Supor-450, 25mm). The residual concentration of phenol was measured with an uv-vis spectrophotometer (Hitachi/ Perkin-Elmer, model 139) at a wavelength of 271 nm. Figure 4 shows the schematic set up f o r the test tube experiments. Test tubes were tightened with t e f l o n septa a i r - t i g h t caps. The phenol solution was bubbled with nitrogen or oxygen p r i o r to experiments. For the gas phase studies, a pressure lok gas syringe was used to withdraw gas from the test tube. The compositions of the gas phase i n the head space were detected by a GC/MS (Hewlett Packard, Model 5890/5970) under an isothermal condition. Study of Intermediates Samples (500 /iL) were dried at room temperature i n a desiccator to evaporate water. After drying, 500 μΐ, of benzene was added to extract phenol from the samples. The residual s o l i d was dissolved i n 10 /iL methanol. Phenol and i t s intermediate compounds were determined by GC/FID and GC/MS techniques.

Results and Discussion Oxygen/Nitrogen Atmosphere Figure 5 shows that oxygen plays a s i g n i f i c a n t role i n phenol oxidation. In a nitrogen atmosphere the percentage of phenol removal i s small compared to that of an oxygen atmosphere. Figure 5 also shows that phenol removal i s i n s i g n i f i c a n t i n the absence of Ti0 . I t has been speculated that ozone may be generated under the oxygen atmosphere. However, this i s not possible since the short UV wavelengths that are responsible f o r ozone production are t o t a l l y absorbed by the pyrex glass tube(34.35). 2

Temperature Effect Results from our previous studies have shown that the photocatalytic oxidation reaction can be complete within 4 hours under 90 °C(35). Figure 6 shows the oxidation of phenol over a temperature ranging from 20 to 50 °C. The results c l e a r l y indicate the importance of temperature. As expected, increases i n temperature greatly raise the rate of phenol decomposition. However the temperature does not exhibit any e f f e c t on phenol decomposition under a nitrogen atmosphere. Oxygen appears to be much important than temperature i n photocatalytic oxidation reactions. Effect of pH The optimum pH f o r phenol oxidation i s at a neutral pH region, e.g. 5-9; a s l i g h t l y small removal at pH 3 was observed (Figure 7). These results agree well with those reported by Matthews(28). Oxidation of 4-chlorophenol, measured by carbon dioxide y i e l d , was the maximum at neutral pH values and decreased or increased as pH values s h i f t e d towards the a c i d i c or the alkaline region. The oxidation of benzoic acid, ethanol, propan-2-ol and methanol i s dramatically reduced at pH 3. The photocatalytic oxidation of the surfactant DBS i s favored i n neutral solutions(36). O l l i s and


TSENG & HUANG



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TSENG & HUANG

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Emerging Technologies in Hazardous Waste ... - ACS Publications

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