The photocatalytic oxidation of sulfur-containing organic compounds using cadmium sulfide and the effect on CdS photocorrosion

Wat. Res. Vol. 25, No. 10, pp. 1273--1278,1991 Printed in Great Britain

0043-1354/91 $3.00+ 0.00 Pergamon Press pie

THE PHOTOCATALYTIC OXIDATION OF SULFUR-CONTAINING ORGANIC COMPOUNDS USING CADMIUM SULFIDE A N D THE EFFECT ON CdS PHOTOCORROSION ALLEN P. DAVIS* ~ and C, P. HUANG~) Department of Civil Engineering, University of Delaware, Newark, DE 19716, U.S.A.

(First received March 1990; accepted in revised form March 1991)

Abstract--The photocatalytic oxidation of sulfur-containing compounds using cadmium sulfide was examined. The reaction rate is approximately zero-order with respect to organic concentration and is greatest under basic conditions. Both of these results are consistent with a rate dependence on the concentration of adsorbed substrate on the photocatalyst surface. Supporting electrolyte type and concentration have little effect on the reaction rate. The presence of sulfur-containing organics reduces the photo-corrosion of CdS. Key words--photocatalyfic oxidation, cadmium sulfide, sulfur-containing organics

INTRODUCTION

Photocatalytic oxidation is a promising process for the treatment of organic containing water and wastewater. The process is efficient and results in complete oxidation, with COz and simple inorganic ions as products. However, the oxidation kinetics are complex and the reaction rate is affected by pH; light intensity; and photocatalyst, oxygen, and substrate concentration. In order to practically utilize this novel treatment process, all of the parameters that affect the reaction should be analyzed and understood. We have previously demonstrated the photocatalytic oxidation of aqueous phenols using illuminated cadmium sulfide (Davis and Huang, 1989, 1990). Cadmium sulfide is an attractive photocatalyst since it is excited by visible light. The absorption of bandgap illumination results in the formation of electron/hole pairs which, upon migration to the particle surface, produce redox chemistry and the organic oxidation: CdS + hv--,e- + h +.

(1)

The greatest detriment encountered with CdS is that this compound undergoes corrosion as the positive holes (h +) also oxidize the solid (Williams, 1960): CdS + 2 h + - . C d 2÷ + S(s).

(2)

The overall reaction with oxygen being (Henglein, 1982; Hsieh, 1987): CdS + 2 02-"*Cd 2+ + SO 2- .

(3)

*Present address: Department of Civil Engineering, University of Maryland, College Park, MD 20742, U.S.A.

This not only results in the destruction of the catalyst, but more importantly, the release of dissolved cadmium into the ambient solution. In order to prevent this corrosion, the organic compound must successfully compete for the holes. Organic compounds containing reduced sulfur have a strong oxidative reactivity, as exemplified by high molecular ozone reaction rates (Hoigne and Bader, 1983a, b) and these compounds should be able to vigorously compete for the interracial hole. Reduced inorganic sulfur compounds (S2-, SO~-) can render CdS stable by successfully reacting with the electron vacancy (Minoura et al., 1976; Inoue et al., 1977; Minoura and Tsuiki, 1978). Organic sulfur compounds should be common in the anoxic environments of both septic wastewater and sediments. In each, microbial activity reduces sulfur to the sulfide state, organic matter is ubiquitous and sulfur-containing organics should be present. Sulfur-containing organic matter is environmentally significant due to offensive odor problems. Two of the compounds used in this study possess significant metal/ligand complexation constants (Table 1). Therefore, according to the surface complexation theory of adsorption, a strong affinity for a cadmium surface site should exist. Adsorption processes are thought to be a major mechanism in controlling photocatalytic oxidation using CdS suspensions (Davis and Huang, 1989, 1990) and this surface affinity may prove significant. One objective of this research is to examine and quantify the photocatalytic oxidation of some reduced sulfur organic substances which should have high oxidation rates. The other is to investigate the

1273

1274

ALLENP. DAVISand C. P. HUANG Table 1. Properties of sulfur-containing organic compounds Molecular u.v. Wavelength Compound Formula weight (rim) Log K* Thioaeetamide (TA) CH3CSNHz 75.14 261 Thiourea (TU) NH2CSNH2 76.12 236 1.33 Thiosemiearbazide (TSC) H2NCSNHNH2 9 I. 13 236 2.28 Dithiobiurea ( D B ) NH2CSNHNHCSNH 2 150.18 247/230 *Complexation constant with M = Cd2+, L = organic. KL= ML/M"L (Martel and Smith, 1974).

interaction of these compounds with the cadmium sulfide surface and to determine the effect their presence has on the photo-corrosion of the CdS photocatalyst. METHODS AND MATERIALS The cadmium sulfide is powdered, electronic grade, purity 99.999% (Aldrich). The N 2 BET surface area is found to be 1.5 m2/g. The crystal structure is the hexagonal greenokite as confirmed by X-ray diffraction. Thioacetamide (TA, Fisher Scientific), thiourea (TU, Pfaltz & Bauer), thiosemicarbazide (TSC, Aldrich) and dithiobiurea (DB, Aldrich) were used as received. A catalyst concentration of 5 g/l was used. Sodium sulfate was used for ionic strength (I = 5 x 10 -2 mol/1) and the solution pH was adjusted with NaOH or H2SO4. Oxidation at pH 3, 5, 7, 9, and 11 was examined. The pH was monitored continuously and maintained with strong base. As the reaction progressed, acid production was clearly evident. The reaction vessel was identical to that described previously (Davis and Huang, 1989, 1990). It is jacketed, with temperature controlled at 25°C. Oxygen was constantly purged through the system. A light intensity of 700 W/m 2 was employed as measured by a YSI radiometer. The illumination was provided by 300 W ELH projection lamps (General Electric). ELH lamps provide a spectrum comparable to that of solar irradiation (Matson et al., 1984). The sulfur-containing compounds have a strong u.v. absorbance and the organic concentration was measured on a Hitachi-Perkin-Elmer u.v.-vis, spectrophotometer at a wavelength of maximum absorption (Table 1). Dissolved cadmium was monitored using a Perkin-Elmer Model 5000 atomic absorption speetrophotometer. Adsorption studies were completed in Pyrex glass test tubes, also at a concentration of 5g/l CdS, in a 10ml solution. The tubes were completely wrapped in aluminum foil to avert any light impingement and prevent any photocatalytic oxidation. The solution pH was adjusted with NaOH or H2SO4. They were allowed to equilibrate by shaking overnight. The next day equilibrium pH was recorded, the samples were filtered, acidified, and the remaining organic concentration was measured. The amount of organic adsorbed was calculated by difference from a blank, containing no CdS, submitted to identical conditions.

may be reflected by the CdS (Dutton, 1958) and this would correspondingly decrease the absorbed light flux. The quantum yield is estimated by dividing the experimental reaction rate by the calculated ideal reaction rate: 0.172 mol/l.h. Estimated initial rates for organic oxidation are approx. 10-4mol/l.h and quantum yields range from 10 -4 to 3.7 × 10-3, indicating strong inefficiencies and recombination processes. RESULTS AND DISCUSSION Initially, it was established that the organic removal is indeed a photocatalytic process. F o r all four compounds, no organic removal was noted under illumination without CdS. A dark organic/CdS suspension demonstrated only slight removal, attributable to adsorption. The combination of CdS catalyst and light effectively removes each organic compound from the solution. The direct reaction of dissolved cadmium with thioacetamide to produce CdS, which is prominent at 90°C (Bowersox and Swift, 1958), is negligible under the low reactant and temperature conditions present during this study. A comparison of the removals of the 4 organics studied is given in Fig. I. At pH 7, the removal follows: TSC > T A > T U > DB. Between TSC and TU, the only two compounds in which complexation constants with Cd(II) could be located, TSC forms the stronger aqueous complex and, following the surface complexation theory of organic adsorption (Schindler and Stumm, 1987), should have the higher surface complex affinity for the CdS. The removal rate of the sulfur-containing compounds is much faster than that of substituted phenols (Davis and Huang, 1990), which take 8-10 times longer under identical conditions.

Quantum yield

100

The quantum yield is calculated by integrating the lamp spectrum (Matson et al., 1984) up to the wavelength of 520 nm, equal to CdS bandgap irradiation. This gives a value for the lamp energy, ET. The use of a 700 W/m 2 light intensity required that the full power light intensity be decreased by a factor of about 2 using a potentiostat. The energy, E, is, therefore, equal to ET/2. The light flux, F, is found from equation (4): F = Ea ~he

(4)

where h is Plancks constant, c the speed of light and a is the area o f the reactor (102era2), giving F = I A 4 x 10~9photons/s. This value corresponds to 0.172mol/l-h in terms of reaction rates presented in this paper, i.e. 500 ml volume and 700 W/m 2 light intensity. Up to 20% of the incident light

pH 7

ii 6o N 20-V Dit hiobiureo

0

I 1

I 2

I 3

i 4 Time(h)

I 5

I 6

I 7

I 8

Fig. 1. Photocatalytic o x i d a t i o n o f f o u r thio-organic compounds. Experimental conditions: 5 g/l CdS, 10 -3 M

organic, hv = 700 W/m 2, pH = 7.

Photocatalytic oxidation of organic compounds -3.0

Table 3. Thioacctamidephotocatalyticoxidationconstants

A .c: -3.5

Initial concentration (mol/I)

First-order rate constant (h-l)

Initial rate (mol/I.h)

5 x 10 - s 1 x 10-4

5.3

0.9 x 10 -4 0.9 x 10-4

~o~-4.0

-45 -5.0

-4.5

1275

I

I

-4.0 -3.5 -3.0 LOg ( concent, rotion, rnol,/I, )

I

-2.5

Fig. 2. Initial oxidation rate versus initial concentration for four sulfur-containing organic compounds, al, Thioacetamid¢; O, thiourea; A, thiosemicarbazide; x , dithiobiurea. Experimental conditions: 5 g/l CdS, hv = 700 W/m2, pH = 7. Figure 2 presents the initial oxidation rates of the four organics as a function of initial organic concentration. It is apparent that this reaction does not follow first-order kinetics. In a plot of this type, the slope represents the reaction order with respect to substrate concentration and is given as x for each compound in Table 2. The oxidation rates are best represented as initial rates since they are essentially independent of concentration. Initial rates and firstorder rate constants for TA are presented in Table 3. Thioacetamide is well characterized by the initial rate, while the first-order value is not constant with respect to concentration. Previous work on photocatalytic oxidation of organics has shown the substrate dependence to follow Langmuir-Hinshelwood kinetics (Davis and Huang, 1989; Augugliaro et al., 1988): - dC - -

dt

kKC -

-

3 x 10 -3

0.05

1.3 x 10-4 1.5 x 10-4 1.1 x 10 _4

X~

X

I

5 x 10-4 1 x 10-3

2.2 0.37 0.15

-

(5)

1 + KC

where C represents the organic concentration and k and K are constants. This equation reduces to a fractional, and eventually zero-order, reaction at high values of K C , consistent with the observed data. This type of relationship suggests a rate controlled by substrate adsorption. The oxidation rate increases with increasing pH (Fig. 3). TA and TU have the strongest pH dependence. Previous work examining the photocatalytic oxidation of some thio-compounds using CdS has noted a maximum reaction rate at pH 9.5 for cysteine and 9.9 for dithiothreitol (Spikes, 1981). In thiourea oxidation by aqueous bromine, under highly acidic conditions, the affinity for electrophilic oxiTable 2. Kinetic expressions for organic photocatalytic oxidation rate = kCX[H+y Compound k x y Thioacctamide 1.8 x 10-s 0.18 -0.21 Thiour~ 3.3 x 10-6 0 -0.17 Thioscmicarbazide 1.7 x 10-4 0.27 -0.11 Dithiobiurea 2.8 x 10-s 0 -0.03

dation by Br2 is diminished by the protonation of the sulfur (Simoyi and Epstein, 1987), but this phenomenon should not be operative under the present conditions. A reaction order with respect to [H +] can be obtained from Fig. 3 rate = k: [H +]y.

(6)

By estimating a linear relationship between the two parameters, an order with respect to pH (y, Table 2) is determined. This value varies from -0.21 in TA to - 0 . 0 3 in DB. The constants in Table 2 are expected to also be a function of light intensity, catalyst concentration, and dissolved oxygen concentration among other parameters as found for phenol photocatalytic oxidation using CdS (Davis and Huang, 1989). It has been shown that substrate adsorption is a prerequisite for photocatalytic oxidation of substituted phenols using CdS (Davis and Huang, 1990). The adsorption profile of TA as a function of pH is given in Fig. 4. Similar data were obtained for the other three compounds. The adsorption is maximum at the highest pH. This is consistent with the kinetic data that also demonstrate the maximum oxidation rate at basic pH. Thioacetamide, which exhibits the strongest pH adsorption dependence, also possesses the highest reaction order with respect to pH. It is reasonable to assume that surface protonation processes are responsible for the small, fractional orders with respect to pH (Stumm and Furrer, 1987). Surface speciation changes and subsequent organic adsorption plays an important role in the oxidation kinetics by altering the degree of adsorption. TSC is [3 X • -3.4 -- 0 -3.0-

!

Thioocetomide Thiourea Thiosemicorbozide Dithiobiur~a

x

~-----'~e

-4.2

-4.6

-5.O

2

[

4

I

I

6

8

I

10

I

12

pH

Fig. 3. Initial oxidation rate versus pH for four thio-organic

compounds.

1276

ALLENP. DAVISand C. P. HUANG (a)

~- 5o 'E

[] 10.3 M x 3 x 10-4M • 5 x 10_5M

4,o

100

/ / ~

~

80-

i

3o

•~ 60

8

~

.~ 20

~

IO

4

5

6

7 8 pH

9

10

11

20

v

100

Electrolyte effects There is a negligible difference in oxidation rate as ionic strength is varied. An advantage of using photocatalytic particles for oxidation is that no dependence of the solution conductivity should be apparent, as in traditional electrochemistry. Rates at ionic strengths of 0.01, 0.05, and 0.3 mol/l are identical; however, a slight reduction in rate is found at

I

1

I

3

2

I

I

4 5 Time(h)

I

o I

6

I

7

8

(b)

8060

~= ,~

Caution must be used when comparing reaction rates of photochemical reactions. The total light flux to the solution is dependent upon the surface area exposed. Additionally, the light flux causes reaction with a certain number of moles of reactant. Therefore, reaction rates must be compared on a mole basis, not a concentration basis. Initial rates determined experimentally are presented in Table 4 where volumes are varied, but the exposed surface area remains constant, giving different volume/surface area (V/SA) ratios. Concentration results apparently give different oxidation rates for different batch solution volumes. In each case, the concentrations of catalyst and reactant were the same, as was the lighted surface area. Therefore, each solution reacted with the same number of moles of thioacetamide. However, differences in volume made rates based on concentration appear variable. If the rates are calculated correctly as mol/surface area, the rates become identical (Table 4). The "thinnest" solution has a smaller rate, apparently because the small depth allows some light to pass through the suspension without being absorbed.

~'~'~'~'~'~'~

NoCLO 4

I

0

Fig. 4. Adsorption profiles of thioacetamide onto dark CdS as a function of pH. 5 g/1 CdS, 10ml solution.

Volume~surface area ratios

~---.~

~o~t

r]NONO3

-

I 12

also adsorbed to a greater extent than TU as suggested by the homogeneous complexation constants.

o

40

~ M~SO~ 20

~

,5 Na2SO4

~

l I 1~'~"-.I t~ 4 5 6 7 8 Time(h ) Fi 8. 5. The effect of supporting electrolyte. (a) Anions, (b) cations. Experimental conditions: 5 g/] C.dS, 10 -3 M organic, hv = 700 W/m2, pH = 7, 1 = 5 x 10-2 M. 0

V?S04I 1 2

I 3

0.003 mol/l. This may be due to a change in activity of the thioacetamide or cadmium sulfide surface resulting in altered interaction between the two. A second possibility may be somewhat more favorable solution/organic compound interactions due to the lower ionic strength, i.e. less salting out of the TA to the solid interface. Generally, the ions used for the supporting electrolyte have little effect on the photocatalytic process. Figure 5(a) and (b) demonstrate TA removal using various electrolytes at the same ionic strength. A slightly increased rate is noted with chloride as compared to the other anions in contrast to TiO2 photoCd(lI) Thioacetomide, pH 7 ~ 10-3 × 3x10 -3

o BLank o 5x10-5 0.001 --,5 10-4 ~.~ ~ --, ~7 5x10-4,

.--.-o ~

o.oool

Table 4. Initial oxidation rates for volume/surface area effect Solution volume (ml)

Initial rate (mol/l •h)

Volume/ surface area (cm)

1000 750 500 250

7.5 x 10 -s 9.5 X 10 -5 t.5 x 10 4 1.8 x 10 -4

10.5 8.0 5.6 3.1

Initial rate (tool/era2. h) 7.9 7.6 8.1 5.4

x 10 -7 X 10 7 x 10 7 x 10 7

000001

I I

l 2

t 3

I I 4 5 Time(h)

I 6

I 7

I 8

Fig. 6. The effect of the presence of thioacetamide on the CdS photo-corrosion. Arrows indicate point of 90% thioacetamide removal. Experimental conditions: 5g/l CdS, hv = 700 W/m2, pH = 7.

Photocatalytic oxidation of organic compounds 0.001

o

Cd (H) Thioacetamide

o.oool

.=

pH

U'° 0.00001

~.~(

o pH 3 13 pH ,5 & pH7

V pH 9 ~ pH 11

]

I

I

I

I

I

I

I

0

1

2

3

4

5

6

7

8

Time (h) Fig. 7. pH dependence of CdS photo-oxidative dissolution during thioacetamide oxidation. Experimental conditions: 5 g/l CdS, 10-3 M thioacetamide (initial), hv = 700 W/m2. catalytic oxidation in which C1- was found to be inhibitory (Augugliaro et al., 1988). A more significant enhancement is found using calcium as the cation. C d S dissolution

Cadmium sulfide dissolution is not completely inhibited by the presence of reduced sulfur compounds. Of the four organic compounds, DB resulted in the least amount of released Cd(II). This compound is the largest and possesses two reduced sulfur groups that would compete for the interfacial holes. The amount of dissolution for the four substrates does not correspond to the order of photocatalytic oxidation rates nor to the amount of adsorption, but should be a function of the reactivity of oxidation intermediates. Figure 6 shows that the presence of TA does inhibit the dissolution; higher organic concentrations result in less released cadmium. Similarly, it is noted that as the organic becomes completely oxidized, the dissolution begins to increase. The point of 90% TA removal is noted on the figure and after this point, a significant increase in Cd(II) is seen. It seems that the TA is competing for the positive hole against lattice sulfide, retarding dissolution, until the TA is no longer present, upon which complete utilization of the holes occurs via corrosive oxidation. A promising note is given in Fig. 7. As with CdS dissolution in the absence of organic compounds (Hsieh, 1987), the Cd(II) released is a strong function of pH. The maximum dissolution occurs under acidic conditions. However, at high pH, the most favorable for sulfur-containing organic oxidation, the photocatalyst corrosion is minimum. CONCLUSIONS The photocatalytic oxidation kinetics of reducedsulfur-containing organic compounds exhibit a nearzero-order reaction rate with respect to substrate concentration and a small rate dependence upon solution pH. An adsorption dependence is assumed

1277

and experimental adsorption data coincide with the pH dependence of the oxidation reaction. The photocatalytic nature of the system presents a concentration removal rate that is dependent upon the volume-to-surface area ratio of the reaction vessel. This must be considered when comparing rates from different reactor configurations. Variations in electrolyte media and concentration have little effect on the reaction which is important since the ionic makeup of waters vary depending on the source and wastewaters may have high concentrations of dissolved solids. The CdS photo-oxidative dissolution is retarded by the presence of the sulfur-containing organics due to strong competition for oxidizing interfacial holes, however, it is not completely inhibited at neutral pH. The organic oxidation is optimum at high pH, under which conditions the CdS dissolution is minimum. Acknowledgements--The research on which this paper is

based was supported in part by the United States Department of the Interior as authorized by the Water Research and Development Act of 1978 (P. L. 95.467). Contents of this publication do not necessarily reflect the views and policies of the United States Department of the Interior, nor does mention of trade names and commercial products constitute their endorsement by the U.S. Government. The award of a University Competitive Fellowship to A.P.D. by the graduate office of the University of Delaware is greatly appreciated.

REFERENCES

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ALLEN P. DAVISand C. P. HUANG

Matson R. J., Emery A. K. and Bird R. E. (1984) Terrestrial solar spectra, solar simulation, and solar short-circuit current calibration: a review. Solar Cells 11, 105-145. Minoura H. and Tsuiki M. (1978) Anodic reactions of several reducing agents on illuminated cadmium sulfide electrode. Electrochim. Acta 23, 1377-1382. Minoura H., Old T. and Tsuiki M. (1976) CdS-electrochemical photocell with S2- ion-containing electrolyte. Chem. Lett. 1279-1282. Sehindler P. W. and Stumm W. (1987) The surface chemistry of oxides, hydroxides, and oxide minerals. In Aquatic Surface Chemistry: Chemical Processes at the ParticleWater Interface (Edited by Stumm W.), pp. 83-110. Wiley, New York.

Simoyi R. H. and Epstein I. R. (1987) Oxidation of thiourea by aqueous bromine: autocatalysis by bromine. J. Phys. Chem. 91, 5124-5128. Spikes J. D. (1981) Selective photooxidation of thiols sensitized by aqueous suspensions of cadmium sulfide. Photochem. Photobiol. 34, 549-556. Stumm W. and Furrer G. (1987) The dissolution of oxides and aluminum silicates; examples of surfacecoordination-controlled kinetics. In Aquatic Surface Chemistry: Chemical Processes at the Particle-Water Interface (Edited by Stumm W.), pp. 197-220. Wiley, New York. Williams R. (1960) Becquerel photovoltaic effect in binary compounds. J. Chem. Phys. 32, 1505-1514.