Solar Energy Vol. 41. No. 1. pp. 55-59, 1988 0038-092X/88 $3.00 ÷ .00 Printed in the U,S.A. Copyright ~ 1988 Pergamon Press pie

REMOVAL OF TOXIC CYANIDE FROM WATER BY HETEROGENEOUS PHOTOCATALYTIC OXIDATION

OVER ZnO

J. DOMI~NECH and J. PERAL Department de Qufmica, Universitat Aut6noma de Barcelona, Cerdanyola, Spain

Abstract--The elimination of cyanide from water by oxidation under UV illumination and using ZnO powder as catalyst has been studied. The study has been carried out by determining the yield of CN- elimination at different irradiation times, initial CN- concentrations, pH, mass of ZnO in suspension, temperatures, and light intensities. The CN- oxidation gives OCN- as an intermediate product, which is further oxidized to CO~3-. The greater yields of the CN- photooxidation are obtained at pH = 11. In addition, at this pH the ZnO is highly stable. The ZnO is a good catalyst even under solar irradiation. It has been observed that, after 2 h of solar exposure, 87% of the initial 10 -a M of CN- has been transformed to CO~3-.

1. INTRODUCTION

Recently, much attention has been paid to the use of semiconductor powders in photocatalytic pro- cesses[ 1 ]. One of the most promising applications of photocatalysis lies in waste-water decontamination. In this way, many works have been carried out that show that this photocatalytic procedure is rather ef- ficient in the elimination of pollutants from water[2].

Cyanide is a highly toxic component, which is present in wastes of a variety of industries. Different methods based on the oxidation of CN- have been proposed (chlorination, electrolytic oxidation, ozon- ation, etc.), all of which present some problems when they are applied in practical cases. In contrast, pho- tocatalysis is less problematic in its application, being a relatively inexpensive method that can utilize solar light as an energy source for the purifcation process.

In the present work, the photocatalytic oxidation of CN- over ZnO is investigated. The ZnO has been chosen because it has shown high efficiency in dif- ferent photocatalytic processes[3, 4]. In this work, a broad study of CN- photooxidation is performed with a special emphasis on the pH influence, the analysis of the final oxidation products, the ZnO stability, and other factors, which completes preliminary studies of Frank and Bard[4] and also more recent studies of CN- photooxidation over TiO215-7] and CdS[8]. The results of this work reflect the high efficiency of the ZnO as photocatalyst for CN- elimination, which is capable of transforming all CN- to CO~-, in contrast to TiO2 for which only OCN- is obtained as the final oxidation product[4, 6]. The viability of this method using ZnO as catalyst is apparent by observing the high yields obtained under solar irradiations.

2. EXPERIMENTAL

Potassium cyanide (Merck) and potassium cyanate (Fluka) were of analytical grade. The ZnO (Probus) was lightly doped as was determined by conductivity measurements by means of a Fluke 6000A conduc-

timeter. The average particle size, determined by scanning electron microscopy, was 0.4 txm. The other chemicals used were, at least, of reagent grade.

Experiments were performed in a thermostated cylindrical Pyrex cell. The light source was a Phillips HPK 125W medium-pressure mercury vapor lamp. The IR fraction of the beam was removed by the water in the double wall of the Pyrex cell. The intensity of the incident light inside the photoreactor, measured employing the uranyl oxalate actinometer[9], was 4.8 10 -6 Einstein ra in- ' , approximately. The light flux was varied from full lamp power by using neutral density filters placed between the lamp and the photoreactor.

Unless otherwise stated, 25 ml of a CN- solution containing 0.2 g of ZnO was used. The mixture was maintained in suspension by means of a magnetic stirrer. After each run, the suspension was centri- fuged, and the cyanide remaining in solution and the corresponding oxidation products were analytically determined. Cyanide and cyanate concentrations were monitored spectrophotometrically (Pye Unicam SP600 spectrophotometer) by measuring the absorbance of the complexes, CN-/barbituric acid-pyridine and OCN-/Cu(II ) -pyr id ine , at 570 and 680 nm, respec- tively. Carbonate concentration was determined by precipitation with Ca(H), determining the excess of Ca(II) by titration with EDTA using murexide as an indicator. The concentration of Zn(II) resulting from ZnO decomposition was determined polarographi- tal ly (Radiometer PO4) polarograph). The experi- mental errors in the analytical determinations were 2% for CN- and CNO- and 5% for CO]- and Zn 2+. Hydrogen peroxide was monitored by oxidation with permanganate and measuring the concentration of ox- ygen formed in solution by means of a Clark type oxygen electrode (Orion 9708).

3. RESULTS AND DISCUSSION

The elimination of CN- ions from aqueous ZnO suspensions by photocatalytic oxidation has been

55

56 J. DOMILN'ECH and J. PEg.M.

studied under different experimental conditions. The mechanism of this photochemical process is based on the absorption, by the semiconductor particles, of light of energy greater than the bandgap of the semicon- ductor, which produces electron-hole pairs (e- - h÷). These photogenerated charges can be very reactive depending on the energy of the conduction and va- lence band edges of the semiconductor and can mi- grate to the particle surface and react with suitable redox species in solution.

In the present case, the following reactions take place at the illuminated semiconductor-electrolyte interface[4, 6]:

ZnO + h~--* ZnO(e- - h ÷) (1)

5 I

-6 e3

4.2 z

1

t {rain)

Fig. I. Variation of CN- concentration in solution with ir- radiation time for different initial CN- concentrations. Mass

of ZnO = 8 g 1-'; pH = 13.3; temperature = 25"C.

ZnO(e- - h ÷) ~ ZnO (2)

1 02 + 2H20 + 2e- ~ H202 + O 2 (3)

CN- + 2OH- + 2h + -o OCN- + H:O (4)

Process (4) leads to the transformation of the CN- ion to a less toxic cyanate ion[10] by reaction with the photogenerated holes. Simultaneously, oxygen in solution is photoreduced by the conduction band electrons giving hydrogen peroxide (process (3)). Il- lumination of a 10 -4 = M solution of CN- in pres- ence of ZnO, in suspension at pH = 13.3, produces H202 at a concentration of 1.03 10 -4 M after 15 min of irradiation. The amount of H202 produced is 100% greater than that produced in absence of CN- in so- lution under the same experimental conditions.

Thermodynamically, the H202 produced from process (3) can oxidize the CN- ions via homoge- neous reaction. In order to determine the efficiency of this process, blank experiments have been per- formed in absence of catalyst. UV illumination of an initial 0.0375-M concentration of CN- in the pres- ence of 0.1 M of H202 at pH = 13.3 and at 25°C originates a decrease of the CN- concentration. For this homogeneous process, an apparent rate constant of 3.4 10 -3 rain -I has been determined, which agrees with the values obtained by Serpone et al.[7]. This value is about 40 times lower than the corresponding rate constant of the CN- heterogeneous photooxi- dation (see later in the text). As a consequence, the amount of CN- oxidized by I-I202 via homogeneous oxidation can be disregarded compared to the amount of CN- oxidized via process (4).

3.1 Kinetics of CN- photooxidation The variation of CN- concentration in solution with

irradiation time for different initial CN- concentra- tions at pH = 13.3 and in the presence of ZnO in suspension is depicted in Fig. 1. As can be seen, the elimination of CN- in solution takes place effi- ciently; after 15 rain of irradiation, the percentage of CN- transformed is greater than 80% in all the cases. In the dark, no adsorption of CN- ions onto ZnO particles has been detected.

From the results of Fig. 1, a first-order kinetics for CN- photooxidation is deduced, with an apparent rate constant of 0.139 rain -1. The same reaction or- der has been obtained for CN- photooxidation over TiO2 but the rate constant determined is lower[6]. According with Rose and Nanjundiah[6] and Arikado et al.[11], a possible mechanism for CN- oxidation involves the formation of a cyanide radical as the rate- determining step.,

CN- + h + ~ CN. (5)

which dimerizes to cyanogen.

2CN. ~ (CN)2 (6)

Finally, cyanogen suffers a dismutation in alkaline solution to give CN- and OCN-[12].

(CN)2 + 2OH- --) CN- + OCN- + H20 (7)

3.2 Influence of the light intensity and of the mass of ZnO

The amount of CN- photooxidized at different light intensities and mass of catalyst in suspension has been determined. Figure 2 shows the variation of the per- centage of CN- transformed after 9 min of irradia- tion, as a function of light flux, for a 10 -4= M initial solution of CN- at pH = 13.3. The slight increase of the percentage of CN- photooxidized observed be- tween 10% and the lamp full power indicates that the process is limited by the e- - h + recombination (pro- eess (2)). On the other hand, the efficiency of the CN- photooxidation depends on the mass of ZnO in suspension. In Fig. 3, the percentage of CN- trans- formed after 9 min of irradiation from a 10 -4 = M initial solution at pH = 13.3, is represented as a function of the mass of catalyst in suspension. The percentage of CN- transformed increases with in- creasing the mass of ZnO, attaining a limiting value of 65% for a suspension of 8 g of ZnO per liter of solution.

70 1001

10

7° I

50

t j 30 .<

2'0 &'0 6'0 8'(D 1(DO % l

Fig. 2. percentage of CN- eliminated as a function of light flux in terms of the fraction of full lamp intensity. Initial CN- concentration = 10 -4 M; mass of ZnO = 8 g I-t; pH

= 13.3; irradiation time = 9 rain; temperature = 25"C.

-.-.-.__..__

9C

I

7C

6C

Heterogeneous photocalalytic oxidation 57

pH

Fig. 4. Variation of the percentage of CN- eliminated with the pH of the suspension. Initial CN- concentration = 10 -~ M; mass of ZnO = 8 g I-; irradiation time = 9 rain; tem-

perature = 25"C.

3.3 Influence of pH Both the flat-band potential of ZnO and the redox

potential of the C N - / O C N - couple show the same variation with the pH (59 mV/pH). Then, from a thermodynamic point of view, no variation of the ef- ficiency of the CN- photooxidation must be ex- pected. However, as is shown in Fig. 4, a strong de- pendence of CN- photooxidafion with pH is observed. As can be seen, the yield of CN- oxidation decreases with increasing pH, especially at pH higher than 12.5.

In fact, it has been shown that the potential of CN- oxidation over platinum, graphite, and TiO2 electrodes does not change with the pHI6, 11, 13]. This is in accordance with the mechanism previously suggested, where the rate-determining step (process (5)) is pH independent. In consequence, the CN- ox- idation becomes less favored with increasing pH, be- cause the difference between the potential of CN- oxidation and the potential of photogenerated holes decreases. Furthermore, at high pH, water oxidation occurs in preference to CN- oxidation, as it has been

made clear from results obtained by electrochemical experiments performed with the rotating ring-disk electrode technique in strongly alkaline media[5].

3.4 Influence of the temperature The variation of the CN- photooxidation yield with

the temperature in the range comprised between 25 and 70°C has been studied. The results for a 10 -4 M initial CN- solution in the presence of ZnO in sus- pension at pH = 13.3, are represented in Fig. 5. In the absence of ZnO, no loss of CN- has been de- tected after 9 rain of agitation in all the range of tem- peratures studied. As Fig. 5 illustrates, the oxidation of the CN- under UV illumination is accelerated with increasing the temperature. The data depicted in Fig. 5 follows an Arrhenius-type behavior with an acti- vation energy of 10.4 KJ mo1-1. This low value is similar to the obtained for other photocatalytic pro- cesses and is characteristic of an electron transfer re- action with rapid faradaic kinetics[ 13-17].

10

f

o.b 0.'08 &2 0 16 0. 0 m(g)

Fig. 3. Percentage of C N - eliminated as a function of the mass of ZnO in suspension. Initial C N - concentrations ffi I0- ' M; solution volume ffi 25 mi; pH ffi 13.3; irradiation

time = 9 min; temperature = 25"C.

90

8O

, 70 Z O

6O

2'5 s'5 7; T(°C)

Fig. 5. Pereentage of CN- eliminated as a function of the temperature. Initial CN- concentration = 10-" M; mass of

ZnO = 8 g l - t ; pH = 13.3; irradiation time = 9 min.

58 J . D O M I h ' q E C H a n d J .

3.5 Products of the CN-" photooxidation According to processes (5) to (7), the product of

the CN- oxidation is OCN-. However, a further ox- idation of OCN- to CO~- and N2 by the photogener- ated holes in the ZnO must be considered:

2OCN- + 8OH- + 6h + ~ 2CO~-

+ N 2 + 3H20 (8)

Table 1. Concentrations of CN-, OCN-, and CO~- for different irradiation times. Mass of ZnO is in suspension

= 8 g 1-'; pH = 3; temperature = 25°C.

t/min [CN-]/M [OCN-]/M [CO~-]

0 10-' - - 9 3.9 10 -3 5.8 10 -3 1.1 10 -3

15 1.8 10 -3 4.9 10 -3 3.1 10 -3 120 <10 -~ <10 -s 10 -2

On the other hand, OCN- can be hydrolyzed in al- kaline solution to give NH3 and CO32-:

OCN- + H20 + OH- --~ CO 2- + NHa (9)

However, the rate constant of the process (9) is very low, ca. 1.081 10 -4 min- ' at 65°C[18].

Figure 6 shows the variation of the OCN- con- centration formed during irradiation in presence of ZnO in the solution. The initial CN- concentration was 10 -4 M and the pH of the suspension was 13.3. As can be seen, the amount of OCN- formed initially increases with an increase in irradiation time, attain- ing a maximum value of 0.53 10 -4 M after 12 min of UV illumination. This is a clear evidence that pro- cess (8) takes place. In fact, CO 2- ion in solution has been detected after illumination. Table 1 summarizes the concentrations of CN- , OCN-, and COl- in so- lution determined after illumination of an initial 10 -2 M solution of CN- in the presence of ZnO at pH = 13.3. These results show the high efficiency of ZnO as a catalyst, which is capable of oxidizing all CN- ions to CO 2-.

3.6 Stability of ZnO It is well known that ZnO is chemically decom-

posed at high pH values giving zincates:

ZnO + 2OH- ~ ZnO~2- + H20 (10)

This chemical dissolution of ZnO is further in- creased in presence of CN- in solution by forming cyanide complexes with Zn(II) ions.

- -

I I I I I I I I

8 12 16 20 24 28 t (rain)

Fig. 6. Concentration of OCN- formed from CN- oxida- tion as a function of the irradiation time. Initial CN- con- centration ffi 10 -4 M; mass of ZnO = 8 g I-'; pH = 13.3;

temperature = 25"C.

Besides these dark processes, which lead to the ZnO dissolution, another source of instability, which corresponds to the anodic decomposition of the ZnO under UV illumination, must be considered. In fact, the anodic decomposition potential of the ZnO, ca. + 0.43 V vs. SCE[19] is more negative than the po- tential of the photogenerated holes in the valence band of the ZnO (+2.7 V vs. SCE at pH = 13120]). As a consequence, ZnO can react with the photogener- ated holes giving Zn(II) and oxygen[21].

1 ZnO + 2h ÷ ~ Zn(II) + ~ O2 (11)

In order to determine the degree of stability of the ZnO in the aqueous environment used in the present work, experiments in the dark and under UV illu- mination have been carried out. Table 2 summarizes the concentration of Zn(II) in solution formed as a result of the ZnO decomposition at different pH and in presence of 10 -4 M of CN-. The data of Table 2 show that the degree of the ZnO decomposition de- creases with decreasing the pH of the solution, be- coming undetectable at pH = 11. On the other hand, at higher pH, a weak photocorrosion is noticed, which only accounts for 3% of the total ZnO decomposed, approximately. Finally, no formation of cyanide complexes of Zn(II) has been detected in solution (initial CN- concentration, 10 -4 M) in the range of pH studied.

3.7 ZnO reutilization Experiments on ZnO reutilization have been per-

formed to test the capacity of the ZnO as a photo- catalyst. A 10 -4 = M CN- solution containing ZnO in suspension has been irradiated; thereafter, the ZnO suspension has been centrifuged and the solid redis- persed in a fresh 10 -4 M CN- solution, which has

Table 2. ZN(H) concentration in solution at different pH. Initial CN- concentrations = 10 -~ M; mass of ZNO in

suspension ffi 8 g 1-'

pH

13.3 12.1 11

Dark [ZnO]/M 6.06 10-" 1.5 10 -4 <10 -6

Irradiation 15 min [ZnO]/M 6.2 10 -4 1.55 10 -4 <10 -6 30 min [ZnO]/M 6.2 10 -4 1.6 10 -4 <10 -6

6

"55 e

It

t j 2

Heterogeneous photocalalytic oxidation

pH:11

o

n

Fig. 7. CN- concentration remaining in solution after UV illumination as a function of the number of runs (see text

for experimental procedure).

been illuminated afterwards at the same experimental conditions. This operation has been repeated several times. In each run, the C N - remaining in solution has been determined. The results for two different series of experiments at pH = 13.3 and 11, respec- tively, are illustrated in Fig. 7. As can be seen, the ZnO shows a certain degree of deactivation, identical at both pH. After each illumination of the sample, the yield of C N - elimination decreases approxi- mately 2.5%.

3.8 Solar irradiations

Solar experiments have been carried out in Bel- laterra (Spain) (alt. 45 m, lat. 41°30'(N) and long. 2°6'(E)) in September 1987. The C N - solutions were placed in cylindrical vessels (diameter 13 cm, height 6 cm) and sufficient ZnO was added to cover all the bottom of the vessel. The experiments have been per- formed in absence of agitation and at a mean tem- perature of 26°C. Figure 8 shows the variation of the

!lCN

3b 6b 9b t Imin)

Fig. 8. Variation of CN-, OCN-, and CO~- concentrations in solution with respect to solar irradiation time. Initial CN- concentration = 10 -2 M; pH = 11; temperature = 26"C.

59

C N - , O C N - , and C O l - concentrations as a function of the solar irradiation time. The initial C N - con- centration was 10 -2 M and the pH of the solution was 11. As Fig. 8 illustrates, the elimination of C N - , which is fully oxidized to CO~3-, takes place with high yield. So, after 2 h of solar irradiation, 87% of the initial C N - was converted to CO]- . The yield of C N - elim- ination decreases with decreasing initial C N - con- centration; for 8 l0 -3 M and 5 10 -3 M initial 72.5% and 701%, respectively.

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