ZnO-catalysed photodegradation of kraft black liquor

1. Photochem. Photobiol. A: Gem.,

ZnO-catalysed Hector

267

78 (1994) 267-213

of kraft black liquor

photodegradation

D. Mansilla+,

Jorge

Villaseiior

and Gabriel

Lkpmtmmtof Biological Sciences, Biotganic Chemistry Laboratov,

Jaime

Baeza

and Juanita

Maturana

Universidad de Taka, CasiUa 747, Talca (Chile)

Freer

Departmentof Chemisby, Chemicai Renewable Resounxs Laboratory, Universidad de Concepckm, Casilla 3-C, Concepckm (Chile)

Nelson

Dur%n

Chemkt!y Institute, Biological Chemirhy Laboramy, Paul0 (Brazil)

Uniwrsidade Esiaduol de Campinos, C.P. 6154 Campinns, CEP 13081-970, Srio

(Received July 26, 1993; accepted October 26, 1993)

Abstract Kraft black liquor was used as a model for effluent from the pulp and paper industry. Photochemical bleaching was attained when dilute samples were photolyzed with UV radiation using ZnO as catalyst. The photochemical mineralization of organic matter was first order and dependent on the oxygen pressure. A catalyst doped with Ag showed little effect on the discoloration or decrease in the chemical oxygen demand (COD) of black liquor. Platinum-impregnated ZnO yielded 100% discoloration after 60 min (190 W m-‘) of IJV radiation. Gel chromatography profiles and COz evolution measurements confirm the photomineralization of organic matter.

1. Introduction

Photochemistry has been widely used in industrial processes and, particularly, in water disinfection. The germicidal action of direct UV irradiation is well known and has permitted the installation of large-scale UV plants for the purification of drinking water in the UK and USA [l, 21. The need for less energy-intensive processes for the disinfection of water and the degradation of organic matter has encouraged scientists to investigate alternatives. Attempts have been made to purify water in the presence of oxidizing agents, such as hydrogen peroxide, combined with UV radiation [3, 41 or methylene blue (as an intermediate for the absorption and transfer of sunlight energy) [S]. Earlier studies have established that, when a semiconductor is exposed to W radiation, it is promoted to an electronically excited state. In the presence of oxygen, the organic matter present in solution should be partially or totally mineralized, with the excited semiconductor acting as catalyst. This subject has been extensively reviewed by Ollis +Author to whom correspondence should be addressed.

lOlO-6030/94/$07.000 1994 Elsevier Sequoia. All rights SSDI 1010-6030(93)03731-U

reserved

ef al. [6], Fox [7] and, more recently, Zeltner et al [8]. When the semiconductor suspended in solution absorbs energy in the range of its band gap, an electron of the valence band (VB) is transferred to the conduction band (CB) forming a positive hole (h+ ) in the valence band Semiconductor -

e-(CB) + h’(VB) (1) The electron-hole pair formed on the catalyst surface and the dissolved oxygen in solution may initiate the mineralization of organic matter (OM), forming carbon dioxide and water. Three possible reaction pathways have been proposed for the photochemical reaction [9, 101: (a) OM is directly oxidized by h+ (VB) forming a cation radical which reacts rapidly with oxygen (eqn. (2)); (b) water is oxidized to hydroxyl radical (OH) by a positive hole, which in turn oxidizes organic matter (eqn. (3)); (c) oxygen is reduced by e- (CB) and the superoxide anion formed will initiate the oxidation of organic matter (eqn. (4)) OM+h+

-

OM+

(2)

H,O+h+

-

OH+H+

(3)

O,+e-

O;(4) Many organic compounds have been oxidized using semiconductor-photocatalysed reactions. So-

268

H.D. Mansilla et al. I ZnO-catalysed photodegradation of baj? black

lar and near-UV light, in the presence of TiOl, have been successfully used in the mineralization of surfactants [11] and several organic contaminants [12]. Phenol and phenol derivatives are oxidized in the presence of UV-irradiated semiconductor catalysts [13-151. Chlorinated pesticides [16], sulfurcontaining organic compounds [17, 181, cellulose derivatives [19], lignin [S, 201, hydrocarbons [21], dioxins [22], kraft effluent [23], organic contaminants [24], water [25] and air contaminated with organic pollutants [26] have also been studied as potential targets for UV light-semiconductor-catalysed reactions. All reactions lead to the mineralization of organic matter. The most widely used catalysts for these reactions are TiOz. ZnO- and CdS; TiOz has shown the highest efficiency. Photocatalysed reactions yielding the microbial degradation of organic matter have also been applied successfully to kraft effluent [23] and organic molecules bonded to humic acid [27]. Therefore photochemical irradiation in the presence of a semiconductor catalyst may have potential in the treatment of some industrial effluents. Wood pulping technology, based on the use of sodium hydroxide and sodium sulfide as ligninremoving chemicals, is known as kraft sulfate cooking. Spent liquor derived from this process (kraft black liquor) contains a large number of low molecular weight compounds, in addition to polymeric constituents, lignin and some polysaccharides. Detailed chemical analysis of black liquor shows a high content of lignm-related phenols [28, 291. The environmental impact of kraft pulp effluent arising from industrial plants, without chlorine bleaching, depends on the nature of the chemicals present in the cooking of wood chips. The discoloration of black liquor may indicate the best conditions for the bleaching of real effluent from unbleached pulp industries. In this work, we investigate the mineralization of kraft black liquor using ZnO as catalyst and various periods of UV light irradiation.

2. Experimental details 2.1. Black liquor The samples used for irradiation were obtained by dilution of black liquor from pinus radiatu wood cooking (1 ml kraft black liquor in 250 ml distilled water). The characteristics of dilution were as follows: chemical oxygen demand (COD) 620 mg 1-l 02; color, 140.4 mg Pt 1-i; pH 9.6; total phenols,

liquor

870 mg 1-l; organic matter dissolved (lyophilized), 0.47 g 1-l; inorganic matter was negligible (determined by calcination). 2.2. Phdolysis Photolyzed samples contained dilute black liquor (1 ml liquor per 250 ml distilled water) and 2% commercial ZnO (GR Merck) which was used without purification. Catalysts impregnated with Pt or Ag were prepared following the methodology described in ref. 8: photodeposition of the metal over the semiconductor using UV radiation under a nitrogen atmosphere. Irradiations were carried out with a General Electric 250 W mercury lamp (HR 250 DX 37/ 40) placed 20 cm from the sample. The fluence rate (2600 FW cm-’ at 254 nm) was determined using a Black-Ray radiometer (Ultra-Violet Prod. Inc.) and corrected for IR radiation with the appropriate filter. Photolysis at A >3OO nm was carried out in a Pyrex round-bottomed flask (50 ml of sample) with magnetic stirring and oxygen bubbling. Photolysis at A>254 nm was performed in a sealed quartz reactor (5 ml of sample) with gas sampling and oxygen flux control via a flowmeter. Irradiation of dilute black liquor under a nitrogen atmosphere, without catalyst, was carried out for correction of the color and COD. 2.3. Analysis Samples irradiated for various periods of time were centrifuged to separate the photocatalyst. The absorption spectra of the filtered solutions were recorded in quartz cells in a Shimadzu UV16OA spectrophotometer. COD, color and total phenols were determined following standardized procedures [30]. All determinations were run twice, or until an error of less than 5% was obtained. Evolution of COz from samples irradiated at A > 254 nm was monitored by gas chromatography using a Shimadzu GC-8A chromatograph equipped with a thermal conductivity detector (TCD). The conditions of analysis were as follows: column, Poropak Q, 2 m, internal diameter (i.d.) $ in; column temperature, 50 “C; detector/injector temperature, 120 “C; filament current, 120 mA; carrier gas, helium at 50 ml min -I. Under these conditions, the retention time of CO, was 120 s. The effect of oxygen partial pressure was examined by mixing helium and oxygen in different proportions in a mixing chamber filled with glass O-rings. The gas mixture was bubbled into the suspension using a total flux of 15 ml min-’ on all runs.

H.D. Mansilla et aL i ZnO-cataiysed photodegradation of krafr black liqwr

The molecular weight profile was obtained on an 80 mm column tiled with Sephadex G-25 using a solution of 5.55 X lop4 M NaOH and 0.1 N LiCl as eluent. The same column was used to determine the profile of the irradiated samples. 3. Results and discussion The effects of irradiation on dilute suspensions of kraft black liquor were studied over two wavelength ranges. When the photolysis of suspensions was carried out with a glass filter (A>300 nm), no effect on color or COD values was found, even after long periods of photolysis. Negligible changes in COD and color values were observed when irradiation was performed in the absence of oxygen or in the absence of a semiconductor. At shorter wavelengths, the color changes of the suspension were evident after a few minutes of irradiation in the presence of a semiconductor and oxygen. Bleaching of the solution was visible after 15 min of irradiation. After long periods of photolysis (2 h), the solution became transparent. The minimum quantity of catalyst required for the efficient destruction of organic matter (measured as COD) was determined. Figure 1 shows the decrease in COD values as a function of fluence (time of irradiation) using various amounts of catalyst. We observed that 0.1 g of catalyst per 5 ml of suspension gives us the best results. Higher quantities of catalyst do not increase the degradation significantly. This means that the appropriate ratio is 0.1 g of ZnO per 2.35 mg of organic

269

matter (organic matter dissolved in 5 ml of solution). The calculated ratio is 42.5 mg of ZnO to each milligram of organic matter. The ratio reported by Ohnishi et aE. [9] for the degradation of alkaline liguin with TiO, was 50 mg catalyst per milligram of lignin. The activity of the catalyst is not modified after several photocycles. After 15 min of irradiation (approximately 50 kJ m-‘, 0.1 g ZnO), a decrease from 620 mg 1-l O2 to 380 mg 1-l 0, was achieved (a reduction of about 40% in COD). After 60 min of photolysis (190 kJ m-‘), a 57% reduction in COD was obtained. This shows that short periods of irradiation can produce major changes in the structure of organic matter and promote the photochemical oxidation of dissolved compounds. The same trend can be observed in color determinations during photocatalysis as shown in Fig. 2. Bleaching reaches 60% after 15 min of irradiation and 80% after 60 min using 0.1 g of ZnO. This indicates that part of the organic matter in the solution is transformed into colorless compounds and not completely destroyed (cf- COD measurements at the same fluences). It has been reported previously that the doping of semiconductors with noble metals enhances their performance as photocatalysts in the decomposition of organic compounds [31-331. This appears to be due to the electron-accepting properties of the metal, which promote the transfer of electrons from the VB to the CB, generating the holes in the catalyst surface and precluding the recombination of the electron-hole pair. In this study, we doped ZnO with platinum or silver in order to

800

600

3 . kO0 E E 200

0i 0

I

1

50

100

1

150 FLUENCE

I

I

I

1

200

250

300

350

LOO

(KJ/m2)

Fig.1.Effect of the amount of ZnO on the COD of dilute black

liquorduring

W

irradiation (5 ml solution, As-254 nm, pH 9.6).

270

H.D. Mantilla et al. / ZnO-catalysed photodepdation

FLUENCE

Fig. 2. Effect of the amount

[KJ/m2)

of ZnO on the color of dilute black liquor during

determine the changes in the efficiency of organic matter degradation. Figure 3 shows that no signiticant difference was found between impregnated and non-impregnated catalyst with respect to COD values. However, moditications were found in the kinetic profile of color changes during photolysis. In Fig. 4 it can be seen that, using ZnO impregnated with platinum, complete elimination of color is attained after 60 min (190 kJ m-“) of irradiation. The ZnO-Pt mixture enhances the efficiency of color elimination but does not promote the total transformation of organic matter into CO, and water.

FLUENCE Fig.

3. COD changes

on photolysis

of kraftblock &pw

UV irradiation

(5 ml, A2254

nm, pH 9.6).

It has been reported that a pH value of 3.5 is the optimum for the photocatalytic decomposition of phenol [9] using TiO, as catalyst, However, other studies have shown that, at high pH, positive holes photogenerated on a semiconductor interact with hydroxyl groups resulting in the formation of hydroxyl radicals (OH- + h’ + OH’), which oxidize organic compounds [17, 25, 341. For this reason, alkaline media have been recommended. In our case, the pH had little influence on the color or COD values when irradiation was performed in the presence of ZnO at short wavelengths.

(KJm2)

with Pt- and Ag-doped

ZnO (5 ml, A > 254 nm, 0.1 g ZnO plus 0.015

g Pt or A& pH 9.6).

H.D. Mansilla et al. I ZnU-cat&wed photodegredation of I&

271

black liquor

60

0

50

100

FLUENCE

Fig. 4. Color changes

on photolysis

250

200

150

350

300

400

(KJ/m2)

with Pt- and Ag-doped

ZnO (5 ml, A>254

nm. 0.1 g ZnO plus 0.015 g Pt or Ag, pH 9.6).

760mm

Hg

380 mm Hg N

3

10

‘0

152 mm Hg

E a 5

52 mm Hg

0

0

3

2

1

TIME Fig. 5. Carbon

dioxide released

during

photolysis

I

5

(min)

at different

The disappearance of phenol derivatives present measured using the solution was in Folin-Ciocalteaus reagent. Total phenols were reduced by about 93% (from 870 mg 1-l to 60 mg 1-l) 15 min after the start of irradiation. This finding was confirmed through UV-visible spectroscopy. The most important peak appearing at 280 nm, characteristic of phenolic groups, decreases significantly during irradiation, indicating phenol degradation. The evolution of CO, in the reaction was measured to confirm photocatalyzed mineralization.

oxygen partial

pressures

(5 ml, A>254

nm, 0.1 g ZnO,

pH 9.6).

Figure 5 shows curves of CO2 evolution ~1s. oxygen partial pressure. The reaction is clearly dependent on oxygen and more effective under conditions of oxygen saturation, Preliminary measurements, under conditions of controlled temperature, indicate that the reaction is first order with respect to oxygen. Gel permeation chromatography is an important tool in the evaluation of the disappearance of organic matter. The profile shown in Fig. 6 indicates major degradation, in which the high molecular

272

H.D. Manrillo et al. I ZnO-catalysed phomdegnadarkm of kmji black liquor

matter occur during the first few minutes of the ZnO-photocatalysed reaction. Acknowledgments The authors thank the International Foundation for Science (Sweden, Grant F/1721-1). Fondo Nacional de Ciencia y Technologia (FONDECYT, Chile, Grant 91-0353), Dirrecibn de lnvestigacidn y Asistencia T&nica (DIAT), Universidad de Talca (UTAL, Grant 310-12), Conselho National de Pesquisa (Brazil) and Funda@o de Apoio & Pesquisa do Estado de Sdo Paulo (Brazil) for financial support. References

Amo::~~, . , . ;;. h_

100

ElUTfON

^

200

VOLUME

(ml)

Fig. 6. Gel permeation chromatography (Sepbadex G-25, 0.1 N LiC1, 5.6X lo-’ N NaOH) before and after irradiation (5 ml, A>254 nm, 0.1 g ZnO, pH 9.6).

weight compounds are transformed into small molecules. This result is in accordance with the other findings discussed above.

4. Conclusions The photodegradation of dilute kraft black liquor was most efficient using ZnO as catalyst at short wavelengths in oxygen-saturated solutions. Under these conditions, the pH does not influence the decomposition kinetics of organic matter. Platinized ZnO was more effective in the discoloration of the solution. Finally, we consider that this method may be useful to treat phenolic and polyphenolic effluents on a short time scale. Rapid changes in color, CO2 evolution, total phenol content and organic

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