A preliminary investigation on the photocatalytic degradation of a model humic acid

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Wal. Sci. reel!. Vol. 33. No.6. pp. 189-194. 1996.. Copyright © 1996 IA WQ. Published by Elsevier SCIence Ltd Pnnted in Great Britain. All nghts reserved. 0273-1223/96 S 15·00 + 0'00

Pergamon

PH: S0273-1223(96)00302-2

A PRELIMINARY INVESTIGATION ON

THE PHOTOCATALYTIC DEGRADATION OF A MODEL HUMIC ACID Miray BekbOlet and GOlhan Ozkosemen Bogazifi University, Institute of Environmental Sciences, 80815 Bebek. Istanbul, Turkey

ABSTRACT The research reported addresses the destructive removal of humic acid in aqueous medium by a photocatalytic oxidation process. Bench scale experiments were carried out using titanium dioxide as the photocatayst and Black Light Fluorescent Lamp as the irradiation source. Following I h irradIalton 40 % TOC and 75% Color4OQ removals were attained for 50 mgIL humic acid solution in the presence of 1.0 mglmL Ti02. The optimum Ti02 loading was found to be 1.0 mg/mL. Acidic medium accelerated the photocatalytic degradation rate whereas a retardation factor of 0.4 was recorded in alkaline medium. The evaluation of the THMFPs of the photocatalytic ally treated humic acid solutions revealed that the destructive removal of humic acid was effective enough to keep the THM levels below the maximum contaminant level of 0.10 mgIL (USEPA). Copyright © 1996 IAWQ. Published by Elsevier Science Ltd

KEYWORDS Humic acid; photocatalytic degradation; THM formation; titanium dioxide. INTRODUCTION Aquatic humic substances are yellow colored degradation products of polymers of natural origin involving enzyme catalyzed depolymerization and oxidation reactions. They possess no readily identifiable structure composed of a heterogeneous group of organic macromolecules of which the main functional groups are carboxylic, phenolic and carbonyl groups. Humic acids are the most important sunlight absorbing species in natural waters leading to the photosensitized oxygenation reactions (Zepp et al., 1977). Recent interest has been focused on the destructive removal of humic acids in natural waters since these substances are known to play an important role in the fate of organic pollutants. Besides the traditional treatment method such as activated carbon adsorption, coagulation precipitation, ion• exchange and biological degradation, advanced oxidation processes (AOPs) are also applied for the removal of humic acids (Medley and Stover, 1983; Arai et al., 1986; Ebenga et al., 1986; Backlund, 1992). The end products of these AOP methods are reported as lower molecular fractions of better biodegradability from the parent high molecular weight molecules. Photocatalytic oxidation process (peO) using semiconductor oxides such as titanium dioxide has gained attention recently (Bahnemann et al., 1994). The main mechanism of the photocatalytic degradation process 189

M. BEKBOLET and G. OZKOSEMEN

190

is the photogeneration of electron-hole pairs. These photo-generated species constitute a redox pair and take part in photo redox reactions. The holes react with the electron donors in the electrolyte to produce powerful oxidizing free radicals such as OH. The simplified mechanism:

Ti02 ~ Ti02· (e cb- + hvbry

Ebg=3.1 eV

(ecb-+hvb+) -+ ecb- + hvb+ e cb- reactions; 02 +

ecb-~02-'

02 + 2ecb- + 2W ~ H202 02- + H20 2 ~ HO' + OH-+ 02 02- + W-+ OOH H202 + ecbH2~

+ hv

~

~

HO+ OHaq•

2HO'

hvb+ reactions; H20 + hvb+ -+ HO' + Haq+ OH- + hvb+ ~ HO' It is widely accepted that the initial reaction takes place via the attack of hydroxyl radical and through the formation and disappearance of reaction intermediates the formation of carbon dioxide constjtutes the end product of the photomineralization. The aim of the study was to investigate the photocatalytic degradation of humic acid and to follow the trihalomethane (THM) formation in relation to the TOC removal. EXPERIMENTAL SECTION Materials The photocatalyst used throughout the experiments was titanium dioxide powder. Ti0 2 Degussa P-25 having a BET surface area of 50 ± 15 m 2/g and mainly in anatase crystal form with average particle size of 30 nm. Commercial humic acid was supplied from Roth. All other chemicals were reagent grade. Analytical Procedures All UV-VIS absorption spectra were recorded using Shimadzu UV-IOO double beam spectrophotometer. TOC values were determined with Shimadzu TOC Analyzer. TOC 500. COD and THMFP values were determined according to the methods given in APHNAWW AlWPCF Standard Methods.

Photocatalytic degradation of a model humic acid

191

Photoreactor The cylindrical pyrex reaction vessel was 7.5 cm in diameter and 3.5 cm in height. The light source was a 125 W Black Light Fluorescent Lamp (BLF) built into a lamp housing that was enclosed in a box. The reaction vessel was illuminated from the top and continuous stirring was provided by means of a magnetic stirrer. Procedure Humic acid solutions were obtained by dilutions of the stock humic acid that was prepared according to the procedure outlined by Urano et al., (1983). The slurry was formed by sonication of the humic acid solution with required amounts of Ti02 powders. The photocatalyst was removed by filtration through 0.45 11m Millipore syringe filter and the clear solution was analyzed according to the procedures outlined above. RESULTS AND DISCUSSION The spectroscopic properties of humic acid (Roth) were determined. The electronic absorption spectra exhibited continuous absorption decreasing with increasing wavelength in the UV -VIS region. The specific absorption coefficient was determined as kh: 6.036 L( mg org q- m- at pH :6.0 and )..:360 nm showing that the commercial humic acid (Roth) can be used as a model compound. Prelimjnary tests Reactions were followed both in the absence of light (dark reaction) and the photocatalyst. The changes in TOC values of the humic acid during the prolonged reaction periods were found to be insignificant as 1% for both of the cases. TOC. COD and Color~ chan~es durjn!: photocatalytic de&radatjon TOC, COD and Color400 changes after a 2 h reaction period for 50 mgIL humic acid solution are given in Table 1. The photocatalyst concentration was kept constant as 1.0 mglmL. The conversion of TOC and COD was 88% following a faster removal of Color400 as 99%. Neither the darkening of the color of the irradiated solutions at the early stages of the photocatalytic degradation nor the formation of a precipitate due to the partial polymerization of reaction intermediates was observed in contrast to the results obtained for the ozonated humic acids (Gilbert, 1988). Table 1. TOC, COD and Color400 changes with respect to irradiation time, Humic acid: 50 mgIL, Ti0 2: 1.0 mglmL Irradiation time, min 0 30 60 90 120

TOC mWL 27.17 21.27 15.31 9.02 3.16

COD mWL 66.10 51.05 34.23 16.53 7.68

Color400 m- I 60.2 38.2 15.1 3.2 0.6

A certain amount of TOC (=3.16 mgIL) was detected even after a 120 min reaction period implies that the destruction of humic substances produces refractory compounds which are oxidized quite slowly. A rough estimation of the destruction rate can be made through the assumption that the decomposition rate of humic substances is proportional to the reactive TOC concentration. Effect of bumic acid concentration The effect of initial substrate concentration on the photocatalytic degradation rate was investigated. The UV• VIS absorption values at 400, 280 and 254 nm were evaluated for humic acid concentrations of 5 mgIL, 10

M. [lEK BO LET and G. OZKOSF \I !':-':

192

mg/L. and 20 mg/L . The ph o t\lcatal y~t concentration was kept con,tant a, 1.0 mg/mL The decl'\('rization of humi c acid fo ll owed the sa me trend- of degradation as the ahsorpti on \·;lIues at 2XO 11m and 254 nm representing the components that wert.: an'malic in ,tructure (Tank 2). Table 2. Effect of humic acid concentration on the removal of color .ml' L'V2XO and CV254. Ti0 2: 1.0 mg/mL

Irradiation time, Ih

[Humic acid] mg/L

Color"tWl

UV)RO

UV Jq

87.9

81.9

802

10

77.1

63 .2

626

20

64.2

49.3

472

5

IrradIation time, 2h

[Humic acid] mglL

Color"IWI

UV 7.RII

UV2~"

5

93 .1

94.4

995

\0

95 .8

92 .1

906

20

88 .2

78.4

75 .8

60 ~ 0

C

-

50

.2 40

~ '"~ "0

g f-

30 20

I

10

0

\0 20 50 5 Initial humic acid concentration

--I

90 0~

i

0

80

70

60

01 .0 mg TlO2/mL

50

....

40

U

10

5

00.5 mg TlO2hnL DO.l mg TlO2hnL

30 '0 20 0

I

0 5

10

20

50

Initial humic acid concentration

Figure I. Effect of photocatalyst concentration at 4 concentratio ns of humic acid.

193

Photocatalytic degradation of a model humic acid

Photocatalyst TiOZ concentration The effect of the photocatalyst concentration was investigated in the range of 0.1 mg/mL, 0.5 mg/mL and 1.0 mglmL (Fig. 1). Humic acid concentrations were in the range of 5mgIL-50 mgIL. TOC degradation percentages were determined as 55% for all of the cases, being fastest for 1.0 mglmL Ti0 2. Further increase to 2.0 mg/mL and 5.0 mg/mL did not exhibit an improved photocatalytic degradation rate. This situation could be explained in terms of hindered light intensity due to the increased opacity of the solutions and deactivation of the activated catalyst molecules by collision with ground state particles. pH effect The pH changes as recorded during the reaction periods were found to be insignificant (0.5 units). The pH effect was investigated in acidic medium (pH:3) and in basic medium (pH: 11) for 50 mgIL humic acid slurry containing 1.0 mglmL Ti0 2. In the acidic medium drastic increases of color 400 removal (to 88.9) and TOC removal (to 80.7) were attained after only 5 min. reaction period. At the natural pH of the humic acid solution (pH:6) the corresponding values were 74.9% and 43.7% after 60 min. of irradiation. Since the pH zpc of Ti0 2 , Degussa P-25 was reported as 6.3, the increase in degradation rate in acidic medium could be explained by the positively charged surface characteristics of the photocatalyst. In acidic medium: Ti02 + H30+ -+ TiOH2+ + H20 TiOH + OH- -+ TiO- +

In basic medium:

H20

The photocatalytic degradation rate in basic medium at pH: 11 was attained by a retardation factor of 0.4 with respect to the degradation rate in natural pH of humic acid solution. THM Formation Trihalomethane formation potentials of the treated humic acid samples were determined. THMFP values after a 2 h reaction period were well below the limit of 100 j.lglL ( USEPA) (Table 3). Table 3. THMFP values of the humic acid samples with respect to the photocatalytic treatment time Humic acid mgIL 5 10 20 50

TOC' mwL 3.41 5.74 9.73 240

THMFP, THMi 34.3 57.8 98.0 241

30mm 23.5 45.7 75.8 124

~gIL

60mm 165 29.2 57.4 94.2

120 min 138 16.4 32.9 14.5

The amount of THM formed was calculated by using the equation developed by Tepeler and BekbOlet, 1994 [THM] =2.05 * 10- 3 [TOC] (pH - 2.8) to.22 Simulated distribution system trihalomethane concentration is the concentration of THMs in a previously chlorinated water sample after storage that represents the time and conditions in the utility's distribution system. There the above given equation is applicable to evaluate the THM concentrations of the

194

M. BEKB6LET and O. 6ZK6sEMEN

photocatalytically treated humic acid solutions. The period of reaction was taken as 24 h, resembling the residence time of water in the distribution system to express the yield at the "consumer's tap".The TOC contents of the irradiated humic acid samples after 30 min were taken as the first indication of THM formation. THM concentrations decreased with irradiation time (Table 4). Table 4. THM values of the humic acid samples with respect to the photocatalytic treatment time

Humic acid

mg/L

TOCj mg/L

5 10 20 50

3.41 5.74 9.73 24.0

ruM THM· 63.6 107 182 448

30 min 43.6 84.7 141 231

~g/L

60 min 30.7 54.2 107 175

120 min 25 .7 30.5 61.0 26.9

CONCLUSION The above explained results indicate that the photocatalytic oxidation process using semiconductor metal oxide, Ti02 might be a possible pretreatment method for the removal of humic acids as THM precursors in natural waters. The effective factors investigated were substrate concentration, photocatalyst loading and pH. The parameters used to follow the photocatalytic degradation were TOC, COD, Color4OQ ,UV 280 and UV254 . The decolorization was 99% and 88% degradation was attained both for COD and TOC. Increasing the photocatalyst concentration also increased the degradation rate. The optimum Ti0 2 concentration might be accepted as 1.0 mglmL since further increase in Ti0 2 didn't improve the degradation rate due to the opacity of the solutions. Acidic medium was found to be more favorable for the removal of humic acid. The evaluation of the THMFP of photocatalytically treated humic acid solutions revealed that the destructive removal humic acid was sufficient to keep the THM levels below the limit of 100 Ilg/L (US EPA). ACKNOWLEDGMENT The support provided by the Research Fund of Bogazi~i University (Project No. 95 Y 0043) is gratefully acknowledged. REFERENCES Arai,H., Arai,M., Sakumoto,A.{ 1986). Exhaustive degradation of humic acid in water by simultaneous application of radiation and ozone. Water Res. 20:7, 885-891. Backlund, P.{ 1992). Degradation of aquatic humic material by ultraviolet light. Ch~mosph~r~, 25: 12, 1869-1878. Bahnemann, D., Cunningham, J.,Fox, M.A., Pelizzetti, E., Pichat, P., Serpone, N. (1994). Photocatalytic treatment of water. In: Aquatic and Suifac~ Photochemistry. G . Helz, R.O. Zepp, D.G. Crosby, (Eds). pp. 261-316. Lewis Pub\. London. Ebenga, J.P., Imbenotte, M., Pommery, J., Catteau, J .P., Erb, F. (I986). Structure and evolution under olonation of a model humic acid. Water Res. 20. 1383-1392. Gilbert, E.{I988). Biodegradability of olonation products as a function of COD and DOC elimination of the example of humic acids. Water Res. 22: 1,123-126. Medley, D.R., Stover, A. (1983). Effects of Olone on the biodegradability of biorefractory pollutants. J. Wat. PolL Control Fed. 55:5, 489-494. Tepeler, B.M., BekbOlet, M. (I994). Tnhaloform formation in drinking water. Water Quality International '94. Abstract Book. p. 126. Budapest. Urano, K., Wada, H .. Takemasa, T. (1983). Empirical rate equation for trihalomethane formation with chlorination of humic substances in water. Watu Res. 17:12 1797-1902. USEPA,Environmental Protection Agengy (1979). Federal Register 44. No.23 I. 68624-68707. Zepp, R.O ., Wolfe, N.L., Baughman, O.L .. Hollins, R.C. (1977). Singlet oxygen in natural waters. Natur~ 267, 421.