photodegradation of surfactants. part xii: photocatalyzed mineralization of phosphorus-containing...

JOURNAL OF MOLECULAR

CATALYSIS Journal of Molecular Catalysis 88 ( 1994) 239-248

Photodegradation of surfactants. Part XII: Photocatalyzed mineralization of phosphorus- containing surfactants at Ti02/H20 interfaces

Hisao Hidaka”, Jincai Zhao, Yasuhito Satoh, Kayo Nohara Department of Chemistry, Meisei University. 2-l-l Hodokubo, Hino, Tokyo 191, Japan

Ezio Pelizzetti Dipartimento di Chimica Analitica, Universita di Torino, Via Pietro Giuria 5, Torino 10125, Italy

Nick Serpone Department of Chemistry and Biochemistry, Concordia University Montreal H3G IM8, Canada

(Received June 30, 1993; accepted October 1, 1993)

Abstract

Phosphorus-containing surfactants of dodecylpoly( oxyetbylene) phosphates and poly- (dodecyldecaoxyethylene)phosphates have been oxidatively mineralized by irradiated TiOZ semiconductor particles. The disappearance of surface activity for the degraded surfactant solutions and the mineralization of surfactants to CO, and PO:- have been observed. In addition to the peroxide and aldehyde intermediates formed, formic acid is observed during the photodegradation process in this work. The possible mineralization mechanism is discussed.

Key words: mineralization; photodegradation; surfactants; titania/water interfaces

1. Introduction

Various surfactants are widely used in households, industrially and in other fields. The amount of liquid and concentrated detergents produced in 1991 was 3.4 X lo6 ton in the USA, 3.9 X lo6 ton in Western Europe, and 0.6 X lo6 ton in Japan [ 11. The production will continue to increase in the near future. Unfortunately, some surfactants are not easily biodegraded by bacteria, and so accumulate and persist in nature for long periods and

*Corresponding author. Fax. ( +81-425)918181.

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240 H. Hidaka et al. /Journal of Molecular Catalysis 88 (1994) 239-248

become one of many direct causes of water pollution. Recently, it was demonstrated that the TiO, photocatalyzed degradation process is one of the promising methods to solve the problems in aquatic pollution. Numerous studies on the photocatalytic oxidation of numerous organic and inorganic pollutants using TiOz have appeared [ 2-91. We have reported extensively on the photodegradation of many types of surfactants catalyzed by a TiOz semiconductor in the past few years [ 10-211. In the photocatalytic process, the adsorption of surfactants on the TiOz surface, the generation of ‘OH radicals at the Ti02/HZ0 interface, the mineralization of the surfactants to CO*, and the intermediate formation of peroxides and aldehydes have all been examined in previous studies, which have been recently reviewed [ 221.

The hydrocarbon groups including alkyl, ethoxyl and aromatic moieties are mineralized to COZ [21,23]. Halogenated compounds are photocleaved to halogen ions [ 3,9,24]. A sulfur-containing group is photo-oxidized to SOi- ions [ 10,251 and a nitrogen-containing groups is photodegraded to NH,‘, NO; and NO; ions 126,271. Reports on the photodegradation of phosphorus-containing compounds have been scarce [ 2528-301. The photooxidation of phosphorus-containing surfactants at TiO,/H,O interfaces has not been examined to date.

In this paper, we report the photooxidative degradation of phosphorus-containing surfactants of dodecylpoly(oxyethylene) phosphates and poly( dodecyldecaoxyethylene) phosphates catalyzed by TiO, semiconductor particles. The disappearance of the surface activity and the mineralization of the surfactants to CO2 and PO:- ions were examined. Moreover, the formation of intermediates, carboxylic acids, are also observed as part of our continuing studies on the photocatalytic degradation process of surfactants.

2. Experimental details

Phosphorus-containing surfactants, dodecylpoly( oxyethylene) phosphates (C,,H&)- (CH,CH,O),-P=O(OH),, n = 0, 2,4, 10) (abbreviated as Cr2EnP) and poly (dodecyldecaoxyethylene)phosphates { [C12H25-O-(CH2CH20) iO],-P=O ( OH)s_,, m = 1,2,3) (abbreviated as [C,,Er,],P), were provided by Toho Chem. Ind. Co. Ltd. For example, C,,E,P expresses dodecyloctaoxyethylene phosphate [ C,,H,,-0-( CH2CH20)8- P=O(OH),} and [C,2E,o]3P stands for tris(dodecyldecaoxyethylene)-phosphate { [ C,,H,,-0-( CH2CH20) rO] 3-P=O}. The TiO, powder employed was kindly supplied by Degussa AG (P-25, mostly anatase powder with a surface area of 55 m’/g). Deionized and doubly distilled water was used throughout.

A dispersion consisting of a surfactant solution (50 ml, 1 mM) and TiO, particles ( 100 mg) was contained in a 76 ml Pyrex glass vessel. After sonication, the suspended solution was irradiated with a mercury lamp (Toshiba SHL-100 UVQ) under magnetic stirring to ensure uniform mixing of the aqueous suspension. The experiments were performed in air- equilibrated suspensions unless the constituents in the gas phase were analyzed, where the vessel was closed with a rubber septum and was purged with pure oxygen for 15 min before illumination. Subsequently, the TiO, particles were removed using a centrifuge and a Millipore filter (0.2 pm). The surface tension was measured at ambient temperature by the De Nouy method. The PO:- ions were determined by ion chromatographic techniques with

H. Hidaka et al. /Journal of Molecular Catalysis 88 (1994) 239-248 241

a I-524 anionic column using tartaric acid ( 1 mM) as the eluent. The formation of carboxylic acids was determined by HPLC methods using a RI detector. The temporal evolution of CO2 during the photodegradation was assayed by gas chromatography using TCD (thermal conductivity detector) detection. Photonic efficiency measurements were carried out in a 2 x 5 cm quartz cell with a surfactant solution ( 1 n&I, 30 ml) together with 30 mg of TiOz powder. Under magnetic stirring, the dispersion was illuminated by a Xe-lamp (USHIO XB-50101AA, 5OOW), equipped with a monochromator (JOBIN YVON H20 UV) to obtain monochromatic irradiated light. The incident light intensity for the wavelength of 334, 366 or 468 nm was measured using a ferrioxalate chemical actinometer [ 3 1 I.

3. Results and discussions

Fig. 1 shows the variations in the surface tension of Cr2EZP solutions during the irradiation process in the presence of TiO, particles. The surface tensions at different initial concentrations increased rapidly with the irradiation time at the initial stage. The system at a low initial concentration (0.25 mM) lost the surface activity immediately and exhibited a definite value of the surface tension after irradiation for 3 h. However, the surface tensions for the solutions at higher initial concentrations (0.5 and 1 .O n&I) increased rapidly within the initial illumination stage, and then decreased again after about 1 h of irradiation, reached a minimum at about 2 h of irradiation and then increased with further irradiation time. This phenomenon was also found in the photodegradation of non-ionic p-nonyl-phenyl- poly( oxyethylene) ether (NPE) surfactants [ 131. We suggest that some intermediate compound having surface activity is produced during the photo-oxidation process and the intermediate also undergoes photodecomposition with further irradiation time. After irradiation for 6 h, the surface tensions for all these samples at different initial concentrations no longer changes and approached the value of water.

301 I I I I I

0 2 4 6 6 10

--@I

Fig. 1. Variations in surface tensions during the photocatalytic oxidation of Cj2E2P solutions (50 ml, 0.25, 0.5 and 1 .O mM) in the presence of TiOz catalyst ( 100 mg) .


The temporal course of the photodegradation of the Ci&P surfactant is shown in Fig. 2. The generation of inorganic PO:- ions was confirmed in the photooxidation of this phosphate surfactant. It should be noted that we reported the total amount of the inorganic phosphorus species in the form of PO:- ion in this paper, in fact, PO:-, HPOZ- ,

H,PO; and H3P04 species exist at their equilibrium concentrations in the degraded solution and the H,PO; ion is predominant since the TiO,/surfactant dispersion becomes acidic ( pH = 4-5) immediately upon illumination. Phosphate ions were produced rapidly in the initial stages of the photo-oxidation and reached a yield of 94% after a 3 h irradiation period. The formation of intermediates, peroxides and aldehydes, in the photodegradation of surfactants has been reported [ 18,191. In this time, the formation of formic acid as an intermediate was also observed in the photomineralization of C,,EPP. The quantity of formic acid formed increased rapidly with irradiation time in the initial stage and then remained unchanged after 5 h of irradiation. Formic acid predominated amongst the possible carboxylic acids. In addition to the intermediates of peroxides and aldehydes, the formation of formic acid in the photodegradation of polyoxyethylene alkyl ethers (C,E,) was also evidenced by NMR measurements [ 19,271, in which the peak of formic acid (8.03 ppm) appeared in the photodegradation process. Mineralization of Ci2E2P to carbon dioxide was relatively slow, compared to the formation of inorganic phosphate and formic acid. After about 20 h of irradiation, the mineralization yield to COZ was about 43%. Further illumination led to more evolution of carbon dioxide.

To the extent that the TiOJsurfactant dispersion becomes acidic immediately under illumination, the c-potential of TiO, particles shifts to more positive values as described previously [ 15,211. Adsorption of anionic CL2E2P molecules onto the TiOZ particles is expected to occur via the negative hydrophilic phosphate group. The catalyzed photodegradation process involves attack of ‘OH (and/or ‘OOH) radicals, generated on the irradiated TiO, surface, on the surfactant molecules as evidenced by spin-trapping ESR data and by peroxide measurements [ 17,181.

hsldation the (h)

Fig. 2. Photodegradation of C&P ( 1 mM; 50 ml) in the presence of TiOl catalyst ( 100 mg)


TiO,z h++e-

h+ +H,O+‘OH+H+

(1)

(2a)

or

h+ +OH- -‘OH (2b)

‘OH + Organics + Products (3)

e- +O,+‘O; s ‘OOH (4)

‘OOH + Organics + Peroxides (5)

The lifetime of the ‘OH radical (or its equivalents) is relatively short (a few microseconds) and therefore is unlikely to migrate far from the TiO, surface; the photodegradation process takes place at the surface or within a few monolayers [ 18,32,33]. In this light, the active oxygen species will attack more easily the moiety of the surfactant molecule closer to the TiOz surface. The a-position to the phosphorus moiety (or -C-OPO:- bond) being nearer the surface of the TiOz particle is expected to be more easily attacked by ‘OH (and/or ‘OOH) radicals. Consequently, PO:- ions formed rapidly in the photodegradation of CL2E2P. Owing to the formation of several other intermediate products (peroxides, aldehydes, carboxylic acids, etc.), the mineralization of C2E2P to carbon dioxide was relatively

slow. The photonic efficiency at different wavelengths for PO:- ion formation during photo-

degradation ( 1 h) of C,,E,P is summarized in Table 1. We could not establish the actual quantum yield owing to light scatter and reflection by TiO, particles, and the complications of the photodegradation process. Therefore, the photonic efficiency (a) [ 341, defined as the number of reactant molecules transformed or product molecules formed divided by the number of incident photons, is reported in this work. The irradiation at h = 334 nm exhibited a photonic efficiency of 0.24, while the u value for illumination by 468 nm light was very small. The photooxidation process involves the generation of electron/hole pairs (see Eq. ( 1) ) , after the TiO, particles have absorbed the photons with an energy above the bandgap of 3.2 eV (about 387 nm) . Consequently, irradiation at wavelengths with an energy below the bandgap (e.g. 468 nm) would be expected and found to be inefficient for the photodegradation of C,,&P.

Formation of PO:- ions in the photodegradation of Ci2E2P is illustrated in Fig. 3. The phosphorus-containing surfactant C,,E2P was converted stoichiometrically to PO:- ions for three differently initial concentrations. At the lower initial concentrations (0.5 and 1 mM) , PO:- ions were easily generated and the mineralization yield reached above 90%

Table 1

Photonic efficiency (g)’

A(m) 334 366 468

(T 0.24 0.11 0.01

“Calculated according to PO:- ion formation.


Fig. 3. Concentration dependence of PO:- ion formation in the photodegradation of C12E2P (50 ml) in the

presence of TiOz particles ( 100 mg) .

8 t (a)

Ci2Ed

--o-en cl . 2k2P

bra&&n the (h)

Fig. 4. Formation of formic acid intermediate in the photooxidation of the used surfactants ( 1 mh% 50 ml)

catalyzed by TiO, particles ( 100 mg).

after 3 h of irradiation. However, a slower conversion rate was observed for a higher initial concentration (2 n&I). The complete transformation to PO:- ions for the solution at an initial concentration of 2 mM required more than 20 h of irradiation. This notwithstanding, the conversion of phosphorus-containing surfactants to PO:- ions is relatively rapid, com-


pared with mineralization of hydrocarbon moieties as reflected by the slow CO2 evolution (see Fig. 6).

Fig. 4 depicts the formation of the formic acid intermediate in the photodegradation of the phosphorus-containing surfactants used. Ci2EnP surfactants having different ethoxyl groups (Fig. 4a) showed nearly identical rates of formic acid formation for Ci2E2P, C,,E,P and Cr2EIOP in the initial stages. However, CIZEIOP with a longer ethoxyl group exhibited a larger amount of formic acid for long irradiation periods. The quantity of formic acid formed in the photooxidation process followed the order: C,&P > Ci2E4P > &E,P > C,&,,P. A maximum formation of formic acid appeared in the photodegradation of Ci2EnP surfactants. The maximum concentration and the required time to achieve the maximum formation of formic acid increased with increasing the number of oxyethylene groups in the C,,E,P structure. Formic acid was scarcely formed in the photodegradation of C&,P. It seems that formic acid is produced easily from the surfactants with an ethoxyl group, compared to those having only a long alkyl chain. Concerning ( Cr2E,,),P surfactants (Fig. 4b), the amount of formic acid intermediate formed increased with an increase in the number of dodecyldecaoxyethylene chains:

(C&K,)~P> (C,&&P>Ci2Ei,,P. Evolution of CO2 as an ultimate mineralization product is shown in Fig. 5. C,*E,,P without

a polyoxyethylene group exhibited a lower rate of CO, evolution, while the greater amount of CO2 was evolved from the surfactants containing a polyoxyethylene moiety (C,,E,P and

Fig. 5. CO2 evolution during the photodegradation of the used surfactants ( 1 mM) in the presence of Ti02 particles

(2 g/l).


tradation tine (h)

Fig. 6. Mineralization yield to CO* in the photodegradation of &E,P at different initial concentra’tions in the

presence of TiO, particles (2 g/l).

CIZEI,,P) (Fig. 4a). The CO;! evolution rate from Ci2E1J’ having a longer polyoxyethylene group was less than that from CL2E4P. After prolonged irradiation (above 50 h), the surfactant having the longer polyoxyetbylene group produced the greater amount of CO* in the following order: Ci2EiOP> C,,E,P> Ci2E$. On the other hand, the surfactants (C,,E,,) 3P, (Ci,E,,J 2P and CIZEl$ exhibited the same tendency of CO2 evolution within 20 h of irradiation (Fig. 5b).

The CO* evolution from the photodegradation of C i2E2P at different initial concentrations is illustrated in Fig. 6. The COZ evolution increased with increasing the initial concentration. The reaction rate should be proportional to the photonic flux and to the coverage of the TiOZ surface by the surfactant molecules. With a constant photonic flux (due to the lamp), the number of molecules converted per time unit increases with the concentration. However, the mineralization yield decreased with increasing the initial concentration since the relative collision opportunity between one surfactant molecule and the photocatalytic sites of TiOz particles decreases with an increase in the initial concentration. At a low initial concentration (0.1 r&I), Ci2EZP was quickly mineralized to COZ and the mineralization yield reached about 80% after irradiation for 8 h. After 20 h of irradiation, tbe yields of Ci2E2P mineralization to COP were 80, 73,42, 20 and 15% for initial concentrations of 0.1, 0.5, 1 .O, 2.0 and 3.0 mM, respectively.

4. Conclusions

Surfactants containing the phosphorus heteroatom, C,,E,P and ( CL2E10)mP, can be easily photodegraded catalytically by irradiated TiOz semiconductor particles. Inorganic PO:- species are generated during the photodegradation. Peroxides, aldehydes, and carboxylic


acid derivatives (mainly formic acid) are formed during the course of surfactant degrada-

tion.

5. Acknowledgments

We thank the Cosmetology Research Foundation for financial support, Toho Chem. Ind. Co. Ltd. for the gift of phosphate surfactants, and Degussa AG for the gift of TiO, catalyst. The authors also thank Mr. H. Nagaoka and Mr. Sakashita for their technical assistance. J.Z. is grateful to the Japan Science Society Foundation.

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photodegradation of surfactants. part xii: photocatalyzed mineralization of phosphorus-containing...

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