The aqueous photochemistry of phenols

Tomsk, 634050, Russia (e-mail: tchon@phys.tsu.ru)

Natural water pollution by organic materials, among of that phenol and its derivatives are the most harmful contamination, increases due to industrial activities and accidents. Resulting from more regulation concerning natural, surface and wastewater, improvements in the existing methods of water treatment and fluorescent analysis, as well as research into new methods are required. In particular, some new techniques have been proposed that utilize UV light of excimer laser. The present study was undertaken in the framework of research needed to increase our knowledge of photolysis mechanism of phenols in water. Phenol constitutes one of the main pollutants to be removed from wastewater. It is a very simple organic compounds, easily soluble in water at different condition of acidity.

This work present the results of the photolysis of phenol in aqueous solution after high-pressure Hg lamps. Also in the given paper the phototransformation of phenols (phenol, 4Clphenol, o-cresol, p-cresol) in solutions at various pH (water with addition KOH and H₂SO₄) is studied. Spectral properties of phenol in water were studied by means of spectrophotometer “Specord M40” for absorption spectra monitoring, and “Hitachi 850” spectrofluorimeter to fix fluorescence spectra. pH medium was measured with pH-meter of 673 model.

Acidity and basicity constant of phenol (pK_a) for the ground S₀ and first excited singlet S₁* states was obtained by means of the known methods [5] using experimental curves of titration. Acidity constant of phenol (pK_a*) for the first excited Frank-Condon state (S₁*^F-C) was calculated via a Forster cycle [1,2]:

pK_a = pK_a* + 2.1× 10^-3 D n ,

where pK_a is the value of acidity for S₀;

pK_a* is the value of acidity for S₁*^F-C;

D n is the difference between the maxima of absorption bands of acid and base forms.

The spectral-luminescent properties of phenol depend on the excitation wavelength and pH We have been studied in [3], it is in agreement with the previous reviews [4,5]. The experiments should contribute to a better knowledge of ionic phenol forms participation in phenols phototransformation processes in the liquid phase.

The value pK_a of phenol decreases from 9.6 in the S₀ state to 5 in the S₁*^F-C state (see Table 2). Thus phenol proton in the S₁*^F-C state of the molecule can be transferred to proton acceptors of the solution. At decay from S₁*^F-C state to equilibrium S₁* state according to Frank-Condon law the phenol molecule relaxation ability approaches to that in ground equilibrium state.

The value of basicity pK_a inferred from changes of the phenol spectral luminescence properties in a slightly acid solution increases at excitation. Hence, the production probability of the form with proton becomes higher (see Table 2). In the S₁*^F-C state this probability is higher than in the equilibrium S₁* state and, more over, in the S₀ state. The excited molecule can exist with equal probability in neutral, cationic and anionic forms in the pH range of 4¸ 9.

Table 1. The spectral luminescence properties of phenols ionic forms (concentration of phenols is 5´ 10^-5 mol L^-1).

		absorption		fluorescence
solution	ðÍ	, nm	, nm	,nm
phenol
water	6.3	210	270	296
water + 10-1mol L-1 ÊÎÍ	12.5	230	290	345
water + 20% Í2SO4	-0.5	240	300	415
4Clphenol
water	6.5	220	280	310
water + 10-1mol L-1 ÊÎÍ	13	240	300	360
water + 20% Í2SO4	-0.9	230	285	415
o-cresol
water	6	210	270	300
water + 5× 10-3mol L-1 ÊÎÍ	10	235	290	340
water + 40% Í2SO4	-1	240	310	400
p-cresol
water	6	220	280	308
water + 5× 10-3mol L-1 ÊÎÍ	10	240	300	350
water + 40% Í2SO4	-1	255	300	405

Table 2. The value of basicity and acidity pK_a of phenols

Our previous experiments [3] have show that the products of phenol phototransformations are quite different at various pH values. Ionic phenol forms were studied [6]. Spectral-luminescence properties of ionic forms of phenols are given in Table I. These ionic forms can participate in phototransformations as intermediate photoproducts. The data on phenol transformations with varying pH levels under identical irradiation conditions (a mercury lamp) have shown that the photoproduction rate in absorption bands around 41000 and 34500 cm^-1 is higher for the anionic phenol forms than for the cationic forms with pH values for the anionic solution decreasing from 10.6 down to 8 (see Table 3). The effect of the phototransformation is to increase the absorption intensity of the anionic phenol form around 240 nm. This is due to photoproduct formation. In the fluorescence spectrum there is a band of low-intensity around 415 nm which occurs both under anionic and cationic irradiation. This fluorescence band can be emitted by cationic phenol form or products of second photodecay. Analysis of the phototransformations of n-benzoquinone studied in [7] suggests that the absorption around 41000 cm^-1 is due to the photoproduction. We have observed more pronounced changes (shift and intensity raise of the absorption band) in the phenol absorption spectra under the UV irradiation from mercury lamps around 45500 cm^-1, while the absorption intensity around 37000 cm^-1 is hardly affected by the irradiation [3]. The phenol absorption around 45500 cm^-1 corresponds to the S_0® S₂ transition [8, 9]. The nature of this absorption band is determined exclusively by the phenyl fragment of the phenol molecule. An increase in the oscillator strength for the S_0® S₂ transition (i.e., the increased absorption intensity observed experimentally by) corresponds to the case where the oxygen-hydrogen bond stretching in the OH group leads to the proton detachment, resulting in a high production probability of the anionic phenol form under irradiation. It was found that the fluorescence band around 345 nm corresponded to the anionic phenol form (see Table I). Hence it follows that the appearance of this band in the fluorescence spectrum, as the exposure time (t_exp) is increased, is evidence of the production of the anionic phenol form. The data on the phenol photolysis in water are given in Table 3. The fluorescence intensity in the band with the maximum around 345 nm is observed to increase as t_exp is increased. The fluorescence is attributed to a photolysis product. At t_exp=60¸ 120 s the fluorescence intensity in the band peaks. On further increase in t_exp the intensity tends to decrease, which results from the photodecay of the photolysis product.

Simultaneously with this process a fluorescence band with the maximum around 415 nm followed by a fluorescence band with the maximum around 440-460 nm is observed (see Table 3). The fluorescence around 415 nm corresponds to the fluorescence band of the cationic phenol form (see Table 1). The fluorescence of the photoproduct around 440 nm is also observed on exposure to unfiltered light from a mercury lamp with t_exp= 6 hrs. On addition of H₂O₂ to the phenol solution, the yield of the product is found to increase at t_{exp< <} 40 min and to decrease at t_exp> 40 min.

It is believed that the fluorescence around 440 nm is due to the production and photodecay of hydroquinone on exposure to the UV radiation. The experiments on the hydroquinone photolysis showed the fluorescence intensity to decrease in the fundamental band around 330 nm, as t_exp was increased. It was also found experimentally that a low intensity band appeared around 440 nm with increase in the exposure time. Under normal conditions (with no irradiation) the neutral hydroquinone solution does not absorb light around 330 nm, while hydroquinone does fluoresce in this band on addition of 5´ 10^-1 mol L^-1 KOH (pH=11.6). This corresponds to the product of the anionic hydroquinone form. It should be noted that the fluorescence of the anionic hydroquinone form, like that of the anionic phenol form, is of low intensity in this band. The fluorescence band at 440 nm, which appears at higher duration of phenol irradiation, coincides to the fluorescence band of the hydroquinone with KOH additions. So this fluorescence band can be emitted by the hydroquinone anion, which can be produced by phenol photodecay to the hydroquinone with proton detachment from the last one. The proton attachment and detachment at phototransformations causes the changes in the pH values (see Table 3). On irradiation of the hydroquinone solution with addition of 20 % H₂SO₄ (pH=-0.5) the absorption is found to increase around 310 nm, and the fluorescence is observed to increase around 440 nm. To sum up, both ionic phenol forms and ionic hydroquinone forms are active contributors to the phenol phototransformations.

Table 3. Spectral-luminescence characteristics of phenol and phenol photoproducts in water exposed to UV mercury lamp.

¹	pÍ,	C	texp,	l abs nm	l fl nm	photoproducts
						fluorescence
						l 1, nm	l 2, nm	l 3, nm	l 4,l 5, nm
					l exc, nm
					270	290	290	333	500
1	6.75	5× 10-3	0 s	270	296
2	7.24	-//-	10 s	270	296	345
3	6.6	-//-	20 s	270	296	345
4	6.5	-//-	30 s	270	296	345
5	6.5	-//-	40 s	270	296	345
6	6.8	-//-	50 s	270	296	345
7	6.4	-//-	60 s	270	296	345
8	6.3	-//-	120s	270	296	345
9	6.3	-//-	3¸ 10 min	270	296	370	415
10	7.3	-//-	15 min	270	296	370	415	460
11	7	-//-	20 min	270	296	370	415	460
12	6.7	-//-	30 min	270	296	370	415	460
13	6.9	-//-	90 min	270	296	-	415	440
14	-	0.1	2hrs 10min	270	-	-	440
15		-//-	3hrs	270	-	-	440
16		-//-	6hrs	270			440

1. The phenol acidity and basicity increases under excitation. As a result, anion, cation and neutral phenol forms can exist in the S₁*^F-C state with equal probability. Phenol photolysis occurs by means of the anion form production in the pure water solution.

2. Parachlorophenol phototransformation strongly depends on the pH of media. Under UV irradiation the photodecay efficiency is higher at pH=10.5, as compared with this one at pH=1.8. Due to chlorine atom presence photodecay of parachlorophenol occurs by means of its ionic forms and the ionic forms of hydroquinone and phenol.

This authors thank Russian Foundation for Basic Research (grand 98-03-03059) and the Ministy of Education of Russia for financial support.

[1] Th. Forster, Z.Elektrochem., 54, pp.42-62, 1950.

[2] I.Y.Martunov, A.B.Demiyshkevich, B.M.Uzhinov et al., “Reactions of proton transfer in the excited electron state of aromatic molecules”, Izv.Akad. Nauk. Uspehi Khim., 46, pp.2-31, 1977.

[3] O.Tchaikovskaya, I.Sokolova, N.Sultimova, “Investigation of phenol phototransformation in aqueous solution by electronic spectroscopy and luminescence methods”, in Sixth International Symposium on Atmospheric and Ocean Optics, Gennadii G.Matvienco, Vladimir P.Lukin, Editors, Proceedings of SPIE Vol. 3983, page numbers 499-504 (1999).

[4] O. Legrini, E. Oliveros, and A.M. Braun, Chem. Rev.93, pp. 657-698, 1993.

[5] G. Kohler and N. Gettof, J. Chem. Soc., Faraday Trans 1., 72, pp. 2101-2107, 1976.

[6] O.Tchaikovskaya, R. Kuznetsova, I.Sokolova, N.Sultimova, Zhyrnal Fizich. Himii, 74, pp. 1812-1815, 2000.

[7] R.T. Kuznetsova, R.M. Fofonova, and V.I. Danilova, Zh. Prikladnoi Spektrosk., 30, pp. 1053-1058, 1979.

[8] G.V. Mayer, O.K.Bazyl, and V.Ya. Artyukhov, et al., Izv. Vyzssh Uchebn. Zaved. Fizika, 42, pp. 3-7, 1999.

[9] O.K.Bazyl, G.V. Mayer, and V.Ya. Artykhov, et al., High Energy Chemistry, 34, pp. 30-36, 2000.