Environ. Sci. Technol. 1991, 25, 494-500
Photolysis of Chloroform and Other Organic Molecules in Aqueous TiO, Suspensions C. Kormann, D. W. Bahnemann,+ and M. R. Hoffmann" W. M. Keck Laboratories, California Institute of Technology, Pasadena. California 91 125 ~
~~~
The photocatalytic degradation of chloroform has been investigated in aqueous suspensions of TiOz over the wavelength range of 310-380 nm. A detailed reaction mechanism has been proposed in which the rate-determining step is the reaction of surface-bound *OH with adsorbed CHCl,. A pH-stat titration technique was developed for the measurement of the rates of degradation of chlorinated hydrocarbons. The quantum efficiency (a = 0.56 a t X = 330 nm) of the degradation of CHCl, was found to be inversely proportional to the square root of the incident light intensity. This relationship can be explained in terms of a direct competition between a second-order recombination of surface-bound 'OH and the rate-determining reaction of surface-bound 'OH with CHCl,. The rates of degradation of several electron donors have been correlated with their computed surface speciation. The results of this study show that the adsorption of electron donors and acceptors to the TiOz surface plays a more important role in determining the rate of the photocatalytic reactions than the effect of pH-dependent Fermi-level shifts. Introduction
The elimination of toxic chemicals such as halogenated aliphatic and aromatic hydrocarbons from wastewater, soil leachates, and groundwater is presently of great concern. In recent years attention has been directed toward the photocatalytic degradation of organic and inorganic molecules in heterogeneous suspensions of metal oxide semiconductors such as TiOs and ZnO (1-6). When these semiconductors are illuminated with light of wavelength X 5 380 nm, excited-state electron and hole pairs are produced, which are capable of initiating a wide variety of chemical reactions. Pruden and Ollis (2) have shown that chloroform is degraded readily in illuminated suspensions of TiOz according to the following stoichiometry: 2CHCl3 + 2H20
hu + 0 2 T~O, 2C02 + 6HC1
(1)
In the present work we examine this photochemical reaction in greater detail. The rate of photodegradation of chloroform and several other organic molecules is studied as function of [CHCl,], pH, [O,], light intensity, and the presence of charged and neutral solutes. The effect of inhibitors on the rate of chloroform degradation is also studied, and the rate of several photocatalytic reactions is correlated with the calculated surface speciation as determined by a surface chemistry code, SURFEQL (7,8). Experimental Procedures
Chemicals and solvents were of reagent grade and used without further purification. The water employed in all preparations was purified by a Milli-Q/RO system (Millipore) resulting in a resistivity of p > 18 MR cm. Chloroform (Baker Analyzed, Photorex grade) was extracted with a 10-fold excess of water a t least three times before use. A saturated CHC1, solution ( [CHC131aq= 62 mM) was prepared by stirring an excess of chloroform in water at
room temperature. Saturation was achieved in >
~ r ~ ' ~ ~ s u , f 1 ~ ~ ~ ~hz['OH,urflZ ~ 3 s u r f l
(17) and see that approaches unity. In the second case, under conditions of high light intensity we can neglect the hr['OHsur~l [CHC13,,,f] term in eq 15 and obtain (18) I, - k2['OH,u,f]2 = 0 Thus, under these conditions the concentration of 'OHsurf is controlled by its second-order recombination to give [.OHsurfl = (Ia/h2)1/2 (19) In addition we see that decreases with increasing light intensity as follows:
(12)
'OHsurf
In turn, the surface-bound hydroxyl radicals ('OHsUrf)are the actual oxidizing agents for chloroform. 'OH radical attack is initiated via the abstraction of a hydrogen atom from the surface-bound chloroform as follows: *OHsurf + CHC13surf
bination as a loss pathway for *OHsurf in illuminated T i 0 2 suspensions. Assuming a photostationary state for *OHsurf,we can write the following steady-state equation:
2'OHsurf H202surf (14) Salvador and Decker (16) have also proposed this recom-
From a single-wavelength illumination experiment a t 330 nm we obtained ar = 0.56 (I, = 4.64 X lo-* einstein L-'s-l and [CHCl,], = 62 pM). Using eq 10 and the associated of 2 X M-1/2 constants we obtain a value of hr/(hz)1~2 From this ratio we see that h, is small compared to h2. With the experimental results presented above we have shown that a variety of solutes affect the rate of chloroform degradation depending on the specific nature of the solute and the pH of the suspension. Simple anions such as C1and HC0,- decrease the photoefficiency of the catalyst at low pH where they can be adsorbed to the positively charged T i 0 2 particle surface (pH,,, = 6.25 for P25 Ti02). However, the same anions have a negligible affect on the measured photoefficiency at high pH since their adsorption to reactive surface sites is prevented by electrostatic repulsion from the negatively charged TiOz particles. The adsorption of the negatively charged anions is stronger the higher the positive surface charge. Thus, we saw a significant reduction in reaction rate as the pH was lowered from 4 to 3 in the presence of C1-. Environ. Sci. Technol., Vol. 25,
No. 3, 1991 497
Inhibition of CHC13 photooxidation by cations, anions, and neutral molecules results from the competition for reactive surface sites and from the competition for reactive intermediates such as 'OHsu,+ While some molecules such as C1-, HC03-, and C2H50Hreact readily with 'OH, other molecules such as F- and Clod-are essentially unreactive and thus have no effect on the net oxidation rate of CHC1,. We can summarize the effects of competitive solutes (i.e., quenchers) on the oxidation of CHC13 by considering a mechanism involving eqs 13 and 2 1
k,
'OHsurf+ QSurfi products (21) The reaction of 'OHsurfwith surface-adsorbed quenchers (QsUrfi) is an inhibition process with a characteristic rate constant of kq. The reaction of eq 21 effectively competes for 'OHsurfand reduces its relative rate with CHC13,ur,. Thus, higher concentrations of quencher molecules present a t the particle surface during illumination will result in more extensive inhibition of chloroform degradation. If we assume that the formation of 'OH,, radicals is the rate-determining step, we can represent mathematically the effect of the quencher molecules on the rate of CHC1, degradation for the photostationary state as follows: d['OHsurfl dt n
kr['OHsurfI [CHC13surfI + [*OHsurflCkq'[Qsurf'I = 0 (22)
0 0
2
6
k and v O ~ ' - are ~ constants for any given experiment where the chloroform concentration and the light intensity are held constant. In order to obtain the surface concentration [QBUrf] of charged quenchers, the electrostatic interaction between the charged surface and the aqueous-phase quenchers has to be calculated. The nature of the metal oxide surface in aqueous solution has been treated in detail previously (17). Surface groups of a metal oxide are amphoteric and the surface acid-base equilibria can be written
%
1=1
From eq 13 the overall rate of chloroform degradation is given by
+ H+
m-TiOH
m-TiOH ---EkqilQsuril
1 0 12 14
Flgure 7. Surface speciation of TiO, particles as function of pH. Calculated with the following primary data: P K , , ~= 2.4, pK,,S = 8.0, p = 0.01 M, surface exchange capacity 0.46 mol g-', surface area 50 m2 g-', and solids concentration 0.5 g L-'. ( 0 )D-TiOH; (0)DTiOH,'; (A) W-TiO-.
B-TiOH,'
kr[CHCl,surfI +
8 PI1
i=l
where
4
m-TiO-
+ H+
d[CHCl,] dt
(-
-'
)
=
We can simplify this latter equation to give
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Environ. Sci. Technol., Vol. 25, No. 3, 1991
(31)
where c T i O H represents the "titanol" surface group. Ka:, Ka;, and the pH,,, (pH,,, = (pK,: + pKa;)/2 = 6.25) may be determined by titration and other experimental techniques (17, 18). The microscopic (Le., apparent) acidity constants in these equations are related with the intrinsic acidity constants via
Ka: = intKai, s e F h + I R T
Taking the double reciprocal of eq 26 we obtain
(30)
(32)
where intKais is the intrinsic acidity constant (i.e., the acidity constant in a hypothetically chargeless environment), +H+ is the effective potential difference between the binding site and the bulk of the solution for H+, and F , R , and T have their usual definitions. The relationship between the surface charge, uo, and the surface potential in a diffuse layer model has been derived previously (19). We utilized the computer code SURFEQL (7) to perform surface equilibrium calculations in an attempt to model the surface chemistry of charged substrates relevant to the conditions of this study. SURFEQL has been used successfully to calculate equilibrium distributions of chemical species in a number of cases (20,21). The set of experimental conditions and surface acidity constants used for the simulations for TiOz surface chemistry are as follows: pKalS= 2.4 (22);PK,,~= 8.0 (22);ionic strength, = 0.01 M; surface exchange capacity = 0.46 mol g-' (23);surface area = 50 m2 8-l (24);solids concentration of 0.5 g L-'. Results of these computations are illustrated in Figure 7, which shows the surface speciation obtained in the absence of surface complexation by ligands (i.e., other than water) present in solution. As illustrated in Figure 7, the neutral surface species D-TiOH is predominent over a broad range of pH. A t pH
