Environ. Sci. Techno/. 1995, 29, 2065-2072
Evidence for a Surface Dual Hole- Radical Mechanism in the Ti02 Photocatalytic Oxidation of 2,4=DichlorophenoxyaceticAcid YUNFU SUN AND JOSEPH J. PIGNATELLO* Department of Soil and Water, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504
The photocatalytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in UV-illuminated aqueous Ti02 suspension was studied at pH 1-12. At pH -3, the initial step is chiefly direct hole (h+) oxidation, while below and (especially) above pH 3, it shifts progressively t o a hydroxyl radical (Horn)-likereaction following rate-limiting h+ oxidation of surface hydroxyls. The hole pathway gives products expected from one-electron oxidation of the carboxyl group-near stoichiometric yield of I4CO2 from [carboxy-14C]2,4-D and high yields of 2,4-dichlorophenol (DCP), 2,4dichlorophenol formate (DCPF), and formaldehyde (HCH0)-and is little affected by 0.1 M methanol or teabutanol. The weak competition by alcohols is proof of a surface reaction since the alcohols scavenge free HO'. The radical pathway results in low yields of 14C02,DCP, DCPF, and HCHO, indicating that attack shifts to the aromatic rings and is strongly inhibited by the alcohols. Solvent kinetic and product D isotope effects at pH 2 and pH 12 are consistent with the dual mechanism. Shift t o the radical mechanism at low pH may result from a lower oxidation potential and/or coordinating ability of the R-COzH, while the shift at high pH is due t o enhanced oxidation potential of the increasingly charged surface, charge repulsion of carboxylate, and/or OH- competition with carboxylate for coordination sites. At pH -3, judging from scavenger effects, the radical mechanism predominates for DCP transformation, whereas the hole mechanism predominates for carboxyl-bearing byproductsformed during late stages of mineralization.
Introduction Photocatalytic oxidation of organic compounds by semiconductor oxides such as Ti02has attracted attention for treating contaminated wastewater among other applications. In the presence of oxygen and near-W light, many organic compounds can be mineralized to carbon dioxide or transformed to less toxic compounds ( 1 ) . The mechanism of this reaction has been intensively studied, and a
0013-936x195/0929-2065$09.00/0
%,
1995 A m e r i c a n Chemical Society
number of reviews are available (2-5). It is well established that photoexcitation of the bulk semiconductor promotes valence band electrons (e-) into the conduction band leaving free electron holes (h+)(eq 1): TiO,
+ light - TiO, (h' + e-)
(1)
Dioxygen normally provides a sink for e-, and its reduced derivatives (HOa', H202, HO') may assist oxidation of the organic. But the primary initiator of oxidation is h+ that escapes recombinationand migrates to the particle surface. It is believed that h+ reacts with surface hydroxyl groups ([OH],- =dissociated (0-OH) or molecular (D QH2) water molecules and their ions) to form a "trapped hole" (O*) on the surface (eq 2) that subsequently oxidizes the organic (eq 3): h+
O*
+ OH,- - O*
+ RH, - R(oxidized)
(2)
(3)
The trapped hole O* is usually describedas an adsorbed hydroxyl radical (OH,'), although its exact nature may be more like I, in which the spin resonates or shifts between hydroxyl and lattice 0 (6):
O*
{Tirv-02--Tirv}-OH'c)(TirvV-u-Tirv}-OH-
I Controversy exists, however, over whether direct h+ oxidation of the organic plays a role (eq 4): h+
+ RH, - R(oxidized)
(4)
While most evidence favors the radical mechanism (eqs 2 and 31, there is suggestive but not compelling evidence in favor of a hole mechanism (7-13)-particularly for carboxylic acids. An assumption among those who favor the hole mechanism is that h+ acts as an electron-transfer oxidant, while O* behaves like free HO'in that it tends to abstract H or add to C-C multiple bonds. Accordingly, h+ oxidation usually has been rationalized on the basis of substrate reactivityand products (7-11).For example,hole oxidation has been invoked to explain the Ti02 photocatalyzed oxidation of oxalate and trichloroacetate ions, whichlackabstractablehydrogens or C-C unsaturation (12). Draper and Fox (131, moreover, observed transient oneelectron oxidized products of 2,4,5-trichlorophenol, N,N,W,N-tetramethyl-p-phenylenediamine, and thianthrene, among other compounds, on Ti02 using diffuse reflectance flash photolysis. Whether the reaction takes place on the surface or in solution has also been controversial. Solution-phase oxidation is possible if the surface generated species were to dissociate as free HO' or if photochemical reactions occurred in solution. Although surface oxidation is often taken for granted, the evidence for it is mixed (2-4). Conformityof rate data to adsorption-reaction (LangmuirHinshelwood) rate laws is inconclusive since similar rate laws describe the reaction of surface oxidantswith dissolved RH or of dissolved oxidants with RH in either phase (5). Discrepancies in the adsorptionconstant between dark and irradiated systemshave also been noted (14). The detection
VOL. 29, NO. 8, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12066
of spin-trapped HO' by ESR (15)is ambiguouswith respect to the origin of the spin. Examples of evidence supporting oxidation on the surface include: (a) the finding that free HO' reacts at diffusion-controlled velocity, and therefore apparently irreversibly, with the Ti02 surface (6')and (b) the observation of current doublingfor some compounds (i.e., two-electron donation to Ti021 UI), which requires that the first oxidation product remain on the surface. Behavior interpreted as consistent with solution-phase oxidation includes the photoelectrochemicalbehavior of Ti02 slurries (16) and the inhibition of furfuryl alcohol oxidation by 2-propanol (17). In this study, we examined the oxidation of 2,4dichlorophenoxyaceticacid (2,4-D)in UV-illuminated Ti02 (anatase) aqueous suspension. This compound is a commercially important herbicide and a component of stockpiled chlorophenoxy herbicide waste materials, such as Agent Orange. Conclusions about the mechanism have been forthcoming based on changes in behavior with pH and comparisons with solution-phase HO' systems.
0
10
20
30
40
50
min 0.100
B
-E
r c.
C
Experimental Section
I I
0.010
4 Materials. 2,4-D was purchased from Aldrich Chemical Y Co. The TiOaphotocatalyst (P25, anatase, average particle size 30 nm, BET surface area 50 m2/@was a gift from Degussa (Dublin, OH). [UL-ring-14Cl-and [c~rboxy-~~Cl2,4-D, 2,4-dichlorophenol (DCP), glyoxylic acid, and 2,40.000 dinitrophenylhydrazine(DNPH)were from Sigma. Others 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 were of the highest purity available. The formate ester of PH DCP (DCPF) was prepared from 1.5 M acetic formic FIGURE 1. (A) First-order plots of W D tranrformrtion in selected anhydride (18) and 1 M DCP in anhydrous diethyl ether at experiments. Ordinate is relative In units. Experiments at pH(D) 2 room temperature for 24 h. The reaction did not go to and pOH(D) 2 were dona at e light intensity of 4 x 1017 and 2.7 x completion, and the concentrationof DCPF was determined 10" quanta s-l L-l, respectively. (B) Dependence of the 24-D by difference from the initial concentration of DCP. transformetion rate constant on pH at a light intensity of 4 x lor7 This solution was used as a standard in experiments in quanta s-l 1-l. which DCPF was quantitated. of HzS04 and diluting gradually into 990 mL of water with Procedures and Analyses. Experiments were carried stirring. To 3 mL of this reagent was added 2 mL of the out in borosilicate vessels irradiated with fluorescent reaction mixture and 3 mL of 80:20 hexane:methylene blacklamps (output, 300-400 nm) at 9 x 10'' quanta s-l chloride, and the mixture was shaken for 30 min. The L-l (determinedby ferrioxalate actinometry) unless stated organic layer was then injected (20 pL) onto the ODs2 otherwise. All experiments employed 0.1 g of Ti02 suscolumn and eluted with 7525 acet0nitrile:water at 1 mL/ pendedin lOOmLofwaterationicstrength,0.2M (NaC104), min with monitoring at 355 nm. A standard was made by and 0.1 mMsubstrateunless specified. The pH was adjusted reacting authentic formaldehyde with DNP reagent. It was with €IC104 or NaOH subsequent to the addition of stock recrystalized from ethanol, dried under vacuum at 50 "C, aqueous solution of the substrate. The system was and dissolved in CH3CN. equilibratedwith stirring in the dark for at least 0.5 h before Glyoxalate was determined by LC on an Aminex HPXillumination. Aliquots of 1-5 mL were withdrawn for 87H ion moderated partition column (300 mm x 7.8 mm) analysis and filtered (0.2pm). Samples from experiments (Bio-RadLaboratories, Hercules, CA)at 65 "C using 0.9 mL/ in which solution radioactivity was determined were min 0.005% H2S04 with detection at 210 nm. DCP and acidified (if needed) to below pH 3 and sparged to remove DCPF were identified in methylene chloride extracts of the 14C02 prior to liquid scintillation counting in Opti-fluor reaction mixture by mass spectroscopy on a quadruple (PackardInstrumentCo.). Samples from reactions in which Hewlett-Packard 5988A spectrometer. DCPF was assayed were adjusted to pH 7 with phosphate and kept frozen to prevent hydrolysis. Results tert-Butanol was determined by gas chromatography The loss of 2,4-D obeyed first-order kinetics over at least using flame ionization detection. 2,4-D, DCP, and DCPF 2 half-lives, as illustrated by the examples in Figure 1A. The were determined by liquid chromatography (LC) on a 25rate constant (kobs) varies with pH in the range 1-12 with cm Spherisorb ODs2 C-18 reversed-phasecolumn (Phase a maximum at pH -3 (Figure 1B). The p& of 2,4-D is 2.9. Sep, Clwyd, U.K.) eluted with 1.5 mL/min 60:40 methanol The pH dependence of kobs is by itself not very useful in water containingabout 0.008% CFBCOZH to separate 2,4-D terms of a delineating mechanism. However, studies from an interfering peak. The effluent was monitored at described below on the identity and yield of certain 230 nm. products, the influence of scavengers, and solvent deuteFormaldehyde was determined as the 2,4-dinitrophenrium isotope effects (H20 vs D20) on products and rates ylhydrazone by a slightly modified method (19). A 1.7 mM indicate that a change in mechanism takes place over this DNPH reagent was prepared by dissolving DNPH in 10 mL pH range. 2006
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 8, 1995
~
:ko.oI
0
.
' I
"
"
.
I
.
'
2 3 4 2,4D First-order half-lives
FIGURE2. Molar ratio of1%Oz releasedto [car60xp~~C]-2,4-Dreacted
( f i during the transformation of 2,4-D.
Products. Figure 2 shows in selected experiments the relative yield (Y)from [c~rboxy-'~C1-2,4-D over the course of the reaction-from less than one-half to more than 4 2,4-Dreaction half-lives. Loss of solution radioactivitywas used as a surrogate for 14C02evolution. At low pH, Yranged from -0.8 to 1.0, meaning that 14C02is released nearly as rapidly as 2,4-Dis reacted. At high pH, by contrast, the low Yof -0.2 at early times indicates that 14C02evolution lags behind 2,4-D transformation. The gradual rise in Yto -0.5 at later times is due to mineralization of intermediates produced in later steps that contain the labeled carboxyl group. Figure 3 shows the yield of 14C02at relatively early times (arbitrarilytaken to be 1 half-life, Y1/2) as a function of pH. The Y1/, decreases on either side of pH 2-3, especially on the alkaline side. These results suggest a progressive shift from a decarboxylation reaction as the initial step at pH 2-3 to some other reaction on either side of pH 2-3. The observed organic products are also consistent with a change in mechanism. The major products at pH 3 are HCHO, DCP, and DCPF (Figure 4A). Glyoxylic acid is a minor product. DCP and HCHO are produced in almost equimolar yields. The combined yields of DCP and DCPF at their maximum ( t= -45 min) account for most (-80%) ofthe reacted 2,4-D. The high yields of carboxy-CO,, HCHO, DCP, and DCPF indicate decarboxylation as the predominant initial step (eq 5): ArOCH,CO,-(H)
101
ArOCH,CO,'
-
ArOCH,' A
+ CO, (5)
In the presence of air, the aryloxymethylene radical A is expected to rapidly form a dioxygen adduct, which then may hydrolyze to DCP and HCHO in equal yield (eq 6). Generation of DCPF (eq 7) can be envisioned by coupling of the dioxygen adduct of A with HOz' (possibly on the surface),followed by decomposition via the Russell mechanism (12): 0 2
A ---c ArOCH,-OO' ArOCH2-OO'
+ HO,'
ArOH
+ HCHO + HO,'
-ArOCH,-OOOOH ArOCH-0 0,
+ + H,O
(6)
(7)
The minor product glyoxylicacid most likely results from oxidant attack on the methylene hydrogens followed by oxygenation and hydrolysis:
FlGURE3. Molar ratio of "CO, released toIcenb0xy-~Fl-2,4-Dreacted at one 2,4-0 half-life ( f i n ) as a function of pH(D) or pOH(D). Error bars represent 2 SD.
ArOCH2C02-(H)
[OI
-
ArOCH'-CO,-(H)
0 2
HZ0
ArOCH(OO')CO,-(H) ArOH CHOCO,-(H)
+
+ HO,'
(8)
The organic products observed at high pH (12) include HCHO, DCP, and glyoxylic acid (Figure 4B). The yield of DCP may represent the combined yields of DCP and DCPF since any DCPF formed would be rapidly hydrolyzed to DCP in strong alkali. Comparison with Figure 4A reveals that DCP DCPF and HCHO are formed in a much lower transient yield at high pH than at low pH. The decreased yield of these compounds and of carboxyl COzcan be taken as evidence that the point of attack shifts away from the carboxylgroup and toward other positions in the molecule. Since glyoxylic acid is a minor product at both low and high pH, it follows that attack shifts largely to the aromatic ring rather than to the methylene hydrogens. Solvent Deuterium Isotope Effects. Figures 1-3 and Table 1 compare reactions in H20 and D20. The kinetic solvent isotope effect (KSIE) is defined as the raio of firstorder rate constants for 2,4-D transformation in the two solvents ( k ~ l k ~The ) . product solvent isotope effect (PSIE) is defined with respect to COZevolution and is reported in Table 1 as the YlI2value in each solvent. A "positive" PSIE will hereafter refer to increased decarboxylation in D20. Under acidic conditions [pH(D) = 21, the KSIE is significant ( k d k D = 2.1 f 0.3) while the PSIE is almost negligible [Yl/2(H20)= 0.88 f0.03; Y1/2(D20)= 0.92 & 0.031. Under alkaline conditions [pOH(D) = 21, the KSIE is negligible ( k ~ l k=~ 1.0 ) f O,l), and the PSIE is slightly positive [Y1/,(D20)= 0.28 k 0.03; &/2(H20)= 0.21 f 0.031. The significance of these results is given later. Effect of Alcohol Scavengers. At pH 3, HO' radical scavengers methanol and tert-butanol had little effect on either the kobs for loss of 2,4-D (Figure5; Table 2) or the rate of 14C02evolution from carboxy-labeled 2,4-D (Figure 6 ) , even when these scavengerswere present at 103times higher concentration. DCP transformation, by contrast, was stronglyinhibited (Figure 7). For example, 0.1 M methanol caused a -20-fold reduction in kobs for DCP (Table 2). At pH 1, the alcohols were about the same or slightly more inhibitory to 2,4-D transformation than at pH 3. At pH 12, however, the alcohols were strongly inhibitory. Figures 6 and 8 show that at pH 2.8 mineralization of [ring-14C]-2,4-D(as loss of 14C in solution) was retarded when methanol was added at 0 or 60 min, but not at 150
+
VOL. 29, NO. 8,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2067
0.20
100
A
pH 2.8 2,4D O
HCHO
v
gyoxylate
4
,.a
DCP DCPF
0.15
80
'5
B
9
E
0.10
40
20 0
0
20
40
0.05
60
80
100
60
80
100
min 100
0.00 30
0
-
60
min 0.25
B
pH 12
1.
0.20
0.15
ai
E
120
90
80
a^ $60 cc" 140
3P
20 0
20
0
40
min \
FIGURE 5. Effect of methanol (A) and tert-butanol (9) on 2,4-D transformation at pH 2.8. \ ~~~
0.10
TABLE 2
Inbibition of 2,hD and DCP Transformation by Alcohols
0.05
kd0.l M ROH)/k,,dno ROH) 2,4-D pH 1 2,4-D pH 2.8 2,4-D pH 12 DCP pH 2.8
0.00 0
30
90
60
120
150
180
0.67 [BBuOHI 0.83 It-BuOH1; 0.63 [MeOHl 0.064 [MeOHl 0.051 [MeOHl
min FIGURE 4. Products of 2,4-D oxidation (fonnaldehyda,glyoxylic acid, 2,4-dichlorophenol, and 2,4-dichlorophenol fonnate) in titania suspensions. (A) pH 2.8; (9) pH 12 (DCPF is rapidly hydrolyzed to DCP).
i
TABLE 1
-
Deutanum Kinetic (KSIE) and Product (PSIE) Solvent Isotope Effects on Transfornation of [c~w~ox~-"C]-~,~=D
Ivv
80
t?l
.-c
.-
60
0
IWMMeOH
0
noalcohol
- nng .''c
....
urbOxy-"C
. .
40
PSIE (as f i = ~A14&.~,,/A2,&D)a~b KSIE (kdk~)' pH(D) = 2 pOH(0D) = 2
2.1 5 0.3 1.0 i 0.1
a Two standard deviations. at 1 2,4-0 half-life.
Hz0
DzO
0.88 f 0.03 0.21 f 0.03
0.92 rt 0.03 0.28 f 0.03
Units of (molequiv) (mol)-';value taken
min. The latter time coincides approximatelywith the time of complete disappearance of DCP in the absenceof alcohol, as observed in photocatalytic transformation of DCP as starting material (Figure 7) or as an intermediate of 2,4-D degradation (not shown). These results suggest that DCP is at least one bottleneck in the mineralization of the 2,4-D ring. These results also indicate that scavengers do not 2068
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 8.1995
4 6 8 hours FIGURE 6. Effect of methanol on mineralization of [rinp14C]- and [c8ribOXp"C]-2,4-D at pH 2.8.
0
2
inhibit mineralization of C of ring origin that are present in intermediates formed during later stages of 2,4-D degradation. We also tested the hypothesis that the weak inhibition by the alcohols at low pH is due to their weak affinity for adsorption-reaction sites compared to 2,4-D. As is often observed in Ti02 photocatalysis, the initial rate of 2,4-D at pH 2.8 is concentration dependent. This dependence can
proof of a surface reaction (3, these results indicate that if reaction does occur on the surface, the lack of inhibition by tert-butanol is not due to its relative inability to adsorb.
Discussion
" 50
0
150
100
200
250
min FIGURE 7. Effect of methanol on DCP transformation at pH 2.8.
Ivv
s6 .-0
0 1 M M d H addedatamw (+
80
O'...O .........
CI
$ v)
untreated)
e. ...........0 .............. 0
. .
60
c
0 40
....0.. ..........
s
......
I
F
20
'C
0
60
180 240 300 360 min FIGURE 8. Mineralizetion of [ring1F1-2,4-D treated end untreated with 0.1 M methanol at various stages (added at arrows) et pH 2.8. 0
120
1s o
1.25
2
8 ,=
1.00
E 0.75 2 L
0.50
The oxidation of 2,4-D between pH 1 and pH 3 is almost unaffected by high concentrations of HO' scavengers. The weak inhibition is compelling evidence that the active oxidant, at least in this pH range, is adsorbed. Reaction of 2,4-D with free HO' generated by the Fenton reaction (eq 10) or by photoreduction of Fe3+(aq)(eq 11) is strongly retarded by methanol (20,21):
+ Fe2++ H,O,
--
+ 0, + 2H' HO' + OH- + Fe3+ [FeOH]2+(aq)+ hv HO' + Fe2+
2Fe3+ H,O,
2Fe2'
-
h+ + OH,-
0.00
20
40 60 l/j2,4-D1(mM")
80
100
FIGURE 9. Langmuir-Hinschelwood fit to the concentration dependence of the 2.44 transformation rate at pH 2.8 (inverse eq 9). Regression coefficient, rZ = 0.997.
be simulatedby the Langmuir-Hinshelwood model, which presumes adsorption of substrate prior to its rate-limiting reaction on the surface: r, =
kK[2,4-D] 1 + K[2,4-D]
In this equation, r, is the initial rate, k is a rate constant, and Kis the binding constant on the illuminated catalyst. Figure9 showsthefittotheinvertedeq9 (i.e., l/r,vs 1/[2,4Dl) for which K is the ratio of intercept to slope. Initial rates of DCP and ferf-butanol were also concentration dependent and fit in the same way. The resulting Kvalues (M-l) followed the order: DCP (8.8 x lo4) > ferf-butanol (2.4 x lo4) > 2,4-D (0.81 x lo4). Although fit to the Langmuir-Hinshelwood model by itself does not constitute
(lob) (11)
Furthermore, the Langmuir-Hinshelwood Kof fen-butanol under photocatalytic conditions is severaltimes lurgerthan that of 2,4-D, indicating that fert-butanol is potentially competitive for adsorption-reaction sites. Indeed, these alcohols compete effectively with DCP for reaction sites. Our results cannot distinguish between prior adsorption and collision with the surface at the moment of reaction. At about pH 3, which is optimum for photocatalytic oxidation of 2,4-D, the observed products (carboxyl CO,, HCHO, DCP, DCPF) and their yields indicate that the first step is one-electron oxidation of the carboxyl group (eqs 5-7). However,the pathway for degradation of 2,4-D clearly changeswith pH. Above and below pH 3, decarboxylation increasingly lags disappearance of 2,4-D (Yl/2 decreases), and the yields of HCHO, DCP, and DCPF greatly decline. This implies that oxidant attack shifts away from the carboxyl group and toward the ring. In concert with this shift is the increasing retardation by HO' scavengers. These results are best rationalized by a dual hole-radical mechanism in which direct h+ oxidation of 2,4-D (eq 4) exists in competitionwith h+ oxidation of surface hydroxyls (eq 2):
0.25 0
(loa)
hf
+ RH,
-
-
O*
(2)
R(oxidized)
(4)
In the context of the dual pathway, it is supposed that (A)Holes carry out electron transfer oxidation, preferring carboxylover otherfunctionalgroups. Holes cannot abstract hydrogen atoms or add to aromatic rings. Preference for the carboxylate group may stem from its ability to coordinate directly with underlying Ti4+, facilitating direct electron transfer (see below). Also, the carboxylate anion is expected to have a higher oxidation potential than the acid. Compounds without carboxylate groups (e.g., DCP) appear to go by the radical pathway since alcohol inhibition is restored. The results in Figure 8, which show negligible alcohol inhibition of ring-14C02evolution duringlate stages of mineralization, are also consistent with a carboxylate preference since the intermediates present then (i.e.,after 2,4-D and DCP are removed) are mostly ring-opened products rich in carboxylate groups. A solution-phase analogy to the surface reaction in eq 4 is the near-W irradiation of Fe3+(aq)/[carb0xy-~~C1-2,4D solutions (21). Normally, the dominant reaction is the VOL. 29, NO. 8.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY 12069
TABLE 3.
hpoctd ul Okmd B o u t a h Kiutic (IUlE) ad )ncct (PSI€) k)wmt Isotopo Effwts for PLotocatdytic Tmthmrtioa of 2,CO tk O r r l H o l e - R d i d Moclulsm speciation conditions
Of
pOH(D) = 2
RC02-
proposed pathway
2,4-D
mostly radical
+, 0,-
0
+
+
expected PSIEbdue to mechanistic shift radical hola
-
+, 0, - (in accord with
KSIE' -0
expected radical KSIE)
observed PSIEb slight
+
+
90% RCOZH mostly hole -0 -0 f, 0, and - mean kdkD > 1, =1, and c l , respectively. * +, 0, and - mean increased, unchanged, and decreased decarboxylation ( M in D20,
pH(D) = 2 a
expected KSIE' for hole radical
respectively.
generation of HO' (eq 111, but in the presence of methanol as HO' scavenger, a slower charge-transfer photolysis of a ferric-2,4-D complex is revealed that gives stoichiometric decarboxylation accompanied by a high yield of DCP (eq 12):
slightly greater decarboxylation. The hole pathway should have no KSIE because H(D)is not involved in the transition state. In order to predict solvent isotope effects on the radical pathway, we must consider the rate law for reaction 2:
(CH,OH)
hv + [ArOCH,COz-Fe12+ Fez+ ArOCH,'
+
+ CO, - -ArOH
(12)
(B) The trapped hole (O*) behaves likefree H O in that it abstracts hydrogen atoms andlor adds to the aromatic ring. Free HO' normally does not react by one-electron transfer due to the high solvent reorganization energy associated with transfer of charge (22). Free HO' produced by either eqs 10 or 11 gives limited decarboxylation of [~arboxy-'~C]-2,4-D (Y -0.2) and is effectively scavenged by methanol (20). (C) Relative rates of reactions 2 and 4 are p H dependent. The TiOzsurface has molecular (D OH,) and dissociated (0-OH) water (5,23,24)and has a point of zero charge at pH -6 (23). The 2,4-D molecule most likely sorbs through the carboxyl group. Sorptiveinteractionsof R-COz-(H) with hydrous oxide surfaces include both outersphere forces (electrostaticattraction to O-OH*+, H-bonding,dispersion) and inner-sphere coordination with the underlying metal ion (0-0-COR). For titania, it has not been established which forces are dominant over which pH region. Innersphere coordination may provide a site for trapping the emerging hole (10) or at least facilitate electron transfer to it. As the pH rises above 3, hole oxidation of OH,- (reaction 2) is increasinglyfavored over hole oxidation of the substrate (reaction 4) owing to one or more of the following: (a) the increasing surface negative charge increases the oxidation potential of the surface; (b) inner-sphere coordination of carboxylate ion is competitively inhibited by OH-; (c) the adsorbed (orcolliding)substrate adopts conformations that place the carboxylate group away from the surface due to growing charge repulsion. As the pH falls below 3, hole oxidation of OH,- becomes increasing important because (a) protonation lowers the oxidation potential of the carboxylate group and (b) inner-sphere coordination of the carboxylate is disfavored. We now consider whether the deuterium KSIE and PSIE are consistent with a dual hole-radical mechanism. The rationale detailed below is put in summary form in Table
We may make the reasonable assumption that oxidation of 0. OH3+ is unimportant. Under alkaline conditions, the k2e term may be ignored. As implied by this equation, isotope effects are possible on the k's as well as on the equilibrium speciation of OH,-. Although individual k s are not obtainable,we canexpect asecondary isotope effect on the one-electron transfer step represented by eq 2, so that kza,kzC,and k 2 d will decrease in DzO.The k2b should remain unchanged. Equilibrium speciation is important because hf reacts faster with negatively charged than uncharged hydroxyls. The speciation of OH,- is governed by the following acid-base equilibria and their respective equilibrium constants: molecular water
-
dissociated water
+
E OH, OH- = H,O Km,Y/LH (14)
E OH-
+
+
0-OH OH- = 0-0HzO KdissZH/l(wH (15)
+
where KmolH is the acidity constant of D QHz, KdissZH is the second acidity constant of 0-OH, and KwHis the acidity constant of bulk water. We may derive an expression that relates the ratio of charged to uncharged water in each solvent. For molecular water
3.
At high alkalinity (pOH =POD = 21, the carboxyl group is fully ionized. The low value of YI/Z indicates that the radical pathway predominates, and thus the rate-limiting step is primarily reaction 2. The data in Table 2 indicate that as the solvent is changed to DzO the rate constant remains unchanged and attack on the substrate shifts to 2070 m ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 8.1995
Likewise for dissociated water at constant [OH-] = [OD-]
From the literature, KwHIKwD = 8.2 at 25 "C (molarscale) (25). Both species 0-OH and 0.. .OHz are stronger acids than water itself (the second acid dissociation constant of the surface is 8; ref 23). Solvent isotope effects on the dissociation of weak acids in water are generally smaller than that on water itself and follow the approximate relationship (25) ApK= PA? - p p = 0.41
+ 0.02pe
(19)
While pKmolH and p&iss2H are not separable,the relationship in eq 19 qualitatively predicts that the expressions in eqs 17 and 18 will be < 1. For example, taking either pK,,lH or p&issZHto be 8 (23)results in ApK = 0.57 and eq 17 or eq 18equal to 0.45. Therefore,the surfacewillhave a greater negative charge in D20 when [OH-] = [OD-]. This makes sense because OD- is a stronger base than OH-. The greater surface charge results in a negative contribution to the overall KSIE due to speciation that acts in opposition to the positive contribution attributable to the rate constants. Thus, the overall expected KSIE depends on the balance between the rate constants and the concentration terms in eq 13 and could be positive, negative, or zero. The observed KSIE is close to nil ( k ~ l = k ~1.0 f 0.11, which suggests a close balance between the effects of rate constants and equilibrium speciation. The weak PSIE under alkaline conditions is consistent with the nil KSIE in that it implies little or no diversion to the hole pathway in Dp0. The zero KSIE for a supposed radical mechanism observed here contrasts with KSIE of 2.8 found by Cunningham and Srijaranai for photocatalytic transformation of 2-propanol on Ti02 (26),for which eq 2 (i.e.,the radical mechanism) was assigned as the rate-limiting step. However,the pH(D)of the 2-propanol suspensions (presumably around neutral)was not precisely controlled as eq 16shows, it is difficult to assess the importance of equilibrium speciationin that case. If it is presumed that around neutral pH the surface charge densityis very low and approximately equal in both solvents, then the terms involving the uncharged species (kzaand kzc) will dominate the overall rate law, and the speciation effect vanishes. Under that assumption, the observed KSIE for 2-propanol reflects a purely kinetic secondary isotope effect on the electron transfer reaction 2. Let us now consider the oxidation of 2,4-D under acidic conditions at pH = pD = 2 where we observed a small positiveKSIE (k~/ko=2.1 f0.3). AtthatpH(D),thecarboxyl group is -90% in the uncharged (-COpH or -C02D) form. The high value of Yli2 (Figure3) indicates that the reaction center is primarily the carboxyl group and that the hole pathway predominates. Hole oxidation of uncharged carboxyl groups will exhibit a secondary KSIE in the transition state of eq 20:
h+ + RCOp-H(D)
HD20
RCO,'
+ H(D),Of
(20)
The radical pathway also will be subject to a KSIE. As discussed above, the k's for all terms in eq 13 involving protonated species will be subject to a KSIE, and this is consistent with the 2-propanol results in Cunningham and Srijaranai (26). Regarding speciation effects, in acidic solution we need only consider neutral and positively charged hydroxyl groups on the surface. We may make the further reasonable assumption that there is no isotope effect on the surface concentration of molecular water [D -0Hzl.
From arguments similar to those above, it can be shown at pH = pD = 2 that
where KdisslHand KdisslDare the first acidity constants of O-OHp+ and O-ODp+, respectively. The &&HI has been estimated to be 10-2.4(23). According to the relationship in eq 19,eq 21 will be equal to 0.35. Hence, the surfacewill have a greater positive charge in Dz0. The influence on the KSIE will be a positive component due to speciation effects that will reinforce a positive component due to rate constant effectsresultingin an overallpositive KSIE. Hence, both reactions 2 and 4 are predicted to show a positive KSIE. While it is difficult to predict their relative magnitudes, we may reasonably expect little if any shift to the hole mechanism on changing the solvent to D20, and this is borne out by the statistically negligible PSIE. Two other plausible mechanisms were considered: a purely radical pathway and a dual surface-solution mechanism. A purely radical pathway must somehow explain the product changes over the pH range, vis a vis decarboxylation vs attack on the ring. These changes might be rationalized if it is supposed that O* preferentially attacks the molecule at the position closest to the surface: it could be argued that O* reacts with the carboxylate group at pH -3 but that other positions in the molecule become more competitive as the pH is lowered [-COpH has a smaller interaction energy than -Cop-] or raised [-Copis repelled from the surface by the increasing surface charge], However, we believe a purely radical pathway is ruled out for the following reasons. (a) The observed pH dependence of the scavenger effect requires that the -COz- group be much more reactive (-lo3- 104-fold)than functionalgroups on the scavengers. This is inconsistentwith the well-known propensity of HO' radicals to abstract H and their weak ability to carry out one-electron oxidation of anions or neutral molecules. As discussed above, decarboxylation of 2,4-D is a minor reaction when free HO' is involved. (b) At high acidity, the expected PSIE is negative (toward less decarboxylation in DpO),whereas the observed PSIE is zero or slightlypositive. Attack of O* on the -COpD group should be disfavored relative to -C02H:that is, other positions in the molecule, which do not exchange deuterium, will be attacked more frequently in D20. (c) The observed PSIE at high alkalinity is in the opposite direction predicted. In Dp0, the surface charge is greater: hence, there is greater repulsion of the carboxylate group in Dp0 than in Hp0 and a higher probability of attack at other positions in the molecule. Finally, we may consider a dual surface-solution mechanism. In this mechanism, it is supposed that O* is the oxidant and that at pH -3 it is surface-bound during its reaction with adsorbed or colliding substrate, but below and (especially)above pH 3, it is progressively dissociated as free HO'. Let us assume this is simply due to decreasing substrate adsorption constant coupled with constant or increasing tendency of O* to desorb. Although we have clear evidencefor a surface reaction at pH 3, we do observe more and more HO-like behavior above and below pH 3, which is plausibly due to solution-phase reactions. A dual surface-solution mechanism is unlikely, however, for the following reasons: (a)We argue that alcohol scavenging of VOL. 29, NO. 8 , 1 9 9 5 /ENVIRONMENTAL SCIENCE &TECHNOLOGY
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0*-presumably of HO-like character-should occur on the surface as well as in solution. (b)At high acidity, a negative PSIE is expected (less decarboxylation in D20)owing to a primary isotope effect for attack of H(D)O on the RC02H(D) bond and consequent favoring of other positions in the molecule. Instead, we found no change or slightlymore decarboxylation.
Acknowledjments We are grateful for the technical help of Marta Day and the financial support of the United States Department of Agriculture, Water Quality Special Grants Program.
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Received for review January 5, 1995. Revised manuscript received April 21, 1995. Accepted May 16, 1995.@
ES950005+ @
Abstract published in Advance ACS Abstracts, July 1, 1995.
