Ind. Eng. Chem. Res. 1993,32,2318-2323
2318
Rates of Photocatalytic Oxidation of Crude Oil on Salt Water on Buoyant, Cenosphere-Attached Titanium Dioxide Maya Nair, Zhenghao Luo, and Adam Heller' Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062
The rate of TiO2-photocatalyzed oxidation of crude oils spilled on aqueous 0.5 M NaCl was determined by measuring the rate of 0 2 uptake. The photocatalyst was attached to 100-pm-diameter fly-ashderived buoyant cenospheres. Partial hydrophobic coating of the cenospheres assured their retention at the air-oil interface. The rate depended on the near-UV (broad band, 365-nm peak) irradiance below 25 W m-2,but varied only mildly with irradiance in the 25-45 W m-2 range. It increased upon wave motion imitating agitation of the liquid, and upon increase of the cenosphere:oil mass ratio. It varied only mildly for different crudes (Arab Heavy, Arab Light, Basrah Light, and Texaco Bruce No. 1). From the measured rates, cleanup times as short as 5-10 days were estimated.
Introduction Titanium dioxide, an n-type semiconductor, is today's most widely used pigment. The rutile form of the pigment has a band gap of 3.0 eV; Le., the pigment absorbs and is excited by light of wavelengths shorter than 414 nm. Its anatase form has a band gap of 3.2 eV, and the pigment absorbs and is excited by light of wavelengths shorter than 387 nm. Each absorbed photon (hv)promotes in the semiconducting particle an electron from the valence to the conductionband, generating an electron (e-)-hole (h+) pair, the hole being an electron vacancy in the valence band of the semiconductor(reaction 1). The hole diffuses
-.
hv e- + h+ (1) or drifts to the particle surface, where it reacts with an adsorbed water molecule to produce an oxidizing hydroxyl radical ('OH) and a proton (reaction 2). Charge neutrality
-
h+ + H20 H++ HO' (2) is maintained through 0 2 reduction to reactive 02'- and through pairs of residual conduction-band electrons and pairs of protons reducing adsorbed molecular oxygen to hydrogen peroxide (reaction 3). Part of the peroxide may 2e- + 2H+
+ 0,
-
H202
(3) decompose to water and 02 (reaction 4). The 'OH radicals
-
2H202 2H20 + 0, (4) initiate oxidation of hydrocarbons to carbon dioxide, water, and water-soluble organic products such as aldehydes, ketones, phenolates, and carboxylates (reactions 5 and 6). This sequence of reactions, where the oxidation of aliphatic RCH2CH2R'+ 'OH
-
R6HCH2R' + H,O
.
R&K"R'
+ O2
-
I
I1
RCOO-
RCOO- + H+ + RCH2'
+ 'OH
R'CH2' + 0 2
+
4
'OH
R'
II
RCHCH2R' + 'OH
RCHCH2R'
0 RCCH2R'
- - 0
I
RCH200'
+
H+
+ O2
R'CHO
+ 'OH
Ro + COP + H 2 0
ROO'
h+ + e-
-
heat
(7)
dation products are italicized in reactions 5 and 6. Water is also formed, but ita formation is difficult to prove because of the large amount present prior to the photooxidation. Reaction 8 where electrons reduce 'OH radicals on the pigment particle surface is, when combined with reaction 2, a recombination reaction equivalent to reaction 7. In summary, reactions 2 followed by reaction 5 results in photoassisted oxidation. Competing reactions 7 and 8 reduce the efficiency of oxidation. e- + H+ + *OH
+
H20
(8)
(5)
0
0
and aromatic hydrocarbons by 02 is assisted by light absorbed in semiconducting particles, is known as photoassisted oxidation (Ollis and Al-Ekabi, H., 1993). It is, like combustion, a free-radical-catalyzed thermodynamically spontaneous process. However, unlike combustion, it proceeds at ambient temperature. Because TiO2photoassisted oxidation of the polymeric binders causes sunlight-exposed paints to "chalk" (Berner and Kreibich, 1982; Colling and Dunderdale, 1982; Eaton, 1984; Voelz et al., 1981), commercially manufactured Ti02 pigment particles are coated with an electrically insulating silicon dioxide or aluminosilicate layer (Iles, 1959; Werner, 1969) through which the charge-carrying electrons and holes cannot diffuse (Heller et al., 1987). When the electrons and holes cannot reach the particle surface, where the holes would react with adsorbed water and the electrons with oxygen, they recombine in a multiphonon relaxation process whereby their electronic excitation energy is converted to vibrational energy of the lattice, i.e., heat (reaction 7). The actually observed photoassisted oxi-
(6)
Crude oils strongly attenuate the near-ultraviolet (UV) photon flux, their absorbance being >lo4 cm-1. Thus, micron-thick crude oil films covering the Ti02 particles considerably slow the rate of solar-assisted oxidation. Because the density of Ti02 (anatase) is 3.84 g ~ m -well ~, above the density of oil or water, the photocatalyst sinks. In order to prevent sinking, or coverage of the photocatalyst by an excessively thick crude oil layer, the particles are attached to a buoyant support (Brock and Heller, 1991). The support's surface must not be photooxidized, as this would cause separation of the Ti02 particles. Hollow ceramic aluminosilicate microspheres, known as cenospheres, as well as hollow borosilicate glass microbeads, are, however, appropriate buoyant supports. Cenospheres
0888-588519312632-2318$04.00/0 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 10,1993 2319 are a sandlike component of the fly ash that is electrostatically collected in coal-burning electric power generating plants. The density of the individual beads is 0.7 f 0.1 g cm4 and their diameter range is 40-200 pm, with the mean diameter being typically 100-150 pm. The thickness of their shell is 8-20 pm. The shells consist primarily of a fused Si02 glass and mullite (3A120~2Si02). Hollow glass microbubbles are engineered to specifications in diameter, density and composition. Because their density can be 1/2-1/3 of that of the cenospheres and, additionally, their diameter can be 2-3 times smaller and their surface area correspondingly larger than that of the cenospheres, the mass of the glass bubbles required is about 7-fold reduced with respect to the mass of the cenospheres. Earlier analysis has shown that coating of about 60% of the surface of microspheres with Ti02 suffices for attainment of maximum solar UV collection and that the particles are good UV collectors even if only 30-50% of their surface is coated with TiO2. Their UV collection efficiency varies relatively little with their ceramic’s composition and shell thickness (Rosenberget al., 1992). In order to maintain the photoactive, partially Ti02 particle coated microspheres at the air-oil interface, i.e., in order to prevent their migration to the air-water interface, part of their uncoated or non-photoactivesurface is made hydrophobic through reaction with an organic (alkyl),(alkoxy)~, silane or with a chlorine-terminated silane (Jackson et al., 1991) such as a Glassclad (Heller et al., 1993a). Approximately 96.0-97.0% of the sea-level solar irradiance consists of photons that are not sufficiently energetic to promote valence band electrons to the conduction band of Ti02 (anatase). Nevertheless, the solar-assisted cleanup can be quite fast even with the residual photons whose wavelengths are in the useful 300390-nm range. For the spherical optics of the buoyant beads the absorbed photon flux is nearly independent of the angle of the sun (Rosenberg et al., 1992). The usual “cosine effect” is absent, but the near-ultraviolet component of sunlight is attenuated by, and therefore depends on, the thickness of the atmosphere that it traverses. This reduces the UV flux relative to the equatorial flux when the sun is at zenith at northern latitudes, in the winter, or in the early morning or late afternoon hours, but only by about a factor of 2. When the sun is at zenith in the Gulf of Mexico or the Persian Gulf, the useful UV irradiance is about 35 W m-2; it is about 15-20 W m-2 during the Alaskan spring, summer, or fall. If the cleanup were ultraviolet irradiance limited, it would be possible to photodissolve a mass of oil spilled on water by an equal mass of beads in less than 1day not only in Singapore, the Persian Gulf, or the Gulf of Mexico, but also in the North Sea or off the coast of Alaska. On sunny days the cleanup process is, however, not limited by ultraviolet irradiance. Its rate can be slowed by the following: (a) aggregation of the hydrophobic microbeads into oil-bound clusters (oxygenmass transport to the interior of the clusters is inadequate and the cluster interiors are not UV exposed); (b) excessive coverage of the cenospheres by the strongly UV-absorbing oil layer; (c) slow mass transport, (d) recombination of photogenerated electrons with holes through reaction 7 or through reaction 8 in combination with reaction 2. Unless the stripping of electrons from the photocatalytic particles through reaction 3 is fast, the particles are electron charged and the photogenerated holes recombineand the hydroxyl radicals are reduced at a rate increasing with the electron density on the particles (reactions 7 and 8). Thus, unless
the 0 2 flux to the particles and reduction kinetics are adequate, the efficiency of solar-assisted oxidation is reduced. In this article we describe oxygen uptake experiments aimed at defining the realizable rates of photooxidation of crude oils. Photooxidation takes place also in the absence of photocatalytic microbeads,through excitation of polycyclic aromatic constituents of crudes (Barth, 1984; Berthou et al., 1985; Hansen, 1975; Nagata and Kondo, 1977;Overton et al., 197%Philpel, 1974;Rontani and Giral, 1990;Tagger et al., 1983;Thominette and Verdu, 1984a,b). This natural process, while destroying carcinogenic polycyclic aromatics, can produce emulsions and toxic products, particularly phenols and polymerization products of phenols. The photocatalytic hydrophobic microbeads prevent emulsification and accelerate the photooxidation of not only the aromatic hydrocarbonsthemselvesbut also of toxic photoproducts, including phenols, that are strongly adsorbed on Ti02 (Heller et al., 1993a,b; Matthews, 1987a,b, 1990;Matthews and McEvoy, 1992a,b; Okamoto et al., 1985; Rontani and Giral, 1990; Trillas et al., 1992).
Materials and Methods Censospheres, i.e., hollow aluminosilicate ceramic microspheres (SLG Type, PQ Corporation, Conshohocken, PA), were used. These aluminosilicate microspheres had an average diameter near 100 pm, and their density was 0.7 f 0.1 g cm3. The cenospheres were coated with Ti02 (Degussa P-25) particles. The P-25 particles’ diameter was 0.1-0.2 pm; each particle consisted of multiple crystallites of 0.02-0.03-pm diameter. The material was of mixed anatase (65%) and rutile (35%) phase. The cenospheres were Ti02 coated by the earlier described high-temperature process and then made oleophilic by treatment with Glassclad 6C (Heller et al., 19938). The crude oil chosen for most of the experiments was Texaco Bruce No. 1. The UV light source consisted of a bank of 40-W fluorescent lamps, phosphor-coated for peak emission at 365 nm. Their emission spectrum was broad (300400 nm) and was skewed to the longer wavelengths. The reactor irradiance was, unless otherwise specified, 46 W m-2. The rate of photoassisted oxidation was followed by measuring the rate of pressure drop and correcting for the rate of C02 evolution. The rate of C02 evolution equaled, for Texaco Bruce No. 1oxidation in air, and for a mixture of 1.5mL of oil and 3.0 g of beads with the reactor orbitally shaken, half of the oxygen uptake. Thus the reported oxygen uptake rates were calculated by multiplying the measured pressure drop by a factor of 1.5. Sealed 200- or 220-mL 7-cm-diameter photoreactors that were water jacketed for temperature control at 23 f 3 “C and connected to a water manometer were used. A volume of 62 f 3 mL of 0.5 M aqueous NaCl was loaded in the reactor, along with a measured volume of oil and a measured weight of photocatalytic cenospheres. The gas volume, including the volume of connecting tubings, was maintained either at 128 f 3 mL, in one reactor, or at 145 f mL in the second. The reactor was flushed with either oxygen or “zero air”, consisting of 21 5% 0 2 , 79% N2 (to be referred to as “air”) and then sealed. In some experiments the fluid-bead mixture was, as indicated, stagnant or gently stirred. In others it was agitated strongly by an orbit shaker operated at 120 rpm that produced wavelike swirling. The pressure drop was measured at 2-30-min intervals for 4.5 h. The concentration of the water-dissolved oxygen was also monitored in part of the experiments using an amperometric 0 2 sensor.
2320 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
Results The rate of 0 2 uptake from air was measured with mild stirring of the bottom aqueous layer in the reactor, without agitating the top oil or cenosphere layer. The measurements, started after 90-min equilibration under illumination, were of 3-h duration. For 0.62 mL of crude oil and 5 g TiOz-coated hydrophobic cenospheres the rate was 5.3 X le7 mol of 0 2 min-l or 2.9 X 103 mol of 0 2 h-' (lb of cenospheres)-l. The relationship between the surface properties of TiOzcoated cenospheres and their photoactivity was probed in experiments under air, without wave-producing shaking, for mixtures of 5 g of cenospheres with 1.25 mL of Texaco Bruce No. 1 oil. When the cenospheres were hydrophilic, Le., their surface was not reacted with Glassclad 6C, the rate of oxygen uptake was 1.0 X mol of 0 2 min-l or 5.4 X lo4 mol of 0 2 h-l (lb of cenospheres)-l. The rate increased when the cenospheres were made hydrophobic mol of 02 by treatment with Glassclad 6C to 5.0 X min-l or 2.5 X lo3 mol of 0 2 h-' (lb of cenosphere)-l. In contrast with the hydrophobic cenospheres, which resided exclusively at the air-oil interface, the hydrophilic cenospheres resided primarily at the oil-water interface and at the water-air interface. The rate of photoassisted oxidation increased when the mass of beads was kept constant and the volume of oil was reduced. When 5 g of cenospheres was applied to 3.75 mL mol of of crude oil, the oxygen uptake rate was 2.0 X 0 2 min-l, corresponding to 1.1 X 10-3 mol h-' (lb of cenospheres)-l. The uptake increased to 4.0 X le7mol of 0 2 min-l corresponding to 2.2 X 103mol of 0 2 h-l (lb of cenospheres)-l when the oil volume was reduced to 2.5 mL, and then further inereased to 5.0 X mol of 0 2 min-' and 2.5 X 103 mol of 0 2 h-l (lb of cenospheres)-l when only 1.25 mL of oil was added. The rate of photoassisted oxidation was mass-transport dependent at 46 W m-2irradiance. When 3 g of photoactive hydrophobic cenospheres were applied to 1.5 mL of oil, mol the oxygen uptake in a stagnant system was 3.5 X le7 mol of 0 2 h-l (lb of 0 2 min-1, corresponding to 3.1 X of cenospheres)-l. With waves, produced by shaking, the mol of 0 2 min-l or 4.0 X rate increased to 4.5 X mol of 02 h-' (lb of cenospheres)-l. The waves did not visibly affect the aggregation of the beads. The rates of photoassisted oxidation of four types of crude oil, Arab Heavy, Arab Light, Basrah Light, and Texaco Bruce No. 1 differed by less than 30% after the initial 90-min equilibration period, during which the differences were larger (Figure 1). When 1.5 mL of one of the crudes was treated with 3 g of photoactive cenospheres and the reactor was shaken to produce waves, the rates of 02 uptake after the initial 90-min period were 4.3 X mol of 0 2 min-l or 3.9 X mol of 0 2 h-l (lb of cenospheres)-l for Arab Light, 3.6 X mol of 0 2 min-l or 3.3 X 103 mol of 0 2 h-1 lb-1 for Arab Heavy, 3.5 X 10-7 mol of 0 2 min-l or 3.2 X mol of 0 2 h-1 lb-' for Basrah mol of 0 2 min-l or 4 X mol of Light, and 4.5 X 0 2 h-' lb-1 for Texaco Bruce No. 1. Replacement of air by oxygen increased the rate of photoassisted oxidation (Figure 2). In the photoassisted oxidation of Texaco Bruce No. 1, with shaking to produce waves and with 3 g of photoactive cenospheres applied to 1.5mL of oil, the sustained rate of oxygen uptake increased by about 36 ?6 to 6.0 X mol of 0 2 min-1, corresponding to 5.6 X 103 mol of 0 2 h-' (lb cenospheres)-l. The increase during the initial 90-min equilibration period was larger, the rate nearly doubling from 7.2 X mol of 0 2 min-l or 6.5 X mol of 0 2 h-' (lb cenospheres)-l to 1.4 X 10-6
e:* .e
E E
2
f" a
a
-130
0
0
.LJV
0
200
100
300
time, min
Figure 1. Pressure drop, measured by the height of the water column of the manometer,in the photoassistedoxidationof crude oils. Solid diamonds,Arab Heavy; open diamonds,Basrah Light; solid circles, Arab Light; open circles, Texaco Bruce No. 1. Conditions: 3.0 g of hydrophobicTiOrcoatedcenosphereswith 1.5mL of crude;measured at 47 W m-2 near-UV (A= 365 nm) broad-band irradiance; in air, with "waves"produced by an orbit shaker.
0
. 0
. 0
.
o
* O
* 0 0
0 0
e
time, min
Figure 2. Enhancement of the rate of photooxidation upon replacement of the air (soliddiamonds) atmosphere by oxygen (open circles). The photooxidized crude is Texaco Bruce No. 1. Other conditions as in Figure 1.
mol of 0 2 min-' or 1.2 X 10-2 mol of 0 2 h-1 (lb of cenospheres)-l. Results of the measurement of the rate of 0 2 uptake as a function of the near-UV irradiance in air and in oxygen are shown, respectively, in Tables I and 11. The mea-
Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2321 Table I. Dependence of the 0 2 Uptake Rate and Photoassisted Oxidation Efficiency in Air on the Near-UV (A,.= = 365 nm) Irradiance initial sustained sustained efficiency, initial rate, efficiency, rate, mol of 0 2 mol of 02 irradiance, mol of 0 2 mol of 0 2 W m-2 min-1 x 107 einstein-1 min-1 x 107 einstein-1 0.038 4.7 0.029 25 6.0 0.029 5.9 0.026 35 6.5 0.024 4.5 0.015 47 7.1 Table 11. Dependence of the Op Uptake Rate and Photoassiited Oxidation Efficiency in Oxygen on the Near-UV (Lx = 365 nm) Irradiance initial sustained sustained efficiency, rate, initial rate, efficiency, mol of 0 2 mol of 02 irradiance, mol of 02 mol of 02 W m-a min-1 x 107 einstein-1 min-1 x 107 einstein-1 4.4 0.066 9 4.4 0.066 5.0 0.051 14 6.6 0.066 0.045 20 11.3 0.065 6.5 0.033 29 11.0 0.053 6.8 6.0 0.018 46 13.7 0.042
After evaporation of their volatile fractions, the nonvolatile oil residue on seawater forms upon agitation an emulsion known as “mousse” or “chocolate mouse”. Formation of the emulsion was observed in experiments on the nonvolatile residue of 1.5 mL of Texaco Bruce No. 1,of which the volatile fraction was removed by heating for 15 h to 100 “C. The hydrophobic photoactive cenospheres completely inhibited the emulsification of the nonvolatile fraction. The rate of 02 uptake from air, a t 48 W m-2 near-W irradiance, was 3.9 X lo-’ mol of 02 min-l in the photoassisted oxidation of the nonvolatile residue of 1.25 mL of Texaco Bruce No. 1 mixed with 5 g of photoactive cenospheres, in the absence of “waves”.
~~~~
0
++
O
O 0 0
X
0 0
E E
+
.
..
n a
+ X
-20
-
II
. “I
-.nn1
I
1
0
100
200
time,
300
min
Figure 3. Dependence of the rate of photoassisted oxidation of Texaco Bruce No. 1 on the irradiance. Open circles, 9 W m-2;solid diamonds, 14 W m-2; solid circles, 20 W m-2; open diamonds, 29 W m-2; crosses, 46 W m-2; oxygen atmosphere; “waves”produced by an orbit shaker.
surements are for 3.0 g of photoactive cenospheres with 1.5mL of Texaco Bruce No. 1and with shaking to produce waves. The uptake of 02 at different levels of irradiance is shown in Figure 3. Under oxygen the photochemical efficiency was independent of irradiance in the initial 90min period below 20 W rnS irradiance and declined at higher irradiance; thus the rate of 02 uptake was proportional to the irradiance below 20 W m-2 and then increased less rapidly with irradiance. The sustained rate was, however, independent of irradiance through the 2046 W rnS range and increased, but not in proportion with the irradiance, in the 9-20 W m-2 range. In air and through the 25-47 W m-2 range both the initial and the sustained rates were nearly independent of irradiance, with the photochemical efficiency declining a t increased irradiance (Table I).
Discussion Noncatalytic photooxidation of crude oil on salt water eliminates part of the near-UV and visible absorbing polycyclic aromatic hydrocarbons, some of which are carcinogens (Barth, 1984; Berthou et al., 1985; Hansen, 1975; Nagata and Kondo, 1977; Overton et al., 1979; Philpel, 1974;Rontaniand Giral, 1990;Tagger et al., 1983; Thominette and Verdu, 1984a,b). Natural photooxidation produces, however, potentially toxic compounds, such as phenols, that may polymerize upon further oxidation to difficult to biodegrade polymeric tars. In contrast, the TiOz-photoassisted oxidation process not only eliminates polycyclic aromatics at an increased rate but also eliminates the phenols (Heller, 1993a; Matthews, 1987a,b, 1990; Matthews and McEvoy, 1992a,b; Okamoto et al., 1985; Rontani and Giral, 1990; Trillas et al., 1992). The hydrophobic TiOz-coated cenospheres are effective photocatalysts. They acceleratethe air oxidation of the crudes, completely prevent tar formation, and also prevent the emulsification of the nonvolatile residue, known as “chocolate mousse” formation (Heller, 1993a,b). The results showthat the rate of photoassisted oxidation can depend on at least four variables: the ratio of cenosphere mass to oil volume, waves, the flux of near-UV photons, and, in the laboratory, the oxygen partial pressure in the atmosphere above the oil. When the cenosphereoil mixture contains too much oil, the cenospheres are covered by an excessively thick ultraviolet light blocking layer. Furthermore, the oil-richmixtures coagulate to large aggregates, in which only the exterior cenospheres are exposed to light and air. The rate of photoassisted oxidation is, however, satisfactory for the 100-150-pmdiameter cenospheres with 2 g applied per mL of oil, Le., 570 lb per barrel of uncollected, unevaporated residue. An equal cleanup rate is expected for 100 lb engineered glass bubbles, which are less dense and smaller, applied per barrel of oil. Outdoor experiments confirm rapid (1-2 week) cleanup at this bubb1e:oil mass ratio. Comparison of the rates of photoassisted oxidation in stagnant water and with waves shows that, in the Gulf of Mexico, the Persian Gulf, or Singapore, where the nearUV irradiance is 25-35 W m-2,the rate will not be limited by light but by mass transport, and will be enhanced by waves. This is notable because other cleanup methods, such as skimming and mechanical collection, are difficult or actually fail in high seas. Although the composition of crude oils,such as the tested Arab Heavy, Arab Light, Basrah Light, and Texaco Bruce No. 1,differed, the rates of their photoassisted oxidation varied only mildly. The relatively mild variation is explained by the assumption that the rates are controlled by either mass transport or by semiconductor surface reactions (reactions 1-4,7, and 8) rather than by the rates of the oil-constituent oxidizing secondary reactions (reactions 5 and 6).
2322 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993
While the rates of photoassisted oxidation of crudes were somewhat higher in oxygen than in air, they did not increase proportionally to the partial presence of oxygen: the rates increased only by -40% when the 0 2 partial pressure was raised 5-fold. Evidently, at high irradiance (2646 W m-2) the reaction rates are limited by other parameters, of which 0 2 diffusion is only one. High photoconversion efficiencies are initially observed in oxygen a t 9-20 W m-2 irradiance, with 1molecule of oxygen being consumed per 15 photons, and at 29 W m-2, where consumption of 1oxygen molecule requires 19photons. In air and at 25 W m-2 irradiance 27 photons are initially needed for the uptake of an 02 molecule. The sustained efficiencies are lower: 35 photons are required in air and about 25 photons in oxygen at 29 W m-2 irradiance for sustained consumption of an 0 2 molecule. As is seen in Table 11, the initial reaction rates, just after equilibration of the aqueous and oil phases with a pure oxygen atmosphere, are light limited, not 0 2 diffusion limited. They are, however, not light limited at high levels of irradiance, or when the 0 2 partial pressure is reduced, or when the sustained photoreaction depletes the 0 2 dissolved in the water and/or oil. In the series of experiments described, the 38cm2surface area reactor compartment contained 3 or 5 g of cenospheres. Thus the cenosphere layer was 17 or 25 cenospheres thick on the average. The cenospheres would have covered, if allowed to spread, 640 and 1070 cm2, respectively. At full spreading the 0 2 uptake rate per pound of beads would have increased by afactor of 17in one reactor and a factor of 25 in the second. Consequently, in the photoassisted oxidation of Arab Light, the rate of 0 2 consumption would have increased from 3.9 X 103to 6.8 X mol of 0 2 h-l (lb of cenospheres)-'. For Texaco to 9 Bruce No. 1 it would have increased from 5.3 X X mol of 0 2 h-l (lb of cenospheres)-l. Improved mass transport upon dispersion of the cenospheres would have further increased the rate of 0 2 uptake. With either 570 lb of cenospheres or with 100 lb of glass bubbles applied per barrel of uncollected oil residue, the projected rate of 0 2 uptake is 2.7 lb of 0 2 h-l for Arab Light and 3.6 lb of 02 h-l for Texaco Bruce No. 1. Complete photoassisted oxidation of one barrel of oil to C02 and water requires uptake of 1500 lb of 0 2 . An oil slick would therefore be completely photooxidized in about 420 h. Because low molecular organic oxidation products are also formed, the actual cleanup rate is faster. If the dissolving fragments had the average carb0n:hydrogen:oxygen ratio of acetate ions, the cleanup would take 120 h. If the average ratio were that for butyrate ions, then cleanup would take only 60 h, in line with the actually observed rates in the first outdoor experiments. The low molecular weight, partially oxidized, and waterdissolved photooxidation products are expected to be rapidly biodegraded by marine bacteria. The rate of biodegradation of an oil layer on water is usually limited by the slow mass transport of highly diluted micronutrients, such as biologically available nitrogen and phosphorus, to the oil-water surface. Dissolving the oil in a large enough volume of micronutrient-containing water overcomes the mass transport rate limitation of biodegradation. Thus the end step of cleaning up an oil spill through photoassisted oxidation may be viewed as a solarassisted biodegradation process.
Conclusion Marine oil spills can be oxidized in a TiO2-photoassisted oxidation reaction using buoyant hollow fly-ash-derived
cenospheres as catalyst carriers. After part of the surface of the cenospheres is reactively coated with a hydrophobic coating, the photocatalytic cenospheres are maintained exclusively where needed, at the air-oil interface. The rate of photoassisted oxidation is light limited only at levels of solar near-UV irradiance below those in normal direct sunlight. In direct sunlight the rates can be mass transport dependent. Both fresh crude oil spills and their nonvolatile residue are oxidized in the photoassisted oxidation process. Tar formation is prevented and the nonvolatile residue is not emulsified in salt water after the oil is mixed with the hydrophobic cenospheres (Heller et al., 1993a,b). Waves enhance the cleanup process by improving mass transport. With either 570 lb of cenospheres or with 100 lb of glass bubbles applied per barrel of oil the cleanup time can be short, requiring about 1 or 2 weeks of direct sunlight, or 2 weeks to 1month of indirect cloud-scattered daylight.
Acknowledgment The photoassisted oil spill cleanup project, partial results of which are presented in this paper, involved collaborations with Prof. J. R. Brock, Lois Davidson, Prof. John G. Ekerd, Prof. Dr. Heinz Gerischer (Fritz Haber Institut, Berlin), Dozent Sten-Eric Lindquist (Uppsala University, Sweden), Jeffery L. Norrell, and Jorg Schwitzgebel. It was supported by the U.S.Department of Energy, Division of Advanced Energy Projects, Office of Basic Energy Sciences.
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Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 2323 (24) Rosenberg, I.; Brock, J. R.; Heller, A. Collection Optics of Ti02 Photocatalyst on Hollow Glass Microbeads Floating on Oil
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