Langmuir 1999, 15, 2071-2076
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The Influence of Mineralization Products on the Coagulation of TiO2 Photocatalyst Kevin E. O’Shea,*,† Enrique Pernas,† and James Saiers‡ Departments of Chemistry and Geology, Florida International University, Miami, Florida 33199 Received June 1, 1998. In Final Form: December 31, 1998 We have investigated the influence of common inorganic and organic anions formed during TiO2 photocatalysis of organic compounds on the coagulation of the photocatalyst. Chloride, sulfate, phosphate, formate, and oxalate were added to aqueous suspensions of TiO2 over a range of pH values and the rates of coagulation determined using photon correlation spectroscopy. The rates of coagulation generally increase with the concentration of ions and are fastest at pH values near the zero point charge (zpc) of the photocatalyst, where electrostatic repulsion among particles is relatively small except for that for the phosphate-containing suspensions. At high concentrations of phosphate the coagulation is relatively slow near the zpc of the TiO2 in clean water. This suggest the characteristics of the TiO2 surface properties are modified through ligand exchange between the surface hydroxyl groups and phosphate or strong surface adsorption of phosphate, such that the zero point charge shifts and the resulting modified particles possess a negative charge when the pH is near the pHzpc of unmodified TiO2. Stirring of the solution dramatically accelerates the coagulation of TiO2 particles, decreasing the available surface area of the catalyst, which is expected to reduce the activity of the catalyst. Sonication was effective for inhibiting coagulation of the photocatalyst, hence maintaining a high surface area which should improve the performance of the photocatalyst relative to mechanical stirring.
Introduction Semiconductor photocatalysis has been used to convert a variety of organic materials through a number of processes including oxidations, reductions, isomerizations, substitutions, and polymerizations.1,2 In the past two decades, TiO2 photocatalysis has been widely investigated for the destruction of aqueous environmental contaminants by way of light-induced redox reactions at the TiO2/ liquid interface.3,4 Heterogeneous photocatalytic systems have also been used to recover metals from wastewater and to produce novel well-dispersed metal-loaded semiconductor catalysts by photodeposition of metallic ions.4,5 While it is generally accepted that these processes are initiated at or near the surface of the photoexcited semiconductor, a limited number of studies have been reported describing the influence of common ions and the role of the surface area on the process.6 Recent reports have shown that the photoactivity of semiconductor catalysts is dependent on the type and size of the particles.7 The majority of studies describing the use of photocatalysis for remediation of pollutants involve the use of † ‡
Department of Chemistry. Department of Geology.
(1) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (2) (a) Kamat, P. V. Chem. Rev. 1993, 93, 267. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Ollis, D. F.; Pelizzetti E.; Serpone, N. Environ. Sci. Technol. 1991, 25, 1523. (4) For an excellent compilation of recent studies, see: Photocatalytic Purification and Treatment of Water and Air; Ollis, D. F., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (5) Prairie, M. R.; Evans, L. R.; Strange, B. M.; Martinez, S. L. Environ. Sci. Technol. 1993, 27, 1776. (6) (a) Abdullah, M.; Low, G.; Matthews, R. W. J. Phys. Chem. 1990, 94, 6820. (b) Tanaka, K.; Capule, M.; Hisanaga, T. Chem. Phys. Lett 1991, 187, 73. (c) Herrmann, J.-M.; Guillard, C.; Pichat, P. Catal. Today 1993, 17, 7. (d) Cunningham, J.; Al-Sayyed, G.; Somkait, S. Aquat. Surf. Photochem. 1994, 317 and references within. (7) (a) Muller, B. R.; Majoni, S.; Memming, R.; Meissner, D. J. Phys. Chem. 1997, 101 (1), 2501. (b) Gerisher, H. Electrochim. Acta 1995, 40, 1277.
TiO2 particles in aqueous media as a slurry. The slurries are generally pretreated with sonication to produce a uniform suspension and mechanically stirred during the photocatalytic process. Johnston and Hocking attributed the acceleration of degradation rates in the TiO2 photocatalytic degradation of pollutants in wastewater with concurrent application of powerful (destructive) sonolysis to cavitational and mass transport effects and sonochemically initiated reactions.8 Our group has reported an enhancement in the rate of photodegradation can be achieved by applying nondestructive sonication during the TiO2 photocatalysis of dimethyl methylphosphonate.9 A plausible explanation for the observed acceleration under the condition of our studies is an enhancement of the adsorption-desorption processes such that the products are desorbed more readily under sonication, increasing catalyst turnover. Alternatively, agglomeration (coagulation) of the catalyst occurs during photolysis and is enhanced by mechanical stirring. Sonication should inhibit coagulation of the photocatalyst and maintain the high surface area, small particle sizes, number of available active sites, and light scattering with respect to the catalyst. Highly oxidized species, such as carboxylic and mineral acids, are produced during photocatalysis and can absorb onto the surface of the catalyst, blocking actives sites and/ or promoting coagulation of individual particles and thus reducing the total surface area and the number of actives sites of the available catalyst. The adsorption and coagulation processes are influenced by the suspension pH, ultimately affecting the rate of the photocatalytic reaction. Since TiO2 photocatalysis of organic pollutants produces (8) Johnston, A. J.; Hocking, P. In Emerging Technologies in Hazardous Waste Management III; Tedder, D. W., Pohland, F. G., Eds.; ACS Symposium Series 518; American Chemical Society: Washington, DC, 1993; p 106. (9) O’Shea K. E.; Aguilar M.; Garcia, I. Res. Chem. Intermed. 1997, 23, 325.
10.1021/la9806808 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/24/1999
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highly oxidized species, including low-molecular-weight organic acids and mineralization products, we chose to study the effects of Cl-, SO42-, PO43-, HCO2-, and C2O42on the coagulation of TiO2. The pH was varied over a wide range to evaluate the influence of the ion speciation on the process. The effects of common inorganic and organic anions, pH, and stirring on the coagulation of TiO2 photocatalyst are also reported. Experimental Materials and Methods Chemicals. Sodium chloride, sodium sulfate, and sodium oxalate were of analytical reagent grade from Fisher Chemical Co. Sodium phosphate monobasic was of analytical reagent grade from Mallinckrodt Chemical Works Co., and sodium formate was of analytical reagent grade from Baker Analyzed Reagent Co. The solutions were prepared with 18 MΩ Millipore water. The titanium dioxide was donated by Degussa. The surface area of the catalyst was ∼50 m2/g, determined on a NOVA BET surface area analyzer. Manufacturer specifications for the P25 TiO2 used for these experiments (P25, lot number 1118) are a BET surface area of 50 ( 15 m2/g, with an average particle size of 30 nm. The specific catalyst has an isoelectric point at a pH value between pH 6 and 7 according to manufacturer specifications and several published reports.10 Apparatus. The measurements of pH were performed with a Corning pH meter 245. The sizes (radius, nm) of TiO2 particlesagglomerates in suspension were determined using photon correlation spectroscopy (PCS). PCS relies on a photon-counting detector which can rapidly track changes of light (photons) intensity resulting from scattering of light by particles in aqueous suspension. The PCS apparatus was a Malvern 4700 system, employing a Coherent Innova 90 laser. Measurements were made at 488 nm with a 90° scattering angle. The precision silica cylindrical optical cell was immersed in the thermostated (25.0 ( 0.05°) index matching liquid (distilled water contained within the cell head). The PCS equipment used for our experiments was calibrated using microspheres of polystrene latex samples prepared in 1 mM KNO3. The particle size and distribution were obtained using the cumulant analysis as described elsewhere.11 General Procedure for Solution Preparation and Measurement. Inorganic and organic ions were studied at two different concentrations (0.06 and 0.01 M) in aqueous suspensions of TiO2 (10 mg/L). The electrolyte solutions were prepared in volumetric glassware using 18 MΩ Millipore water. Powdered TiO2 was added to the electrolyte solution, and the resulting suspension was sonicated (15 W) for approximately 5 min. The pH was adjusted by adding the conjugate acid or base of the corresponding substrate. An aliquot of the resulting suspension was transferred and sonicated for an additional minute prior to measurement of the particle size. Low-energy sonication of the TiO2 suspension was performed with a VC 60 Sonic & Materials 20 kHz instrument equipped with a half inch horn, which was submerged in the slurry. The solution was immediately transferred to a quartz cuvette for particle size analysis, and the measurement taken 1 min after sonication was terminated at given time intervals. The particle sizes were measured as a function of time and typically ranged from an initial size of ∼100 nm which rapidly grew to 1000 nm. The number of TiO2 particles in suspension was calculated from the experimentally obtained radius of TiO2 particles, and the aggregate size distribution is assumed to be Smoluchowskian in order to analyze the data.12 (10) The pHzpc for the Degussa P25 TiO2 is known to be between pH 6 and 7 according to the manufacturer and several published reports, and the slight variations within this range are likely the result of different techniques and/or the experimental conditions used to determine the zpc. (a) Foissy, A.; M′pandou, A.; Lamarche, J.; JaffrezicRenault, N. Collids Surf. 1982, 5, 363. (b) Jaffrezic-Renault, N.; Pichat, P.; Foissy, A.; Mercier, R. J. Phys. Chem. 1986, 90, 2733. (11) Herrington, T. M.; Midmore, B. R. Powder Technol. 1991, 65, 251. (12) The shape of the TiO2 P25 catalyst by Degussa is spheroidal, as measured by electron microscopy by the manufacturer. Analogous applications of Smoluchowski’s theorem have been experimentally verified using rapidly aggregating hydrophobic suspensions, including TiO2.11
O’Shea et al.
Theory Kinetics of Coagulation.11-13 The rate constant for the coagulation process k is indicative of the changes in surface area of TiO2 and is expected to influence the photocatalyst’s activity. The rates of coagulation were estimated from the change of particle size as a function of time. The particle size was measured at specific times using photon correlation spectroscopy (PCS), as detailed in the Experimental Section. The number of particles at the initial time can be estimated by
Ni ) [mVaNA]/M
(1)
where m is the mass concentration of TiO2, M is the molar mass of TiO2, Va is the volume of the aliquot, and NA is Avogadro’s number. Spherical shape and uniformity of TiO2 particles were assumed for the sake of model simplicity,12 such that the volume (size) of the particle can be represented by
V ) 4/3πr3
(2)
where r is the particle radius. The number of particles at a specified time Nt can be expressed in terms of particle volumes, such that
Nt ) Ni/[Vt/Vi]
(3)
where Vt represents the volume of the particle at a given time and Vi is the particle volume at the initial time. By substituting (2) into (3), the number of particles at a given time can be written as
Nt ) Ni/[rt/ri]3
(4)
where rt is the average radius of TiO2 particles at a given time and ri is the average radius of TiO2 particles at the initial time. The rate of particle coagulation depends on the frequency of collisions. According to Stumm and Morgan,13 the time-dependent decrease in the concentration of particles (coagulation) in a monodisperse suspension promoted by collisions can be expressed as a second-order rate law
dNt/dt ) kNt2
(5)
where k is the rate constant of coagulation [(particle number/volume)-1 (minute)-1]. Integration of (5) leads to
1/Nt ) 1/N0 + kt
(6)
The coagulation of TiO2 particles is consistent with a second-order process if a linear relationship is observed for the plot of 1/Nt versus time. The slope represents the rate constant of the coagulation process k and can be used to compare the coagulation of TiO2 particles under a variety of conditions. Results and Discussion Control experiments were conducted on suspensions of TiO2 (10 mg/L) without added electrolytes to obtain rates of coagulation as a function of pH, as shown in Figure 1. The pHzpc of the specific TiO2 catalyst used in these experiments is known to be between pH 6 and 7.10 The solution pH was adjusted by adding minimal amounts of (13) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley & Son Inc.: New York, 1981; Chapter 10.
Coagulation of TiO2 Photocatalyst
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Figure 1. Control experiment: Rate of coagulation as a function of pH (TiO2, 10 mg/L).
acid or base in order to minimize changes in the ionic strength (
