ARTICLE pubs.acs.org/IECR
Photocatalytic Membrane Contactors for Water Treatment Izumi Kumakiri,*,† Spyros Diplas,† Christian Simon,† and Pawel Nowak‡ † ‡
SINTEF, P.O. Box 124 Blindern, NO-0314 Oslo, Norway Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 Krakow, Poland ABSTRACT: A three-phase catalytic membrane contactor (CMC) was applied in the photocatalytic degradation of organic substances in water. Particles of commercial TiO2 (P25 and ST01) were deposited on porous ceramic materials. Platinum was deposited onto TiO2 by the photoreduction method. The activity of the photo-CMC was studied with respect to removing from water model contaminants such as formic acid, oxalic acid, and humic acid. The influence of contacting nitrogen and oxygen with water through the membrane on the efficiency of the removal of those contaminants was investigated. The highest degradation rate was observed under UV (black light) irradiation with simultaneous oxygen supply. A combination of Pt with a photocatalyst improved the lifetime of Pt, as the photocatalysis maintains Pt in the metallic state by its reductive action and prevents Pt poisoning due to the generation of reactive oxygen species. No membrane fouling was observed under the conditions studied. The initial catalytic activity was preserved during several months of the tests and several months of storage in air.
1. INTRODUCTION Requirements related to the water treatment technologies depend on the water origin and quality, which is a challenge. In Norway, surface water is a common source for the production of potable water and the removal of natural organic matters (NOMs) is of primary importance.1,2 While the major part of the NOMs can be removed by coagulation or by filtration, small to medium sized NOMs are difficult to remove completely. NOMs give water yellowish to brown color and can form carcinogens by reacting with chlorine,3 which is a conventional chemical applied for the purpose of disinfection during potable water production. Water pollution by persistent substances is another growing concern as slowly biodegradable and nonbiodegradable pollutants remain in the environment and spread to wider areas. Some substances are toxic even at small concentrations on the parts per million or even parts per billion level. There are several regulations, such as the Stockholm convention on persistent organic pollutants (POPs),4 aiming toward reducing the discharge of toxic chemicals to the environment. Persistent chemicals are widespread in the environment and can be found in surface water and groundwater in different places.57 Simple, robust, economical, and environmentally friendly water treatment technology is a key for sustainable development. The wet air oxidation (WAO) and catalytic wet air oxidation (CWAO) processes are applied to treat industrial effluents with low to medium concentrations of nonbiodegradable substances. The process requires high temperatures (120300 °C) and pressures (5200 bar)8,9 associated with acute reactor corrosion problems. Photocatalysis is an interesting alternative process that can remove organic substances at almost ambient temperature and pressure by oxidation.10,11 Titanium dioxide is the most frequently and widely studied photocatalyst for water treatment and is often applied as a slurry. One of the advantages of a slurrytype reactor is the large catalytic surface area available. It, however, requires extra facilities to separate photocatalysts from treated water. Therefore, the immobilization of the photocatalyst r 2011 American Chemical Society
on a supporting material has been considered to circumvent this problem. There are major problems associated with the immobilized photocatalyst. These comprise mainly the insufficiently strong catalystsupport adhesion and the difficulties in designing a reactor having a high ratio of surface area per volume of water and with a sufficiently intensive mass transport. Several photocatalytic modules are commercially available today.12 A module, consisting of photocatalytic fibers made by a “bleed out” technique,1315 gives an interesting solution toward achieving high surface area and strong adhesion between photocatalysts and supporting material. Oxygen is consumed during the oxidation of organics in water; however, the solubility of oxygen in water is very low at ambient pressure. Therefore, oxygen transport can easily be the limiting step. Air or oxygen bubbling is often applied to increase the oxygen transport that subsequently increases the reaction rate.16,17 Photocatalytic degradation can also be enhanced by supplying oxidants. Synergy effect arising from the combination of the photocatalyst with oxidants, such as adding ozone18,19 and other chemicals,20,21 was also reported. Oxygen or oxidant should be supplied continuously to the photocatalysts as they are consumed during the photocatalytic reaction. An overdose will not only increase the operation costs but also may require posttreatment to eliminate the excess oxidant from clean water. To achieve a higher reaction rate with a lower amount of oxidant, an efficient contact of oxygen or oxidant, photocatalyst, and light is required. We have been working on catalytic membrane contactors (CMCs), where a porous membrane separates the water stream from the air stream.4,5 A liquidgas interface is maintained inside the asymmetric membrane by controlling the gas-side pressure higher than the liquid-side pressure as illustrated in Figure 1. Received: December 11, 2010 Accepted: March 23, 2011 Revised: March 15, 2011 Published: April 08, 2011 6000
dx.doi.org/10.1021/ie102470f | Ind. Eng. Chem. Res. 2011, 50, 6000–6008
Industrial & Engineering Chemistry Research
Figure 1. Schematic image of a photocatalytic membrane contactor. Phase 1, water containing organic substances; phase 2, oxidant flow, e.g., air or oxygen. Interface of two phases is maintained in the membrane by the over pressure at phase 2.
Gas molecules in phase 2 dissolve in liquid (phase 1) at the interface, diffuse to the catalytic surface, and are consumed by the catalytic reaction. The formed substances may diffuse to both phases. It has been shown that the CMC configuration facilitates the mass transport. For example, the improved oxygen transport could result in a higher oxidation rate especially at low temperature.22 We believe that the CMC configuration provides an interesting alternative from the reactor design point of view, especially in the case of an advanced oxidation process (AOP) where several oxidation processes are combined.21 In this paper, a catalytic membrane contactor was integrated with the UV radiation and a photocatalyst. The radiation was applied from the water side as shown in Figure 1. The catalytic activity of two commercial TiO2 powders was compared. PtTiO2 and oxygen were chosen as the model photocatalyst and oxidant, respectively. Photocatalytic membrane contactors were prepared and applied for the oxidation of organic substances in water in the dark and under UV radiation.
2. EXPERIMENTAL SECTION 2.1. Membrane Preparation. Two different types of support were used in this study: disk-shaped porous R-alumina supports (37 mm diameter, 4 μm pore size) which were provided by Keranor, Norway, and porous R-alumina tubes (250 mm length, 10 mm outer diameter, 12 μm pore size) provided by Pall-Exekia, France. Commercial TiO2 particles were mechanically deposited on the supports, followed by a heat treatment in air at 300 °C for 3 h. Two different TiO2 particles were compared: ST01 consisting of anatase phase with 7 nm primary particle size provided by Ishihara Sangyo Co., Japan, and P25 provided by EvonicDegussa, Germany, which was a mixture of anatase (e.g., 78%), rutile (e.g., 14%), and amorphous (e.g., 8%) phases.23 Pt was deposited by a photoreduction method using H2PtCl6 aqueous solution (Sigma-Aldrich, purity 99.9%, 0.025 mmol of Pt/L). After reduction, the membrane was washed with water. Mass changes of the membrane after the TiO2 deposition and upon the washing process were measured. As a reference, a membrane without any photocatalyst but with Pt catalyst was prepared by immersing the porous Ralumina tube in H2PtCl6 aqueous solution followed by reduction in hydrogen at 200 °C. The reference sample was named PtCMC. The tube was provided by Pall-Exekia having the same geometry as above, with additional mesoporous layers (50 and 800 nm pore sizes) on top of the coarse body (12 μm pore size). Details of this procedure may be found elsewhere.24
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Formed membranes were characterized by scanning electron microscopy and energy dispersive X-ray analysis (SEMEDX) using a Jeol, JSM-5900 LV microscope and X-ray photoelectron spectroscopy (XPS). Prior to the XPS measurements the samples after having been immersed in the H2PtCl6 aqueous solution were exposed to UV radiation for 2 and 6 h in the aqueous formic acid solution under constant N2 flow to attain the photoreduction conditions. XPS was performed on a KRATOS AXIS ULTRADLD spectrometer using Al KR (hν = 1486.6 eV) radiation. The spectra were acquired at 0° angle of emission in the hybrid mode (i.e., using both electrostatic and magnetic lenses). In the hybrid mode the area of analysis is determined by the slot aperture (approximately 700 μm 300 μm). Both survey and high resolution spectra were registered at pass energies of 160 and 20 eV, respectively. The analysis was performed under ultrahigh vacuum conditions with the pressure in the analysis chamber being 109 Torr. Charge compensation was achieved by using low energy electrons from a flood gun, and charge correction was done using the low energy component of the C 1s high resolution spectrum which was assigned to CC/CH at 285 eV. This energy correction resulted in the Ti 2p3/2 being positioned at 459.2. ( 0.1 eV, in accordance with the reported literature values for TiO 2.25 2.2. Catalytic Activity Tests. Batch tests were performed for the rapid evaluation of membranes in order to select the type of catalyst suitable for the membrane contactor application. The membrane was immersed in 20 g of stirred solution and placed under a UV lamp. Three black lamps (Sylvania, black light blue (BLB) 8 W, 356 nm as the main wavelength) were combined and used as the radiation source. The light intensity at the membrane surface was approximately 0.3 mW/cm2. Blank tests with support without any catalyst deposited were performed to evaluate the contribution of any process, such as evaporation or adsorption, to the concentration change. Figure 2 shows a schematic view of a membrane contactor setup. The tubular membrane was sealed with Viton O-rings. The membrane reactor was made in quartz glass to permit UV radiation to penetrate through. Four black lamps (Sylvania, black light blue (BLB) 8 W) were placed around the membrane reactor. Both the membrane reactor and the lamps were covered by a stainless-steel box to avoid the influence of other light sources. Oxygen gas was supplied to the inner side of the membrane with a flow rate of 60 NmL/min controlled by a digital flow meter. The gas side was pressurized up to 2 bar by a digital pressure controller. Aqueous solution of the studied acid was supplied to the outer side of the membrane with a controlled flow rate (1 L/min). The liquid after flowing over the outer side of the membrane flowed back to the feed tank. One liter of solution was used for each test. Experiments were performed at room temperature. Formic acid (FA; Fluka, 98%), oxalic acid (OA; Sigma-Aldrich, purity >99%) and humic acid sodium salt (HA; Sigma-Aldrich, technical grade) were dissolved in water to prepare model solutions of 1003800 mg/L (FA), 70 mg/L (OA), and 20 mg/L (HA), respectively, and were used as solutions. Humic acid has a broad molecular weight distribution with an estimated average molecular formula of C187H186O89N9S1.26 The concentration of humic acid was suitably selected in order to represent the humic substance concentration in potable water. The concentration change was measured by UVvis analysis (Shimadzu, UV-1800). 6001
dx.doi.org/10.1021/ie102470f |Ind. Eng. Chem. Res. 2011, 50, 6000–6008
Industrial & Engineering Chemistry Research
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Figure 2. Schematic images of the photocatalytic membrane contactor test rig (top) and location of black light lamps (bottom).
expressed by the Langmuir equation: Cm ¼ Csat
KCs 1 þ KCs
ð2Þ
In dilute solutions (KCs , 1), eq 2 becomes Cm ¼ KCsat Cs
ð2'Þ
and eq 1 can be written as V Figure 3. Schematic image of concentration gradient.
2.3. Model. The degradation of a substance by a photocatalytic membrane will create a boundary layer at the surface of the membrane as illustrated in Figure 3. The concentration change of the bulk solution can be described as follows:
V
dC ¼ AhðC Cs Þ ¼ AkCm dt
ð1Þ
where A is the surface area of the membrane, C is the bulk concentration, Cs is the concentration of the solution at the membrane surface, Cm is the concentration of the substance adsorbed at the membrane surface, h is the mass transfer coefficient, and k is the kinetic constant. Assuming that the photocatalytic reaction occurs with the substances adsorbed on the surface and assuming that the reaction occurs under kinetic control, which is an often used assumption, adsorption will be at equilibrium and could be
dC ¼ AhðC Cs Þ ¼ AkCm ¼ Ak0 Cs dt
ð1'Þ
With the initial conditions C = C0 at t = 0, eq 10 can be solved giving 1 0 C B At C B C C ¼ C0 expB A @ 1 1 þ 0 V h k
ð3Þ
When the kinetic constant is much smaller than the mass transfer coefficient (k , h), eq 3 can be simplified to Ak0 t C Ak0 ð4Þ C ¼ C0 exp f ln ¼ t V C0 V Accordingly, if we plot the logarithm of the concentration ratio, ln(C/Ct), against time, a linear relation should be obtained for the case of dilute solution. The reaction rate increases with bulk concentration, reaching a limitation at high concentration due to adsorption saturation. 6002
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Industrial & Engineering Chemistry Research
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Figure 4. SEM images of the (a) surface of the membrane porous support, (b) commercial TiO2 (ST01) on the porous support with Pt deposition, and (c) cross section of the PtTiO2 (ST01) on porous support. (d) EDX analysis of the area close to the surface of (b).
3. RESULTS AND DISCUSSION 3.1. Membrane Morphology. A significant stripping (e.g., >50%) of the TiO2 particles was often observed during the heat treatment process and during the testing of the catalytic activity, when the TiO 2 deposition was larger than 2 mg/cm2. By contrast, when the TiO 2 deposition was less than 2 mg/ cm2, the loss of TiO2 during the first flushing with water was less than 0.03 mg/cm2 . Mass changes in further washing were negligible. No traces of TiO2 were detected in the water used for washing as proved by the lack of absorption at 380 nm wavelength27 in the UVvis spectrum. These results suggest a strong adhesion of TiO2 particles to the support at a TiO2 deposition below 2 mg/cm2. Figure 4 shows the surface of the support before (Figure 4a) and after (Figure 4b) TiO2 deposition. The surface of the support was totally covered by TiO2 particles. The thickness of the TiO 2 layer was about 1 μm as observed by SEM in Figure 4c. A tubular membrane was soaked in water, and gas was supplied to the inner side of the membrane by increasing the pressure. No gas permeation to the water side was observed with a pressure difference between the gas side and the water side lower than 1 bar. The pore size of the TiO2 layer can be estimated from the
following equation:28 d ¼ 4σðcos θÞ=ΔP
ð5Þ
where σ is the surface tension of water, θ is the contact angle between water and membrane, and d is the pore diameter corresponding to the pressure difference across the membrane (ΔP). Assuming complete wetting (cos θ = 1), the pore size (d) of the TiO2 layer was estimated to be smaller than 1.5 μm. The support had pores of 12 μm in diameter, which corresponds to capillary pressure of 0.24 bar. Accordingly, applying an overpressure between 0.24 and 1 bar at the gas side will maintain the liquidgas interface just below the TiO2 layer in the membrane, as illustrated in Figure 1. Figure 4d shows the SEMEDX analysis of TiO2 membrane with Pt deposited by photoreduction. The mass gain of the membrane after Pt deposition was negligible (
