Degradation of organic compounds in photocatalytic membrane reactors has been one of the first application in the combination of photocatalysis and membranes. The possibility of using a membrane photoreactor for degradation of organics, developing a hybrid system in which the photocatalytic reaction and the separation of the desired product occur in only one device is of high interest. Some results obtained in this application are reported in the following. The ability of cross-flow ultrafiltration (UF), combined with photocatalytic reactions, to separate TiO2 photocatalysts from treated water in photocatalytic drinking water treatment was investigated by Lee et al (2001). The effect of natural organic matter (i.e., humic acids) and cross-flow velocities on UF fluxes and organic removal was explored with and without UV irradiation in the photocatalytic reactor. The interaction between the two solutes in the system, humic acids and TiO2 photocatalysts, played a significant role in the formation of dense cake layers at the membrane surface, leading to a greater flux decline during ultrafiltration of TiO2 particles. According to visual observations of the used membranes and the estimation of back-transport velocities of the solutes, a substantial amount of TiO2 deposited on the membrane induced a higher amount of humic acids to accumulate at the membrane through the adsorption onto TiO2 particles. The humic-acid-laden TiO2 particles offered more than four times higher specific cake resistance with a substantially increased compressibility coefficient than TiO2 particles alone. The higher the cross-flow velocities, the greater the UV254 removal achieved. This was because the rise of cross-flow velocities contributed to the reduction of concentration polarization at the membrane surface, thereby resulting in a decrease of the driving force for humic acids to pass through the membrane. When photocatalytic reactions took place with UV illumination, UV254 removal efficiencies of the permeate were improved markedly, and also the permeate flux was kept at a constant level without any sign of fouling. Although humic acids were not completely mineralized by photocatalysis, the degradation of the humic acids helped to enhance the UF flux, as they were transformed to less adsorbable compounds. A photocatalytic reaction system, composed of solution and gas spaces divided by a thin Teflon film and TiO2-coated mesh or cloth, for the treatment of contaminated aqueous solutions was developed to be operated with enhanced aeration without bubbling of air in the solution (Villacres et al 2003). First, the photocatalytic activities of TiO2 particles immobilized on two kinds of support material, stainless steel mesh (SSM) and fiberglass cloth (FGC), were investigated for photocatalytic oxidation of 2-propanol, as a model volatile organic compound, dissolved in aerated aqueous solution. The TiO2 particles immobilized on both support materials exhibited photocatalytic activity to oxidize 2-propanol into acetone and carbon dioxide (CO2), and the activity levels of the TiO2 particles immobilized on the two kinds of support materials were comparable. Presumably due to the presence of a small amount of metal species originating in SSM that might work as reduction catalysts, molecular hydrogen H2 was also liberated on the TiO2-immobilized SSM. Results of analysis of weight loss after photoirradiation suggested that the stability of the TiO2-immobilized FGC was better than that of the TiO2-immobilized SSM. On the basis of these results, FGC was employed in construction of a photocatalytic reactor equipped with an O2permeable Teflon membrane in order to make oxygen pass from a gas space to a solution space and to keep the surface of the immobilized TiO2 photocatalyst, facing an aqueous solution containing volatile organic compounds, saturated with dissolved O2. From the results of photocatalytic oxidative decomposition of 2-propanol, it was clarified that the surfaces of TiO2 particles could be sufficiently supplied with O2 from the gas space through the membrane to accelerate the oxidation. Dyes are organic compounds used in textile, food and drug industries, and their abatement represents one of the main problems in the treatment processes because generally they are very stable toxic compounds. Two commercial azo-dyes, i.e. Congo Red (C32H22N6Na2O6S2) and Patent Blue (C27H31N2NaO6S2), in aqueous solution were degraded in a photocatalytic membrane reactor (Molinari et al 2004) by using TiO2 Degussa P25 as the catalyst. Different system configurations and irradiating sources were studied, and the influence of some operational parameters such as the pressure in the membrane cell and the initial concentration of the substrates were determined. A comparison between suspended and entrapped TiO2 was also done. The experimental results showed a satisfactory degradation efficiency of the photocatalytic membrane process. The influence of various parameters (e.g. feed concentration, recirculation rate) has been discussed to obtain high reaction rates, operating stability and high membrane rejection, both for substrates and by-products. Congo Red was photodegraded with higher rate under the same experimental conditions probably due to its higher adsorption onto the catalyst surface. It was possible to treat successfully highly concentrated solutions (500 mg/L) of both dyes by means of a continuous process obtaining good values of permeate fluxes (30-70 L/m2h); this could be interesting for industrial applications. The reactor containing the suspended photocatalyst was significantly more efficient than the reactor containing the catalyst entrapped into the membrane. An advanced oxidation process (AOP) with reactor capacity of 150 L, using ultraviolet (UV) radiation and TiO2 photocatalyst, was evaluated for the destruction of toxic organic chemical, bisphenol A (BPA) by Thiruvenkatachari et al (2005). TiO2, in the form of powder, was suspended as slurry in the water avoiding the commonly adopted practice of immobilizing it onto a carrier material such as glass, concrete or ceramics. Adsorption of BPA by TiO2 was evaluated and was performed as a pretreatment to AOP. The combined effect of ozone with the AOP process was also studied. Coupling ozone with UV/TiO2, brought about a synergistic effect on BPA degradation and 10 ppm of BPA and the intermediate organic compounds were completely removed within three hours. The highlight of this study was the simultaneous degradation of BPA and separation of TiO2 particles from water after the photocatalytic treatment, in order to obtain reusable quality water. Separation of TiO2 particles was carried out by a unique two stage coagulation and settling process followed by submerged hollow fiber microfiltration membrane technique. With initial turbidity of 4,000 NTU, the turbidity of the final permeate water was well below 0.1 NTU. Some of the main advantages of this hybrid treatment system include the possibility of large scale treatment, complete and efficient BPA and its organic intermediates degradation, easy separation and reuse of TiO2 that resulted free from chemical coagulant contaminants, reusable quality water and potentiality for continuous operation with simple process modifications. A study of the photodegradation of different pharmaceuticals [furosemide, ranitidine (hydrochloride), ofloxacine, phenazone, naproxen, carbamazepine and clofibric acid] in aqueous medium at various pHs by using a batch photoreactor and a photocatalytic membrane reactor working in recirculation regime was carried out by Molinari et al (2006). Polycrystalline TiO2 was used as the photocatalyst, and different membranes (NTR 7410, PAN GKSS HV3/ T, N 30 F, NF PES 10) were tested. A different adsorption of the substrates onto the catalyst surface was observed owing to the hydrophilic/ hydrophobic character of the catalyst, depending on the pH. The photodegradation of the seven molecules in the batch reactor was successfully carried out and the behaviour was in accordance with pseudo-first order kinetics. Furosemide and ranitidine were selected to carry out the study of rejection and photodegradation in the hybrid membrane system. The permeate flux of the treated water was in the 31.5-60.0 L/(hm2) range for NTR 7410 membrane, whereas rejection values in the range 10-60% for furosemide and 5-30% for ranitidine in the dark (without photoreaction) were found. The degradation in the hybrid membrane photoreactor showed that the photocatalyst was retained by the membrane in the reaction ambient, while the membrane rejection towards the pollutants was not very satisfactory. A net decrease of the rejection down to 0 was observed in the contemporary presence of light, photocatalyst and oxygen. The combination of a dialysis membrane and a photocatalytic reactor into an original membrane photoreactor (MPR) to mineralize organic compounds contained in artificial turbid waters which were obtained by using natural clay named bentonite was studied by Azrague et al (2007). Various systems have been described in the literature, combining photocatalysis with pressure-driven membrane techniques, such as nanofiltration (NF) and ultrafiltration (UF), but these systems can lead to membrane fouling. Only the combination of photocatalysis and membrane distillation avoids this problem, but it needs energy to reach pervaporation phenomena. The MPR system worked at ambient temperature, with the membrane used as a barrier for particles and to extract the organic compounds from the turbid water without transmembrane pressure. Thus it was possible to separate the polluted turbid water from the photoreactor compartment and to separate TiO2 continuously from the treated water. The photocatalytic reaction and dialysis were studied separately before the MPR process was developed. A model pollutant, 2,4-dihydroxybenzoic acid (2,4-DHBA), was mineralized from turbid waters by photocatalysis. By means of the membrane, the TiO2 remained in the photoreactor compartment without filtration and bentonite was kept away from the photoreactor. The degradation of toxic organic compounds such as trichloroethylene (TCE) in water, using a combined photocatalysis/microfiltration (MF) system was focused by Choo et al (2008). The performances of the hybrid system were investigated in terms of the removal efficiency of TCE and membrane permeability, in the presence or absence of humic acids and at different pHs. The fate of TCE during the photocatalytic reactions was checked in order to evaluate the feasibility of the photocatalytic membrane reactor (PMR). Greater TCE degradation (>60%) was achieved with an increase in the TiO2 dosage (up to 1.5 g/L) in PMR, but a substantially large TiO2 dosage brought about a decrease in TCE degradation efficiency. The photocatalytic decomposition of TCE appeared to be more effective at acidic pH conditions than at a neutral or alkaline pH. The alkalinity and the addition of humic acids into the feedwater did not produce a significant effect on TCE degradation, while humic acids (whose dose was 1 mg/L as TOC) in the feedwater played a role in a decline of permeability by 60%. Membrane permeability in the PMR was also affected by tangential velocities. An improvement of 60% in flux was achieved when the tangential velocity increased from 0.19 to 1.45 m/s. This occurred because flow regimes can govern the deposition of TiO2 particles on the membrane surface. References Azrague K, Aimar P, Benoit-Marquie F, Maurette MT(2007) A new combination of a membrane and a photocatalytic reactor for the depollution of turbid water. Applied Catalysis B: Environmental 72:197-204 Choo KH, Chang DI, Park KW, Kim MH (2008) Use of an integrated photocatalysis/hollow fiber microfiltration system for the removal of trichloroethylene in water. Journal of Hazardous Materials 152:183-190 Lee SA, Choo KH, Lee CH, Lee HI, Hyeon T, Choi W, Kwon HH (2001)Use of ultrafiltration membranes for the separation of TiO2 photocatalysts in drinking water treatment. Industrial & Engineering Chemistry Research 40:1712-1719 Molinari R, Pirillo F, Falco M, Loddo V, Palmisano L (2004) Photocatalytic degradation of dyes by using a membrane reactor. Chemical Engineering and Processing 43:1103-1114 Molinari R, Pirillo F, Loddo V, Palmisano L (2006) Heterogeneous photocatalytic degradation of pharmaceuticals in water by using polycrystalline TiO2 and a nanofiltration membrane reactor. Catalysis Today 118:205-213 Thiruvenkatachari R, Kwon TO, Moon IS (2005) A total solution for simultaneous organic degradation and particle separation using photocatalytic oxidation and submerged microfiltration membrane hybrid process. Korean Journal of Chemical Engineering 22:938-944 Villacres R, Ikeda S, Torimoto T, Ohtani B (2003) Development of a novel photocatalytic reaction system for oxidative decomposition of volatile organic compounds in water with enhanced aeration. Journal of Photochemistry and Photobiology A: Chemistry 160:121-126
Photocatalytic membrane reactor in the degradation of organic compounds
MOLINARI, Raffaele
2013-01-01
Abstract
Degradation of organic compounds in photocatalytic membrane reactors has been one of the first application in the combination of photocatalysis and membranes. The possibility of using a membrane photoreactor for degradation of organics, developing a hybrid system in which the photocatalytic reaction and the separation of the desired product occur in only one device is of high interest. Some results obtained in this application are reported in the following. The ability of cross-flow ultrafiltration (UF), combined with photocatalytic reactions, to separate TiO2 photocatalysts from treated water in photocatalytic drinking water treatment was investigated by Lee et al (2001). The effect of natural organic matter (i.e., humic acids) and cross-flow velocities on UF fluxes and organic removal was explored with and without UV irradiation in the photocatalytic reactor. The interaction between the two solutes in the system, humic acids and TiO2 photocatalysts, played a significant role in the formation of dense cake layers at the membrane surface, leading to a greater flux decline during ultrafiltration of TiO2 particles. According to visual observations of the used membranes and the estimation of back-transport velocities of the solutes, a substantial amount of TiO2 deposited on the membrane induced a higher amount of humic acids to accumulate at the membrane through the adsorption onto TiO2 particles. The humic-acid-laden TiO2 particles offered more than four times higher specific cake resistance with a substantially increased compressibility coefficient than TiO2 particles alone. The higher the cross-flow velocities, the greater the UV254 removal achieved. This was because the rise of cross-flow velocities contributed to the reduction of concentration polarization at the membrane surface, thereby resulting in a decrease of the driving force for humic acids to pass through the membrane. When photocatalytic reactions took place with UV illumination, UV254 removal efficiencies of the permeate were improved markedly, and also the permeate flux was kept at a constant level without any sign of fouling. Although humic acids were not completely mineralized by photocatalysis, the degradation of the humic acids helped to enhance the UF flux, as they were transformed to less adsorbable compounds. A photocatalytic reaction system, composed of solution and gas spaces divided by a thin Teflon film and TiO2-coated mesh or cloth, for the treatment of contaminated aqueous solutions was developed to be operated with enhanced aeration without bubbling of air in the solution (Villacres et al 2003). First, the photocatalytic activities of TiO2 particles immobilized on two kinds of support material, stainless steel mesh (SSM) and fiberglass cloth (FGC), were investigated for photocatalytic oxidation of 2-propanol, as a model volatile organic compound, dissolved in aerated aqueous solution. The TiO2 particles immobilized on both support materials exhibited photocatalytic activity to oxidize 2-propanol into acetone and carbon dioxide (CO2), and the activity levels of the TiO2 particles immobilized on the two kinds of support materials were comparable. Presumably due to the presence of a small amount of metal species originating in SSM that might work as reduction catalysts, molecular hydrogen H2 was also liberated on the TiO2-immobilized SSM. Results of analysis of weight loss after photoirradiation suggested that the stability of the TiO2-immobilized FGC was better than that of the TiO2-immobilized SSM. On the basis of these results, FGC was employed in construction of a photocatalytic reactor equipped with an O2permeable Teflon membrane in order to make oxygen pass from a gas space to a solution space and to keep the surface of the immobilized TiO2 photocatalyst, facing an aqueous solution containing volatile organic compounds, saturated with dissolved O2. From the results of photocatalytic oxidative decomposition of 2-propanol, it was clarified that the surfaces of TiO2 particles could be sufficiently supplied with O2 from the gas space through the membrane to accelerate the oxidation. Dyes are organic compounds used in textile, food and drug industries, and their abatement represents one of the main problems in the treatment processes because generally they are very stable toxic compounds. Two commercial azo-dyes, i.e. Congo Red (C32H22N6Na2O6S2) and Patent Blue (C27H31N2NaO6S2), in aqueous solution were degraded in a photocatalytic membrane reactor (Molinari et al 2004) by using TiO2 Degussa P25 as the catalyst. Different system configurations and irradiating sources were studied, and the influence of some operational parameters such as the pressure in the membrane cell and the initial concentration of the substrates were determined. A comparison between suspended and entrapped TiO2 was also done. The experimental results showed a satisfactory degradation efficiency of the photocatalytic membrane process. The influence of various parameters (e.g. feed concentration, recirculation rate) has been discussed to obtain high reaction rates, operating stability and high membrane rejection, both for substrates and by-products. Congo Red was photodegraded with higher rate under the same experimental conditions probably due to its higher adsorption onto the catalyst surface. It was possible to treat successfully highly concentrated solutions (500 mg/L) of both dyes by means of a continuous process obtaining good values of permeate fluxes (30-70 L/m2h); this could be interesting for industrial applications. The reactor containing the suspended photocatalyst was significantly more efficient than the reactor containing the catalyst entrapped into the membrane. An advanced oxidation process (AOP) with reactor capacity of 150 L, using ultraviolet (UV) radiation and TiO2 photocatalyst, was evaluated for the destruction of toxic organic chemical, bisphenol A (BPA) by Thiruvenkatachari et al (2005). TiO2, in the form of powder, was suspended as slurry in the water avoiding the commonly adopted practice of immobilizing it onto a carrier material such as glass, concrete or ceramics. Adsorption of BPA by TiO2 was evaluated and was performed as a pretreatment to AOP. The combined effect of ozone with the AOP process was also studied. Coupling ozone with UV/TiO2, brought about a synergistic effect on BPA degradation and 10 ppm of BPA and the intermediate organic compounds were completely removed within three hours. The highlight of this study was the simultaneous degradation of BPA and separation of TiO2 particles from water after the photocatalytic treatment, in order to obtain reusable quality water. Separation of TiO2 particles was carried out by a unique two stage coagulation and settling process followed by submerged hollow fiber microfiltration membrane technique. With initial turbidity of 4,000 NTU, the turbidity of the final permeate water was well below 0.1 NTU. Some of the main advantages of this hybrid treatment system include the possibility of large scale treatment, complete and efficient BPA and its organic intermediates degradation, easy separation and reuse of TiO2 that resulted free from chemical coagulant contaminants, reusable quality water and potentiality for continuous operation with simple process modifications. A study of the photodegradation of different pharmaceuticals [furosemide, ranitidine (hydrochloride), ofloxacine, phenazone, naproxen, carbamazepine and clofibric acid] in aqueous medium at various pHs by using a batch photoreactor and a photocatalytic membrane reactor working in recirculation regime was carried out by Molinari et al (2006). Polycrystalline TiO2 was used as the photocatalyst, and different membranes (NTR 7410, PAN GKSS HV3/ T, N 30 F, NF PES 10) were tested. A different adsorption of the substrates onto the catalyst surface was observed owing to the hydrophilic/ hydrophobic character of the catalyst, depending on the pH. The photodegradation of the seven molecules in the batch reactor was successfully carried out and the behaviour was in accordance with pseudo-first order kinetics. Furosemide and ranitidine were selected to carry out the study of rejection and photodegradation in the hybrid membrane system. The permeate flux of the treated water was in the 31.5-60.0 L/(hm2) range for NTR 7410 membrane, whereas rejection values in the range 10-60% for furosemide and 5-30% for ranitidine in the dark (without photoreaction) were found. The degradation in the hybrid membrane photoreactor showed that the photocatalyst was retained by the membrane in the reaction ambient, while the membrane rejection towards the pollutants was not very satisfactory. A net decrease of the rejection down to 0 was observed in the contemporary presence of light, photocatalyst and oxygen. The combination of a dialysis membrane and a photocatalytic reactor into an original membrane photoreactor (MPR) to mineralize organic compounds contained in artificial turbid waters which were obtained by using natural clay named bentonite was studied by Azrague et al (2007). Various systems have been described in the literature, combining photocatalysis with pressure-driven membrane techniques, such as nanofiltration (NF) and ultrafiltration (UF), but these systems can lead to membrane fouling. Only the combination of photocatalysis and membrane distillation avoids this problem, but it needs energy to reach pervaporation phenomena. The MPR system worked at ambient temperature, with the membrane used as a barrier for particles and to extract the organic compounds from the turbid water without transmembrane pressure. Thus it was possible to separate the polluted turbid water from the photoreactor compartment and to separate TiO2 continuously from the treated water. The photocatalytic reaction and dialysis were studied separately before the MPR process was developed. A model pollutant, 2,4-dihydroxybenzoic acid (2,4-DHBA), was mineralized from turbid waters by photocatalysis. By means of the membrane, the TiO2 remained in the photoreactor compartment without filtration and bentonite was kept away from the photoreactor. The degradation of toxic organic compounds such as trichloroethylene (TCE) in water, using a combined photocatalysis/microfiltration (MF) system was focused by Choo et al (2008). The performances of the hybrid system were investigated in terms of the removal efficiency of TCE and membrane permeability, in the presence or absence of humic acids and at different pHs. The fate of TCE during the photocatalytic reactions was checked in order to evaluate the feasibility of the photocatalytic membrane reactor (PMR). Greater TCE degradation (>60%) was achieved with an increase in the TiO2 dosage (up to 1.5 g/L) in PMR, but a substantially large TiO2 dosage brought about a decrease in TCE degradation efficiency. The photocatalytic decomposition of TCE appeared to be more effective at acidic pH conditions than at a neutral or alkaline pH. The alkalinity and the addition of humic acids into the feedwater did not produce a significant effect on TCE degradation, while humic acids (whose dose was 1 mg/L as TOC) in the feedwater played a role in a decline of permeability by 60%. Membrane permeability in the PMR was also affected by tangential velocities. An improvement of 60% in flux was achieved when the tangential velocity increased from 0.19 to 1.45 m/s. This occurred because flow regimes can govern the deposition of TiO2 particles on the membrane surface. References Azrague K, Aimar P, Benoit-Marquie F, Maurette MT(2007) A new combination of a membrane and a photocatalytic reactor for the depollution of turbid water. Applied Catalysis B: Environmental 72:197-204 Choo KH, Chang DI, Park KW, Kim MH (2008) Use of an integrated photocatalysis/hollow fiber microfiltration system for the removal of trichloroethylene in water. Journal of Hazardous Materials 152:183-190 Lee SA, Choo KH, Lee CH, Lee HI, Hyeon T, Choi W, Kwon HH (2001)Use of ultrafiltration membranes for the separation of TiO2 photocatalysts in drinking water treatment. Industrial & Engineering Chemistry Research 40:1712-1719 Molinari R, Pirillo F, Falco M, Loddo V, Palmisano L (2004) Photocatalytic degradation of dyes by using a membrane reactor. Chemical Engineering and Processing 43:1103-1114 Molinari R, Pirillo F, Loddo V, Palmisano L (2006) Heterogeneous photocatalytic degradation of pharmaceuticals in water by using polycrystalline TiO2 and a nanofiltration membrane reactor. Catalysis Today 118:205-213 Thiruvenkatachari R, Kwon TO, Moon IS (2005) A total solution for simultaneous organic degradation and particle separation using photocatalytic oxidation and submerged microfiltration membrane hybrid process. Korean Journal of Chemical Engineering 22:938-944 Villacres R, Ikeda S, Torimoto T, Ohtani B (2003) Development of a novel photocatalytic reaction system for oxidative decomposition of volatile organic compounds in water with enhanced aeration. Journal of Photochemistry and Photobiology A: Chemistry 160:121-126I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
