A photocatalytic process combined with membrane operations is a set of reduction and oxidation photocatalytic reactions occurring in the presence of a photocatalyst (see photocatalytic process) and a membrane giving rise to one or more products starting from organic or inorganic substrate(s) (see photocatalytic membrane reactors). Photocatalysis and, in particular, heterogeneous photocatalysis is a process of great potentiality for pollutants abatement. To improve the overall performance of the photoprocess, heterogeneous photocatalysis has been often combined with physical or chemical operations, which affect the chemical kinetics and/or the overall efficiency. Various possibilities to couple heterogeneous photocatalysis with other technologies to photodegrade organic and inorganic pollutants dissolved in actual or synthetic aqueous effluents have been described by Augugliaro et al. (2006). Thesecombinations increase the photoprocess efficiency by decreasing the reaction time with respect to the separated operations or they decrease the cost with respect to heterogeneous photocatalysis alone, generally in terms of light energy. Depending on the operation coupled with heterogeneous photocatalysis, two categories of combinations exist. When the coupling is with ultrasonic irradiation, photo-Fenton reaction, ozonation, or electrochemical treatment, the combination affects the photocatalytic mechanisms thus improving the efficiency of the photocatalytic process. When the coupling is with biological treatment, membrane reactor, membrane photoreactor, or physical adsorption, the combination does not affect the photocatalytic mechanisms but it improves the efficiency of the overall process. The choice of the coupling is related to the type of wastewater to be treated. A synergistic effect, giving rise to an improvement of the efficiency of the photocatalytic process, has been reported in the literature for many cases. The influence of the type of nanofiltration membrane, initial concentration of pollutant and pH on the photodegradation rate was investigated by Molinari et al. (2002) in discontinuousand continuous configurations in a photocatalytic membrane system with the lamp immersed in the suspension inside the photoreactor by using polycrystalline TiO2 (Degussa P25) as the catalyst and humic acids, organic dyes, 4-nitrophenol as the pollutants. Two membranes were tested, i.e. NF-PES-010 (Celgard, Germany) of polyethersulphone and NTR-7410 (Nitto-Denko, Japan) of sulphonated polyethersulphone. The last one was chosen for all of the photoreactivity experiments because permeability and rejection tests indicated that it was able to hold both catalyst and small molecules carrying the same membrane charge (negative), thanks to the Donnan Exclusion. Despite the fluxes ranged between 20 and 40 L h-1 m-2 in operating conditions at 6 bar and these values were interesting for application purposes, the rejections of NTR-7410 nanofiltration membrane, obtained during operation of the membrane photoreactor in the degradation of humic acids, patent blue dye and 4-nitrophenol, were significantly lower than those obtained in the absence of photodegradation, probably because of the small molecular size of the by-products and of the intermediate species produced during the photodegradation process. This means that in order to select a suitable membrane, rejection should be determined during operation of the photoreactor. A batch-recirculated photoreactor was combined with a hollow-fibre membrane ultrafiltration (UF) unit for heterogeneous photocatalysis applications.The operation and modelling of the UF process on the performance of the integrated photoreactor-UF process was studied by Sopajaree et al. (1999). Methyleneblue and titanium dioxide (Degussa P-25) were used as the test pollutant and photocatalyst, respectively. The influence of cross-flow velocity, transmembrane pressure, and TiO2 dose on the permeate flux through the hollow fibre membrane was described. The data were modelled on the basis of concentration polarization and gel layer formation at the membrane surface/feed slurry boundary. The photocatalytic degradation of natural organic matter (NOM) is an attractive option in the treatment of drinking water (Choo et al., 2008). Theperformance of a submerged photocatalytic membrane reactor (PMR) was investigated with regard to the removal of NOM and the control of membrane fouling. The efficiency of a hybrid system combining photocatalysis and membrane filtration in a single module was investigated. Low-pressure submerged hollow fiber membranes were used to retain the TiO2 particles in the system by Chin et al. (2007) duringthe photodegradation of Bisphenol A (BPA) as the model pollutant. Aeration was applied in the submerged membrane photoreactor (SMPR) to provide: i) mixing, ii) dissolved oxygen, iii) mechanical agitation (to prevent agglomeration of TiO2 particles), and iv) shear forces (to remove TiO2 particles from the membrane surface).It was found that intermittent permeation enhanced the sustainability of the submerged membranes but showed no effect on the photoactivity of the system. An intermittence aeration frequency (IF) of 0.1 was sufficient to reduce the fouling rate of the membrane under the experimental conditions. The SMPR appears to be very effective and can achieve removal of low-concentration organics (such as BPA) in a compact, low-energy system.The combined photocatalytic and membrane processes have been more recently addressed also towards the selective production of high value molecules in mild experimental conditions by using suitable photocatalysts, solvent and starting substrates and membranes (Molinari et al. 2009a; Molinari et al. 2009b, Augugliaro et al. 2010) References Augugliaro V,Litter M,Palmisano L,Soria J(2006) The combination of heterogeneous photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY C-PHOTOCHEMISTRY REVIEWS 7: 127-144. Augugliaro V, Palmisano L (2010) Green oxidation of alcohols to carbonyl compounds by heterogeneous photocatalysis (2010) ChemSusChem 3:1135–1138 Chin SS, Lim TM,Chiang K,Fane AG(2007) Hybrid low-pressure submerged membrane photoreactor for the removal of bisphenol A. DESALINATION 202: 253-261 Choo KH,Tao R, Kim MJ (2008) Use of a photocatalytic membrane reactor for the removal of natural organic matter in water: Effect of photoinduced desorption and ferrihydrite adsorption. JOURNAL OF MEMBRANE SCIENCE 322: 368-374 Molinari R,Borgese M, Drioli E,Palmisano L,Schiavello M(2002) Hybrid processes coupling photocatalysis and membranes for degradation of organic pollutants in water. CATALYSIS TODAY 75:77-85 Molinari R, Caruso A, Palmisano L (2009a) Photocatalytic membrane reactors in the conversion or degradation of organic compounds. In: Drioli E, Giorno L(eds) Membrane operations, Innovative separations and transformations.Wiley-VCH, Weinheim, pp 335-361 Molinari R, Caruso A, Poerio T (2009b) Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor.CATALYSIS TODAY 144: 81-86 Sopajaree K, Qasim SA,Basak S,Rajeshwar K(1999) An integrated flow reactor-membrane filtration system for heterogeneous photocatalysis. Part II: Experiments on the ultrafiltration unit and combined operation. JOURNAL OF APPLIED ELECTROCHEMISTRY 29: 1111-1118
Photocatalytic processes, membrane operations
MOLINARI, Raffaele
2013-01-01
Abstract
A photocatalytic process combined with membrane operations is a set of reduction and oxidation photocatalytic reactions occurring in the presence of a photocatalyst (see photocatalytic process) and a membrane giving rise to one or more products starting from organic or inorganic substrate(s) (see photocatalytic membrane reactors). Photocatalysis and, in particular, heterogeneous photocatalysis is a process of great potentiality for pollutants abatement. To improve the overall performance of the photoprocess, heterogeneous photocatalysis has been often combined with physical or chemical operations, which affect the chemical kinetics and/or the overall efficiency. Various possibilities to couple heterogeneous photocatalysis with other technologies to photodegrade organic and inorganic pollutants dissolved in actual or synthetic aqueous effluents have been described by Augugliaro et al. (2006). Thesecombinations increase the photoprocess efficiency by decreasing the reaction time with respect to the separated operations or they decrease the cost with respect to heterogeneous photocatalysis alone, generally in terms of light energy. Depending on the operation coupled with heterogeneous photocatalysis, two categories of combinations exist. When the coupling is with ultrasonic irradiation, photo-Fenton reaction, ozonation, or electrochemical treatment, the combination affects the photocatalytic mechanisms thus improving the efficiency of the photocatalytic process. When the coupling is with biological treatment, membrane reactor, membrane photoreactor, or physical adsorption, the combination does not affect the photocatalytic mechanisms but it improves the efficiency of the overall process. The choice of the coupling is related to the type of wastewater to be treated. A synergistic effect, giving rise to an improvement of the efficiency of the photocatalytic process, has been reported in the literature for many cases. The influence of the type of nanofiltration membrane, initial concentration of pollutant and pH on the photodegradation rate was investigated by Molinari et al. (2002) in discontinuousand continuous configurations in a photocatalytic membrane system with the lamp immersed in the suspension inside the photoreactor by using polycrystalline TiO2 (Degussa P25) as the catalyst and humic acids, organic dyes, 4-nitrophenol as the pollutants. Two membranes were tested, i.e. NF-PES-010 (Celgard, Germany) of polyethersulphone and NTR-7410 (Nitto-Denko, Japan) of sulphonated polyethersulphone. The last one was chosen for all of the photoreactivity experiments because permeability and rejection tests indicated that it was able to hold both catalyst and small molecules carrying the same membrane charge (negative), thanks to the Donnan Exclusion. Despite the fluxes ranged between 20 and 40 L h-1 m-2 in operating conditions at 6 bar and these values were interesting for application purposes, the rejections of NTR-7410 nanofiltration membrane, obtained during operation of the membrane photoreactor in the degradation of humic acids, patent blue dye and 4-nitrophenol, were significantly lower than those obtained in the absence of photodegradation, probably because of the small molecular size of the by-products and of the intermediate species produced during the photodegradation process. This means that in order to select a suitable membrane, rejection should be determined during operation of the photoreactor. A batch-recirculated photoreactor was combined with a hollow-fibre membrane ultrafiltration (UF) unit for heterogeneous photocatalysis applications.The operation and modelling of the UF process on the performance of the integrated photoreactor-UF process was studied by Sopajaree et al. (1999). Methyleneblue and titanium dioxide (Degussa P-25) were used as the test pollutant and photocatalyst, respectively. The influence of cross-flow velocity, transmembrane pressure, and TiO2 dose on the permeate flux through the hollow fibre membrane was described. The data were modelled on the basis of concentration polarization and gel layer formation at the membrane surface/feed slurry boundary. The photocatalytic degradation of natural organic matter (NOM) is an attractive option in the treatment of drinking water (Choo et al., 2008). Theperformance of a submerged photocatalytic membrane reactor (PMR) was investigated with regard to the removal of NOM and the control of membrane fouling. The efficiency of a hybrid system combining photocatalysis and membrane filtration in a single module was investigated. Low-pressure submerged hollow fiber membranes were used to retain the TiO2 particles in the system by Chin et al. (2007) duringthe photodegradation of Bisphenol A (BPA) as the model pollutant. Aeration was applied in the submerged membrane photoreactor (SMPR) to provide: i) mixing, ii) dissolved oxygen, iii) mechanical agitation (to prevent agglomeration of TiO2 particles), and iv) shear forces (to remove TiO2 particles from the membrane surface).It was found that intermittent permeation enhanced the sustainability of the submerged membranes but showed no effect on the photoactivity of the system. An intermittence aeration frequency (IF) of 0.1 was sufficient to reduce the fouling rate of the membrane under the experimental conditions. The SMPR appears to be very effective and can achieve removal of low-concentration organics (such as BPA) in a compact, low-energy system.The combined photocatalytic and membrane processes have been more recently addressed also towards the selective production of high value molecules in mild experimental conditions by using suitable photocatalysts, solvent and starting substrates and membranes (Molinari et al. 2009a; Molinari et al. 2009b, Augugliaro et al. 2010) References Augugliaro V,Litter M,Palmisano L,Soria J(2006) The combination of heterogeneous photocatalysis with chemical and physical operations: A tool for improving the photoprocess performance. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY C-PHOTOCHEMISTRY REVIEWS 7: 127-144. Augugliaro V, Palmisano L (2010) Green oxidation of alcohols to carbonyl compounds by heterogeneous photocatalysis (2010) ChemSusChem 3:1135–1138 Chin SS, Lim TM,Chiang K,Fane AG(2007) Hybrid low-pressure submerged membrane photoreactor for the removal of bisphenol A. DESALINATION 202: 253-261 Choo KH,Tao R, Kim MJ (2008) Use of a photocatalytic membrane reactor for the removal of natural organic matter in water: Effect of photoinduced desorption and ferrihydrite adsorption. JOURNAL OF MEMBRANE SCIENCE 322: 368-374 Molinari R,Borgese M, Drioli E,Palmisano L,Schiavello M(2002) Hybrid processes coupling photocatalysis and membranes for degradation of organic pollutants in water. CATALYSIS TODAY 75:77-85 Molinari R, Caruso A, Palmisano L (2009a) Photocatalytic membrane reactors in the conversion or degradation of organic compounds. In: Drioli E, Giorno L(eds) Membrane operations, Innovative separations and transformations.Wiley-VCH, Weinheim, pp 335-361 Molinari R, Caruso A, Poerio T (2009b) Direct benzene conversion to phenol in a hybrid photocatalytic membrane reactor.CATALYSIS TODAY 144: 81-86 Sopajaree K, Qasim SA,Basak S,Rajeshwar K(1999) An integrated flow reactor-membrane filtration system for heterogeneous photocatalysis. Part II: Experiments on the ultrafiltration unit and combined operation. JOURNAL OF APPLIED ELECTROCHEMISTRY 29: 1111-1118I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
