Photocatalytic Membranes Immobilizing Titanium Dioxide Research Articles

Soil adsorption behaviour and photomineralization by photocatalytic membranes immobilizing titanium dioxide of atrazine and intermediates

The soil adsorption behaviour of atrazine and its photodegradation intermediate 4,6-diamino-2-chloro-1,3,5- s-triazine (I), was evaluated by measuring partition coefficients on five representative European topsoils using classical batch experiments. From this study, (I) appears to be less immobilised than atrazine itself, especially for typical agricultural soils. Consequently, (I) has a higher tendency to reach the saturated zones, and hence, the groundwater. In order to remove atrazine and its degradation products from groundwater prior to making it drinkable, photocatalytic degradation onto membranes immobilizing 30±3 wt.% TiO2 has been investigated as a possible technology to mineralize these waters satisfactorily. For this purpose, experiments have been carried out using a pilot-plant facility, with 0.02 - 0.10 mmol L-1 solutions of atrazine, as well as of compound (I) and of 2,4,6-trihydroxy-1,3,5-s-triazine (II). When employing an excess of hydrogen peroxide as the oxidizing agent, the monitoring of chemical oxygen demand (COD), as well as control by HPLC, showed that (II) was a photostable degradation product, owing to its resistance to attack by hydroxyl radicals. On the other hand, when kinetic runs were carried out onto membranes in the presence of dissolved oxygen only or of sub-stoichiometric quantities of H2O2, (II) could be quantitatively mineralized. A mechanism has been proposed, based on the possible direct involvement of holes in the irradiated semiconductor surface, together with superoxide anions, these latter being produced by the reaction of conduction band electrons with dissolved oxygen. This mechanism, which is inhibited by an excess of hydrogen peroxide, owing to its reaction yielding hydroxyl radicals by any route, is also substantiated by previous experiments, carried out in the presence of ozone, with and without photocatalytic promoters.

Nonlinear Modelling of Kinetic Data Obtained from Photocatalytic Mineralisation of 2,4‐Dichlorophenol on a Titanium Dioxide Membrane

Photomineralisation of 2,4‐dichlorophenol (DCP) in aqueous solutions (10.0–100.0 mg/L of C) was systematically studied at 318 ± 3 K, in an annular laboratory‐scale reactor, by photocatalytic membranes immobilizing titanium dioxide, as a function of substrate concentration, and absorbed power per unit length of membrane. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed (first series of experiments). At a fixed value of irradiance (1.50 W⋅cm−1), other series of mineralization experiments were repeated (second series of experiments) by carrying out only analyses of chemical oxygen demand (COD), in order to compare modelling results of the two sets of experiments. In both sets of experiments, stoichiometric hydrogen peroxide was used as oxygen donor. For the first series of experiments, a kinetic model was employed, already validated in previous work, from which, by a set of differential equations, four final optimised parameters, k1 and K1, k2 and K2, were calculated. By these parameters, the whole kinetic profile could be fitted adequately. The influence of irradiance on k1 and k2 could be rationalised very well by this four‐parameter kinetic model. Modelling of quantum yields, as a function of irradiance, could also be carried out satisfactorily. As has been found previously for other kinds of substrates, modelling of quantum yields for DCP mineralization is consistent with kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading finally to carbon dioxide, paralleled by a second competition kinetics involving superoxide radical anion. For the second series of experiments, on the contrary, the Langmuir‐Hinshelwood model was employed. Uncertainties of COD analyses, coupled with discrepancies of this model and with its inability to reproduce kinetics up to complete mineralization, are underlined.

Nonlinear modelling of data in photomineralization kinetics of organic micropollutants by photocatalytic membranes immobilizing titanium dioxide in membrane reactors

AbstractKinetic fitting of substrate disappearance and of total organic carbon (TOC) mineralization of organic micropollutants, in water and air, by photocatalytic membranes immobilizing titanium dioxide, was carried out. A model was used in which mineralization of substrate to CO2 is supposed to occur, with kinetic constant k1, through one single intermediate, mediating the behaviour of all the numerous real intermediates formed in the path to CO2, kinetic constant of formation of the latter being k2. A competitive Langmuirian‐type adsorption of both substrate and ‘intermediate’ was also supposed to be operative, as expressed by pseudo‐thermodynamic constants K1 and K2 respectively, these constants possessing a, partly at least, kinetic significance. Nonlinear models could be fitted to data by using the least‐squares method. The very satisfactory matching is shown for the laboratory‐scale mineralization kinetics of methane, as model molecule of aliphatic contaminants, both in the gas phase and in aqueous solution. Furthermore, in pilot plant experiments, using phenol, as model molecule of aromatics, modelling of quantum yields was carried out, as a function of concentration and of adsorbed radiant power. Kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading to mineralization, was considered, paralleled by a second competition kinetics due to superoxide anion radical and its conjugate acid, equally leading to mineralization. In this model the contribution of hydroxyl radicals to mineralization decreases with irradiance, while the contribution of superoxide anion radical and its conjugate acid increases. If the regression equations of these two contributions are considered together, in a linear combination, the surface model perfectly fits the experimental data. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of Irradiance, Flow Rate, Reactor Geometry, and Photopromoter Concentration in Mineralization Kinetics of Methane in Air and in Aqueous Solutions by Photocatalytic Membranes Immobilizing Titanium Dioxide

Photomineralization of methane in air (10.0–1000 ppm (mass/volume) of C) at100%relative humidity (dioxygen as oxygen donor) was systematically studied at318±3 K in an annular laboratory-scale reactor by photocatalytic membranes immobilizing titanium dioxide as a function of substrate concentration, absorbed power per unit length of membrane, reactor geometry, and concentration of a proprietary vanadium alkoxide as photopromoter. Kinetics of both substrate disappearance, to yield intermediates, and total organic carbon (TOC) disappearance, to yield carbon dioxide, were followed. At a fixed value of irradiance (0.30 W⋅cm-1), the mineralization experiments in gaseous phase were repeated as a function of flow rate (4–400 m3⋅h−1). Moreover, at a standard flow rate of 300 m3⋅h−1, the ratio between the overall reaction volume and the length of the membrane was varied, substantially by varying the volume of reservoir, from and to which circulation of gaseous stream took place. Photomineralization of methane in aqueous solutions was also studied, in the same annular reactor and in the same conditions, but in a concentration range of 0.8–2.0 ppm of C, and by using stoichiometric hydrogen peroxide as an oxygen donor. A kinetic model was employed, from which, by a set of differential equations, four final optimised parameters,k1andK1,k2andK2, were calculated, which is able to fit the whole kinetic profile adequately. The influence of irradiance onk1andk2, as well as of flow rate onK1andK2, is rationalized. The influence of reactor geometry onkvalues is discussed in view of standardization procedures of photocatalytic experiments. Modeling of quantum yields, as a function of substrate concentration and irradiance, as well as of concentration of photopromoter, was carried out very satisfactorily. Kinetics of hydroxyl radicals reacting between themselves, leading to hydrogen peroxide, other than with substrate or intermediates leading to mineralization, were considered, and it is paralleled by a second competition kinetics involving superoxide radical anion.

Laboratory‐scale photomineralization of n‐alkanes in gaseous phase by photocatalytic membranes immobilizing titanium dioxide

Kinetics of photocatalytic oxidation of methane, ethane, and n‐heptane, to yield intermediates, and photomineralization of intermediates, to yield carbon dioxide and water, was studied in the gaseous phase, at 308 ± 2 K, by a laboratory‐scale photoreactor and photocatalytic membranes immobilizing 30 ± 3 wt.% of TiO2, in the presence of aerosolized stoichiometric hydrogen peroxide as oxygen donor, and at a relative humidity close to 100%. The whole volume of irradiated solution was 4.000 ± 0.005 L, the ratio between this volume and the geometrical apparent surface of the irradiated side of the photocatalytic membrane was 3.8 ± 0.1 cm, and the absorbed power was 0.30 W/cm (cylindrical geometry). The pinetic parameters of the present work substantially coincide with those of the same molecules previously studied in aqueous solution, within the limits of experimental uncertainty. Photocatalytic processes thus appear to be controlled by interface phenomena, which are ruled kinetically, and apparently also thermodynamically, by concentration gradients, independently on diffusion and other processes in the aqueous or gaseous bulk, if turbulence in these phases is adequately assured.

EPR spin-trapping of hydroxyl radicals onto photocatalytic membranes immobilizing titanium dioxide, and spin adduct competition, as a probe of reactivity with aqueous organic micropollutants

Spin-trapping of photogenerated •OH radicals, with electron paramagnetic resonance (EPR) detection of spin adducts, has been evidenced on to photocatalytic membranes, prepared by photografting, and immobilizing 30±3 wt.% TiO 2 semiconductor. By employing the known rate constant for the reaction of •OH radicals with the spin-trap (DMPO), rate constants for the reaction of •OH radicals with a given substrate could be obtained by a competition kinetic scheme, using the initial rates of the first step of photodegradation of the substrate. This method used, for a parallel series of runs, also membranes, immobilizing 7 wt.% of trialkyl vanadates as photocatalytic promoters together with the semiconductor, able to yield unitary quantum yields for the photogeneration of •OH radicals from semiconductor. Quantum yields for the non-photocatalytically promoted semiconductor membrane could thus be obtained. This method has been validated with phenol, 4-chlorophenol, 2,4-dichlorophenol, atrazine, propazine, prometryn, ametryn, and formate ion, and compared with a similar technique, set up recently in the literature, based on production of •OH by homogeneous photolysis of H 2O 2 and direct EPR measurements of the rate of spin adduct formation.

Photobleaching and photomineralization of azobenzene and substituted azobenzenes in aqueous solution by photocatalytic membranes immobilizing titanium dioxide

The kinetics of photobleaching (by spectrophotometric analysis) and integral photomineralization (by total organic carbon (TOC) analysis) of azobenzene ( I) and substituted azobenzenes in aqueous solution were followed in laboratory-scale runs on photocatalytic membranes immobilizing 30±3 wt.% of semiconductor TiO 2. Experiments were carried out by the technique described in preceding papers of this series, employing stoichiometric hydrogen peroxide as the oxygen donor. The following azobenzenes were examined: (4-diethylamino)-phenylazobenzene ( II), 4′-(((4-diethylamino)phenyl)azo) benzoic acid ( III), 4′-(((2-amino-5-diethylamino)phenyl)azo) benzoic acid ( IV), 4′-(((2-acetamido-4-diethylamino)phenyl)azo) benzoic acid ( V), 4′-(((4-dimethylamino)phenyl)azo) benzenesulphonic acid, sodium salt ( VI) and 4′-(((2-acetamido-4-diethylamino)phenyl)azo) benzenesulphonic acid, sodium salt ( VII). From the Langmuir-Hinshelwood treatment of the initial rate data as a function of the initial concentration ((0.10−1.0) × 10 −3M), the kinetic parameter k and the pseudo-thermodynamic parameter K for photobleaching were obtained. With regard to photobleaching, I and II were certainly the most reactive, followed by IV. The remaining molecules showed a photo-oxidation rate of one-third to one-quarter of that of I chosen as reference structure. Consequently, the presence of an amino group in the 4-position ( II) does not stabilize the azobenzene structure against photo-oxidation leading to bleaching, whereas the same group in the 2-position ( IV) decreases the photobleaching rate by about 40% when a carboxylic group is also present in the I′-position. Acetylation of this amino group, such as in V, decreases the photobleaching rate more markedly. With regard to photomineralization, it was observed that, when photobleaching was virtually complete, a certain amount of TOC was already mineralized. The maximum amount of TOC remaining at the end of photobleaching ranged from about 90% to about 30%, varying with the dye structure and initial concentration as well as with the power and type of irradiation source. By examining the TOC concentration profiles as a function of the substituted azobenzene structure, the following hypotheses were proposed: 1. 1. during the photobleaching period, the ring containing the diethylamino group breaks down (more markedly if further amino or acetamido groups are present in the structure), and photomineralization of the other ring occurs more slowly: 2. 2. both rings break down, within certain limits, during photobleaching; however, the aliphatic fragments containing carboxyl or sulphonic groups are mineralized more slowly. The fact that a small initial plateau in the TOC profile is followed by another more evident plateau at the end of the photobleaching period, for both VI and VII, suggests that hypothesis (1) is more probable in these cases. When the second plateau is reduced to a sigmoidal curve or an inflection point, hypothesis (2) also needs to be considered.

Mathematical modelling of pilot-plant photomineralization of chlorophenols in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide

Photomineralization of 4-chlorophenol, 2,4-dichlorophenol, and pentachlorophenol, in 9.9×10 −4−3.8×10 −5 M aqueous solutions, in the presence of stoichiometric hydrogen peroxide, was studied using PHOTOPERM® CPP/313 membranes containing immobilized 30±3 wt.% TiO 2, by following analytically both disappearance of absorbable chloro-organics (AOX), and mineralization of total organic carbon (TOC) content. Experiments were carried out in a PHOTOPERM®WP pilot plant, using monochromatic irradiation (254 nm) in the absorption range of semiconductor, with an absorbed power of 22±1 W. The initial rate of photodegradation, both regarding AOX and TOC analysis, was studied as a function of the initial concentration of substrate using the linearized form of the Langmuir-Hinshelwood equation, from which the rate constants κ and apparent adsorption contants ϰ were evaluated. These parameters, which are unable to fit the whole photomineralization kinetic curves, because they account only for initial rates, were employed, as starting values, to optimize, by numerical integration, a kinetic model which considers appearance and disappearance of all intermediates, as if they were represented by a hypothetical single molecule, mediating all of them. In this way, two couples of parameters, κ 1 and ϰ 1, κ 2 and ϰ 2 were obtained, relative to the two successive steps of the model (substrate disappearance and mineralization), able to reproduce the whole kinetics satisfactorily. Mean quantum yields of organic carbon mineralization, calculated by κ 2 parameters, reach 75, 71, and 31 % for the chlorinated phenols in the order reported above, with respect to their maximum allowable quantum yields. These values are from 3 to 5 times greater than those measured in laboratory-scale experiments thus showing the excellent performance of the pilot-plant employed.

Mathematical modelling of photomineralization of phenols in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide

Photomineralization of phenol, 2,6-dimethyl phenol, 1,2,3-benzenetriol, 4-chloro phenol, and 2,4-dichloro phenol, in 9.9×10 −3 − 5.6×10 −5 M aqueous solutions, in the presence of stoichiometric hydrogen peroxide, was studied, using PHOTOPERM® CPP/313 membranes containing immobilized 30±3 wt.% TiO 2, by analysis of total organic carbon (TOC) content. With phenol and 2,6-dimethyl phenol also the rate of disappearance of the substrate molecules was measured fluorimetrically. Polychromatic or monochromatic irradiation was carried out (radiant power in the absorption range 145 and 8 W respectively). The initial rate of photodegradation was studied as a function of the initial concentration of substrate using the linearized form of the Langmuir-Hinshelwood equation, from which the rate constants k and apparent adsorption constants K were evaluated. These parameters, which are unable to fit the whole photomineralization kinetic curves, were employed to optimize, by numerical integration, a kinetic model which considers appearance and disappearance of all intermediates, as if they were represented by a hypothetical single molecule, mediating all of them. By this way, two couples of parameters, k 1 and K 1, k 2 and K 2 were obtained, relative to the two successive steps of the model (substrate disappearance and mineralization), able to reproduce satisfactorily well the whole kinetics. Experimental data, as compared to modelling, show evidence of a main rate determining path, and of another, minor but not unsignificant, parallel path leading to a faster mineralization. Mean quantum yields of organic carbon mineralization, calculated by k 2 parameters, reach 10–20% of the maximum allowable efficiencies.

Pilot-plant photomineralization of dichloromethane and tetrachloroethene in aqueous solution, by photocatalytic membranes immobilizing titanium dioxide and photopromoters

The TiO 2-mediated photomineralization of 5.0×10 −6−5.0×10 −7 M aqueous solutions of dichloromethane (DCM), tetrachloroethene (PCE), and mono-, di-, and trichloroethanoic acids (MCEA, DCEA, and TCEA respectively) was studied at 296 ± 2 K, using PHOTOPERM® CPP/313 membranes containing immobilized 30 ± 3 wt.% TiO 2, and, in some of the runs with PCE, 7 wt.% of photocatalytic promoters based on Co III, V V, and Fe III organometallic compounds. Disappearance of both substrate and total organic carbon (TOC) was examined, as well as chloride ion formation. The apparent reaction order being unit in the range of concentration considered, half times for these three kinetic processes have been measured in a PHOTOPERM® WW membrane module, fitted with 1 m 2 of membrane, with an absorbed radiating power of 31 W, and saturated with air. With PCE, the marked decrease of rate (to 1 25 ) with decreasing O 2 concentration (to 2–3 ppm), and the positive effects of promoters or of applying an anodic bias (enhancement of rate up to about 3 1 ) were ascertained. The role of oxygen as scavenger for photogenemted electrons is much preferable to that of adsorbed substrate to scavenge these conduction band electrons, both to explain the efficient action of photopromoters as oxidant scavengers or oxygen transporters, and redox processes involving formation of chlorinated intermediates, such as DCEA, during radical reactions, as TOC disappearance from DCEA, MCEA, and TCEA proceeds at a much slower rate than that experimentally observed for PCE.

Pilot-plant-scale photodegradation of phenol in aqueous solution by photocatalytic membranes immobilizing titanium dioxide (PHOTOPERM® process)

The TiO 2-mediated photodegradation of phenol was studied at 298 ± 2 K or at 315 ± 5 K (with the ratio between the hydrogen peroxide added and the stoichiometric amount ( N) in the range 0 – 3), using PHOTOPERM® CPP/313 membranes containing immobilized 30 ± 3 wt.% TiO 2. The rate of photodegradation was studied as a function of: i) initial concentration of substrate (8.76x10 −6 − 0.01 M), ii) flow rate (0.4 – 5 m 3/h), iii) apparent (geometrical) membrane surface (30 – 190 cm 2/cm of central axis of radiation field), iv) kind of radiation source (low and high pressure mercury arc lamps, with fixed or variable radiant power, in the absorption range of semiconductor, of 31 W and 136–680 W respectively), v) radiation intensity. Disappearance of both phenol and total organic carbon (TOC) was examined. The operations of two pilot-plants, differing in range of flow rates, and membrane arrangement, are critically compared. The apparent reaction orders as a function of concentration are also compared and discussed, on the basis of kinetic parameters resulting from both pilot-plant- and laboratory-scale experience. Advantages of the membrane process are highlighted.

Laboratory- and pilot-plant-scale photodegradation of chloroaliphatics in aqueous solution by photocatalytic membranes immobilizing titanium dioxide

The TiO2-medicated photodegradation of chloroaliphatics (dichloromethane, trichloroethene and mono-, di- and trichloroethanoic acids) was studied at 308±2 K (with the ratio between the hydrogen peroxide added and the stoichiometric amount (N) in the range 0–30), using PHOTOPERMTMCPP/313 membranes containing immobilized 30%±3% TiO2, at laboratory scale (radiant power in the absorption range, 145 W) and in a pilot plant (radiant power in the absorption range, 31 W). In addition to this semiconductor, some proprietary photocatalytic systems, including stabilized preparations containing CoIII, VV and FeIII organometallic compounds, were immobilized in the photocatalytic membranes. The initial rate of photodegradation was studied as a function of the initial concentration of the substrates (5.0×10-2–5.0×10-7M) using the linearized form of the Langmuir-Hinshelwood equation, which was well fitted by the membranes over the whole range of concentration, and from which the rate constants k and equilibrium adsorption constants K were evaluated. The contributions of processes (a)-(d) ((a) photolysis of micropollutant, independent of the presence of membranes and oxidizing agent; (b) photo- degradation due to UV and hydrogen peroxide; (c) photodegradation due to the semiconductor immobilized in the membrane; (d) the same as (c), but in the presence of a promoting photocatalytic system), which occur simultaneously in our experimental conditions, were measured. The contribution of process (d), particularly for a synergistic mixture of tri-(tert-butyl) and tri-(isopropyl) vanadate(V) or iron (III) potassium oxalate as photocatalysts, is greater than that of all other processes, while UV degradation in the presence of hydrogen peroxide (b) represents one-tenth or less of the whole. The rationalization of k values for this integrated membrane process is discussed on the basis of their dependence on the membrane surface and on the square root of the radiation intensity.

Degradation of some chloro-aliphatic water contaminants by photocatalytic membranes immobilizing titanium dioxide

The TiO2-mediated photodegradation of chloroethanoic acid, trichloroethene and tetrachloroethene, chosen as model molecules of aliphatic chloro-organics was studied at 303±2 K by employing polymeric membranes photografted on cellulose to immobilize the TiO2 (3.0–5.6 mg cm−2). The radiant flux in the absorption range and the volume-surface ratio in the photoreactor cell were kept constant at 2.5±0.2 mW cm−2 and 1.1±0.1 mL cm−2 respectively. The initial rate of photodegradation was studied as a function of the initial concentration of reactants by the linearized form of the Langmuir—Hinshelwood equation, by which rate constants k and equilibrium adsorption constants K were evaluated. Values of k (5.7±0.5 μmol h−1) are independent of the chemical nature of the three reactants investigated, in agreement with other literature findings. Values of K are compared with those measured in aqueous suspensions of TiO2 with the same chloro-organics and with the values predicted by a mechanism involving hydroxyl radicals as the primary oxidant adsorbed on the photocatalyst surface. Satisfactory performance and potential technological advantages of photocatalytic membranes are discussed.