Use of a batch rotating photocatalytic contactor for the degradation of organic pollutants in wastewater

Applied Catalysis B: Environmental 30 (2001) 49–60

Use of a batch rotating photocatalytic contactor for the degradation of organic pollutants in wastewater Noel A. Hamill, Lawrence R. Weatherley1 , Christopher Hardacre∗ School of Chemistry, The Queen’s University of Belfast, Belfast BT9 5AG, UK Received 21 May 2000; received in revised form 27 July 2000; accepted 5 August 2000

Abstract Aqueous solutions of a chlorinated VOC, 3,4-dichlorobut-1-ene, as well as other pollutants, may be mineralised to carbon dioxide, water and hydrochloric acid using a sealed rotating photocatalytic reactor. The effect of pH, dissolved oxygen concentration, light intensity, pollutant concentration and rotation speed on the degradation rate have been investigated as well as competition kinetics with methanol. This reactor may be optimised to minimise competition effects in mixed solutions. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Photocatalytic; Wastewater; Dichlorobutenes; Rotating disc

1. Introduction Semiconductor photocatalysis has attracted a considerable amount of research interest, especially in the field of wastewater treatment [1–3]. Photocatalysts utilise photons with energies greater than the band gap energy of the semiconductor to create a charge separation (electron-hole pairs). Subsequent diffusion of the charge carriers through the bulk to the surface of the semiconductor particle allows interaction with solvent molecules (e.g. water, in slurries) or adsorbed species (pollutants). The main product of these interactions are hydroxyl radicals (OH• ) which are the primary oxidants of wastewater organics [1]. The mineralisation of most organics to water, carbon ∗ Corresponding author. Tel.: +44-28-9027-4592; fax: +44-28-9038-2117. E-mail address: [email protected] (C. Hardacre). 1 Present address: Department of Chemical and Process Engineering, University of Canterbury, Christchurch, New Zealand.

dioxide and mineral acids is achieved by successive hydroxyl radical attack and fragmentation. As a method of water purification it has considerable advantages over existing technologies. It destroys toxins rather than merely transferring them to another phase (e.g. activated carbon adsorption, gas sparging), and does so without the use of potentially hazardous oxidants (e.g. ozonolysis, chlorination). Removal of chlorinated hydrocarbons is of particular importance in industrial effluent treatment and has been shown to respond well [1] to photocatalytic treatment. There are a significant number of problems associated with photocatalysed treatment of wastewater which have prevented full commercialisation. For example, mass transfer of oxygen, adsorption of organic substrates on titania particles, light penetration and electron-hole recombination all lead to low photoefficiencies and reduce the economic viability of the process. Reactor design can alleviate some of the problems and increase the efficiency of the photocatalysed process. A variety of designs and the problems asso-

0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 0 ) 0 0 2 1 9 - 8

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ciated with these have been recently reviewed [4,5]. Many incorporate the use of fixed films in order to reduce the need for filtration, including the novel swirl flow reactor [6] and the photo-CREC unit (a variation on the classical annular reactor) [7]. Dionysiou et al. [8] have recently reported a rotating disc photocatalytic reactor similar to the reactor reported in this study, which has been employed for the degradation of chlorophenols. In the present case, we report a study on the degradation of a volatile organic, 3,4-dichlorobut-1-ene (DCB). Dichlorobutenes are extremely toxic to marine life even at parts per billion levels [9] and are used extensively, for example in the manufacture of Neoprene. In this study, we discuss the effects of mass transfer and the use of such a reactor for combinations of pollutants. We show that this type of reactor is capable of good reaction efficiencies, compared with slurry reactors, for a range of substrates, and most importantly that it may be used for VOCs where air stripping may be a problem.

2. Experimental 2.1. RPC Fig. 1 shows a schematic of the RPC used in this study and Table 1 summarises the reactor dimensions. Titania was supported on four glass discs using the procedure outlined below, and the discs are rotated. Between each pair of discs 3 UV lamps (3× Phillips PL-S 9W/10, emitted power per lamp: 1.9 W, λmax = 365 nm) irradiate the catalyst surface. On rotation, a thin film of liquid becomes entrained on the disc

Table 1 Summary of the dimensions of the RPC shown in Fig. 1 Length of trough (m) Width of trough (m) Number of discs Diameter of discs (m) Thickness of discs (m) Distance between discs (m) Submerged fraction of disc (m) Total entrained film area (m2 ) Catalyst coated area (m2 ) Catalyst film loading (kg m−2 )

0.19 0.11 4 0.10 0.003 0.045 0.032 0.04 0.041 0.003

from the bulk solution. Illumination and hence reaction takes place in the headspace. Glass windows, which transmitted 89% of the incident light at 365 nm, were utilised. Variation of the rotation speed of the discs (5–140 rpm) was accomplished by the application of a variable voltage (6–24 V) to a Maxon DC motor fitted with a gearbox (either a 30:1 or 200:1 rotation ratio). The emitted radiance of the lamps was measured using an ABLE meter fitted with UVA filter. The light intensity was varied using 100 ␮m cellulose acetate filters (transmittance 80% at 365 nm) placed between the lamp and the windows. 2.2. Catalyst preparation Roughened glass discs were immersed in alkali (1 M NaOH) for 15 min to clean and etch the surface, washed with copious amounts of deionised water and dried at 100◦ C. The disc was mounted vertically on the rotating axle and partially immersed in a sonicated Degussa P25 titania slurry (10 g/l). Before re-entry

Fig. 1. Schematic diagram of the rotating photocatalytic reactor used.

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into the solution, the entrained films on the discs were dried at 300◦ C in hot air whilst rotating at 5 rpm. This procedure deposited an even layer of TiO2 on the discs. They were subsequently calcined at 450◦ C for 30 min [10,11]. 2 Rinsing with deionised water resulted in no change in TiO2 loading. The TiO2 loading was calculated as 0.3 mg/cm2 . Prior to reaction, the titania was removed from the two non-illuminated disc sides by carborundum abrasion. 2.3. Reagents Analar grade reagents were used throughout. 3,4 dichlorobut-1-ene (99.6%) was supplied by DuPontDow Elastomers, and Dyactive black (DB) was supplied by Ciba-Geigy. For oxygen treatment, 99.999% O2 was supplied by BOC. Distilled deionised 18.2 M water was used to prepare the stock solutions. In all cases Degussa P25 TiO2 was used as received. 2.4. Reaction procedure Typically, the required amount of 3,4 dichlorobut-1ene (DCB) was added to distilled water, pre-saturated with air and the mixture vigorously agitated before transfer to the RPC which was subsequently sealed gas tight. For oxygen saturated runs, oxygen was flushed through the RPC and bubbled through the water prior to DCB addition. The discs were rotated at 20 rpm for 90 min to allow for DCB adsorption on the discs and headspace saturation with DCB vapour. Headspace saturation occurred within 10 min and was verified by stable peak size in GC analysis of gaseous samples. Aliquots were taken and analysed as described below. If required, the pH was adjusted with 10 M KOH or conc. HClO4 to the desired value which was monitored using a Metrohm 713 pH meter. Following each experimental run, the discs were washed with copious amounts of deionised water, the trough filled with distilled water and the discs were illuminated for at least 1 h to ensure removal of adsorbed orga2 Calcining to 450◦ C is likely reduce the activity of the catalyst film because of morphological changes or incorporation of Na+ or Si4+ cations into the catalyst layer. This reduction is likely to be of the order of 30% at 450◦ C, see [10,11]. In this reactor, this temperature was necessary to ensure good mechanical stability of the films.

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nics. Air stripping experiments were performed using a similar procedure without illumination and in an open vessel. 2.5. Slurry phase reactions All slurry phase reactions were performed using a similar procedure to the RPC and were carried out in a magnetically stirred, water cooled Pyrex cylindrical reactor shown in Fig. 2. Following air saturation, the solution was placed in the reactor and the catalyst, Degussa P25 titanium dioxide (0.5 g/l), added and stirred for 90 min to allow adsorption/desorption equilibration. Again, aliquots were taken and analysed as described below. Illumination was provided by 8× Phillips TL 8 W/05 UV lamps (emitted power per lamp: 1 W, λmax = 365 nm). 2.6. Analysis methods Aqueous samples were taken at regular intervals and were extracted with an equal amount of isooctane containing 292 ppb tetrachloroethene as an internal standard. For other aqueous DCB concentrations, the organic to aqueous volume ratio could be altered with no adverse effect on response linearity. The isooctane sample was analysed using a Pye Unicam 300 series gas chromatograph with electron capture detection (GC-ECD). For slurry phase experiments the aliquots were filtered through a stainless steel “in-line” filter using Whatman WCN 5.0 ␮m pore diameter filter membranes. DCB, 2-chlorobutadiene and 2,3-dichlorobutane were detected using the GC-ECD. Other intermediates were measured using headspace sampling using a Micromass GC-MS for analysis. CO2 levels were determined using headspace sampling and acidified bulk solution samples monitored using a Fisons MS. For all species detected the total number of molecules were calculated using Henry’s law constants assuming gas–liquid equilibrium. Phenol concentrations were monitored using a diode array Beckman Gold HPLC coupled with a Hypersil BDS column (C18 phase, 150 mm × 4.6 mm) and 1:1 methanol: water mobile phase. Dyactive black concentrations were monitored using an HP diode array 5890A UV–VIS spectrometer.

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Fig. 2. Schematic diagram of the slurry phase reactor used.

Fig. 3. Concentration and ln concentration vs. time curves for 0.2 mM DCB degraded in a slurry reactor and in the RPC at 136 rpm.

Unless otherwise stated all rate constants quoted for the RPC and slurry phase reactions are in the absence of mass transfer effects, i.e. above 90 rpm for the RPC. The pseudo-first-order (apparent) rate constants are calculated from the gradient of a plot of ln(Ci /Ci0 ) against time. 3

3 From [8], the rate is given by R = kK C /1+K C +K C a A A A A0 S S and the pseudo-first-order rate constant is defined as kdeg = kKA /1 + KA CA0 + KS CS where k is the true rate constant, KA is the adsorption constant for the substrate A, KS is the adsorption constant for the solvent molecules and Ci and Ci 0 are the concentrations of i and the initial concentration of i, respectively.

3. Results and discussion 3.1. 3,4-Dichlorobut-1-ene degradation Fig. 3 shows the concentration–time curve and the pseudo-first-order plot for a 0.2 mM solution of DCB degraded in the RPC at 136 rpm and in the slurry reactor under air saturation. The pseudo-first-order rate constants (kdeg ) for the RPC and slurry are 0.0242 and 0.0330 min−1 , respectively. The agreement with pseudo-first-order kinetics is common for photocatalytic reactors and agrees well with the findings of Dionysiou et al. [8] for a similar reactor.

N.A. Hamill et al. / Applied Catalysis B: Environmental 30 (2001) 49–60 Table 2 %Carbon present as intermediates at maximum concentration for 1 mM DCB degraded at 20 rpm Intermediate Acetaldehyde Acetic acid Acetone Butadiene 2-Chlorobutadiene Chloroform 2,3-Dichlorobutane Dichloromethane

%C at tmax

tmax (min)

45 >1 0.3 0.4 0.2 0.03 0.2 0.009

120–250 400 360–400 110 0 >250 320 600

Table 2 shows the intermediates formed and Fig. 4 shows the carbon mass balance variation as a function of reaction time at 20 rpm. It should be noted that the low rotation speed was used to increase the concentrations and number of intermediates found for identification purposes and that these are lower at higher speeds because of increased reaction efficiency towards mineralisation, see below. Although a number of intermediates are observed, with the exception of acetaldehyde, these are found only in trace amounts. The maximum concentration of 2-chlorobutadiene was detected in the aqueous phase at the start of the reaction, but was not present in the starting material. This is a common product formed via a base-catalysed dehalogenation of DCB in aqueous media as reported by many researchers, for example [12]. Two electron transfer induced dehalogenation could account for the presence of butadiene directly from DCB. However, it is also possible that reductive coupling of acetaldehyde could form butadiene via aldol condensation

Fig. 4. Variation in percentage of carbon attributable to CO2 and the overall observed carbon mass balance with time for the degradation of 1 mM DCB at 20 rpm.

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reactions [13–15]. Acetaldehyde is likely to be formed from hydroxyl radical attack on the double bond of DCB followed by oxidative cleavage of the C(2)–C(3) bond, possibly via oxygen addition. This results in acetaldehyde and a chloroacetyl chloride derivative which would hydrolyse rapidly giving chloroacetic acid (this was not detectable using the analysis method used in this study). It is possible that the latter may be responsible for the trace amounts of chlorinated methanes although these are normally only observed under oxygen lean conditions [16,17]. Acetic acid may be formed from the oxidation of acetaldehyde. The formation of acetone may be from either acetic acid or acetaldehyde, both reactants have been shown to form acetone over TiO2 [13,18,19]. Haloalkanes are generally less reactive than unsaturated species and hence once formed, 2,3-dichlorobutane accumulates. There are a number of possible pathways for its formation, for example, reductive rearrangement of DCB via a three-membered ring intermediate. The reductive nature of the latter process is supported by the large increase in rate of 2,3-dichlorobutane intermediate formation under anoxic conditions in the slurry reactor. In contrast to the destruction of DCB, the mineralisation to CO2 shows an initial increase followed by a plateau after 320 min reaction. At this point ∼30% of the total carbon present is in the form of CO2 . As the reaction proceeds, the total accountable carbon mass gradually decreases and a high proportion of carbon is present as acetaldehyde, reaching a maximum of 45%. Since the RPC is a sealed vessel, volatile intermediates, such as acetaldehyde, are not removed from the system via air stripping, and a gas–liquid equilibrium is set up. Under these conditions, the concentration of reactively formed acetaldehyde builds up and becomes high enough to effectively compete with DCB for catalyst surface sites. At extended reaction times, the level of CO2 remains constant whilst the level of acetaldehyde decreases slowly. Under these conditions, i.e. a sealed vessel with a high concentration of acetaldehyde, TiO2 -catalysed aldol condensation reactions dominate over the photoxidation of acetaldehyde and the products block surface sites preventing mineralisation occurring but still reducing the level of acetaldehyde [13,20]. These reactions result in a build-up of strongly adsorbed condensation products which may deactivate the catalyst.

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Fig. 5. Variation in pseudo-first-order rate constant with rotation speed following the degradation of 0.2 mM DCB in the RPC.

3.2. Effect of rotation speed The effect of rotation speed with respect to kdeg is shown in Fig. 5. Three regions are clearly seen region 1, 0–20 rpm, region 2, 20–90 rpm and region 3, >90 rpm. Sharp steps in the rate of DCB removal are observed between each of these regions. Plataeaus in the rate are found within these regions with pseudo-first-order rate constants, k deg = 0.0128 min−1 at 5 rpm, k deg = 0.0177 min−1 at 67 rpm and k deg = 0.0242 min−1 at 136 rpm. The most likely explanation for each of the steps is a change in mass transfer properties of the system. Within region 1, the rate constant is found to rise rapidly and level off between 7–20 rpm. At 20 rpm, a sharp increase is observed. This type of change is known to accompany changes in the mixing within the bulk solution. Although the dimensionless mixing time (ωtmix ) of the bulk solution remains constant within the laminar and turbulent flow regimes for a mixed system, between these regimes, a sharp change occurs [21]. Ideally for any Newtonian fluid, the transition between laminar and turbulent flow occurs at a value of the Reynolds numbers, Re = 2100 [22]. For a rotating disk contactor, the Reynolds number is given by Re =

ωdρx µ

(1)

Eq. (1) relates the rotation speed ω (min−1 ) to the disc diameter d (m), density of the solution, ρ (kg m−3 ), viscosity of the solution µ (kg m−1 min−1 ), and disc

spacing, x (m). Between regions 1 and 2, i.e. where the first step occurs, the Re rises from a value of 1500 at 20 rpm to 2400 at 32 rpm. This indicates that the low rate of degradation in region 1, compared with region 2, is mainly because of poor mixing within the bulk solution at low speeds, i.e. in the laminar flow regime. At increased rotation speeds, the bulk solution transforms into the turbulent flow regime and much improved mixing and transport to the catalyst surface occurs. Above 30 rpm, mixing within the bulk solution is efficient and no longer rate limiting. Between regions 2 and 3, i.e. at 90 rpm, a similar step is observed, again indicative of mass transfer limitation. This is likely to be because of changes in the nature of gas–liquid–solid contacting within the entrained film on the discs. Two theories have been developed for mass transport within liquid films in rotating disc contactors, namely penetration theory and film theory [22]. In penetration theory, a concentration gradient exists within the entrained film with poor transport perpendicular to the disc surface. In film theory, eddies and turbulent mixing occur within the liquid film ensuring good mixing and good transport to the disc surface. The amalgamation of these models by Toor and Marchello [23] into the film-penetration theory allows the transition between regimes to be calculated using the variation in the rate of air stripping with rotation speed. DCB is a VOC which is easily air stripped from solution and, in the present study, was used to calculate the transition point between film and penetration theory, as described below. The rate of air stripping is governed by the expression RS = kL a(C0 − Ceqm )

(2)

Eq. (2) relates the rate of air stripping of the VOC (Rs ) to the overall mass transfer coefficient (kL ), the specific contact area (a), and the concentrations of VOC in liquid initially and at equilibrium C0 and Ceqm . Fig. 6 shows the effect of rotation speed on the experimentally determined stripping constant kL . A sharp break in the gradient is observed at 90 rpm corresponding to the transition point between the penetration and film theories. Penetration theory predicts that the mass transfer co-efficient increases with ω0.5 and experimentally, at rotation speeds below 90 rpm, kL ∝ ω0.47 . Film theory predicts that the mass transfer co-efficient increases with ω1.5 and experimentally, at

N.A. Hamill et al. / Applied Catalysis B: Environmental 30 (2001) 49–60

Fig. 6. Variation in volumetric mass transfer coefficient with rotation speed following the air stripping of 0.2 mM DCB in the RPC. The gradients indicated in each case represent the power dependence x, of kL on ω, i.e. kL ∝ ωx .

speeds above 90 rpm, kL ∝ ω1.5 , Fig. 6. In all cases, the mass transfer coefficient is found to be invariant with initial concentration and is only a function of disc rotation speed, in agreement with Hseuh et al. [24]. The agreement in the transition points found in the air stripping and the degradation experiments, indicates that the sharp change in the rate of degradation is related to changes in the mass transfer within the entrained film. Below 90 rpm, the RPC is operating under penetration theory governed conditions and the mass transfer coefficient for O2 and DCB is independent of film thickness. Above 90 rpm, film theory is obeyed and in this region, mass transfer is not thought to be rate limiting since good mixing within the bulk solution as well as within the entrained film is present. In region 3, a photon limited process is thought to be operating, as described below. However, these experiments do not allow a distinction to be made between rate limitation due to gas–liquid mass transfer or liquid–solid mass transfer in region 2.

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The invariance with O2 is consistent with the explanation suggested above to explain the variation of DCB degradation with rotation speed. Using the air stripping experiments described above, it is possible to estimate the flux of oxygen to the catalyst surface. The rate of oxygen transfer is much greater than the rate of destruction of DCB, even at the lowest rotation speeds studied. For example, at 5 rpm, the maximum oxygen flux is 2.5 × 10−6 mol dm−3 s−1 , whereas the rate of degradation of DCB is 3.25 × 10−8 mol dm−3 s−1 , i.e. ∼75 times greater. Since the gas to liquid mass transfer of oxygen is not thought to limit the photodegradation kinetics within the RPC, the rate limitation in region 2 is likely to be because of mass transfer within the entrained film between the liquid and solid. 3.4. Effect of light intensity Fig. 7 shows the variation in degradation rate with light intensity variation at 20 and 136 rpm unlike all previous results which were measured using the maximum light intensity. At 20 rpm a gradual increase is found for the degradation rate constant up to 2 mW/cm2 whereas further increases in intensity do not alter the rate. At 136 rpm, increases in light intensity continue to increase the DCB degradation rate. The explanation given for the variation in rate with rotation speed also explains this result. At low speeds, mass transfer of DCB to the surface of the catalyst

3.3. Effect of O2 partial pressure Little effect on kdeg was observed for air saturation compared with O2 saturation in the RPC at a variety of rotation speeds (typical variation <4%). This is analogous to the slurry studies where again little change was observed except at very low O2 partial pressures where a reductive versus an oxidative process was observed [25]. This shows that O2 diffusion is not rate limiting in this range of rotation speeds.

Fig. 7. Effect of light intensity on pseudo-first-order rate constants at 20 rpm (solid line) and 136 rpm (dotted line) for the degradation of 0.2 mM DCB in the RPC.

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limits the rate of degradation and thus increases in light intensity beyond a critical value have little effect. Ohko et al. have also reported this phenomenon for supported TiO2 films for the gas phase photocatalytic oxidation of 2-propanol [26]. With increased photon flux, the reaction rate became increasingly mass transfer limited until it was purely mass transfer controlled and the rate remained constant. At higher rotations speeds, i.e. in region 3, mass transfer is no longer thought to be rate limiting since the system is now photon limited. Under these conditions, the expected power-law dependence of kdeg with light intensity is observed [27–29]. The light intensity variation may be related to the rate via the apparent quantum yield or photonic efficiency (φ 0 ) as defined in Eq. (3) [30]: φ0 =

Moles of reactant converted/unit time Moles of incident photons/unit time

(3)

The photonic efficiency measured at 2 mW/cm2 in the RPC for DCB degradation was 0.039. Increasing the photon flux reduced the photonic efficiency at all speeds. Above 90 rpm, i.e. where the reaction is photon limited, at a light intensity of 5.6 mW/cm2 , the photonic efficiency decreased to 0.027. This is because of the increased electron-hole recombination rate at higher photon flux which results in the fractional order variation in light intensity described above [27]. 3.5. Effect of pH pH effects can be dramatic in photocatalytic reactions. The effect in the present case was studied by comparing rates of degradation at pH 2 and at the natural pH for the system, ∼4.6. On lowering the pH, a 23% increase in kdeg was observed, contrary to the results shown for slurry reactions where a concomitant decrease in pH reduces kdeg . It is likely that this discrepancy is a result of changes in the slurry characteristics, for example, catalyst aggregation [31]. These effects are unlikely to change the immobilised film and variations in surface adsorption properties with pH would be expected to change both the slurry and RPC similarly. Alkaline pH studies could not be performed because of the inherent instability of the oxide films under these conditions [32].

Table 3 Comparison of pseudo-first-order rate constants following successive degradation reactions of 0.05 and 1 mM DCB at 20 rpm in the RPC No. of batch

kdeg (min−1 ), initial [DCB] = 0.05 mM

kdeg (min−1 ), initial [DCB] = 1 mM

1 2 3 4

0.0229 0.0241 0.0252 0.0237

0.0096 0.0090 0.0088 0.0085

3.6. Film degradation The catalyst films were examined for catalyst deactivation using consecutive batches of solution without regeneration of the catalyst surface between reactions. These tests were performed at 20 rpm, under conditions where mass transfer limitation occurs, in order to maximise the intermediates formed, the residence time on the catalyst surface and the likelihood of non-oxidative processes occurring. Each reaction was stopped at the half-life of the reaction so that adsorbed reactants and intermediates might still be expected to be present on the catalyst surface. Table 3 shows the effect of four batches of solution on the kdeg at an initial concentration of 0.05 and 1 mM DCB. At low levels of DCB, little variation in kdeg was observed with consecutive batches of reactant. At 1 mM DCB, the kdeg fell from 0.0096 to 0.0085 min−1 after four cycles. In order to evaluate whether catalyst loss from hydraulic abrasion was responsible, the discs were dried and re-weighed following a series of experiments. After an estimated 65,000 rotations the discs had lost approximately 2.5% (by weight) of the original catalyst loading. The loss occurred predominantly in the first 8000 rotations and implied that this was because of loosely bound material being washed off, as opposed to constant abrasion. Although the decrease in rate of degradation may be because of a build up of competing recalcitrant intermediates, the change observed is small and may also just be related to catalyst removal. 3.7. Competition effects The rate of degradation of DCB was studied in the presence of methanol. Table 4 shows the pseudofirst-order rate constants in the presence (kMeOH ) and absence (kdeg ) of methanol for solution containing

N.A. Hamill et al. / Applied Catalysis B: Environmental 30 (2001) 49–60 Table 4 Pseudo-first-order rate constants for the degradation of 0.2 mM DCB in the presence (kMeOH ) and absence (kdeg ) of 30 mM methanol in the slurry reactor and in the RPC at 20 and 136 rpm Reactor setup

kMeOH (min−1 )

kdeg (min−1 )

Slurry RPC @ 20 rpm RPC @ 136 rpm

0.0069 0.0037 0.0025

0.0330 0.0130 0.0244

[DCB] = 0.2 and [MeOH] = 30 mM at 20 and 136 rpm in the RPC and in the slurry reactor. In slurry phase reactions, methanol competes with DCB for adsorption sites and is preferentially degraded leading to an induction period prior to significant DCB degradation whilst the alcohol is reacting. This general phenomenon is also observed for RPC. At low speeds, 20 rpm, the pseudo-first-order rate constant of DCB degradation in the presence of methanol is a factor of 3.5 slower than for DCB in the absence of the methanol whereas at higher speeds, 136 rpm, this factor increases to 9.7. This may be compared with the slurry reactor which has a factor of 4.8. Contrary to the general observation that increasing rotation speed leads to an increase in degradation rate, in the presence of methanol, the DCB degradation rate actually decreases. It is inherent in the design of the RPC that as the reaction only occurs on exposure of the catalyst to the light, degradation only occurs in the entrained film on rotation into the headspace. In the headspace, the catalyst is exposed to a static liquid film with little lateral mixing with the bulk solution. Hence, the maximum amount of pollutant in the entrained liquid film at any given time is fixed and small. At low speeds, this enables the methanol to be degraded without being replenished from the bulk solution, thus freeing adsorption sites so that the DCB present in the entrained film may subsequently be degraded, i.e. the methanol is degraded in less than the headspace residence time. At higher speeds, although the overall degradation rate of methanol has increased, this is due to the catalyst surface being continually supplied with methanol from the bulk solution without decreasing the concentration in the entrained film significantly, i.e. the methanol is no longer degraded in less than the headspace residence time. Since the entrained liquid film never becomes methanol lean, it continues to block sites till such time as the methanol has been

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reduced significantly in the bulk solution. Under these conditions, the surface concentration of DCB is low and only becomes significant at extended reaction times when significant degradation of methanol has occurred. At higher speeds, the rate is still increasing for the overall degradation, i.e. methanol + DCB, although the selectivity towards DCB removal has decreased. This effect has significant consequences in the industrial application of the RPC where it is necessarily the case that a variety of species will be present. Species which adsorb strongly will necessarily be degraded more quickly than species which are weakly bound owing to surface concentration effects and site blocking. If easily biodegradable species such as methanol block the sites, the more recalcitrant species will be untreated. In most photocatalytic reactor designs, it is not possible to tailor the surface residence time of the pollutants whilst exposed to the UV radiation. However, because of the small volumes involved in the entrained film and the poor mixing between the films in the headspace and the bulk solution, the RPC can be optimised for a particular effluent. The weakly adsorbed or recalcitrant species may then be degraded in the presence of other more strongly adsorbed or more easily oxidised species. 3.8. DCB degradation compared with alternative pollutants The degradation of phenol and a highly UV absorbing dye, DB with an absorption coefficient, ε = 105 l mol−1 cm−1 (@ 365 nm) has been studied using the RPC. Table 5 summarises the initial rates and Table 5 Initial rates and pseudo-first-order rate constants for the degradation of 0.2 mM/1 mM phenol and 0.3 mM/3.3 mM DB in the slurry reactor and in the RPC at 136 rpma Reaction

rRPC,ini (nmol dm−3 s−1 )

kRPC (min−1 )

kslurry (min−1 )

0.2 mM phenol 1 mM phenol 0.3 mM DB 3.3 mM DB

21.3 33.8 22.9 12.9

(0.15b ) (0.75) (0.25) (2.58)

0.0085 0.0027 0.0055 0.0003

0.0064 – 0.0071 –

a The rates have been calculated from the equilibrium concentrations in solution (shown in brackets) following dark adsorption. b The values in brackets are expressed in mM.

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pseudo-first-order rate constants for various concentrations of phenol and DB. Although both the dye and phenol were degraded as expected using the RPC and the initial rates of destruction increases with phenol concentration, the initial rate for DB decreases with increasing concentration. The decrease in rate with increasing DB concentration suggests that there is reduced UV light penetration to the catalyst. This is common for dye pollutants and, as a consequence, photocatalysis is usually ineffective other than at low concentrations of dye — typically 0.03 mM [4,5]. In the present study, however, even at concentrations of 3.3 mM, the RPC efficiently catalyses the degradation of the dye. Degradation proceeds at such high concentrations owing to increased penetration depth through the thin entrained liquid film which increases the efficiency of the light transmission to the catalyst surface. As film thickness decreases with decreasing rotation speed, it becomes advantageous to operate the RPC at low rotation speed in the treatment of effluents of a highly absorbing nature. The high rate of phenol degradation in the RPC may be because of a decrease in the formation of strongly adsorbed intermediates. Direct electron transfer to benzoquinone is thought to cause a build-up of irreversibly adsorbed quinones and polymerisation products [33]. Catechol, in particular, displays strong chemisorption to surface hydroxyl sites [34]. This generally occurs under anoxic conditions. At high rotation speeds, where the RPC operates under photon limiting conditions, there is little possibility of an anoxic zone occurring on the disc owing to the excellent oxygen mass transport within the entrained film and hence direct electron transfer and the formation of surface poisons are prevented. 3.9. Comparison with other reactors Dionysiou et al. [8] have reported a similar reactor design to the RPC for the degradation of chlorinated phenols in water. Their reactor contained only one disk coated with TiO2 using an adhesive and was a continuous flow design using an open vessel. As in the present study, this reactor proved to be highly effective for the degradation of pollutants and found little variation in the rate of degradation with the adhesive used for the coating was observed. Unlike the present

study, no variation was observed with rotation speed. Dionysiou et al. report that at 12 rpm a power-law dependence on the light was observed and hence even at the lowest speed, mass transfer did not limit the reaction rate. This difference is because of the size of the discs and the surface roughness employed in each study. In region 1, described above, the transition between laminar and turbulent flow in the bulk solution occurs between 20 and 32 rpm, corresponding to R e = 1500–2400. Dionysiou et al. only studied rotation speeds above 12 rpm and in the single disc design, assuming the spacing between the discs is associated with the disc to vessel wall distance, Re ∼ 2250 at 12 rpm. As stated above, the transition between laminar to turbulent flow should theoretically occur at a Re ∼ 2100 and, therefore, both studies could be limited by poor mixing in the bulk solution. In each case, however, the method of preparation of the catalyst films differed markedly. An increased surface roughness is likely to occur in the preparation using an adhesive compared with a slurry-coated film, as was employed in the present study. The greater roughness would lower the Reynolds number for the transition from laminar to turbulent flow, hence the rotation speed at which it occurs. This difference coupled with the flow system explains the absence of mass transfer effects in the previous study associated with bulk solution mixing. The second transition point partially occurs because of a change in the film thickness with increased peripheral velocity, however, this is not the only factor involved in conversion from penetration theory to film theory. It has been reported that larger discs undergo this transition at much lower speeds than smaller discs. For example, Hsueh et al. [24] used discs of diameter 38 cm for the air stripping of VOC and found that the transition from penetration to film theory occurred at a peripheral velocity of 0.08 m s−1 , this transition occurs in the present system at a peripheral velocity of 0.5 m s−1 . Since Dionysiou et al. used discs of diameter 50 cm, the transition would occur at a similarly slow speed. Direct comparison between the two reactors shows good agreement in the pseudo first order rate constants. At 0.2 mM, kdeg (2,4,6-trichlorophenol) = 0.007 min−1 which decreases to 0.0013 min−1 at 1 mM. Our results are in good agreement with the phenol pseudo-first-order rate constant varying from

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0.0085 min−1 at 0.2 mM to 0.0027 min−1 at 1 mM, see Table 5. It should be noted that Dionysiou et al. [8] showed that phenol and 2,4,6-trichlorophenol have similar pseudo-first-order rate constants. By screening the bulk solution from the light, we have demonstrated that photodegradation only occurs in the headspace of the RPC. The disc rotation effectively creates periodic illumination of the catalyst, and, therefore, the phenomenon of controlled periodic illumination (CPI) may be contributing to the photonic efficiencies observed. CPI has been shown by several studies to dramatically increase the photoefficiency of TiO2 photocatalysed processes. Upadyha and Ollis [35] theorise that dark recovery time allows more time for the rate-limiting step (namely the adsorption of O2 on to the catalyst surface and/or the transfer of electrons to adsorbed O2 ) to occur. However, Ohko et al. [36] define light intensity as the average time between photon impacts and therefore describe CPI as merely an enhancement associated with the lowering of light intensity. Noble et al. [37] have shown that for a fully immersed vertical rotating disc contactor the CPI effect was not observed and that increases in photoefficiency could be attributed to mitigation of mass transfer limitation (the dark time allowed accumulation of oxygen at the surface with associated rate enhancement during illumination). This may be particularly true in the present reactor where mass transfer processes strongly contribute to the reaction rate. 3.10. Scale up As described above, although mineralisation was found to occur in our system, CO2 levels were found to be only 30% of the total carbon mass balance and acetaldehyde reached levels of 45%. This is a direct result of the sealed vessel, which essential for the degradation of VOCs such as DCB. Dionysiou et al. [8] also reported that mineralisation did occur but, in their study, an open vessel was used. The build up of reactively formed VOCs, such as acetaldehyde, and the possibility of strongly adsorbed surface species deactivating the catalyst limits the general applicability of this sealed design for the treatment of VOCs. The effect would require reactors to be used in tandem. For example, one reactor could be cleaned, by photoxidising the surface bound species in the presence

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of clean water, whilst the other performed the normal water treatment process. It is unlikely that the RPC could be used with caustic effluent streams owing to the instability of the titania films, but in such cases buffered aqueous streams could be used. Dionysiou et al. [8] showed that adding phosphate buffers had little detrimental effect on the degradation efficiency in their rotating disc reactor. In both studies the most likely final concept would be to retrofit these units to existing rotating biological contractors as a pre-treatment step in order to degrade biological poisons prior to contact with the enzyme degradation units.

4. Conclusion The RPC has been shown to effectively degrade a range of substrates and the rate of degradation increases with rotation speed up to 90 rpm in the present study. Both volatile and involatile pollutants may be degraded with equal efficiency. In the case of VOCs, the need for a sealed vessel to prevent air stripping may cause problems because of the build up of gas phase pollutants, such as acetaldehyde, which may subsequently poison the catalyst. Owing to the thin entrained liquid films, UV absorption by substrate molecules or intermediates is reduced in comparison with slurry phase reactors and hence the RPC may be employed successfully for dye effluent treatment. By using the mass transfer limitations, it is possible to optimise the reactor performance for combinations of pollutants and alleviate some of the problems found when treating recalcitrant pollutants in the presence of easily oxidised or strongly adsorbed molecules such as methanol.

Acknowledgements We thank DuPont Dow Elastomers and the QUESTOR centre for their support and the Department of Education for Northern Ireland for a Distinction Award to N.A.H. We thank Degussa for the generous donation of the TiO2 P25 used in this study. We would like to thank Dr. M.G. Burnett and Dr. D. Rooney for useful discussions in the preparation of this manuscript.

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