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The photodegradation kinetics of aqueous sodium oxalate solution using TiO2 catalyst
Applied Catalysis A: General 175 (1998) 221±235
The photodegradation kinetics of aqueous sodium oxalate solution using TiO2 catalyst Jimmy Bangun, Adesoji A. Adesina* Reactor Engineering and Technology Group, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 12 January 1998; received in revised form 13 May 1998; accepted 17 June 1998
Abstract The kinetics of the photodegradation of sodium oxalate have been investigated in a semi-batch annular photoreactor using commercial titania as the catalyst. Sodium oxalate is an important toxic pollutant in Bayer liquor during alumina processing and its removal and possible rejuvenation of the caustic solution is vital to plant economics. Experiments carried out under UV irradiation (250±310 nm wavelength) over 3 h runs showed that initial solution pH, light intensity, oxygen partial pressure, catalyst loading and oxalate concentration all have strong effects on the decomposition rate. The catalyst exhibited a relatively low activation energy of about 45.8 kJ molÿ1. The degradation rate decreased exponentially with pH, but both O2 and catalyst loading went through a maximum at about 45% O2/N2 and 1.5 gcat lÿ1, respectively. The rate±oxalate concentration behaviour indicates that the catalyst has a high adsorption constant for oxalate species. This was attributed to the increased density of positively charged sites at pH values below the isoelectric point (pH6). The essential features of the proposed mechanism involve aqueous phase dissociation of sodium oxalate followed by adsorption of the C2 O2ÿ 4 ions onto positively charged sites. However, adsorption of O2 to electron-rich sites yields the superoxide ions which are protonated to give highly active peroxy radicals. The data suggested a bimolecular surface reaction between adsorbed oxalate and peroxy species as the rate-controlling step in the homogeneous±heterogeneous mechanism. The subsequent production of HCOÿ 3 ions gave increased pH with time-on-stream. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Photocatalytic degradation; Sodium oxalate; Titania; Bayer liquor
1. Introduction High molecular weight organic compounds associated with bauxite are converted to sodium salts (mostly carbonate and oxalate) during caustic digestion in the Bayer process for alumina production. This results in substantial loss (>10%) in caustic solution *Corresponding author. Fax:+61-2-9385-5966; e-mail: [email protected]
which should be recycled to the digestion unit. While sodium carbonate may be easily converted with lime treatment to NaOH and CaCO3, the industry still faces considerable challenges in the removal of sodium oxalate. The ef®cient disposal of, and if possible recovery of, caustic solution from sodium oxalate is vital to Australia's alumina industry ± the nations's third largest export earner. Due to its low solubility in Bayer liquor, sodium oxalate readily crystallises during alumina precipitation. This interference with
0926-860X/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00223-3
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agglomeration leads to increased generation of alumina trihydrate ®nes and hence poor hydrate classi®cation ef®ciency. Many of the oxalate control techniques used in industry are based on physical separation or conversion to less harmful substances ± the manganese dioxide process or liquor calcination. Recently, a patented microbiological process [1], the Rotating Biological Contactor, in which the oxalate can be removed within three weeks has been described. Photocatalytic destruction of organic pollutants in aqueous systems (whether with oxygen or anaerobically) has been successfully utilised for the complete mineralisation of toxic recalcitrant compounds at relatively faster rates than other conventional thermal, catalytic or biological pathways and under very mild conditions of temperature and pressure [2±10]. It is now increasingly evident that the photocatalytic degradation route is the most attractive of the advanced waste-water treatment and water puri®cation processes. The opportunity for solar energy utilisation and low reactor volume per unit throughput are additional incentives. In a previous paper [11], we demonstrated that the light-assisted decomposition of sodium oxalate over a titania catalyst was indeed accompanied by an increase in pH with time-onstream, suggesting the production of a caustic solution. Thus, photocatalysis offers itself as a useful technology to improve production economics and environmental strategy in the minerals processing industry. In anticipation of multiphase photoreactor optimisation, we now report a detailed kinetic investigation of this reaction. Most photocatalytic reactions proceed via redoxtype mechanisms [6±8,12±14]; thus the pH of the feed solution has a signi®cant effect on reaction rate. Trillas et al. [15] observed that the photodegradation of 2, 4 dichlorophenoxyacetic acid was fastest at a slurry pH of 3, while phenol oxidation in aqueous titania suspension has an optimum pH of 7 [14,16,17]. Gimenez et al. [9] have also noted that the photoreduction of Cr(IV) using titania powder was highest at pH1, with catalyst deactivation setting in at pH>4 due to fouling by chromium hydroxides. On the contrary, an increase in pH improved the photocatalytic degradation of malonic acid, attaining a ``plateau'' at pH7. In fact, maximum CO2 production rate was obtained under this condition, since malonic acid contains two ioni-
sable hydrogens which can be abstracted at pK12.85 and pK25.69. Interestingly, Vidal et al. [18] saw no discernible effect of slurry pH during the photodegradation of ethylbenzene over TiO2. The role of oxygen partial pressure in photomineralisation has also been investigated. Lu et al. [19] reported that optimum gas composition was 20% O2 for the oxidation of dichlorovos. Mills et al. [20] found that the photo-oxidative destruction rates of salicyclic has a turning point at 60% O2. In many cases, O2 adsorbs to give the superoxide ion which is the precursor for surface peroxy or hydroxy radicals. Oxygen also plays an important role as an electron scavenger, thus reducing electron-hole recombination rates and hence improved quantum ef®ciency [17]. The mass transfer characteristics of the bubble column reactor are also in¯uenced by O2 distribution in the liquid phase. In general, bubbles travelling at optimum velocity give high mass transfer rates. Whilst a low gas ¯ow rate introduces gas±liquid transport resistance, very high ¯ow rates engender bubble coalescence and slug formation and poor reactor performance due to reduced residence time. Hydrodynamics and transport effects in multiphase reactors constitute the subject of several classic treatises [21±23]. Speci®c issues pertinent to the present work are taken up in a later section. Substrate concentration also affects photocatalytic rate. Wei and Wan [17] reported the inhibitory role of initial phenol concentration on the reaction rate constant. CO2 formation during photocatalytic mineralisation of malonic acid over titania has a ®rst order dependency at low initial organic acid concentration (<2 mM), but a zero order kinetics was observed at higher values [13]. Dhananjeyan et al. [24] reported a similar rate-concentration behaviour for the photodegradation of thymine. In conventional slurry reactors, conversion generally increases with catalyst loading and a plot of the reciprocal of rate against the reciprocal catalyst loading gives a straight line which may be used for diagnosing various transport resistances. However, the role of catalyst concentration in multiphase photoreactive systems is somewhat diffused due to light scattering and other secondary effects at high catalyst loading [17]. The photo-oxidation rate of malonic acid increased with catalyst loading up to 0.8 g lÿ1 and thereafter remained constant at this limiting value even for TiO2 content of up to 3 g lÿ1 [13]. The
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photocatalytic destruction of phenol also followed a similar pattern [17]. Temperature appears to have only a mild effect on photocatalytic rates, since many reactions reportedly possess low activation energy, EA. The oxidation of salicylic acid over titania has an EA of about 4.6 kJ molÿ1 between 298 to 323 K [20], while the photo-induced production of H2 and CO2 from aqueous ethylene glycol has values of 15 and 18 kJ molÿ1, respectively [25]. However, the multiphase nature of a photocatalytic reaction system can easily induce transport-disguised kinetics and render meaningful mechanistic evaluation dif®cult. If it is unambiguously established that the reaction is intrinsically characterised by a low activation energy, this will offer a raison-d'etre for the role that light plays in providing alternative energetically-favourable sites for facile surface reaction on the photocatalyst and why reactions otherwise kinetically-constrained (under conventional catalytic temperatures) now proceed with relative ease at low temperatures (<333 K). Active sites (holes and electrons) are generated on the surface of semiconductor oxides when illuminated with light of wavelength greater than the band gap energy. The effect of light intensity is, however, not well understood. Light intensity is often determined using ferrioxalate actinometry, although the introduction of modern digital radiometers may now permit more accurate measurements. Wei and Wan [17] observed that phenol oxidation rates followed a parabolic behaviour up to about 1.61 mEinstein sÿ1 and subsequently showed a ®rst order dependency on light intensity at higher values. Okamoto et al. [26], however, recorded a square root relationship for the same reaction. The square root dependency was also seen in the case of malonic acid oxidation [13]. From the foregoing, it is evident that a comprehensive kinetic study should include the effects of all six factors: namely, pH, light intensity, catalyst loading, oxygen partial pressure, oxalate concentration and temperature on the photocatalytic reaction rate. 2. Experimental 2.1. Materials and methods High purity (AnalaR grade) sodium oxalate and titania (>99% anatase) were obtained from Aldrich
223
and used as supplied in all runs. Milli-Q triply distilled and deionised water was used to prepare the feed solution. NaOH and HCl were used to adjust the pH of the slurry prior to reaction. Ultrapure O2 and N2 gases were obtained from BOC Gases, Sydney. A Waters Associate programmable ion chromatograph (model 590) with a conductivity detector was employed to measure oxalate concentration. CO2 photogeneration was determined from an on-line GOW-MAC gas chromatograph with a TCD. Continuous pH monitoring was obtained from a TPS Digital pH meter. Light intensity was measured by a IL1400 radiometer/detector (International Light, Massachusetts, USA) calibrated for 265±332 nm. 2.2. Apparatus Kinetic data were collected from the annular photoreactor shown in Fig. 1. The reactor was equipped with a commercial UVarc lamp driven by a three-level setting power supply (low200 W, medium300 W, high400 W). Spectral analysis from the local vendor showed a signi®cant fraction of the radiation in the 250±310 nm range. The UV lamp was enclosed in a double-walled quartz hollow U-tube (OD4 cm) through which water was circulated at 750 ml minÿ1 as coolant to maintain reaction isothermality and to remove IR fraction of the incident rays. The lamp was suspended at the top with a silicon rubber-bung tightly ®tted as a lid to the outer reactor Pyrex casing (ID10 cm). The external surface of the lamp was continuously ¯ushed with N2 to remove ozonised air in the space between the U-tube and the lamp. A thermocouple and pH probe were inserted as shown to register the reactor temperature and pH, respectively, while electronic mass ¯ow controllers were used to regulate gas ¯ow rates. 3. Results and discussion 3.1. Analysis of mass transport conditions Blank runs carried out in the presence of oxygen and under a variety of anaerobic conditions showed that oxygen, catalyst and UV light are necessary to achieve measurable oxalate decomposition rates. The reactor was operated in the semi-batch mode (con-
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Fig. 1. Photoreactor assembly.
tinuous upward O2/N2 supply at 500 ml minÿ1) at a total pressure of about 101 kPa. In a multiphase reactor, various transport resistances could falsify true kinetics, thus, operating conditions were chosen to ensure that gas±liquid, liquid±solid and intraparticle
transfer limitations were negligible. As may be seen from Fig. 2, gas ¯ow rate ceased to have any effect on rate above 400 ml minÿ1 thus justifying operation at 500 ml minÿ1. It is, however, conceivable that at gas ¯ow rates higher than the range used here
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225
Fig. 2. Effect of gas flow rate on photodegradation.
(>1200 ml minÿ1), bubble coalescence may become signi®cant and the attendant large bubbles and shorter residence times may result in a drop in conversion. Although the volumetric gas±liquid mass transfer coef®cient, kLaL, in a bubble-column reactor is a complex function of sparger design, ¯ow regime, physicochemical characteristics of the system, gas velocity, catalyst concentration and particle size,
Deckwer et al. [27] have pointed out that reactors with sintered plate distributors are generally endowed with the best kLaL values. Thus, a combination of high gas ¯ow rate and a 20-micron sintered glass plate distributor used in this study ensured the absence of external transport resistance. In fact, using the correlation of Akita and Yoshida [28], kLaL was estimated as 5.9710ÿ4 sÿ1 (cf. Eq. (1)).
Table 1 Values of various parameters Parameter
Value
CAb (based on Henry's law, pAHxA) H DAB Deff dT g "g ÿrexp g L P ST STW L g ug utp gdP2 P ÿ L =18L
310ÿ7 mol cmÿ3 at PO2 0.3 atm 4.75104 for O2 at 303 K 210ÿ5 cm2 sÿ1 0.1DAB 10 cm 981 cm sÿ2 3.510ÿ5 710ÿ6 mol gcatÿ1 minÿ1 (at 30% O2) 1.210ÿ3 g cmÿ3 1.0 g cmÿ3 3.9 g cmÿ3 72 dyne cmÿ1 72 dyne cmÿ1 0.85 cP 0.018 cP 0.106 cm sÿ1 2.310ÿ7 cm sÿ1
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k L aL
0:6D0:5 AB
ÿ0:12 ÿ0:62 L ST dT0:17 g0:93 "1:1 g L L
concentration-time plot was based on analysis of aliquots taken at 5 min intervals over a 2±3 h period. (1)
with the variables as de®ned in Section 5. Table 1 gives the values of the speci®ed parameters. For a well-mixed system, this kLaL value predicts conversion of 100%, suggesting that gas±liquid mass transfer was not rate-limiting under these conditions. The average catalyst particle diameter for the commercial TiO2 powder was 0.35 micron and hence, intraparticle transport resistance should be minimal. To be sure, the generalised Thiele modulus, n for an nth order kinetics may be given by: 2n
nÿ1 kn dp2 Sa P CAS
4Deff
;
(2)
where the particle surface concentration, CAS, may be taken as the bulk liquid concentration CAb in the absence of external transport resistance. For unknown kinetics, Eq. (2) may be rewritten as: 2exp
ÿrexp dp2 p 4CAb Deff
;
(3)
Using the data in Table 1, exp was estimated as 1.5210ÿ2. For such small values of exp (<10ÿ1) the effectiveness factor, , is essentially unity irrespective of particle shape. It may therefore be concluded that intraparticle transport effect is practically nil. Additionally, the liquid±solid mass transfer coef®cient, kS, evaluated from the Kobayashi±Saito correlation [29] given in Eq. (4), was a high 1.143 cm sÿ1. This signals the absence of liquid±solid limitation. " #1=3 dp3 p ÿ L g k s dp dp ug L 0:112 2 0:212 : DAB L DAB L (4) Incidentally, the operational parameters put the ¯ow situation in the multiphase reactor in the so-called homogeneous bubble ¯ow regime [21], where the hydrodynamic characteristics facilitated excellent mass transport. Indeed, the presence of well-dispersed ®ne bubbles in the reactor was clearly visible during all runs. Initial rate data were obtained from the slope of the concentration-time pro®le in the ®rst 30 min of each run. This curve was often linear in this region. The
3.2. Effect of slurry pH The photoreactivity of TiO2 is determined by the redox potential of the reacting medium. The isoelectric point, IEP, of titania in water is about 6; thus pH values above and below this point were investigated. Fig. 3 shows that the photodegradation rate dropped almost exponentially with increase in slurry pH. At low pH (<6), the catalyst surface is predominantly positively charged. The relatively high concentration of positive sites and increased electrostatic attraction between anions and such sites probably improved the surface coverage of oxalate species, and hence gave high degradation rates. Conversely, at high pH values, surface negativity will inhibit photocatalytic rates. Additionally, the solubility of sodium oxalate decreases with increased alkalinity [30]; consequently, at low pH, the liquid phase concentration of free oxalate ions will be high. This situation would facilitate higher surface concentration of the adsorbed oxalate species and accordingly better reaction rates. Incidentally, since the photodecomposition leads to pH rise with time on-stream [11], product inhibition sets in. From the foregoing, it appears that the pH value played a signi®cant role both in the homogeneous (dissociation of Na oxalate) and surface (creation of titania sites with adequate electrical polarity) steps of the proposed mechanism. 3.3. Effect of initial oxalate concentration The oxalate concentration in the feed was varied between 0 and 3 mM to cover the typical oxalate content in industrial liquor (ca. 10ÿ5±10ÿ4 M). It is evident from Fig. 4 that the rate of photocatalytic degradation rose very rapidly for dilute concentrations (<0.5 mM) ± since the curve must pass through the origin, before changing to a lower value at higher concentration. It was dif®cult to obtain additional data points in the low oxalate concentration range due to reproducibility problems with the ion chromatograph under these conditions. Even so, it would seem that the catalyst has a large surface capacity for oxalate anions, although this itself may be a function of the pH employed. A pH value of 2 used for this run may
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227
Fig. 3. Variation of photocatalytic degradation rate with slurry pH.
Fig. 4. Influence of initial oxalate concentration on rate.
also partly explain the high adsorption constant for species due to the abundance of positively C2 O2ÿ 4 charged sites, consistent with our previous observation on the role of slurry pH. Clearly, the adsorption of oxalate ions may be ruled out as the rate-controlling step (rcs) in this study. Indeed, Sclafani et al. [8] have pointed out that signi®cant interactions between the various physicochemical factors and the redox reac-
tions may make it impossible to establish a unique correlation for the photoreactivity of the semiconductor catalyst. 3.4. Effect of oxygen The in¯uence of O2 partial pressure on initial rate is displayed in Fig. 5. Although oxygen content was
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Fig. 5. Effect of oxygen composition on rate.
varied, the total gas ¯ow rate (O2/N2 mixture) bubbled through the reactor was kept constant at 500 ml minÿ1 to ensure identical ¯uid dynamics. The rate increased with oxygen partial pressure, as may be expected for an aerobic reaction; nonetheless, a turning point is seen at about 45% O2 beyond which enrichment in oxygen was detrimental to the degradation. Such an optimum is symptomatic of competitive inhibition between the two reactants (oxygen and oxalate ions) for adsorption on the same site. It is known that O2 adsorbs on electron-rich sites (eÿ cb ) during photo-oxidation; hence, competitive inhibition may not be the obvious reason for the down-turn in rate at high oxygen partial pressures. Upon adsorption, the proto nation of the superoxide, Oÿ 2 adspecies by H ions to yield highly reactive peroxy (HO2 ) radicals which are thought to be the initiators in the oxidative photodecomposition of organics [12±17] leads to a rapid drop in solution acidity. This in turn decreases the dissociation of sodium oxalate as well as the concentration of positively charged vacant sites, and hence a reduction in the surface concentration of oxalate ions (in accordance with Le Chatelier's principle). This scenario would therefore be responsible for the lower degradation rates seen at higher O2 partial pressures. 3.5. Effect of catalyst loading Experiments to determine the effect of catalyst loading were carried out at the optimum pH (2)
and oxygen partial pressure (45% O2). Intuitively, since the photocatalyst is necessary for measurable degradation rates, high titania content should be bene®cial. However, the maximum amount of catalyst that can be kept in suspension Wmax for a bubblecolumn reactor may be estimated from [21]: Wmax 6:8 10ÿ4 C dT ug G ST "g ÿ0:23 L G ug L ÿ0:18 "g utp
ÿ3 ; (5a) ug where C, a viscosity correction factor, and the gas hold-up "g are given by C 0:232 ÿ 0:1788 ln L 0:1026 ln L 2
(5b)
and "g
ug STW L =ST 1=3 : 30 2ug
(5c)
Using the values provided in Table 1 for this system, Wmax was calculated as 13 wt% (130 gcat lÿ1) for ¯ow in Stoke's regime (Rep<0.4) The data presented in Fig. 6 are clear evidence of the nonlinear dependency of rate on catalyst concentration. Wei and Wan [13] had made a similar observation during photocatalytic degradation of phenol over titania,where an optimum catalyst loading of 1±3 g lÿ1 was obtained. The exact value seemed to vary with the type of titania and reactor con®guration.
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229
Fig. 6. Influence of catalyst loading on rate.
The anatase form of TiO2 reportedly exhibited better photoactivity than the rutile type. Also, the in¯uence of catalyst loading was unnoticeable in low capacity reactors (<100 cm3). The present photoreactor has an annular volume of 1.25 l in which titania (>99% anatase) was suspended. Thus, an optimum catalyst loading of 1.5 g lÿ1 under the prescribed operating conditions is consistent with observations on other organic pollutants. From the pro®le shown in Fig. 6, the increase in rate with catalyst loading (below the optimum value) may be due to improved concentration of active sites with increase in number of particles in solution. However, as particle concentration increases, particle±particle interactions may produce deleterious effects such as light shielding and agglomeration. Light shadowing may be due to a decrease in the photons incident on the catalyst surface at high solids content, while agglomeration results from collision between grains [31]. Thus, in spite of the increase in catalyst loading, the poor light-harvesting and loss in particle external surface area leads to a drop in surface concentration of active sites (h vb and eÿ ) and a decline in photodecomposition rate beyond cb the `critical' catalyst loading. However, since particle± particle interaction (e.g. agglomeration) is itself a function of medium acidity, it is conceivable that the optimum catalyst loading would also vary with initial pH.
3.6. Effect of temperature Several studies have shown that photocatalytic reactions are generally characterised by low activation energy values. Fig. 7 shows that a relatively low activation energy, EA, of about 45.8 kJ molÿ1 obtained for sodium oxalate photodecomposition corroborates this widely observed feature. It would seem that although surface processes are involved (as in conventional heterogeneous catalysis) and one would expect the overall activation energy to be a `hybrid' of various elementary steps, the energetically-signi®cant photogeneration (of active sites) step is temperature-insensitive and hence the reduction in activation energy, at least in the Arrhenius-sense. Additionally, the migration of the photogenerated holes and electrons to the particle surface is essentially a low activation energy diffusional process. Although the present study gave a higher activation energy than that of salicyclic acid (4±5 kJ molÿ1) and ethylene glycol (15±18 kJ molÿ1) [20,25], it is a credible estimate since mass transfer effects were negligible. In particular, the overall activation energy is a re¯ection of both homogeneous solute dissociation step and heterogeneous surface reaction steps; thus sodium oxalate, which is a less soluble compound than salicylic acid and ethylene glycol, would exhibit a somewhat higher EA value, as we have observed.
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Fig. 7. Arrhenius plot.
(6)
of is practically constant at 0.1005 cmÿ1. For a bubble-column photoreactor, the catalyst in the annular space is exposed to an average light intensity, IAV, given by: R R0 Ir dr : (7) IAV RRRi 0 Ri r dr
at each of the three different power ratings. The value
For our reactor the annular containing the slurry is
3.7. Effect of light intensity The commercial lamp used exhibited a characteristic exponentially decaying light intensity with radial distance (from source) as shown in Fig. 8 which can be represented by: I ÿ I0 eÿ r
Fig. 8. Radial profile for light intensity of the UV lamp.
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
3 cm thick; thus R03 cm and Ri0, so Eq. (7) yields: IAV 2 1 eÿ R0 1 R0 2 2ÿ (8) I0 R0 and upon substitution provides IAV/I082%. Thus, it is seen that even with this relatively large annular volume (1.25 l) the average particle is exposed to more than 80% of the maximum intensity available. Light intensity affects the degree of light absorption by the photocatalyst and hence the rate of reaction. Fig. 9 shows that the degradation rate increased linearly with light intensity. Previous workers have observed orders ranging from 0.5 to 1 in titania suspensions during photo-oxidation of organic acids [13,17]. Apart from this set of runs, a light intensity at 300 W (medium) was used in all other experiments. 3.8. Mechanistic conjecture and kinetic modelling In order to prescribe a plausible sequence of elementary steps consistent with these experimental observations, we adopt the formal procedure outlined in Fogler [32]. The data shown in Fig. 3 for the in¯uence of pH on degradation rate may be described by an exponential rate law of the type: ÿroxal Aeÿb1 pH ;
(9)
where regression analysis provides these parameter estimates: A1.5710ÿ2 mmol gcatÿ1 minÿ1 and b10.315 with a correlation coef®cient, Rÿ0.99. However, since pH ÿlog10 CH
k1 Coxal 1 Koxal Coxal
ÿroxal
k2 Poxy
1 Koxy Poxy 2
;
(13)
where k28.323210ÿ4 mmol gcatÿ1 minÿ1 kPaÿ1 and the oxygen equilibrium adsorption constant, Koxy3.002710ÿ2 kPaÿ1 with a correlation coef®cient of 0.94. Although the catalyst may not be explicitly regarded as a reactant, its effect as shown in Fig. 6 may also be modelled by an equation structurally similar to Eq. (13), namely: 0 ÿroxal
kcat Ccat
! Ccat 2
;
(14)
where the parameters, kcat is a pseudo-rate constant for the catalyst and ! is an attenuation factor which accounts for the apparent volumetric light shielding effect and the squared denominator is indicative of particle±particle interaction. Values for these parameters were determined: kcat2.963910ÿ2 mmol gcatÿ1 minÿ1 and !0.8486 gcat lÿ1 with a good data ®t possessing a correlation coef®cient of 0.98. The composite rate equation at constant catalyst loading and pH may therefore be written as: krxn Coxal Poxy
ÿroxal
(11)
where krxn is a function of pH and catalyst loading and is given by:
where bb1/2.303. Similarly, the data shown in Fig. 4 may be represented by a Langmuir±Hinshelwood (LH) equation for oxalate ion adsorption on positively charged sites as ÿroxal
data. Appropriate inversion of Eq. (12) followed by linear regression of the data gave R0.94, while the oxalate adsorption constant, Koxal is 11.627 l mmolÿ1 and k1 has a value of 0.1557 l gcatÿ1 minÿ1. The nonlinear effect of O2 shown over the entire range of partial pressure as seen in Fig. 5 may be captured by the LH expression, written:
(10)
then ÿroxal ACHb ;
231
(12)
to accommodate behaviour at low and high oxalate concentrations. Clearly, at very dilute oxalate concentrations, (KoxalCoxal 1) the model approaches a straight line through the origin as required by the
1 Koxal Coxal 1 Koxy Poxy 2
krxn A k1 k2 CH0:137
kcat Ccat
! Ccat 2
:
;
(15a)
(15b)
It is apparent that Eq. (15a) may arise from a Langmuir±Hinshelwood mechanism involving dual adsorption sites. This is consistent with the proposition that oxalate ions are chemisorbed on positively charged sites, while oxygen is adsorbed on electronrich centres. Thus we propose the following mechanism for the photocatalytic degradation of sodium oxalate:
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Fig. 9. Effect of light intensity on photocatalytic rate.
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Na2 C2 O4 2Na aq C2 O2ÿ 4 aq homogeneous dissociation TiO2 light
eÿ cb
(16)
h nb
photogeneration of active sites H 2 O h nb OH ads ÿ O 2 eÿ cb O2 ads
H aq
Oÿ 2 ads
H aq
(18) (19)
HO2 ads
(20)
ÿ C2 O2ÿ 4 aq hnb C2 O4 ads
C2 Oÿ 4 ads
HO2 ads
(17)
HCO3 ads
(21)
COÿ 3 ads
(22)
COÿ 3 ads H aq HCO3 ads
(23)
HCO3 ads
(24)
Na aq
HCOÿ 3 aq
HCOÿ 3 aq
h vb
NaHCO3
ÿ h vb ecb TiO2 uncharged heat
(25) (26)
It may be readily shown from formal LH model considerations that Eq. (15a) pertains to the situation where the rate-controlling step is the surface reaction between the adsorbed oxalate and peroxy radical (cf. Eq. (22)). The squared denominator arises because the peroxy surface radicals ± in the rate-limiting step- are formed from two consecutive reactions. Although hydroxy radicals (from the decomposition of hydrogen peroxide) may also participate in the reaction sequence, peroxy species are probably more active and predominant. This matter, however, is inconsequential to the derivation of the rate model structurally identical to Eq. (15a) where we have assumed that most abundant reactive intermediates ± MARI ± are adsorbed oxalate and peroxy species with Eqs. (19)±(21) in quasi-equilibrium and all others considered rapid. In particular, the electronhole recombination step (i.e. Eq. (26)), reduces the apparent quantum ef®ciency de®ned as the ratio of the moles of organic compound degraded to the moles of incident photons.
loading, oxalate concentration, oxygen partial pressure, temperature and light intensity all in¯uence the rate of reaction. However, pH appeared to have the most signi®cant effect, suggesting that the redox potential of the reaction medium is important in determining photoreactivity of the titania catalyst. Whilst reaction rate increased monotonically with oxalate concentration, the behaviour with respect to both catalyst loading and oxygen partial pressure is described by a unimodal function with optimum values at 1.5 g lÿ1 and 45% O2, respectively. The reaction is, however, characterised by a relatively low activation energy of 45.8 kJ molÿ1. Analysis revealed that mass transport effects were not responsible for this feature. Langmiur±Hinshelwood consideration of the data suggests the possibility of a bimolecular surface reaction as the rate-controlling step. The dual-site mechanism proposed requires liquid phase homogeneous dissociation of the sodium oxalate, followed by adsorption of oxalate ions onto photogenerated holes (positively charged sites) and the formation of superoxide oxygen ions on the electron-rich sites. Peroxy surface radicals produced from the protonation of adsorbed oxygen ions presumably interact with the adsorbed oxalate species (in the rcs) to yield hydrogen carbonate ions which are responsible for the increased alkalinity of the solution with time-on-stream. A formal LH kinetic model based on this mechanism is deducible from the rate data collected. 5. Nomenclature aL A b, b1 CAb CAs CH
4. Concluding remarks The kinetic analysis of the photocatalytic degradation of sodium oxalate showed that pH, catalyst
233
dp dT DAB
gas±liquid interfacial area per unit volume of reactor (cmÿ1) constant in Eq. (9) empirical parameters in Eqs. (9) and (11) bulk oxygen concentration in the liquid phase (mol cmÿ3) surface concentration of oxygen (mol cmÿ3) concentration of hydrogen ions (mol cmÿ3) catalyst particle diameter (cm) reactor diameter (cm) diffusivity of oxygen in water at 303 K (cm2 sÿ1)
234
Deff EA g I I0 kL kn ks Koxal Koxy Poxy ÿroxal ÿr0 oxal Sa ST STW ug utp Wmax Greek "g exp
g L L P !
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effective diffusivity inside the particle (cm2 sÿ1) activation energy (kJ molÿ1) acceleration due to gravity (cm sÿ2) light intensity (mEinstein mÿ2 sÿ1) maximum light intensity (mEinstein mÿ2 sÿ1) liquid film mass transfer coefficient (cm sÿ1) nth order rate constant liquid-to-catalyst mass transfer coefficient (cm sÿ1) adsorption equilibrium constant for oxalate ions adsorption equilibrium constant for oxygen oxygen partial pressure (kPa) photodegradation rate of sodium oxalate (mmol gcatÿ1 minÿ1) photodegradation rate of sodium oxalate, (mmol lÿ1 minÿ1) surface area per unit weight of catalyst (cm2 gÿ1) liquid surface tension (dyne cmÿ1) water surface tension (dyne cmÿ1) superficial gas velocity to reactor (cm sÿ1) particle terminal velocity in reactor (cm sÿ1) maximum catalyst loading suspended in a bubble column reactor (g lÿ1) parameter in Eq. (6) gas hold-up experimental Thiele modulus wettability factor (essentially 1 for most catalysts) effectiveness factor gas viscosity (g cmÿ1 sÿ1) liquid viscosity (g cmÿ1 sÿ1) liquid density (g cm)ÿ3 particle density (g cmÿ3) parameter in Eq. (14)
Acknowledgements This project was supported by an Australian Research Council grant. The authors are also indebted
to Kate Nasev and Andrew Chau for assistance with the analytical instruments.
References [1] Q.R.J. Durber, R.A. Morton in: Proceedings of the 21st Australasian Chem. Eng. Conference, vol. 2, Perth, Australia, 1994, p. 483. [2] M. Schiavello (Ed.), Photocatalysis and Environment: Trends and Applications, Kluwer Academic Publishers, Dordrecht, 1988. [3] E. Pelizzetti, N. Serpone (Eds.), Photocatalysis: Fundamentals and Applications, Wiley, New York, 1991. [4] M. Nair, Z. Luo, A. Heller, Ind. Eng. Chem. Res. 32 (1993) 2318. [5] R. Matthews, S.R. McEvoy, J. Photochem. Photobiol. A 66 (1992) 355. [6] R. Matthews, S.R. McEvoy, J. Photochem. Photobiol. A 64 (1992) 231. [7] S. Cheng, S.-J. Tsai, Y.-F. Lee, Catal. Today 26 (1995) 87. [8] A. Sclafani, L. Palmisano, M. Schiavello, J. Phys. Chem. 94 (1990) 829. [9] J. Gimenez, M.A. Aguado, S. Cervera-March, J. Mol. Catal. A 105 (1996) 67. [10] K.T. Ranjit, T.K. Varadarajan, B. Viswanathan, J. Photochem. Photobiol. A 96 (1996) 181. [11] H. Wang, A.A. Adesina, Appl. Catal. B 14 (1997) 241. [12] H. Hori, F.P.A. Johnson, K. Koike, O. Ishitani, T. Ibusuki, J. Photochem. Photobiol. A 96 (1996) 171. [13] Y. Inel, A.N. Okte, J. Photochem. Photobiol. A 96 (1996) 175. [14] M. Trillas, M. Pujol, X. Domenech, J. Chem. Tech. Biotech. 55 (1992) 85. [15] M. Trillas, J. Peral, X. Domenech, Appl. Catal. B 5 (1995) 377. [16] V. Augugliaro, V. Loddo, L. Palmisano, M. Schiavello, J. Catal. 153 (1995) 32. [17] T.-Y. Wei, C.-C. Wan, Ind. Eng. Chem. Res. 30 (1991) 1293. [18] A. Vidal, J. Herrero, M. Romero, B. Sanchez, M. Sanchez, J. Photochem. Photobiol. A 79 (1994) 213. [19] M.-C. Lu, G.-D. Roam, J.-N. Chen, C.P. Huang, J. Photochem. Photobiol. A 76 (1993) 103. [20] A. Mills, C.E. Holland, R.H. Davies, D. Worsley, J. Photochem. Photobiol. A 83 (1994) 257. [21] P.A. Ramachandran, R.V. Chaudhari, Three-Phase Catalytic Reactors, Gordon and Breach, New York, 1983. [22] A. Gianetto, P.L. Silveston (Eds.), Multiphase Reactors, Hemisphere, Toronto, 1986. [23] S. Whitaker, A.E. Cassano (Eds.), Concepts and Design of Chemical Reactors, Gordon and Breach, New York, 1986. [24] M.R. Dhananjeyan, R. Annapoorani, S. Lakshmi, R. Renganathan, J. Photochem. Photobiol. A 96 (1996) 187. [25] G.R. Bamwenda, S. Tsubota, T. Kobayashi, M. Haruta, J. Photochem. Photobiol. A 77 (1994) 59.
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[30] K. Tricklebank, Ph.D. thesis, University of New South Wales, Sydney, Australia, 1992. [31] Z.-H. Wang, Q.-X. Zhang, J. Photochem. Photobiol. A 75 (1993) 105. [32] H.S. Fogler, Elements of Chemical Reaction Engineering, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1992.
The photodegradation kinetics of aqueous sodium oxalate solution using TiO2 catalyst Jimmy Bangun, Adesoji A. Adesina* Reactor Engineering and Technology Group, School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received 12 January 1998; received in revised form 13 May 1998; accepted 17 June 1998
Abstract The kinetics of the photodegradation of sodium oxalate have been investigated in a semi-batch annular photoreactor using commercial titania as the catalyst. Sodium oxalate is an important toxic pollutant in Bayer liquor during alumina processing and its removal and possible rejuvenation of the caustic solution is vital to plant economics. Experiments carried out under UV irradiation (250±310 nm wavelength) over 3 h runs showed that initial solution pH, light intensity, oxygen partial pressure, catalyst loading and oxalate concentration all have strong effects on the decomposition rate. The catalyst exhibited a relatively low activation energy of about 45.8 kJ molÿ1. The degradation rate decreased exponentially with pH, but both O2 and catalyst loading went through a maximum at about 45% O2/N2 and 1.5 gcat lÿ1, respectively. The rate±oxalate concentration behaviour indicates that the catalyst has a high adsorption constant for oxalate species. This was attributed to the increased density of positively charged sites at pH values below the isoelectric point (pH6). The essential features of the proposed mechanism involve aqueous phase dissociation of sodium oxalate followed by adsorption of the C2 O2ÿ 4 ions onto positively charged sites. However, adsorption of O2 to electron-rich sites yields the superoxide ions which are protonated to give highly active peroxy radicals. The data suggested a bimolecular surface reaction between adsorbed oxalate and peroxy species as the rate-controlling step in the homogeneous±heterogeneous mechanism. The subsequent production of HCOÿ 3 ions gave increased pH with time-on-stream. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Photocatalytic degradation; Sodium oxalate; Titania; Bayer liquor
1. Introduction High molecular weight organic compounds associated with bauxite are converted to sodium salts (mostly carbonate and oxalate) during caustic digestion in the Bayer process for alumina production. This results in substantial loss (>10%) in caustic solution *Corresponding author. Fax:+61-2-9385-5966; e-mail: [email protected]
which should be recycled to the digestion unit. While sodium carbonate may be easily converted with lime treatment to NaOH and CaCO3, the industry still faces considerable challenges in the removal of sodium oxalate. The ef®cient disposal of, and if possible recovery of, caustic solution from sodium oxalate is vital to Australia's alumina industry ± the nations's third largest export earner. Due to its low solubility in Bayer liquor, sodium oxalate readily crystallises during alumina precipitation. This interference with
0926-860X/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00223-3
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agglomeration leads to increased generation of alumina trihydrate ®nes and hence poor hydrate classi®cation ef®ciency. Many of the oxalate control techniques used in industry are based on physical separation or conversion to less harmful substances ± the manganese dioxide process or liquor calcination. Recently, a patented microbiological process [1], the Rotating Biological Contactor, in which the oxalate can be removed within three weeks has been described. Photocatalytic destruction of organic pollutants in aqueous systems (whether with oxygen or anaerobically) has been successfully utilised for the complete mineralisation of toxic recalcitrant compounds at relatively faster rates than other conventional thermal, catalytic or biological pathways and under very mild conditions of temperature and pressure [2±10]. It is now increasingly evident that the photocatalytic degradation route is the most attractive of the advanced waste-water treatment and water puri®cation processes. The opportunity for solar energy utilisation and low reactor volume per unit throughput are additional incentives. In a previous paper [11], we demonstrated that the light-assisted decomposition of sodium oxalate over a titania catalyst was indeed accompanied by an increase in pH with time-onstream, suggesting the production of a caustic solution. Thus, photocatalysis offers itself as a useful technology to improve production economics and environmental strategy in the minerals processing industry. In anticipation of multiphase photoreactor optimisation, we now report a detailed kinetic investigation of this reaction. Most photocatalytic reactions proceed via redoxtype mechanisms [6±8,12±14]; thus the pH of the feed solution has a signi®cant effect on reaction rate. Trillas et al. [15] observed that the photodegradation of 2, 4 dichlorophenoxyacetic acid was fastest at a slurry pH of 3, while phenol oxidation in aqueous titania suspension has an optimum pH of 7 [14,16,17]. Gimenez et al. [9] have also noted that the photoreduction of Cr(IV) using titania powder was highest at pH1, with catalyst deactivation setting in at pH>4 due to fouling by chromium hydroxides. On the contrary, an increase in pH improved the photocatalytic degradation of malonic acid, attaining a ``plateau'' at pH7. In fact, maximum CO2 production rate was obtained under this condition, since malonic acid contains two ioni-
sable hydrogens which can be abstracted at pK12.85 and pK25.69. Interestingly, Vidal et al. [18] saw no discernible effect of slurry pH during the photodegradation of ethylbenzene over TiO2. The role of oxygen partial pressure in photomineralisation has also been investigated. Lu et al. [19] reported that optimum gas composition was 20% O2 for the oxidation of dichlorovos. Mills et al. [20] found that the photo-oxidative destruction rates of salicyclic has a turning point at 60% O2. In many cases, O2 adsorbs to give the superoxide ion which is the precursor for surface peroxy or hydroxy radicals. Oxygen also plays an important role as an electron scavenger, thus reducing electron-hole recombination rates and hence improved quantum ef®ciency [17]. The mass transfer characteristics of the bubble column reactor are also in¯uenced by O2 distribution in the liquid phase. In general, bubbles travelling at optimum velocity give high mass transfer rates. Whilst a low gas ¯ow rate introduces gas±liquid transport resistance, very high ¯ow rates engender bubble coalescence and slug formation and poor reactor performance due to reduced residence time. Hydrodynamics and transport effects in multiphase reactors constitute the subject of several classic treatises [21±23]. Speci®c issues pertinent to the present work are taken up in a later section. Substrate concentration also affects photocatalytic rate. Wei and Wan [17] reported the inhibitory role of initial phenol concentration on the reaction rate constant. CO2 formation during photocatalytic mineralisation of malonic acid over titania has a ®rst order dependency at low initial organic acid concentration (<2 mM), but a zero order kinetics was observed at higher values [13]. Dhananjeyan et al. [24] reported a similar rate-concentration behaviour for the photodegradation of thymine. In conventional slurry reactors, conversion generally increases with catalyst loading and a plot of the reciprocal of rate against the reciprocal catalyst loading gives a straight line which may be used for diagnosing various transport resistances. However, the role of catalyst concentration in multiphase photoreactive systems is somewhat diffused due to light scattering and other secondary effects at high catalyst loading [17]. The photo-oxidation rate of malonic acid increased with catalyst loading up to 0.8 g lÿ1 and thereafter remained constant at this limiting value even for TiO2 content of up to 3 g lÿ1 [13]. The
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
photocatalytic destruction of phenol also followed a similar pattern [17]. Temperature appears to have only a mild effect on photocatalytic rates, since many reactions reportedly possess low activation energy, EA. The oxidation of salicylic acid over titania has an EA of about 4.6 kJ molÿ1 between 298 to 323 K [20], while the photo-induced production of H2 and CO2 from aqueous ethylene glycol has values of 15 and 18 kJ molÿ1, respectively [25]. However, the multiphase nature of a photocatalytic reaction system can easily induce transport-disguised kinetics and render meaningful mechanistic evaluation dif®cult. If it is unambiguously established that the reaction is intrinsically characterised by a low activation energy, this will offer a raison-d'etre for the role that light plays in providing alternative energetically-favourable sites for facile surface reaction on the photocatalyst and why reactions otherwise kinetically-constrained (under conventional catalytic temperatures) now proceed with relative ease at low temperatures (<333 K). Active sites (holes and electrons) are generated on the surface of semiconductor oxides when illuminated with light of wavelength greater than the band gap energy. The effect of light intensity is, however, not well understood. Light intensity is often determined using ferrioxalate actinometry, although the introduction of modern digital radiometers may now permit more accurate measurements. Wei and Wan [17] observed that phenol oxidation rates followed a parabolic behaviour up to about 1.61 mEinstein sÿ1 and subsequently showed a ®rst order dependency on light intensity at higher values. Okamoto et al. [26], however, recorded a square root relationship for the same reaction. The square root dependency was also seen in the case of malonic acid oxidation [13]. From the foregoing, it is evident that a comprehensive kinetic study should include the effects of all six factors: namely, pH, light intensity, catalyst loading, oxygen partial pressure, oxalate concentration and temperature on the photocatalytic reaction rate. 2. Experimental 2.1. Materials and methods High purity (AnalaR grade) sodium oxalate and titania (>99% anatase) were obtained from Aldrich
223
and used as supplied in all runs. Milli-Q triply distilled and deionised water was used to prepare the feed solution. NaOH and HCl were used to adjust the pH of the slurry prior to reaction. Ultrapure O2 and N2 gases were obtained from BOC Gases, Sydney. A Waters Associate programmable ion chromatograph (model 590) with a conductivity detector was employed to measure oxalate concentration. CO2 photogeneration was determined from an on-line GOW-MAC gas chromatograph with a TCD. Continuous pH monitoring was obtained from a TPS Digital pH meter. Light intensity was measured by a IL1400 radiometer/detector (International Light, Massachusetts, USA) calibrated for 265±332 nm. 2.2. Apparatus Kinetic data were collected from the annular photoreactor shown in Fig. 1. The reactor was equipped with a commercial UVarc lamp driven by a three-level setting power supply (low200 W, medium300 W, high400 W). Spectral analysis from the local vendor showed a signi®cant fraction of the radiation in the 250±310 nm range. The UV lamp was enclosed in a double-walled quartz hollow U-tube (OD4 cm) through which water was circulated at 750 ml minÿ1 as coolant to maintain reaction isothermality and to remove IR fraction of the incident rays. The lamp was suspended at the top with a silicon rubber-bung tightly ®tted as a lid to the outer reactor Pyrex casing (ID10 cm). The external surface of the lamp was continuously ¯ushed with N2 to remove ozonised air in the space between the U-tube and the lamp. A thermocouple and pH probe were inserted as shown to register the reactor temperature and pH, respectively, while electronic mass ¯ow controllers were used to regulate gas ¯ow rates. 3. Results and discussion 3.1. Analysis of mass transport conditions Blank runs carried out in the presence of oxygen and under a variety of anaerobic conditions showed that oxygen, catalyst and UV light are necessary to achieve measurable oxalate decomposition rates. The reactor was operated in the semi-batch mode (con-
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Fig. 1. Photoreactor assembly.
tinuous upward O2/N2 supply at 500 ml minÿ1) at a total pressure of about 101 kPa. In a multiphase reactor, various transport resistances could falsify true kinetics, thus, operating conditions were chosen to ensure that gas±liquid, liquid±solid and intraparticle
transfer limitations were negligible. As may be seen from Fig. 2, gas ¯ow rate ceased to have any effect on rate above 400 ml minÿ1 thus justifying operation at 500 ml minÿ1. It is, however, conceivable that at gas ¯ow rates higher than the range used here
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225
Fig. 2. Effect of gas flow rate on photodegradation.
(>1200 ml minÿ1), bubble coalescence may become signi®cant and the attendant large bubbles and shorter residence times may result in a drop in conversion. Although the volumetric gas±liquid mass transfer coef®cient, kLaL, in a bubble-column reactor is a complex function of sparger design, ¯ow regime, physicochemical characteristics of the system, gas velocity, catalyst concentration and particle size,
Deckwer et al. [27] have pointed out that reactors with sintered plate distributors are generally endowed with the best kLaL values. Thus, a combination of high gas ¯ow rate and a 20-micron sintered glass plate distributor used in this study ensured the absence of external transport resistance. In fact, using the correlation of Akita and Yoshida [28], kLaL was estimated as 5.9710ÿ4 sÿ1 (cf. Eq. (1)).
Table 1 Values of various parameters Parameter
Value
CAb (based on Henry's law, pAHxA) H DAB Deff dT g "g ÿrexp g L P ST STW L g ug utp gdP2 P ÿ L =18L
310ÿ7 mol cmÿ3 at PO2 0.3 atm 4.75104 for O2 at 303 K 210ÿ5 cm2 sÿ1 0.1DAB 10 cm 981 cm sÿ2 3.510ÿ5 710ÿ6 mol gcatÿ1 minÿ1 (at 30% O2) 1.210ÿ3 g cmÿ3 1.0 g cmÿ3 3.9 g cmÿ3 72 dyne cmÿ1 72 dyne cmÿ1 0.85 cP 0.018 cP 0.106 cm sÿ1 2.310ÿ7 cm sÿ1
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k L aL
0:6D0:5 AB
ÿ0:12 ÿ0:62 L ST dT0:17 g0:93 "1:1 g L L
concentration-time plot was based on analysis of aliquots taken at 5 min intervals over a 2±3 h period. (1)
with the variables as de®ned in Section 5. Table 1 gives the values of the speci®ed parameters. For a well-mixed system, this kLaL value predicts conversion of 100%, suggesting that gas±liquid mass transfer was not rate-limiting under these conditions. The average catalyst particle diameter for the commercial TiO2 powder was 0.35 micron and hence, intraparticle transport resistance should be minimal. To be sure, the generalised Thiele modulus, n for an nth order kinetics may be given by: 2n
nÿ1 kn dp2 Sa P CAS
4Deff
;
(2)
where the particle surface concentration, CAS, may be taken as the bulk liquid concentration CAb in the absence of external transport resistance. For unknown kinetics, Eq. (2) may be rewritten as: 2exp
ÿrexp dp2 p 4CAb Deff
;
(3)
Using the data in Table 1, exp was estimated as 1.5210ÿ2. For such small values of exp (<10ÿ1) the effectiveness factor, , is essentially unity irrespective of particle shape. It may therefore be concluded that intraparticle transport effect is practically nil. Additionally, the liquid±solid mass transfer coef®cient, kS, evaluated from the Kobayashi±Saito correlation [29] given in Eq. (4), was a high 1.143 cm sÿ1. This signals the absence of liquid±solid limitation. " #1=3 dp3 p ÿ L g k s dp dp ug L 0:112 2 0:212 : DAB L DAB L (4) Incidentally, the operational parameters put the ¯ow situation in the multiphase reactor in the so-called homogeneous bubble ¯ow regime [21], where the hydrodynamic characteristics facilitated excellent mass transport. Indeed, the presence of well-dispersed ®ne bubbles in the reactor was clearly visible during all runs. Initial rate data were obtained from the slope of the concentration-time pro®le in the ®rst 30 min of each run. This curve was often linear in this region. The
3.2. Effect of slurry pH The photoreactivity of TiO2 is determined by the redox potential of the reacting medium. The isoelectric point, IEP, of titania in water is about 6; thus pH values above and below this point were investigated. Fig. 3 shows that the photodegradation rate dropped almost exponentially with increase in slurry pH. At low pH (<6), the catalyst surface is predominantly positively charged. The relatively high concentration of positive sites and increased electrostatic attraction between anions and such sites probably improved the surface coverage of oxalate species, and hence gave high degradation rates. Conversely, at high pH values, surface negativity will inhibit photocatalytic rates. Additionally, the solubility of sodium oxalate decreases with increased alkalinity [30]; consequently, at low pH, the liquid phase concentration of free oxalate ions will be high. This situation would facilitate higher surface concentration of the adsorbed oxalate species and accordingly better reaction rates. Incidentally, since the photodecomposition leads to pH rise with time on-stream [11], product inhibition sets in. From the foregoing, it appears that the pH value played a signi®cant role both in the homogeneous (dissociation of Na oxalate) and surface (creation of titania sites with adequate electrical polarity) steps of the proposed mechanism. 3.3. Effect of initial oxalate concentration The oxalate concentration in the feed was varied between 0 and 3 mM to cover the typical oxalate content in industrial liquor (ca. 10ÿ5±10ÿ4 M). It is evident from Fig. 4 that the rate of photocatalytic degradation rose very rapidly for dilute concentrations (<0.5 mM) ± since the curve must pass through the origin, before changing to a lower value at higher concentration. It was dif®cult to obtain additional data points in the low oxalate concentration range due to reproducibility problems with the ion chromatograph under these conditions. Even so, it would seem that the catalyst has a large surface capacity for oxalate anions, although this itself may be a function of the pH employed. A pH value of 2 used for this run may
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227
Fig. 3. Variation of photocatalytic degradation rate with slurry pH.
Fig. 4. Influence of initial oxalate concentration on rate.
also partly explain the high adsorption constant for species due to the abundance of positively C2 O2ÿ 4 charged sites, consistent with our previous observation on the role of slurry pH. Clearly, the adsorption of oxalate ions may be ruled out as the rate-controlling step (rcs) in this study. Indeed, Sclafani et al. [8] have pointed out that signi®cant interactions between the various physicochemical factors and the redox reac-
tions may make it impossible to establish a unique correlation for the photoreactivity of the semiconductor catalyst. 3.4. Effect of oxygen The in¯uence of O2 partial pressure on initial rate is displayed in Fig. 5. Although oxygen content was
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Fig. 5. Effect of oxygen composition on rate.
varied, the total gas ¯ow rate (O2/N2 mixture) bubbled through the reactor was kept constant at 500 ml minÿ1 to ensure identical ¯uid dynamics. The rate increased with oxygen partial pressure, as may be expected for an aerobic reaction; nonetheless, a turning point is seen at about 45% O2 beyond which enrichment in oxygen was detrimental to the degradation. Such an optimum is symptomatic of competitive inhibition between the two reactants (oxygen and oxalate ions) for adsorption on the same site. It is known that O2 adsorbs on electron-rich sites (eÿ cb ) during photo-oxidation; hence, competitive inhibition may not be the obvious reason for the down-turn in rate at high oxygen partial pressures. Upon adsorption, the proto nation of the superoxide, Oÿ 2 adspecies by H ions to yield highly reactive peroxy (HO2 ) radicals which are thought to be the initiators in the oxidative photodecomposition of organics [12±17] leads to a rapid drop in solution acidity. This in turn decreases the dissociation of sodium oxalate as well as the concentration of positively charged vacant sites, and hence a reduction in the surface concentration of oxalate ions (in accordance with Le Chatelier's principle). This scenario would therefore be responsible for the lower degradation rates seen at higher O2 partial pressures. 3.5. Effect of catalyst loading Experiments to determine the effect of catalyst loading were carried out at the optimum pH (2)
and oxygen partial pressure (45% O2). Intuitively, since the photocatalyst is necessary for measurable degradation rates, high titania content should be bene®cial. However, the maximum amount of catalyst that can be kept in suspension Wmax for a bubblecolumn reactor may be estimated from [21]: Wmax 6:8 10ÿ4 C dT ug G ST "g ÿ0:23 L G ug L ÿ0:18 "g utp
ÿ3 ; (5a) ug where C, a viscosity correction factor, and the gas hold-up "g are given by C 0:232 ÿ 0:1788 ln L 0:1026 ln L 2
(5b)
and "g
ug STW L =ST 1=3 : 30 2ug
(5c)
Using the values provided in Table 1 for this system, Wmax was calculated as 13 wt% (130 gcat lÿ1) for ¯ow in Stoke's regime (Rep<0.4) The data presented in Fig. 6 are clear evidence of the nonlinear dependency of rate on catalyst concentration. Wei and Wan [13] had made a similar observation during photocatalytic degradation of phenol over titania,where an optimum catalyst loading of 1±3 g lÿ1 was obtained. The exact value seemed to vary with the type of titania and reactor con®guration.
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229
Fig. 6. Influence of catalyst loading on rate.
The anatase form of TiO2 reportedly exhibited better photoactivity than the rutile type. Also, the in¯uence of catalyst loading was unnoticeable in low capacity reactors (<100 cm3). The present photoreactor has an annular volume of 1.25 l in which titania (>99% anatase) was suspended. Thus, an optimum catalyst loading of 1.5 g lÿ1 under the prescribed operating conditions is consistent with observations on other organic pollutants. From the pro®le shown in Fig. 6, the increase in rate with catalyst loading (below the optimum value) may be due to improved concentration of active sites with increase in number of particles in solution. However, as particle concentration increases, particle±particle interactions may produce deleterious effects such as light shielding and agglomeration. Light shadowing may be due to a decrease in the photons incident on the catalyst surface at high solids content, while agglomeration results from collision between grains [31]. Thus, in spite of the increase in catalyst loading, the poor light-harvesting and loss in particle external surface area leads to a drop in surface concentration of active sites (h vb and eÿ ) and a decline in photodecomposition rate beyond cb the `critical' catalyst loading. However, since particle± particle interaction (e.g. agglomeration) is itself a function of medium acidity, it is conceivable that the optimum catalyst loading would also vary with initial pH.
3.6. Effect of temperature Several studies have shown that photocatalytic reactions are generally characterised by low activation energy values. Fig. 7 shows that a relatively low activation energy, EA, of about 45.8 kJ molÿ1 obtained for sodium oxalate photodecomposition corroborates this widely observed feature. It would seem that although surface processes are involved (as in conventional heterogeneous catalysis) and one would expect the overall activation energy to be a `hybrid' of various elementary steps, the energetically-signi®cant photogeneration (of active sites) step is temperature-insensitive and hence the reduction in activation energy, at least in the Arrhenius-sense. Additionally, the migration of the photogenerated holes and electrons to the particle surface is essentially a low activation energy diffusional process. Although the present study gave a higher activation energy than that of salicyclic acid (4±5 kJ molÿ1) and ethylene glycol (15±18 kJ molÿ1) [20,25], it is a credible estimate since mass transfer effects were negligible. In particular, the overall activation energy is a re¯ection of both homogeneous solute dissociation step and heterogeneous surface reaction steps; thus sodium oxalate, which is a less soluble compound than salicylic acid and ethylene glycol, would exhibit a somewhat higher EA value, as we have observed.
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Fig. 7. Arrhenius plot.
(6)
of is practically constant at 0.1005 cmÿ1. For a bubble-column photoreactor, the catalyst in the annular space is exposed to an average light intensity, IAV, given by: R R0 Ir dr : (7) IAV RRRi 0 Ri r dr
at each of the three different power ratings. The value
For our reactor the annular containing the slurry is
3.7. Effect of light intensity The commercial lamp used exhibited a characteristic exponentially decaying light intensity with radial distance (from source) as shown in Fig. 8 which can be represented by: I ÿ I0 eÿ r
Fig. 8. Radial profile for light intensity of the UV lamp.
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
3 cm thick; thus R03 cm and Ri0, so Eq. (7) yields: IAV 2 1 eÿ R0 1 R0 2 2ÿ (8) I0 R0 and upon substitution provides IAV/I082%. Thus, it is seen that even with this relatively large annular volume (1.25 l) the average particle is exposed to more than 80% of the maximum intensity available. Light intensity affects the degree of light absorption by the photocatalyst and hence the rate of reaction. Fig. 9 shows that the degradation rate increased linearly with light intensity. Previous workers have observed orders ranging from 0.5 to 1 in titania suspensions during photo-oxidation of organic acids [13,17]. Apart from this set of runs, a light intensity at 300 W (medium) was used in all other experiments. 3.8. Mechanistic conjecture and kinetic modelling In order to prescribe a plausible sequence of elementary steps consistent with these experimental observations, we adopt the formal procedure outlined in Fogler [32]. The data shown in Fig. 3 for the in¯uence of pH on degradation rate may be described by an exponential rate law of the type: ÿroxal Aeÿb1 pH ;
(9)
where regression analysis provides these parameter estimates: A1.5710ÿ2 mmol gcatÿ1 minÿ1 and b10.315 with a correlation coef®cient, Rÿ0.99. However, since pH ÿlog10 CH
k1 Coxal 1 Koxal Coxal
ÿroxal
k2 Poxy
1 Koxy Poxy 2
;
(13)
where k28.323210ÿ4 mmol gcatÿ1 minÿ1 kPaÿ1 and the oxygen equilibrium adsorption constant, Koxy3.002710ÿ2 kPaÿ1 with a correlation coef®cient of 0.94. Although the catalyst may not be explicitly regarded as a reactant, its effect as shown in Fig. 6 may also be modelled by an equation structurally similar to Eq. (13), namely: 0 ÿroxal
kcat Ccat
! Ccat 2
;
(14)
where the parameters, kcat is a pseudo-rate constant for the catalyst and ! is an attenuation factor which accounts for the apparent volumetric light shielding effect and the squared denominator is indicative of particle±particle interaction. Values for these parameters were determined: kcat2.963910ÿ2 mmol gcatÿ1 minÿ1 and !0.8486 gcat lÿ1 with a good data ®t possessing a correlation coef®cient of 0.98. The composite rate equation at constant catalyst loading and pH may therefore be written as: krxn Coxal Poxy
ÿroxal
(11)
where krxn is a function of pH and catalyst loading and is given by:
where bb1/2.303. Similarly, the data shown in Fig. 4 may be represented by a Langmuir±Hinshelwood (LH) equation for oxalate ion adsorption on positively charged sites as ÿroxal
data. Appropriate inversion of Eq. (12) followed by linear regression of the data gave R0.94, while the oxalate adsorption constant, Koxal is 11.627 l mmolÿ1 and k1 has a value of 0.1557 l gcatÿ1 minÿ1. The nonlinear effect of O2 shown over the entire range of partial pressure as seen in Fig. 5 may be captured by the LH expression, written:
(10)
then ÿroxal ACHb ;
231
(12)
to accommodate behaviour at low and high oxalate concentrations. Clearly, at very dilute oxalate concentrations, (KoxalCoxal 1) the model approaches a straight line through the origin as required by the
1 Koxal Coxal 1 Koxy Poxy 2
krxn A k1 k2 CH0:137
kcat Ccat
! Ccat 2
:
;
(15a)
(15b)
It is apparent that Eq. (15a) may arise from a Langmuir±Hinshelwood mechanism involving dual adsorption sites. This is consistent with the proposition that oxalate ions are chemisorbed on positively charged sites, while oxygen is adsorbed on electronrich centres. Thus we propose the following mechanism for the photocatalytic degradation of sodium oxalate:
232
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
Fig. 9. Effect of light intensity on photocatalytic rate.
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
Na2 C2 O4 2Na aq C2 O2ÿ 4 aq homogeneous dissociation TiO2 light
eÿ cb
(16)
h nb
photogeneration of active sites H 2 O h nb OH ads ÿ O 2 eÿ cb O2 ads
H aq
Oÿ 2 ads
H aq
(18) (19)
HO2 ads
(20)
ÿ C2 O2ÿ 4 aq hnb C2 O4 ads
C2 Oÿ 4 ads
HO2 ads
(17)
HCO3 ads
(21)
COÿ 3 ads
(22)
COÿ 3 ads H aq HCO3 ads
(23)
HCO3 ads
(24)
Na aq
HCOÿ 3 aq
HCOÿ 3 aq
h vb
NaHCO3
ÿ h vb ecb TiO2 uncharged heat
(25) (26)
It may be readily shown from formal LH model considerations that Eq. (15a) pertains to the situation where the rate-controlling step is the surface reaction between the adsorbed oxalate and peroxy radical (cf. Eq. (22)). The squared denominator arises because the peroxy surface radicals ± in the rate-limiting step- are formed from two consecutive reactions. Although hydroxy radicals (from the decomposition of hydrogen peroxide) may also participate in the reaction sequence, peroxy species are probably more active and predominant. This matter, however, is inconsequential to the derivation of the rate model structurally identical to Eq. (15a) where we have assumed that most abundant reactive intermediates ± MARI ± are adsorbed oxalate and peroxy species with Eqs. (19)±(21) in quasi-equilibrium and all others considered rapid. In particular, the electronhole recombination step (i.e. Eq. (26)), reduces the apparent quantum ef®ciency de®ned as the ratio of the moles of organic compound degraded to the moles of incident photons.
loading, oxalate concentration, oxygen partial pressure, temperature and light intensity all in¯uence the rate of reaction. However, pH appeared to have the most signi®cant effect, suggesting that the redox potential of the reaction medium is important in determining photoreactivity of the titania catalyst. Whilst reaction rate increased monotonically with oxalate concentration, the behaviour with respect to both catalyst loading and oxygen partial pressure is described by a unimodal function with optimum values at 1.5 g lÿ1 and 45% O2, respectively. The reaction is, however, characterised by a relatively low activation energy of 45.8 kJ molÿ1. Analysis revealed that mass transport effects were not responsible for this feature. Langmiur±Hinshelwood consideration of the data suggests the possibility of a bimolecular surface reaction as the rate-controlling step. The dual-site mechanism proposed requires liquid phase homogeneous dissociation of the sodium oxalate, followed by adsorption of oxalate ions onto photogenerated holes (positively charged sites) and the formation of superoxide oxygen ions on the electron-rich sites. Peroxy surface radicals produced from the protonation of adsorbed oxygen ions presumably interact with the adsorbed oxalate species (in the rcs) to yield hydrogen carbonate ions which are responsible for the increased alkalinity of the solution with time-on-stream. A formal LH kinetic model based on this mechanism is deducible from the rate data collected. 5. Nomenclature aL A b, b1 CAb CAs CH
4. Concluding remarks The kinetic analysis of the photocatalytic degradation of sodium oxalate showed that pH, catalyst
233
dp dT DAB
gas±liquid interfacial area per unit volume of reactor (cmÿ1) constant in Eq. (9) empirical parameters in Eqs. (9) and (11) bulk oxygen concentration in the liquid phase (mol cmÿ3) surface concentration of oxygen (mol cmÿ3) concentration of hydrogen ions (mol cmÿ3) catalyst particle diameter (cm) reactor diameter (cm) diffusivity of oxygen in water at 303 K (cm2 sÿ1)
234
Deff EA g I I0 kL kn ks Koxal Koxy Poxy ÿroxal ÿr0 oxal Sa ST STW ug utp Wmax Greek "g exp
g L L P !
J. Bangun, A.A. Adesina / Applied Catalysis A: General 175 (1998) 221±235
effective diffusivity inside the particle (cm2 sÿ1) activation energy (kJ molÿ1) acceleration due to gravity (cm sÿ2) light intensity (mEinstein mÿ2 sÿ1) maximum light intensity (mEinstein mÿ2 sÿ1) liquid film mass transfer coefficient (cm sÿ1) nth order rate constant liquid-to-catalyst mass transfer coefficient (cm sÿ1) adsorption equilibrium constant for oxalate ions adsorption equilibrium constant for oxygen oxygen partial pressure (kPa) photodegradation rate of sodium oxalate (mmol gcatÿ1 minÿ1) photodegradation rate of sodium oxalate, (mmol lÿ1 minÿ1) surface area per unit weight of catalyst (cm2 gÿ1) liquid surface tension (dyne cmÿ1) water surface tension (dyne cmÿ1) superficial gas velocity to reactor (cm sÿ1) particle terminal velocity in reactor (cm sÿ1) maximum catalyst loading suspended in a bubble column reactor (g lÿ1) parameter in Eq. (6) gas hold-up experimental Thiele modulus wettability factor (essentially 1 for most catalysts) effectiveness factor gas viscosity (g cmÿ1 sÿ1) liquid viscosity (g cmÿ1 sÿ1) liquid density (g cm)ÿ3 particle density (g cmÿ3) parameter in Eq. (14)
Acknowledgements This project was supported by an Australian Research Council grant. The authors are also indebted
to Kate Nasev and Andrew Chau for assistance with the analytical instruments.
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