Flow-injection determination of organic contaminants in water using an ultraviolet-mediated titanium dioxide film reactor

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Analytrca Chrmrca Acta, 231 (1990) 13-20 Elsevler Science Pubhshers B.V , Amsterdam - Pnnted m The Netherlands

Flow-injection determination of organic contaminants in water using an ultraviolet-mediated titanium dioxide film reactor GARY K-C LOW * and R W MATTHEWS

Centrefor Adoanced Analytrcal Chemistry, CSIRO Dwrsron of Fuel Technology, Lucas Heights Research Laboratones,

PMB

7, Menar,

NS W 2234 (Au.stralra)

(Received 8th August 1989)

ABSTRACT A novei reactor for use m the ffow-mJect]on determmatlon of orgamc solutes m water was constructed by first lmmobttizmg a tIim tiim of tltamum dioxide onto the Inner surface of a length of TetetTon tubmg then wrappmg the treated tubmg around a near-UV tllummatmg source The reactor was mstalled after the InJector port of the flow

system t3gamc compoundk mjected- m the !Towmg stream were oxldlzed- photocataiytlcaiiy to carbon dloxlde, wfuch was subsequentiy momtored by a conductlvlty detector To optmuze the sensltlvlty of detectlon, a number of reactor parameters, such as temperature, conflguratlon and catalyst loadmg, were studied The techmque was apphed to alcohols, formaldehyde, ethylene oxide and smgle-cell algae The detectlon hnut IS 1 x 10m9 M methanol (m 20 ~1).

Previous work in this laboratory [l-4] onstrated that many organic compounds

has demcan be

using aqueous suspensions of titanium dioxide illurnmated with near-UV light or using the titanium dioxide as a stationary phase. In most instances, carbon dioxide and water are the main products. The degradation of the orgamc solutes is due to photocatalytic oxidation via a mechanism which 1s believed to involve positively charged holes formed on the surface of the excited titanium dioxide 15-q and ambient oxygen. ‘There are advantages in attaching the titanium dioxide to a stationary support, as the solution for oxidation may be passed contmuously over the lllummated photocatalyst to some appropriate detector. This has been done using glass tubing as the stationary support arranged as a spiral around a fluorescent tube [2,4]. Other materials may provlde more useful supportmg configurations and in this paper we deal with titanium dloxlde attached decomposed

to the Inner wall of Teflon tubing. A mmiature spiral reactor was manufactured using Teflon tubmg treated in this way. The small volume of the reactor was suitable for coupling with a flow-mJection analysis (FIA) system for the detection of the oxidized products. The reactor column was mstalled after the Injector, and the organic solutes were chemically modified before detectlon. Although the use of reactors in FIA has been reported elsewhere [&lo], they were largely restricted to enzyme-lmmoblhzed apphcatlons.

EXPERIMENTAL

Instrumentation The configuration of the FIA system used m this study is shown schematically in Fig. 1. Only a single channel pump 1s needed (Model 590, Mdhpore-Waters, Mllford, MA), as the reaction occurs

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Fig 1 Conflguratlon of the FIA system 1, Carrier solvent reservoir. 2, pump, 3. two-way \wltch valve. 4, Rhcodync 7020 InJectIon valve 5. gds mjectmn valve (see Inset), 6. reactor, where 66 = black-hght fluorescent tube with tltamum dloxlde-lmmoblllzed Teflon tubmg wrapped around It. 6b = air-coohng tube and 6c = reflectant foil enca\lng. 7. Waters Model 430 conduLtlvlty detector. 8. Integrator. 9. solvent wdste Inset 1, Perapex block with 10 mm diameter flow chdnnel drllled Into It. 2, Mmlnert screw-up. push-button valve (Pierce. Rockford, IL), 3, septum. 4, compresvon xrew Arrow\ lndlcate the dIrectIon of solvent flow

enttrely wtthm the tttantum dtoxrde-tmmobtlized Teflon reactor. The two qectton valves used m thts apparatus were controlled by a two-way swttchtng valve Normally, the fixed-loop 20-~1 qector (Item 4, Fig. 1) (Model 7020, Rheodyne, Cotatt, CA), was employed, but for calibratton of the FIA system wtth carbon dtoxrde, the second InJector (Item 5, Ftg 1) had to be used m order to obtatn reproducrble results. The detatled constructton of thts Injector IS shown m the mset m Ftg 1. Gas-ttght syrmges were used wtth the latter Injector The reactor conststed of an approprtate length of Teflon tubing wtth tmmobthzed titanturn dtoxtde. wrapped around a 32 5 mm dtameter NEC 20-W black-hght blue fluorescent tube (mounted m a standard 20-W fluorescent tube domesttc holder), an au cooler (whtch conststed essentially of a ptece of copper tubing wtth graduated holes) connected to the laboratory atr supply and a reflectant fotl casing. The temperature of the reactor during operation was contmuously tagged using a temperature probe connected to the temperature-

momtormg port of an Alpha 800 laboratory conducttvtty meter (CHK Engmeertng. Sydney). The photo-oxtdtzed products from the reactor were detected by a Waters 430 conductrvtty detector, whrch tn turn was connected to a reporting integrator (Hewlett-Packard, Avondale, PA) Immohrlrzutwn tulxng

of

trtunrum droxr&

on

Tt~flotl

Teflon tubes of I d. 0 9 and 1.25 mm were used throughout. The tubing was washed with 50 ml of 1% (w/w) mtrtc actd and 100 ml of purtfted water before being dried to constant weight In an evacuated oven (Thermoltne Screnttftc Equtpment, Wethertll Park, NSW). An ultrasontcated suspension of tttamum dtoxtde (1 g tn 20 ml water) was then syrtnge-drawn mto the Teflon tubing. one end of which was temporarily blocked wtth a glass plug before bemg transferred to a vacuum oven set at 50” C and a pressure of 360 mmHg for 30 mm. The oven temperature was then Increased to 100°C and the pressure reduced to 0 mmHg for a further 60 mtn. When this process was repeated.

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the carrier gases were of htgh-purity grade obtained from C.I.G. The water for all soluttons was obtained from a M&pore Mini-Q water purification system and had a spectftc resistance greater than 18 MS2 cm. The alga Chlorella pyrenozdosa was cultivated to a density of 3.9 X lo9 ml-‘. The solution was diluted lo-fold and a 20-~1 aliquot was used for FIA. The carrier solvent was Milli-Q water. For the analysis of low concentrations of organics (< 100 ng per injection), the water was further purified by passing it through the titanium dioxide-mediated photocatalytic oxidation water purifier [4]; 40 ml of Mtlli-Q water at a time were recirculated through the system with a continuous bleed of oxygen. WAVELENGTH

Inml

Rg 2 UV spectra of (A) tltamum dioxide-coated uncoated Teflon tubmg, (C) blank cell

and (B)

layers of titanium dioxide were built up on the inner surface of the tubing. Figure 2 shows a typical UV spectrum of a piece of Teflon tubing coated with titanmm dioxide. It is difficult to estimate how much of the light has been transmitted through the titamum dioxide surface, as the spectrum represents the net transmtttance of the whole tube. Scholten et al. [ll] pointed out that diffuse radiation transfer and internal reflectance can considerably enhance the transmittance of UV light in Teflon tubing. The UV spectra were measured with a Varian DMS 300 UV spectrophotometer. A 2 mm width, 10 mm light-path cell was used with a 25 mm length of titanium dioxide-coated Teflon tubing (1.25 mm o.d.) inserted into the centre of the light path. Materzals Tttanium dioxide was Degussa P25 grade, BET surface area 50 * 15 m* gg’, average particle size 30 nm, primarily anatase. The organic compounds were of laboratory-reagent grade. Carbon dioxtde used for cahbratton of the FIA system was Commonwealth Industrial Gases food grade (C.I.G., Sydney). Helium and oxygen for the sparging of

RESULTS

AND DISCUSSION

Reproduczbzlzty of response, detectzon level and sample throughput rate Figure 3 illustrates the changes in detection signals when different concentrattons of methanol were injected into the FIA system using different conditions of the reactor. Clearly, the presence of a small amount of titanium dioxide on the mner surface of the reactor considerably enhanced the detection of the organic solute. In the absence of the photocatalyst, the detection of methanol decreased by a factor of almost 10 and when methanol was injected into the titanium dioxideimmobthzed reactor without the UV source turned on it produced small, broad, negative peaks. The broadness of these peaks may be attnbuted to the adsorption of the neutral methanol molecules on the titanium dioxide. The signals obtained from the injection of methanol into the UV-illuminated reactor with no photocatalyst immobilized on it appear to be derived from the sum of a sharp positive peak and a broad negative peak. Figure 4(A) illustrates the signals obtained when different concentrations of methanol were inJected into the FIA system. Clearly, the number of inlections that can be made is dependent on the flowrate of the carrier solvent. In a typical situation where the flow-rate was set at 3 ml nun-‘, 30

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Fig 3 Response from uqectlon of solutlons contammg dlfferent concentrations of methanol usmg different condltlons of the reactor. Event 1, signals obtamed on 150-cm Teflon cod wlthout any tltamum dioxide lmmoblhsed but with UV hght on, event 2, responses from qectlons of the same solutions on the Teflon cod with tltamum dloxlde coating (9 5 mg) and with the l&t turned off, and event 3, responses from mJectlons of the same soluttons but usmg Teflon tubmg coated with tltamum coatmg and having the hght turned on. Absolute concentration of methanol injected (A) 317 5, (B) 190, (C) 63 5 ng. The flow-rate was set at 3 0 ml mu- ‘, detector sensltlvlty at 0 2 PS full-scale deflectlon (f s d.) and chart speed 2 5 mm mu-’ The temperature of the reactor was mamtamed at 43O C

inJections of successtvely increasing or decreasing concentration of methanol gave excellent reproducibility. A relative standard deviation of 1.5% was found for twelve separate 20-~1 mlecttons of an 8 PM solutton spread over a period of 5 h wtth the temperature of the reactor maintained at 30 k 0S”C. The detection hnut (three times the noise) was 20 ~1 of 10 nM methanol solution with the flowrate set at 3 ml mini ’ and 0.05 p.7 full-scale detector setting. At the same instrumental settings, 20 ~1 of 15 nM formaldehyde may also be detected.

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Effect of oxygen concentration rn the carrier solvent The importance of a sufftcient concentration of oxygen m the carrier solvent is illustrated in Fig. 4(B). When the carrier solvent was sparged with helium to displace a large fraction of the dissolved oxygen, the sensittvtty of detection was reduced. Under this condition, solutions containing different concentrattons of methanol gave almost tdentical signals. This observation strongly indicates that the concentration of dissolved oxygen m the carrier solvent governs the extent of photo-oxidation. It appears that dissolved oxygen in the carrier solvent is adsorbed on the titanium dioxide surface where tt interacts with the conductton band electrons, thereby inhibiting the pair recombmation of valence band holes with these electrons [12,13]. The recombinatton reaction dunmishes the photocatalytic activity. Augugliaro et al. [12] have previously demonstrated that the rate of photocatalyttc oxidation of phenol in aqueous solutions is dependent on the oxygen partial pressure. The rate of carbon dioxide production increases proportton-

Fig. 4 (A) Results obtamed from successive mjections of solutions contammg different concentrations of methanol (1) 127, (2) 254, (3) 381, (4) 508, (5) 635 ng The earner solvent flow-rate was set at 3 ml mu-’ and the detector at 1 0 PS f s d (B) Results obtamed after the carrier solvent had been sparged with hehum for 30 mm Methanol concentrations as m (A), detector set at 0.4 PS f s d The length of the reactor was 150 cm with 9 0 mg of tltamum dloxlde coatmg

FIA OF ORGANIC

CONTAMINANTS

IN WATER

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marginal mcrease in response with an increase in temperature, whereas at higher concentration the oxidation is only partial and an increased temperature causes a significantly higher proportion of the solute to be oxidized.

Fig. 5 Effect of reactor temperature on response Solutions wtth different concentrations of methanol were Injected (A) 6 1, (B) 3 05, (C) 1 53, (D) 0 76, (E) 0 36 pg The flow-rate was set at 3 ml/mm, detector sensltlvlty at 2 5 pS f s d , reactor length 150 cm and 9 0 mg of tttanmm dloxtde coatmg

ally with increasing oxygen pressure m the solution. The usual practice in LC of spargmg helium m the mobile phase solvents to overcome bubble problems is therefore not recommended here. In our laboratory, this problem is not usually encountered in FIA and solvent degassmg 1s rarely warranted. However, to obtain reproducible dayto-day results, the carrier solvent was sparged with hehum for 30 mm and a fixed volume of oxygen was then bubbled through it to give a carrier solvent with a fixed concentration of dissolved oxygen. Effect of temperature on detection senatrvrty Figure 5 illustrates the effect of temperature on sensittvity when 20 ~1 of solutions containing different concentrations of methanol were qected mto the FIA system. For the same volume mlection of a solution, the response increased with mcreasmg temperature. However, a solution of higher methanol concentration was more sensitive to temperature changes. A linear relationship was obtamed when the slope (representing detector response per o C) of each line m Fig. 5 was plotted against the concentration of the InJected solutions. At lower concentrations, almost total oxtdation of the orgamc solutes in the solution allows only a

Effect of flow-rate of earner solvent on sensltrvlty As demonstrated by Fig. 6, the sensrttvity was reduced with mcreasmg flow-rate of the carrier solvent. This was true for all the reactor conftgurations used. The results obtained here were different to those observed previously for the closed system [2], where the rate of destruction of an orgamc solute was enhanced wtth increasing flow-rate. In single-pass, contmuous flow systems, as here, a low flow-rate of the carrier solvent means a longer residence time for the inJected slug of organic solute m the reactor, thereby causing a larger percentage of the solute to be oxidized photocatalytically. Reactor length An obvious alternative to increasing the residence time of the injected solute in the reactor is to Increase its length. Figure 7 illustrates the increase m the rate of photocatalytic oxidation with

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Fig 6 Effect of carrier solvent flow-rate on response of qectlon of 6 1 pg of methanol. Reactor length (A) 300, (B) 210, (C) 120, (D) 87, (E) 67 5 cm The detector senslttvlty was mamtamed at 2.5 PS f s d and the temperature at 48O C, a 150-cm length of Teflon tubmg was coated with 11 5 mg of tltamum dtoxlde

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Fig. 7 Effect of reactor length on response for 6 1 pg of methanol Injected at different earner solvent flow-rates (A) 1, (B) 2, (C) 3, (D) 4, (E) 5 ml mu-’ Detector settmg and reactor conflguratlons as m Fig 6

increasing length of the reactor. For a low flow-rate of 1 ml mu-‘, a reactor length of ca. 120 cm (curve A m Fig. 7) would be sufficient to give total oxidation of the injected slug of methanol. However, an increase in the flow-rate of the carrier solvent would substantially decrease the signal; tlus decrease in response cannot be compensated for simply by mcreasmg the reactor length, because of the band diffusion effect of the injected slug. A compromise between flow-rate of the carrier solvent and reactor length is necessary for optimum results.

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light-scattering properties of the semiconductor) mto the reactor reduces the same process. However, solutions of higher concentration were more sensitive to the weight of the titanium dioxide on the Teflon tubing. This observation may be explained in terms of availability of adsorption sites on the titanium dioxide. At low concentrations, there are already sufficient sites available for the injected solute and any further increase would not significantly increase the rate of photocatalytic oxidation, whereas for mjectton from solutions of higher concentration, sites are available for only a fraction of the total solute present and any mcrease m adsorption sites will therefore increase the rate of oxidation. This is especially true for slug injection in the angle-pass, continuous flow system, m which the migrating solutes m the slug contmuously occupied a new zone on the titamum dioxide surface as they passed through the reactor. It has been demonstrated [14] that the rate of oxidation of an organic solute on a photocatalyst is proportional to the fraction of the surface of the semiconductor covered by the solute molecules. Alcohol determmatron Using the system in Fig. 1, methanol, ethanol, propan-l-01, butan-l-01 and amyl alcohol were analysed, using a flow-rate of carrier solvent of 3 ml mu-‘. Figure 9 shows a plot of detector

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Tztamum dloxlde layer The effect of the amount of titanium dioxide on the surface of the Teflon tubing is shown m Fig. 8. In the absence of the photocatalyst, a negligible response was observed for all injections of solutions of different concentration. A sharp increase in the signal resulted from the immobilization of the initial layer of titanium dioxide and was followed by the gradual levelhng of the response with addition of further layers. It has been pointed out previously [3] that a dichotomous effect will occur with each additional loading of titanium dioxide on the tubing. Increasmg surface adsorption sites enhances photocatalytic oxidation, but decreasing penetration of the light (shieldmg of the light because of the opacity and

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Fig 8 Effect of tltamum dloxlde loadmg on response Dlfferent concentrations of methanol were Injected (A) 6 1, (B) 3 05, (C) 1 53, (D) 0 76, (E) 0 37 pg. A reactor length of 150 cm was used Detector as m Fig 7, temperature, 38” C

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Fig 9 Responses from mJectlons of dtfferent concentrations of (0) methanol, (0) ethanol, ( +) propan-2-01, (*) butan-l-01 and ( x ) tertiary amyl alcohol The detector was set at 2 5 PS f s.d , reactor length, 150 cm, with 9 6 mg of tItanturn dloxlde coatmg, mamtamed at 41° C The solvent flow-rate was set at 3 ml mu-’

response against concentration for each alcohol injected (in 20 ~1). Two lines can be clearly drawn through these data points. The first fits results of the mjection of solutions containing methanol and the second approximately fits the data points from the injection of solutions containing other alcohols. If total oxidation is occurrmg one would expect the response to be directly proporttonal to carbon content and the lower results for methanol reflect this. However, alcohols of different chain lengths have been reported to have different rates of photocatalytic oxidation [15] and the energy input may have been insufficient to ensure total oxidation in all instances. Confirmation of incomplete oxidation is given by the results in Fig. 10, showing detector response (for mjections of solutions of the same concentration) vs. the carbon number of the alcohol. The levellmg m responses of these curves depends on the flow-rate of the carrier solvent. At a flow-rate of 1 ml min-’ (curve A) the detector response increases with mcreasmg carbon chain length m the alcohol. However, as the flow-rate is mcreased (curves B-E) the differences in detector response among the alcohols soon disappear, with plateaumg of the curves from C, upwards. There is, however, a significant increase in response on going from C, to C, at all flow-rates. The implication is that longer carbon chain alcohols are incompletely oxidized at higher flow-rates.

tert-Butanol gave a response five times less than that of methanol. The significant difference in these results is probably due to the extremely stable tert-butanol radicals and is the SubJect of further investigation. The position of the hydroxy group m the alcohol molecule appears to be tmportant in regulating the rate of the photocatalytic oxidation. Matthews [15] reported that the rate of photocatalytic decomposition of propan-2-01 is much less than that of propan-l-01 and is only half that of ethanol. Fraser et al. [16] proposed that propan-2-01 reacts as a hole scavenger on the titamum dioxide surface to produce acetone, which has an extremely slow rate of photocatalytic oxidation. Determmatron of other orgamc solutes Some immediate applications of this FIA system are for the detection of low levels of orgamc contammants m sterile waters used for therapeutic purposes and the monitoring of residual sterilants such as ethylene oxide and formaldehyde in rmsing solutions of kidney dialysis machmes after sterilization. In thts study, it was found that formaldehyde is at least twice as reactive as methanol, and ethylene oxide is 50% more reactive than ethanol. In view of this, the FIA system devised here can adequately handle the detection of the

ALCOHOL

CARBON

NUMBER --)

Fig. 10 Response vs carbon number m the alkyl group of alcohols at different flow-rates (A) 1, (B) 2, (C) 3, (D) 4. (E) 5 ml mu-’ 20 ~1 of a 2 mM solution of each alcohol were used Detector and reactor condltlons as m Fig 9

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perrmsstble residual concentrations of these compounds [17,18]. Also encouragmg is the response which was obtained with the single-cell alga Chlorella pyrenodosa. At a flow-rate of 3 ml mm’, a significant response (10% full-scale deflection when the detector is set at 2.5 @) can be obtained even with an mlectton containing as few as 8 cells m 20 ~1. Thts response is due to the photocatalytic degradation of lipopolysacchartdes and hpoproterns, which form the outer layer of the cell walls m the algae. It offers a potential method for momtoring endotoxin levels of stenle inlectton soluttons and substitutton fluid for haemoftltratton, and could replace the cumbersome biological procedure [19].

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REFERENCES

10 11 12 13

Conclwons Titanium dioxide-coated Teflon tubing, illummated with an approprtate UV source, can be employed as a universal solid reactor m FIA for the detection of organic solutes m aqueous solutions. In the FIA system and low-power reactor described here, in most instances only a fraction of the organic compound is oxidized and the magnitude of the response of a solute depends on us rate of photocatalytic oxidation. As has been demonstrated, however, the response can be mcreased by increasing any of several physical parameters of the reactor, such as its length, titanium dioxide loading and temperature. At present tlus system can be used for determmmg low concentrations of a single organic solute in aqueous solutions after it has been calibrated with the same compound.

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R W Matthews, Water Res., 20 (1986) 569 R W Matthews, J Phys. Chem , 91 (1987) 3328 R W Matthews, Aust J Chem , 40 (1987) 667 R W Matthews, Solar Energy, 38 (1987) 405 C.K. Gratzel, M Jlrousek and M Gratzel, J Mol Catal, 39 (1987) 347 K Takag, T FuJloka, Y Sawalu and H Iwamura, Chem Lett , 7 (1985) 913. H Kawaguchl, Environ. Technol Lett., 5 (1984) 471 B.C Madsen and J K Murphey, Anal Chem, 53 (1981) 1924 L. Anderson, Anal Chum Acta, 119 (1970) 123. T Yao, J Flow InJectIon Anal, 2 (1985) 115 AHMT~ SchoLte~ PLM Welhng, UATh Brmkman and R.W Frel, J Chromatogr , 199 (1980) 239 V. Augugharo, L Pahntsano, A Sclafam, C Mmero and E. Pehzzettl, Toxlcol Environ Chem , 16 (1988) 89. C Kormann, D W Bahnemman and M R Hoffman, Envlron. SCI Technol , 22 (1988) 89. M Barbem, C. Mmero, E. Pehzzettl, E Borgarello and N Serpone, Chemosphere, 16 (1988) 2225 R W Matthews, m M Scluavello (Ed ), Photocatalysls and Environment Trends and Applications, Kluwer, Dordrecht, 1988, p 682 I M Fraser and J.R MacCullum, J. Chem. Sot, Faraday Trans 1, 82 (1986) 2747 C W Bruch, m G.B Phdhps and W S Mdler (Eds ), Industnal Stenhzatlon, Intematlonal Symposmm, Amsterdam, 1972, Duke Umverslty Press, Durham, NC, 1973. S. Sprmg, Investlgatlon of the IZlsks and Hazards Assoclated with Hemodtalysls Devtces, Report No PB80-215403, Mmneapohs MedIcal Research Foundation, MN Regonal Kidney Disease Program, NTIS, U.S Department of Commerce, 1980, p 338-349 H. Tommaga, S. Tanaka and N Tommaga, Nephron, 42 (1986) 128