The rapid destruction of microcystin, a cyanobacterial toxin, using a titanium dioxide photocatalyst is observed; the process is extremely efficient with high concentrations of toxin completely undetectable within 10-40 min, depending on the initial concentration

Lawton, Linda A.

Münch, Burkhard

Robertson, Peter K.J.

Rouzade, Julien

English

M�nch, Burkhard

Open Access Institutional Repository at Robert Gordon University

807060504030201000 5 10 15 20 25 30 35 40t / minConcentration / µM c200c150c100c75c50Destruction of cyanobacterial toxins by semiconductor photocatalysisPeter K. J. Robertson, Linda A. Lawton, Burkhard Mu¨nch and Julien RouzadeSchool of Applied Science, The Robert Gordon University, St Andrew Street, Aberdeen, UK AB25 1HGThe rapid destruction of microcystin, a cyanobacterial toxin,using a titanium dioxide photocatalyst is observed; theprocess is extremely efficient with high concentrations oftoxin completely undetectable within 10–40 min, dependingon the initial concentration.Cyanobacterial (blue-green algal) toxins are extremely toxicnaturally occurring substances which display hepato- andneuro-toxic behaviour.1,2 Several genera of freshwater cyano-bacteria including Microcystis, Oscillatoria, Anabaena andAphanizomenon produce a range of potent natural toxins. Thesetoxins can be classified in two general categories, neurotoxins[e.g. anatoxin-a, anatoxin-a(s) and saxitoxins] and hepato-toxins, namely microcystins which are the subject of thisinvestigation. The heptapeptide microcystins are a family of ca.50 congeners which exhibit potent inhibition of proteinphosphatase 1 and 2A resulting in a range of clinical symptoms.Acute doses of microcystin, such as that recently experiencedby patients undergoing dialysis, results in death due to hepaticfailure.3 Longer-term exposure to sub-lethal levels, however, issuspected of promoting primary liver cancer. The incidence ofcyanobacterial blooms in freshwaters, including drinking waterreservoirs, appears to have increased over the past few decadesdue to rising nutrient levels caused by intensive farmingpractices, sewage generation and detergent usage.4 Micro-cystins are chemically very stable5 and conventional watertreatment processes fail to remove them, furthermore the use ofmore advanced methods such as granular carbon filtration andphotochemical degradation have shown only limited efficacy.6,7The use of semiconductor photocatalysis for destruction ofenvironmental pollutants is a well established technique withthe mineralisation of a wide range of materials being reported.8When semiconductors are illuminated with light of an appro-priate wavelength, electrons are promoted from the occupiedvalence band (producing oxidising sites) to the unoccupiedconductance band (producing reducing sites). It is believed thathydroxyl radicals are generated at the valence band whichpromote the mineralisation of the pollutants.9We have discovered that this technique is extremely effectivein the destruction of microcystin-LR (molecular weight 994)2one of the most commonly occurring cyanobacterial toxins.This toxin, a heptapeptide with the generic structure of amicrocystin and leucine (L) and arginine (A) in the variablepositions, was purified from a natural sample of Microcystisaeruginosa.10 Aqueous solutions of microcystin-LR (pH 4, 306K) were illuminated in the presence of air and a TiO2 catalyst(1% m/v solution of TiO2 Degussa P-25) using a xenon UVlamp (280 W UVASpot 400 Lamp, Uvalight Technology Ltd,spectral output 330–450 nm). The investigation was performedon a series of solutions containing varying concentration ofmicrocystin-LR. The concentrations (mm) used to evaluate thephotocatalytic destruction of microcystin-LR greatly exceededthat which occurs naturally (nm–pm). This, however, enabled usto directly quantify the toxin by HPLC (as described else-where11) without multi-step processing which is necessary toquantify the much lower levels found in the environment. Veryrapid destruction of microcystin-LR was observed for allconcentrations (Fig. 1) even at the highest level tested (200 mm)with the toxin virtually undetectable after 40 min. It was foundthat the initial rate of decomposition was influenced by theinitial toxin concentration (Table 1). In fact, at concentrationsbelow those shown the rate of destruction was so rapid thatmicrocystin-LR was undetectable by the first sampling point (2min). These data suggest that the much lower concentrationswhich can be present in drinking water will be rapidly removedby this technique. The samples that were illuminated withoutTiO2 displayed no degradation. As was observed with the initialrates of destruction, the specific rates and specific activities forthe process were also reduced with increasing toxin concen-tration.The Langmuir–Hinshelwood kinetic rate model has beenwidely applied to the initial rate of the photocatalytic destruc-tion of a wide series of compounds.12 Eqn. (1) displays theLangmuir–Hinshelwood kinetic rate law where r0 is the initialrate of disappearance of the substrate and C0 is the initialFig. 1 The destruction of a range of concentrations of microcystin-LR usinga titanium dioxide photocatalyst. Samples were quantified by HPLC usingthe previously described method of Lawton et al. (ref. 11).Table 1 Rates of decomposition of microcystin-LR in the presence of TiO2photocatalystSpecific rate/Initial toxin Specific activity/ mm cm22 min21concentra- Initial rate/ mm min21 cm22 mm photon21tion/mm mm min21 (3 1026)a (3 1023)b50 5.95 108.3 33.475 7.20 130.8 40.4100 9.51 155.1 47.9150 12.22 233.2 72.1200 14.60 265.5 82.0a The specific activity represents the rate of destruction per unit area ofcatalyst assuming a catalyst surface area of 50 m2 g21. b The specific raterepresents the rate of destruction per unit area of catalyst per photonabsorbed assuming the number of photons absorbed by the reactor as 1 31024 photons min21 determined using potassium ferrioxalate actinometry(ref. 19).Chem. Commun., 1997 3930.180.160.140.120.100.080.060.040.020.000.00 0.01 0.02 0.03 0.04 0.05 0.06 0.071 / C01 / r010987654321002468101214n2s / µmol g–1Ceq / µmolsubstrate concentration, k is the rate constant of the reaction andK is the Langmuir adsorption constant.kKC0r0 = (1)1 + kKC0The constants k and K may be determined from a plot of 1/r0vs. 1/C0. If the system obeys the Langmuir–Hinshelwood modelthis plot should be linear. The intercept of the line provides uswith 1/k while the slope is equal to 1/kK. We examined thesuitability of applying this model to our system. The reciprocalof initial rate vs. initial concentration values for the microcystindecomposition reactions were plotted (Fig. 2) and a linearrelationship was observed, confirming that our system doesindeed follow the Langmuir–Hinshelwood model. From thegraph we determined that k and K for the destruction ofmicrocystin were 19.23 mm min21 and 2.9 3 1022 dm3 mmol21respectively. Rate and adsorption constants reported for thephotocatalytic destruction for other organic compounds arecomparable with those reported here.8,13–15 Examples include4-chlorophenol (k = 79.3 mm min21, K = 4.88 3 1023dm3 mmol21),13 benzene (k = 39 mm min21, K = 1.8 3 1022dm3 mmol21)14 perchloroethylene (k = 34 mm min21, K = 8.63 1023 dm3 mmol21)14 and phenol (k = 12.9 mm min21,K = 2.19 3 1022 dm3 mmol21).15Dark adsorption isotherms, for the adsorption of microcystinon TiO2, were prepared in order to compare the value of theLangmuir adsorption constant determined by this method withthat of the constant determined using the initial rates method forthe photocatalytic model. Universal bottles containing the toxinsolution plus TiO2 were shaken at room temperature in the darkusing a flask shaker for 24 h. The dissolved microcystin-LR wasseparated from the catalyst and quantified by HPLC. The darkadsorption isotherm for microcystin-LR was plotted (Fig. 3) andthe K value was found to be 2.9 3 1024 dm3 mmol21. This is100 times lower than the value obtained using the initial ratesmethod. Similar differences have been observed by otherworkers.16,17 It has been proposed that this may be due tophotoadsorption reactions at the TiO2 surface or that decom-position of the substrate may not occur exclusively at thecatalyst surface but may be promoted by hydroxyl radicalsreleased from the catalyst surface.16,17Preliminary results from our laboratory have shown that thissystem can also be successfully applied to the destruction ofother classes of cyanobacterial toxins.18 The efficient removalof microcystins and the benefits of the reported removal of otherorganic contaminants, indicates that photocatalysis is anextremely attractive treatment method for potable watersupplies.We thank J. McElhiney and B. Cornish for assistance withHPLC analysis throughout the project, and S. M. Henderson andC. Edwards (Biotage UK, Hertford) for their collaboration inthe purification of microcystin-LR.References1 W. W. Carmichael, Sci. Am., 1994, 2, 64.2 L. A. Lawton, C. Edwards, K. A. Beattie, S. Pleasance, G. J. Dear andG. A. Codd, Nat. Toxins, 1995, 3, 50.3 J. Dunn, Br. Med. J., 1996, 312, 1183.4 C. S. Reynolds, in Advances in Botanical Research, ed. J. A. Callow,Academic Press, London, 1987, vol. 13, p. 63.5 K. Tsuji, S. Nalto, F. Kondo, N. Ishikawa, M. F. Watanabe, M. Suzukiand K.-I. Harada, Environ. Sci. Technol., 1994, 28, 173.6 K. Tsuji, T. Watanuki, F. Kondo, M. Wanatabe, S. Suzuki, H.Nakazawa, M. Suzuki, H. Uchida and K. Harada, Toxicon, 1995, 33,1619.7 K. Himberg, A.-M. Keijola, L. Husvirta, H. Pyysalo and K. Sivonen,Water. Res., 1989, 23, 979.8 M. R. Hoffmann, S. T. Martin, W. Choi and D. W. Bahnemann, Chem.Rev., 1995, 95, 69.9 A. Mills, R. H. Davies and D. Worsley, Chem. Soc. Rev., 1993, 93,417.10 C. Edwards, L. A. Lawton, S. M. Coyle and P. Ross, 1996, 734, 163.11 L. A. Lawton, C. Edwards and G. A. Codd, Analyst (London), 1994,119, 1525.12 D. F. Ollis, E. Pelizzetti and N. Serpone, in Photocatalysis, Funda-mentals and Applications, ed. N. Serpone and E. Pelizzetti, Wiley, NewYork, 1989, p. 603.13 R. W. Matthews, J. Catal., 1988, 111, 264.14 C. S. Turchi and D. F. Ollis, J. Catal., 1989, 119, 483.15 R. W. Matthews and S. R. McEvoy, J. Photochem. Photobiol. A: Chem.,1992, 66, 355.16 J. Cunningham and G. Al-Sayyed, J. Chem. Soc., Faraday Trans., 1990,86, 3935.17 A. Mills and S. Morris, J. Photochem. Photobiol. A: Chem, 1993, 71,75.18 J. Rouzade, Internal Report, The Robert Gordon University, Aberdeen,Scotland, 1996.19 C. G. Hatchard and C. A. Parker, Proc. R. Soc. London, A, Math. Phys.Sci., 1956, 235, 518.Received, 25th November 1996; Com. 6/07965BFig. 2 Plot of the reciprocal of the initial rate (1/r0) vs. the reciprocal of theinitial concentration (1/C0)Fig. 3 Dark adsorption isotherm of microcystin-LR on TiO2. The number ofmoles of microcystin-LR attached per gram of TiO2 is n2S while Ceqrepresents the equilibrium concentration of microcystin after 24 h.394 Chem. Commun., 1997

Destruction of cyanobacterial toxins by semiconductor photocatalysis.

The rapid destruction of microcystin, a cyanobacterial toxin, using a titanium dioxide photocatalyst is observed; the process is extremely efficient with high concentrations of toxin completely undetectable within 10-40 min, depending on the initial concentration.</p

Robertson, Peter K J

Queen's University Belfast Research Portal

Destruction of cyanobacterial toxins by semiconductor photocatalysis

https://rgu-repository.worktribe.com/preview/294313/ROBERTSON%201997%20Destruction%20of%20cyanobacterial%20toxins.pdf

Destruction of cyanobacterial toxins by semiconductor photocatalysis.

Abstract

Similar works

Full text

Available Versions

Open Access Institutional Repository at Robert Gordon University

Queen's University Belfast Research Portal