Summary in English


Ilisz István

Vizek szerves mikroszennyezõinek fotokatalitikus oxidációja

József Attila Tudományegyetem, Szeged

1998



Bevezetés

Az utóbbi évtizedekben a hidroszféra növekvõ mértékben elszennyezõdik. A kommunális, az ipari, illetve a mezõgazdasági eredetû szennyvizek nagy része ugyan hatékonyan tisztítható a konvencionális biológiai, illetve fizikai-kémiai szennyvízkezelési eljárásokkal, az élõvizekbe azonban egyre nagyobb mennyiségben jutnak olyan toxikus (elsõsorban ipari eredetû) anyagok, amelyek nem távolíthatóak el e hagyományosnak számító vízkezelési módszerekkel. Ezen súlyosbodó környezeti probléma eredményes megoldásához a kémiai víztisztítási technológiák fejlesztése is szükségszerûvé vált. Az új kihívások a kémiai úton végrehajtott víztisztítás területének rohamos fejlõdését indították el, és már manapság is a gyakorlati életben jól hasznosítható eredményeket szolgáltatnak. Ezen új, reaktív gyökök generálásán alapuló módszereket összefoglaló néven nagyhatékonyságú oxidációs eljárásoknak ("Advanced Oxidation Processes") nevezik.

A nagyhatékonyságú oxidációs eljárások között az 1970-es évek vége óta sorra jelennek meg az ún. heterogén fotokatalitikus eljárások, amelyek szilárd félvezetõk megfelelõ hullámhosszúságú megvilágításán alapulnak. Jelentõségük a napenergia kémiai energiává való átalakításában és a rezisztens kémiai szennyezõ anyagok eltávolításában keresendõ. A terület fontosságának a nagyszámú publikációban is realizálódó intenzív kutatás a fokmérõje.
 

Célkitûzés

A heterogén fotokatalitikus reakciók területén megjelenõ jelentõs számú publikáció ellenére a fotofizikai lépéseket követõ kémiai folyamatok és reakciókinetikai jellegzetességeik mindmáig csak részben tisztázottak.

Különösen sok figyelmet szenteltek az aromás vegyületek, fõként a fenol, illetve a fenolszármazékok vizsgálatának. Ezért választottam én is a fenolt, illetve származékait modellvegyületként vizsgálataimhoz. Döntésemben mind környezetvédelmi vonatkozások, mind pedig az irodalomban meglevõ bizonytalanságok, ellentmondások behatóbb megismerése, esetleges feloldása egyaránt szerepet játszott. Az aromás vízszeny-nyezõk heterogén fotokatalitikus lebomlásának kémiai mecha-nizmusa és kinetikai jellegzetességeinek tanulmányozása mellett célom volt a hidrogén-peroxid, mint ezen folyamatok egyik lehetséges köztiterméke, átalakulásának vizsgálata.
 

Eredmények

H2O2 fotokémiai átalakulására vonatkozó eredmények

Közeli ultraibolya sugárzással gerjesztett TiO2 tartalmú vizes szuszpenzióban különbözõ körülmények között tanulmányoztam a hidrogén-peroxid fotokémiai bomlását.
 

Megállapítottam:

A fenolbomlás kinetikájára vonatkozó eredmények

Különféle elektronakceptorok hatását vizsgáltam a fenolátalakulás kinetikájára közeli-UV-fénnyel megvilágított TiO2 szuszpenziókban. Megállapításaim:


A fenolátalakulás kémiai mechanizmusára vonatkozó eredmények

A fenolátalakulás mechanizmusára vonatkozó kísérleti eredményeim alapján megállapítottam:


Egyéb aromás termékek átalakulására vonatkozó eredmények

A fenol átalakulásában képzõdõ köztitermékek (katechol, hidrokinon, 1,2,4-trihidroxi-benzol, 1,4-benzokinon), illetve egy fenolszármazék (4-klór-fenol) heterogén fotokatalitikus reakcióinak vizsgálatával megállapítottam:



István Ilisz
 

Heterogeneous photocatalytic oxidation of water pollutants

Ph.D. thesis, József Attila University, Szeged

1998



Introduction

The appearance of compounds that are difficult to degrade by conventional chemical and/or biological methods (toxic, mutagenic, carcinogenic pollutants) in natural and waste waters recently created a pressing need for the development of efficient water-treatment processes. The search for a solution of this problem has involved extensive examinations in the field of advanced oxidation processes (AOPs).

Since the early 1970s, there has been immense interest in the photochemistry of semiconductors, because of their beneficial features relating to the photocatalytic decontamina-tion of water and the utilization of solar energy. In the well-known process of excitation of a semiconductor, absorption of a photon with suitable energy is followed by promotion of an electron from the conduction band to the valence band, leaving behind a positive charge vacancy, i.e. a hole. Therefore an irradiated solid semiconductor can provide available charges (electrons and positive holes) for the redox transformations of either organic or inorganic contaminants. In order for a photochemical change to proceed, on the one hand the charge-transfer reaction must compete efficiently with the recombination process. On the other hand the band edge positions of the applied semiconductor must be appropriate to the investigated redox system. Among the examined semiconductors TiO2 (anatase) usually possesses the best features: photostable, inexpensive and non-toxic. Thus the TiO2 based heterogeneous photocatalysis became one of the most promising possibilities in the water-treatment processes.

Although the continuously growing interest in semiconductor photochemistry has resulted in the appearance of a huge number of papers in the literature, there are still crucial uncertainties in our understanding.

In the present doctoral thesis, I set out to investigate the possibilities of the practical applications of heterogeneous photocatalytic processes in water treatment. The decom-position of several model compounds was investigated in near-UV-irradiated aqueous unbuffered TiO2 (anatase) suspensions.
 

The photochemical behavior of hydrogen peroxide

Although much effort has been devoted to the investigation of H2O2 photogeneration, far less attention has been paid to the basic photochemical transformations of H2O2 in irradiated TiO2 suspensions, and few of the papers give a detailed description of the observed kinetics. Hence, I considered it useful to study the kinetic behavior of H2O2 in near-UV-irradiated aqueous unbuffered TiO2 suspensions. The effects of the H2O2 concentration, the catalyst loading, the dissolved oxygen concentration and the light intensity on the initial photodecomposition rate were studied.

The rapid photocatalytic decomposition of H2O2 was observed in a UV-irradiated aqueous TiO2 suspension. It was found that the heterogeneous decomposition can be satisfactorily described in terms of the initial rates obtained from linear regression fits to the tirrad vs. cH2O2 curves. The linear dependence observed between the irradiation intensity and the phototransformation rate demonstrates that the photons absorbed by the semiconductor particles determine the decomposition rate. The initial rate of decomposition of H2O2 is strongly affected by the catalyst loading up to a saturation limit. This is due to the increased number of active sites until all the photons reaching the reactor are absorbed by the semiconductor. For a description of the dependence of the initial rates on the initial concentrations, Langmuir-Hinshelwood and Freundlich treatments were applied. The Freundlich treatment proved to be a suitable model for the characterization of this reaction system. A decomposition rate was established with a formal reaction order of 0.21± 0.01, as a consequence of the adsorption of H2O2 on the surface of the photocatalyst. Dissolved oxygen as a potential electron scavenger does not have a significant effect on the degradation rate.

Consequently, the H2O2 presumably formed in irradiated TiO2 suspensions may be expected to be only a short-lived intermediate, which can not accumulate under the applied conditions.
 

Investigation of the photodecomposition of phenol and its derivatives

Our results, relating to the photodecomposition of organic compounds (phenol and its derivatives) show that without any efficient electron scavenger present, no phototransformation of organics is observed, except for the reduction of p-benzoquinone.

The degradation of phenol under different reaction conditions obeys apparently zero-order kinetics, although at high conversions deviation is observed from this model. The Langmuir-Hinshelwood model was successfully applied to describe the effects of O2 and the initial substrate concentration on the initial rate of phenol decomposition. A comparison of the adsorption constants of O2 and phenol reveals that O2 binds more strongly than phenol to the given anatase sample.

During phenol photooxidation in the presence of O2, the rate-determining step is the electron capture by adsorbed O2. Under the conditions of this study both dissolved O2 and its reduction forms play only a minor role (apart from electron trapping) in the decomposition of phenol.

In the presence of dissolved O2 as an electron scavenger the degradation of all of the examined model compounds can occur. Enhancing the concentration of dissolved O2 results in enhanced photooxidation. Both the appearance of hydroxylated products, which is in good accordance with the generally accepted degradation mechanism and the presented kinetics support that in the presence of dissolved O2 the OH. radical is the main oxidizing species. The degradation is significantly inhibited by the presence of OH.-scavengers (e.g. methanol).

Besides the identified aromatic intermediates (hydroquinone, catechol, 1,2,4-trihydroxybenzene), the

formation of presumed ring-opening products occurs simultaneously. Based on indirect evidences these compounds are presumably aliphatic acids and aliphatic oxocompounds.

Addition of silver ion as (a more efficient) electron scavenger results in a significantly enhanced photodegradation rate, which is due to the beneficial effects of Ag+ on the charge separation efficiency. In the presence of Ag+ the only intermediate detected was p-benzoquinone, which can accumulate in the reaction mixture as long as the Ag+ reduction proceeded. After all the Ag+ has been reduced, the reduction of p-benzoquinone to hydroquinone occurs. In these systems the inhibition effect of methanol is not relevant. The experimental results confirm that in the presence of Ag+ the direct hole oxidation is the main degradation pathway and the hole-scavenging reactions determine the overall transformation rate.

Although the presence of H2O2 can open up an efficient route for the degradation of phenol without the addition of any other electron acceptor (i.e. the H2O2 can act as efficient electron acceptor), the presence of H2O2 as an additional radical source in air-equilibrated TiO2 suspensions did not result in significantly enhanced mineralization. During the degradation of phenol in the presence of H2O2 both the degradation kinetics and the formation of primary intermediates occur similarly as in the presence of dissolved O2. The significant but not complete inhibition observed in the presence of OH.-scavenger indicates that in the presence of H2O2 both OH. and positive holes contribute to the degradation process.
 

Publications

1. A. Dombi, Gy.Wittmann, I.Ilisz, Gyökgenerálási eljárások a vízkezelésben alkalmazott nagy-hatékonyságú oxidációs eljárásokhoz, Múzeumi Füzetek 6 (1997) 58.
2. I. Ilisz, K. Fõglein, A. Dombi, The photochemical behavior of hydrogen peroxide in near UV-irradiated aqueous TiO2 suspensions, J. Mol. Catal. A. Chem.,135 (1998) 55.
3. I. Ilisz, Zs. László, A. Dombi, Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. I. Effect of charge-trapping species on the degradation kinetics, Appl. Catal. A., 180 (1999) 25.
4. I. Ilisz, A. Dombi, Investigation of the photodecomposition of phenol in near-UV-irradiated aqueous TiO2 suspensions. II. Effect of charge-trapping species on the product distribution, Appl. Catal. A., 180 (1999) 35.
5. Zs. László, I. Ilisz, G. Peintler, A. Dombi, VUV intensity measurement for a 172 nm Xe excimer lamp using oxygen actinometry, Ozone Sci. Technol., 20 (1998) 421.


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