<p>Kao što je poznato, RS-2-(4-hlor-o-toliloksi)propionska kiselina (MCPP), (4-hlor-2-metilfenoksi)sirćetna kiselina (MCPA) i 3,6-dihlorpiridin-2-karboksilna kiselina (klopiralid) su herbicidi sa veoma širokim spektrom dejstva, a pored toga su rastvorljivi u vodi, teško biorazgradljivi i prema literaturnim podacima su, nažalost, veoma često prisutni herbicidi u pijaćoj vodi. Proces heterogene fotokatalize uz primenu TiO<sub>2</sub>i UV zračenja se pokazao kao veoma pogodan način za njihovo uklanjanje iz vode. Međutim, zbog velikog energetskog procepa od 3,2 eV (anataze-oblik), odnosno, 3,0 eV (rutil-oblik), veoma mali udeo bliskih UV zraka iz sunčeve svetlosti (oko 3−4%) biva iskorišćen u toku fotokatalitičkog procesa, što ukazuje na to da je TiO<sub>2 </sub>praktično neaktivan u prisustvu sunčeve svetlosti. Na osnovu literaturnih podataka je zapaženo da postoji mogućnost fotorazgradnje pojedinih supstrata u prisustvu TiO<sub>2 </sub>primenom vidljive svetlosti. Na primeru MCPP je ispitana aktivnost TiO<sub>2 </sub>Degussa P25 kao fotokatalizatora u prisustvu vidljive svetlosti. Na osnovu refleksionih spektara je utvrđeno da MCPP adsorbovan na TiO<sub>2 </sub>Degussa P25 apsorbuje vidljivi deo spektra (λ ≥400 nm). Nastali prelazni kompleks je potvrđen FTIR merenjima. Efikasnost TiO2Degussa P25 primenom vidljive svetlosti je upoređena sa sunčevim i UV zračenjem, kao i direktnom fotolizom u prisustvu pomenutih izvora zračenja. Brzina fotokatalitičke razgradnje MCPP primenom vidljive svetlosti iznosi 0,86 μmol dm<sup>−3</sup>min<sup>−1</sup>, što je oko 4 puta brže u poređenju sa direktnom fotolizom. Nadalje je ustanovljena optimalna masena koncentracija katalizatora od oko 8 mg cm<sup>−3</sup>, koja je znatno viša u poređenju sa primenom UV zračenja. Razlog je najverovatnije različit mehanizam fotorazgradnje koji se odvija primenom vidljivog i UV zračenja. Naime, prisustvo 2-metil-2-propanola (poznatog hvatača <sup>•</sup>OH-radikala) praktično ne utiče na brzinu fotokatalitičke razgradnje MCPP p rimenom vidljive svetlosti, što ukazuje da se mehanizam razgradnje MCPP primenom vidljive svetlosti ne odvija posredstvom <sup>•</sup>OH-radikala, za razliku od onog uz primenu UV zračenja.</p><p>S obzirom da se katalizator TiO<sub>2 </sub>Degussa P25 uz primenu vidljive svetlosti nije <br />pokazao kao naročito efikasan kada je u pitanju razgradnja sva tri herbicida i imajući u vidu da se u poslednje vreme iz razloga praktične primene sve više pribegava procesu dopovanja TiO<sub>2 </sub>različitim tipovima metala (alkalnih, zemnoalkalnih, prelaznih i dr.) i nemetala (halogenida, halkogenida i dr.), u okviru ove doktorske disertacije je ispitana aktivnost N-TiO<sub>2 </sub>(sintetisanih mokrim i suvim putem) i TiO<sub>2 </sub>(rutil) dopovanog sa različitim količinama Fe<sup>3+</sup>-jona (0,13−1,48 at.%) pri razgradnji herbicida MCPP i MCPA primenom vidljive svetlosti. Pored toga je ispitana efikasnost TiO<sub>2 </sub>(anataze) takođe dopovanog sa različitim količinama Fe<sup>3+</sup>-jona (0,71−1,80 at.%) na primeru MCPP. </p><p>Poredeći N-TiO<sub>2 </sub>(sintetisan mokrim putem) i N-TiO<sub>2 </sub>(sintetisani suvim putem), primećeno je da je u drugom slučaju efikasnost katalizatora veća oko 2 puta. Isto tako je zapažena u slučaju MCPP nešto veća fotokatalitička aktivnost N-TiO<sub>2 </sub>(sintetisani suvim putem) u poređenju sa TiO<sub>2 </sub>(anataze). Kada je u pitanju MCPA aktivnost sva tri katalizatora je veoma slična. Pored toga je zapažena veća efikasnost N-TiO<sub>2 </sub>(sintetisan mokrim putem) u poređenju sa TiO<sub>2 </sub>Degussa P25 (oko 1,5 puta) i oko 5 puta u odnosu na direktnu fotolizu, dok su N-TiO<sub>2 </sub>(sintetisani suvim putem) oko 3 puta efikasniji u poređenju sa TiO<sub>2 </sub>Degussa P25 i oko 10 puta u poređenju sa direktnom fotolizom. Brzina solarne razgradnje je preko 100 puta manja nego primenom vidljivog i UV zračenja, što je posledica različitih intenziteta pomenutih izvora ozračivanja i različitih uslova pri kojima je vršena razgradnja. Ustanovljena je optimalna masena koncentracija N-TiO<sub>2 </sub>(sintetisan mokrim putem) od 4 mg cm<sup>−3</sup>.</p><p>Prilikom razgradnje MCPP i MCPA je nađeno da je brzina veća kada se kao katalizator koristi TiO<sub>2 </sub>(rutil) u poređenju sa Fe-TiO<sub>2 </sub>i da sa povećanjem količine Fe<sup>3+</sup>-jona fotokatalitička aktivnost uglavnom opada. Kada je kao fotokatalizator korišćen TiO<sub>2 </sub><br />(anataze) dopovan različitim količinama Fe<sup>3+</sup>-jona (od 0,71 do 1,80 at.%), razgradnja <br />MCPP je u svim slučajevima znatno sporija u odnosu na TiO<sub>2 </sub>(anataze).</p><p>S obzirom da su prema literaturnim podacima kinetika i mehanizam fotokatalitičke <br />razgradnje klopiralida nepoznati, ispitana je njegova stabilnost pri različitim eksperimentalnim uslovima. Tokom ispitivanja uticaja pH kako u prisustvu, tako i u odsustvu dnevne svetlosti u intervalu pH od 1,0−9,0, nađeno je da ni u jednom slučaju ne dolazi do razgradnje supstrata u periodu od sedam meseci koliko je proces praćen. Takođe je ispitana kinetika fotokatalitičke razgradnje klopiralida primenom UV i vidljivog zračenja u prisustvu TiO<sub>2 </sub>Degussa P25, kao i direktna fotoliza primenom oba izvora zračenja. Nađeno je da je brzina fotokatalitičke razgradnje primenom UV zračenja veća oko 5 puta u odnosu na direktnu fotolizu. Za praćenje toka fotokatalitičke razgradnje klopiralida je izabrana pH-vrednost od 3,2. Nadalje je zapaženo da se u ispitivanom opsegu početnih koncentracija supstrata (0,5–3,0 mmol dm<sup>−3</sup>) kinetika fotokatalitičke razgradnje klopiralida može opisati pseudo-prvim redom. Pri ispitivanju uticaja masene koncentracije katalizatora (0,5–8 mg cm<sup>−3</sup>) na brzinu razgradnje klopiralida, ustanovljena je optimalna masena koncentracija primenjenog fotokatalizatora od oko 4 mg cm<sup>−3</sup>. Izračunata ukupna prividna energija aktivacije iznosi 7,74 kJ mol<sup>−1</sup>. Pored toga, prisustvo kiseonika ubrzava reakciju 2 puta, dok dodatak elektron-akceptora kao što su (NH<sub>4</sub>)<sub>2</sub>S<sub>2</sub>O<sub>8</sub>, H<sub>2</sub>O<sub>2 </sub>i KBrO<sub>3 </sub>pokazuje značajan i različit efekat na kinetiku fotokatalitičke razgradnje klopiralida. Pri ispitivanju uticaja etanola, kao hvatača slobodnih radikala, nađeno je da se heterogena fotokataliza odvija uglavnom preko <sup>•</sup>OH-radikala.</p><p>Na osnovu LC–MS/MS (ESI+) merenja ustanovljeno je prisustvo nekoliko intermedijera: 3,6-dihlor-piridin-2-ol, 3,6-dihlor hidroksipiridin-2-karboksilna kiselina i 3,3',6,6'-tetrahlor-2,4'-bipiridin-2'-karboksilna kiselina. Na osnovu identifikovanih intermedijera, kao i kinetičkih rezultata, predložen je mogućput mehanizma fotokatalitičke razgradnje klopiralida.</p><p>Prilikom ispitivanja uticaja strukture molekula na brzinu razgradnje, konstatovano <br />je da u slučaju klopiralida praktično ne dolazi do fotokatalitičke razgradnje u prisustvu TiO<sub>2 </sub>(anataze) i N-TiO<sub>2 </sub>(sintetisani suvim putem) uz primenu vidljive svetlosti, kao i u slučaju TiO<sub>2 </sub>Degussa P25. Pored toga, primenom TiO<sub>2 </sub>(rutil) i Fe-TiO<sub>2 </sub>kao fotokatalizatora, sa povećanjem količine Fe<sup>3+</sup>-jona od 0,13 do 1,27 at.% raste brzina razgradnje klopiralida, ukazujući da strukturne osobine supstrata utiču na brzinu njihove razgradnje.</p> / <p>It is well known that RS-2-(4-chloro-o-tolyloxy)propionic acid (MCPP), (4-chloro-2- methyl-phenoxy)acetic acid (MCPA) and 3,6-dichloropyridine-2-carboxylic acid (clopyralid) are herbicides of wide activity spectrum. They are soluble in water, hardly biodegradable and, unfortunately, often present in drinking water.</p><p>Heterogeneous photocatalysis by application of TiO<sub>2 </sub>and UV radiation proved to be very suitable for their removal from water. However, due toits large energy gap, i.e. 3.2 eV (anatase-form) and 3.0 eV (rutile-form), a very small fraction of sunlight in the near UV range (about 3–4%) is used during photocatalytic process, which is an indication of TiO<sub>2 </sub>inactivity in the presence of this light source. Some literature data report on the possibility of photodegradation of certain substrates by visible light in the presence of TiO<sub>2</sub>. MCPP served as substrate for testing TiO<sub>2 </sub>Degussa P25 photocatalytic activity in the presence of visible light. On the basis of reflection spectra it was established that MCPP adsorbed on TiO<sub>2 </sub>Degussa P25 was absorbing visible spectrum radiation (λ ≥400 nm). The existence of thus formed charge-transfer complex was confirmed with FTIR analysis. The efficiency of TiO<sub>2 </sub>Degussa P25 with application of visible light was compared to sunlight and UV radiation, as well to directphotolysis in the presence of these light sources. The rate of MCPP photocatalytic degradation by means of visible light is 0.86 μmol dm<sup>−3 </sup>min<sup>−1</sup>, which is about 4 times faster than direct photolysis. In addition, the optimal catalyst concentration of about 8 mg cm<sup>−3</sup>, much higher than using UV radiation, <br />was established. The reason is, probably, a different mechanism of hotodegradation in the presence of visible and UV irradiation. Namely, the presence of 2-methyl-2-propanol (well-known <sup>•</sup>OH radical scavenger) has practicallyno effect on the rate of MCPP photocatalytic degradation using visible light, which points that this degradation mechanism does not involve <sup>•</sup>OH radicals, in contrast to that established for UV radiation.</p><p>Since the catalyst TiO<sub>2 </sub>Degussa P25 with application of visible light was not very <br />efficient in degradation of all three herbicidesand in view that nowadays is very popular doping process of TiO<sub>2</sub> with different types of metals (alkali, alkaline-earth, transition, etc.) and non-metals (halogen, chalcogen, etc.), in the scope of this Ph.D. thesis activities of N-TiO<sub>2 </sub>(synthesized by wet and dry procedure) and TiO<sub>2 </sub>(rutile) doped with various amounts of Fe<sup>3+ </sup>(0.13–1.48 at.%) in degradation processes of herbicides MCPP and MCPA using visible light were studied. In addition, the efficiency of TiO<sub>2 </sub>(anatase) doped with various amounts of Fe<sup>3+ </sup>(0.71–1.80 at.%) was also tested for MCPP degradation. When comparing N-TiO<sub>2 </sub>(synthesized by wet procedure) and N-TiO<sub>2</sub> (dry procedure), it was observed that in the latter case the catalyst efficiency was about two times higher. In this case for MCPP was also observed somewhat higher photocatalytic activity of N-TiO<sub>2</sub> (synthesized by dry procedure) in comparison with TiO<sub>2</sub>. When activities of all three catalysts towards MCPA are compared, the results are very alike. In addition, higher efficiency of N-TiO<sub>2 </sub>(wet procedure) comparing to TiO<sub>2 </sub>Degussa P25 (about 1.5 times) and about 5 times in comparison to direct photolysis were recorded, while N-TiO<sub>2</sub> (dry procedure) was about 3 times more efficient than TiO<sub>2 </sub>Degussa P25 and about 10 times in comparison with direct photolysis. The rate of solar degradation is about 100 times lower than by application of UV and visible radiation, as a consequence of various intensities of the mentioned light sources and different conditions of photodegradation. An optimal concentration of N-TiO<sub>2</sub> (wet procedure) of 4 mg cm<sup>−3</sup><br /> was established.</p><p>During degradation of MCPP and MCPA it was observed that the rate is higher if TiO<sub>2 </sub>(rutile) was applied comparing to Fe-TiO<sub>2 </sub>and with increasing amount of Fe<sup>3+ </sup>photocatalytic activity mostly decreases. When TiO<sub>2</sub> (anatase) doped with various amounts of Fe<sup>3+ </sup>(0.71 to 1.80 at.%) was applied for MCPP degradation, the process was much slower than with undoped catalyst.</p><p>Since we have not found relevant literature data on kinetics and mechanism of photocatalytic degradation of clopyralid, its stability in different experimental conditions was tested. In investigating of influences of pH (1.0–9.0) both in presence and in absence of daylight, in no cases decomposition was observed during seven months experiments. Also, the kinetics of photocatalytic degradation of clopyralid using UV and visible irradiation in the presence of TiO<sub>2 </sub>Degussa P25 and in direct photolysis by application of both irradiation sources was studied. It was found that the rate of photocatalytic decomposition using UV radiation was 5 times higher comparing to direct photolysis. For clopyralid photocatalytic monitoring a pH value of 3.2 was chosen. In addition, in the investigated concentration range (0.5–3.0 mmol dm<sup>−3</sup>) the photocatalytic degradation kinetics of clopyralid in the first stage of the reaction follows approximately a pseudo-first kinetic order. In investigation of influence of catalyst concentration (0.5–8 mg cm<sup>−3</sup>) on the rate of clopyralid degradation the highest reaction rate was observed at 4 mg cm<sup>−3 </sup>of catalyst concentration The apparent activation energy of the reaction being 7.74 kJ mol<sup>−1</sup>. The absence of molecular oxygen resulted in a significant decrease (about 2 times) in the rate of clopyralid photodegradation. The effect of the presence of (NH<sub>4</sub>)<sub>2</sub>S<sub>2</sub>O<sub>8</sub>, H<sub>2</sub>O<sub>2 </sub>and KBrO<sub>3</sub>, acting as electron acceptors along with molecular oxygen affects clopyralid photocatalytic degradation considerably and indifferent ways. By studying the effect of ethanol as a hydroxyl radical scavenger it was shown that the heterogeneous catalysis takes place mainly via <sup>•</sup>OH radicals.</p><p>LC−MS/MS (ESI+) monitoring of the process showed that several pyridine-containing intermediates are formed: 3,6-dichloropyridin-2-ol, 3,6-dichloro hydroxypyridine-2-carboxylic acid and 3,3',6,6'-tetrachloro-2,4'-bipyridine-2-carboxylic acid. Based on the identified intermediates and overall kinetic results, a probable photocatalytic degradation mechanism was proposed. </p><p>Finally, in the case of clopyralid it was established that practically no degradation <br />occurs in the presence of TiO<sub>2 </sub>(anatase) and N-TiO<sub>2</sub> (dry procedure) with visible light and also with TiO<sub>2 </sub>Degussa P25. Besides of that, by using TiO<sub>2 </sub>(rutile) and Fe-TiO<sub>2 </sub>as photocatalysts it was noted that increasing the concentration of Fe<sup>3+ </sup>from 0.13 to 1.27 at.% comes to increasing photodegradation rate of clopyralid. Results indicate that differences in molecular structure of chosen compound, influence obtained photocatalytic activity to a great extent.</p>
| Identifer | oai:union.ndltd.org:uns.ac.rs/oai:CRISUNS:(BISIS)16610 |
| Date | 08 July 2009 |
| Creators | Šojić Daniela |
| Contributors | Abramović Biljana, Jovanović Ljiljana, Čomor Mirjana |
| Publisher | Univerzitet u Novom Sadu, Prirodno-matematički fakultet u Novom Sadu, University of Novi Sad, Faculty of Sciences at Novi Sad |
| Source Sets | University of Novi Sad |
| Language | Serbian |
| Detected Language | Unknown |
| Type | PhD thesis |
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