Φωτοχημική και φωτοκαταλυτική αποδόμηση και εκτίμηση της τοξικότητας επιλεγμένων παρασιτοκτόνων και προϊόντων διάσπασής τους

Abstract

Intensive cultivation, due to increased needs and economical reasons, requires a constant struggle against losses caused by weeds, insects or diseases. The agents used for this purpose are the various forms of pesticides: insecticides, herbicides, fungicides etc. However, the widespread use and disposal of pesticides leads to the pollution of surface waters, which has sharply increased and poses a major environmental threat. Therefore, the need for effective technologies, which are able to reduce pesticide input into water runoff or able to completely restore polluted water, is urgent and compulsory. Possible treatment methods comprise physical entrainment, biodegradation and chemical reactions. Detoxification technologies should be considered successive when they achieve complete decomposition of pollutants and avoid the formation of toxic end- and by-products. According to this point of view Advanced Oxidation Processes (AOPs) offer an attractive alternative as they are destructive methods and they do not simply transfer pollution from one phase to another. Many oxidation processes are currently employed, including photochemical degradation processes (UV/O₃, UV/H₂O₂), photocatalysis (TiO₂/UV, Photo-Fenton’s reagents) and chemical oxidation processes (O₃, O₃/H₂O₂, H₂O₂/Fe²⁺). Encouraging laboratory scale data has come up and some industrial scale tests have shown that this group of techniques can most of the times offer an integrated solution to the problem of pesticide pollution, either by using them individually or in combination with other conventional techniques, like biological treatment methods. Among the organic compounds used for pest control carbamates constitute an important class of substances of environmental concern. Carbamate insecticides are substituted esters of carbamic acid (NH₂COOH) with aliphatic or aromatic substituents on the oxygen or nitrogen atoms. In general, the aryl esters of N-methyl-substituted carbamic acid are insecticides and the esters of N-aryl-carbamic acids are selective herbicides. Carbamates draw growing attention because they are widely used as substitutes of organochlorine pesticides due to their rapid action and moderate persistence in the environment. Their toxic effects are ought to the inhibition through carbamylation of the enzyme acetyl cholinesterase at nerve endings. Unlike to organophosphorus pesticides, the inhibition caused by carbamates is reversible, so they are considered to be less toxic to mammals. In the present study, the hydrolytic, photolytic and photocatalytic degradation of three carbamate pesticides, namely carbaryl, molinate and pirimicarb, were investigated. The photocatalytic methods which were used were heterogeneous photocatalysis in the presence of TiO₂ slurry and photocatalysis using variations of the photo-Fenton system. The objectives of the study were: I) to examine the pesticide degradation through hydrolysis and photolysis, II) to study the efficiency of the photocatalytic methods for the oxidation and disappearance of the target compounds, III) to evaluate the kinetics of the photocatalytic oxidation, IV) to examine the effect of various parameters that affect the efficiency and the rate of the oxidative process, V) to evaluate the degree of mineralization of selected pesticides, VI) to investigate the evolution of toxicity during the degradation process and at the end of the treatment and VII) to identify the organic intermediates formed during the oxidative degradation. According to the results of the study, the main conclusions that could be drawn are summarized as follows: The herbicide molinate and insecticide pirimicarb are rather stable against hydrolytic degradation, in all pH regions, and they exhibit slower degradation rates when the hydrolysis is investigated in natural waters coming from a river or a lake. The insecticide carbaryl is very sensitive to the hydrolytic degradation in alkaline or neutral pH, while it is rather stable in acidic pH. As far as heterogeneous photocatalysis in the presence of titanium dioxide is concerned, complete disappearance of the pesticides is achieved and titanium dioxide P-25 was proved to be more efficient photocatalyst than titanium dioxide Hombikat UV-100. Parameters such as concentration of the catalyst, the pH value and the addition of oxidants (hydrogen peroxide or potassium peroxidisulfate) affect the degradation rate. The photooxidation of the organic compounds can be described by pseudo-first order kinetics according to the Langmuir-Hinshelwood model. The mineralization of all three pesticides requires higher illumination periods, but can be achieved when aquatic solutions in deionized water are concerned. Complete mineralization in natural water solutions is not achieved, under certain experimental conditions. However, toxicity of the treated solutions is significantly reduced or complete detoxification is achieved in shorter illumination times comparing to the treatment time required for mineralization. This fact suggests the formation of organic intermediates that are present in such concentration levels that prove to be less toxic than the mother compound. On the other hand, photo-Fenton system has also proved to be rather efficient when pesticide disappearance is concerned. The absence of illumination is detrimental for the treatment, since Fenton reagent is unable to oxidize the target compounds in a considerable extent, when the same concentrations of reagents, as in the case of photo-Fenton, are used. The concentration and the valence of iron or the concentration of hydrogen peroxide and the way of adding it are parameters affecting the degradation rate and efficiency of the process. The effect of counterions of Fe³⁺ and H⁺ was also investigated. Zero valet iron was studied as an alternative source of iron and the results were rather encouraging. The degradation of all three pesticides was also achieved in the case of natural waters. Mineralization, although required longer illumination and higher hydrogen peroxide consumption, was achieved. The profile of toxicity reduction is analogous to that acquired during the heterogeneous photocatalysis. The identification of organic intermediates was carried out using LCMS/MS technique in the case of molinate and pirimicarb and GC-MS technique for carbaryl. Twelve organic byproducts were detected in the case of cararyl, five in the case of molinate, including several isomers, and eleven in the case of pirimicarb. Some slight differences were observed when comparing the intermediates formed during the heterogeneous and homogeneous photocatalysis. The LC-MS/MS technique is considered quite advantageous, in cases that can be applied, since no pre-concentration and preparation stage is required. Comparing the oxidation methods that are applied, it can be concluded that, even though both are effective for treating aqueous solutions contaminated with the selected pesticides, the photo-Fenton reaction has proved to be more flexible and more efficient in the case of more complex substrates such as natural waters.

Σκοπός της παρούσας διατριβής ήταν η μελέτη της αποδόμησης τριών παρασιτοκτόνων της κατηγορίας των καρβαμιδικών, η φωτοκαταλυτική επεξεργασία υδατικών τους διαλυμάτων και η ανίχνευση και ο προσδιορισμός των προϊόντων διάσπασής τους. Μελετήθηκε η αποδόμηση των παρασιτοκτόνων μέσω της υδρόλυσης σε τέσσερα διαφορετικά υδατικά περιβάλλοντα (δισ-απιονισμένο νερό, νερό από δύο διαφορετικούς ποταμούς και λιμναίο νερό) και σε διαφορετικές αρχικές τιμές pH. Επιπλέον, εξετάστηκε η φωτολυτική διάσπασή τους με τη χρήση τεχνητής πηγής ακτινοβολίας, συγκεκριμένα λυχνίας Hg υψηλής πίεσης, τοποθετημένης σε φίλτρο από pyrex, ώστε να δεσμεύεται η ακτινοβολία με μήκη κύματος <290 nm. Η φωτοκαταλυτική επεξεργασία των υδατικών διαλυμάτων πραγματοποιήθηκε με τη μέθοδο της ετερογενούς φωτοκατάλυσης με τη χρήση διοξειδίου του τιτανίου (TiO₂) και με τη μέθοδο της ομογενούς φωτοκατάλυσης με τη χρήση του συστήματος photo-Fenton (Fe³⁺ ή 2⁺/H₂O₂/UV). Όσον αφορά στη μέθοδο της ετερογενούς φωτοκατάλυσης η έρευνα επικεντρώθηκε στα εξής σημεία: I) στην εκτίμηση της κινητικής της αποδόμησης των επιλεγμένων παρασιτοκτόνων, IΙ) στην εξέταση διαφόρων παραγόντων που επηρεάζουν τη διαδικασία διάσπασης των προς μελέτη ουσιών, όπως η συγκέντρωση και ο τύπος του καταλύτη, η αρχική συγκέντρωση των παρασιτοκτόνων, το αρχικό pH των διαλυμάτων και η επίδραση της προσθήκης οξειδωτικών ουσιών στο ρυθμό και την απόδοση της επεξεργασίας, ΙΙΙ) στον καθορισμό και στις παραμέτρους που επηρεάζουν το βαθμό ανοργανοποίησης των ενώσεων που μελετώνται και ΙV) στην εκτίμηση της τοξικότητας της αρχικής ένωσης και των ενδιάμεσων προϊόντων διάσπασης που προκύπτουν κατά τη διεργασία αποδόμησης. Σχετικά με τη μέθοδο της ομογενούς φωτοκατάλυσης με τη χρήση του συστήματος photo-Fenton (Fe³⁺ ή 2⁺/H₂O₂/UV), ακολουθήθηκε παρόμοιο σκεπτικό, ενώ οι παράμετροι που εξετάστηκαν αφορούν τη συγκέντρωση των ιόντων σιδήρου, τη συγκέντρωση του οξειδωτικού, το pH, το σθένος των ιόντων σιδήρου και τα ανόργανα ιόντα. Τελικό στάδιο αποτέλεσε η ανίχνευση, ταυτοποίηση και προσδιορισμός των κύριων προϊόντων μετασχηματισμού που προκύπτουν κατά την επεξεργασία των επιλεγμένων παρασιτοκτόνων και με τις δύο φωτοκαταλυτικές μεθόδους που επιλέχθηκαν. Πραγματοποιήθηκε σύγκριση των ενδιάμεσων προϊόντων που προκύπτουν από τις δύο τεχνικές, ερευνήθηκε και έγινε προσπάθεια να ποσοτικοποιηθεί η εξέλιξη σχηματισμού και διάσπασής τους κατά την πορεία της επεξεργασίας και επιχειρήθηκε η πρόταση ενός πιθανού μηχανισμού διάσπασης των προς μελέτη ουσιών.