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.
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 LC-MS/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.