Electro Catalytic Decompsition Of Captan And Methoxychlor Pesticide Residues Using Cyanocobalamin
Muya, Catherine Njambi
MetadataShow full item record
The current study involved electrocatalytic reduction of the organochlorine pesticide residues which are, (3aR,7aS)-2-[(trichloromethyl)sulfanyl]-3a,4,7,7a-tetrahydro-lHisoindole- l,3(2H)-dione (Captan) and 1,1, I-trichloro-2,2-bis(p-methoxy phenyl) ethane (Methoxychlor) pesticide residues in homogenous medium (acetonitrile/water 1:1) and aqueous micellar solution of Sodium dodecyl sulfate (SDS) surfactant. The study therefore involved the use of cyanocobalamin as a catalyst to lower the reduction potentials. The studies were carried out by use of cyclic voltammetry method. A three electrode system was used with Pyrolytic graphite and Glassy carbon as the working electrodes, Platinum wire as the counter electrode and the Saturated Calomel Electrode (SCE) as the reference electrode. Potassium ferricyanide was used as a standard for instrument calibration. Preliminary work also involved cyclic voltammetric study of cyanocobalamin. Direct reductions of (3aR,7aS)-2-[(trichloromethyl)sulfanyl]-3a,4,7,7atetrahydro- lH-isoindole-l,3(2H)-dione (captan) and 1,1, I-trichloro-2,2-bis(p-methoxy phenyl) ethane (methoxychlor) pesticides were carried out using glassy carbon and pyrolytic graphite as working electrodes. The observed reduction potentials were approximately -1.34 ± 0.07 V for captan and -1.31 ± 0.03 V for methoxychlor using pyrolytic graphite electrode. The electrocatalytic reduction of the pesticide residues using cyanocobalamin was then carried out. The obtained voltammograms gave lower reduction potential of -0.800 ± 0.054 V for Captan and -0.88 V for methoxychlor in 0.1 M KN03 in acetonitrile/water, respectively. Therefore, a lowering of potential for reduction of captan and methoxychlor of about 0.54 V and 0.43 V was achieved respectively, which is an indication of energy saving. Captan and methoxychlor had low solubility in the SDS solutions. This influenced the concentration of the substrate at the surface of electrode. As a result the calculated current efficiency was lower than that of the reduction in 0.1 M KN03 in acetonitrile/water (1: 1). The results of cyclic voltammetry at glassy carbon and at pyrolytic graphite for catalytic electrode reduction of captan were compared in order to establish the most suitable electrode with higher current efficiency under the same conditions. The electrocatalytic reduction of captan by cyanocobalamin in 0.1 M KN03 in acetonitrile/water (1: 1) using pyrolytic graphite electrode gave current efficiency of 8.31 ± 0.19, while that obtained for glassy carbon in the same solution medium and under the same reaction conditions gave lower values of 1.41 ± 0.046. The current efficiency at pyrolytic graphite was therefore higher suggesting higher surface concentration and unhindered orientation of the substrate on the pyrolytic graphite electrode. The pesticides were then decomposed in the presence of cyanocobalamin by bulk electrolysis method for four hours each at constant potential of 1.0 V. The product yield was determined by UVNis Spectrophotometry. The UVNIS spectrum for captan gave an intense absorption peak at 278 nm. The decomposed substrate gave three peaks at 260.5 nm, 223.6 nm and 214.9 nm which indicated the presence of decomposed products. The UVNIS spectrum for methoxychlor gave an intense absorption peak at wave length of 283.440 nm. High performance liquid chromatography (HPLC) was carried out for the pesticide standards and the decomposed samples. The observed retention time of captan standard was 3.3 minutes and its decomposed products had 4.0 minutes and 4.3 minutes, respectively. The chromatograms for methoxychlor gave retention time of 3.8 minutes for the standard and the products, 3.2 minutes and 3.4 minutes, respectively. The captan sample after decomposition was also analysed by Thin layer chromatography, which indicated the presence of two decomposition products with different Rf values of 0.727 and 0.636 respectively, while captan standard gave Rf value of 0.152. This confirmed the decomposition reaction. The study demonstrated effective lowering of the reduction potentials of captan and methoxychlor in the presence of cyanocobalamin. The method is therefore economical and environmentally friendly and can greatly contribute in alleviating the levels of captan and methoxychlor pesticide residues in the water bodies and in contaminated swimming pools and improve their quality.