The summer school was held at the Department of Chemistry, Makerere University, Kampala Uganda. Originally, the course was planned to last for two
weeks, i.e. from the 8th to the 19th of July. Due to demand by the participants, the course was extended for literally twice the period to the 2nd of August.
Students continued with their experiments even during the week in which the ANCAP Regional Symposium was held, i.e. 21st - 25th, after the symposium sessions were over.
Course Tutor: Prof. Peter Nkedi-Kizza , University of Florida, USA
1. Mr. A.J. Mmochi Tanzania
2. Mr. K. Gomoro Ethiopia
3. Mr. M. O. Vincent Kenya
4. Mr. M. Mulugeta Ethiopia
5. Mr. T. Hannington Uganda
6. Mr. F. Kengara Kenya
7. Mr. M. Mihale Tanzania
8. Mr. J. Wasswa Uganda
9. Mr. H. Mwevura Tanzania
10. Mr. B. Robert Uganda
11. Mrs. M. B. Sunny Uganda
The course was based on the sorption and transport of hydrophobic organic chemicals, mostly pesticides, in the environment. The theoretical and practical aspects of sorption, desorption, degradation and leaching of these chemicals in soil were studied. Attention was on the fact that though most existing data and theories for sorption of these chemicals on soil and sediment deal with aqueous solutions, there is an increasing concern on groundwater contamination from land disposal and land treatment of organic chemicals. This calls for characterizing sorption and transport of hydrophobic organic compounds from solutions containing mixtures of water and various water-soluble organic solvents.
Three experiments were designed to cater for sorption, degradation and
leaching of three pesticides: diuron, atrazine and carbaryl. Sorption of
pesticides in soils in aqueous solvent was done for all the pesticides.
Sorption of the pesticide in mixed solvents (methanol and water) was done
using only diuron.
Degradation and leaching experiments were performed using carbaryl and atrazine, respectively.
Five Ugandan soils of differing texture, organic matter content, cation exchange capacity were used in the experiments. All the experiments were based on the solvophobic theory developed by Rao et. al.
Sorption of pesticides in aqueous media and in presence of co solvent was necessary for comparison because simple extrapolation of the existing theories and data may be neither feasible nor desirable. It is a well known phenomenon that solubility and sorption in aqueous solutions are inversely related. This was also applied in mixed solvents, and it was shown that sorption increased exponentially as the mole fraction of the co solvent increases. That is sorption coefficient of organic compound decreases with increasing fraction of co solvent. The experiments showed that for a given sorbate - solvent combination, the slopes of log sorption coefficient, log Km vs fraction cosolvent (fc) plots were essentially identical for all soils indicating that in mixed solvents, soil properties have little effects on the sorption of hydrophobic chemical.
Since our ideal goal in applying pesticides is to ensure that they end up being mineralized, a degradation experiment was set for the same soils by assuming the degradation of the carbaryl is first order. On the assumption that no degradation of the adsorbed pesticides occurs, known weights of the soil was added in E-flasks containing the pesticide dissolved in methanol. Triplicate soil samples were prepared for each soil type. The samples were stirred with a stainless steel for about 5 min to evenly distribute the pesticide. Soils were sampled after stirring and the concentration of carbaryl was determined. Soils were sampled at 0.1h 1, 3, 5 and 7 d and extracted to determine the amount of carbaryl remaining in the soil. A buffer solution (80:20 methanol: 1 mol per L trichloroacetic acid/sodium acetate) was used to extract the pesticide. Two extractions were done for each sub-sample and the extracts were combined and analyzed using reverse phase HPLC -UV technique. Because of and limited time it was not possible to evaluate the degradation rate as well as the half life of the pesticide in this tropical soil.
Atraznie was used in setting this experiment, known weights of the soils were used for the experiment. The experiment was set such that the solution of pesticide was to pumped through the soil and then collected in a reservoir. Potassium bromide was used to calibrate the system before the atrazine leaching experiment. HPLC pump was used to pump the pesticide solution through the soil and the "elute" was collected in different test tubes ready for analysis. Again the analysis was done using the HPLC -UV method.