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.