3.1. Optimization of Leaching of Malathion from the Soil
Leaching of malathion was carried out using groundwater in the pH range of 4.5 to 8.5 to arrive at optimum pH. The concentration values as a function of pH are plotted in Figure 1. From the figure it is clear that leaching was low at pH 4.5 (30%) and gradually increased with pH. It was observed that at pH 8.2 about 80% of the spiked malathion was leached from the soil and thereafter percentage of leaching of malathion from soil remained nearly constant. Recovery of malathion from the soil was verified from the results of freshly prepared in-house soil standard and the trends were similar. The cause of preferential leaching of malathion in groundwater at pH 8.2 could be due to preferential binding of OH– over the malathion with the organic matter of the soil. Once the pesticide enters the soil it is partitioned between soil particles and soil solution. In soil particles, malathion particularly binds with the soil organic matter. If organic matter is associated with soil particles, then it is favor- able for binding pesticides. However when pH is increased from 4.5 to 8.2, the adsorbed pesticide molecules on soil organic matter are desorbed [1,19].