Comparison of Pesticide Sorption by Physicochemically Modified Soils with Natural Soils as a Function of Soil Properties and Pesticide Hydrophobicity

Abstract

The objectives of this paper were to determine the efficiency of physicochemically modified soils with a surfactant in the sorption of pesticides, the stability against washing of the pesticides sorbed, and the effective sorption capacity of surfactant adsorbed by soils as a function of pesticide hydrophobicity and soil characteristics. Five soils of different characteristics and five pesticides (penconazole, linuron, alachlor, atrazine and metalaxyl) with different Kow values were selected and octadecyltrimethylammonium bromide (ODTMA) was chosen as model of cationic surfactants. Sorption-desorption isotherms were obtained and constants Kf and Kfd for natural soils (from Freundlich equation) and K and Kd for ODTMA-soils (from linear equation) were determined. Sorption on ODTMA-soils was higher than on natural soils. K increased 27–165 times for penconazole, 22–77 times for linuron, 7–14 times for alachlor, 9–23 times for atrazine, and 21–333 times for metalaxyl in relation to Kf. Sorption coefficients normalized to 100% of total organic matter (TOM) from organo soils KOM (K 100/%TOM), were always higher than those from natural soils KfOM (Kf 100/%OM), indicating that the organic matter (OM) derived from the ODTMA (OMODTMA) had a greater sorption capacity than the OM of the natural soil. KOM values were also higher than the Kow (octanol/water distribution coefficient) value for each pesticide. The similarity of the high KOM values for the sorption of each pesticide by the five soils and the linearity of isotherms point to a partitioning of the pesticides between surfactant and water. The use in this work of different soils and various pesticides, unusual in this type of investigation, allowed us to obtain equations to know the sorbed amount of a given pesticide by the surfactant-modified soils as a function of the OM content derived from the cation and the Kow of the pesticide. The results obtained are of interest when it becomes necessary to increase the sorption capacity of soils with low OM contents with a view to delaying pesticide mobility in soils from pollution point sources (high concentration in small area), and preventing the pollution of waters.