Sorption-desorption of the nematicide fenamiphos sulfoxide in relation to residence time in soil

Abstract

The nonfumigant nematicide, fenamiphos [Nemacur^R , ethyl-3-methyl-4-(methylthio) phenyl (l-methyl-ethyl) phosphoramidate] has been increasingly used for controlling nematodes in Hawaii pineapple and various other crops in other states since the ban of the effective fumigants OBCP and EDB. Previous study indicated that fenamiphos in soil transformed rapidly into a persistent product, fenamiphos sulfoxide (f. sulfoxide), which is then slowly transformed to fenamiphos sulfone (f. sulfone): half lives of fenamiphos, f. sulfoxide and f. sulfone in one Hawaii soil were 3, 80 and 60 days, respectively. Sorption of f. sulfoxide is much less than that of fenamiphos, and slightly less than f. sulfone (Lee et al., 1986). The objective of the study was to investigate the sorption behavior of f. sulfoxide in both the surface soil and subsoil of selected important agricultural soils in Hawaii (Wahiawa series) and California (Salinas series) in relation to pesticide residence time. For this objective sorption methodology was initially developed, and then sorption-desorption behavior of f. sulfoxide with time was studied using equilibrium batch desorption and dynamic column leaching systems. Two solution:soil ratios, 2:1 and 5:1, were tested to determine the appropriate ratio in the sorption measurement of f. sulfoxide for Wahiawa soil, 0-20 cm, 40-60 cm and 100-120 cm, and Salinas soil, 0-15 cm and 115-130 cm. Sorption of f. sulfoxide at the 5:1 ratio showed appropriate mixing, while sorption with the 2:1 ratio indicated insufficient mixing during the various batch equilibrium times (4, 12, 24 and 48 hours). An appropriate equilibration time was determined using the four times and four concentrations of f. sulfoxide solution (0.1, 1.0, 5.0 and 10 ~mol/L). Sorption increased over all equilibration times after apparent equilibrium at 4 hours, indicating incomplete equilibrium within 48 hours. Transformation of f. sulfoxide is probably responsible for the increase between 24 hours and 48 hours. The recommended equlibration time is 24 hours, since it may eliminate the insufficient sorption and avoid undesirable transformation as well. A simple and precise sorption method, which measures sorbed phase directly (mass balance method) was developed to study the sorption onto low-sorptive soil, such as subsoils with low organic carbon. The mass balance method greatly improved precision of sorption measurement on the deepest Wahiawa subsoil, which had the lowest sorption; the coefficient of variation (CV) for Kd measurement was 20.3% for the batch method and 8.4% for the mass balance method. The sorption partition coefficients (Kd and Kf) of f. sulfoxide from the equilibrium desorption in batch suspension were measured on Wahiawa soils (0-20 and 40-60 cm) and Salinas soils (0-15 and 115-130 cm) after incubation for 1, 8, 15, 30, 60 and 90 days in an unsaturated condition at 23 and 4°C. The Kd and Kf increased with time on Wahiawa soils at 23°C and 4°C, and on Salinas surface soil at 23°C. The Kd and Kf did not change on Salinas surface soil at 4°C and on the Salinas subsoil at both 23 and 4°C. Various mechanisms responsible for the increase in Kd over time were quantified by computing the percentage of total increase in Kd at a given residence time that was due to (1) transformation of f. sulfoxide to f. sulfone, (2) diminished level of pesticide residue and (3) conversion of labile pesticide to a nonlabile form. The increase in Kd over time was accounted for principally by conversion of labile pesticide to a nonlabile form in all soils. The increase in sorption partitioning over time indicates that the use of sorption parameters obtained from freshly added pesticide in a conventional batch suspension measurement might result in estimating more rapid leaching of pesticide than would actually occur. However, results from the dynamic column measurements led to different conclusions. A column measurement was conducted to study the effect of pesticide residence time on dynamic desorption of the pesticide in relation to changes in equilibrium partitioning over time. The soils treated with f. sulfoxide (8.0 mg/kg soil) were leached with 0.002 N CaS04 solution after 1- and 30-day incubations in an unsaturated condition at 4°C. The dynamic desorption of pesticide 30 days after incubation was retarded relative to I-day residence time in Wahiawa soils, 0-20 cm and 40-60 cm, while little change was observed in Salinas soils, 0-15 cm and 115-130 cm. The slower desorption for the longer residence time agreed with residence time effects on sorption partition coefficients measured by equilibrium batch desorption. However, the experimental dynamic desorption of f. sulfoxide from small columns was much faster than that calculated from sorption-desorption theory. In fact, equilibrium theory suggested that equilibrium pesticide displacement should have been faster than measured displacement if the dynamics of leaching were controlled by desorption kinetics and diffusion from sorption sites to pores with mobile water. The opposite result was obtained in the leaching experiment: measured displacement was much faster than that computed by may not have attained equilibrium during the 1- and 30-day incubations. Overall, sorption partitioning of f. sulfoxide increases over time in some soils. However, actual dynamic desorption and leaching of the pesticide may be faster than that indicated by equilibrium sorption-desorption theory when the pesticide is applied to an initially dry field soil, which remains unsaturated prior to a leaching event.