Sorption behavior of methamidophos in a heterogeneous alluvial soil profile

Laboratory batch sorption experiments were performed to study the effects of soil components and solution pH on methamidophos sorption in an alluvial soil profile. Soil samples were collected at depths (0–30 cm (surface soil), 30–60 cm, 60–90 cm and 90–120 cm). Our results show that methamidophos sorption increases with increasing solution pH, indicating that amino groups in methamidophos may be actively involved in the sorption processes. While methamidophos sorption in the surface soil is mainly controlled through partitioning into soil organic matter, soil minerals (e.g., clays, Al/Fe oxides) play an important role on the sorption of methamidophos in subsurface soils. The batch sorption experiments also indicate that the mineral surfaces (e.g., clay or Al/Fe oxides) exhibit much greater sorption affinity for methamidophos at low methamidophos concentrations compared to the soil organic matter although the soil organic matter significantly increases the sorption capacity of soils.The sorption isotherms were non-linear in subsurface soils, and were best described with a linearized Langmuir equation, indicating specific interactions between specific surface groups in soils (e.g., Al/Fe oxides) and methamidophos. Despite the heterogeneity of soils used in the study, a simple isotherm model with spatially averaged isotherm parameters may be sufficient in describing the sorption processes in the whole soil profile.