Non-equilibrium sorption during the movement of pesticides in soils

Abstract

Sorption and movement of two ionic herbicides (2,4-D and atrazine) and two non-ionic insecticides (phorate and terbufos) in an allophanic (Patua silt loam) and a non-allophanic (Tokomaru silt loam) soil were examined using 14C-labelled pesticides. For sorption measurements, a range of concentrations of pesticide solutions in 0.1 M calcium chloride were shaken with soil samples at a soil:solution ratio of 1:10 for 4 h at 25°C. The movement of pesticides was examined using re-packed soil columns following a step input of 2,4-D and tritiated water (3H2O) and a pulse input of 2,4-D, atrazine, phorate and terbufos. A convection-dispersion equation (CDE), either with an equilibrium or a bicontinuum non-equilibrium sorption process, was used to simulate the measured effluent breakthrough curves (BTCs) obtained by simultaneous displacement of a non-sorbed solute (3H2O) and a sorbed solute (2,4-D). The Patua soil sorbed more pesticide than did the Tokomaru soil. This is attributed to the larger amounts of organic matter and the presence of short-range order clays (allophane) in the former soil compared to the latter. Kinetic sorption data for pesticides showed an initial rapid rate followed by a slower rate of sorption. In column experiments, the pesticide in the leachate appeared later in the Patua than in the Tokomaru soil. Movement of pesticides in soils decreased with an increase in Kd values. The step-function experiments showed a symmetrical BTC for the non-sorbed solute (3H2O) with a sigmoidal shape, whereas there was an asymmetrical BTC with extensive tailing for the sorbed solute (2,4-D). The CDE with an equilibrium sorption process adequately described the 3H2O BTC, but failed to simulate the BTC for 2,4-D. The CDE with a bicontinuum non-equilibrium sorption process provided a good description of the BTC for 2,4-D. Diffusion of pesticides into sorbent organic matter was considered to be the likely mechanism for the observed sorption non-equilibrium during the movement of pesticides in soils.