Quantifying dynamic desorption of 3,5,6-trichloro-2-pyridinol in loamy farmland soils

Reliable estimate of the release of adsorbed pesticide from soil particles is crucial to evaluating the pesticide fate, mobility, efficacy, and remediation. In this study, the dynamics of TCP (3,5,6-trichloro-2-pyridinol) desorption, the main degradation product of chlorpyrifos and triclopyr, is explored quantitatively by the breakthrough curve (BTC) experiment with the tracer of Br^- in the loamy farmland purple soil sampled from Sichuan Basin of southwestern China. TCP in the outflow originates from two sources: dissolved TCP in pore water and desorbed TCP from soil particles by infiltrating water. The dissolved TCP is considered proportional to the amount of Br^- because both TCP and Br^- are dissolved in water uniformly. According to the mass balance equation, the desorbed TCP are estimated and the typical patterns of dynamic TCP desorption are revealed. Characteristics of TCP desorption are compared between packed and undisturbed soil columns as well as between different planting types. The dynamics of the proportion of desorbed TCP during the breakthrough process are characterized. In particular, the high heterogeneity of the undisturbed soil may be responsible for the observed fluctuation of desorbed TCP in the outflow. Additionally, the obtained increase-decrease pattern of the desorbed rate of TCP released from the soil shows that most models proposed to simulate the desorption processes are not appropriate, because these models display a monotone decreasing trend, such as the Noyes-Whitney Rule and other release kinetic models (zero order, first order, Higuchi and Korsmeyer-Peppas model, etc.). After a comparison among linear model, Gamma distribution and Weibull distribution, the CDF of gamma distribution is identified as a better method to describe the proportion of desorbed TCP in outflow. Therefore, this study provides an alternative method to measure the dynamic desorption process of TCP in different environment of the purple soil, and their affecting factors are also identified. These results are useful in quantifying the leaching of the TCP in the field, in support of the prevention of agricultural non-point pollution of pesticides.