Adsorption and Desorption Behavior of Metalaxyl in Intensively Cultivated Acid Soils

Abstract

Metalaxyl adsorption and desorption behavior in acid soils were evaluated via batch and stirred-flow chamber experiments. On the basis of batch experiments (adsorption curves of the Giles C-type), metalaxyl has a low affinity for acid soils. Also, as derived from batch and stirred-flow chamber tests, its adsorption in acid soils is dictated mainly by their organic matter and clay contents. The high correlation between these two variables makes it rather complicated to resolve their effects. Metalaxyl adsorption occurs largely (80-99%) via fast adsorption reactions. On the other hand, the pesticide is desorbed in variable proportions (30-100%). The desorption parameters obtained by fitting the results to a pseudo-first-order reaction were correlated with no edaphic variable; however, the q(0)/q(max) ratio, which is a measure of reversibility in the adsorption-desorption process, exhibited significant negative correlation with the organic matter and clay contents.

Metalaxyl mobility in acid soils: evaluation using different methods

“…Therefore, the three methods tested in this study provided the same qualitative result, i.e., the soil with higher organic matter and clay content has a higher capacity to adsorb metalaxyl. This result agrees with previous work (Andrades et al 2001;Bermúdez-Couso et al 2011a;Marín-Benito et al 2009b), and therefore, the three methods are adequate to compare the metalaxyl adsorption capacity from different soils.…”

“…In general, the percentage of metalaxyl desorbed also increased as the amount of metalaxyl previously added increased, ranging between 9.2 and 37.8 % in Soil 1 and between 21.7 and 42.6 % in Soil 2. This behavior suggests that the strength of the metalaxyl's bond to the soil diminishes as the amount of metalaxyl adsorbed in the soil increased (Bermúdez-Couso et al 2011a). Results also show a high irreversibility of adsorption processes because more than 50 % of the previously adsorbed metalaxyl remained in the soil after the desorption experiments in all cases.…”

“…These percentages of metalaxyl desorption were higher than those found in the batch equilibrium test. In a previous work, Bermúdez-Couso et al (2011a) suggested that these differences could be attributed to the continuous removal of desorbed metalaxyl in the stirred flow chamber experiments, whereas in the batch experiments, the metalaxyl desorption could be limited by the amount present in solution. Higher percentages of desorption using stirred flow chamber experiments than using batch experiments were also found for other organic contaminants (Bermúdez-Couso et al 2011b) and for heavy metals (Fernández-Calviño et al 2010).…”

“…In the laboratory, the samples were air-dried, sieved (\2 mm) and stored in polyethylene jars until analysis. The soil samples' characteristics were analyzed using the same methods employed in a previous study (Bermúdez-Couso et al 2011a in which the methods used for the general characterization are described in detail. The general characteristics of the soils used in the experiments are shown in ESM Table 2.…”

“…Most of these studies were performed at equilibrium using a batch technique (Arias et al 2006;Baglieri et al 2011;Marín-Benito et al 2009aMonkiedje and Spiteller 2002;Sharma and Awasthi 1997;Sukul and Spiteller 2001), leaching experiments using soil columns (Marín-Benito et al 2009b) or a combination of both techniques (Andrades et al 2001;Fernandes et al 2003). However, studies on the characteristics of the kinetic of metalaxyl adsorption-desorption process are scarce (Bermúdez-Couso et al 2011a). More detailed studies on the kinetic characteristics of the adsorption and desorption processes of pesticides in the soil are important as they are not an instantaneous processes.…”