Adsorption-desorption and hysteresis phenomenon of tebuconazole in Colombian agricultural soils: Experimental assays and mathematical approaches
Introduction
Because of the different physico-chemical properties of insecticides, herbicides, fungicides, and other biocides applied in crops and elsewhere, specific knowledge of the adsorption-desorption process in soils is essential to estimate the irreversibility of the adsorption (hysteresis) and the amount of the chemical that can be volatilized, degraded, leached, and retained by soil particles (Flores et al., 2009, Gebremariam et al., 2012, Mosquera-Vivas et al., 2016a, Mosquera-Vivas et al., 2016b). Transport parameters can be derived from these processes and used in pesticide-leaching models and/or indices to estimate the mobility of the substance throughout the vadose zone (Singh et al., 2016, Vanclooster et al., 2000). The simple Groundwater Ubiquity Score GUS index based on two widely studied aspects of pesticides in the soil: persistence (half-life) and soil organic carbon-adsorption coefficient could be used to estimate the leaching potential of these compounds (Gustafson, 1989, Mosquera-Vivas et al., 2016a, Mosquera-Vivas et al., 2016b). The adsorption in both the dynamic and stagnant regions of the porous media of soil are assumed to be instantaneous while the adsorption process is considered linear (van Genuchten and Wierenga, 1976, Vanclooster et al., 2000). The coefficients of distribution and the can be measured directly by the batch method, soil columns, and soil thin-layer chromatography, and they can also be estimated using regression with other partition coefficients such as n-octanol-water partition coefficient and the water solubility (Delle Site, 2001, Gao et al., 1996, Wauchope et al., 2002) to use in environmental-fate models (Juraske et al., 2011, Binder et al., 2016).
The irreversibility or hysteresis is calculated by mathematical equations with a single-point desorption and/or the desorption isotherms for each initial concentration (Huang et al., 1998, Mamy and Barriuso, 2007). Mamy and Barriuso (2007) pointed out that the desorption data of pesticides can be difficult to apply because the desorption parameter is often highly dependent on the initial concentration used for desorption measurements. In addition, these authors reported that when the desorption parameter was independent of the initial concentration of the adsorbed pesticide, desorption was regulated by adsorption mechanisms (irreversible binding onto specific soil adsorption sites) and one desorption isotherm at an initial concentration would be sufficient to describe the adsorption-desorption process.
Tebuconazole is a highly effective fungicide for controlling soil-borne diseases, foliar fungi and other plant pathogens in the onion, rice, potato, tomato, bean, fruit, and flower crops in Colombia (ICA, 2016) and worldwide (Strickland et al., 2004). In the last five years, the presence of the fungicide has caused increasing concern in the scientific sector and has been studied in mammals (animals and humans) (Zhou et al., 2016), fish (Altenhofen et al., 2017, Liu et al., 2016), soil and soil fauna (Álvarez-Martín et al., 2016a, Álvarez-Martín et al., 2017, Boithias et al., 2014, Druart et al., 2011, Li et al., 2015, Li et al., 2012, García-Santos and Forrer, 2011; Storck et al., 2016, Wang et al., 2016, Yu et al., 2012), water bodies (Caldas et al., 2010, Kahle et al., 2008, Sancho et al., 2016), small agricultural streams (De Gerónimo et al., 2014, Rabiet et al., 2010, Schäfer et al., 2008, Stamatis et al., 2010), and in numerous food products (Li et al., 2012, Muñoz et al., 2017, Patyal et al., 2013, Strada et al., 2012). The European Committee for Risk Assessment categorized tebuconazole from acute toxicity 4 to toxicity 1 and from aquatic chronic II to I (RAC, 2013). Furthermore, there are still significant gaps in knowledge regarding its environmental fate, as recently reported by the European Food Safety Authority (EFSA) (EFSA, 2014, Papadopoulou et al., 2016, Storck, 2016). In general, it is known that triazole fungicides have moderately lipophilic molecules—that is, tebuconazole has a water solubility of 36 mg L−1 and log of 3.7 (Gámiz et al., 2016, Tomlin, 2009), indicating that they are moderately or strongly adsorbed in soils (Jamet and Eudeline, 1992). The tebuconazole adsorption was controlled by the soil organic matter (SOM) and clay faction (Čadcová et al., 2013a, Cadcová et al., 2012). Also, the addition of organic amendments has been found to increase its retention in temperate soils (Álvarez-Martín et al., 2016b, Fernandes et al., 2006, Fenoll et al., 2011, Herrero-Hernández et al., 2011, Marín-Benito et al., 2012, Rodríguez-Cruz et al., 2007). Tebuconazole exhibits high to low mobility in soil (Aldana et al., 2011, Čadcová et al., 2013a, Cadcová et al., 2012, EFSA, 2014, Gámiz et al., 2016, Herrero-Hernández et al., 2011, Vallée et al., 2014) and tends to be pH independent in temperate soils (EFSA, 2014, Čadková et al., 2013b).
High hysteresis values show that the mass transfer of the pesticide from the solution to the soil (adsorption) occurs more readily than the mass transfer from the soil to the solution (desorption). A low rate of desorption may lead to physico-chemical entrapment of adsorbing pesticide in soil particles (Boivin et al., 2005, Mosquera-Vivas et al., 2016b, Yu et al., 2010), and thus the mobility and biocide activity of a chemical in field diminishes over time. This phenomenon is less studied (Gebremariam et al., 2012), but available information on desorption of triazoles showed hysteresis (Marín-Benito et al., 2012, Singh, 2002) and high retention rates (30–60%) in the soil after a single washing (Singh, 2002).
The adsorption process, irreversibility and the leaching potential of tebuconazole in agricultural and tropical soils are poorly understood and the knowledge of pesticide dynamics under temperate conditions may not be valid for soils in tropical climates. Therefore, to better understand the behavior of tebuconazole in some tropical soils, the aims of this study were i) to evaluate empirically the retention and hysteresis phenomena of the fungicide at surface and subsurface of Inceptisols and Histosol soils from a tropical region (highlands of Colombia) and ii) to assess the suitability of four different mathematical approaches to estimate the adsorption isotherms based on the mass balance, n-octanol-water partition coefficient () and aqueous solubility (), which may be used for future studies on the environmental fate of pesticides in tropical and temperate soils. These approaches could be very useful in the estimation of pesticide leaching using models whose parameters include the coefficients of distribution, since the initial concentration and percentage of adsorption of the pesticide can be used to obtain the isotherm of adsorption, which can in turn be used to obtain the coefficient of distribution. The percentage of adsorption can be calculated with a minimum of three experimental assays.
Section snippets
Pesticide
The non-labeled tebuconazole [(RS)-1-p-chlorophenyl-4,4-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl) pentan-3-ol] with a purity of 98% was supplied by Dr. Ehrenstorfer GmbH (Augsburg, Germany). The phenyl-ring-U-14C tebuconazole (4.8 MBq mg−1 and 93.4% radiopurity) were purchased from the Institute of Isotopes Co. Ltd (Budapest, Hungary).
Soils
The adsorption and irreversibility of tebuconazole were assessed in Inceptisols and Histosol agricultural soils from Colombia. The samples of Inceptisols (I01 and
Adsorption assays of tebuconazole
The experimental curves for the adsorption kinetics of tebuconazole in the Inceptisols I01, I02 and Histosol H03 soils at the surface (0–10 cm) and subsurface (40–50 cm) are shown in Fig. 1. The fungicide at zero equilibration time (22 min) showed instantaneous adsorption in all soil layers due to a rapid retention of tebuconazole in accessible sites on the solid surface, where the van der Waals, hydrophobic and electrostatic interactions (π-π stacking interactions) might occur between the
Conclusions
Empirical adsorption and desorption parameters of tebuconazole and its mobility in tropical agricultural Inceptisols and Histosol soils at different depths are presented for the first time, for use in future studies on the fate of tebuconazole. The retention of the fungicide in the soils depended on the amount and type of soil organic matter, where the greater organic carbon content and aryl-C increased the adsorption coefficient and irreversibility. The hysteresis phenomenon was controlled by
Acknowledgement
This work was supported by the International Atomic Energy Agency (IAEA) [Project number: Col 05/22]. The authors would like to thank Dr. Ronnie Juraske and Esperanza R. Taboada for their expert support and Malherbología Laboratory (Universidad Nacional de Colombia, Bogotá) for its collaboration.
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