Sorption and desorption of indaziflam degradates in several agricultural soils

Processes regulating pesticide fate in the environment are influenced by the physicochemical properties of pesticides and soils. Sorption and desorption are important processes as they regulate the movement of pesticides in soil. Although sorption-desorption is widely studied for herbicides, studies involving their metabolites in soil are scarce. Sorption and desorption of indaziflam metabolites (indaziflam-triazinediamine (FDAT), indaziflam-triazine-indanone (ITI) and indaziflam-carboxilic acid (ICA)) were investigated in six Brazilian (BRA) soils and three United States (USA) soils with different physicochemical properties. The Freundlich equation described sorption of the metabolites for all soils (R2 > 0.98; 1/n ~ 1). Sorption order (Kf) was ITI > ICA > FDAT. Mean values of Kf,oc were 453, 289, and 81 (BRA) and 444, 48, and 48 (USA) for metabolites ITI, ICA, and FDAT respectively. Desorption was hysteretic for all metabolites in all soils. These results suggest that these metabolites fall in the classification range of mobile to moderately mobile in soils.


Introduction
The fate and distribution of agrochemicals in the soil water environment are largely determined by sorption-desorption processes.Previous herbicide research has focused predominantly on the interaction of the parent compound with soil, while studies involving their metabolites are in short supply.Because data are unavailable for model input, it is often assumed that the metabolite sorption coefficient is equal to that of the parent compound.Studies on the metabolites of imidacloprid have demonstrated that this assumption can be inaccurate (Papiernik et al., 2006).For example, of three imidacloprid metabolites evaluated, two showed sorption higher than the parent compound and one lower (Cox et al., 1997;Papiernik et al., 2006).Alternatively, the idea that as a compound degrades, its byproducts are smaller and more polar, and, therefore, more mobile is also riddled with exceptions.The hydroxylated metabolites of atrazine (Clay and Koskinen, 1990) and florasulam (Krieger et al., 2000) show greater adsorption to soil than the parent compound.Experimental metabolite sorption data is, therefore, imperative for performing environmental assessments of herbicides.While information regarding herbicide metabolite behavior in soil is scarce overall, no data exist for the relatively new herbicide, indaziflam.

Sorption
Sorption studies were performed in triplicate using the batch equilibration method.Preliminary studies (Alonso et al., 2011) showed that to sorb 20-80 % of the initial chemical in solution on the soil (for greater analytical accuracy), a ratio of 4 g soil to 10 mL (4:10) solution was acceptable for the ITI metabolite, whereas for FDAT and ICA, the soil:solution ratio was 10 g to 10 mL (1:1) for each sample.Glass centrifuge tubes (35-mL) with Teflon lined caps containing soil and radioactive solutions were mixed using a vortex mixer (30 s) and shaken for 24 h on a tabletop shaker (20 ± 2 o C) in the dark.Preliminary kinetic studies (Alonso et al., 2011) showed that equilibrium was reached in < 24 hrs.Sample tubes were then centrifuged (20 min at 370 g) and 3-mL of supernatant was removed from each sample for analysis.A 1-mL aliquot of the removed supernatant was mixed with 5 mL of a scintillation cocktail (EcoLite, cocktail, ICN Biomedicals, Costa Mesa, CA) and the concentration of 14 C of each metabolite in solution was determined by liquid scintillation counting with a Packard 1500 counter (Packard Instruments, Downer Grove, IL).The amount of chemical sorbed to the soil was calculated as the difference between the initial solution concentration and the supernatant concentration Major Minor ---------------% ----------------

Desorption
Immediately following the sorption experiments, desorption experiments were performed for each metabolite at the lowest and highest initial concentrations: 0.06 and 1.91 µmol L −1 for FDAT, 0.03 and 0.95 µmol L −1 for ITI, and 0.03 and 0.92 µmol L −1 for ICA on all nine soils.Desorption was determined by replacing the 3-mL aliquot of removed supernatant for sorption analysis with 3 mL of 0.01 N CaCl 2 .The soils were then resuspended using a vortex mixer, shaken for 24 h, and centrifuged.The supernatants were analyzed as previously described and again replaced with CaCl 2 .This was repeated for a total of three desorption steps.All studies were done in triplicate.

Sorption and Desorption Model
Sorption coefficients K f and 1/n were calculated from the slope and intercept of the log form of the Freundlich equation: Log where [Cs] is the concentration (µmol kg −1 ) of the metabolite sorbed onto soil after equilibration and [Ce] is the metabolite concentration (µmol L −1 ) after equilibration.Sorption coefficients were normalized to soil OC contents to give K f-oc (L kg −1 ), where K f-oc = (K d / % OC)*100.The hysteresis coefficient, H, for the sorption-desorption isotherms was determined using the formula H = (1/ n desorption )/(1/n sorption ), where 1/n sorption and 1/n desorption are the Freundlich slopes from the sorption and desorption isotherms, respectively (Barriuso et al., 1994).
Sorption distribution coefficients K d (L kg −1 ), where were also calculated for the lowest and highest initial solution concentrations and normalized to soil OC contents to give K oc (L kg −1 ), where K oc = (K d / % OC)*100.K d and K oc values were used to compare sorption between soils at initial solution concentrations with sorption calculated after the third desorption step.The concentration of metabolite remaining in the 0.01 N CaCl 2 solution after the third desorption step was taken as [C e ], and the concentration of metabolite sorbed, [C s ], was assumed to equal the initial concentration minus the amount remaining in the solution and the amount removed in the previous desorption steps.

Statistical Analysis
For each metabolite, Pearson Product Moment Correlation analyses were performed charting the sorption coefficients against selected soil physicochemical properties using the SigmaPlot (SysStat © for Windows, Version 10).For all correlation analyses, soils were divided into two groups, tropical soils (BRA) and glacial soils (USA).Linear regression analyses were performed using the same software package.

Sorption
The empirical Freundlich equation described the sorption behavior of the metabolites FDAT (Table 2, Figure 2), ITI (Table 3, Figure 3), and ICA (Table 4, Figure  4) for all soils (R 2 > 0.98).Average 1/n values were 0.99 for FDAT, and 0.95 for ITI and ICA indicating sorption was minimally dependent on initial solution concentration.As 1/n values did not differ significantly from one another, K f values were used for comparisons.In general, the order of sorption magnitude for the indaziflam metabolites in the present study was FDAT < ICA < ITI.K f values for FDAT, the least sorbed metabolite, were 0.14 to 3.0 in Oxisols and 0.63 to 0.85 in Mollisols.ICA K f values of Oxisols ranged from 0.51 to 13.1, whereas       between K f and OC, the normalization of K f for organic carbon (% OC) reduced the variability of sorption coefficients.Variation range in K f,oc was ~12x in contrast to the ~21x variation for K f .The ICA K f values of Oxisols had significant positive correlation (p < 0.05) with % OC (r = 0.76**), and an equation describing K f as a function of % OC was calculated where K f = -2.1738+ 5.0867 % OC (R 2 = 0.57).ICA sorption was not correlated with clay content or pH of these soils.In the Mollisols, significant negative correlations (p < 0.05) were observed for ICA K f values with pH (r = -0.93**)and clay content (r = -0.99**).Linear equations describing K f as a function of soil pH (K f = 4.0454 -0.411 pH, R 2 = 0.87), clay contents (K f = 3.6604 -0.1413 % Clay, R 2 = 0.99), and % OC (K f = -0.4103+ 0.7174 % OC, R 2 = 0.88) were calculated.Although not significant, the normalization of K f or K d for organic carbon content reduced sorption variability between soils by approximately half, from ~30 to ~16.The low ICA sorption is presumably related to the anionic character of the metabolite at the pH of the suspensions.The anionic acid group is repelled by the negative charge of Oxisols, or weakly retained as the molecular species by most soil and sediment components in glacial soils (FAO, 2000).According to calculations, pKa varied from 3.70 to 4.36, depending on the group, triazine ring or the acid group (Chemaxon, 2014).
ITI K f was positively correlated (p < 0.05) with organic carbon (% OC) (r = 0.68**) and with clay content (% Clay) (r = 0.74**) of the Oxisols.Linear equations describing K f as a function of carbon content (K f Sorption of FDAT was lower as compared to indaziflam, presumably due to differences in hydrophobic interactions.Using a theoretical pKa for FDAT of 4.10 (Chemaxon, 2014), a higher %age of cationic molecules would be expected as compared to indaziflam (pKa = 3.5), and greater sorption.However, the pKa is not the only factor that affects sorption.There was no correlation (p < 0.05) between low FDAT K f values and the physicochemical properties (ie.pH and %age clay) of the soils.Although there was no significant correlation   Although its physicochemical characteristics are similar to soil BRA7, its K f was ~3 times lower.Similar behavior was observed for the sorption of the parent compound (indaziflam) for the same soil (Alonso et al., 2011).It is surprising that there was no correlation between sorption and pH.The theoretical pKa for triazine indanone is 4.10 (Chemaxon, 2014), and the %age cationic molecules at a given pH would be expected to contribute to sorption.However, as was mentioned above, the pKa is not the only factor that affects the sorption.
For the three Mollisols studied, there were significant incidences of correlation between the ITI Freundlich sorption coefficients and soil pH, % OC and % clay.The pH (r = -0.85**)and clay content (r = -0.99**)were negatively correlated with K f , whereas the organic carbon content correlated positively with the K f for these soils (r = 0.98**).Linear equations describing K f as a function of soil pH (K f = 27.2005-2.48 pH, R 2 = 0.72), clay contents (K f = 26.3058-0.9275 % Clay, R 2 = 0.99), and % OC (K f = -0.9101+ 4.9621 % OC, R 2 = 0.97) were significant.The negative correlation observed between K f and clay content is possibly a result of the positive correlation between K f and the organic carbon content of these soils overshadowing the contribution of the clay.The normalization of both K f and K d for % OC (K f,oc ) reduced the coefficients' range of variability from ~7.5x to ~4x.
When the sorption of metabolites is very low, such as is the case with FDAT, the chemical properties of the soil will bear little or no relation to the mobility of the compound.On the other hand, when the metabolite presents higher sorption, the main chemical property contributing to increased sorption is the carbon content, which showed significant correlation with the sorption of these compounds in most cases.This correlation is reiterated by the fact that the normalization of K f and K d to carbon contents K f,oc and K oc, respectively, reduced the variability between the coefficients.
The main soil variable that was negatively correlated with the sorption of metabolites was pH.The inverse relationship between pH and sorption suggests that the compounds may be partially ionized in the soil solution.ITI and FDAT have N-groups, which may be protonated at low pH, while ICA may be dissociated at high pH.On the other hand, compounds that are not ionized in the soil tend not to show any correlation between sorption and pH.
The inconsistent effects of clay content on the sorption of metabolites, showing both positive (Oxisols) and negative correlation (Mollisols), suggests that the influence of the organic fraction and pH of these soils are the main characteristics affecting sorption.In Mollisols organic carbon was inversely related to clay contents and pH.Therefore, sorption increased with the increase in % OC and the decrease in pH, which resulted in apparent negative correlation between K f and the clay fraction of these soils.Negative correlation between K f and the clay fraction of these soils was also observed for the parent compound (indaziflam) (Alonso et al., 2011).

Desorption
Desorption of the three metabolites was found to be hysteretic for all soils studied, 1/n desorption < 1/n sorption (Tables 2, 3 and 4). Figures 2, 3 and 4 show representative desorption isotherms of the metabolites in four soils at the lowest and the highest initial concentrations used in the study.Hysteresis coefficients, H, ranged from 0.27 to 0.49 (in 0.06 µmol L −1 ) and from 0.12 to 0.67 (in 1.91 µmol L −1 ) for Oxisols, and from 0.46 to 0.56 (in 0.06 µmol L −1 ) and from 0.45 to 0.49 (in 1.91 µmol L −1 ) for Mollisols (Table 2).
It is difficult to ascribe the degree of hysteresis to a particular soil property.Although H showed a significant positive correlation (p < 0.05) with Oxisol K f values (r = 0.71**) (described by the equation H = 0.2784 -0.0998K f , R 2 = 0.50), no correlation was found between H and the physicochemical characteristics of the Oxisols in this study.In contrast, in the Mollisols negative correlation was observed between H and % OC (r = -0.78**)and positive correlation between H and % Clay (r = 0.66**) and between H and K f (r = 0.61**).Multiple correlation was described by the following linear equations: H = 0.5739 -0.0477 % OC, R 2 = 0.60; H = 0.3392 + 0.0075 % Clay, R 2 = 0.43; and H = 0.3272 + 0.1947 K f , R 2 = 0.38.No correlation was found between H and pH of the Mollisols.

Potential Mobility
As a result of the lower sorption of the FDAT, ITI, and ICA metabolites compared to the parent compound, Sci.Agric.v.73, n.2, p.169-176, March/April 2016 it is expected that their mobility will be higher than indaziflam.Based on the criteria established by the Food and Agriculture Organization of the United Nations (FAO) (FAO, 2000), FDAT K f-oc variation found in this study (Table 2), from 23 to 283 L kg −1 for tropical Oxisols and from 29 to 82 L kg −1 for glacial Mollisols, would be classified as FDAT as mobile in all soils studied except BRA11, in which it is considered moderately mobile.Based on K f-oc values (Table 3), ITI would be classified as moderately mobile in Oxisols (K f-oc = 196-706 L kg −1 ) and Mollisols (K f-oc = 422-482 L kg −1 ).ICA K f-oc values for Oxisols ranged from 101 to 638 L kg −1 and from 39 to 62 L kg −1 for Mollisols (Table 3).Thus, the metabolite ICA would be classified as mobile to moderately mobile in Oxisols, and considered mobile in Mollisols (FAO, 2000).
Although sorption initially regulates soil-pesticide interactions, desorption also influences the potential mobility of these molecules and the risk of environmental contamination.Desorption represents the magnitude of the sorbed fraction that can return to the solution by reaching a new equilibrium.This typically occurs when a portion of the chemical initially present in solution is shifted in the soil profile or removed by adsorption, degradation, leaching, or volatilization.Therefore, to fully understand compound availability as a function of time, it is necessary to account for the desorbed fraction reentering the solution.
Desorption of the three metabolites is hysteretic, which means it does not occur to the same degree as sorption, 1/n sorption >> 1/n desorption , and a large portion of sorbed metabolites is not easily desorbed or is desorbed slowly.Desorption hysteresis is caused by a variety of mechanisms for a large number of herbicide-soil systems (Wauchope et al., 2002).Weber et al. (1998) suggest that hysteresis can most likely be attributed to slow desorption and retention of sorbed molecules within highly condensed organic matrices.The highest coefficients of hysteresis were found for FDAT, indicating that along with being the least sorbed, this metabolite also has the highest desorption potential among those studied as a result of being partially protonated in the soil solution.The FAO mobility classification system (FAO, 2000) does not take this desorption hysteresis into account.Because the hysteretic behavior of chemicals reduces their mobility, the potential of contamination is likely overestimated.
A variety of transport models have been used to predict the potential leaching depth of a chemical in soil.The sorption coefficient, K d , is the most commonly used retardation factor and the depth of leaching is directly proportional to the magnitude of K d .This coefficient does not take into account hysteresis, if it occurs during desorption.For FDAT, use of the sorption K d would overpredict the depth of leaching by a factor of 1.5 to 2 as compared to using the sorption coefficients for the third desorption equilibration (K d-des3 ) as indicated by the K d-des3 / K d ratio (Table 2).Similarly, depending on the soil, potential depth of leaching could be overestimated by a factor of 2 for both ITI and ICA.
The evaluation of the potential leaching of these metabolites to ground water may vary according to environmental conditions, particularly with regard to their persistence in the soil.The more persistent these chemicals are, the greater the likelihood of them becoming major contaminants.Thus, complementary studies to determine the half-life of these compounds in the soil should be carried out with the aim of fully understanding their potential to become contaminants.
In summary, the three metabolites showed distinct behavior in terms of sorption in the soil, which suggests that they should be treated as isolated molecules.The three compounds are less sorbed than the parent compound, which indicates that they have a higher leaching potential in the soil, especially in those with low carbon content and high pH.Hysteresis was observed in desorption experiments indicating an imbalance in the sorption-desorption process, which may signify the presence of strongly sorbed fractions and the formation of bound residues.

Table 1 −
Physicochemical properties of soils.
-0.0171 + 4.6889 % OC, R 2 = 0.46) and K f as a function of clay content (K f = 1.4215 + 0.1342 % Clay, R 2 = 0.54) were calculated.The poor fit observed for both cases is directly related to low sorption by soil BRA8.