3.1. Adsorption Kinetics of Sulfentrazone Isolated and Mixed with Glyphosate Formulations
In order to understand the sorption kinetics of sulfentrazone in the soil, we used and compared two models: pseudo-first-order and pseudo-second-order. The pseudo-first-order is described by the following equation: ln (qe − qt) = ln(qe) −
k1t, where qe is the amount of the compound absorbed at the equilibrium time, qt is the amount of the compound absorbed at time t, and
k1 is the constant of the sorption rate (min
−1). The second model is described by the following equation: (t/qt) = (t/qe) + [1/(
k2 qe
2)], where qe is the amount of the compound absorbed at the equilibrium time, qt is the amount of the compound absorbed at time t, and
k2 is the constant of the sorption rate (g mg
−1 min
−1) [
25].
The nonlinear pseudo-first-order and pseudo-second-order models presented a high R
2 for all of the treatments, varying between 0.96 to 0.99 (
Table 2). The parameters qe and k for the pseudo-first-order and pseudo-second-order models were significant at
p-value ≤ 0.05 (
Table 2). The pseudo-first-order was slightly better-adjusted than the pseudo-second order. For the evaluation of the differences among the applications of sulfentrazone alone and mixed with glyphosate formulations, the pseudo-first-order was chosen.
The good adjustment of the kinetic models allowed us to understand the adsorption of the pesticides to the soil correctly. For sulfentrazone, both isolated and mixed, the first-order nonlinear model showed better performance in explaining the adsorption kinetics of this herbicide. Sorbates that exhibit high affinity to an adsorbate fit better with pseudo-first-order models due to the exponential adsorption early in the exposure periods. This behavior was observed for sulfentrazone alone and mixed in the studied soil (
Figure 1). In this work, 80% (the mean value of the treatments) of the herbicide molecules were adsorbed by the soil at 54.1, 28.3, 29.2, and 30.5 min for sulfentrazone, sulfentrazone + Roundup Ready
®, sulfentrazone + Roundup Ultra
®, and sulfentrazone + Zapp Qi
®, respectively.
The presence of the glyphosate formulations did not alter the equilibrium time of sulfentrazone (
Figure 1). The time required for the balance between the amount of sulfentrazone sorbed to the soil and present in the solution was 4 h for all of the treatments (
Figure 1). Initially, sulfentrazone was rapidly adsorbed by organic and mineral colloids in the soil, with the adsorption of 8.5 mg kg
−1 of the herbicide 0.5 h after the beginning of the agitation of the tubes containing the soil and 10 mg L
−1 sulfentrazone. This higher initial adsorption to the soil was observed for different organic pesticides due to the high availability of sites capable of adsorbing organic molecules [
26,
27,
28]. Passos et al. (2013) [
22] also reported a similar behavior for sulfentrazone in different Brazilian soils after the first few minutes of exposure between the herbicide and the soil.
After the exponential adsorption of sulfentrazone, a second phase is initiated with slower sorption of the herbicide to the soil. The occupation of the sites available for the adsorption of sulfentrazone and the repulsion promoted by the absorbed herbicide hamper the sorption of new molecules of the herbicide present in the solution, reducing the adsorption rate [
29,
30]. Despite the slow process, sulfentrazone continued to be adsorbed to the soil for four hours under stirring. At that point, a third phase was initiated, and the herbicide could not bind to the soil. The third phase is when the concentrations between the amount sorbed and in the solution become constant [
22]. For the sulfentrazone alone and mixed with the glyphosate formulations, this equilibrium was reached after four hours. A period of six hours was chosen in order to ensure equilibrium between the sulfentrazone concentrations in the soil and the solution.
3.2. Sorption and Desorption Isotherms of Sulfentrazone Alone and Mixed with Glyphosate Formulations
The Freundlich isotherm was adjusted to describe the sulfentrazone’s sorption alone and mixed with glyphosate formulations (
Figure 2). The RSME (Root Mean Square Error) values ranged from 0.03 to 0.08, and the Kfs values ranged from 1.34 to 2.34 (
Table 3).
The values of Kfs obtained were similar to those observed in another study that evaluated the adsorption capacity of sulfentrazone in Brazilian soils [
22]. The isotherms had a high adjustment, and allowed the comparison of the sorption capacity of isolated and mixed sulfentrazone through the sorption coefficient obtained for each treatment.
The presence of the formulated Zapp Qi
®, Roundup Ready
®, and Roundup Ultra
® products caused increases in the Kfs values compared to that with sulfentrazone alone (
Table 3). Higher Kfs values indicate that the formulations increased the sulfentrazone sorption to the soil. Therefore, it is evident that the glyphosate or other inert ingredients that make up the formulated product affected the interaction between the sulfentrazone molecule and the soil, altering the natural retention process of sulfentrazone to the soil. Rd allows the classification of the adsorption of compounds in binary systems in three cases: (i) if the Rd > 1, there is synergism; (ii) if the Rd < 1, there is antagonism; and (iii) if the Rd ≈ 1, there is no interaction [
31]. For the adsorption of sulfentrazone to the soil in binary systems, it is possible to observe that the glyphosate formulations acted as synergists, with Rd values equivalent to 1.46, 1.65, and 1.76 for the Zapp Qi
®, Roundup Ready
®, and Roundup Ultra
® formulations, respectively (
Table 3).
There are no reports on the role of glyphosate formulations in increasing sulfentrazone sorption to the soil. Despite the synergistic effect between the sulfentrazone and glyphosate formulations, this behavior was not observed for other herbicide mixtures. For example, Mendes et al. (2018) [
32] did not note differences in mesotrione sorption in seven Brazilian soils when it was applied alone or mixed with S-metolachlor.
In the soil solution, the phosphate group of the glyphosate molecule can be rapidly sorbed to the iron and aluminum oxides present in clay colloids [
33]. Once it is adsorbed to the soil, glyphosate can increase the sorption of sulfentrazone due to the presence of chemical groups in both molecules that allow the establishment of hydrogen bonds between them. These groups can be identified when their molecular structures are evaluated (Electronic
Supplementary Material-2 ). Although sulfentrazone also binds to iron and aluminum oxides due to its zwitterionic ion behavior in neutral form [
34,
35,
36], the sulfentrazone is adsorbed more slowly than glyphosate, favoring the formation of a soil-glyphosate-sulfentrazone complex. This interaction between glyphosate and sulfentrazone would be possible since sulfentrazone is adsorbed more slowly than glyphosate, forming a soil-glyphosate-sulfentrazone complex. The possible hydrogen bonds between glyphosate and sulfentrazone may also explain the difference observed for the Kfs values of sulfentrazone mixed with glyphosate formulations.
The Roundup Ultra formulation presents the lowest molecular weight, and 10 hydrogen donors and acceptors (
Table 4). The lower molecular weight of glyphosate in the Ultra formulation may allow a higher number of molecules to bind to the soil than other formulations. The lower molecular weight of a pesticide has already been associated with higher soil sorption [
37]. Therefore, the greater number of the glyphosate molecules of the Ultra formulation bound to the soil can increase sulfentrazone sorption via hydrogen bonds, explaining the higher Kfs value.
Different behaviors were observed when comparing the Ready and Zapp Qi formulations. The Ready formulation had the highest molecular weight, and the sulfentrazone sorption for this formulation was higher than that for Zapp Qi (
Table 3 and
Table 4). This fact may be related to the lower number of sites in the Zapp Qi formulation to establish hydrogen bonds (9) compared to the number in Ready (10), reducing the sulfentrazone adsorption. Radian et al. (2015) [
37] correlated the physicochemical properties of several pesticides with their soil sorption, and showed that the molecule with the highest number of hydrogen donor and acceptor sites has a high correlation with the sorption of these organic compounds.
The increase in the sorption of sulfentrazone when mixed with the Ultra
®, Ready
®, and Zapp Qi
® formulations may reduce the mobility of this herbicide in soils, and consequently the risk of leaching and surface runoff [
38,
39]. From an environmental point of view, mixed applications have been shown to be safer than isolated sulfentrazone applications. However, it is essential to evaluate this herbicide’s desorption process under the conditions of isolated and mixed use for a better understanding of its mobility in the soil [
40].
The Freundlich models obtained for the desorption of sulfentrazone alone and mixed with glyphosate formulations demonstrate a low RSME value (0.03 to 0.07), and the parameters of Kfd and ‘n’ were significant for the proposed model (
Table 5). The good fit of the Freundlich model for desorption, similarly to sorption, indicates that the coefficients Kfd and ‘n’ can be used to explain the behavior of sulfentrazone in the soil.
The presence of glyphosate formulations increased the Kfd values compared to that with sulfentrazone alone (
Table 5). The Kfd values for sulfentrazone were 1.9-, 3.7-, and 11.2-fold greater in the Ready, Zapp Qi, and Ultra formulations, respectively, than in the isolated treatment (
Table 5). The Kfd value refers to the herbicide’s ability to remain sorbed to the soil after shaking in the extractive solution. Therefore, soils with high Kfd values have a lower desorption capacity for an herbicide [
41]. The presence of the formulated products reduced the sulfentrazone desorption, and this phenomenon may be associated with a higher affinity of sulfentrazone to the soil in the presence of glyphosate, as observed in the sorption tests. The electrostatic and hydrogen bonds that may have occurred between the soil-glyphosate-sulfentrazone complexes may have increased the bonds’ stability, requiring higher energy for removal of the sulfentrazone absorbed.
The Ultra formulation provided the lowest desorption and the highest sulfentrazone sorption to the soil. This fact indicates that the interactions occurring between sulfentrazone and the soil in the presence of this formulation are stable, and that the presence of the Ultra formulation does not increase the horizontal and vertical mobility of this herbicide. However, it was possible to observe that the sulfentrazone and Ready formulation showed greater desorption than that of the treatment mixed with Zapp Qi, even allowing the higher initial sorption of sulfentrazone.
The Ultra and Zapp Qi formulations have a higher number of rotational bonds than that in the Ready formulation; as such, after establishing the interactions between glyphosate and sulfentrazone, the glyphosate molecules from the Ultra and Zapp Qi formulations can more easily exhibit intramolecular rotations. Some work has demonstrated the role of rotational bonds in increasing the complexity and crystallization of molecules, indicating that this feature may increase the stability of interactions between organic molecules [
42,
43,
44,
45]. Thus, this mechanism may have reduced the amount of sulfentrazone desorbed in the Ultra and Zapp Qi formulations relative to the Ready formulation.
The hysteresis index for the treatments ranged from 0.88 to 0.90 (
Table 5). According to Bariusso et al. (1994) [
46], the values for the hysteresis index—ranging from 0.7 to 1.0—indicate that it is not possible to confirm the existence of this phenomenon. This fact was observed for sulfentrazone independent of the applied treatment, so it is possible to conclude that the sorption and desorption processes occur with the same intensity in this work’s conditions.
The evaluation of the sorption and desorption of sulfentrazone alone and mixed with glyphosate showed that these processes were altered and could affect the dynamics of this pesticide in soils. The lower sorption and higher desorption of the herbicide in the soil increase the risk of the contamination of freshwater and groundwater, and thus use under these conditions should be avoided [
39]. The presence of the glyphosate formulations increased the sorption and reduced the desorption of sulfentrazone compared to its application alone, so according to these parameters, the risk of environmental contamination did not increase due to the mixing of these products. However, assays that estimate the leaching of herbicides in the soil should be carried out in order to confirm these results. Among the methods used to evaluate leaching, simulation in columns is a good way to quantify the leaching potential of pesticides [
47,
48].
3.3. Estimated Leaching of Sulfentrazone Applied Alone and Mixed with Glyphosate Formulations
The sulfentrazone leaching in the different treatments was evaluated in 50 cm depth columns. However, the sulfentrazone was only detected and quantified up to 20–25 cm deep (
Table 6). The isolated sulfentrazone reached the 10–15 cm layer on the columns with a concentration of 0.35 mg kg
−1 (
Table 6). The Ultra formulation reduced the sulfentrazone leaching compared to that of the other treatments, with quantification only until a depth of 10 cm (
Table 6). The highest concentration of sulfentrazone was observed when combined with the Ultra formulation at a concentration of 1.25 mg kg
−1 (
Table 6).
Theoretically, increasing the sorption and reducing the desorption of an herbicide reduces the molecule’s mobility, preventing it from leaching to deeper layers of soil [
48,
49]. This effect was observed for sulfentrazone when mixed with Ultra. In this treatment, higher sorption and lower desorption were observed, reflecting lower sulfentrazone leaching. Environmentally, this behavior is favorable, since it is possible to reduce the risk of sulfentrazone leaching, ensuring a safer application of this pesticide [
31].
The highest leaching of sulfentrazone was observed in the treatment with the Ready formulation, reaching the 20–25 cm depth layer (
Table 6). Unlike Ultra, the increase in sorption and reduction in sulfentrazone desorption promoted by the presence of the Ready formulation did not cause any less leaching of the molecule. This behavior observed for Ready seems contradictory; however, more leaching may be observed in conditions of the greater sorption and lower desorption of a pesticide. The Ready formulation delayed the sulfentrazone adsorption process in the soil compared to the process in the other treatments. The K
1 and K
2 values for sulfentrazone + Roundup Ready
® (2.75 and 1.67, respectively) were lower than those for sulfentrazone alone and mixed with other formulations (
Table 2). The constants K
1 and K
2 indirectly indicate the sorption rate of sorbets to adsorbents [
25]. Consequently, more sulfentrazone may still be in the soil solution at the time of the rain simulation when it is mixed with the Ready formulation. This fact raises the probability of herbicide leaching, as observed for sulfentrazone mixed with Roundup Ready.
Khan and Brown (2017) [
49] reported a similar effect for propyzamide. The authors observed higher leaching of the formulated product propyzamide compared to that for the same herbicide in its pure form, even with the formulated product exhibiting increased sorption. The authors attributed the increased leaching of the formulated product to the surfactants that can remain associated with the active substance, retarding the sorption process and thus increasing the leaching.
The sulfentrazone initially sorbed to the soil when it was combined with Ready formulation may initially reduce the number of available molecules in the soil solution. However, when the adsorption is not of high stability, the herbicide can gradually return to the soil solution and be leached after rainfall. Reddy and Locke (1998) [
50] demonstrated that the higher sorption of sulfentrazone in no-tillage soils compared to conventional tillage soils may prolong sulfentrazone persistence due to the lower amount of herbicide available in the soil. However, sulfentrazone was able to return to the soil solution, increasing the amount available for leaching. Evidence that may support this behavior is the higher total amount detected (in the sum of all layers) of sulfentrazone (1.5-fold higher) in the sulfentrazone + Roundup Ready
® treatment compared to the sulfentrazone treatment alone (
Table 6). The lower sorption observed when sulfentrazone was applied alone may allow the degradation of the herbicide in the first few hours, reducing the amount of sulfentrazone leached into the soil profile.
Although the simulations using PVC columns do not reflect the real scenario in agricultural fields, the results demonstrate a perspective of the risks involved in the mixed application of these herbicides. When applied in a mixture with Zapp Qi
®, these results showed that the leaching of sulfentrazone reached a depth similar to that of sulfentrazone alone (
Table 6). However, a larger total amount was quantified in the sulfentrazone + Zapp Qi
® treatment than with sulfentrazone alone (
Table 6). Similar to that in the Ready formulation, the higher sulfentrazone sorption in the presence of Zapp Qi
® reduced the amount of the herbicide in the soil solution that was available for degradation, but the lowest desorption rate in the sulfentrazone + Zapp Qi
® treatment (2-fold higher than that in the sulfentrazone + Ready treatment) minimized the effects of rainfall on the sulfentrazone leaching.