Adsorption isotherms
To understand the interactions between atrazine and soil, as well as revealing the adsorption mechanism, adsorption isotherms were quantitatively established to analyze the process of atrazine transfer from the solid phase to the liquid phase [41]. The adsorption isotherm of atrazine in both soils were depicted at different temperatures, as shown in Fig. 1. Results revealed that the adsorption capacity of atrazine was significantly different in the two soils, which might be attributed to the difference in physical and chemical properties between the two soils [30–31, 22]. The adsorption curves for atrazine in the two soils were both L-type, with absorption increasing sharply initially and then showing a gradually increasing trend at 288, 298 and 308K (Fig. 1a, b). For each isotherm, the soil equilibrium adsorption capacity increased with increasing equilibrium concentration, due to the adsorption driving force increasing with the enhancement of equilibrium concentration [42]. The adsorption capacity of soil to atrazine decreased with the increase in temperature within an initial atrazine concentration range from 0 to 30 mg·L-1, especially in the Wushan soil (Fig. 1b). In addition, the equilibrium adsorption capacity also decreased with increasing temperature, suggesting that the atrazine adsorption process in the two soils was an exothermic process [27]. Generally, the adsorption capacity of atrazine in both soils was not very large, demonstrating that atrazine can easily penetrate the soil vadose zone and pollute groundwater.
The adsorption mechanism of atrazine in soil is likely to be related to soil properties. During the adsorption process, monolayer adsorption may occur and heterogeneous energy distribution of active sites on the soil surface [42]. In the assessed range of atrazine concentrations soil adsorption of atrazine exhibited a basic linear relationship, indicating that atrazine mainly adsorbs to soil organic matter via hydrophobic effects [43]. This type of distribution function is mainly related to the composition of soil organic matter and soil characteristics: (1) hydrogen bonds are formed between the neutral atrazine molecules and active centers on the surface of soil colloids; (2) ionic atrazine adsorbs on colloidal surfaces such as soil organic polyelectrolyte clay minerals; (3) atrazine can form complexes with carboxyl groups and free hydroxyl groups in soil humic acid on the surface of soil colloidal micelles [27, 44]. Therefore, adsorption models have been further used to analyze the mechanism of adsorption.
Linear, Friedrich or Langmuir adsorption models have commonly been used to describe analytical adsorption isotherms [45]. In order to clarify the overall adsorption process of atrazine in the two soils, Langmuir and Freundlich sorption models have been used. The Langmuir isotherm model [46] assumes that the energy adsorbed on the surface of the adsorbent is uniform, the adsorbent does not migrate in the surface plane and there is no interaction between the adsorbed molecules [47–48], the equation for which is described as follows in Eq. (2): (see Equation 2 in the Supplementary Files)
where, qe is the amount of atrazine adsorbed by soil (mg·kg-1); qm is the maximum amount of atrazine adsorbed (mg·kg-1); Kl (L·mg-1) is a constant related to the adsorption energy; and Ce is the equilibrium concentration of atrazine (mg·L-1);
The Freundlich isotherm model is used to describe heterogeneous surface equations such as the heterogeneity of the adsorbent surface, the adsorption energy and the exponential distribution of the adsorption point. The adsorption data fits the Freundlich isotherm [49] and its expression described in Eq. (3) as follows: (see Equation 3 in the Supplementary Files)
where, Kf and n are the Freundlich constants that refer to relative capacity and adsorption intensity, respectively.
The Freundlich and Langmuir models were used to fit the adsorption data of atrazine in soil, with the parameters depicted in Table 3. In all cases, the correlation coefficient (R2) of the Langmuir and Freundlich isotherm models exceeded 0.99, indicating that both models fit well to the adsorption of atrazine in both soil samples. According to the Freundlich model adsorption constant (KF), the adsorption capacity of the Wanzhou soil to atrazine was higher than Wushan soil and according to the Langmuir model, the sorption capacities (qm ) of Wanzhou soil was higher than that of Wushan soil, which is consistant with the change in KF. These results also demonstrate that the adsorption mechanism of atrazine in soil is related to both physical and chemical properties of the soil [19, 42].
The qm of the atrazine adsorbed by Wanzhou soil and Wushan soil decreased with increasing temperature, suggesting that the adsorption amount was negatively correlated with temperature. For the Freundlich model, n < 1 represents unfavorable absorption conditions and n > 1 represents favorable sorption conditions. In this study, the lowest n values for atrazine adsorbed by Wushan soil at the three temperatures of 298, 308, and 318 K were 1.49, 1.13, and 1.07, respectively. The n values were greater than 1 in all experiments, suggesting that sorption in both soil samples were favorable. The atrazine KF value for soil adsorption decreased with increasing temperature from 298 to 318 K. Moreover, the values of KF were less than 5 in all experiments, implying that atrazine has a weak adsorption capacity in these two soils, that atrazine is highly mobile in the soil-water environment and likely to cause groundwater pollution [28, 50].
Table 3 Freundlich and Langmuir isotherm constants for atrazine sorption in the assessed soil samples.
|
Samples
|
T(K)
|
Freundlich
|
|
Langmuir
|
KF
|
N
|
R2
|
|
Q max
|
KL
|
R2
|
Wanzhou soil
|
298
|
2.05
|
1.55
|
0.988
|
|
63.69
|
0.056
|
0.998
|
|
308
|
1.55
|
1.17
|
0.994
|
|
55.87
|
0.037
|
0.989
|
|
318
|
1.31
|
1.14
|
0.996
|
|
32.89
|
0.040
|
0.992
|
Wushan soil
|
298
|
1.77
|
1.49
|
0.981
|
|
48.07
|
0.066
|
0.995
|
|
308
|
1.36
|
1.13
|
0.993
|
|
34.60
|
0.075
|
0.997
|
|
318
|
1.02
|
1.08
|
0.988
|
|
17.79
|
0.162
|
0.993
|
Based on further analysis of the Langmuir equation, the dimensionless parameter of the equilibrium or sorption intensity (RL) was calculated using Eq. (4) as follows [51–52]: (see Equation 4 in the Supplementary Files)
where, Co is the initial concentration of atrazine (mg L-1). The change in soil adsorption strength (RL) indicates the type of isotherm and adsorption state, as per the following classification criteria:RL > 1 indicates unfavorable adsorption; RL=1 indicates linear adsorption; 0 <RL < 1 indicates favorable adsorption;RL = 0 indicates irreversible adsorption [51, 53].
In this study, the RL values for atrazine adsorption by Wanzhou soils were 0.37, 0.47 and 0.45, while for Wushan soils they were 0.33, 0.31, and 0.17 at 298, 308, and 318 K, respectively. The RL values for atrazine indicate that it could be well adsorbed by the two assessed soils. With increasing temperatures, the reaction rate decreased, indicating that the adsorption of atrazine in low temperature environments was easier than in high temperature environments. Results imply that the two soils have a certain affinity for atrazine, but absorption was negatively correlated with temperature [50]. In addition, low temperature conditions can promote the adsorption of atrazine in both soil samples, which is consistent with the results of the Freundlich model.
Adsorption kinetics
Adsorption kinetics are one of the main properties controlling the absorption rate and adsorption efficiency of solutes and are highly valuable when revealing the migration and transformation rate of atrazine in soil environments [28]. In this study, we used adsorption kinetics to assess the effects and dynamics of contact time on the adsorption capacity of atrazine in the two soil samples as shown in Fig. 2. As shown, the adsorption kinetics of atrazine in the two soils were similar, with both undergoing distinct stages of rapid adsorption, slow adsorption and equilibrium adsorption. Within 60 min, the adsorption rate of atrazine in the both soils were increased and the adsorption capacity showed a sharp increase because of physical adsorption to mineral surfaces [50, 54]. The faster initial sorption rate might be attributed to chemical bonds and hydrogen bonds between the atrazine and minerals [25]. With an increase in contact time, the adsorption rate decreased and the adsorption process gradually approached equilibrium within 48 h. The reason for this phenomenon may be due to the high initial atrazine concentration increasing the molecular collision frequency, allowing the dissolved state atrazine molecules to rapidly adsorb onto the surface of soil particles. In contrast, the molecular collision frequency decreased with low initial atrazine concentrations, resulting in a slower adsorption rate [27]. In addition, the rapid adsorption phase of atrazine to soil was determined by the large pores on the surface of particles, while the slow adsorption phase was determined by micropores inside the soil particles. Slow adsorption processes may be related to the region where organic compounds gradually enter soil micropores or where soil organic matter is highly crosslinked, covering the adsorption sites on the internal surfaces of minerals [55]. This phenomenon is compatible with the reported results of adsorption reactions of organic pollutants on porous media particles, showing the reaction can generally be divided into a rapid reaction phase and a slow reaction phase [56].
The adsorption process of atrazine to soils is complicated, as a result of the interactions between organic matter and inorganic minerals in soil [25, 57]. The rapid adsorption of organic pollutants can be attributed to their distribution in soil organic matter and on the surface of minerals. Physical adsorption in terms of the intermolecular interaction force, is mainly manifested by the van der Waals force, dipole force, and hydrogen bonding force, with these effects usually completed in a relatively short time period [44]. In order to reach the adsorption site on the surface of soil particles, hydrophobic organic pollutants need to overcome the diffusion of water molecules on the surface of soil particles into soil micropores and diffuse into the pores of the soil particles through the matrix [44, 58]. The rapid adsorption process of atrazine mainly occurs due to the presence of organic matter contained in the soil, as well as the distribution and physical adsorption of the mineral surface [44].
In order to describe the characteristics and mechanism of atrazine adsorption in soil, a pseudo-second-order kinetic model was employed, which can be described according to Eq. (5) as follows [59]: (See Equation 5 in the Supplementary Files)
The integration of the Eq. (5) with the initial condition, = 0 at t = 0 leads to linear Eq. (6) as follows: (see Equation 6 in the Supplementary Files)
where, qt is the adsorption capacity of atrazine at time t (mg·kg-1)k2 is the pseudo-second-order reaction rate constant (kg·(mg·min)-1) qe is the amount of atrazine (mg·kg-1) adsorbed by soils at equilibrium.
The linearized pseudo-second-order kinetic model of atrazine adsorption to soil and the assessed parameters are presented in Fig. 3 and Table 4. According to the fitting correlation coefficient (R2), the pseudo-second-order kinetic model exhibited good consistency with experimental data and could describe the dynamic processes of atrazine adsorption in both soils, with R2 values almost equal to 1 for both soil samples. This indicates that the adsorption behavior of atrazine is obvious and that the rate during the adsorption process is controlled by chemical processes [59–60]. The pseudo-second-order kinetic model covers all processes of adsorption, such as external liquid film diffusion, internal particle diffusion and surface adsorption, among others [43], which can reflect the adsorption mechanism of atrazine in soil more accurately and comprehensively.
Table 4 Kinetic parameters for atrazine adsorption in soils.
|
Kinetic models
|
Parameters
|
Soil samples
|
WZ
|
WS
|
Pseudo-second-order kinetic model
|
k2 (kg (mg·min)-1)
|
0.0489
|
0.0782
|
|
R2
|
0.9997
|
0.9986
|
Weber–Morris intra-particle diffusion model
|
kp (mg (kg min0.5)-1)
|
0.14
|
0.05
|
|
c(mg·kg-1)
|
14.01
|
10.64
|
|
R2
|
0.8081
|
0.5958
|
Generally, the adsorption kinetics of particles or molecules onto the surface of soils mainly includes two phases of transport and attachment separation [60–62]. The transport of particles is based on diffusion, which is caused by the attraction of the surface to the adsorbate [63]. However, the pseudo-second-order kinetic model does not identify the diffusion mechanism of atrazine soil adsorption.
Therefore, in order to better understand the factors affecting adsorption kinetics, the Weber-Morris intraparticle diffusion model [40] was applied to simulate experimental data, which was calculated according to Eq. (7) as follows: (See Equation 7 in the Supplementary Files)
where, kid (kg·(mg·min0.5)-1) is the intraparticle diffusion rate constant; and c is a constant.
The linearized Weber–Morris intra-particle diffusion kinetic model and its parameters for atrazine sorption in soils are depicted in Table 3. According to previous studies, qt and t0.5 in the interparticle diffusion model exhibit a linear relationship. Based on the intercept through the origin point, results indicated that the interparticle diffusion process of substances is not the only control step for adsorption rate, with other mechanisms potentially playing an important role [42, 60, 64–65]. From Fig. 4 and parameters in Table 3, it was found that the linear relationship between qt and t0.5 was not reasonable and that plots do not pass through the point of origin. This indicates that the intragranular diffusion process of atrazine was not the main control process of adsorption in both soils. The adsorption process involves a certain degree of boundary layer control and the adsorption rate is also affected by extragranular diffusion processes, such as surface adsorption and liquid film diffusion [42–43,60].
Thermodynamic parameters of adsorption
Thermodynamic parameters help describe the energy changes involved in the adsorption process [42]. In order to clarify the adsorption mechanism of atrazine in soil, the adsorption status of atrazine at different temperatures (288, 298 and 308 K) were analyzed and the influence of temperature on the equilibrium adsorption coefficient was assessed based on the standard Gibbs free energy ( ) formula (Eq. (8)). Furthermore, the relevant thermodynamic parameters of standard enthalpy ( ), and standard entropy ( ) were calculated using Eq. (9) as follows [60]: (see Equations 8 and 9 in the Supplementary Files)
where, is the adsorption equilibrium constant; R is the gas molar constant (8.314 J·(mol K)-1); and T is absolute temperature. According to the slope and intercept of the plot versus 1/T, the values of and were obtained.
In general, the represents the spontaneity of the chemical reaction and is used to evaluate whether the adsorption reaction occurs spontaneously [66]. The thermodynamic parameters of atrazine adsorption by the two soils are shown in Table 5, with values of being < 0, indicating the sorption of atrazine by soils was spontaneous. The ranked order of the absolute values for atrazine absorption by soil at different temperatures were as follows: 298 K > 308 K > 318 K. The adsorption of atrazine in the soil increased with the increase in temperature, indicating adsorption at higher temperatures was easier than at low temperatures. It is generally believed that when the values of are between 0 and −20 kJ/mol, physical adsorption is occurring with van der Waals forces playing a dominant role, resulting in the adsorption effect being small and desorption occurring easily. When the values of are between −80 and −400 kJ/mol, chemical adsorption is occurring with chemical bonding playing a dominant role, resulting in a larger adsorption energy and a higher tendency for irreversible adsorption [42, 67]. In this study, the values of were between −0.418 kJ/mol and −0.849 kJ/mol for Wanzhou soil and between −0.222 kJ/mol and −0.794 kJ/mol for Wushan soil indicating that the adsorption of atrazine by both soil samples occurred by physical adsorption and confirming that the adsorption of the atrazine by soil is feasible and spontaneous.
Table 5 Thermodynamic parameters for sorption of atrazine onto soils at different temperatures.
|
Samples
|
T
|
(kJ·mol-1)
|
(J·(mol·K)-1
|
(kJ·mol-1)
|
Soil (Wanzhou)
|
288
|
-0.849
|
16.9
|
-5.38
|
298
|
-0.598
|
|
|
308
|
-0.418
|
|
|
Soil (Wushan)
|
288
|
-0.794
|
28.7
|
-9.27
|
298
|
-0.309
|
|
|
308
|
-0.222
|
|
|
The standard enthalpy changes ( ) for the adsorption of atrazine in WanZhou and WuShan soils were -5.38 and -9.27 kJ·mol-1, respectively. The value of was less than 0, indicating that the solid phase of atrazine adsorbed from water to the soils was via an exothermic process, releasing large amounts of heat [68]. This result showed that adsorption was mainly based on physical properties, which is consistent with the results of . Studies have previously reported that when the standard enthalpy ranges from 40 to 120 kJ·mol-1, adsorption occurs via a chemical process, otherwise adsorption is via physical processes [69]. Therefore, the adsorption of the atrazine in this study appeared seems to occur via physical sorption. The standard entropy changes ( ) of the adsorption of atrazine in Wanzhou and Wushan were 16.9 and 28.7 J·(mol· K)-1, respectively. The positive values of indicate that soils had an affinity for atrazine and also reflect changes in the structures of atrazine and soils. In addition, the positive value indicated that randomness of the solid–liquid interface increased during the adsorption process.