1. Introduction
Phenoxyalkanoic acid herbicides (PAAHs;
Table 1) were first used in the 1940s [
1] and since then, they have been widely applied throughout the world [
2]. Although PAAHs are post-emergence herbicides, significant amounts of these compounds reach the soil surface, surface water, and groundwater, and their adsorption should be analyzed to predict pesticide behavior and environmental impact. In the top soil layer, PAAHs are adsorbed mainly on organic matter, and sorption is negatively correlated with soil pH [
3,
4]. The contribution of soil inorganic components to the overall sorption of PAAHs increases with soil depth.
The adsorption PAAHs in soils decreases in the following order: 2,4-DB > MCPB >> 2,4-D > MCPA >DCPP-P > MCPP-P [
5,
6]. Taking into account the data from European Union dossiers [
7] the adsorption of 2,4-DB expressed by the adsorption distribution coefficient normalized to organic carbon (
KOC) can exceed 1000 mL/g, whereas the maximum adsorption of MCPP-P is around 150 mL/g. PAAHs are quickly adsorbed in soils, and in batch experiments, adsorption usually reaches equilibrium within 4 to 12 h [
6]. PAAHs are also easily desorbed from soils. In soils with an organic matter content of > 1%, typically 40% to 80% of the adsorbed PAAHs can undergo desorption. In soils with an organic matter content of < 0.3%, the effectiveness of desorption reached 100% [
6]. Thermodynamic parameters have been rarely determined for the adsorption of PAAHs in soils. In the work by Shariff [
8], the standard enthalpy (
) of the adsorption of 2,4-D in soils ranged from -28.6 kJ/mol to -18.5 kJ/mol. These values indicate that physical adsorption, including hydrophobic sorption, van der Waals interactions, H-bonding, water-bridging, and/or anion exchange, should be the predominant process for 2,4-D [
6].
Humic substances play an important role in protecting the environmental against contamination because the contaminants are incorporated into the matrix of humic substances through different mechanisms [
9]. The chemical behavior of humic substances is determined mostly by carboxylic and phenolic functional groups, and humic molecules have a negative charge that is pH-dependent due to the partial dissociation of these acid groups [
10]. Fulvic acids (FAs) and humic acids (HAs) are humic substances with different solubility in water. Fulvic acids are water-soluble at any pH, whereas HAs are water-soluble at alkaline pH. Therefore, 0.1 M NaOH or 0.1 M Na
4P
2O
7 solutions are most often used to extract these fractions from soil, and the obtained extracts are acidified to pH 1.0-1.5 with HCl, which leads to the sedimentation of HAs [
11,
12]. The role played by FAs and HAs in the sorption of PAAHs in soil has not been fully elucidated to date due to the limited number of studies investigating the sorption of PAAHs on isolated fractions of humic substances [
10,
13,
14,
15,
16,
17].
According to Haberhauer et al. [
18], Kah and Brown [
19], Ćwieląg-Piasecka et al. [
17], as well as our recent study [
5], the most important interactions between the neutral forms of PAAHs and humic substances should involve H-bonding as well as van der Waals and hydrophobic interactions. In a study by Khan [
13] The
of 2,4-D adsorption on HAs at pH 3.3-3.6 (at this pH, the adsorption of its neutral form should be dominant) was 5.69 kJ/mol, i.e. remained in the physical adsorption range [
20,
21]. For the anionic forms of PAAHs, electrostatic interactions should also occur with their deprotonated carboxyl groups. Based on the pKa (negative logarithm of the dissociation constant;
Table 1) values of the six PAAHs that have been authorized for use in the European Union [
22], as well as the assumption that most soils have a pH of 5 to 8, only the anionic forms of 2,4-D, MCPA, DCPP-P and MCPP-P, and the anionic and neutral forms of 2,4-DB and MCPB are adsorbed at pH > 5. Our recent study [
5] has revealed that at low pH, FAs and HAs adsorb the neutral forms of all PAAHs, whereas at pH > 5, pure FAs are unable to adsorb the anionic forms of any PAAHs, and pure HAs can adsorb only the anions of 2,4-DB and MCPB. For example, the
KOC values for MCPB, 2,4-D and MCPP-P adsorbed on FAs examined in the current study reached 2482.2, 937.0, and 814.0 mL/g, respectively, at pH 2.9 and 548.8, 45.8 and 3.5 mL/g, respectively, at pH 5.7. In turn, the
KOC values for MCPB, 2,4-D and MCPP-P adsorbed on HAs examined in this study reached 659.1, 108.2, and 101.6 mL/g, respectively, at pH 2.9, and 170.7, 0.0 and 0.0 mL/g, respectively, at pH 7.1.
However, herbicide adsorption in soil depends on the interactions between organic matter, Fe and Al oxyhydroxides and clay minerals [
14,
18,
23]. According to Piccolo and Stevenson [
24], HAs and FAs can interact with metal ions to form metal-organic complexes in soils. Studies on the adsorption of 2,4-D [
25] and DCPP [
16] on FAs after coagulation with Al
3+ indicated that the anionic forms of these herbicides were bound to FAs mostly via an Al
3+ bridge. Larrivee et al. [
25] provided evidence that metal ions (Al
3+ and Pb
2+) interact strongly with organic complexing agents (FAs and 2,4-D) and suggested that negatively-charged FA components can interact with negatively-charged herbicides (such as 2,4-D) through positively-charged metal ion bridges. According to Elkins et al. [
16], Al
3+ species can form complexes with DCPP and/or FAs, and complexation reactions play an important role in decreasing herbicide degradation and mobility in soils, and in determining the environmental fate and bioavailability of aluminum. Jin et al. [
26] investigated the interactions between HAs and an Al
3+ coagulant and demonstrated that at pH 5, the activated functional groups of HAs formed complexes with Al
3+ species. However, at pH 7 surface complexation occurred, and the activated functional groups were absorbed on amorphous aluminum hydroxide (Al(OH)
3).
In our recent study [
5], when FAs were coagulated with sufficiently large amounts of Al
3+, the adsorption of the anionic forms of all PAAHs increased and reached a maximum at pH ~ 5.0-5.5, and it decreased to zero at pH ~ 7.0-7.5. The adsorption of large amounts of Al
3+ species by HAs resulted in high adsorption of 2,4-D, MCPA, DCPP-P, and MCPP-P anions, whereas the adsorption of 2,4-DB and MCPB anions was much lower in comparison with their adsorption on pure HAs. Moreover, the adsorption of the anionic forms of PAAHs on HAs was similar in the pH range of ~ 5 to 7.5. For example, the
KOC values for MCPB, 2,4-D, and MCPP-P adsorbed at pH 5.7 in examined in this study FAs complexed with Al
3+ species reached 7923.1, 825.1, and 1123.2 mL/g, respectively. In turn, the
KOC values for MCPB, 2,4-D and MCPP-P adsorbed at pH 7.1 on the examined HAs complexed with Al
3+ species reached 59.4, 1168.4 and 1139.5 mL/g, respectively.
Our previous study involved mainly batch experiments which were not sufficient to explain the observed adsorption phenomena. These mechanisms should be further explored because they play a key role in the retention of PAAHs in soil and their bioavailability to soil bacteria. In addition, a recent study by Wu et al. [
2] suggests that the transport of PAAHs adsorbed on the surface of water-soluble humic substances, which have a chemical structure similar to that of FAs, contributes significantly to the contamination of groundwater and surface water with these herbicides.
Therefore, the aim of the present study was to elucidate the adsorption mechanisms between FAs or HAs, Al3+ species, and the molecular and anionic forms of PAAHs based on the results of Fourier-transform infrared (FTIR) spectroscopy, the values of the thermodynamic parameters of PAAH adsorption, and molecular modeling.
5. Conclusions
This study demonstrated that the adsorption mechanisms of PAAHs on pure FAs and HAs and on FAs and HAs complexed with Al3+ involved physical adsorption mechanisms. This observation was confirmed by the obtained thermodynamic parameters of adsorption and FRIR spectra.
At low pH, the neutral forms of PAAHs were bound mainly via H-bonds between their nondissociated carboxyl groups and the carboxyl or hydroxyl groups of FAs and HAs. Moreover, the presence of hydrophobic and π– π stacking interactions with FAs and HAs was observed, mostly in the case of MCPB and 2,4-DB molecules. These derivatives of butyric acid have five rotatable bonds (the remaining ones have three), which significantly increases their conformational abilities.
The highest conformational abilities of MCPB and 2,4-DB also promoted the adsorption of their anionic forms on surface HAs at high pH, despite the fact that HA surfaces were partly negatively charged due to the dissociation of their carboxyl groups. The anionic forms of MCPB and 2,4-DB were adsorbed on HAs mainly via hydrophobic and π–π stacking interactions and H-bonds.
The complexation of FAs and HAs with Al3+ created a positive charge on their surfaces, mainly due to the formation of Al3+ bridges with two dissociated carboxyl groups of FAs and HAs. Both types of humic substances formed outer-sphere complexes with Al3+. The complexation of FAs with Al3+ promoted the formation of outer-sphere complexes between the anionic forms of PAAHs and the outer-sphere complexes of FAs with Al3+. In the created PAAH–Al3+–FA ternary complex, the electrostatic interactions between the ─COO¯ group of PAAHs, [Al(H2O)6]3+ , and two ─COO¯ groups of FAs were dominant. The above complex should exist at a pH up to 7-7.5 because at pH 7-7.5, only Al(OH)3 and small amounts of [Al(OH)4]+ exist in aquatic solution.
The complexation with Al3+ led to the coagulation of FAs and HAs, which largely decreased the surface of these complexes and, consequently, the number of their available sorption sites. This is the main reason why the adsorption of the anionic forms of MCPB and 2,4-DB was lower on HAs+Al3+ than on pure HAs (none of the anionic forms of PAAHs was adsorbed on pure FAs, and only the anionic forms of MCPB and 2,4-DB on pure HAs).
Contrary to adsorption on FAs+Al3+, the adsorption of the anionic forms of PAAHs was independent on pH and occurred even at pH > 8. The study demonstrated that the adsorption of the anionic forms of PAAHs on HAs+Al3+ was associated with a slight increase in pH. Therefore, it was postulated that the mechanisms of adsorption on HAs involved a ligand exchange between a loosely bound hydroxyl group of hydrolyzed Al3+ complexed with this adsorbent and the anionic form of the herbicide. However, in this case, adsorption took place only in the presence of sufficiently strong hydrophobic and π-π stacking interactions supported by H-bonds, and the ligand exchange mechanism was only an accompanying process. When the above conditions are fulfilled, the proposed mechanism could occur in the pH range of ~ 5.5-8.0, independently of pH.