Paszko Tadeusz, Matysiak Joanna, Spadotto Claudio A, Boguta Patrycja, Skic Kamil
Department of Chemistry, University of Life Sciences, Akademicka 13, 20-950 Lublin, Poland.
Embrapa Digital Agriculture, Av. André Tosello, 209, Campinas 13083-886, São Paulo, Brazil.
Int J Mol Sci. 2024 Nov 26;25(23):12699. doi: 10.3390/ijms252312699.
Our recent study demonstrated that fulvic and humic acids are the major contributors to the adsorption of phenoxyalkanoic acid herbicides in soils. At very low pH, the neutral forms of these herbicides are bound directly to fulvic and humic acids, whereas at higher pH, their anionic forms are adsorbed mainly via bridges created by Al species. The number of active sorption sites associated with Al species complexed with fulvic acids is pH-dependent, whereas the number of corresponding sites in humic acids is pH-independent. Based on the results of the FTIR analysis, research into adsorption thermodynamics, and molecular modeling, an attempt was made in the present study to explain the adsorption mechanisms of six phenoxyalkanoic herbicides used currently in the European Union on the surfaces of the above fractions of humic substances. The obtained values of standard enthalpy (ΔH0) for the adsorption of the anionic forms of phenoxyalkanoic herbicides on fulvic or humic acids complexed with Al were in the range of physical adsorption, i.e., from -8.4 kJ/mol to -2.9 kJ/mol for the former, and from -5.3 kJ/mol to -2.4 kJ/mol for the latter. The study demonstrated that the neutral forms of phenoxyalkanoic herbicides were bound to humic substances mainly via H-bonds, π-π stacking interactions, and hydrophobic interactions. Al species were complexed with fulvic and humic acids to form outer-sphere complexes. Ternary outer-sphere complexes were also created between the anionic forms of phenoxyalkanoic acid herbicides and positively charged Al species complexed with fulvic acids. The mechanisms of adsorption on humic acids involved a ligand exchange between a loosely bound hydroxyl group of hydrolyzed Al 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. These findings elucidate why phenoxyalkanoic herbicides are mobile in the soil profile and are often rapidly degraded in soils.
我们最近的研究表明，黄腐酸和腐殖酸是土壤中苯氧基链烷酸类除草剂吸附的主要贡献者。在极低的pH值下，这些除草剂的中性形式直接与黄腐酸和腐殖酸结合，而在较高的pH值下，它们的阴离子形式主要通过铝物种形成的桥进行吸附。与黄腐酸络合的铝物种相关的活性吸附位点数量与pH值有关，而腐殖酸中相应位点的数量与pH值无关。基于傅里叶变换红外光谱（FTIR）分析、吸附热力学研究和分子模拟的结果，本研究试图解释目前欧盟使用的六种苯氧基链烷酸类除草剂在上述腐殖物质组分表面的吸附机制。苯氧基链烷酸类除草剂阴离子形式在与铝络合的黄腐酸或腐殖酸上吸附的标准焓（ΔH0）值处于物理吸附范围内，即前者为-8.4 kJ/mol至-2.9 kJ/mol，后者为-5.3 kJ/mol至-2.4 kJ/mol。该研究表明，苯氧基链烷酸类除草剂的中性形式主要通过氢键、π-π堆积相互作用和疏水相互作用与腐殖物质结合。铝物种与黄腐酸和腐殖酸络合形成外层络合物。苯氧基链烷酸类除草剂的阴离子形式与与黄腐酸络合的带正电荷的铝物种之间也形成了三元外层络合物。在腐殖酸上的吸附机制涉及与该吸附剂络合的水解铝的松散结合羟基与除草剂阴离子形式之间的配体交换。然而，在这种情况下，吸附仅在存在由氢键支持的足够强的疏水和π-π堆积相互作用时发生。这些发现阐明了为什么苯氧基链烷酸类除草剂在土壤剖面中具有移动性，并且在土壤中经常迅速降解。
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