Soybean root exudates and low molecular weight organic acids on uniconazole adsorption in clay minerals: Mechanisms and environmental implications

The environmental behavior of uniconazole, a widely used plant growth regulator, is strongly influenced by its interactions with clay minerals in soil, yet the influence of root exudates remains poorly understood. Based on our preliminary research, we hypothesized that natural soybean root exudates and low molecular weight organic acids (LMWOAs) promote the adsorption of uniconazole in clay minerals. To examine this overarching hypothesis, we integrated batch equilibrium experiment and molecular spectroscopy with density functional theory (DFT) simulations. At 20  mmol L⁻¹, exudates and LMWOAs enhanced adsorption by 12.8–51.7 %, with stronger effects on montmorillonite (17.6–51.7 %) than kaolinite (12.8–30.9 %). Structurally complex acids (oxalic, malic, citric) and root exudates promoted adsorption across 5–80  mmol L⁻¹ via strong Al complexation, whereas simpler acids (tartaric, acetic) inhibited adsorption at higher concentrations due to site competition. Mechanistic analyses revealed that electrostatic attraction, coordination complexation, LMWOA bridging, ion exchange, and hydrogen bonding enhanced chemisorption, with slight mechanistic differences between exudates and single acids. DFT simulations demonstrated that oxalic acid increased adsorption energy, stabilizing interactions, facilitating hydrogen bond formation, and enhancing electrostatic attraction. Adsorption isotherms and kinetics followed the Freundlich, D-R, and pseudo-second-order models, suggesting a multilayer chemisorption mechanism. This study provides a mechanistic insight into how soybean root exudates and LMWOAs enhance uniconazole adsorption on clay minerals, highlighting the critical role of organic acid structure and concentration in modulating herbicide–clay interactions and influencing their environmental fate, raising concerns about the potential accumulation of plant growth regulators in soil ecosystems.