Structure-Dependent Adsorption of Triterpenoid Saponins to Soil Minerals and Implications for Environmental Mobility

Saponins are natural plant metabolites with surface-active and bioactive properties against plant pests, making them promising biopesticides. However, their environmental fate in soil remains poorly understood. This study investigated the adsorption behavior of three triterpenoid saponins: the two monodesmosidic α-hederin and hederacolchiside A1, and the bidesmosidic hederacoside C, on common soil components including clay minerals (kaolinite, montmorillonite), metal oxides (gibbsite, goethite), black carbon, and topsoil. Batch adsorption experiments assessed the influence of saponin structures, sorbent properties and environmental factors. All saponins exhibited unexpectedly strong sorption (Kd> 10³ L/kg in topsoil), with α-hederin showing the highest affinity (Kd = 229×10³ L/kg on goethite), attributed to its moderate hydrophobicity (log Kow ~4.4), short sugar chain, and -COOH/-OH interactions. In contrast, the bulkier, more polar hederacoside C (log Kow ~ –1.2) showed weaker sorption. Sorption isotherms followed Freundlich behavior and was enhanced at acidic pH (~50% increase) for α-hederin and hederacolchiside A1 due to protonation of carboxylic acid groups (pKa ~4.7–4.9), while hederacoside C (pKa ~12.5) remained unaffected. Salt and fulvic acid reduced α-hederin adsorption (up to 80%), likely due to ion exchange and competitive complexation. Desorption studies showed α-hederin was strongly retained (<20% desorption), particularly on metal oxides. Scenario-based modeling indicates that under realistic application (50 μM), α-hederin and hederacolchiside A1 remain largely immobile, while hederacoside C may leach in low-sorption soils. These findings highlight the combined role of saponin structure and soil mineralogy in regulating environmental mobility, with implications for biopesticide design and risk assessment.