Structure-Dependent Sorption of Triterpenoid Saponins to Soil Constituents 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 unclear. This study investigated the sorption properties of three triterpenoid saponins: two monodesmosidic α-hederin and hederacolchiside A1 saponins, and the bidesmosidic hederacoside C saponin, on common soil constituents including clay minerals (kaolinite, montmorillonite), metal oxides (gibbsite, goethite), black carbon, and topsoil. Batch sorption experiments assessed influences of structures, sorbent properties and environmental factors. All saponins exhibited unexpectedly strong sorption (distribution coefficient (Kd)> 10³ L/kg on topsoil), with α-hederin showing the highest affinity (Kd = 229 × 10³ L/kg on goethite), attributed to its moderate hydrophobicity (octanol-water partition coefficient, log Kow ∼ 4.4), short sugar chain, and interactions involving carboxyl (-COOH) and hydroxyl (-OH) functional groups. In contrast, more polar hederacoside C (log Kow ∼ –1.2) showed weaker sorption with Kd of 1.56 × 10³ to 22.7 × 10³ L/kg. Sorption isotherms followed Freundlich behavior and increased by ∼50% at acidic pH for α-hederin and hederacolchiside A1 due to protonation of carboxylic acid groups (acid dissociation constant, pKa ≈ 4.7–4.9), whereas hederacoside C lacking carboxylic acid groups remained unaffected. Salts and fulvic acid reduced α-hederin sorption (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 at realistic saponin biopesticide doses (50 micromolar (µM)), α-hederin and hederacolchiside A1 remain largely immobile, whereas hederacoside C may slightly 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.