Natural toxins are a diverse group of rather polar, multifunctional and often ionizable organic compounds.^1,2 While generally understudied, some exemplary compound classes can be found both in the soil and aqueous environment. Concentrations, e.g., for pyrrolizidine alkaloids, vary from a few ng/L to several hundred µg/L in surface waters^3,4 or can reach up to more than 1000 µg/kg in clayey soils.⁴

By systematically quantifying sorption coefficients to major geosorbents, the compounds’ mobility in soils can be assessed to allow for reliable environmental exposure/risk assessment. For the purpose of systematic high-throughput analysis of sorption coefficients, we optimized a column chromatography setup^5,6 to investigate natural toxin sorption to organic carbon and clay mineral sorbents (kaolinite, montmorillonite). A set comprising over 100 natural toxins from 30 different subclasses was studied under changing environmental conditions with regards to pH as well as the type of inorganic ion and ionic strength (IS) in the aqueous solution.

Soil organic carbon (SOC) sorption of analytes which are neutral under experimental conditions is largely governed by hydrophobic partitioning and H-donor/-acceptor interactions. In contrast, strong effects of variable pH and IS suggest that electrostatic interactions majorly contribute to the sorption affinity of multifunctional, ionizable compounds. Cation exchange (CE) is the major sorption mechanism for cationic natural toxins stressed by the significant decrease in sorption in presence of competing inorganic cations in solution. Enhanced sorption is additionally striking for (partially) cationic N-heterocyclic natural toxins with aromatic moieties. Those compounds undergo cation-/π-interactions that significantly increase their affinity to SOC. For analytes with complexing ligands on neighboring C-atoms, pH and IS effects point to ternary complex formation involving Ca²⁺. When comparing cation exchange capacity normalized sorption coefficients for SOC and clay minerals, both cationic N-heterocyclic aromatic and complexing analytes sorb less or equally strong to clays, while those compounds merely dominated by CE show up to ten times larger sorption coefficients. Overall, varying sorption affinities of natural toxins highlight that the sorption of multifunctional, ionizable organic compounds is the result of a complex interplay of various interaction mechanisms. These are not only dependent on the compounds’ properties, but also the sorbent characteristics and composition of the aqueous medium. Our results allow to disentangle those complex mechanisms to pinpoint key structural moieties dominating natural toxin mobility and provide valuable insights for understanding and modelling environmental transport and fate processes. The large dataset additionally allows to test existing modeling approaches for their applicability for multifunctional, ionizable compounds in general.

[1] Schönsee and Bucheli; 2020. J. Chem. Eng. Data 65, 1946−1953
[2] Günthardt, et al; 2018. J. Agric. Food. Chem. 66, 7577-7588
[3] Günthardt, et a.; 2020. CHIMIA 74, 129–135
[4] Hama and Strobel; 2019. RSC Advances 9, 30350-30357
[5] Schenzel, et al; 2012. Environ. Sci. Technol. 46, 6118-6126
[6] Metzelder and Schmidt; 2017. Environ. Sci. Technol. 51, 4928-4935