Disentangling Mechanisms in Natural Toxin Sorption to Soil Organic Carbon

Abstract

Natural toxins are multifunctional, often ionizable organic compounds increasingly detected in the environment. Surprisingly little is known about their interactions with soil organic carbon, although sorption largely controls transport, bioavailability, and dissipation. For a set of 117 natural toxins from 36 compound classes the pH-dependent organic carbon–water distribution coefficient (D oc) was quantified using a soil column chromatography approach under changing conditions with regards to pH, ionic strength, and the major inorganic cation in solution. Natural toxins could be assigned to groups with either hydrophobic partitioning or specific interactions (complexation reactions, cation exchange) as dominating sorption mechanisms. The complex interplay of interactions in the sorption of natural toxins was equally influenced by sorbate, sorbent, and solution specific characteristics. High variability in sorption was particularly observed in the presence of Ca2+ resulting in D oc being enhanced by a factor of 10 when the pH was increased from 4.5 to 6. Sorbates following this trend contain either functional groups able to form ternary complexes via Ca2+ or aromatic moieties adjacent to protonated N presumably stabilizing cation exchange reactions. Although sorption was often stronger than predicted, investigated natural toxins were highly mobile under all considered conditions.

Sorption and Mobility of Charged Organic Compounds: How to Confront and Overcome Limitations in Their Assessment

“… 43 Thus, measurements of IOC sorption to pure soil constituents (SOM, clay minerals, black carbon) at specific pH and ionic strength conditions may offer a solid base for improved IOC mobility estimates. Although no one single “standard” SOM exists, a growing sorption data set on IOC has become available for Pahokee peat, 60 , 61 and many processes such as influence of ionic strength, hardness, and pH dependency are relatively constant for other SOM types. 60 …”

“…43 Thus, measurements of IOC sorption to pure soil constituents (SOM, clay minerals, black carbon) at specific pH and ionic strength conditions may offer a solid base for improved IOC mobility estimates. Although no one single "standard" SOM exists, a growing sorption data set on IOC has become available for Pahokee peat, 60,61 and many processes such as influence of ionic strength, hardness, and pH dependency are relatively constant for other SOM types. 60 In future approaches, D OC could be complemented by pHdependent K CEC for cations, and extended to pH-and ionicstrength-dependent sorption measurements of key scenarios (e.g., a soil with a low OC content, a high CEC, and a low ionic strength would likely show large discrepancies between D OC and K CEC ).…”

Effects of Different Liming Materials on the Uptake of Cadmium and Mineral Nutrients in Cacao Plants

“…The leaf Zn concentration follows a pattern similar to the case of Cd, with significant increases for the Ca and Ca–Mg liming materials. This can be due to increased competition for the deprotonated functional groups and the smaller Ca cation outcompeting the zinc group cations . Due to the large increase in soil Ca concentration, sorption sites overlap decreasing the soil capacity to bind trace elements …”

“…This can be due to increased competition for the deprotonated functional groups and the smaller Ca cation outcompeting the zinc group cations. 54 Due to the large increase in soil Ca concentration, sorption sites overlap decreasing the soil capacity to bind trace elements. 55 Since Cd uptake into the plant is related to Zn and Ca transporters, 56 there is a strong correlation between the concentration of these elements (see the correlation matrix and discussion in Table S3 and Supporting Note 1 in the SI), for instance, leaf Zn and leaf Cd (R = 0.92), leaf Ca and leaf Cd (R = 0.90), and leaf Ca and leaf Zn (0.92).…”

Natural toxins: environmental contaminants calling for attention

“…Thus, cyanobacterial blooms cause direct water contamination due to toxic secondary metabolites, while exudation from plants and rain wash off transfer toxins to soils from where they leach to surface waters and groundwater. A high fraction of natural toxins are polar and even charged, with octanol-water partition coefficients, log K ow , typically in the range from − 5 to 3 [9,10]. Hence, they are highly mobile in soils and sediments (Table 1).…”