Disentangling Mechanisms in Natural Toxin Sorption to Soil Organic Carbon

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 (Doc) 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 Doc 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.ISSN:0013-936XISSN:1520-585

Phytotoxin sorption to clay minerals

Background: Phytotoxins of various classes and origin are often found in their cationic form in the soil environment and thus, their overall soil behavior may be strongly affected by all geosorbents presenting cation exchange capacity (CEC). In addition to soil organic carbon (SOC), clays may exhibit great potential as sorbents for cationic organic chemicals. Therefore, 52 compounds of the major phytotoxin classes alkaloids, terpenoids and steroids were investigated with regard to their sorption behavior to the clay minerals kaolinite (low CEC) and montmorillonite (high CEC) by means of continuous flow column sorption experiments as a high-throughput alternative to traditional batch sorption experiments. Results: In total, sorption coefficients log D [L kg ] were quantifiable for 26 phytotoxins on kaolinite (log D > 0.1) and 33 on montmorillonite (log D > 0.5). They ranged from 0.14 ± 0.09 for the pyrrolizidine alkaloid senkirkine on kaolinite to 3.05 ± 0.03 for the indole alkaloid brucine on montmorillonite. Although maximum sorbed concentrations lay well below the CEC for both clay minerals, sorption non-linearity was observed in some cases where as little as 0.1% of all cation exchange sites were occupied. Contrary to the expectations, sorption non-linearity could not be wholly explained by saturation of available sorption sites; for protonated tertiary amines with aromatic moieties, cooperative sorption seemingly took place and the results indicated a significant increase in sorption affinities within a very limited concentration range. Comparing montmorillonite and SOC, notable differences in preferences of cationic sorbates were observed between phytotoxins with and without aromatic moieties (e.g., isoquinoline versus pyrrolizidine alkaloids) as well as between N-heterocycles and N-heteroaromatics in particular (e.g., strychnine versus gramine; both indole alkaloids). Conclusions: Overall, clay sorption seems a result of the interplay of charge location on the sorbent and various structural features of the sorbates. To confirm observed tendencies towards cooperative sorption for certain cationic phytotoxins, further studies with higher concentrations are needed. Nevertheless, obtained sorption coefficients indicate that a high proportion of phytotoxin sorption in soils may be attributed to clay minerals. Thus, clay minerals possess the ability to decrease total cationic phytotoxin environmental mobility. clay clay clay −1ISSN:2190-4715ISSN:2190-470

"Is there anybody else out there?" - First Insights from a Suspect Screening for Phytotoxins in Surface Water

To protect themselves, plants can produce toxic secondary metabolites (phytotoxins) that appear with widely varying structures and negative effects. These phytotoxins often show similar properties as known aquatic micropollutants in terms of mobility, persistence, toxicity, and possibly also ecotoxicity. However, their occurrence in surface waters remains largely unknown, which is also due to unknown ability of available screening approaches to detect them. Therefore, we performed a target and suspect screening based on a persistence-mobility prioritization for phytotoxins in small Swiss creeks using high resolution mass spectrometry. In total, three of 26 targets were detected, three of 78 suspects tentatively identified, and six suspects fully confirmed by reference standards. To the best of our knowledge, it is the first time that three different plant secondary metabolite classes are detected in the same surface water sample. Estrogenic isoflavones were detected at 73% of the sites with formononetin as main toxin, which is in agreement with previous studies. Furthermore, pyrrolizidine alkaloids and the indole alkaloid gramine were detected. Especially pyrrolizidine alkaloids might be critical due to their production by various plants including the invasive Senecio inaequidens, and their known importance in food and feed safety. Based on these first screening results, different phytotoxin classes should be assessed for their ecotoxicological effects and considered in future water monitoring.ISSN:0009-429

Target screening of plant secondary metabolites in river waters by liquid chromatography coupled to high-resolution mass spectrometry (LC–HRMS)

Background Substantial efforts have been made to monitor potentially hazardous anthropogenic contaminants in surface waters while for plant secondary metabolites (PSMs) almost no data on occurrence in the water cycle are available. These metabolites enter river waters through various pathways such as leaching, surface run-off and rain sewers or input of litter from vegetation and might add to the biological activity of the chemical mixture. To reduce this data gap, we conducted a LC–HRMS target screening in river waters from two different catchments for 150 plant metabolites which were selected from a larger database considering their expected abundance in the vegetation, their potential mobility, persistence and toxicity in the water cycle and commercial availability of standards. Results The screening revealed the presence of 12 out of 150 possibly toxic PSMs including coumarins (bergapten, scopoletin, fraxidin, esculetin and psoralen), a flavonoid (formononetin) and alkaloids (lycorine and narciclasine). The compounds narciclasine and lycorine were detected at concentrations up to 3 µg/L while esculetin and fraxidin occurred at concentrations above 1 µg/L. Nine compounds occurred at concentrations above 0.1 µg/L, the Threshold for Toxicological Concern (TTC) for non-genotoxic and non-endocrine disrupting chemicals in drinking water. Conclusions Our study provides an overview of potentially biologically active PSMs in surface waters and recommends their consideration in monitoring and risk assessment of water resources. This is currently hampered by a lack of effect data including toxicity to aquatic organisms, endocrine disruption and genotoxicity and demands for involvement of these compounds in biotesting.ISSN:2190-4715ISSN:2190-470

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

Permanently charged and ionizable organic compounds (IOC) are a large and diverse group of compounds belonging to many contaminant classes, including pharmaceuticals, pesticides, industrial chemicals, and natural toxins. Sorption and mobility of IOCs are distinctively different from those of neutral compounds. Due to electrostatic interactions with natural sorbents, existing concepts for describing neutral organic contaminant sorption, and by extension mobility, are inadequate for IOC. Predictive models developed for neutral compounds are based on octanol-water partitioning of compounds (Kow) and organic-carbon content of soil/sediment, which is used to normalize sorption measurements (KOC). We revisit those concepts and their translation to IOC (Dowand DOC) and discuss compound and soil properties determining sorption of IOC under water saturated conditions. Highlighting possible complementary and/or alternative approaches to better assess IOC mobility, we discuss implications on their regulation and risk assessment. The development of better models for IOC mobility needs consistent and reliable sorption measurements at well-defined chemical conditions in natural porewater, better IOC-, as well as sorbent characterization. Such models should be complemented by monitoring data from the natural environment. The state of knowledge presented here may guide urgently needed future investigations in this field for researchers, engineers, and regulators.ISSN:0013-936XISSN:1520-585