Predicting Organic Cation Sorption Coefficients: Accounting for Competition from Sorbed Inorganic Cations Using a Simple Probe Molecule

Abstract

With the increasing number of emerging contaminants that are cationic at environmentally relevant pH values, there is a need for robust predictive models of organic cation sorption coefficients (K). Current predictive models fail to account for the differences in the identity, abundance, and affinity of surface-associated inorganic exchange ions naturally present at negatively charged receptor sites on environmental solids. To better understand how organic cation sorption is influenced by surface-associated inorganic exchange ions, sorption coefficients of 10 organic cations (including eight pharmaceuticals and two simple probe organic amines) were determined for six homoionic forms of the aluminosilicate mineral, montmorillonite. Organic cation sorption coefficients exhibited consistent trends for all compounds across the various homoionic clays with sorption coefficients (K) decreasing as follows: K > K ≥ K > K ≥ K > K. This trend for competition between organic cations and exchangeable inorganic cations is consistent with the inorganic cation selectivity sequence, determined for exchange between inorganic ions. Such consistent trends in competition between organic and inorganic cations suggested that a simple probe cation, such as phenyltrimethylammonium or benzylamine, could capture soil-to-soil variations in native inorganic cation identity and abundance for the prediction of organic cation sorption to soils and soil minerals. Indeed, sorption of two pharmaceutical compounds to 30 soils was better described by phenyltrimethylammonium sorption than by measures of benzylamine sorption, effective cation exchange capacity alone, or a model from the literature (Droge, S., and Goss, K. Environ. Sci. Technol. 2013, 47, 14224). A hybrid approach integrating structural scaling factors derived from this literature model of organic cation sorption, along with phenyltrimethylammonium K values, allowed for estimation of K values for more structurally complex organic cations to homoionic montmorillonites and to heteroionic soils (mean absolute error of 0.27 log unit). Accordingly, we concluded that the use of phenyltrimethylammonium as a probe compound was a promising means to account for the identity, affinity, and abundance of natural exchange ions in the prediction of organic cation sorption coefficients for environmental solids.

Hydrophobic Solvation of Gases (CO2, CH4, H2, Noble Gases) in Clay Interlayer Nanopores

“…Instead, we find that clay surfaces have a significant hydrophobic character at the atomistic scale, as previously noted in the context of cation-exchange selectivity. , This hydrophobic character is modulated by the presence of exchangeable cations and by the templating of interfacial water by the clay surface. Our results have broader relevance to the adsorption of hydrophobic solutes (including organic molecules) at clay–water interfaces, for example, during sedimentary rock diagenesis, soil formation, and contaminant migration in the subsurface …”

“…Our results have broader relevance to the adsorption of hydrophobic solutes (including organic molecules) 44 at clay−water interfaces, for example, during sedimentary rock diagenesis, 45 soil formation, 46 and contaminant migration in the subsurface. 47 ■ METHODOLOGY Molecular Dynamics Simulations. MD simulations were carried out to examine the partitioning of dissolved gases between clay interlayer water and bulk liquid water.…”

Evaluation of Existing Models to Estimate Sorption Coefficients for Ionisable Pharmaceuticals in Soils and Sludge

“…The availability of only a small set of poorly validated corrective increments to estimate D OC,IE and K CEC,CLAYS suggest these need to be updated and extended with new data to cover more complex multifunctional structures that are common in ionisable pharmaceuticals. Building on the work of Droge and Goss [30], Jolin et al [36], demonstrated that you can predict organic cation sorption using a simple probe molecule, phenyltrimethylammonium, in combination with structural scaling factors derived from the Droge and Goss [30] approach. This hybrid approach offers promising advances in mechanistic derived sorption coefficients for organic cations, however, the need for an experimentally determined K d value for the probe molecule in your soil type is a limiting factor in the Jolin et al [36] approach.…”

“…Models were selected that account for chemical speciation by incorporating terms related to matrix properties (e.g., soil pH and lipophilicity corrected for pH (log D)). Whilst additional sorption models have been published that specifically account for the presence of ionisable functional groups and are based on mechanistic understanding of sorption to soils, these models often require detailed soil properties which are not commonly reported in standard sorption studies (e.g., ter Laak [29]) or experimentally determined sorption coefficients of a probe compound in the soil of interest (e.g., Jolin et al [36]). It was, therefore, not possible to evaluate all existing sorption models as input data needed for these models was not available in the iPiE database or could not be computed experimentally.…”

Sorbent-modified biodegradation studies of the biocidal cationic surfactant cetylpyridinium chloride

“…Isotherms for sorption of CPC to SiO 2 and illite were plotted with a fixed slope of 1 (Fig. 2), as sorption is expected to be linear at concentrations well below the cation-exchange capacity (CEC) (Droge and Goss, 2013a;Jolin et al, 2016Jolin et al, , 2017. Two independent experiments using SiO 2 were in good agreement; the fitted curve indicates a log K D of 3.5 and spans two orders of magnitude, with highest concentration (47 μg/L) close to the NIEC-STP (100 μg/L).…”