Molecular dynamics (MD) simulations were performed to calculate free energies of sorption (ΔG sorb) of cationic aromatic amines to Ca-montmorillonite. We applied the linear interaction energy (LIE) method, well-established in the biochemistry field, to derive ΔG sorb. We obtained a mean average error of 0.3 kcal mol–1 within the compound training set and an error of 0.41 kcal mol–1 for a validation test set. We were able to reproduce absolute ΔG sorb values for a variety of compound structures, including the zwitterionic antibiotic oxytetracycline. MD simulations also provided atomistic level insights into the underlying driving forces that modulate sorption. Importantly, our approach provides a compelling alternative to polyparameter linear free energy relationship methods, which have shown limited success in capturing the sorption of ionogenic compounds with polar and/or charged moieties. We conclude that the LIE method can be used as a robust and tractable method to predict ΔG sorb within families of organic cations bound to aluminosilicate clay minerals.

Column chromatography was evaluated as a method to obtain organic cation sorption isotherms for environmental solids while using the peak skewness to identify the linear range of the sorption isotherm. Custom packed HPLC columns and standard batch sorption techniques were used to intercompare sorption isotherms and solid-water sorption coefficients (Kd) for four organic cations (benzylamine, 2,4-dichlorobenzylamine, phenyltrimethylammonium, oxytetracycline) with two aluminosilicate clay minerals and one soil. A comparison of Freundlich isotherm parameters revealed isotherm linearity or nonlinearity was not significantly different between column chromatography and traditional batch experiments. Importantly, skewness (a metric of eluting peak symmetry) analysis of eluting peaks can establish isotherm linearity, thereby enabling a less labor intensive means to generate the extensive data sets of linear Kd values required for the development of predictive sorption models. Our findings clearly show that column chromatography can reproduce sorption measures from conventional batch experiments with the benefit of lower labor-intensity, faster analysis times, and allow for consistent sorption measures across laboratories with distinct chromatography instrumentation.

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.

Current predictive models of organic cation sorption assume that sorbates interact with all sites on aluminosilicate minerals in the same manner. To examine whether differences in aluminosilicate structure and the resultant changes in electrostatic potential influence the sorption of organic cations, seven smectites were chosen with different proportions of isomorphic substitutions (origin of clay charge) located in octahedral versus tetrahedral layers and with the presence or absence of aluminosilicate interlayers. Sorption coefficients for 14 benzylamine derivatives with systematic differences in compound structures were collected to understand the possible influence of aluminosilicate mineralogy. Benzylamine compounds with methyl group substitution on the charged amine or with electron-donating or -withdrawing ring substituents displayed decreases in cation exchange-normalized sorption coefficients (K CEC ), by up to one order of magnitude, between hectorite (100% isomorphic substitution in the octahedral layer) and nontronite (100% isomorphic substitution in the tetrahedral layer). To understand this difference across aluminosilicates, stochastic molecular models of the various aluminosilicate minerals with interlayers were performed. These models showed that negative charge density associated with tetrahedral sites results in high positive electrostatic energy barriers within the interlayer, creating a penalty for compounds with positive charge spread over a larger compound surface area as occurs from primary to quaternary amines. Conversely, clays with charge originating from octahedral sites produce low electrostatic potential barriers within the interlayer, decreasing the penalty for quaternary amine sorption. Trends for nine cationic pharmaceutical compounds, which varied in size, group alkylation, and/or polar substituents, demonstrated similar decreases in K CEC values to aluminosilicate minerals with high electrostatic energy barriers. Overall, aluminosilicate mineralogy was found to exert a large influence (0.5−1 order of magnitude in sorption coefficients) on organic cation sorption. The application of atomistic electrostatic potential mapping of both sorbent and sorbate structures provided insights to explain trends in sorption coefficients that could not be described by the basic electrostatic potential theory or by assuming that sorbate structure moieties yielded additive sorption contributions.

Procedures for removing harmful radiation from interior and exterior surfaces of homes and businesses after a nuclear or radiological disaster may generate large volumes of radiologically contaminated waste water. Rather than releasing this waste water to potentially contaminate surrounding areas, it is preferable to treat it onsite. Retention barrels are a viable option because of their simplicity in preparation and availability of possible sorbent materials. This study investigated the use of aluminosilicate clay minerals as sorbent materials to retain (137)Cs, (85)Sr, and (152)Eu. Vermiculite strongly retained (137)Cs, though other radionuclides displayed diminished affinity for the surface. Montmorillonite exhibited increased affinity to sorb (85)Sr and (152)Eu in the presence of higher concentrations of (137)Cs. To simulate flow within retention barrels, vermiculite was mixed with sand and used in small-scale column experiments. The GoldSim contaminate fate module was used to model breakthrough and assess the feasibility of using clay minerals as sorbent materials in retention barrels. The modeled radionuclide breakthrough profiles suggest that vermiculite-sand and montmorillonite-sand filled barrels could be used for treatment of contaminated water generated from field operations.