Aluminosilicate Mineralogy and the Sorption of Organic Cations: Interplay between Electrostatic Barriers and Compound Structural Features

Abstract

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.

Phytotoxin sorption to clay minerals

“…This sorbate is almost two times heavier than the average analyte and thus potentially showed a size related restriction to sorb on (otherwise dominating) interlayer sorption sites. It was previously highlighted that the presence of various polar functional groups in addition to the protonated amine functionality mainly affects sorption to clay minerals due to sorbate size rather than polarity, thus limiting interlayer sorption [30]. The steroidal alkaloid CEV with eight polar functional groups (ester-, epoxy-, hydroxyl-groups) represents an excellent example for this observation.…”

“…1) were indicative of cooperative sorption suggesting enhanced sorbate affinities with increasing surface coverage [11,29]. Similar ranges for n and sigmoidal isotherms were obtained in previous Table 2 (continued) log D kao = sorption coefficient to kaolinite; log D mont = sorption coefficient to montmorillonite; subscript CEC depicts the sorption coefficients normalized to the cation exchange capacity of the different clay minerals; values < 0.1 or < 0.5 are below the operational range of the method; errors are expressed by the standard deviation of experiments; numbers in parentheses = the number of individual retention measurements with quantifiable sorption coefficient for each phytotoxin; for explanation of abbreviations in column "ID" see Table 1; NA = not analyzed Table 3 Fitting parameters for semi-quantitative isotherms describing sorption of phytotoxins to montmorillonite studies on cationic aromatic amines and clay minerals with interlayers in particular [29,30]. All compounds that showed this behavior in our study are rather planar, contain tertiary amine functionalities that were protonated under the experimental conditions and additionally possess aromatic moieties in proximity to the charge.…”

“…3) and must be attributed to structural features of the clay minerals. The mineralogy of different clay minerals was shown to affect sorption behavior of organic cations, particularly for higher order amines or structurally complex sorbates [30]. It was postulated that the position of the negative charge in clay minerals may play a role in sorption of organic cations.…”

“…The charge in montmorillonite predominantly originates from isomorphic substitutions in the mineral structure when clay lattice metal ions are replaced by other cations of lower charge. As about two-thirds of the negative charge are located in the internal octahedral layers of montmorillonite [31], it exhibits an electrostatic barrier for organic cation uptake [30]. In kaolinite the charge mainly results from pH-dependent (de-)protonation of surface functional groups on edge sites (Fig.…”

“…2) [31]. Additionally, sorbates may be more constrained in sorption to interlayer sorption sites on montmorillonite which would affect structurally complex, bulky molecules in particular [30]. The only compound with a sorption affinity to kaolinite that was larger (by more than 0.5 log units) in comparison with montmorillonite after normalization is the steroidal alkaloid cevadine (CEV).…”