Interactions among K⁺−Ca²⁺ Exchange, Sorption of m-Dinitrobenzene, and Smectite Quasicrystal Dynamics

Abstract

The fate of organic contaminants in soils and sediments is influenced by sorption of the compounds to surfaces of soil materials. We investigated the interaction among sorption of an organic compound, cation exchange reactions, and both the size and swelling of smectite quasicrystals. Two reference smectites that vary in location and amount of layer charge, SPV (a Wyoming bentonite) and SAz-1 were initially Ca- and K-saturated and then equilibrated with mixed 0.01 M KCl and 0.005 M CaCl2 salt solutions both with and without the presence of 200 mg L(-1) m-dinitrobenzene (m-DNB). In general, sorption of m-DNB increased with the amount of K+ in the system for both clays, and the SPV sorbed more m-DNB than the SAz-1. Sorption of m-DNB increased the preference of Ca-SPV for K+ relative to Ca2+ but had little effect on K+-Ca2+ selectivity for K-SPV. Selectivity for K+ relative to Ca2+ was slightly higher for both K-SAz-1 and Ca-SAz-1 in the presence of m-DNB than in its absence. Distinct hysteresis loops were observed for the K+-Ca2+ cation exchange reactions for both clays, and the legacy of having been initially Ca- or K-saturated influenced sorption of m-DNB by SPV but had little effect for SAz-1. Suspension X-ray diffraction was used to measure changes in d-spacing and the relative thickness of smectite quasicrystals during the cation exchange and m-DNB sorption reactions. The results suggest that interactions among cation exchange and organic sorption reactions are controlled byan inherently hysteretic complex feedback process that is regulated by changes in the size and extent of swelling of smectite quasicrystals.

Sorption of Non-ionic Aromatic Organics to Mineral Micropores: Interactive Effect of Cation Hydration and Mineral Charge Density

“…This facilitated the specific interactions of polar functional groups (−NO 2 ) of 1,4-DNB with K + and/or polarized cation-bridging water molecules as well as the cation−π interaction between K + and the planar surface of PX, which has an aromatic π–donor system. It also resulted in less obscuration of zeolite surfaces between exchangeable cations and, hence, makes uncharged regions of the basal siloxane surfaces (hydrophobic nanosites) accessible for organics. ,, While for zeolite with too high of a charge density (SiO 2 /Al 2 O 3 ratio of 5.4), the distance between the sorbed cations was very close, with water molecules in the micropores strongly bonded as a result of the overlap of cationic electric effects. Organic sorption was strongly inhibited by both K + and Ca 2+ , because the large hydration shell of metal cations may intrude or shield the sorption sites for organics in pore spaces, with only a very weak sorption at the uncharged regions being allowed.…”

“…This is because they could control clay interlayer environments and interact with sorbed organic contaminants. 18,20,29−31 Previous studies have focused on the hydration effects of metal ions on the sorption behaviors of organics, such as nitroaromatic compounds (NACs), 22,26,30,32,33 pesticides, 20,23 and organic cations (C X H Y N amines), 34,35 in clay−water systems. It has revealed that sorption of organics to minerals that were exchanged with weakly hydrated cations (e.g., K + , Cs + , and NH 4 + ) was significant.…”

“…Metal ions commonly found in the natural environment (Na + , K + , Ca 2+ , and Mg 2+ ) can often replace the original cations in minerals via ion exchange and dramatically influence the sorption of organic contaminants at MWIs. This is because they could control clay interlayer environments and interact with sorbed organic contaminants. ,,− Previous studies have focused on the hydration effects of metal ions on the sorption behaviors of organics, such as nitroaromatic compounds (NACs), ,,,, pesticides, , and organic cations (C X H Y N amines), , in clay–water systems. It has revealed that sorption of organics to minerals that were exchanged with weakly hydrated cations (e.g., K + , Cs + , and NH 4 + ) was significant.…”

“…It has revealed that sorption of organics to minerals that were exchanged with weakly hydrated cations (e.g., K + , Cs + , and NH 4 + ) was significant. However, the presence of strongly hydrated cations (e.g., Ca 2+ and Mg 2+ ) led to lower sorption. − ,,− ,, This is due to the fact that uncharged siloxane surfaces between charged sites on mineral surfaces are relatively hydrophobic and can interact with nonpolar moieties of organic contaminants that access these mineral surfaces. When minerals are saturated with strongly hydrated divalent cations, the water cluster surrounding exchangeable cations prohibits organic molecules from approaching the hydrophobic domains, thus weakening their interactions.…”

“…Up to now, although the hydration effect of metal ions on the sorption behaviors of organics to mineral micropores has been extensively studied, ,,,,,− the interactive role of surface chemistry, especially the surface charge density of mineral micropores on these processes, was not fully understood. Boyd and co-workers have done a lot of work on the sorption of organics on minerals, especially smectite, and provided a significant mechanistic understanding on the effect of cation hydration and surface charge density on the sorption behaviors. − For example, they found that the sorption of NACs to a variety of smectite surfaces was controlled largely by the hydration characteristics of the exchangeable cations, which regulates cation–nitroaromatic complexation. , However, the surface charge densities of the clay minerals that they used were usually limited in the intermediate range [e.g., cation exchange capacity (CEC) = 80–132 cmol/kg]. ,,,, Recently, Droge et al reported that surface charge densities of the minerals (with a relatively wider range) and the presence of inorganic cations Na + and Ca 2+ in solution significantly influenced the ion exchange sorption of a wide variety of organic cations (C X H Y N amines) to phyllosilicate clay minerals, such as illite, kaolinite, and bentonite. , However, for non-ionic organics, the main sorption mechanism is no longer ion exchange but one or more interactions, including specific interactions with the exchangeable cations or water molecules surrounding the cations, EDA interactions, and non-specific van der Waals interactions with the neutral siloxane surface. ,− ,, How metal ions affect sorption of non-ionic organics to mineral micropores, especially related to surface charge density at sufficiently low or high charge density, needs further investigation.…”