Competitive Adsorption Processes at Clay Mineral Surfaces: A Coupled Experimental and Modeling Approach

Abstract

Quantification of adsorption processes on clay mineral surfaces is often necessary to predict the extent and the evolution of contaminants’ migration in surficial and underground environments. Many studies have been dedicated to retention measurement as a function of pH and ionic strength in relation with the two main identified adsorption processes for clay minerals, that is, cation exchange on their basal surfaces and surface complexation on their edge surfaces. The latest process has been repeatedly assessed as an effective retention mechanism at circumneutral pH conditions, which often prevail in natural environments. This assessment must however be tempered by the lack of information about competitive processes that can take place with the numerous chemical species present in natural settings, compared to simplified systems investigated in laboratory experiments. In this study, we quantified experimentally the competition between Pb2+, Co2+, Zn2+, and Mg2+ for specific adsorption on montmorillonite edge surfaces. Zn2+ was an effective competitor with Pb2+ and Co2+, and our results showed also unambiguously the influence of Mg2+ concentration levels on the specific adsorption of Pb2+ and Co2+. Because of the high ionic strength used in the experiments, cation exchange with Mg2+ was dismissed as a possible reason for such competition process, leaving specific competitive adsorption on edge surfaces as a unique explanation for our observations. Modeling of Pb2+ adsorption data with a state-of-art electrostatic complexation model for montmorillonite edge surfaces, supported by state-of-the-art, made it possible to distinguish two types of possible competition driving forces: Zn2+ competition for adsorption site occupancy but also detrimental changes in surface electrostatic potential following Mg2+ adsorption on sites neighboring those of Pb2+ adsorption. Mg2+ competition observed in our experiment should apply in most of clayey environments. Consequently, adsorption data obtained on pure clay mineral phases, and the associated models that have been built based on these data without considering the geochemical background in competitive species, may overestimate the retention properties of clay minerals when applied to natural settings.

Acid–Base Properties of Cis-Vacant Montmorillonite Edge Surfaces: A Combined First-Principles Molecular Dynamics and Surface Complexation Modeling Approach

“…Nevertheless, we consider that our cis-vacant model is improved compared to the previously published trans-vacant model by Tournassat et al, because a good agreement between predicted and measured proton charge was found (Figure ) and because our cis-vacant model takes into account the most recent insights on clay edge surface structure and reactivity. First, the cis-vacant structure at montmorillonite edges was confirmed with high resolution electron transmission microscopy measurements . And second, relative proportions of edge surface directions were made available from AFM measurements .…”

“…First, the cis-vacant structure at montmorillonite edges was confirmed with high resolution electron transmission microscopy measurements. 34 And second, relative proportions of edge surface directions were made available from AFM measurements. 54 Our cis-vacant model could be further improved with the addition of other types of substitutions, of which the p K a values remain to be determined using FPMD calculations.…”

“…Trans-octahedra have OH groups on opposite corners, while cis -octahedra have OH groups on adjacent corners. , Consequently, a TOT layers may be either trans-vacant with pure trans -octahedra in the octahedral sheet or cis-vacant with half cis -octahedra and half trans -octahedra (Figure ). Montmorillonite is a typical dioctahedral smectite that exhibits either cis- or trans-vacant structures, but cis-vacant structures are the most common in natural samples widely used in research studies, such as Wyoming montmorillonites available at the Source Clays Repository or Kunipia montmorillonite. , The cis-vacant structure of crystallographic planes was observed for Kunipia montmorillonite by using scanning transmission electron microscopy . Tournassat et al pointed out that the theoretical estimates of intrinsic p K a values were carried out on the basis of a trans-vacant model, whereas experimental titration and adsorption data were obtained on montmorillonite samples with a cis-vacant structure.…”

“… 33 , 34 The cis-vacant structure of crystallographic planes was observed for Kunipia montmorillonite by using scanning transmission electron microscopy. 34 Tournassat et al 21 pointed out that the theoretical estimates of intrinsic p K a values were carried out on the basis of a trans-vacant model, whereas experimental titration and adsorption data were obtained on montmorillonite samples with a cis-vacant structure. Because the structure of a clay mineral determines its intrinsic physical and chemical properties, the differences between cis- and trans-vacant configurations inevitably lead to different surface properties.…”

A deep learning neural network approach for predicting the factors influencing heavy-metal adsorption by clay minerals

“…Over the past several decades, the adverse impacts of heavy metals and organic pollutants on animals and plants have been studied and discussed extensively, and it has become clear that the treatment of water containing heavy metals requires urgent attention (Wang et al , 2006). Clay materials play a significant role in the adsorption of heavy metals in water due to their large surface areas and charged surfaces (Gomes et al , 2021; Orucoglu et al , 2022; Xu et al , 2022). Previous studies have shown that a given heavy metal can be adsorbed by several clay minerals (Liu, 2005; Erdoğan et al , 2021; Ma et al , 2021; Sarabadan et al , 2021; Silva et al , 2021; Barrientos-Velazquez et al , 2022; Werling et al , 2022).…”