Adsorption of Hg (II) on Ca-bentonite

The adsorption of Hg²⁺ on Ca and Na-bentonite was followed via ASV, results are shown in Fig. 1 and 2 respectively as the black circles. Additionally, a theoretical model was used to identify bond interactions. This model is based on molecules interactions, where the adsorption process is carried out by adsorption desorption interactions between cations competing by active sites, this model is represented as described by Chen and Frank, is presented in Eq. 4 ^[36].

Fig. 1 Adsorption of Hg²⁺ onto Na-bentonite. Carried out at atmospheric pressure, 25°C, and neutral pH. 

Fig. 2 Adsorption of Hg²⁺ onto Ca-bentonite. Carried out at atmospheric pressure, 25°C, and neutral pH. 

where K_a represents the adsorption constant, K_d desorption constant, and C₀ represents initial concentration. q_m represents the equilibrium adsorption capacity of the clay, N₀ the adsorbed concentration by available adsorption sites, and q_t relates total quantity of metal adsorbed by the clay in a time t.

Experimental data of adsorption process represented in Fig. 1 and 2 by the black line (Fig. 1, black circles) where fitted to the model of Chen and Frank. As can be seen in fig. 1 and 2, calcium bentonite has higher sorption capacity, for this purpose this clay was used for modeling and next calculations. Follow this plot thermodynamic parameters k_a, k_d, q_m, N₀ y ∆Gº where calculated, these values are shown in Table 1.

In terms to understand experimental results, further analysis was done using Gaussian 09, results are described below.

Structural and computational models

Bentonite is a complex structure with a highly variant composition, for this reason its principal structure, montmorillonite, was modelled ^[37]. The isomorphic substitution of montmorillonite prevents the formation of a structure with an explicit localization for the substitutions in the crystallographic data, for this reason the montmorillonite cell model studied was based on the de Mignon et al., 2010 report ^[38]. The cell parameters of Ca-montmorillonite described by Viani et al., 2002 ^[39] (a = 5.18 Å, b = 8.98 Å, c = 15.00 Å, α = 90 Å, 90 Å = β, γ = 90 Å) were taken and duplicate in three directions to form a new cell dimensions a = 11.26676 Å, b = 17.46034 Å, c = 26.8225 Å after adjusting the content of atoms. Calcium atoms were removed, and the hydrated cations of interest were place in their vacated sites.

Montmorillonite has a diffuse negative charge distribution resulting from isomorphous substitution in the octahedral layer. Such distribution may not reflect the replacement of hydrated cations located between the layers ^[5], so this net negative charge was neutralized by the addition of hydrogen atoms in the montmorillonite crystal model

Hydration of Ca ²⁺ and Hg ²⁺

The structure of the hydration sphere of the ions Ca²⁺ and Hg²⁺ is fundamental to understanding their behavior in a given system. Thus, there are many theoretical and experimental reports where probable hydration numbers for Ca²⁺ and Hg²⁺ are described. In the specific case of Ca²⁺, the more favorable hydration numbers are 6 and 7 that may alloy a second layer which can improve hydration stability. ^[4,40-46] A similar situation arises for Hg^{2+ ([47-54]}. Initially four hydration systems were proposed for Ca²⁺ and Hg²⁺ ions: penta, hexa, hepta and octa-hydrated whose resulting structures were taken as the basis for a second hydration sphere (10 water molecules) for those ions.

Computational results

Quantum mechanical (QM), molecular mechanics (MM), molecular dynamics (MD) methods as well as Monte Carlo (MC) simulation techniques have been successfully applied to the behavior of clay materials as well as various hydrated cations during adsorption. The method of theory density functional (DFT) has proven particularly useful in simulation systems of interest for montmorillonite ^[5], Ca²⁺ and Hg²⁺ hydrated cations ^[4,6] and complex cation-montmorillonite. ^[11,55-57] The system model for montmorillonite was optimized using molecular mechanics (UFF) ^[12,13] using the software Gaussian 09. ^[35] While optimizing the geometries of the Ca²⁺ and Hg²⁺ hydrated systems using the method of DFT by the functional B3LYP ^[15] and Lanl2dz ^[16] base was conducted with the same software. The structures resulting from the optimization process (montmorillonite and hydrated cations) were combined to create the complex hydrated-montmorillonite cation (hydrated cations were located between the layers of montmorillonite). Additionally, it optimized with oniom (B3LYP / LANL2DZ technique: UFF) and implemented in the Gaussian 09 program ^[58]. All structures resulting from the optimization process were subjected to frequency analysis to ensure that they correspond to a minimum (local) and to estimate the values of enthalpy and free energy of reaction ^[59].

Geometries

According to the simulation performed for hydration of Ca²⁺ and Hg²⁺, these cations may form structures with those added water molecules. However, after 6 water molecules, the systems showed a preference for maintaining the coordination number ^[38]²²²² in its first hydration layer and subsequently forming a second layer with the remaining water molecules (Fig. 3).

Fig. 3 Coordination Modes for Ca²⁺ and Hg²⁺ ions in the presence of 1 to 8 molecules of water, wherein a preference for a first coordination sphere formed by six water molecules is observed. Upper: Hydration for Ca²⁺; Lower hydration for Hg²⁺ (B3LYP / LANL2DZ). 

In the case of the decahydrated systems for Ca²⁺ and Hg²⁺, the same trend as previously described occurs, promoting bonding of six water molecules in the first hydration layer and the remainder on a second layer for both cations (Fig. 4).

Fig. 4 Structures resulting from the optimization process for decahydrated Ca²⁺ and Hg²⁺ systems where a similar behavior is perceived described in Figure 3. Top: Hydration for Ca²⁺; lower for Hg²⁺ (B3LYP / LANL2DZ). The links of the first hydration sphere are marked for clarity. 

During the optimization process, the hydrated montmorillonite cation complex, showed that previously described coordination spheres are not changed even though the water molecule hydrated cation are oriented to form hydrogen bonds with the oxygen atoms of the montmorillonite (Fig. 5).

Fig. 5 Illustrative examples of the structures resulting from the modeling of complex-montmorillonite hydrated cation (B3LYP / LANL2DZ: UFF). Top: Resort Ca²⁺ penta (left) and octahydrate (right) with montmorillonite; Bottom: Resort Ca²⁺ decahydrated with montmorillonite. 

Enthalpies and free energies of reaction.

The usual way of determining the enthalpies of reaction is through the determination of the heats of formation considering the appropriate sums and differences.

From equation (5) the enthalpies of reaction for the proposed models can be established from equation (6).

where Ca²⁺·H₂O-Montmorillonite and Hg²⁺·H₂O-montmorillonite complexes correspond to the hydrated cation-montmorillonite while Ca²⁺·H₂O and Hg²⁺·H₂O represent hydrated cations. A similar process can be applied to determine free energies of reaction. Thus, the respective combinations for both Ca²⁺ and Hg²⁺ were analyzed. As was expected, each of the complexes present several values Δ_rH° and Δ_rG° (B3LYP/Lanl2dz:UFF). However, those systems have two coordination spheres between the montmorillonite layers, where the first hydration sphere consists of 6 water molecules and the molecules remaining to a second to give the same signs for Δ_rH° and Δ_rG°. Specifically, the octahydrate cation-montmorillonite model gave values of ΔrH° = (57.53 kJ / mol and ΔrG°= (60.76 kJ / mol. These values are relatively close to those described experimentally. Thus, this is possible that this complex contributes, preferentially, to Hg²⁺ adsorption on bentonite as suggested in Fig. 6.

Fig. 6 Preferable combination by Δ_rH° and Δ_rG° values of possible complexes involved in the adsorption of Hg²⁺ on bentonite. Top left: Ca²⁺ octahydrate-montmorillonite; top right: Hg²⁺ octahydrate. Lower left: Hg²⁺ octahydrate-montmorillonite complex; bottom left: Ca²⁺ octahydrate. 

It is possible to observe, in Fig. 6, that the Hg²⁺ hydration sphere changes after simulation in solution when it is located between montmorillonite layers. The difference between the experimental values and those obtained by simulation may correspond to the simplification of the proposed models as well as the characteristics of the calculation variables and methods that were used.