Cation Hydration Energy and Surface Hydration Have Crucial Role On Thermodynamics of Clay Swelling

The swelling capacity and stability of clay play a crucial role in various areas ranging from cosmetics to oil extraction; hence change in their swelling behavior after cation exchange with the surrounding medium is important for their efficient utilization. Here we focus on understanding the role of different hydration properties of cation on the thermodynamics of clay swelling by water adsorption. We have used mica as the reference clay, Na+, Li+, and H+ ions as the interstitial cations, and performed grand canonical Monte Carlo simulations of water adsorption in mica pores (of widths d = 4−40 Å). We found that water adsorption in Na-, Li- and H-mica pores is qualitatively similar; however significant quantitative differences are observed, especially at smaller d. Higher water density in H-mica pores (ρH) was expected due to the smaller size of H+ ions having higher hydration energy. However, a counter-intuitive trend of ρLi > ρNa > ρb (bulk density) > ρH was observed due to adsorption energy where the contribution of mica framework atoms was also found to be significant. The disjoining pressure (Π), swelling free energy (∆Ωex), and several structural properties of confined water and ions were calculated to perform thermodynamic analysis of the system. Our detailed calculations have captured the structural evolution of ions and water, especially the transitions from mono- to bi- and multilayer as a function of d. Oscillatory behavior in the Π and ∆Ωex profiles with diminishing to zero for d ≥ 11 Å is observed in all three mica systems. A shift in the location of global minima of ∆Ωex towards the higher d values and ∆Ωex becoming more repulsive is observed in the increasing order of hydration energy of Na+, Li+, and H + ions. The ∆Ωex for Na-mica is characterized by global minima at d = 6 Å corresponding to crystalline swelling, a significant barrier for crystalline swelling from d = 6 to 9 Å and lower for crystalline (d = 9 Å) to osmotic swelling (d > 12 Å). For Li-mica, the energy barrier for crystalline to osmotic swelling is lesser compared to the Na-mica system, whereas for H-mica the ∆Ωex > 0 for all d thus favoring osmotic swelling. We found that the hydration of cations by surface atoms plays a key role in the thermodynamics of clay swelling. The Na+ ions hydrate more number of surface oxygens, act as anchors, and hold the mica pore at d = 6 Å by sharing hydrating water with ions of opposite sides forming an electrostatically connected bridge of mica Na-water-Na-mica. The Li+ ions do hydrate surface oxygen atoms, albeit lesser number and sharing of hydration shell with nearby Li+ ions is also minimum. Both, surface hydration and water sharing, is minimum in the H+ ion case, as they are mostly present in the center of the pore as diffusive ions; thus exerting a consistent osmotic pressure on the mica frameworks, favoring swelling.

Effects of Electronegativity and Hydration Energy on the Selective Adsorption of Heavy Metal Ions by Synthetic NaX Zeolite

The adsorption capacity of synthetic NaX zeolite for Pb2+, Cd2+, Cu2+ and Zn2+ in single and multi-component systems were investigated. The effects of electronegativity and hydration energy on the selective adsorption, as well as potential selective adsorption mechanism of the NaX zeolite for Pb2+, Cd2+, Cu2+ and Zn2+ were also discussed. The maximum adsorption capacity order of the heavy metals in the single system was Pb2+ > Cd2+ > Cu2+ > Zn2+, and this could be related to their hydration energy and electronegativity. The values of the separation factors (α) and affinity constant (KEL) in different binary systems indicated that Pb2+ was preferentially adsorbed, and Zn2+ presented the lowest affinity for NaX zeolite. The selective adsorption capacities of the metals were in the order, Pb2+ > Cd2+ ≈ Cu2+ > Zn2+. The trend for the selective adsorption of NaX zeolite in ternary and quaternary systems was consistent with that in the binary systems. Pb2+ and Cu2+ reduced the stability of the Si-O-Al bonds and the double six-membered rings in the NaX framework, due to the high electronegativity of Pb2+ and Cu2+ than that of Al3+. The selective adsorption mechanism of NaX zeolite for the high electronegative metal ions could mainly result from the negatively charged O in the Si-O-Al structure of the NaX zeolite, hence heavy metal ions with high electronegativity display a strong affinity for the electron cloud of the oxygen atoms in the Si-O-Al. This study could evaluate the application and efficiency of zeolite in separating and recovering certain metal ions from industrial wastewater.

The mechanism and regulation of the electrosorption selectivity of inorganic anions during capacitive deionization

Explore and verify the influence of hydration energy and valence on selectivity of anions, and the role of anion's shape

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The energy expenditure in the 5 types of modern tennis players

This project analyzes and categorizes the 5 types modern tennis players: striker from the baseline, counterattack from the baseline, complete in all court, serve and volley, attacking player, through the assessment of body hydration, energy expenditure, and matches statistics.