Investigating the Influence of Impurity Defects on the Adsorption Behavior of Hydrated Sc3+ on the Kaolinite (001) Surface Using Density Functional Theory

In natural kaolinite lattices, Al3+ can potentially be substituted by cations such as Mg2+, Ca2+, and Fe3+, thereby influencing its adsorption characteristics towards rare earth elements like Sc3+. Density functional theory (DFT) has emerged as a crucial tool in the study of adsorption phenomena, particularly for understanding the complex interactions of rare earth elements with clay minerals. This study employed DFT to investigate the impact of these three dopant elements on the adsorption of hydrated Sc3+ on the kaolinite (001) Al-OH surface. We discerned that the optimal adsorption configuration for hydrated Sc3+ is Sc(H2O)83+, with a preference for adsorption at the deprotonated Ou sites. Among the dopants, Mg doping exhibited superior stability with a binding energy of -4.311 eV and the most negative adsorption energy of -1104.16 kJ/mol. Both Mg and Ca doping enhanced the covalency of the Al-O bond, leading to a subtle shift in the overall density of states towards higher energies, thereby augmenting the reactivity of the O atoms. In contrast, Fe doping caused a pronounced shift in the density of states towards lower energies. Compared to the undoped kaolinite, Mg and Ca doping further diminished the adsorption energy of hydrated Sc3+ and increased its coordination number, while Fe doping elevated the adsorption energy. This study offers profound insights into understanding the role of dopant elements in the adsorption of hydrated Sc3+ on kaolinite.

In-Depth Study on the Effects of Impurity Ions in Saline Wastewater Electrolysis

Concentration followed by electrolysis is one of the most promising ways for saline wastewater treatment, since it could produce H₂, Cl_2, and an alkaline solution with deacidification potential. However, due to the diversity and difference of wastewater, knowledge on the suitable salt concentration for wastewater electrolysis and the effects of mixed ions are still lacking. In this work, electrolysis experiments of mixed saline water were conducted. The salt concentration for stable dechlorination was explored, with in-depth discussions on the effects of typical ions such as K⁺, Ca²⁺, Mg²⁺, and SO₄^2-. Results showed that K⁺ had a positive effect on the H₂/Cl₂ production of saline wastewater through accelerating the mass transfer efficiency in the electrolyte. However, the existence of Ca²⁺ and Mg²⁺ had negative effects on the electrolysis performance by forming precipitates, which would adhere to the membrane, reduce the membrane permeability, occupy the active sites on the cathode surface, and also increase the transport resistance of the electrons in the electrolyte. Compared to Mg²⁺, the damaging effect of Ca²⁺ on the membrane was even worse. Additionally, the existence of SO₄^2- reduced the current density of the salt solution by affecting the anodic reaction while having less of an effect on the membrane. Overall, Ca²⁺ ≤ 0.01 mol/L, Mg²⁺ ≤ 0.1 mol/L and SO₄^2- ≤ 0.01 mol/L were allowable to ensure the continuous and stable dechlorination electrolysis of saline wastewater.