THERMODYNAMIC CALCULATIONS OF ION-EXCHANGE SORBATION OF IRON IONS ON CLINOPTYLOLITE

Abstract

Increased content of iron in water, impairs its quality and, in many cases, makes it impossible to use such water for home and industrial needs. Although iron is the basis of many biochemical processes in the human body, its excess content can have many negative effects. According to WHO standards, the permissible content of iron in drinking water is 0.3 mg/l. The high iron content is observed in the natural waters of many regions ofUkraineand other countries. This phenomenon is often not connected with anthropogenic influence but caused by the peculiarity of the geomorphological structure of the area. Therefore, the development of new and improvement of the existing methods of water desalination (iron removal) is a pressing issue.The methods of water desalination can be divided into four different categories: traditional techniques, biological methods, processes based on membrane technologies, and methods based on nanotechnologies. Among the traditional methods, one of the main approaches is ion-exchange.In this work, quantum-chemical calculations of iron ion exchange processes on clinoptilolite (structural type HEU) and its modified forms are carried out. The calculations used the software xTB. The semi-empirical GFN2-xTB method was chosen as the primary method, based on a modified density functional theory, namely the Density Functional based Tight Binding (DFTB) approach.Iron atoms were considered in oxidation states 2 and 3. Also, they considered the possible electronic states of each of the ions and sorption products (low spin state - multiplicity 1 and 2 in the case of Fe2+ and Fe3+, respectively, and high spin state - multiplicity 5 and 6 in the case of Fe2+ and Fe3+, respectively).Fe(H2O)6n+ + Zeo-X(H2O)a = X(H2O)6 + Zeo-Fe(H2O)b + y (H2O)6 (1) The thermodynamic calculations of the ion exchange (1) indicate the possibility of the forward reaction run (ΔG <0). In the case of monovalent sodium exchange ions, ion exchange is possible both Fe2+ and Fe3+. In the case of double-charge calcium exchange ions, ion exchange is possible for Fe3+ ions. Ion exchange involving Fe3 + ions is much more energetically favorable than Fe2+ ions.