Synthesis of zeolitic material from green tuff stone cake and its adsorption properties of silver (I) from aqueous solution

Abstract

A zeolitic adsorbent was synthesized from waste green tuff stone cake by hydrothermal treatment, and its adsorption of silver ions from aqueous solution was examined for application in silver ion removal from polluted industrial waters and as a bactericide. The waste cake was composed mainly of SiO2 and Al2O3 in the form of silicate and aluminosilicate minerals, such as quartz, anorthite, muscovite and clinochlore. The cake was treated with 0–10 M NaOH solution at 80, 120 and 160 °C for 24 h, and analcime, hydroxysodalite and hydroxycancrinite were synthesized. A product with high silver adsorption ability, including analcime, was obtained in 4 M NaOH solution at 160 °C. The concentrations of Si and Al in the solution during the reaction in 4 M NaOH solution at 160 °C explain the synthesis of analcime zeolite phase due to the dissolution of quartz and anorthite in the cake, and Ag adsorption of the product depends on analcime zeolite crystals in the product. The optimal pH for silver adsorption for the product was found to be around 5. The equilibrium silver adsorption capacity was measured and extrapolated using the Langmuir and Freundlich isotherm models, and experimental data were found to fit the Langmuir model. The calculated maximum adsorption capacity of 1.10 mmol/g shows that this product is suitable for silver uptake from aqueous media. Adsorption kinetics was tested for pseudo-first- and -second-order reactions, and the rate constants of adsorption for these kinetic models were calculated. Adsorption experiments demonstrated that the adsorption process corresponds to a pseudo-second- rather than -first-order kinetics model. With decreasing aqueous solution temperature, the adsorption kinetics become slower and the amount of silver ion adsorbed increases. The thermodynamic values, ΔG°, ΔH° and ΔS°, indicated that adsorption was an exothermic and spontaneous process.