Zeolitisation of perlite fines: mineralogical characteristics of the end products and mobilization of chemical elements

This study examines the synthetic zeolites produced from perlite waste. Zeolitisation was carried out in autoclaves with NaOH solutions at 100°–140°C. The volcanic glass was converted to zeolite-Pc, zeolite-V and hydroxysodalite. The rate of conversion of the glass and the transformation of zeolites depended on temperature, time and NaOH concentration. The principal chemical changes observed between the initial glass and the end product involved the removal of Si and K and the uptake of Na. The amount of Si and K released increased with increasing alkalinity, reaching a steady state in the whole temperature range. The amount of Si at the steady state decreased with increasing temperature, while that of K followed an opposite trend. Therefore, the SiO2:Al2O3 ratio of the solid phase at steady state increased with increasing temperature. The amount of Na taken up increased with increasing alkalinity, increasing the Na2O:Al2O3 ratio and decreasing the SiO2:Na2O ratio of the solid phase, as reaction progresses. The crystallinity of zeolite-Pc increased with the amount of Na2O present in solution up to a maximum value, then decreased rapidly. The zeolite-Pc initially formed had a high SiO2:Al2O3 ratio, which then decreased gradually until zeolite-V was formed. Both zeolites were replaced by hydroxysodalite at higher alkalinity. Zeolite-V was metastable and converted to zeolite-Pc after prolonged exposure to the atmosphere. The average cation exchange capacity (CEC) of the zeolite products is 3.0 meq/g and equilibrium was attained very fast, suggesting that the products can be efficiently used for removal of ammonia from agricultural wastewater. At the described experimental conditions, hydroxysodalite was the stable Na-rich, Si-poor and zeolite-Pc was the stable Si-richer phase.