Effects of modifications of natural zeolites upon removal of manganese from aqueous solutions

Abstract

A series of natural zeolites with different compositions were modified by NH4NO3), alkali treatment (NaOH) and addition of aluminum (Al2(SO4)3). The natural and modified samples were characterized by XRF, XRD, N2 sorption, FTIR, NH3-TPD, SEM-EDS and TGA. The adsorption capacity of natural and modified zeolites was determined for manganese removal. Ion-exchange with NH4+ of NZ leads to the exchange of the Na+ and Ca2 + cations and the partial exchange of the Fe3+ and Mg2 + cations. However, the treatment of the NZ with NaOH and addition of aluminum into the zeolites lead to insignificant change of almost all cations irrespective of composition of natural zeolites. While the treatment of the NZ with NaOH causes significant dealumination and desilication of zeolites, introduction of aluminum increases their aluminum content. FTIR and TGA results showed that decationization, dealumination and desilication give rise to a decrease in hydrophilic nature of NZ. However, manganese adsorption of samples enhances their hydrophilic nature. Surface area and pore volume of zeolites increase with ion exchange and aluminum introduction and decrease with alkali treatment with NaOH. In addition, the treatment with NaOH gives rise to partial collapse of structure with significant desilication and dealumination depending on the concentration of NaOH and composition of zeolite. Ion exchange and aluminum introduction increase surface area and pore volume of samples as well as surface acidity, whereas alkali treatment with NaOH decreases them due to partial collapse of structure with significant desilication and dealumination. The NH3-TPD results showed that treatments alter the acidity of zeolite. While alkali treatment and addition of aluminum decreases the number of weak and medium Brønsted/Lewis acid sites, the number of strong acid sites increases through the introduction of Na+, the removal of silica and the formation of extra framework aluminum species. All modifications, ion exchange, alkali treatment, and aluminum introduction increases significantly the manganese adsorption capacity of natural zeolites depending on composition of zeolites. The pseudo-second-order kinetic model for the adsorption of manganese on all natural and modified zeolites fits well.