Adsorption of ammonia by base activated bentonite clay kinetic and equilibrium studies

TL;DR: In this paper, the effect of the adsorption conditions (adsorbent dose, temperature, pH, adsorbate concentration and contact time) on the amount of removed ammonia from the aqueous solution was investigated.

Abstract: Nitrogen compounds are essential elements for living organisms. The increased concentration of ammonia in ionized (NH4+) and non-ionized form (NH3) in surface waters causes negative effects on aquatic organisms, since it enters into the nitrogen cycle, and thus contributes to the eutrophication of water. The presence of higher concentrations of ammonia in surface waters than natural ones is the result of faecal contamination through the sewage system or runoff from arable areas which were treated with fertilizer. Water contaminated with ammonia can be purified by various methods: biological (nitrification-denitrification method), oxidation with chlorine, chemical precipitation, membrane filtration, etc. One of the alternative methods is removal by adsorption with natural materials such as clay and zeolite. Bentonite clay was marked off as a suitable adsorbent of cationic pollutants due to the permanent negatively charged surface. Тhe textural properties (specific surface, porosity) and structural properties of bentonite clay (the distance between the layers, the type of ions in the space between the layers) and consequently its adsorption capacity for pollutants is changed with the activation by acid or base and intercalation. There is very little literature data on the application of bentonite clay for removal of ammonium ions. Therefore, the aim of this work is activating the raw bentonite clay with a base and its application as an adsorbent for removal of ammonium ions from aqueous solutions. The influence of the adsorption conditions (adsorbent dose, temperature, pH, adsorbate concentration and contact time) on the amount of removed ammonia from the aqueous solution was investigated. Equilibrium data were analyzed by Langmuir, Freundlich and Dubinin-Radushkevich isotherms. The best agreement with experimental data was obtained by Langmuir isotherm (R2 = 0.987). The following kinetic models were applied: the pseudo-first order, pseudo-second order, and the intra-particle diffusion model. The pseudo-second order model is most suitable for describing the adsorption of ammonium ions onto the base activated bentonite from aqueous solutions (R2 = 0.997). Removal of ammonium ions by activated clay is a pH-dependent and endothermic process, the adsorption degree of pollutants is increased with rising temperature. The highest adsorption capacity of the base activated bentonite clay of 26.82 mg/g was obtained in a solution of pH 6 and a temperature of 40 °C, at an adsorbent dose of 2 g/dm3 and аt an adsorbate concentration of 100 mg/dm3. By activating the adsorbent in a 1 M NaOH solution, the adsorption ability of the bentonite clay towards ammonium ions is improved. The results of this study show that the base activated bentonite clay is a potential adsorbent material for ammonia, i.e. ammonium ions.