Use of halloysite nanotubes to reduce ammonium concentration in water and wastewaters

Abstract

Halloysite nanotubes (HNTs) were used for the reduction of ammonium concentration in aqueous solutions under various experimental conditions. The results indicated that HNTs exhibit fast adsorption rates and high removal capacity for the NH4+ ions at neutral pH. The removal efficiency of the treated halloysite depends on the mass/volume ratio and the initial concentration of NH4+ ions. The adsorption capacity of the material was examined for different mass/volume ratios at initial concentration <40 mg/L and it was found that in some cases the material exhibit up to 30% removal efficiency. Halloysite clay removal efficiency was also examined in a 4–10 pH range and it was found that it is increasing continually upon increasing pH values. Additionally the adsorption efficiency is strongly and negatively related to temperature. Langmuir adsorption models were applied to describe the equilibrium isotherms. Estimation of thermodynamic parameters such as enthalpy (ΔH°), entropy (ΔS°) and change in free energy (ΔG°) confirms that the process is spontaneous and exothermic. Adsorption experiments were also performed in aqueous NH4+ solutions in the presence of other cations and it was found that the removal efficiency for NH4+ follows the order Na > K > Zn > Cu > Mn > Ca > Fe > Mg. The above results indicate that halloysite clay mineral can be considered as a new material for use in large and/or industrial-scale applications for water and wastewater purification purposes.