Effect of Pretreatment on Ammonium Adsorption Properties of Sepiolite: Equilibrium, Kinetics and Thermodynamic Studies

Sepiolite is considered as a selective adsorbent to remove ammonium from wastewater. However, ammonium adsorption capacity under different pretreatment conditions has not been well documented. In this study, batch experiments were performed to investigate the ammonium adsorption capacity of low-strength wastewater by modified sepiolite. Sepiolite was conditioned with an acid, HCl, HNO3 or H2SO4, then thermally treated. Results showed that the modification process enhanced ammonium adsorption and sepiolite treated with 1N HNO3 followed by calcination at 400°C exhibited superior adsorption property. Adsorption capacity increased with contact time and initial concentration but decreased with increasing temperature and sepiolite dosage. A maximum adsorption capacity of 143 mg/g was achieved at pH 8 and a dosage of 1/70 g sepiolite/mL solution. In contrast, an increase of dosage from 1/70 to 1/10 improved the ammonium adsorption efficiency from 45 to 75%. Overall, the ammonium adsorption followed the pseudo-second-order kinetics. The equilibrium data were best fitted with Redlich Peterson isotherm. Through thermodynamic analysis, the ammonium adsorption was not spontaneous with sepiolite dosage of 1/70. However, the spontaneity of adsorption was feasible with ΔG° of −2.5 kJ/mol at 25°C and a dosage of 1/10. This study provided a potential pathway for ammonium removal during wastewater treatment.