Investigations on the mechanism, kinetics and isotherms of ammonium and humic acid co-adsorption at low temperature by 4A-molecular sieves modified from attapulgite

Abstract

Attapulgite (ATP) is a type of natural magnesium aluminum silicate mineral and has been applied as an adsorbent to remove organic pollutants and heavy metals in aqueous solution. This study investigated the 4A-molecular sieve modified from ATP on simultaneous adsorption of ammonium and humic acid (HA). The properties of raw and alkali treated ATP were compared such as surface morphologies, crystallinity phase and removal efficiency. The influences of 4A-molecular sieve dosage, pH values, mixing speed, initial contaminants' concentration, solution temperature and adsorption time on removal of ammonium and HA were thoroughly evaluated. The results indicated that the removal efficiency of ammonium at the low temperature of 5 °C increased dramatically using alkali modified ATP comparing with that using the raw ATP. The optimum 4A-molecular sieve dose was 1 g L⁻¹ and the maximum adsorption of ammonium was obtained at natural water's pH, while the maximum HA adsorption was obtained at around pH 4. The adsorption isotherms and kinetics for adsorption of ammonium and HA were also studied to obtain the adsorption mechanisms. It was found that the temperature did not have a significant influence on the removal of ammonium and HA. The ammonia has likely been removed from water through electrostatic adsorption and ion-exchange whereas the humic acid might be eliminated via a mechanism of electrostatic adsorption and cation-bridge effect. These results revealed that 4A-molecular sieves modified from ATP have a significant potential for applications of ammonium removal at low temperature.