Rubidium and cesium ion adsorption by an ammonium molybdophosphate–calcium alginate composite adsorbent

A composite spherical adsorbent was prepared with ammonium molybdophosphate (AMP), sodium alginate (NaALG), and calcium chloride. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the composite adsorbent. The adsorption of rubidium and cesium ions onto the composite adsorbent in aqueous solutions was investigated comprehensively by varying the initial metal ion concentration, pH, ionic strength, and temperature. The adsorption kinetics of both rubidium and cesium was described by the first-order and second-order kinetic models. The second-order rate constant and the initial adsorption rate increase with increasing temperature. In general, the equilibrium adsorption amount of both rubidium and cesium increases with the increase in initial metal ion concentration, but decreases with increasing ionic strength and temperature. Maximum adsorption of rubidium and cesium occurs in the solution with an equilibrium pH value of 3.5–4.5. Under similar conditions, cesium shows a higher adsorption amount than rubidium. The composite adsorbent is easy to prepare and highly porous. It has a fast adsorption rate and an adsorption capacity of 0.58 and 0.69 mmol g−1 for rubidium and cesium, respectively. The composite adsorbent is effective for the adsorption of rubidium or cesium ions from solutions containing some other alkali metal ions, such as sodium ions.