Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II)

The traditional treatment for disused adsorbents is usually recovery for recycling or direct discarding. In this paper, a more efficient and economical method is introduced to treat disused adsorbents. Chitosan hydrogel beads (CS) are proven to be effective biosorbents for the removal of many heavy metal ions from aqueous solutions. The CS after the saturated adsorption of metal ions are usually recovered in dilute acidic solutions. However, in this work, the disused CS after the removal of copper(II) (Cu(II)) were not recovered but were directly applied to adsorb phosphate from the aqueous solutions. The CS after the adsorption of Cu(II) were stable and suitable for phosphate adsorption within a wide pH range. The results of pH effects on the adsorption capacities of phosphate indicated that the maximum was achieved at around pH 5.0 near 28.86 mgP/g. In the various forms of phosphates, the Cu(II)-loaded CS preferred to adsorb dihydrogen phosphate. Further adsorption equilibrium and kinetics study showed that the adsorption behaviors were mainly ascribed to chemical adsorption. The adsorption isotherm was well described by the Langmuir equation. The adsorption kinetics followed both pseudo first-order and second-order models. The effects of electrolytes on the adsorption of phosphate indicated that the additional NaCl and Na2SO4 both slightly decreased the adsorption capacities for the anions’ competition of adsorption active sites.