Nickel(II)-modified chitosan hydrogel: A hybrid sorbent for removal of phosphate anions from water

A novel nickel-modified chitosan sorbent (CsNi), highly efficient in phosphates removal from aqueous solutions, was proposed. Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and point of zero charges of sorbent were employed to determine its structure and properties. Batch sorption experiments allowed to identify the CsNi high phosphate sorption capacity, exceeding 62.5 mg/g (with the removal efficiency of 73 % - for optimal experimental conditions: pH 7, sorbent dose 10 g/dm³, temperature 293 K, contact time 1440 min). The equilibrium data for the new CsNi sorbent and aqueous solution of phosphates fitted well with the Freundlich, Redlich-Peterson, Hill, Sips, Toth, and Khan sorption isotherms (R² = 0.988-0.992), suggesting sorbate distributions other than a creation of monolayer on the heterogeneous sorption surface. The phosphates sorption on the new sorbent resulted from the electrostatic attraction between the negatively charged H₂PO₄^- or HPO₄^2- anions and the positively charged CsNi surface, formation of stable complexes between these anions and nickel(II) species, and the anions direct sorption by interacting with the free and active chitosan amino groups through hydrogen bonding (-NH₃⁺ --- ^-OPO(OH)₂). The results highlighted the promising potential of the CsNi hydrogel for phosphate anions removal from contaminated water.