Adsorption for phosphate by crosslinked/non-crosslinked-chitosan-Fe(III) complex sorbents: Characteristic and mechanism

Fe(III)-doped chitosan (CTS-Fe) and crosslinked Fe(III)-chitosan (CTS-Fe-CL) composites were developed for the removal of phosphorus from aqueous solution. The characteristics of adsorbents, performance on adsorption and pathway of phosphate transformation were investigated in batch studies. The SEM images and hardness tests showed that CTS-Fe-CL exhibited a three-layer structure with well-developed pore size, while CTS-Fe had more compact structure and stronger strength than CTS-Fe-CL. The low pH value was favorable for the adsorption of phosphate on the chitosan-Fe complex. Moreover, raw CTS-Fe showed better adsorption capacity than after crosslinking in a wide pH range. XPS and FTIR analyses demonstrated that under acidic conditions, the route of the adsorption process was the reaction between phosphate and functional groups (NH2 and NH3+) by electrostatic attraction and iron hydrolysates by ligand exchange, respectively. In addition, Fe(III) could release large quantities of hydrogen ions during the hydrolysis reaction process, which could enhance the protonation and electrostatic attraction under acidic conditions. Owing to the increase of challenging OH− ions, the electrostatic attraction weakened and ligand exchange was the dominant mechanism for phosphate adsorption under alkaline conditions. Moreover, the maximum phosphate adsorption capacity for CTS-Fe and CTS-Fe-CL was 15.7 and 10.2 mg-P g−1 at 303 K, respectively. The present study revealed the phosphate adsorption mechanism on Fe-modified chitosan under different pH conditions.