Sulphate-modified chitosan hydrogel beads a recyclable bio-adsorbent for Cu(II): Experimental and theoretical studies...

This study was designed to develop a chemically modified chitosan-based sustainable adsorption system for soluble metal pollutants. Here, chitosan hydrogel beads modified with sulphate-sulphuric acid presented a high adsorption capacity toward copper ions and higher reusability due to enhanced low pH tolerance. Though the adsorption process was independent of incubation temperature, it varied with the pH of solution. The equilibrium adsorption data fitted well with Langmuir isotherm, and one gram sulphate-sulphuric acid-modified chitosan hydrogel beads (SSmChHgBs) adsorbed 80.3 ± 0.5 mg copper(II) ions. Kinetic study revealed the completion of >98 % adsorption within the first 120 min. The experimental data ascertained the multistep rate-controlling processes. The initial sharp adsorption step was controlled mainly by the pseudo-second order model followed by a slow intraparticle diffusion-controlled step to reach the equilibrium. FTIR, XPS, and EDAX spectra revealed the binding of copper ions with amine and hydroxyl groups at C-3 position of the glucosamine unit of chitosan in a pendant fashion by displacing the sulphate-containing moiety of SSmChHgBs. EPR spectra and DFT study indicated the formation of 1:1 chitosan‑copper complex with square planner geometry. The desorption of copper from the loaded beads was achieved by lowering the pH of the solution to 1.0. The SSmChHgBs could be reused more than six times for the removal of Cu (II) from water without much loss of its efficacy.