Chitosan selectivity for removing cadmium (II), copper (II), and lead (II) from aqueous phase: pH and organic matter effect

The aim of this study was to investigate the selectivity of chitosan for cadmium, copper and lead in the presence and absence of natural organic matter (NOM) in different pH solutions. Adsorption isotherms of one and three adsorbates at initial concentration of 5–100 mg/L were carried out in batch reactors at pH 4, 5, or 7 and 25 °C in reactive and clarified water. The chitosan employed had a MW of 107.8 × 10 3 g/mol and degree of acetylation (DA) of 33.7%. The chitosan adsorption capacity at pH 4 in reactive water was 0.036, 0.016, 0.010 mmol/g for Pb 2+, Cd 2+, and Cu 2+, respectively, and it decreased for Pb 2+ and Cd 2+ in clarified water. Conversely, experiments carried out in clarified water showed that the cadmium adsorption capacity of chitosan was enhanced about three times by the presence of NOM at pH 7: an adsorption mechanism was proposed. Furthermore, it was found that the biosorbent selectivity, in both reactive and clarified water at pH 4, was as follows Cu 2+ > Cd 2+ > Pb 2+. Finally, the preliminary desorption experiments of Cd 2+ conducted at pH 2 and 3 reported 68 and 44.8% of metal desorbed, which indicated that the adsorption mechanism occurred by electrostatic interactions and covalent bonds.