Chemically Modified Chitosan Bio-Sorbents for the Competitive Complexation of Heavy Metals Ions: A Potential Model for the Treatment of Wastewaters and Industrial Spills

Heavy metal pollution of water sources has become one of the most serious environmental and health problems nowadays. Chitosan (CHI) and derivatives can be used in the complexation and adsorption of heavy metals in water. Hence, this study compared the influence of CHI and carboxymethyl chitosan (CMC) ligands on the competitive complexation between toxic heavy metal ions with opposite charges Cr(VI) and Cd(II) at different pH values as an alternative and ecologically sustainable proposal for industrial spills of heavy metals into aquatic environments. The properties of the synthesized products and CHI were extensively characterized by several spectroscopy techniques, the competitive kinetics of complexation of metal ions with CHI and two type of CMC (with methanol, CMC 40_ME, and with ethanol, CMC 60_ET) was evaluated by wavelength dispersive X-ray fluorescence spectroscopy (WDXRF). The results clearly indicated the influence of the pH and ligands on the competitive complexation of the cationic or anionic ions. The results demonstrated that O -carboxymethylation of chitosan has occurred with a degree of functionalization of (1.20 ± 0.02) and (0.88 ± 0.02) for CMC 40_ME and CMC 60_ET, respectively, leading to the formation of CMC soluble in alkaline medium (pH range of 3.5 ≤ pH ≤ 6.5 and 4.0 ≤ pH ≤ 8.0 for CMC 40_ME and CMC 60_ET, respectively). In alkaline media, complexation of CMC 40_ME is approximately 10% higher (147.2 ± 0.9 mg g −1 ) than CMC 60_ET (99.8 ± 2.9 mg g −1 ) with Cd 2+ ions at pH 8.5, and approximately 21% lower [(95 ± 2 mg g −1 ) and (96.3 ± 2.8 mg g −1 ) for CMC 40_ME and CMC 60_ET, respectively] than CHI (121 ± 6 mg g −1 ) for Cr x O y z− ions at pH 3.0. The kinetic analysis showed variations for each ion and a significant difference regarding the complexant towards the negatively charged ions. The CMC 40_ME, in any analyzed pH, the complexation occurred during the first 45 min of the process. These results showed that CMC, as polydentate functional ligand, was more efficient than CHI especially for the complexation of cations in basic media. Therefore, these systems appear to be attractive alternatives for the containment of industrial spills of heavy metals in wastewater.