Towards an in-depth experimental and theoretical understanding of the cadmium uptake mechanism on a synthesized chitin biopolymer

This work analyzes the adsorption of cadmium, a potentially toxic pollutant, on biopolymers synthesized from shrimp shells, as a promising new adsorbent. Spectroscopic analysis, such as FTIR, ATD/ATG, XRD, and SEM/ EDX techniques, were used to characterize chitin before it was exposed to cadmium ions. Experimental data indicate that Cd(II) adsorption proceeds with pseudo-second-order model kinetics and is influenced by increasing temperature. In the equilibrium, the maximum adsorption capacity obtained was 58.82 mg g-1 at a temperature of 308 K. Experiments to obtain adsorption data were performed at T = 298-318 K. The saturated adsorption capacities of Cd(II) range from 57.40 to 59.90 mg g-1. The application of the physics-statistics model indicates that the Cd(II) atoms are adsorbed on the surface of the chitosan forming a monolayer. In addition to that, the results also show that the adsorption affinity increases with the system temperature, resulting in higher affinity and an increase of 30% in the adsorption capacity. The adsorption energy was found to be around 2.5 kJ mol-1, indicating that the adsorption is physical and endothermic. The entropy was found to quickly increases (reaching a maximum value of 6 x 10-22 kJ mol- 1 K-1) at low concentrations followed by the equilibrium after the 40 mg L-1, indicating that the equilibrium is quickly reached. The Gibbs free energy indicates that the process is spontaneous (ranging from -1.364 to -1.451 kJ mol-1) and that the energy tends to remain constant after 15 mg L-1. Last, the results indicate that the Cd(II) is adsorbed due to dipole-dipole interactions and possible coordination bounds. The results of tests on the adsorption of cadmium by chitin showed that this biopolymer could replace other more expensive adsorbents.