Biosorption of Cd from aqueous solutions by shrimp shell

Nowadays, extensive attention has been paid to removal and remediation of heavy metals from polluted sites. Biosorption is a cost-effective method for removing heavy metals from aqueous solutions. In this study, biosorption of Cd has been examined by shrimp shell.
The adsorption of Cd on shrimp shell was studied as a function of pH (Cd concentration: 8.0 mg L-1) in the range of 3.0-8.0, and as a function of metal concentration (Cd concentration in the range of 15-100.0 mg L-1) using a 24h batch equilibration experiments. The amounts of Cd sorbed on the adsorbent were calculated from the difference between the metal concentration in the blank and equilibrium concentration of Cd in the solutions. Chemical species of Cd(II) in the solutions was also predicted using Visual MINTEQ, a computer code developed to simulate equilibrium processes in aqueous systems. Infrared spectrums of untreated and Cd-loaded adsorbent were obtained using a Fourier transform infrared spectrometer.
The results of pH dependent experiments showed that the percent removal of Cd on the sorbent increased with the increase of suspension pH. The experimental data showed that shrimp shell used in this study removed more than 95% Cd from solution at pH=7.5. The Freundlich and Langmuir isotherms were described satisfactorily equilibrium data. Maximum sorption capacity (qmax) of the sorbent was 5.1 mg g-1. Pseudo second order kinetic model provided a perfect fit for the kinetic data of Cd sorption onto shrimp shell. Biosorption kinetics of Cd consisted of two steps: An initial rapid step where adsorption was very fast. The second phase is the gradual biosorption stage before the Cd uptake reached equilibrium. The equilibrium was achieved in 30 min for initial metal concentration of 5 mg L-1 and in 240 min for initial metal concentration of 50 mg L-1 where 91 % and 92 % of Cd were adsorbed, respectively. Based on the estimates obtained by Visual MINTEQ, the Cd2 and Cd(OH) species were the dominant species present in solutions in pH ≤ 8.5 and pH≥ 8.5, respectively. Infrared spectrum analysis of adsorbent before and after adsorption showed that the N atoms of the amine functional groups played a major role in shrimp shell for adsorption of Cd.
Based on the equilibrium sorption and kinetic information obtained for Cd removal by this sorbent, it may be concluded that shrimp shell is an effective sorbent for Cd and may be considered for environmental remediation in future studies.