Characterization of Citric Acid-Modified Clam Shells and Application for Aqueous Lead (II) Removal

Due to the presence of chitin (a type of animal polysaccharides), clam shells (CS) exhibited a promising potential as the adsorbent for lead or Pb removal especially after surface modification by citric acid. Both porosity and surface areas of the citric acid-modified clam shell (CACS) were significantly increased as compared to the untreated CS. Fourier transform infrared (FTIR) spectra indicated that carboxyl groups were increased on the surface of CACS. Zeta potential and X-ray photoelectron spectroscopy (XPS) analysis showed that electrostatic interaction and ion exchange were the main adsorption mechanism. The adsorption capacity of CACS was enhanced by 34.9–61.7 % compared to that of CS. The Pb (II) adsorption capacity was increased from 4.848 to 4.929 mg Pb·g⁻¹ with a temperature increase from 10 to 50 °C, respectively, suggesting that the adsorption process was endothermic. The Pb (II) adsorption kinetics on CACS well fitted the pseudo-second-order model, while the adsorption isotherm for Pb (II) could be described by the Langmuir equation. The presented work shed new insight into the application of natural materials as low-cost and effective absorbents to remove aqueous heavy metals such as Pb (II).