Biosorption of Cd²⁺ from aqueous solution by Ca²⁺/Mg²⁺ type Citrus paradisi Macf. peel biosorbents

Grape fruit (Citrus paradisi Macf.) peel (GP) was used as raw material to prepare two novel biosorbents: CaGP (Ca²⁺ type) and MgGP (Mg²⁺ type). Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and N₂ adsorption-desorption isotherms were used to characterize prepared adsorbents. Cd²⁺ biosorption by CaGP, MgGP and GP was investigated systematically by studying the effects of pH, biosorption time and initial concentration on the biosorption of Cd²⁺. The results showed that biosorption efficiencies of Cd²⁺ on CaGP and MgGP increased with increase in pH, and the highest removal of Cd²⁺ was occurred at pH 6.0. Meanwhile, Cd uptake capacity increased with contact time, and could reach equilibrium within 20 min. The rates of biosorption of Cd²⁺ on three prepared biosorbents were found to best-fit pseudo-second-order equation. Experimental isotherms were well fitted by Langmuir and Freundlich isotherms models. Moreover, according to the Langmuir equation, the maximum biosorption capacities (q_m) of Cd²⁺ on CaGP and MgGP were found to be increased by 46.3% and 27.0%, respectively, compared with GP. The present study demonstrated that the waste grape fruit peel after simple Ca²⁺ or Mg²⁺ treatment could be used as a potential biosorbent for Cd²⁺, which indicated modified novel inactive/dead biological materials could remove Cd²⁺ from water.