Competitive Effect of Zinc and Cadmium on the Biosorption of Chromium by Orange Waste.

Batch experiments were conducted to test orange waste (OW), an agricultural solid waste byproduct from the orange juice manufacturing industry, as adsorbent for binary solutions of Cd²⁺-Cr³⁺ and Zn²⁺-Cr³⁺. Fourier transform infrared spectroscopy (FTIR) and the point of zero charge (pH_pzc) were used to identify the functional groups on the OW surface involved in biosorption. The biosorption equilibrium data for both binary-metal solutions were obtained and fitted to various isotherm models. The extended Sips and the non-modified Redlich-Peterson isotherm models gave the best fit for the experimental data. According to the extended Sips model, the maximum biosorption capacity of OW was 0.573 mmol·g⁻¹ for Cd²⁺, 0.453 mmol·g⁻¹ for Zn²⁺, and 1.96 mmol·g⁻¹ for Cr³⁺. The sorption capacity dropped to 0.061 mmol·g⁻¹ for Cd²⁺ and to 0.101 mmol·g¹ for Zn²⁺ in their binary systems with Cr³⁺ for the higher initial metal concentrations in the solution. However, the maximum sorption capacity of chromium was only slightly affected by the presence of Cd²⁺ or Zn²⁺. For both binary systems, the presence of a second metal ion in the solution always conduces to a reduction in the sorption of the other metal in the solution. The presence of Cr³⁺ decreased the sorption of Cd²⁺ and Zn²⁺ more than vice versa. Conclusively, effective removal of Cr³⁺ ions from an aqueous solution can still be achieved in the presence of Cd²⁺ or Zn²⁺. [ABSTRACT FROM AUTHOR]