Removal of Traces of Silver Ions from Aqueous Solutions Using Coconut Husk as a Sorbent

Abstract

Sorption of silver ions onto coconut (Cocos nucifera) husk has been studied in detail using radiotracer technique. Maximum sorption (98%) of Ag ions (8.23 × 10⁻⁴ M) onto sorbent surface is achieved from 4.5 mL of 0.1 M HNO₃ solution within 20 min using 200 mg of coconut husk. The sorption data follow the Freundlich and Dubinin‐Radushkevich (D‐R) isotherms. The values of Freundlich characteristic constants 1/n = 0.98 ± 0.09 and of A_m = 0.35 ± 0.05 m mole g⁻¹ and D‐R constants β = − 0.006207 ± 0.000439 kJ² mole⁻², X_m = 0.57 ± 0.08 m mole g⁻¹, and E = 9.0 ± 0.3 kJ mole⁻¹ have been computed. The variation of sorption with temperature yields thermodynamic parameters ΔH = 61.3 ± 5.4 k J mole⁻¹, ΔS = 241.9 ± 18.7 J mol⁻¹ K⁻¹, and ΔG = − 10.6 ± 0.09 k J mole⁻¹ at 298 K. The positive enthalpy and negative value of free energy reflect the endothermic and spontaneous nature of sorption respectively. Sulphate, Al(III), Ni(II), and Ba(II) enhance the sorption significantly whereas Ce(III) and citrate cause its suppression appreciably. To check the selectivity of the coconut husk, the sorption of a number of metal ions on the sorbent surface has been measured under optimized conditions. The results indicate that coconut husk can be used to separate Ag ions from Cs(I), Zn(II), Cr(III), Co(II), and Se(IV).

Keywords:

• Ag(I) ions
• Sorption
• Radiotracer technique
• Thermodynamics
• Sorption isotherms
• Coconut husk

Acknowledgment

The technical assistance of Mr. M.H. Chaudhry is gratefully acknowledged.