Potential of Rice Husk for the Decontamination of Silver Ions from Aqueous Media

Abstract

The sorption behavior of silver ions on rice husk has been investigated in detail. Various physico-chemical parameters were optimized to simulate the best conditions in which this material can be used as an adsorbent. Maximum adsorption was observed at 0.001 mol L⁻¹ of acid solutions (HNO₃, H₂SO₄ and HClO₄) using 0.5 g of adsorbent for 9.27 × 10⁻⁵ mol L⁻¹ silver concentration in fifteen minutes equilibration time. The adsorption of silver was decreased with the increase in the concentrations of all the acids used. The kinetic data indicated an intraparticle diffusion process with sorption being pseudo-second order. The determined rate constant k₂ was 14.707 ± 1.832 mol g⁻¹ min⁻¹. The adsorption data obeyed the Freundlich, Langmuir, and Dubinin-Radushkevich isotherms over the silver concentration range of 1.85 × 10⁻⁴ to 1.16 × 10⁻³ mol L⁻¹. The characteristic Freundlich constants, that is, 1/n = 0.38 ± 0.033 and K = 0.271 ± 0.104 m mol g⁻¹ whereas the Langmuir constants Q = (1.504 ± 0.054) × 10⁻² m mol g⁻¹ and b = (16.582 ± 2.227) × 10³ dm³ mol⁻¹ have been computed for the sorption system. The sorption mean free energy from the Dubinin-Radushkevich isotherm is 12.16 ± 0.82 kJ mol⁻¹ indicating ion-exchange mechanism of chemisorption. The uptake of silver increases with the rise in temperature (283–333 K). Thermodynamic quantities, namely, ΔG, ΔS, and ΔH have also been calculated for the system. The sorption process was found to be endothermic. The effect of other cations and anions on the adsorption of silver has also been studied.

Keywords:

• kinetics
• rice husk
• silver adsorption
• sorption isotherms
• thermodynamics

ACKNOWLEDGEMENTS

Shagufta Zafar is thankful to the Higher Education Commission, Islamabad, Pakistan, for financial support during the present work.

Notes

*Concentration expressed in ng g⁻¹.

**Determined by AAS.

Experimentally measured q_e = 0.166 mg g⁻¹.