Sorption profile of Cd(II) ions onto coconut husk

Abstract

The removal of Cd(II) ions by coconut (Cocos nucifera) husk has been studied extensively optimising all parameters controlling their accumulation on solid surface. Maximum sorption (84%) of Cd(II) ions (2.29×10 -5 M) onto sorbent surface is achieved from deionized water in 20 min. using 100 mg of coconut husk/4.5 cm3. The accumulation of Cd(II) ions monitored over a ∼ 400-fold concentration (2.30×10 -6-9.18×10-4 M) followed Freundlich and Dubinin-Radushkevich (D-R) isotherms. The Freundlich constants l/n = 0.92±0.4 and Cm = 52±22 mmole g-1 and D-R parameters β = - 0.0068±0,0002 kJ2 mole-2, Xm= 35.1±6.8 mmole g-1 and E = 8.5±0.1 k J mole-1 have been evaluated. The variation of temperature in the range of 278-323 K under optimized conditions yielded thermodynamic entities ΔH = 18.1±0.6 k J mole-1, ΔS = 74±2 J mole-1 K-1 and ΔG298K = - 3.8 ± 0.04 k J mole-1. The positive enthalpy and negative free energy reflect the endothermic and spontaneous nature of sorption respectively. Among the ions tested EDTA, borate, oxalate and Eu(III) increased the sorption, while ascorbate, Ce(III) and Sb(III) suppressed the sorption. The investigation regarding selectivity of coconut husk reveals that Cd(II) ions can be separated from Zn(II), Se(IV), Tc(VII) and I(I). This study indicates the potential of coconut husk for the removal of Cd(II) ions from aqueous solutions/industrial effluents.