Adsorption Characteristics of Cr(III) Ions onto Coconut Husk from Aqueous Solution

Abstract

The adsorption of chromium(III) ions onto coconut ( Cocos nucifera) husk has been studied using radiotracer and batch techniques. Maximum adsorption (ca. 91%) of Cr(III) ions (2.59 × 10−5 M) onto the adsorbent surface from deionised water was achieved in 30 min when 100 mg coconut husk was employed. The Langmuir, Freundlich and Dubinin—Radushkevich (D—R) isotherms all provided an accurate fit of the adsorption data. The Langmuir constants, i.e. Q = 18.25 ± 0.55 μmol/g and b = (2.57 ± 0.15) x 104 l/mol, were computed. Similarly, application of the D—R isotherm led to an adsorption capacity of 0.25 ±0.03 mmol/g, a β value of −0.005352 ± 0.000360 kJ2/mol2 and an adsorption energy of 9.67 ± 0.33 kJ/mol. Use of the Freundlich isotherm allowed the constants 1/n = 0.85 ± 0.05 and A = 58.6 ± 36.8 mmol/g to be estimated. Studies of the variation of adsorption with temperature gave ΔH = 10.8 ± 0.8 kJ/mol, ΔS = 48.8 ± 2.7 J/(mol K) and ΔG = −4.6 ± 0.03 kJ/mol at 25°C. Addition of the divalent ions Ba, Co, Pb, Ni and sulphate to the aqueous solution led to an increase in adsorption whilst the presence of borate, carbonate and oxalate ions reduced the adsorption significantly. The use of Zr(IV), I(I), Se(IV) and Tc(VII) ions led to a low adsorption capacity. The results showed that coconut husk can be used to separate Cr(III) ions from all these various ions via a single or multistage operation.