The potential of cost-effective coconut husk for the removal of toxic metal ions for environmental protection

Coconut (Cocos nucifera) husk, an agricultural waste, has been thoroughly investigated for the removal of toxic Cd(II), Cr(III) and Hg(II) ions from aqueous media. The parameters like nature and composition of electrolyte, concentration of toxic ions, dosage of coconut husk, and equilibration time between the two phases were optimized for their maximum accumulation onto the solid surface. The effect of common ions on the uptake of metal ions has been monitored under optimal conditions. The variation of retention of each metal ion with temperature was used to compute the thermodynamic quantities ΔH, ΔS and ΔG. The values 18.1±0.6 kJ mol−1, 74±2 J mol−1 K−1, and −3.8±0.04 kJ mol−1 at 298 K; 10.8±0.8 kJ mol−1, 48.8±2.7 J mol−1 K−1, and −4.6±0.3 kJ mol−1 at 298 K; and −37.4±2 kJ mol−1, 105±7 J mol−1 K−1 and −2.58±0.5 kJ mol−1 at 298 K were obtained for Cd(II), Cr(III) and Hg(II) ions, respectively. The sorption data were analysed by applying different sorption isotherms. The sorption capacity and energy were evaluated for each metal ion. The values of the Freundlich constants 1/n and Cm were 0.92±0.04 and 52.6±22.2 m mol g−1; 0.85±0.05 and 56.0±0.03 m mol g−1; and 0.88±0.03 and 6.84±0.45 m mol g−1 for Cd(II) Cr(III) and Hg(II) ions, respectively. Similarly, the Dubinin–Radushkevich (D–R) constants β, Xm, and E were evaluated for the three metal ions. To check the selectivity of the sorbent, sorption of a number of elements was measured under similar conditions. Separation of Zn(II) from Cd(II); Cr(III) from I(I), Zr(IV), Se(IV), and Hg(II) from Se(IV) and Zn(II) can be achieved using this sorbent. This cheap material has potential applications in analytical chemistry, water decontamination, industrial effluent treatment and in pollution abatement.