Sorptive potential of sunflower stem for Cr(III) ions from aqueous solutions and its kinetic and thermodynamic profile.

The sorptive potential of sunflower stem (180-300mum) for Cr(III) ions has been investigated in detail. The maximum sorption (>/=85%) of Cr(III) ions (70.2muM) has been accomplished using 30mg of high density sunflower stem in 10min from 0.001M nitric and 0.0001M hydrochloric acid solutions. The accumulation of Cr(III) ions on the sorbent follows Dubinin-Radushkevich (D-R), Freundlich and Langmuir isotherms. The isotherm yields D-R saturation capacity X(m)=1.60+/-0.23mmolg(-1), beta=-0.00654+/-0.00017kJ(2)mol(-2), mean free energy E=8.74+/-0.12kJmol(-1), Freundlich sorption capacity K(F)=0.24+/-0.11molg(-1), 1/n=0.90+/-0.04 and of Langmuir constant K(L)=6800+/-600dm(3)mol(-1) and C(m)=120+/-18micromolg(-1). The variation of sorption with temperature (283-323K) gives DeltaH=-23.3+/-0.8kJmol(-1), DeltaS=-64.0+/-2.7Jmol(-1)K(-1) and DeltaG(298k)=-4.04+/-0.09kJmol(-1). The negative enthalpy and free energy envisage exothermic and spontaneous nature of sorption, respectively. Bisulphate, Fe(III), molybdate, citrate, Fe(II), Y(III) suppress the sorption significantly. The selectivity studies indicate that Cr(III), Eu(III) and Tb(III) ions can be separated from Tc(VII) and I(I). Sunflower stem can be used for the preconcentration and removal of Cr(III) ions from aqueous medium. This cheaper and novel sorbent has potential applications in analytical and environmental chemistry, in water decontamination, industrial waste treatment and in pollution abatement. A possible mechanism of biosorption of Cr(III) ions onto the sunflower stem has been proposed.