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

Abstract

The sorptive potential of sunflower stem (180–300 μm) for Cr(III) ions has been investigated in detail. The maximum sorption (≥85%) of Cr(III) ions (70.2 μM) has been accomplished using 30 mg of high density sunflower stem in 10 min from 0.001 M nitric and 0.0001 M 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 ± 0.23 mmol g −1, β = −0.00654 ± 0.00017 kJ 2 mol −2, mean free energy E = 8.74 ± 0.12 kJ mol −1, Freundlich sorption capacity K F = 0.24 ± 0.11 mol g −1, 1/ n = 0.90 ± 0.04 and of Langmuir constant K L = 6800 ± 600 dm 3 mol −1 and C m = 120 ± 18 μmol g −1. The variation of sorption with temperature (283–323 K) gives Δ H = −23.3 ± 0.8 kJ mol −1, Δ S = −64.0 ± 2.7 J mol −1 K −1 and Δ G 298k = −4.04 ± 0.09 kJ mol −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.