Adsorption Isotherms and Thermodynamics Study of Cd (II), Cr (III) and Cr (VI) binding by Natural Goethite and Synthetic Goethite Adsorbents

Abstract

Batch adsorption studies of some heavy metal ions; Cd (II), Cr (III) and Cr (VI) ions were conducted on a natural goethite (NGT) and compared same with a synthetic goethite (SGT). The influence of NGT particle size, initial adsorbate concentration, initial solution pH, adsorbent dosage and temperature on the adsorption efficiency was investigated using batch equilibrium assays. The adsorption data was fitted into three common isotherm models; Langmuir, Freundlich and Temkin models. Thermodynamic parameters such as ΔG, ΔH, ΔS and isosteric heat of adsorption (ΔHr) were estimated. Results of the investigation showed that, adsorption processes were dependent on the various equilibrium parameters investigated.The adsorption behavior was essentially the same, although, SGT performed more excellently than NGT. The result of isotherm modeling revealed the order of fittings;Langmuir > Temkin >Freundlich. The adsorption process was found to be feasible and spontaneous (ΔG = –ve).Also, ΔH andΔS for the processes are shown thus; Cd (II) and Cr (III) ions(ΔH = +ve, ΔS = -ve) and Cr (VI) ion (ΔH = +ve, ΔS =+ve). Additionally, the best fitting isotherm model, magnitudes of ΔH and ΔHr suggested a combined physisorption and chemisorptions mechanism.Therefore, combined results of the study could provide useful information on the adsorption characteristics of these heavy metal ions onto goethite surfaces.

Keywords: Adsorption, Natural goethite, Synthetic goethite, Isotherm, Thermodynamics