A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade

Novel modified Ni/Fe layered double hydroxides with different morphology of spherical - like shape were fabricated via using urea as a ligand and glycerol (Ni/Fe LDH/GL) with Ni:Fe molar ratios of 2:1 by the simplest co -precipitation method. Also, for comparison purposes, Ni/Fe LDH was synthesized to be used as a control one. A suggested interpretation for the morphology change was also given. The materials were characterized by X-ray diffraction (XRD), The Fourier transform infrared (FT - IR) spectroscopy, field emission scanning electron microscopy (FESEM), EDX for elemental analysis, high resolution transmission electron microscopy (HRTEM), Brunauer, Emmett, and Teller (BET) equation, particle size distributions and Zeta potential measurements. In addition, the synthesized materials were used as adsorbents for removal of potassium dichromate from aqueous solutions under various experimental conditions. The adsorption of Cr (VI) was strongly pH dependant and the pHPZC was studied. Kinetic studies were evaluated through different models including, pseudo first and second orders, mixed 1, 2 orders, intra particle diffusion and Avrami models. For adsorption isotherms, two-parameter models (Langmuir, Freundlich and Temkin) and three parameter models (Sips, Langmuir-Freundlich and Tooth) were investigated showing maximum adsorption capacity of 50.43 mg/g and 136.05 mg/g for Ni/Fe LDH and Ni/Fe LDH/GL, respectively. Also, the effect of temperature was investigated at (23, 35, 45, 55 °C) and the thermodynamic parameters (∆H°, ∆S° and ∆G°) were calculated showing exothermic and spontaneous adsorption process. The effect of coexisting anions (Cl-, SO42- and HPO42-) and humic acid at different concentrations on the removal efficiency of dichromate ions was investigated. Chemical stability and recyclability of these adsorbents were also studied. The intermolecular hydrogen bonds formation between dichromate ion, urea, glycerol, LDH was explored by Monte Carlo simulation This study suggested that the modified Ni/Fe LDH/GL materials were promising nanoadsorbents for efficient potassium dichromate removal.

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Novel synthesis of Ni/Fe layered double hydroxides using urea and glycerol and their enhanced adsorption behavior for Cr(VI) removal

Hexavalent chromium removal from aqueous solution by adsorbents synthesized from groundwater treatment residuals

In this study, locally available biomasses obtained from avocado kernel seeds (AKS), Juniperus procera sawdust (JPS) and papaya peels (PP) were investigated as adsorbents for the removal of Cr(VI). The study employed a batch method to investigate the efficiency of the adsorbents. Parameters that influence adsorption such as: pH, adsorbent dose, initial concentration of Cr(VI), contact time and temperature have been studied. Optimum parameters for the adsorption of 5mg L –1 Cr(VI) were: pH: 1, adsorbent dose: 0.5 g, contact time: 160 min and temperature: 313 K. The equilibrium data was best fitted to the Freundlich adsorption isotherm model. The results of kinetic models showed that the pseudo-second-order kinetic model was found to correlate well to the experimental data. The results of thermodynamic parameters showed that the adsorption process is feasible. The positive values of the entropy change suggest increased randomness at solid-liquid interfaces during the adsorption. The efficiencies of the adsorbents towards the removal of chromium from real samples were also examined and were found to remove chromium from waste water. KEYWORDS  Adsorption, chromium (VI), avocado kernel seed, papaya peel, Juniperus procera sawdust, local adsorbent, adsorption kinetic, Thermodynamic of adsorption.

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Kinetic and Thermodynamic Studies of the Adsorption of Cr(VI) onto Some Selected Local Adsorbents

Magnetic adsorption separation (MAS) process: An alternative method of extracting Cr(VI) from aqueous solution using polypyrrole coated Fe3O4 nanocomposites

In this study, the adsorption potential of activate d carbon and bentonite for removal of Cr(VI) ions f rom wastewater has been investigated. The study involves batch type ex periments to investigate the effects of initial con centration, adsorbent dose, agitation speed, contact time, temperature an d pH of solution on adsorption process and the eval uation of optimum conditions. The adsorption process has fit pseudo-second order kinetic models. Langmuir and Freundlich adsorption isotherm models were applied to analyze adsorption data, and both were found to be applicable to this adsorption process. Thermodynamic parameters, e.g., ∆G ◦ , ∆S ◦ and ∆H ◦ of the on-going adsorption process have also been calculated and found the sorption process as endothermic. Finally, it can be seen that bentonite was found to be more effective for the removal of Cr(VI) than activated carbon, under the same experi mental conditions.

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Manal Ahmed

Papers

Adsorption Studies on the Removal of Hexavalent Chromium-Contaminated Wastewater using Activated Carbon and Bentonite