Abstract
The aim of this study is to report batch adsorption results of Cd (II) and Cr (VI) onto Algerian bentonite. The equilibrium adsorption process was achieved at about 120 min contact time. The kinetic and isotherms aspects of Cr (VI) and Cd (II) adsorption were investigated by varying operation parameters such as pH (0.5 - 6), initial Cd (II) and Cr (VI) concentrations (50 - 200 mg/l) and temperature (293.15 K - 343.15 K). The optimum conditions obtained were: pH = 6.0 and 4.0 for Cd (II) and Cr (VI) respectively, initial concentrations of Cd (II) and Cr (VI) = 50 mg/L and T = 293.15 K. The kinetic of the adsorption process was studied by application of the most important kinetic models namely the pseudo-first order, the pseudo-second order and Elovich equations. The results showed that the pseudo-second order model fitted well the adsorption data of Cr (VI) whereas, and the Cd (II) adsorption data fitted best the Elovich equation. The equilibrium data fitted best the Langmuir isotherm, and the maximum adsorption capacity was determined through this model and was found to be 13.17 and 12.61 mg/g for Cd (II) and Cr (VI) respectively. The temperature had a reverse effect on the Cd (II) and Cr (VI) adsorption; our results showed that the removal efficiency increased to82.4% and 55.70% for Cd (II) and Cr (VI)when the temperature decreasedto 293.15 K.The negative figures of the Gibbs free energy ΔG°ads values range from －15.23 kJ/mol to －14.37 kJ/mol for Cd (II) and from －9.70 kJ/mol to －9.64 kJ/mol for Cr(VI) at 293.15 K to 343.15 K showed that the adsorption process is spontaneous and favourable. These results confirmed the ability of the low-cost Algerian natural bentonite to efficiently and competitively adsorb the two toxic elements investigated: cadmium (II) and chromium (VI).
Highlights
Many heavy metals have toxic effects at high concentration [1] [2]
The most common oxidation states of chromium are trivalent Cr (III) and hexavalent Cr (VI)) which are invariably found in various industrial processes
We focused on the adsorption process, because it is inexpensive, widely applicable and has been used in the water industry for the removal of colour, odour and organic matters [13]-[17]
Summary
Many heavy metals have toxic effects at high concentration [1] [2]. Some of them, at trace level and under a specific valence play an important role as micronutrients in organismsThe presence of heavy metals such as cadmium and chromium in industrial wastewater and effluents has been a great concern due to their high toxicity and adverse accumulation characteristics [3] [4].The most common oxidation states of chromium are trivalent Cr (III) and hexavalent Cr (VI)) which are invariably found in various industrial processes. Chromium (VI) compounds are much more toxic than Chromium (III) ones [5] [6] The former are reported to be a powerful carcinogen capable of modifying the deoxyribonucleic (DNA) transcription process in both animals and humans which result in important chromosome aberrations [7]. It is, essential to remove Cr (VI) from wastewaters of electroplating, cement, leather tanning and paint industries which may contain up to hundreds of mg/L of chromium though the tolerance limit of Cr (VI) for discharge into inland surface waters is only 0.1 mg/L and in waste water at the level of 0.05 mg/l [8] [9]
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