Adsorption du Cr (VI) sur la lignine activée

Abstract

La présente étude porte sur les potentialités de la lignine issue de la cuisson sulfate de l’alfa grass à éliminer le Cr (VI) des solutions aqueuses. Les variations du pH, de la concentration initiale du Cr (VI), de la température, du temps de contact, de la dose de l’adsorbant et leurs effets sur la capacité d’adsorption sont vérifiés expérimentalement. L’adsorption du Cr (VI) sur la lignine est maximale en milieu acide de pH = 2. Elle est optimale à une concentration initiale Cr (VI) de 180 mg•L-1, une dose de lignine de 4 g•L-1 et à un temps de contact maximal de 80 min. Les résultats de l’étude sont exploités à l’aide des équations de Langmuir, de Lagergren et d’Arrhenius afin de déterminer les divers paramètres d’équilibre tels que la capacité maximale d’adsorption, l’énergie d’adsorption, d’interaction et d’activation et les constantes d’équilibre adsorbat-adsorbant. L’application du modèle de Langmuir pour les données expérimentales d’isotherme d’adsorption a permis d’obtenir une capacité maximale d’adsorption de 75,8 mg•g-1 à 40 °C. Le paramètre sans dimension (RL) dont les valeurs sont situées entre 0,060 et 0,818 (0<RL<1) indique une adsorption du Cr (VI) par la lignine effective. Les paramètres thermodynamiques obtenus révèlent que l’adsorption est spontanée et endothermique. L’interaction de la lignine avec les ions chromate est rapide et sa cinétique suit l’équation de vitesse de sorption réversible de deuxième ordre de Lagergren dont les constantes de vitesse d’adsorption à l’équilibre augmentent en même temps que la température.Cette étude confirme que la lignine possède un potentiel d’adsorption important faisant d’elle un moyen efficace pour l’élimination du Cr (VI) des solutions aqueuses.Activated lignin with a surface area of 1023 m2•g-1 was prepared from sulfate lignin that was treated by 30% H2O2 and carbonized at 300°C, in order to test the adsorption of chromium (VI) from aqueous solution. The influence of contact time, pH, initial concentration of adsorbent and adsorbate, and temperature on the adsorption capacity were investigated. The maximum removal of Cr (VI) was found to be 92% at pH = 2 and with a contact time of 80 min. Optimal concentrations of lignin and Cr (VI) were found to be 4 g•L-1 and 180 mg•L-1, respectively. The adsorption kinetics were tested according to pseudo-first-order and pseudo-second-order equations. The analytical data fitted a pseudo-second-order equation well, and the rate of removal of chromium was found to increase with increasing temperature. The activation energy for the adsorption process was found to be 18.19 kJ•mol-1. The Langmuir and Freundlich adsorption isotherm models were applied to describe the isotherm and to obtain the isotherm constants for the adsorption of Cr (VI) on lignin. These constants and correlation coefficients of the isotherm models were calculated and compared. Results indicated that Cr (VI) uptake could be described by the Langmuir adsorption model. The maximum adsorption capacity (qm) of Cr (VI) on lignin was 75.8 mg•g-1 at a ...

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