Modelling synergistic sorption of Cr(VI), Cu(II) and Ni(II) onto exhausted coffee wastes from binary mixtures Cr(VI)-Cu(II) and Cr(VI)-Ni(II)

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Simultaneous sorption of Cr(VI), Cu(II) and Ni(II) onto exhausted coffee waste from binary mixtures was investigated. Equilibrium studies were carried out in binary mixtures in which Cr(VI) and the second metal ion concentration was varied between 0.5 and 12. mM and 0.5 and 8. mM, respectively. Equilibrium isotherms for each metal of the binary mixtures were determined by fixing the initial concentration of the second metal ion. Results showed that both components of the binary mixture exert a synergistic effect on each other. The best fit for Cr(VI) experimental data was provided by the Sips isotherm model while the Langmuir isotherm was the most effective to fit Cu(II) and Ni(II) experimental data. A multi-component model based on Sips and Langmuir isotherms models has been developed for describing the cooperative effect between the components of Cr(VI)-Cu(II) and Cr(VI)-Ni(II) binary mixtures. The proposed model fits successfully the experimental data. The relationships between variables of the sorption process determined in this work allow calculating both Cr(VI) and Cr(III) concentration at equilibrium and estimating a break concentration from which Cr(VI) appears in solution. The present work could be helpful for the design and prediction of sorption processes performance especially when chromium is forming a binary mixture with metal cations and its sorption is in part or totally based on its reduction to the trivalent form ​
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