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.
2013 Elsevier