Equilibrium studies on the adsorption of Co(II) and Ni(II) by modified silica gels: One-component and binary systems

The BiLangmuir and Redlich–Peterson models as well as their extended equations were tested for the modeling of one- and two-component adsorption equilibrium of Co(II) and Ni(II) ions on EDTA- and DTPA-modified silica gel. An approach consisting in initialization of parameter values and non-linear parameter estimation by minimization of two objective functions (MPSD and ERRSQ) was proposed to assure that equilibrium parameters obtained related to the global minimum. A usage of the experimentally obtained value of maximum adsorption capacity as an initial guess made an iteration procedure well-posed and independent of the Ki guess values. None of the parameters obtained for one-component systems satisfactorily helped in prediction of the two-component adsorption equilibrium. The application of the extended BiLangmuir model identified the coexistence of low- and high-energy active sites on EDTA/DTPA-silica gel that showed an unequal ability for Co(II) and Ni(II) adsorption.

• EDTA- and DTPA-silica gel were good adsorbents for Co(II) and Ni(II).
• Affinity of the surface groups toward Ni(II) was clearly higher than toward Co(II).
• BiLangmuir model and its extended form described the adsorption data well.
• Initial guesses, error function, and the amount of fixed parameters affected the simulations.
• Parameters from one-component modeling could not be used to predict the two-component behavior.