Isotherms for the adsorption of Cu(II) onto lignin - comparison of linear and non-linear methods

Equilibrium studies were carried out for the adsorption of Cu(II) onto Kraft lignin as an adsorbent. The experimental data were fitted to the Freundlich, Langmuir and Redlich-Peterson isotherms by linear and non-linear methods. Comparison of linear and non-linear regression methods was given in selecting the optimum isotherm for the experimental data. The coefficient of correlation r(2) and chi-square test, chi(2) were used to select the best linear theoretical isotherm. The best linear model is Redlich-Peterson isotherm model, where r(2) = 0.985 and chi(2) = 0.02. In order to predict the error ERRSQ, HYBRD, MPSD, ARE and EABS were used. Moreover, by minimizing these error functions the optimal values of parameters and also the optimum isotherm was found. The Redlich-Peterson isotherm was found to be the best representative for adsorption of Cu(II) on the adsorbent in the cases when ERRSQ, HYBRD and MPSD functions were used. Their coefficients of determination were 0.986, 0.985 and 0.984, respectively and chi-square was 0.02 in all cases. Freundlich isotherms obtained by minimization of the ERRSQ, HYBRD, MPSD, ARE and EABS function showed very good agreement with experimental data. In all cases the coefficients of determination were greater than 0.91. Besides, it was observed that non-linear isotherm models were better for representation of equilibrium data than linearized models.