Kraft lignin as an adsorbent to remove heavy metal ions from water

Abstract

© 2014 by Nova Science Publishers, Inc. All rights reserved. There is increasing interest in the development of new, economically viable, applications of lignin, a waste product from the cellulose and paper industries. One of the promising applications for lignin is adsorption of heavy metal ions and other pollutants from water and waste water. The different functional groups in lignin provide binding sites for heavy metal ions. The absorptive capacity of lignin depends on its origin, type of metal ions and the adsorption conditions. The effectiveness of an adsorbent is estimated by its capacity, adsorption rate, mechanical strength, possibility of regeneration and reusability. Adsorption capacity is determined on the base of equilibrium experiments and adsorption isotherm. Equilibrium models often provide some insight into the adsorption mechanism, surface properties and affinity of the adsorbent. Several adsorption isotherm equations can explain solid-liquid adsorption systems. The required amount of an adsorbent for the treatment of some polluted water can be calculated from the adsorption isotherms. The more precise an isotherm describes the adsorption, the more accurate the calculated adsorbent dosage. Least squares refinement is the most widely used technique for determining the best fit adsorption isotherm. However, the non-linear regression method is the best way for selecting the optimum isotherm. In nonlinear regression all of the isotherm's parameters are fixed in the same axis, which does not alter the error distribution, unlike linear technique. Isotherm models for Cr(VI) adsorption were analyzed and compared using non-linear two-parameter equations of Freundlich, Langmuir, Temkin, Dubinin-Radushkevich and non-linear three-parameter equations of Redlich-Peterson, Sips and Toth. For Cr(VI) adsorption the best twoparameter isotherm are Langmuir and Dubinin-Radushkevich isotherms. Of the threeparameter isotherms the Redlich-Peterson isotherm is the best. Generally, Redlich- Peterson isotherm was found to be the best representative for Cr(VI) adsorption for Kraft lignin. To test Kraft lignin as an adsorbent for copper(II) ion, the experimental data were fitted to the Freundlich, Langmuir and Redlich-Peterson isotherms by linear and nonlinear methods. The coefficient of correlation and Chi-square test were used to select the best linear theoretical isotherm. The best linear model is Redlich-Peterson isotherm model as in the case of Cr(VI) adsorption. ERRSQ, HYBRD, MPSD, ARE and EABS functions were used to predict the error. Moreover, by minimizing these errors the optimal values for the parameters and the optimum isotherm models were found. The Redlich-Peterson isotherm was found to best describe adsorption of Cu(II) on the Kraft lignin when ERRSQ, HYBRD, MPSD functions were used. Further, lignin adsorption capacities for Cr(VI), Cu(II), Cd(II) and Zn(II) were compared. It was established that in terms of values of Langmuir maximum adsorption capacities, the adsorbed heavy metal ions followed the order: Cr(VI)>Cd(II)>Cu(II)>Zn(II). The results of variance analysis (ANOVA) for adsorption capacities in whole range of initial metal concentrations indicated that the adsorption of Cr(VI) ions on lignin very significantly deviates from other heavy metals investigated.