Hypothesis: Lignin-based adsorbents are one of the common materials for removal of heavy metal ions in wastewater treatment and the adsorption of metals extracted in mines. Therefore, it seems that industrial lignin and its carboxymethylated derivative are able to adsorb zinc and copper cations from aqueous solutions under different operating conditions.
Methods: Firs, industrial lignin was carboxymethylated by reacting with sodium chloroacetate. After carboxymethylation of lignin, the characterization tests including FTIR spectroscopy, 1H NMR spectroscopy, acid-base titration, pHpzc (pH point of zero charge) and zeta potential tests were performed. Concentration of ions in aqueous solutions was determined by atomic adsorption technique.
Findings: The results of structural characterization tests showed that the reaction of lignin with sodium chloroacetate removed the aromatic hydroxyl groups of lignin, while the concentration of carboxyl groups increased. Thus, it was found that lignin was carboxymethylated successfully. The results of zinc (II) and copper (II) ions adsorption showed that the maximum adsorption of copper ion in comparison with zinc ion occurs in the presence of the modified industrial lignin adsorbent at a pH close to 4. The kinetic study results showed that the adsorption of both ions follows a pseudo second-order model. In the isotherm studies, the Sips model showed the best performance for both the industrial and modified lignin samples. Maximum adsorption capacities for the zinc and copper ions by carboxymethylated lignin were observed to be 0.25 and 0.37 mmol/g, respectively, showing a significant improvement in comparison with unmodified kraft lignin. However, selectivity of the copper ion decreased in contrast. This improvement could be mainly attributed to the significant increase in active sites on carboxymethylated lignin.