Non-linear adsorption characteristics of modified pine wood sawdust optimised for adsorption of Cd(II) from aqueous systems

The facile modification of pinewood sawdust, with maleic acid, to produce a sorbent aimed at metal ion adsorption, was tested via the batch adsorption of Cd(II) ions from aqueous solution. The sorbent was characterized for the pH of zero point charge (pH(ZPC)), spectroscopic evaluation of the surface functionalization, structural and morphological features. Factors affecting adsorption behavior, such as adsorbent dose, pH of solution, contact time and Cd(II) ion concentration were investigated. Results obtained show the adsorption rate to be comparatively fast, with equilibrium achieved after similar to 35 min. Subsequent analysis, showed Langmuirian behavior and a monolayer adsorption capacity of 180.4 mg g(-1), at pH 6; while data derived from two-parameter and three-parameter isotherm models was evaluated using non-linear regression methods, with error analysis, to determine the most appropriate model and allow prediction of optimum parameters. The Sips isotherm model proved the most appropriate in describing the experimental data obtained in the study; with a low level of heterogeneity in the adsorption sites occupied suggesting the interaction of the metal ions is preferential with the added carboxylate moieties only. Additionally, the rate of adsorption was analysed using a range of kinetic models (pseudo-first-order, pseudo-second-order, Bangham and Elovich) in their non-linear forms to provide insight into the adsorption mechanism and showing pseudo-second order behavior is observed, indicating two processes are key in the adsorption process, likely diffusion to the surface and subsequent adsorption on the carboxylate moieties incorporated by chemical modification. In conclusion, the sorbent produced in this study offers high potential for the removal of Cd(II) ions from aqueous solution due to the carboxylic functionalities incorporated into the material, which is optimized by solution pH and adsorbent dose.