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

A chemically-modified adsorbent (SDTMA), produced from the treatment of pinewood sawdust with maleic acid, was tested in batch adsorption experiments to remove Cd(II) ions from aqueous solution. SDTMA was characterised for the pH of zero point charge (pHZPC), spectroscopic evaluation of the surface functionalisation, structural and morphological features. Factors affecting adsorption behaviour, such as adsorbent dose, pH of solution, contact time and Cd(II) ion concentration were investigated. Results obtained show that adsorption rate to be comparatively fast, with equilibrium achieved after ~35 min. Subsequent analysis, showed Langmuirian behaviour and a monolayer adsorption capacity of 180.4 mg g-1, at pH 6; while data derived from two-parameter (Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R)) and three-parameter (Redlich-Peterson (R-P), Toth Khan and Sips) 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. Additionally, the rate of adsorption was analysed using a range of kinetic models (pseudo-first-order, pseudo-second-order, intra-particle diffusion, Bangham and Elovich) in their non-linear forms to provide insight into the adsorption mechanism and showing pseudo-second order behaviour is observed. In conclusion, the SDTMA adsorbent material 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 optimised by solution pH and adsorbent dose.