Comparative Adsorption of Zn2+ from Aqueous Solution Using Hydroxylated and Sulphonated Biochars Derived from Pulp and Paper Sludge

Thermally robust hydroxylated biochar (HBC) and sulphonated biochar (SBC) were synthesised from paper and pulp sludge (PPS) and used for the adsorption of Zn2+ from synthetic wastewater through batch experiments. FTIR analyses proved successful incorporation of the hydroxyl and sulphonic functional groups in HBC and SBC, respectively. The effects of initial solution pH, initial Zn2+ concentration, solution temperature and equilibrium contact time were investigated. The removal efficiency of Zn2+ increased with increase in both solution temperature and initial Zn2+ concentration. Adsorption of Zn2+ was greatest at pH 3. HBC and SBC removed 38-99% and 68-90% of Zn2+ from solution, respectively. Zn2+ adsorption on SBC followed both Langmuir (R-2 = 0.994) and Freundlich isotherm models (R-2 = 0.999), while adsorption on HBC followed the Freundlich model (R-2 = 0.989). Zn2+ adsorption on both biosorbents followed pseudo-second-order kinetics (R-2 = 0.994-0.999). The increase in enthalpy of adsorption indicated the adsorption process was endothermic and a decrease in Gibbs free energy signified the spontaneity of adsorption. Positive entropy change values imply that the adsorbed Zn2+ ions are randomly distributed over the adsorbent surface. The research demonstrated that although their adsorption mechanisms had salient differences, HBC and SBC can effectively remove Zn2+ from wastewater. Development of HBC and SBC from PPS provides potential low-cost biosorbents for water and wastewater, while simultaneously minimising the environmental and public health risks associated with current disposal practices of PPS.