Qualitative assessments of the biomass from oil palm calyxes and its application in heavy metals removal from polluted water.

Graphical abstract Highlights • The adsorbent has a highly ordered, multilayer-like composite structure. • The sorption process followed the pseudo-second-order kinetic model. • Adsorption of Cd²⁺, Ni²⁺ and Cr³⁺ ions onto the surface of OPC was heterogeneous. • Thermodynamic studies suggested physical adsorption for the removal of, Pb²⁺. • Physico-chemical process may be involved in the adsorption of Cd²⁺, Ni²⁺ and Cr³⁺. Abstract The efficiency of oil palm calyxes (OPC) in removing heavy metals from aqueous solution is presented in this report. Qualitative analyses showed the participation of hydroxyl, carboxylate and methoxyl groups in the binding process. OPC surface was observed to be amorphous, highly ordered and multilayered. Quantitative analyses indicated that the removal of Cd²⁺, Pb²⁺ and Ni²⁺ occurred at fast kinetics, with uptake of 38.24, 47.66 and 32.61 mg/g respectively, after 5 min. Optimum Cr³⁺ adsorption was however not reached after 3 h, indicating slow kinetics. Pseudo-second-order model best described the kinetics of the adsorption, suggesting chemisorption as the rate-limiting step. Metal uptake increased with rise in adsorbate concentration. However, no further rise in uptake was observed with increase in Pb²⁺ concentration beyond 200 mg/L. Data from the sorption of Cd²⁺, Ni²⁺ and Cr³⁺ were best described by Freundlich isotherm, while those of Pb²⁺ obeyed Langmuir isotherm, with maximum adsorption capacity (q max) of 120.48 mg/g. Thermodynamic studies indicated that the removal of Cd²⁺, Ni²⁺ and Cr³⁺ was exothermic and occurred with decreased disorderliness, while Pb²⁺ removal was endothermic with increased entropy. The negative values of ΔG° indicated the spontaneity of the biosorption of Cd²⁺, Pb²⁺, Ni²⁺ and Cr³⁺ onto OPC. [ABSTRACT FROM AUTHOR]