African almond (Terminalia catappa L) leaves biochar prepared through pyrolysis using H3PO4 as chemical activator for sequestration of methylene blue dye

Abstract

African almond leaves (ALs) were used for the preparation of sustainable and eco-friendly adsorbent through chemical activation, followed by pyrolysis at 700 oC for adsorption of methylene blue (MB) dye from aqueous solution. Phosphoric acid activated African almond leaves biochar (PALB) was characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), pH point zero charge (pH pzc) and X-ray diffraction (XRD) techniques. The influence of various adsorption parameters was investigated for determination of optimum conditions for sequestration of MB dye. Equilibrium adsorption isotherm and kinetic models were equally assessed. Mesoporous PALB with surface area 816 m2/g and radius 1 nm has excellent removal efficiency due to removal of >98% MB dye from aqueous solution at room temperature 303 K, pH 8, initial dye concentration of 50 mg/L, adsorption dose of 0.2 g/L and contact time 30 min. Adsorption isotherm and kinetic were best fitted with Freundlich and pseudo-first order (PFO) kinetic models. Positive value of enthalpy (ΔHo) showed that the adsorption process was endothermic, while the negative value of free energy (ΔGo) suggested that the process was spontaneous and the positive value of entropy (ΔSo) indicated increase in randomness at adsorbate/adsorbent interface. PALB displayed a good monolayer adsorption capacity (Q max) of 263.95 mg/g when compared to those of other adsorbents reported in the literature for removal of MB dye from aqueous solution. This study established potential ability of PALB as eco-friendly adsorbent for adsorption of MB dye from aqueous solution.