Synthesis, characterisation and methyl orange adsorption capacity of ferric oxide–biochar nano-composites derived from pulp and paper sludge

A Fe2O3–biochar nano-composite (Fe2O3–BC) was prepared from FeCl3-impregnated pulp and paper sludge (PPS) by pyrolysis at 750 °C. The characteristics and methyl orange (MO) adsorption capacity of Fe2O3–BC were compared to that of unactivated biochar (BC). X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed the composite material was nano-sized. Fourier transform infrared (FTIR) spectroscopy revealed the presence of hydroxyl and aromatic groups on BC and on Fe2O3–BC, but Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) porosity were lower for Fe2O3–BC than BC. Despite the lower BET surface area and porosity of Fe2O3–BC, its MO adsorption capacity was 52.79 % higher than that of BC. The equilibrium adsorption data were best represented by the Freundlich model with a maximum adsorption capacity of 20.53 mg g−1 at pH 8 and 30 min contact time. MO adsorption obeyed pseudo-second-order kinetics for both BC and Fe2O3–BC with R2 values of 0.996 and 0.999, respectively. Higher MO adsorption capacity for Fe2O3–BC was attributed to the hybrid nature of the nano-composites; adsorption occurred on both biochar matrix and Fe2O3 nanocrystals. Gibbs free energy calculations confirmed the adsorption is energetically favourable and spontaneous with a high preference for adsorption on both adsorbents. The nano-composite can be used for the efficient removal of MO (>97 %) from contaminated wastewater.