Beneficiation of pulp and paper mill sludge: production and characterisation of functionalised crystalline nanocellulose

The beneficiation of sludge from pulp and paper mills to produce high-value products such as crystalline nanocellulose will alleviate the challenges associated with conventional methods of sludge disposal, such as landfilling and incineration. In addition, the use of sludge will reduce the consumption of fresh raw materials in the synthesis of nanocellulose which is usually produced from high-purity cellulose pulps. In this study, fibres were cleaned and separated from sludge and then converted to crystalline nanocellulose using ammonium persulphate under optimised oxidative conditions. To extend potential applications of the crystalline nanocellulose produced, the crystalline nanocellulose was functionalised with zinc oxide, silver and hydroxyapatite to prepare crystalline nanocellulose-zinc oxide, crystalline nanocellulose-silver and crystalline nanocellulose-hydroxyapatite nano- and micro-composites powders using the sol–gel process. Transmission electron microscopy, field-emission scanning electron microscopy, X-ray diffraction and thermo-gravimetric analysis were used to investigate the properties of crystalline nanocellulose and functionalised crystalline nanocellulose. The transmission electron microscope and field-emission scanning electron microscope coupled with energy-dispersive X-ray spectroscopy confirmed the synthesis of crystalline nanocellulose, and inorganic nanoparticles. Functionalised samples (crystalline nanocellulose-zinc oxide, crystalline nanocellulose-silver and crystalline nanocellulose-hydroxyapatite) showed better thermal stability than pure crystalline nanocellulose. This implies that the modified inorganic crystalline nanocellulose composites could be used in applications where thermal stability is desirable. The cost of production is economically viable as the raw material cost is cheaper compared to the use of wood pulp.