Waste Cassava Peel and Almond Hull Valorisation via Microwave Processing: A Source of Chemicals, Materials and Bioenergy

Unavoidable food supply chain wastes are an under-utilised source of renewable materials, chemicals and (bio)energy. This thesis investigates the microwave processing (both conventional pyrolysis and hydrothermal) of waste cassava peel (CP) and almond hull (AH) as a source of microfibrilated celluloses (MFCs) (materials), sugars and bio-oil (chemicals) and biochar (bio-energy). Particular focus is given to the production of MFC and their application as adsorbents for Cr(VI) and methylene blue (MB) remediation.
A range of MFC's was prepared via acid-free hydrothermal microwave processing at different temperatures (120, 170 and 220 oC) of both virgin CP or AH and their residue from pretreatment with either ethanol or heptane. ATR-IR, TGA, XRD, solid-state 13C CPMAS NMR, zeta potential, HPLC, CHN, TEM and SEM analyses were performed to ascertain structural changes in the production of MFCs. Significant changes were seen between MFCs processed at 170 oC with respect to 220 oC, i.e. significant increases in the Crystallinity Index, thermal decomposition by TGA and visual darkening of the materials synonymous with the leaching of amorphous cellulose and possible pyrolytic processes. AH MFCs presented excellent hydrogel and film forming capabilities.
CP MFC (CP 220 oC-30 minutes) was shown to have superior performance in the adsorption of Cr (VI) reaching 180.73 mg/g via ultrasound treatment at pH 2. However, in the uptake of MB, CP MFC (CP120 oC-30 minutes) had a superior performance (199.06 mg/g). It was found that for both MFCs, the kinetic processes can be accurately predicted by pseudo-second order rate kinetics.
Bio-oil produced via the microwave pyrolysis of AH had a high heating value of 34.37 MJ/kg whilst char derived from CP gave a value 25.64 MJ/kg by bomb calorimetry, indicating that both have potential as alternative biofuels.