Towards the production of adsorbents from mixed domestic discarded materials using heat pipe pyrolysis

Abstract

Water contamination and municipal waste management are two problems of global significance. The amount of municipal waste generated is expected to increase over the coming century as populations increase and become more urban in nature as developing countries become more developed. Furthermore, water contamination will also become an increasingly significant problem as developing countries become more industrialised. For this reason, solutions are needed to remove contamination from water, and to reduce the amounts of municipal waste being disposed of in unsustainable ways. The main objective of this thesis was to assess the adsorption ability of biochar produced from mixed municipal discarded material (MMDM) in a novel heat pipe reactor. Another novelty of this thesis is that the use of heat pipes limits the pyrolysis temperature for safety reasons, dependent on the working fluid used. Consequently, biochar produced in the heat pipe reactor contains more oxygen containing functional groups. These interact with aqueous metals in complexation interactions and participate in hydrogen bonding with functional groups present in some organic contaminants. The biochar adsorbent was characterised using scanning electron microscopy, energy dispersive x-ray analysis, x-ray diffraction crystallography, Fourier transmission infra-red spectroscopy, and Raman spectroscopy. Following characterisation, batch adsorption experiments were conducted using either copper, methylene blue or tetracycline as the target adsorbate. Batch adsorption experiments were analysed using kinetic, diffusion and isothermal models. This thesis used the non-linear equations of adsorption kinetics and isotherms to remove the transformation errors caused through using the linear kinetic and isothermal models that are commonly used in literature. This thesis shows that biochar produced using MMDM in a heat pipe reactor can produce an adsorbent that is comparable to other biochar in literature. Copper adsorption reached a maximum of 6.3mg/g, methylene blue adsorption reached a maximum of 7.3mg/g, and tetracycline adsorption reached a maximum of 9.84mg/g. Typically, Elovich and pseudo second order kinetic models were the best fitting adsorption kinetics, with the Langmuir isotherm best describing copper adsorption, and the dual mode isotherm best describing the methylene blue and tetracycline adsorption. Biochar produced from MMDM is shown to be a promising adsorbent material that could be used in various water treatment applications. Further activation/functionalisation could produce an adsorbent material from discarded materials that could rival current commercial activated carbons. This highlights how this material could be used in a holistic circular economy approach to waste management and water treatment.