Melia, Patrick Matthew (2019) Development of biochar-based materials for the recovery of phosphorus from wastewater. (PhD thesis), Kingston University, .
Abstract
Phosphorus (P) is a irreplaceable element essential for life. It is relied upon, as mineral-derived phosphate fertilisers, for maintaining global agricultural productivity. Excess P in surface water causes eutrophication, where hypoxic effects and algal toxins degrade the environment and are hazardous to health. Therefore, the removal of P from wastewater, before the discharge of effluents to the environment, is important. Additionally, due to the finite nature of geological sources of mineral P, it is now crucial that P is not only removed from wastewater but captured in forms suitable for its direct reuse as recovered fertiliser. There are various routes to achieving this, however approaches developed can be limited to large wastewater treatment plants or have limited ability to remove P to the low concentrations (<1 mg P/L) increasingly required by legislation. Biochars, as adsorbents for the recovery of P, are attractive due to their known native soil ameliorant and carbon sequestration properties. Biochar-based materials saturated with P could have further benefit as being a recovered P fertiliser. This thesis has critically reviewed the feasibility and trends in the various technological approaches for the recovery of P from wastewater treatment plants and also the state-of-the-art in the development of biochar and biochar-based materials for the capture of P. Complimentary characterisation approaches suitable for biochar analysis related with its environmental uses were assessed before subsequent work investigated the forces driving the capture of P with biochar derived from various sources. This work found that the concentration of Ca and Mg within various unmodified biochars was the main driver of P adsorption, with CO[sub]2 activation (carried out to increase biochar surface area and porosity) providing only a limited improvement. Building on this initial work, the thesis has culminated in the development of novel modified biochars with enhanced properties for the efficient recover of P. Various biochar composites removed >90% of P from an initial concentration of 10 mg P/L and presented moderate release of P after extraction at mildly acidic and basic pH, which attempted to replicate soil solutions. Upon further study through isotherm and kinetics experiments the adsorption was found to occur at a rapid rate (reaching equilibrium within <5 min); whilst maximum capacities were modest in most cases (10-15 mg P/g composite), the heat of adsorption was found to be higher than commonly reported in the literature. This thesis has made a significant contribution to the analysis, use, and mechanisms involved in biochar-P interactions, and has furthered the development of biochar composites with improved properties for P recovery through novel routes.
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