Engineered Porous Carbon Adsorbents for Radionuclide Remediation: A Study of Chemical and Physical Factors

Accidental release of radioactive material to the environment poses a widespread threat to health of the biosphere, including to humans though contamination of food or water sources, or though inhalation of airborne radioactive particles. The development of targeted, functionalised adsorbents and remediation materials which have the versatility to work effectively in varying groundwater conditions, often containing high quantities of dissolved matter, is necessary. Biopolymers are such a class of materials which are well suited to remediation and immobilisation of contaminants such as radionuclides from groundwater and soil. Not only do they possess promising physicochemical characteristics such as extensive hierarchical porosity, surface functionality and recalcitrance, they are inherently compatible with environmental systems. They can be further functionalised or activated, or incorporated into monolithic composites with specific, engineered functionality and morphology for enhanced uptake and removal of radionuclides. Despite this promise, they are poorly understood at a mechanistic level, in part, due to their amorphous nature which makes analysis of molecular scale processes difficult. Therefore consistent bulk behaviour is more difficult to predict. Understanding the underpinning physical and chemical features of biopolymers and their composites is a crucial step to both further optimisation and deployment of such a material in a remediation setting. Several functionalised biopolymers and monolithic composites were created for strontium uptake and immobilisation. Both the physical and chemical factors governing uptake behaviour were examined. The binding mechanism of strontium was examined using X-ray absorption spectroscopy and paired with bulk strontium uptake isotherm data. High and rapid uptake capacities were achieved to functionalised biochar with even higher uptake achieved to novel biochar-alginate hydrogel composites. EXAFS fitting results indicated biochars and hydrogels alike exhibit an inner sphere binding mechanism to engineered biopolymer adsorbents, indicating strong binding to the adsorbent. Pores, specifically macro pores play a crucial role in mass transport of radionuclides to/from active adsorption sites. They can also prevent pore blocking or fouling during adsorption. The pore architecture of a range of functionalised biochars was investigated quantitatively using X-ray tomography, revealing the pore tuning effect of several common activators on the macro pore space. Each choice of biochar-activator combination yields distinct pore architecture, which can be selected in response to varying application or conditions.