Adsorptive Removal of Volatile Anaesthetics from Waste Medical Gas

Abstract

Depending on the type of volatile anaesthetic (VA) agent used and anaesthetising strategy, the application of inhalation anaesthetics may constitute up to 63% of the carbon footprint from operating theatres. Volatile anaesthetics have high global warming potential. They are metabolised only minutely by patients with the rest released inevitably to the atmosphere causing global warming, and in some cases, ozone depletion. Activated carbons (ACs) provide an option for removing the VAs in the waste gas, reducing their ecological footprint. While the adsorption efficiency of ACs for polar molecules like VAs may be limited, AC surface could be modified to incorporate appropriate surface functionalities for enhanced adsorption. Additionally, the relationship between different oxygenated functional groups and sevoflurane adsorption into activated carbon remains unexplored. This thesis aims to gain a deeper understanding and insights on the adsorption of the commonly used inhalational VA, sevoflurane, on activated carbon surfaces to improve sevoflurane adsorption. The selected commercial granular activated carbon, E-GAC, was modified using various approaches, such as (a) aqueous oxidation using nitric acid, ammonium persulfate and hydrogen peroxide; (b) oxidative hydrothermal surface modification; and (c) nitrogenation using ammonia solution. The surface physical and chemical properties of the oxidised samples were characterised and associated with adsorption of sevoflurane measured at fixed conditions (bed depth = 10 cm, inlet concentration = 528 mg/L, and flow rate = 3L/min). The adsorption conditions were determined from breakthrough analyses of continuous fixed-bed adsorption of sevoflurane using selected commercial activated carbons, including E-GAC. The Conductor-like Screening Model for Real Solvents (COSMO-RS) model was applied to predict the interaction between sevoflurane molecule and surface functionality. The changes in surface physical properties did not show a clear relationship with sevoflurane adsorption. However, these surface modifications incorporated varying degrees of oxygen or nitrogen functional groups on activated carbon surfaces. There were occurrences of the removal and incorporation of surface functional groups during the surface modifications. The experimental and COSMO-RS findings suggested that surface oxygen groups with dominant hydrogen bond acceptor character, for instance, ether and carbonyl groups, enhanced sevoflurane adsorption. In contrast, surface oxygen groups with dominant hydrogen bond donor character, for instance, carboxyl groups, adversely affected sevoflurane adsorption. The findings of the parametric study from the oxidative hydrothermal surface modification suggested the disappearance and reformation of oxygen functional groups containing C‒O structures (as in hydroxyl and ether groups) when treatment temperature increased from 150°C to 200°C, and when treatments were conducted above 200°C, respectively. The re-formation of certain surface oxygen groups might lead to the development of oxygen functionalities that improve sevoflurane adsorption. On the other hand, nitrogen functional groups possessing C‒N‒C structures demonstrated a higher tendency for sevoflurane to adsorb to activated carbon. The insights on adsorption tendency of sevoflurane molecule to surface functional groups are useful to tailor activated carbons for improved sevoflurane adsorption. For over 150 years, the mainstay of good anaesthesia for surgical procedures has been VAs. For reasons of cost, environmental concerns, or simply elegance, hospitals aspire to reduce the emission of VAs by minimising the amounts used during anaesthesia. Despite that, considerable VAs are still released to the atmosphere unabated. One possible solution to this problem would be the installation of VA recovery technology to existing anaesthesia systems, capturing VAs before discharging the waste gas into the atmosphere. Although adsorption technology has high technology readiness level for adsorption of VAs, the current societal perception and legislations do not embrace the application of reclaimed and purified VAs on patients. Also, it is due to the conflicts of commercial interests with the VAs’ manufacturers. While the recycling of reclaimed VAs is not feasible, the reclaimed VAs could be either used as fluorinated feedstock for another process or deconstructed to inert forms before disposal. Regardless of the routes, they share an eventual aim of reducing the emission of these greenhouse gases to the atmosphere.