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.