(727c) Molecular Insights into Adsorption Behavior of Naphthenic Acids in Carbonaceous Materials

Nowadays, the largest bitumen deposit in the Athabasca oil sands region (AOSR) is mainly recovered by steam assisted drainage process, during which vast amount of steams are injected to produce bitumen. The oil sands processed-affected waters (OPSW) from extraction may result in a series of environmental concerns, such as acute and chronic toxicity affecting aquatic and mammalian species. In fact, OPSW consist of a complex mixture of organic and inorganic constituents, among which, naphthenic acids (NAs) are the primary toxic components. In addition, NAs can cause the severe corrosion in extraction equipment, pipelines, and storage tanks, increasing the operational costs. Therefore, it is imperative to remove NAs from OSPW to make oil sands operations more environmentally and economically sustainable.

Adsorption in carbonaceous materials such as activated carbon (AC) is a cost-effective and efficient method to remove NAs from OSPW, thanks to their high specific surface area. The high-surface-area AC can effectively adsorb NAs in OSPW to increase their concentration to trigger specific microbial metabolism mechanisms to preferably accelerate the compound-specific biodegradation. While there have been some experimental measurements on NAs equilibrium adsorption isotherm on ACs, the underlying mechanisms of NAs adsorption in nanoporous ACs cannot be elucidated by macroscopic experiments. Molecular simulations can explore the length scales that are not accessible to experiments to provide important understanding about adsorption behavior of NAs in ACs from molecular perspectives.

In this work, we use molecular dynamics (MD) simulations to study the adsorption behavior of two different NA species (cyclohexanoic and heptanoic acid) immersed in water in slit carbon nanopores. We explicitly study the effect of pore size and hydroxyl groups on carbon surface. For pristine carbon nanopores, heptanoic acid molecules incline to be parallel to the surface, while cyclohexanoic acid molecules tend to show a relatively weaker adsorption on the surface. The neighboring molecules bundle together due to hydrophobic interactions, while their carboxyl head groups are hydrated by water molecules. In carbon nanopores with hydroxyl groups, both NAs can form hydrogen bonding with surface functional groups. Heptanoic acids no longer align parallelly to the surface. It is worth noting that when the concentration of heptanoic acid is high, they can aggregate to form a cluster with their hydrophobic tails pointing inward and hydrophilic groups pointing outward to be hydrated by water and surface hydroxyl groups. In contrast to heptanoic acid, cyclohexanoic acid still has a strong adsorption on the surface with few of them dispersed in water. In addition, the aggregate of cyclohexanoic acid can hardly form even at a high concentration. The different adsorption behaviors between the two NAs can be attributed to the varying structural properties. The interactions among cyclohexanoic acids is weaker than those among heptanoic acids. It is because the linear hydrophobic tails of heptanoic acids impose strong hydrophobic interactions, while the ring-like structure in cyclohexanoic acids suffer configurational entropy loss when they aggregate.

Our work should shed lights on the NAs adsorption mechanism in ACs and provide important insights into manufacturing high-performance carbonaceous materials for OSPW treatment.