(702f) The Structural Properties of Micropollutants in Activated Carbons in Relation to Greywater Treatment: A Molecular Dynamics Study

Waste water reclamation for reuse is a promising measure to alleviate water scarcity and the associated burden on water supply. Greywater from toilet flushing, laundry and irrigation has been identified as a potential stream for water recycle, thanks to its substantial amount (50%~80% of the household wastewater) and the lower concentrations of organic matter and pathogens comparing to black water. Despite of its reuse potentials, there are environmental concerns over the persistent organic micropollutants, including pharmaceuticals and preservative as well as personal care products (PPCPs). Despite the trace amount, these compounds are biologically active and may accumulate in tissues of living organisms, imposing a negative impact on the environment and ecosystem. Thus, appropriate handling and treatment of greywater to remove micropollutants are required prior to their discharge or reuse.

Activated carbons (ACs) are often employed to remove various micropollutants, due to their high specific surface area and adsorption capacity. Applications of powdered activated carbon (PAC) and granular activated carbon (GAC) for greywater treatment have been reported in substantial experimental studies. While the laboratory results demonstrate the removal potentials of ACs, they fail to elucidate the adsorption mechanisms of micropollutants from a molecular perspective. In this endeavor, molecular dynamic (MD) simulation has been proven to be a promising and powerful option to reveal the structural and dynamic properties.

In this work, we use MD simulations to study the structural properties of three typical micropollutants (ethylparaben, nonylphenol and triclosan) immersed in water in slit carbon nanopores at 300 K and 1 bar. Through our simulation work, the distributions of micropollutants and their interactions with carbon surface are explicitly studied. Moreover, the effect of hydroxyl groups on the surface is also investigated. Owing to the Van der Waals and π-π interactions, ethylparaben and nonylphenol molecules tend to be parallel to the surface in pristine carbon nanopores. Besides, nonylphenol molecules are adsorbed on the surface, while they incline to aggregate parallelly due to hydrophobic interactions. Most triclosan molecules exhibit a unique configuration with one benzene ring parallel to the surface, while the other one is perpendicular to the surface, due to π-π interactions with other triclosan molecules. Triclosan molecules form aggregation due to the π-π interactions. For the carbon surface with hydroxyl groups, all three compounds form hydrogen bonds with the surface functional groups, breaking the layered structures observed in pristine carbon nanopores. Furthermore, they likely form clusters with different morphologies in the vicinity of the surface by varying mechanisms. The ethylparaben molecules present a tilted configuration with their benzene rings parallel to each other owing to the π-π interaction. Interestingly, the nonylphenol molecules incline to form a brush type structure with hydrophilic groups forming hydrogen bonding with hydroxyl groups on the pore surface, while they align parallel to each other due to strong hydrophobic interactions. A weaker π-π interaction is also observed among the benzene rings in nonylphenol molecules. For triclosan molecules, they form a smaller cluster due to relatively weak π-π interaction and configurational entropy loss. In addition, some dispersed triclosan molecules are distributed on the surface through interactions between either carboxyl groups or chloride atoms and surface functional groups.

This work advances the understanding of the adsorption behavior of micropollutants in ACs and the corresponding treatment technologies.