Molecular Dynamics Simulaitons For Adsorption Of Dissolved Organic Matter On Carbon Nanomaterials

Carbon nanomaterials （CNMs） are widely used in our daily production and life due totheir desirable characters. When released to the environment, CNMs can pose detrimentaleffects to both the environment and human health. To evaluate the environmental risks ofCNMs, it is important to investigate their environmental behaviors. Previous studies show thatCNMs can suspend in water, especially in natural water. Moreover, it has been reported thattheir suspensions are even more stable when they adsorb dissolved organic matter （DOM）,organic contaminants or biomacromolecule in the water. However, due to the complexity ofthe DOM structure and component, with the existing experimental and theoretical methods,investigations on the morphology and mechanisms of the interactions between CNMs andDOM are rather difficult. In this study, a method for molecular modeling molecular dynamics（MD） was employed to simulate the adsorption of DOM on CNMs. Moreover, as an emergingorganic contaminant which can also be adsorbed by CNMs, the adsorption of antibiotics onCNMs was also demonstrated. C₆₀, single-walled carbon nanotubes （SWNTs）,9lowmolecular weight organic acids （LOAs）, tannic acid and sulfamethoxazole were chosen as thetarget compounds. The effects of species, different dissociative forms and the concentrationsof DOM and sulfamethoxazole to their adsorptions on CNMs were elucidated. Theseinvestigations were dedicated to reveal the mechanisms of the interactions between CNMsand DOM/sulfamethoxazole, and to provide primary data for the evaluation of the exposedbehavior of CNMs in water.DOM is a complex mixture, which includes high-molecular-weight organic acids andlow-molecular-weight organic acids. However, data on the effect of LOAs on the stability ofCNMs in water is rather limited. Hereby, the interactions of9LOAs with C₆₀in vacuum andwater were simulated. Predictive models on the interaction energy between LOAs and C₆₀were developed. MD simulations indicate that LOAs could be well adsorbed on C₆₀both invacuum and water conditions. Moreover, the aromatic LOAs have stronger interactions withC₆₀than the aliphatic LOAs, and the LOA molecules have stronger interactions with C₆₀thanthe corresponding LOA anions. The models of the interaction energy between LOAs and C₆₀show good goodness-of-fit and robustness. On the other hand, the dominating interactions ofLOAs with C₆₀are dispersion, induction and hydrophobic interactions.To evaluate the effect of real water molecules on the adsorptions of LOAs on C₆₀, theadsorptions of citric acid and gallic acid on C₆₀in water were also simulated with MD. Theseresults show that both acids were also firmly adsorbed on C₆₀, and the dominating interactionsare van der Waals and hydrophobic interactions. However, gallic acid has stronger interactions with C₆₀than citric acid. Furthermore, hydrogen bonds were formed when citric acid andgallic acid interacted with water. Nevertheless, hydrogen bonds are not the dominatinginteractions, besides citric anion has stronger interactions with C₆₀than citric molecule.The interactions of high-molecular-weight organic acids with CNM are stronger, whichlead to their significant higher stability on the CNMs in water. However, the mechanisms ofinteractions between DOM and CNMs are still unclear. Therefore, in this dissertation, theadsorptions of tannic acid at different concentrations on SWNTs were simulated, with thediameter of SWNTs as another influence factor. The results show that tannic acid could bewell adsorbed on SWNTs, and their dominating interactions are Ï€-Ï€ and hydrophobicinteractions. Moreover, multilayer adsorption phenomena of several tannic acids on SWNTswere detected, and bridge effect was observed between the water molecule and the tannicacids. Furthermore, the sorption affinity of SWNTs for gallic acid was enhanced with anincreasing surface area of SWNTs, and shows no direct relation to the diameter of SWNTs.Antibiotics can be adsorbed by CNMs, hence, their environmental behavior should beevaluated together than individually. The adsorptions of sulfamethoxazole on SWNTs andcarboxylic SWNTs were simulated respectively, and the effects of different dissociative formsof sulfamethoxazole, concentration of sulfamethoxazole and ionic strength on the adsorptionswere considered. Moreover, the effect of oxygen mass percent of carboxylic SWNTs on theadsorptions was also investigated. The results show that the orders of interactions of differentdissociative forms of sulfamethoxazole with SWNTs were: cation> molecule> anion. Themain interactions of sulfamethoxazole with SWNTs are Ï€-Ï€ and hydrophobic interactions.However, the effects of concentration of sulfamethoxazole and ionic strength on theadsorptions were no significant, while the interactions of sulfamethoxazole with SWNTs arestronger than with carboxylic SWNTs. Sorption affinity of the molecules of carboxylicSWNTs for sulfamethoxazol increases with an increasing oxygen mass percent of carboxylicSWNTs, and the sorption affinity of the dissociative form of carboxylic SWNTs is not relatedto the oxygen mass percent of carboxylic SWNTs. Hydrogen bonds betweensulfamethoxazole adsorbed by carboxylic SWNTs and water occurs more than those betweensulfamethoxazole adsorbed by SWNTs and water. Carboxyl group of SWNTs only reduce thesurface area of SWNTs, but it does not affect the adsorption affinity of SWNTs.