O papel da água na adsorção do diclofenaco em nanotubos de carbono

Abstract

Emerging Contaminants (ECs) are a special class of pollutants that are not removed from water by methods currently used in treatment for human use. Furthermore, these compounds do not yet have specific regulations regarding the maximum concentration allowed in water resources, making their disposal in nature a serious problem. However, ECs are essential compounds for the functioning of society as we know it today. Examples include agrochemicals, personal care products, pharmaceuticals, caffeine, and other molecules. Although they are necessary for society, there is a growing number of studies pointing out the dangers of the presence of ECs in the environment. The occurrence of pharmaceuticals in water, for example, is known to be harmful to aquatic biota. In particular, the drug diclofenac stands out, a non-steroidal anti-inflammatory drug that, because it is relatively cheap and does not require a medical prescription to purchase, is widely used to treat pain and inflammation resulting from injuries. However, its presence in water bodies, caused by incorrect disposal of medicines and even human excrement, has already been proven to be harmful to some species of fish and aquatic biota. Carbon nanotubes (CNTs) have promising applications for removing the drug diclofenac potassium from water through the adsorption process. To clarify this process, studies were conducted with Classical Molecular Dynamics to evaluate how CNT geometry factors affect the interaction between the nanotube and diclofenac. A series of simulations were carried out varying the diameter, length, number of walls of the nanotubes and the distribution of water within them. The analyzes carried out on the trajectories resulting from the simulations consist of calculations of the radial position of diclofenac over time and the position of the molecule along the axial axis of the nanotube as this radial distance decreases and analyzes of the distribution and density of water around the CNTs and the diclofenac. For single-walled nanotubes (SWCNTs), it is noted that adsorption occurs faster the larger the radius of the nanotube. It was found that the drug performs a random walk through the simulation box until it “encounters” the CNT, which explains the faster adsorption for cases with a larger adsorbent surface area as a purely statistical consequence. On the other hand, double-walled nanotubes (DWCNTs) with the same external radius as the largest SWCNTshowed worse results, being less favorable for adsorption. Evidence indicates that the water structure around the DWCNT acts as a barrier to the pollutant’s approach to the nanomaterial wall - thus, one way to improve adsorption may be to functionalize the CNT surface to reduce the barrier generated by structured water. These results assist in the design of highly efficient membranes for removing emerging contaminants from water.