Theoretical study on the rejection of solutes with low molecular weight by Carbon NanoTubes

The removal of pollutants with low molecular weight and the recovery of compounds with high added value are current challenges in the wastewater treatment. The choice or the design of innovative nanomaterials for membrane preparation is crucial to address these issues. In this frame, CNTs are promising nanostructures since they combine amazing hydrodynamic properties1 with the capability to reject low molecular weight solutes due to their tailored inlet. Thus, in this contribution, the CNTs rejection ability towards some uncharged solutes coming from pharmaceutical industrial water and olive mill wastewater was investigated at a theoretical level. In the rejection of small organic solutes by NF/RO membranes, size exclusion plays an important role. In the wake of works concerning the NF modelling2,3, a molecular descriptor, i.e. the minimum cross-section (MCS), is here proposed exploiting the geometry optimizations of the solutes by an ab-initio method. In details, starting from the ab-intio molecular geometries, their MCS are calculated via an homemade algorithm; then, based on MCS, a geometric criterion able to predict the rejection capability of CNTs is proposed. The criterion, called Maximum Circumference Diameter (MCD), provides the internal diameter of single- or multi- walls CNTs suitable to achieve a complete rejection of the target solutes by a size exclusion mechanism. In order to validate the proposed criterion, an accurate analysis at Quantum Mechanics level was conducted by using specific configurations of Tyrosol molecules as obtained by Molecular Dynamics simulations4 inside a (8,8) SWNT. The QM calculations showed that due to significant distortions of the Tyrosol phenyl ring the rejection by (8,8) SWNT occurs in agreement with the prediction of the proposed geometric criterion. Moreover, QM calculations revealed that the hydrogen bonds5, formed during the Tyrosol self-assembly in the SWNT, cannot compensate the energy loss per molecule due to phenyl ring distortion. Also, Tyrosol MD configurations in a larger SWNT (10,10) were investigated always at QM level. Conversely to the case of (8,8) SWNT, in this case lower values of energy, in the range of noncovalent bonds5, is required to the solutes to go in the SWNT. Finally, it worth noting that the proposed novel criterion is completely free from fitting or adjustable parameters, therefore, it can be extended to predict the molecular rejection by any nanoporous; in fact CNTs can be regarded as ideal pores.