Paper title: Salt Rejection and Water Transport through Boron Nitride Nanotubes
Journal: Small (2009) Vol. 5, No. 19, 2183-2190

Author(s): Tamsyn A. Hilder, Daniel Gordon, and, Shin Ho Chung
*Computational Biophysics Group, Research School of Biology, Australian National University, ACT 0200, Australia

Summary:
Boron nitride nanotubes (BNNTs) are not as popular as carbon nanotubes (CNTs), specially in the field of desalination. However, it has been shown that BNNTs are better because they can achieve higher salt rejection and, at the same time, higher water flux.

The authors used molecular dynamics simulation (MDS) to study (5,5), (6,6), (7,7) and (8,8) BNNTs (arm-chair type BNNTs). They were constructed by using a hexagonal array of alternating boron and nitrogen atoms that were rolled up to form a tube, just like when making CNTs. In making the BNNTs, aside from the BNNT unit cell parameters, the physical parameters of CNTs were used so that the authors simply had to change each carbon atom to either a boron or nitrogen one.

MD simulations were carried out using NAMD and VMD. The system was composed of a BNNT, a silicon nitride matrix, water molecules, and, sodium and chloride ions. The nanotube was situated in between 2 water reservoirs and embedded on the silicon nitride membrane. The silicon nitride membrane was constructed by simply following the procedure presented in the NAMD/VMD Tutorials section of the UIUC website. The simulation box dimensions were 4.5 by 4.0 by 6.4 nm3. Other simulation requirements such as ensemble (constant T and P), number of molecules, concentration of NaCl, force field, free energy, hydrostatic pressure, initial positions and movement of molecules were also discussed in the paper.

The paper has theoretically shown the efficiency of desalination with the use of BNNT-embedded silicon nitride membranes. But among the BNNTs used in this study, (5,5) BNNT is considered as the best type because it can not only conduct water molecules but also can reject 100% of the ions with concentration as high as 1 M. The paper also says that salt rejection is a function of BNNT radius and water structure within the BNNT. Increasing or decreasing the radius affects the ionic selectivity of the BNNTs

Contribution and application:
Inclusion of CNTs in desalination membranes is gaining popularity since its introduction in the early ‘90s. This is due to the ability of CNTs to allow faster fluid flow rates. Just like CNTs, boron nitride nanotubes have also proven that they can achieve salt rejection and water permeation. Actually, BNNTs are better in these two aspects than CNTs and they resemble the efficiency of biological water channels (aquaporin) more. The use of BNNTs in the areas of membrane development and desalination is relatively new. However, it has a huge potential of improving the efficiency of SWRO desalination process.



By: Hannah Ebro
hannah@gist.ac.kr