Paper title:

Molecular simulation of carbon nanotube membrane for Li+ and Mg2+ separation

 

Journal:

Journal of Membrane Science Volume 444, 1 October 2013, Pages 327?331

 

Author/s:

Dengfeng Yang[a], Qingzhi Liu[b], Hongman Li[a], Congjie Gao[a]

 

[a] College of Chemistry and Chemical Engineering, Ocean University of China, No.238 SongLing road, Qingdao city 266100, China

[b] College of Chemistry and Pharmaceutical Science, Qingdao Agriculture University, No. 700 Changcheng road, Qingdao city 266109, China

 

Summary:

This original research article studied the transport of lithium and magnesium ions through carbon nanotube (CNT) membranes, with the goal of separating these similarly-sized ions in solution.

 

In this study, ion conductance through armchair CNTs from (8,8) to (11,11) was investigated at hydrostatic pressure differences of 100 and 200 MPa. Two water boxes were placed on both sides of the CNT membrane. The net concentration of the LiCl or MgCl₂ solution was 250 mM. Potential of mean force (PMF) for ions passing through the CNTs were determined based on the method of Adaptive Biasing Forces (ABF) and consequently quantified the energy barrier to ion conductance.

 

It was found that even though the diameters of the Li⁺ and Mg2⁺ are similar, they experienced different energy barriers when entering a CNT. Lithium face a much smaller barrier than magnesium, making it easier for Li ions to pass through CNTs. The difference was caused by the difference in the hydration numbers of the 2 ions, wherein Li⁺ has 4 water molecules in the 1^st hydration shell while Mg²⁺ has 6. When entering the CNT, some of the hydrating water molecules should be removed from the ions in order for them to pass through.

 

Separation of the 2 ions was achieved using different combinations of CNT size and applied pressure. It was found that there should be at least 8 kJ/mol PMF difference in order to achieve ion separation. Under 100 MPa hydrostatic pressure difference, (9,9) and (10,10) CNTs were able to extract lithium ions while under 200 MPa, (8,8) to (10,10) CNTs can separate both lithium and magnesium ions with high ion fluxes.

 

Axial density profiles were also obtained in the study. They showed that there was a higher density at the entrances of the pores, showing the accumulation of ions at the openings. The use of axial density profiles was effective because it was able to provide clear and visual interpretation of the ion separation capacity of the membranes.

 

 

Contribution and application:

This study has validated the importance of studying the concept of PMF. Using PMF profiles, energy barriers to ion or water flow through a CNT could be measured and analyzed. PMF can also be used to quantify the energy barriers to water and ion flow during the SWRO process using a novel CNT and a conventional polymeric membrane.

 

By: Hannah Ebro

hannah@gist.ac.kr