Abstract
Nanocomposite membranes are strongly desired to break a trade-off between permeability and selectivity. This work reports new thin film nanocomposite (TFN) forward osmosis (FO) membranes by embedding aluminosilicate nanotubes (ANTs) into a polyamide (PA) rejection layer. The surface morphology and structure of the TFN FO membranes were carefully characterized by FTIR, XPS, FESEM and AFM. The ANTs incorporated PA rejection layers exhibited many open and broad “leaf-like” folds with “ridge-and-valley” structures, high surface roughness and relatively low cross-linking degree. Compared with thin film composite (TFC) membrane without ANTs, the TFN membrane with only 0.2 w/v% ANTs loading presented significantly improved FO water permeability, selectivity and reduced structural parameters. This promising performance can be mainly contributed to the special ANTs embedded PA rejection layer, where water molecules preferentially transport through the nanochannels of ANTs. Molecular dynamic simulation further proved that water molecules have much larger flux through the nanotubes of ANTs than sodium and chloride ions, which are attributed to the intrinsic hydrophilicity of ANTs and low external force for water transport. This work shows that these TFN FO membranes with ANTs decorated PA layer are promising in desalination applications due to their simultaneously enhanced permeability and selectivity.
Highlights
The global water crisis and fresh water shortage have become one of the most essential issues in the world [1]
Molecular dynamic simulation further proved that water molecules have much larger flux through the nanotubes of aluminosilicate nanotubes (ANTs) than sodium and chloride ions, which are attributed to the intrinsic hydrophilicity of ANTs and low external force for water transport
It should be further pointed out that as the ANTs loading increases to 0.2 w/v%, the Js /Jw value declines and the water flux remarkably increases. It suggests that both the salt rejection and water permeability of thin film nanocomposite (TFN) membranes can be improved by embedding appropriate contents of ANTs into the PA rejection layer, which are always a trade-off in nanocomposite membranes. This result is in accordance to that of Carbon nanotubes (CNTs) incorporated TFN membranes reported by Song et al [24], which may be attributed to the thin tubular structures of CNTs and ANTs
Summary
The global water crisis and fresh water shortage have become one of the most essential issues in the world [1]. Zang et al [32] studied the transport properties of water, methanol, and ethanol through a single-walled aluminosilicate nanotube and showed that water diffusion in ANTs is comparable to bulk phase due to the large pore diameters These works described a clear physical image for the structure and self-diffusion behavior of water molecules in ANTs. several questions were still not well addressed: (1) What are the effects of ANTs on the water permeability and selectivity of TFN membranes in FO process without any hydraulic pressure? The effects of ANTs on the morphology and performance of TFN membranes were investigated by embedding different contents of the synthesized ANTs. It should be pointed out that the ANTs embedded PA rejection layer endowed the TFN membranes with both enhanced water permeability and solute rejection due to the characteristic physicochemical structures of ANTs. molecular dynamics simulation was applied to study the competitive transport of water molecules, sodium cations, and chloride anions through the nanotubes. These ANTs embedded TFN FO membranes could be promising in desalination application
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