A review of water treatment membrane nanotechnologies

Abstract

We thank the reviewers for their insightful comments and we have revised the above referenced manuscript in response. In addition, we addressed each reviewer comment in an itemized fashion in the attached "Response to Reviewer Comments."We have uploaded high resolution (600 dpi minimum) TIF files for each image in the article. We have kept the images in color for the online version, but please convert these to grey-scale for the print version. We have ensured that no information is lost from the figure upon conversion.We feel we have adequately addressed all reviewer comments and hope that you will accept our review paper for publication in Energy & Environmental Science without further revision. Figure 6. Percolation Threshold was removed because it also seemed to provide superfluous detail. Section 3.2.1. Mixed Matrix MembranesSelections have been deleted due to excessive explanation of mixed matrix theory; the detail of background theory here appeared to be out of proportion with that in other sections. The important points from the deleted paragraphs have, however, been shortened and inserted into the present introduction so that major points are not lost. Also, it was re-emphasized that details about gas separation membranes were included only to show the past application and strength of these materials now being re-engineered for water treatment purposes. Section 3.3.3. Isoporous Block Copolymer MembranesSelections have been deleted, again when out of proportion detail in background explanation was found. Within the particular works cited and described, some specific details have been removed or minimized in order to highlight the most important achievement of each work. Lengthy block copolymer names have also been removed in most cases, in lieu of the type (i.e., tri-block, homopolymer) so that the individual methods described can be seen in a more general system framework. Some of the extensive deleted sections have been shortened/rewritten and inserted in the section so that significant points are not lost, but only the excess detail. DiscussionThe discussion section has been significantly lengthened and clarified. Each technology is summarized to shed light upon the benefits and hurdles of each material that led to its individual ranking in our system. Each score is justified and explained within our metric.Throughout the text, an effort was made to reduce excess detail and highlight the major hurdles achieved by each researcher cited. Primary accomplishments and future needs for each material were further emphasized throughout the body of this paper.Specific responses to the referees follows. RESPONSES TO REVIEWER COMMENTS REFEREE 1Within their extensive review the authors state that in their review "… the published literature describing … membrane technologies (i.e. nanotechnologies) is critically reviewed and discussed within the context of conventional membrane materials used in the relevant water treatment applications" and "a ranking system was developed …" (all citations from the Abstract) Yet...

Two-dimensional covalent triazine framework as an ultrathin-film nanoporous membrane for desalination

“…In particular, new membranes with enhanced water permeability compared to currently available polyamide--based membranes may have an important role to play in lowering energy consumption. 4 Several nanostructured materials have been previously investigated as promising membrane candidates such as carbon nanotubes (CNTs) 5,6,7 and zeolites 8,9 . Despite CNTs' high water permeability, 5 membranes based on CNTs exhibit poor salt rejection, and it remains challenging to fabricate membranes with dense, well--aligned nanotubes.…”

“…Despite CNTs' high water permeability, 5 membranes based on CNTs exhibit poor salt rejection, and it remains challenging to fabricate membranes with dense, well--aligned nanotubes. 6,7 Zeolite membranes, on the other hand, possess three--dimensional networks that can effectively reject salt ions 8 but show relatively low water permeability. 9 Recently, nanoporous one--atom--thick graphene, the thinnest possible membrane made from matter, has drawn considerable attention and been shown computationally to provide significantly enhanced permeability compared to polyamide while maintaining excellent ability to reject salt.…”

“…From our calculations, we predict that the water permeability of CTF--1 can be as high as 64.2 L cm --2 day --1 MPa --1 , two to three orders of magnitude larger than currently available polyamide--based membranes. 28 Thanks to the porous nature of CTF materials, the pore density of CTF membranes is extremely high (i.e., ~5 x10 13 pores per cm 2 for CTF--1) and this high density substantially facilitates water permeation through the membrane. As a comparison, the pore density of the nanoporous graphene membrane recently achieved experimentally by Surwade et al is estimated to be ~10 12 pores per cm 2 .…”

Nanomembrane Materials Based on Polymer Blends

“…They are densely packed polymer membranes with organic or inorganic nanoparticles introduced into their structure. MMMs seek to take advantage of both the low cost and ease of fabrication of organic polymeric membranes, and the mechanical strength and functional properties of inorganic materials [46]. Zimmerman et al [42] wrote about MMMs as a way to push the limitations of polymeric membranes for gas separation.…”

Reverse osmosis as one-step wastewater treatment: a case study on groundwater pollution

“…Water molecules, on the other hand, pass freely through the membrane, thus creating a purifi ed product stream. The water produced (Permeate) satisfi es most of the water quality standard parameters 8, 10, 12 .…”