Tiny tubes de­sali­nate water faster, cheaper

Chemical Industry Digest - - New Developments -

Cur­rently, de­salin­i­sa­tion is too ex­pen­sive and en­ergy in­ten­sive for large-scale fea­si­bil­ity. The study, pub­lished in the jour­nal ‘Sci­ence’, showed that car­bon nano tubes are bet­ter at de­salin­i­sa­tion than any other ex­ist­ing method.

A car­bon nan­otube is like an im­pos­si­bly small rolled

up sheet of pa­per, about a nanome­ter in di­am­e­ter. For com­par­i­son, the di­am­e­ter of a hu­man hair is 50 to 70 mi­crom­e­ters — 50,000 times wider. The tube’s minis­cule size, ex­actly 0.8 nm, only al­lows one water mol­e­cule to pass through at a time. This sin­gle-file lineup dis­rupts the hy­dro­gen bonds, so water can be pushed through the tubes at an ac­cel­er­ated pace, with no bulk­ing, the study said.

To con­duct the re­search, Meni Wa­nunu, As­so­ci­ate Pro­fes­sor of Physics at North­east­ern Uni­ver­sity in Bos­ton and post doc­toral stu­dent Robert Hen­ley col­lab­o­rated with sci­en­tists at the Lawrence Liver­more Na­tional Lab­o­ra­tory in Cal­i­for­nia. In ad­di­tion to be­ing pre­cisely the right size for pass­ing sin­gle water mol­e­cules, car­bon nan­otubes have a neg­a­tive elec­tric charge. This causes them to re­ject any­thing with the same charge, like the neg­a­tive ions in salt, as well as other un­wanted par­ti­cles. The find­ing offers a novel sys­tem that could have ma­jor im­pli­ca­tions for the future of water se­cu­rity.

A sim­i­lar study which was done at Lawrence Liver­more Na­tional Lab­o­ra­tory, Cal­i­for­nia, by Ramya H. Tunuguntla and col­leagues re­vealed that at just the right size, car­bon nan­otubes can fil­ter water with bet­ter ef­fi­ciency than bi­o­log­i­cal pro­teins. They found that car­bon nano tubes with a width of 0.8 nanome­ters out­per­formed aqua­por­ins, a class of bi­o­log­i­cal pro­teins, in fil­ter­ing ef­fi­ciency by a fac­tor of six.

These nar­row car­bon nan­otube porins (nCNTPs) were still slim enough to force the water mol­e­cules into a sin­gle-file chain. The re­searchers at­tribute the dif­fer­ences be­tween aqua­por­ins and nCNTPS to dif­fer­ences in hy­dro­gen bond­ing - whereas pore-lin­ing residues in aqua­por­ins can do­nate or ac­cept H bonds to in­com­ing water mol­e­cules, the walls of CNTPs can­not form H bonds, per­mit­ting unim­peded water flow.

The nCNTPs in this study main­tained per­me­abil­ity ex­ceed­ing that of typ­i­cal salt­wa­ter, only di­min­ish­ing at very high salt con­cen­tra­tions. Lastly, the team found that by chang­ing the charges at the mouth of the nan­otube, they can al­ter the ion se­lec­tiv­ity.

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