Biomedical Engineering Reference
In-Depth Information
19.3.2 Advanced Membranes
In. approximately. the. past. 5. years. many. new. classes. of. membranes. with. functional.
nanoscale. components,. or.those.inspired. by.biological. systems,. have. been.developed. as.
discussed. in. a. recent. review. article.
47
. One. kind. of. advanced. membranes. is. the. fouling-
resistant. membrane
48,49
. with. integrated. polymeric. brushes. and. nanostructures,. which.
allow. it. to. have. both. hydrophilic. and. hydrophobic. regions. that. mitigate. fouling.. A. sec-
ond.kind.of.advanced.membrane,.composed.of.aligned.carbon.nanotubes.(CNTs),.exhibits.
super-high.lux.for.water.transport.
18,19
.The.measured.luxes.were.found.to.be.up.to.three.
orders.of.magnitude.higher.than.those.predicted.by.conventional.Hagen-Poiseuille.low.
theory.. These. aligned. CNT. membranes. were. shown. to. have. average. tube. diameters. of.
1.6 nm
18
.and.ultra-smooth,.hydrophobic.walls.that.do.not.follow.the.basic.no-slip.condi-
tion,.thereby.letting.luids.slide.along.the.wall.in.an.essentially.frictionless.coniguration,.
permitting.high.lux.through.the.CNTs..While.water.lows.nearly.effortlessly.through.the.
aligned.CNTs,.the.energy.cost.is.expended.in.the.water.molecules.entering.the.CNTs,.and.
in.the.modiication.of.the.mouths.of.the.CNTs.to.reject.small.salt.ions,.which.increases.the.
energy.needed..The.inal.energy.usage.for.these.membranes.is.still.to.be.determined.for.
concentrated.salt.solutions.with.a.high.rejection.potential.
19.3.3 Forward Osmosis
Desalination.by.the.process.of.forward.or.direct.osmosis.(FO).employs.a.highly.concen-
trated. solution. (also. referred. to. as. the. draw. solution). to. create. an. osmotic. pressure. that.
extracts.freshwater.from.salinated.water.across.a.semipermeable.membrane.
50
.Unlike.the.
RO. processes,. because. forward. osmosis. uses. osmotic. pressure. as. the. driving. force,. this.
method.operates.at.small.or.no.hydraulic.pressure,.which.reduces.the.amount.of.observed.
fouling.
17,51
.The.reliance.on.osmotic.pressure.necessitates.that.a.major.factor.in.the.choice.
of.the.draw.solution.is.the.osmotic.pressure.of.the.draw.solution.
17
.The.lack.of.hydraulic.
pressure.allows.forward.osmosis.setups.to.be.uncomplicated,.with.relatively.little.mem-
brane.support,.as.the.only.force.the.low.has.to.overcome.is.the.few.bars.provided.by.the.
membrane.itself
17
.as.the.physical.barrier..Possibilities.for.the.draw.solution.are.numerous;.
for.water.desalination.solutions.of.sulfur.dioxide,.aluminum.sulfate,.glucose,.potassium.
nitrate,. mixtures. of. glucose. and. fructose,. and. mixtures. of. ammonia. (NH
3
). and. carbon.
dioxide.(CO
2
).gases.have.all.been.suggested.as.draw.solution.candidates.
17
.However,.many.
draw.solutions.add.an.after.taste.or.an.odor.requiring.post-treatment..Sugar.(glucose.and.
fructose.primarily).has.been.a.popular.draw.solution.due.to.the.minimal.need.for.exten-
sive. post-treatment. of. desalinated. water.. The. major. energy. used. in. FO. is. to. remove. the.
draw.solution..For.draw.solutions.of.NH
3
.and.CO
2
,.which.both.are.soluble.in.water,.low-
quality. thermal. energy. can. be. used. to. decrease. the. solubility. and. decrease. the. energy.
needed.to.volatilize.the.NH
3
.and.CO
2
,.to.remove.as.a.gas.and.reuse.upstream..A.major.
hindrance. to. this. technology. has. been. the. development. of. an. appropriate. membrane.
52
.
Ideally,. FO. membranes. should. be. able. to. allow. a. high. lux. of. water. while. retaining. a.
high.rejection.rate.of.dissolved.solids,.demonstrating.compatibility.with.both.the.feedwa-
ter.and.the.draw.solution,.and.handling.the.mechanical.stresses.imposed.by.the.osmotic.
pressures.experienced.during.the.desalination.process.
52
.Currently.available.commercial.
membranes,.such.as.cellulose.triacetate.(CTA).membranes,.are.not.compatible.with.many.
of.the.preferred.draw.solutions..However,.due.to.the.low.pressure.across.the.membrane,.
fragile. membranes. made. from. aquaporins,. which. are. nature's. perfect. nanochannel. and.
only.permit.water.to.low.through,.can.potentially.be.used.for.FO.
