Recent Advances in Water Desalination through Biotechnology and Nanotechnology - Bionanotechnology: Global Prospects - page 375

19.3.1 Microfluidic and Nanofluidic Concentration Polarization for Desalination

Concentration. polarization. in. most. water. desalination. systems. is. considered. to. be. a. loss.
term,.and.immense.resources.and.effort.have.been.employed.in.better.understanding.and.
mitigating.polarization.effects. 15,44,45 .Recent.research.has.revealed.that.the.imbalances.cre-
ated. by. concentration. polarization. (enrichment. and. depletion. regions). can. be. sustained.
indeinitely.when.triggered.across.a.nanochannel.connecting.two.microchannels. 46 .Utilizing.
this.fact,.researchers.demonstrated.the.ability.to.desalinate.water.using.a.setup.as.shown.
in.Figure 19.7,.thereby.exploiting.polarization.for.separations..However,.for.eficient.sepa-
ration,. pre-treatment. was. needed. for. the. removal. of. Ca 2+ . ions,. and. physical. iltration. for.
elimination. of. precipitation. and. large. debris.. The. membrane-less. system. that. was. tested.
was. primarily. driven. by. electrokinetic. lows. and. achieved. a. 99%. salt. rejection. ratio. 5 . In.
addition.to.achieving.seawater.desalination,.this.system.was.shown.to.remove.most.solid.
particles,.microorganisms,.and.biomolecules.due.to.their.charged.nature. 5 .The.energy.con-
sumption.of.this.system.for.the.actual.separation.process.is.modest.due.to.the.delection.of.
ions,.rather.than.physical.displacement.across.a.membrane,.and.the.low.low.resistance.of.
microchannels,.with.an.energy.consumption.of.approximately.5.Wh/L.for.low.rates.of.0.25.
μl/min. 5 .Although.considerable.challenges.lie.ahead.for.the.scaling.up.of.a.system.using.
the. concentration. polarization. technique,. massive. parallelization. of. the. proof-of-concept.
device.is.estimated.to.deliver.approximately.180-288 ml/min.for.a.small-scale.system. 5

A B

V

Gravity-fed
sea water

Nano-
channel

GND

Repulsion
zone

V

Desalted

GND

V

V V

Micro-
channels

GND

SG desalter

GND

Concentrate Fresh

(a)

V

ANY charged
species

Desalted
water

V +
pressure

Sea water

V

Ion-depletion
boundary

∆ V

Nanojunction

GND GND

(b)

FIGURE 19.7
(A).Small-scale.setup.for.concentration.polarization.desalination..(From.Shannon,.M.A.,. Nature Nanotechnology ,.
5,. 248-250,. 2010.). (B). Schematic. of. microchannel-nanochannel. prototype. tested. for. water. desalination. using.
concentration.polarization..(From.Kim,.S.J.,.et.al.,. Nature Nanotechnology ,.5,.297-301,.2010.)