Environmental Engineering Reference
In-Depth Information
7.2.4 s
and
F
ilters
Brownian.diffusion.is.the.dominant.mechanism.governing.the.transport.of.nanopar-
ticles.through.the.granular.ilter.
As.they.pass.through.the.ilter,.nanoparticles.are.
removed.from.the.luid.stream.by.several.processes,.including:
.
1.. Brownian.diffusion.causes.the.nanoparticles.to.agglomerate.into.larger.par-
ticles.or.to.agglomerate.with.the.ilter.grains.
.
2.. Nanoparticles. are. immobilized. by. gravitational. sedimentation. because.
their.density.is.higher.than.that.of.the.ilter.medium,.or.the.low.velocity.is.
reduced.within.the.ilter.bed.
.
3.. Nanoparticles. are. intercepted. by. physical. contact. with. the. ilter. medium.
[30].
Attachment.of.particles.to.the.ilter.medium.is.affected.by.a.variety.of.
forces,.described.by.the.term.“attachment.eficiency,”.as.discussed.further.
below.[31].
The.attachment.eficiency.(a).is.the.ratio.of.the.rate.of.particle.deposition.to.the.
rate. of. particle. collisions. with. the. ilter. medium. [31].. This. parameter. is. governed.
by.various.phenomena,.including.van.der.Waals.forces,.the.forces.of.solvency,.and.
electrostatic.repulsive.forces.(see.Chapter.6).
When.a.is.less.than.unity,.conditions.
are.not.conducive.to.particle.attachment.
When.a.equals.unity,.no.barriers.to.particle.
attachment.exist.
When.a.is.greater.than.unity,.particles.may.be.attracted.to.the.sur-
face.of.the.ilter.medium.over.small.distances.
However,.for.very.small.nanoparticles.
less.than.2.nm.in.size,.the.relative.effects.of.the.forces.governing.the.parameter.a.can.
be.unpredictable.and.different.from.those.of.larger.particles.
If.smaller.nanoparticles.
aggregate.to.form.colloidal.material,.as.has.been.observed.for.C60.fullerenes.and.
some.other.particles,.the.behavior.of.the.material.within.a.granular.ilter.will.differ.
from.the.response.predicted.based.on.the.size.of.the.original.manufactured.particle..
Therefore,. researchers. have. concluded. that. direct. measurement. of. the. mobility. of.
nanoparticles.is.currently.the.most.accurate.means.by.which.to.quantify.their.behav-
ior.in.porous.media.[32].
Nanoparticle.mobility.within.a.porous.medium.is.a.function.not.only.of.size,.but.
also.of.surface.chemistry.[32].
Lecoanet,.Bottero,.and.Wiesner.[30].conducted.labo-
ratory.experiments.to.quantify.the.mobility.of.eight.different.manufactured.nanoma-
terials.in.a.porous.medium.of.glass.beads,.which.the.researchers.indicated.would.be.
representative.of.a.water.treatment.plant.ilter.or.a.sandy.groundwater.aquifer.
Their.
results.indicated.that.different.forms.of.nanoparticles.with.the.same.composition.have.
different.mobilities.
For.example,.of.the.carbon-based.particles.tested,.single-walled.
nanotubes.and.fullerols.(hydroxylated.C60).passed.through.the.porous.medium.more.
rapidly. than. the. colloidal. aggregate. form. of. C60
.
known. as. nano-C60.
The. solubi-
lized.forms.of.the.particles.are.more.mobile.than.the.suspended.form.[33].
Conditions.in.the.waste.stream,.such.as.pH.and.ionic.strength,.will.also.affect.
the.behavior.of.nanoparticles.in.water.and.the.attachment.eficiency.of.nanoparticles.
passing.through.a.ilter.medium.[31].
As.noted.above,.Fortner.et.al..[26].observed.
that.the.pH.and.ionic.strength.of.water.affect,.respectively,.the.particle.size.and.sta-
bility.of.the.nano-C60.in.solution.
