Environmental Fate and Transport - Nanotechnology and the Environment - page 125

be.attracted.to.one.another.and.to.other.environmental.constituents..(The.term.“phy-
sisorption”.refers.to.adsorption.as.a.result.of.van.der.Waals.forces.) Nanoparticles.
will. tend. to. agglomerate. unless. this. physisorption. is. inhibited. As. the. size. of. the.
agglomerates. increases,. buoyancy. is. reduced. and. the. force. of. gravity. causes. the.
particles. to. settle. out. of. suspension. If. the. nanoparticles. have. similar. electrostatic.
surface. charges,. however,. the. repulsive. force. will. counter. the. attraction. resulting.
from.van.der.Waals.forces.and.keep.particles.in.suspension. Nanoparticles.also.can.
adsorb.to.natural.organic.matter..That.may.either.increase.the.particles'.buoyancy.
or.disrupt.subsequent.agglomeration,.thereby.allowing.the.nanoparticles.to.remain.
suspended. Other.environmental.interactions.such.as.dissolution.or.biodegradation.
also.can.reduce.the.concentration.of.nanoparticles.in.suspension. As.a.result.of.the.
various.forces.acting.on.nanoparticles,.which.become.even.more.complex.than.this.
simple.conceptual.model.when.considering.transport.through.soil,.the.concentration.
of. nanoparticles.in. solution. does. not. remain. at. equilibrium. but. changes. over. time.
and.over.distance.from.the.discharge.point.
Sections.6.2.and.6.3.describe.the.forces.that.affect.the.fate.and.transport.of.nanopar-
ticles. (Section.6.6.lists.the.symbols.used.in.mathematical.equations.in.those.sections.)
As.with.any.model,.the.mathematics.can.approximate.only.real-world.complexities.
The. nanoparticles'. characteristics. such. as. a. shape. or. variance. in. composition. will.
affect.the.material's.chemical.properties. Further,.the.environmental.characteristics.
of.the.suspending.medium.such.as.the.pH,.hardness,.mineral.content,.ionic.strength,.
types.and.amounts.of.dissolved.organic.matter,.and.especially.the.characteristics.of.
sediment/soil.will.affect.the.environmental.fate.and.transport.of.nanomaterials. Sec-
tion.6.4.summarizes.research.indings.regarding.the.fate.and.transport.of.the.target.
nanomaterials,.which.account.for.the.effects.of.some.of.those.characteristics.
. natuRe oF nanoMateRials in the enViRonMent
Special.considerations.unique.to.predicting.the.fate.and.transport.of.nanomaterials.
can.be.divided.into.two.general.groups:.(1).those.related.to.the.physical.manifesta-
tion.of.the.materials,.and.(2).those.related.to.special.chemical.properties.that.affect.
their.reactivity.and.interactions.with.their.surroundings. Each.is.discussed.below.
6.2.1 p hysiCal m aniFestatiOn OF n anOmaterials : p artiCle s iZe
d istributiOn and F OrmatiOn OF m Obile s uspensiOns
Nanoparticles.can.form.suspensions.in.air.or.water,.and.can.be.transported.through.the.
environment.in.such.suspensions. The.suspension.of.nanoparticles.is.not.an.equilibrium.
phenomenon,.but.depends.in.part.on.the.particle.size.and.changes.in.particle.size.that.
result.from.collisions.and.reactions.in.the.environment,.as.discussed.below. Other.fac-
tors.that.affect.the.suspension.of.nanoparticles.are.discussed.in.subsequent.sections.
With.few.exceptions,.preparations.of.nanomaterials.are.not.of.uniform.particle.
size. Rather,. nanopreparations. consist. of. a. distribution. of. varying. particle. sizes.
When.a.nanomaterial.is.released.into.a.luid.environment,.such.as.air.or.water,.the.
size.distribution.will.begin.immediately.to.change.as.the.result.of.differential.settling.