Environmental Engineering Reference
In-Depth Information
10.1.3 e
FFeCts
OF
p
artiCle
s
iZe
How.does.the.particle.size.relate.to.the.reactivity.of.nZVI?.As.described.in.Chap-
ter.2,.nanoparticles.may.behave.differently.than.their.bulk.counterparts.due.to.the.
increased.relative.surface.area.per.unit.mass.and/or.the.inluence.of.quantum.effects..
As.discussed.below,.the.typical.particle.sizes.of.nZVI.and.experience.with.granular.
ZVI.provide.insight.into.why.nZVI.can.be.so.effective.
For.a.metal.such.as.iron,.quantum.effects.on.physical.and.chemical.properties.
are.negligible.above.a.particle.size.of.approximately.5.nm..(For.metal.oxides,.which.
have.a.lower.electron.density,.quantum.effects.may.become.evident.at.particle.sizes.
between.10.and.150.nm.[12].).Therefore,.given.the.typical.particle.sizes.of.commer-
cially.available.nZVI,.quantum.effects.are.probably.negligible..The.effectiveness.of.
nZVI.must.relate,.then,.to.particle.size.rather.than.to.quantum.behavior.
Previous.work.with.granular. (not. nano). ZVI. showed.that.the. rate.of.reductive.
dehalogenation.is.relatively.independent.of.contaminant.concentration.and.depends.
strongly. on. the. surface. area. of. the. iron. catalyst. [2].. The. smaller. the. particle,. the.
higher.the.percentage.of.the.total.number.of.atoms.on.the.surface.of.the.particle,.and.
thus.the.higher.the.reactivity..A.comparison.of.degradation.rates.for.carbon.tetra-
chloride.treated.by.granular.ZVI.and.nZVI.showed.that.the.higher.reaction.rate.with.
nZVI.resulted.from.the.high.surface.area,.not.from.a.greater.relative.abundance.of.
reactive.sites.on.the.surface.of.nZVI.or.the.greater.intrinsic.reactivity.of.surface.sites.
on.nZVI.[6,.12]..Some.data.suggest.that.reaction.with.nZVI.can.generate.different.
products. than. reaction. with. granular. ZVI,. although. the. mechanisms. causing. this.
apparent.difference.are.not.yet.understood.[12].
Over. time,. agglomeration. increases. the. effective. particle. size.. This. has. been.
observed,. as. described. above,. in. aged. reagent. samples.. Increases. in. particle. sizes.
can.limit.the.mobility.of.the.nZVI.because.larger.particles.cannot.remain.suspended.
in.and.transported.by.the.groundwater..Consideration.of.the.primary.physical.forces.
acting.on.nZVI.particles.suspended.in.water,.as.discussed.in.Section.6.2.1.and.shown.
in. Figure.6.4,. suggests. that. less. than. half. the. particles. above. 80. nm. in. size. will.
remain.in.stable.suspension..Phenrat.et.al..[81].studied.the.agglomeration.of.nZVI.in.
laboratory.experiments..They.found.that.agglomeration.occurred.in.two.stages..Dur-
ing.the.irst.stage,.the.nZVI.particles.rapidly.agglomerated.to.form.discrete.microm-
eter-sized.clusters..These.clusters.then.linked.to.form.chain-like.fractal.structures.in.
the.second.stage..The.rate.of.agglomeration.depended.on.the.particle.concentration.
and.was.affected.by.the.magnetic.forces.between.particles,.in.addition.to.the.forces.
discussed.in.Chapter.6..Agglomeration.occurred.rapidly:.for.a.2.milligram.per.liter.
(2.mg/L).solution.of.20-nm.nZVI.particles,.the.irst.stage.of.agglomeration.occurred.
in. 10. min.. These. results. illustrate. why. some. nZVI. reagents. are. modiied,. by. the.
inclusion.of.polymers.or.other.additives,.to.limit.agglomeration.
10.1.4
I
n
S
Itu
r
emediatiOn
with
n
Zvi
Manufacturers.typically.ship.nZVI.reagents.to.a.site.in.a.concentrated.slurry..It.may.
be.shipped.at.a.high.pH.or.under.nitrogen.atmosphere.to.limit.passivation..Workers.
at.the.site.dilute.this.slurry.to.the.desired.concentration..As.described.for.two.case.
studies.in.Section.10.1.6,.this.concentration.is.on.the.order.of.2.grams.per.liter.(g/L)..
