Environmental Engineering Reference
In-Depth Information
organic.matter.at.varying.concentrations.in.laboratory.solutions.and.at.background.
concentrations.in.actual.river.water.samples,.the.carbon.nanotubes.remained.in.a.sta-
ble,.dispersed.state.for.more.than.one.month..The.natural.organic.matter.appeared.to.
be.a.better.stabilizing.agent.than.sodium.dodecyl.sulfate,.a.surfactant.often.applied.
in.industrial.processes.to.stabilize.carbon.nanotubes.[18].
Studies. by. Chen. and. Elimelech. [14]. found. a. similar. response. with. fullerenes.
where.a.concentration-dependent.inverse.effect.of.a.was.observed..The.presence.of.
excess.humic.acid.appeared.to.increase.the.electrostatic.hindrance.to.agglomeration..
This.increased.the.critical.coagulation.concentration.(i.e.,.the.minimum.concentra-
tion.of.a.coagulant.necessary.to.suppress.the.double-layer.charge.and.allow.particles.
to.agglomerate).from.8.0.to.19.4.mM.MgCl
2
..This.result.is.consistent.with.and.pre-
dictable.based.on.DLVO.kinetics..However,.Chen.and.Elimelech.also.observed.that.
high.calcium.concentrations.above.the.critical.coagulation.concentration.(40.mM).
increased.the.rate.of.agglomeration.of.C60
.
fullerenes.relative.to.that.in.an.untreated.
suspension..Although.not.addressed.in.the.article,.this.higher.rate.of.agglomeration.
can.be.accounted.for.by.changes.in.collision.kinetics.relative.to.the.humic.acid.plus.
fullerene.
P
(
r
)/a.values.
Nowack.reviewed.the.literature.on.C60.fullerenes.naturally.occurring.in.ancient.
geologic. materials. and. concluded. that. “the. stability. of. fullerenes. under. geologic.
conditions.for.hundreds.of.millions.of.years.shows.that.they.are.truly.recalcitrant.in.
the.environment.”.While.pure.fullerenes.are.nearly.insoluble.in.water,.under.certain.
conditions. fullerenes. will. form. polymorphic. hexagonal. unit. cell. agglomerates. in.
water. referred. to. as. nano-C60.. These. agglomerates,. approximately. 25. to. 500. nm.
in. size,. carry. a. strong. negative. charge. [19].. The. physical. and. chemical. properties.
of.the.agglomerate.nano-C60,.such.as.color,.hydrophobicity,.and.reactivity,.are.sig-
niicantly. different. as. the. result. of. the. differing. crystalline. structure. that. can. be.
manipulated.by.controlling.the.solution.pH.and.the.rate.at.which.water.is.added..The.
critical.coagulant.concentration.for.these.nano-C60.agglomerates.is.in.excess.of.500.
mM.NaCl,.indicating.a.signiicant.increase.in.electrostatic.hindrance.to.agglomera-
tion. relative. to. the. individual. fullerenes. [20].. Such. dramatic. deviations. in. surface.
properties.as.the.result.of.polymorphism.further.emphasize.the.importance.of.nano-
material.characterization.in.predicting.environmental.fate.and.transport.
Researchers.at.the.Georgia.Institute.of.Technology.also.have.studied.the.photo-
chemical.reactions.that.affect.fullerenes.in.an.aqueous.system..In.their.studies,.they.
prepared.various.polymorphic.agglomerate.forms.of.fullerenes.in.water:.nano-C60.
suspension. prepared. by. solvent. exchange,. nano-C60. suspension. prepared. by. soni-
cation,.C60.stabilized.in. water.with.polymers. and. surfactants,.and. C60.stabilized.
in.water.by.natural.organic.matter..They.evaluated.the.photochemical.reactivity.of.
the.various.dispersed.forms.of.C60.by.measuring.the.production.of.reactive.oxygen.
species.(speciically.the.singlet.oxygen.and.superoxide.radical.anion)..The.research-
ers. found. that. the. photochemical. reactivity. of. the. fullerenes,. or. the. ability. of. the.
particles.to.mediate.energy.and.electron.transfer,.was.a.function.of.the.polymorphic.
nature.of.the.nanomaterial.and.the.characteristics.of.the.stabilizing.molecules.[21].
Other.researchers.at.Purdue.University,.funded.by.the.U.S..EPA's.National.Cen-
ter.for.Environmental.Research.(NCER).for.the.period.from.2007.to.2009,.are.study-
ing. the. photodegradation. of. fullerenes. and. single-walled. carbon. nanotubes.. The.
