Environmental Engineering Reference
In-Depth Information
inadvertently.that.vary.in.content.and.concentration.between.manufacturers..Work.
by. Plata. et. al.. [9]. illustrates. this. point.. They. evaluated. the. co-products. of. nano-
tube.synthesis.by.testing.various.samples.of.commercially.available,.puriied.carbon.
nanotubes..Samples.of.SWCNTs.contained.iron,.cobalt,.and.molybdenum.(used.to.
catalyze.nanotube.synthesis).at.1.3.to.4.1%.total.metals..The.samples.also.variously.
contained.chromium,.copper,.and.lead.at.0.02.to.0.3.parts.per.thousand..Such.impu-
rities.could.affect.the.toxicity.of.a.sample.of.SWCNTs.
Third,.some.nanomaterials.contain.fundamentally.toxic.materials..A.recent. in
vitro .study.using.human.lung.epithelial.cells.[10].showed.that.cobalt.and.manganese.
entering.the.cell.as.nanoparticles.showed.eight.times.the.toxicity.of.their.respective.
water-soluble. metal. salts,. purportedly. because. the. latter,. as. ions,. could. not. enter.
the. cells.. This. so-called. “Trojan-horse”. mechanism. also. may. operate. with. quan-
tum.dots.produced.for.medical.applications,.which.are.essentially.spherical.heavy.
metal.alloys.coated.with.a.material.such.as.an.immunoreactive.protein.intended.to.
have.a.speciic.biological.activity..If.white.blood.cells.engulfed.these.quantum.dots,.
the. coating. could. be. broken. down. by. degradative. enzymes. and. the. heavy. metals.
released. into. the. cytoplasm. of. the. cell.. The. central. core. of. the. quantum. dot. then.
becomes.bioavailable.and.therefore.able.to.manifest.toxicity.to.various.components.
within.the.cell.
The.design.of.experiments.that.measure.toxicity.also.can.inluence.the.results..
Just.as.with.traditionally.toxic.materials,.the.form.used.for.dosing.a.nanomaterial.can.
throw.into.question.whether.an.experiment.is.really.scientiically.valid..If.a.nanoma-
terial.is.practically.insoluble.in.water,.then.many.of.the.doses.used.in.experiments.
may.not.be.applicable.to.real-world.situations..In.fact,.one.can.ind.studies.reported.
in.the.literature.that.use.doses.or.concentrations.that.may.not.be.realistic.should.a.
nanomaterial.enter.a.waste.stream..For.example,.C60.fullerenes.are.very.insoluble.
in.water..To.test.the.toxicity.of.fullerenes,.researchers.have.used.a.successive.series.
of.water-insoluble.solvents.or.other.artiicial.means.(as.discussed.in.Section.8.4.1).to.
get.them.into.aqueous.suspension..Consequently,.many.researchers.question,.as.they.
have.for.decades.about.conventional.toxic.compounds,.“Will.studies.performed.in.
the.laboratory.be.applicable.to.what.might.happen.in.the.ield?”
Concerns.about.the.toxicity.of.nanomaterials.can.be.put.in.a.broader.perspec-
tive..With.regard.to.aquatic.systems,.one.group.of.researchers.[11].stated.that.“[t]he.
increasing.worldwide.contamination.of.freshwater.systems.with.thousands.of.indus-
trial. and. natural. chemical. compounds. is. one. of. the. key. environmental. problems.
facing.humanity.”.This.statement.does.not.acknowledge.the.fact.that.natural.waters.
have.some.ability.to.self-purify.[12]..Ordinary.processes.that.are.always.at.work.in.
nature.naturally.cleanse.the.water.column:.oxygenation.of.running.waters,.sorption.
of.pollutants.by.suspended.sediment.and.subsequent.iltration.by.wetlands,.complex-
ation.by.particulate.or.dissolved.organic.matter,.microbial.mineralization.of.pollut-
ants,.and.puriication.by.ilter-feeding.organisms..Thus,.any.discussion.of.potential.
environmental. effects. of. nanotechnology. must. consider. the. fate. and. transport. of.
those.materials.in.the.environment,.which.may.limit.an.organism's.exposure.
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