Environmental Engineering Reference
In-Depth Information
will.not.be.removed.during.settling,.unless.by.the.action.of.coagulants.or.locculants.
or.by.the.adsorption.of.the.nanoparticles.onto.large.particles.[3]. For.further.discus-
sion.of.the.forces.affecting.the.settling.of.nanoparticles,.see.Chapter.6.
7.2.2 
C OagulatiOn and  F lOCCulatiOn
Coagulation.and.locculation.are.typically.used.to.remove.solids.in.water.treatment;.
certain.wastewater.treatment.applications.can.include.these.processes. Coagulation.
can. facilitate. the. removal. of. nanomaterials. prior. to. sedimentation. or. membrane.
separation.[3].
Coagulation.refers.to.the.net.reduction.in.electrical.repulsive.forces.at.particle.
surfaces.to.allow.them.to.agglomerate. In.a.treatment.plant,.operators.rapidly.mix.
a.coagulant.(such.as.aluminum.or.iron.salts,.or.long-chain.polyelectrolytes).into.the.
water.to.destabilize.colloids. Flocculation.is.the.process.of.aggregating.those.par-
ticles.by.chemical. bridging.between.particles. After.the.coagulation.step,.water.is.
slowly.mixed.to.allow.particles.to.collide.and.loc.to.form. Sedimentation.removes.
the.loc,.or.membrane.separation.can.be.used.to.polish.the.water.
Huang. et. al.. [22]. performed. jar. tests. to. evaluate. the. optimal. dosage. of. the.
coagulant.poly-aluminum.chlorate.(PACl).and.the.optimum.pH.required.to.remove.
nanoscale. silica. from. chemical. mechanical. polishing. wastewater. generated. from.
semiconductor.manufacturing. Prior.to.use,.the.silica.present.in.the.polishing.slurry.
has.a.uniform.particle.size.of.100.nm. After.the.polishing.process,.the.colloidal.sil-
ica.particles.present.in.the.wastewater.range.in.size.from.78.to.205.nm.and,.without.
pretreatment,.can.penetrate.and.clog.the.microiltration.membrane. The.researchers.
found.that.supernatant.from.the.jar.tests.had.the.lowest.turbidity.when.the.pH.was.
around. 6. and. the. concentration. of. PACl. was. greater. than. 10. mg/L. At. pH. 6,. the.
PACl.acts.to.neutralize.the.negatively.charged.silica.and.to.destabilize.the.colloidal.
particles. Supernatant.representative.of.the.range.of.optimal.conditions.identiied.in.
the.settleability.tests.was.then.subjected.to.ilterability.testing.by.measuring.the.time.
to.pass.50.mL.of.the.supernatant.through.the.microiltration.membrane. This.testing.
conirmed.that.a.pH.of.6.and.a.PACl.concentration.of.30.mg/L.produced.the.shortest.
iltration.time..The.coagulation.enlarged.the.particle.size.such.that.nearly.all.the.par-
ticles.were.greater.than.4000.nm.in.diameter. Although.subsequent.microiltration.
through.a.500-nm.membrane.removed.approximately.95%.of.the.silica,.silica.still.
remained.in.the.treated.wastewater.at.a.concentration.of.44.mg/L.[22].
Kvinnesland.and.Odegaard.[23].studied.the.effect.of.different.polymers.on.the.
coagulation. and. locculation. of. humic. substances. present. in. water. primarily. as.
nanoparticles.less.than.100.nm.in.size. For.the.purposes.of.their.study,.they.deined.
coagulation.as.the.process.by.which.the.nanoparticles.formed.aggregates.that.could.
be.removed.by.a.100-nm.ilter,.and.locculation.as.the.process.by.which.the.particles.
further.agglomerated.for.removal.by.an.11,000-nm.ilter. The.researchers.found.that.
the.ive.different.polymers.achieved.the.same.maximum.removal.of.nanoparticles.
via.coagulation.(approximately.95%.removal). The.coagulation.was.achieved.by.the.
addition. of. cationic. charge. regardless. of. the. type. of. polymer. applied. Removal. of.
the.humic.substances.by.locculation.varied.according.to.the.charge.density.of.the.
different.polymers.[23].
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