Environmental Engineering Reference
In-Depth Information
expected.distribution..The.occurrence,.distribution,.and.mobility.of.arsenic.
are.dependent.on.the.interplay.with.geochemical.factors.such.as.redox.poten-
tial.(Eh),.pH.or.acidity,.redox.pairs,.competing.anions,.and.reaction.kinetics.
StabilizationofArsenic
General Principle and Mechanisms of Stabilization
In.general.terms,.stabilization.is.a.process.where.additives.are.mixed.with.
waste.to.convert.the.hazardous.constituents.into.a.form.that.minimize.their.
rate.of.migration.and.level.of.toxicity..Stabilization.is.conducted.by.the.addi-
tion.of.reagents.that.improve.the.handling.and.physicochemical.properties.
of.the.waste,.decrease.the.surface.area,.which.transfers.pollutants,.and.limit.
the.solubility.and.toxicity.of.the.contaminants..Although.limiting.the.solu-
bility.and.toxicity.is.not.highly.anticipated.in.practical.application,.stabiliza-
tion. is. regarded. as. one. of. the. most. effective. and. eficient. methods. for. the.
immobilization.of.contaminants.in.waste.
The. fundamental. mechanisms. of. stabilization. are. mainly. one. or. more.
of. the. following. phenomena:. macroencapsulation/microencapsulation,.
adsorption,. precipitation,. and. detoxiication.. With. macroencapsulation,.
contaminants.in.the.pores.of.waste.are.physically.held.in.a.large.structural.
matrix. whereas. microencapsulation. is. the. phenomenon. where. the. pollut-
ants.are.entrapped.in.the.crystalline.structure.of.the.matrix..Adsorption.is.
a.mechanism.where.contaminants.are.electronically.bonded.to.additives.by.
van.der.Waals.forces.or.hydrogen.bonding..In.the.precipitation.process,.con-
taminants.from.the.waste.form.precipitates.with.additives,.such.as.hydrox-
ide,.carbonate,.silicate,.and.sulide..These.precipitates.then.become.a.part.of.
the. material. structure. contained. in. the. stabilized. mass.. Detoxiication. is. a.
mechanism.that.changes.the.constituents.of.a.toxic.chemical.into.less.toxic.
or.nontoxic.forms..For.example,.the.change.of.oxidation.state.from.As(III).to.
As(V).during.stabilization.lowers.the.solubility.and.toxicity.of.arsenic..The.
use.of.these.mechanisms.with.different.stabilizing.agents.will.be.discussed.
in.the.next.chapter.
Stabilizing Agent
Cement
Cement.is.used.as.the.principal.reagent.in.stabilization,.with.Portland.cement,.
which.is.a.mixture.of.limestone.and.clay.formed.at.high.temperature,.being.
the. most. commonly. used. for. stabilization.. Portland. cement. is. composed.
of. particles. with. a. size. ranging. between. 1. and. 50.μm,. mainly. comprising.
tri-calcium. silicate. (45%-60%),. di-calcium. silicate. (15%-30%),. tri-calcium.
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