Biomethylation of Arsenic in Contaminated Soils - Environmental Contamination: Health Risks and Ecological Restoration - page 251

inputs.should.create.a.larger.and.more.active.microbial.community.to.trans-
form. soil. As. in. to. volatile. forms.. Indeed,. Dobran. and. Zagury. (2006). who.
found.As.methylated.products.were.not.found.in.soils.with.limited.nutrients.
available.for.microbial.growth.
Subsequent. studies. on. naturally. occurring. substrates. such. as. animal.
manure.have.provided.evidence.that.these.inputs.can.affect.As.volatilization..
Additions.of.fresh.manure.to.soil.can.increase.the.bioavailability.of.As.spe-
cies. that. appear. to. be. linked. to. higher. rates. of. volatilization. (Walker. et. al..
2004)..Mohapatra.et.al..(2008).studied.the.effect.of.cow.dung.on.As.volatiliza-
tion.of.methanogenic.bacteria.in.an.anaerobic.digester..They.reported.a.sur-
prisingly.large.methylation.yield.of.35%.under.the.most.suitable.conditions.
(Table.12.1)..Moreover,.the.volatilization.rates.decreased.when.rates.of.manure.
inputs.reached.a.threshold,.which.they.attributed.to.the.formation.of.meth-
ane.that.could.inhibit.the.formation.of.volatile.As.and.led.to.the.production.
of.non-volatile.compounds..Similarly,.Edvantoro.et.al..(2004).added.manure.
and.urine.to.soils.contaminated.by.As.and.DDT..Their.5.month.experiments.
demonstrated.that.increasing.manure.applications.caused.more.As.losses.
Investigations.of.model.compounds,.most.often.on.pure.cultures,.have.been.
done.to.determine.the. mechanisms.that.drive.As.volatilization..Turpeinen.
et.al..(2002).showed.that.a.very.simple.carbon.source.(0.2%.glucose).did.not.
stimulate. As. methylation. or. induce. the. reduction. of. soluble. As. in. soils..
Conversely,. Gao. and. Burau. (1997). showed. that. cellulose,. a. more. complex.
and.recalcitrant.compound,.stimulated.the.soil.microbial.community.which.
resulted.in.elevated.evolution.of.volatile.As..There.was.a.linear.relationship.
( R 2 .=.0.99).between.cellulose.application.rate.(ranging.from.0%.to.5%.wet.wt.).
and.volatile.arsine.production.in.the.soil.freshly.spiked.with.10.mg.inorganic.
As/kg.soil.over.a.70.day.incubation..However,.in.the.same.study,.the.addi-
tion.of.cellulose.to.soil.containing.organic.As.as.sodium.cacodylate.caused.a.
decrease.in.demethylation.of.CA..The.authors.suggested.the.microorganism.
preferred.readily.released.C.from.cellulose.over.that.in.cacodlyate.
In. pure. culture,. Huysmans. and. Frankenberger. (1991). found. that. a.
Penicillium . sp.. was. stimulated. to. convert. MMAA. from. TMA. when. amino.
acids.(phenylalanine,.isoleucine,.and.glutamine).were.added.to.pure.culture.
media.. Conversely,. this. same. study. showed. that. carbohydrates. and. sugar.
acids.suppressed.this.process.
Incubation Time and Rates of As Volatilization
Reported.incubation.times.for.As.biomethylation.varies.from.days.to.months.
(Table. 12.1).. Rates. of. As. volatilization. varied. considerably. across. various.
studies..This.is.likely.to.the.highly.variable.environmental.conditions,.types.
of. organic. substrates. that. may. or. may. not. have. been. used,. soil. types,. and.
other.unknown.factors.
In.general,.anaerobic.incubation.times.used.in.published.studies.tend.to.
be.longer.than.aerobic.incubation..Mohapatra.et.al..(2008).studied.the.rate.of.