Biomethylation of Arsenic in Contaminated Soils - Environmental Contamination: Health Risks and Ecological Restoration - page 232

and. underground. water.. Arsenic. occurs. naturally. in. oxide,. hydrous. oxide,.
sulide,.phosphate,.and.other.minerals.(Garelick.et.al..2009)..In.soil.the.con-
centration.of.As.ranges.from.0.1.to.40.mg/kg,.with.a.median.concentration.of.
total.As.being.6.0.mg/kg.worldwide.(Bowen.1979)..Volcanic.activity,.thermal.
springs,.or.anthropogenic.activities.such.as.mining.and.industrial.production.
can.transport.or.concentrate.As.in.the.environment.(Nordstrom.2002).
Arsenic.is.carcinogenic.(Rosen.1971).and.has.been.ranked.the.number.one.
hazardous.substances.since.1997.in.the.U.S..Comprehensive.Environmental.
Response,. Compensation,. and. Liability. Act. (CERCLA). (Agency. for. Toxic.
Substance. and. Disease. 2010).. Inorganic. arsenic. is. classiied. by. the. U.S..
Environmental. Protection. Agency. as. a. class. A. human. carcinogen. (EPA.
1998).. Additionally,. arsenic. compounds. are. poisonous. and. classiied. into.
three. broad. toxicological. categories:. as. arsine. gas,. inorganic. compounds.
(solid.and.solution.phases),.and.organic.As.
Arsenic. has. four. states. of. oxidation. (V,. III,. 0,. or. -III). which. controls. its.
bioavailability.and.toxicity.to.organisms..However,.in.the.environment.the.
inorganic.forms.of.arsenate.(AsO 4 −3 ).(V).and.arsenite.(AsO 2 −2 ).(III).dominate.
(Cullen.and.Reimer.1989),.with.arsenite.being.more.toxic.(Lerman.et.al..1983,.
Vahidnia.et.al..2007)..Arsenate.is.toxic.because.of.its.similarity.to.phosphate,.
causing.phosphorylation.inhibition.(Moore.et.al..1983,.Coddington.1986).and.
forming. ADP-arsenate. that. is. unstable. (Crane. and. Lipmann. 1953,. Gresser.
1981,. Moore. et. al.. 1983).. Arsenite. causes. toxicity. by. reacting. with. dithiols.
such.as.glutaredoxin.and.with.sulfhydryl.groups.of.proteins.(Oehme.1972,.
Knowles.and.Benson.1983).
Direct. contamination. of. soils. with. As. can. occur. from. several. human.
activities..In.agriculture.arsenical.pesticides.and.herbicides.have.been.major.
sources.of.As.of.soils.(Smith.et.al..1998,.WHO.2001,.Liao.et.al..2005)..Currently,.
approximately. 50%. of. arsenic. production. is. pesticides,. with. organic. forms.
being.the.most.important.forms.(Matschullat.2000)..Another.important.and.
extensive.As.source.is.pressure-treated.wood.
From. the. 1970s. until. about. 2004,. 90%. of. the. wood. used. in. the. United.
States.for.outdoor.applications.was.treated.with.chromated-copper.arsenate.
(CCA)..Although.this.was.banned.in.2004.in.the.United.States,.some.seven.
billion.board.feet.was.used.widely.in.fences,.home.decks,.play.structures,.
and.other.uses.(Raloff.2004)..Arsenic.in.CCA-treated.wood.structures.and.
poles.can.leach.into.soils.or.as.CCA.wood.decomposes.As.is.released.to.soils.
(Zagury.and.Pouschat.2005)..Additionally,.CCA-treated.wood.manufactur-
ing. plants. have. caused. incidental. contamination. of. soil. with. As. (Zagury.
and.Pouschat.2005).
Incidental.soil.ingestion.by.children.is.an.important.exposure.pathway.for.
assessing.public.health.risks.associated.with.exposure.to.As-contaminated.
soils..The.importance.of.soil.ingestion.by.children.as.a.health.issue.has.been.
reported. by. many. researchers. and. highlights. the. importance. of. this. path-
way. in. terms. of. subsequent. chemical. exposure. (Basta. et. al.. 2001,. Scheckel.
et.al..2009,.Hale.et.al..2010).