Toxicity and Bioavailability of Heavy Metals and Hydrocarbons in Mangrove Wetlands and Their Remediation - Environmental Contamination: Health Risks and Ecological Restoration - page 382

Strong. wastewater. (10. NW). discharge. resulted. in. a. shift. of. the. positive.
relationship.between.Fe.plaque.and.ROL.to.a.negative.one.(Figure.17.4),.sug-
gesting.that.excessive.Fe.plaque.served.as.a.barrier.to.ROL.(Pi.et.al..2010b)..
The.concentrations.of.Fe.plaque.on.the.root.surface.could.reach.5-10.times.
the. concentrations. of. the. surrounding. sediments. (Sundby. et. al.. 1998).. Fe.
plaque.also.acted.as.a.physical.barrier.to.reduce.plant.uptake.of.metal.toxic-
ity.(Otte.et.al..1989,.Batty.et.al..2000)..Nevertheless,.the.role.of.Fe.plaque.in.
the.reduction.of.plant.uptake.of.metal.toxicity.is.still.arguable,.as.the.results.
were.not.always.convincing.and.conclusive.(Ye.et.al..1998,.Zhang.et.al..1998)..
For. phytoremediation. of. heavy. metals. using. wetland. plants,. the. net. effect.
of.the.presence.of.plants,.particularly.roots.and.associated.features,.such.as.
ROL. and. Fe. plaque,. on. the. behavior. of. heavy. metals. (e.g.,. mobilization. or.
immobilization).should.be.further.evaluated.
Other. factors,. such. as. sediment. texture,. organic. matter,. and. salinity,.
would. also. affect. the. bioavailability. of. heavy. metals. in. mangrove. sedi-
ments..The.clay-like.nature.of.mangrove.sediments.could.provide.a.physi-
cal. trap. for. ine. particulates. and. heavy. metals.. High. contents. of. organic.
matter,.especially.the.humic.substances.in.mangrove.sediments,.also.pro-
vided.strong.adsorptive.properties.in.binding.heavy.metals..Salinity.may.
also. affect. the. binding. of. pollutants. in. sediments.. Paalman. et. al.. (1994).
reported. that. when. river. water. mixed. with. seawater,. due. to. increases. in.
chloride.concentrations,.heavy.metals.such.as.Cd.get.mobilized.from.the.
sediment.and.dissolve.later.into.chloro-complexes..Tam.and.Wong.(1999b).
also.reported.that.mangrove.sediments.receiving.wastewater.prepared.in.
freshwater.had.slightly.higher.concentrations.of.nutrients.and.heavy.met-
als.and.larger.enrichment.factors.than.that.treated.with.saline.wastewater.
(salinity.15%).
Hydrocarbons
Mangrove. sediments. are. often. anaerobic. or. anoxic. just. a. few. centimeters.
below. the. surface. (Mitsch. and. Gosselink. 2000),. which. reduces. aerobic.
degradation. of. hydrocarbons,. especially. polycyclic. aromatic. hydrocar-
bons. (PAHs),. and. as. a. result,. they. accumulate. and. persist. in. deep. layers.
of.the.sediment.for.up.to.20.years.(Burns.et.al..1994,.2000)..ROL.of.wetland.
plants.has.the.potential.to.signiicantly.alter.both.microbial.and.chemical.
processes. in. the. rhizosphere,. such. as. increasing. the. aerobic. respiration.
(Schussler.and.Longstreth.1996).and.aerobic.degradation.and.the.transfor-
mation. of. environmental. pollutants. such. as. PAHs. (St-Cyr. and. Campbell.
1996,.Visser.et.al..2000).
Compared. to. plants,. microorganisms. may. play. a. more. signiicant. role.
in. removing. and. degrading. organic. pollutants.. Ke. et. al.. (2003a). found.
that. the. uptake. of. pyrene. (PYR),. a. four-ring. PAH,. by. B. gymnorrhiza . and.
K. candel ,.only.contributed.to.0.65%-0.88%.of.PYR.removal.and.that.most.
of. the. removal. (84.8%-92.5%). was. due. to. microbial. degradation.. There. is.