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12. Environmental science of
metals in the environment:sources, fate and transport
Rodica G. StanescuPolitehnica University of Bucharest
IasiRomania
June 2009
Gheorghe Asachi Technical University of IasiInstitute of International Health, Michigan State University
Polytechnic University, BucharestAlexandru Ioan Cuza University of IaiInstitute of Public Health, Bucharest
John E. Fogarty International Center, U.S. National Institutesof Health
http://www.fic.nih.gov/index.htmhttp://www.uaic.ro/uaic/bin/view/Main/http://en.wikipedia.org/wiki/Image:Universitatea_Politehnica_Bucuresti_logo.png8/3/2019 12 Environmental Science of Metals in the Environment Sources, Fate and Transport[2]
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What We Know
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Source, Fate and Transport
ROCKS AND MINERALS
chemically altered andunaltered materials
weathering
(im)mobilization
Rock characteristics
Environment characteristics
Ion characteristics
Environment characteristics
(chemical)
IONS, OTHER METAL SPECIES
RECEPTOR
Ecosystems
transport
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Terrestrial Abundances of theElements
From Mineral and Energy Resources,Douglas G. Brookins, (1990).
From Planet Earth, Casare Emiliani,(1992).
CRUST: ~0.5% of Earths total mass
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What We Know
The abundant metallic - aluminum, iron, calcium,magnesium, sodium, potassium, and titanium --bound up in the various silicates, carbonates,
oxides, and other rocky materials that make upthe crust.
Silicates
- extremely stable in the chemical sense, and
- it requires prodigious amounts of energy to dissociateordinary rocks in order to retrieve pure metals fromthem.
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Metal Specificity
Metals cannot be destroyed by biological orchemical processes
Metals can only be reduced by physical removal (e.g.,leaching, biological uptake)
Metal speciation can influence metal distribution andbioavailability within the environment
Some metals are essential elements
Speciation (forms), transformations, and
geochemical environment need to be consideredwhen evaluating potential effects of metals onthe environment
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Heavy Metals
usually taken to mean those metallic elements that havea density equal to or greater than 6.0 g/cm3:
Cu, Cd, Cr, Hg, Ni, Pb and Zn
occur NATURALLY in soils, sedimentary deposits and
water bodies, normal background concentrations of these metals exist.
are considered to be contaminants relative tobackground concentrations
are considered to be pollutants relative to regulatedlimits of concentrations, where their concentrations haverisen to such an extent that they present a real orpotential risk to living organisms.
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Geoavailability
that portion of achemicalelements or acompounds totalcontent in anearth material
that can beliberated to thesurficial or near-surfaceenvironment (orbiosphere)throughmechanical,chemical, orbiologicalprocesses
(from Smith and
Huyck (1999))
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Weathering
Codrington, Stephen. Planet Geography 3rd Edition(2005)
Physical weathering Chemical weathering
Weathering is the breakdown and alteration of rocks andminerals at or near the Earth's surface into products thatare more in equilibrium with the conditions found in thisenvironment. Pidwirny, M. (2006). "Weathering". Fundamentals
of Physical Geography, 2nd Edition.
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Weathering Rate
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Chemical Weathering
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Processes and Geochemical Conditions thatRedistribute Cationic Dissolved Metals
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Mobility of Elements
Mobility refers to the capacity of an element tomove within fluids after dissolution
difficult to quantitatively predict in surficial
environmentsNeeds to be considered in a relative sense
empirically compare the behavior of elementsunder changing environmental conditions
geochemical barriers
Controlling factors include pH, solubility,sorption, and redox conditions
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Factors Affecting Mobility Speciation
Ions Cr(III), Cr(VI),As(III), As(V)
Complexes(Inorganicligands, Organicligands)
Mineralogy
Geochemical factorspH
Eh
Dissolved oxygen
Water chemistry
Microbial activity
Sorbents
Competition fromother ions
There are several computerprograms that perform chemical
speciation calculations(e.g., MINEQL+; MINTEQA2;
PHREEQ; GEOCHEM).
Speciation models andThermodynamic databases
http://chess.ensmp.fr/chemsites.html
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Mobility of Elements
in the surficial environmentis a function of ionicpotential (ratio of oxidationnumber to ionic radius)
from Rose et al. (1979)
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Relative Mobility of Chemical Elements underDifferent Environmental Conditions
(from Smith (2007))REE = Rare Earth Elements
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Metal Speciation
Key to understanding metal mobility, bioavailability,and toxicity
Different chemical species of a given metal often have differentmobility behavior and toxicological effects
Distinct chemical species are chemical compounds that differ inisotopic composition, conformation, oxidation or electronic state,or in the nature of their complexed or covalently boundsubstituents*
* from Templeton et al. (2000)
Physical and chemical properties of metals at the atomiclevel are responsible for differences in theirenvironmental behavior
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General Characteristics in AquaticSystems
after Smith and Huyck (1999)
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Metal Sorption Reactions
Largely control the fate of many traceelements in natural systems
Are strongly pH-dependent Are a function of metal-complex formation
and ionic strength At many mining sites there are abundant
iron-and aluminum-oxide precipitates can act as effective sorbents for a variety of
metals self-mitigating capacity of MIW (Mining Influenced
Waters) i.e., with pH increasing, dissolved iron can precipitate
and sorb trace elements
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Redox-Sensitive Metals
Generally undergo a change in mobilityunder different oxidizing or reducingconditions
uranium is immobile under reducing conditionsbut can be mobile under oxidizing conditions
Atmospheric oxygen generally is the primaryoxidant, and organic matter generally is the
primary reductant at mining sites, other reductants may include FeS,
FeS2, Fe2+, Mn2+, or H2S
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Metal Complexes
A metal complex - formed by thedonation of a lone pair of electronsfrom a negative ion, e.g. CN, ormolecule, e.g. H2O, to that metal.
The species donating the electronpair : a ligand.
The bond : co-ordinate chemicalbond.
The number of ligands : co-ordination number.
Ligands capable of co-ordinatingmore than once are termed
polydentate ligands, and arereferred to as chelating agents.(ethylenediaminetetraacetic acid)(EDTA)
Z=30
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Heavy Metal PollutantsSources and Cycles
pollutant concentration in agiven location typicallydepends on geochemicalcycling over multiple inputsources and pathways, withinter-reservoirexchange.
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Examples of metalforms or species that
are of potentialenvironmental and
toxicologicalimportance.
General schememetal speciation in
solution.
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Complex Ions in Sea Water
Metals Redox
condition
Insoluble
Compd
pKsp Soluble complex (%)
Cd (II) Oxidative CdCO3 13.59 CdCl+(56.5), CdCl2
0(15.2), CdCl42- (10.0),
CdCl64-
(9.1), CdCl3-
(9.0)
Cd (II) Reductive CdS 26.96 Cd(HS)20(97.2), Cd(HS)3
-(2.2), Cd(HS)42-(0.1),
Cd(HS)+(
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Heavy Metals in Soil
exchange withother cations on thesurface of thecolloidal clays andbecome adsorbed.
form complexeswith humic and fulvicacid groups.
react with anions
present and beprecipitated out ofsolution.
From emsi.stanford.edu/aboutEMSI.html
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pH role
mobility and bioavailability primarily determined by pH
mobility and bioavailability are enhanced under acidicconditions
Balance between acid-generating and acid-consumingreactions depends on:
relative rates of those reactions
accessibility of minerals that contribute to thosereactions
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pH role - Ficklin Diagram
from Plumlee et
al. (1999)
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Metal Speciation
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Bioavailability
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Metal Hydroxide
Solubility
Metal Sulfide
Solubility
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Single Variable Diagram: pH
What is the most abundant species of iron in natural waters?
Source: DeGraff, 2007 Understanding and Responding to Hazardous Substances at Mine Sites in the Western US
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Single Variable Diagrams:
pH
Most abundant species of arsenic innatural waters
Influence of pH on As distribution?
Source: DeGraff, 2007 Understanding and Responding to Hazardous
Substances at Mine Sites in the Western USHFO = hydrous ferric oxide
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Redox Potential
As Desorption andDissolutionduring changes ofthe reducingconditions
Insoluble Fe Soluble Fe
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Two Variable Diagrams:pE-pH
What is the most
abundant species ofarsenic in natural waters?
H3AsO4 H2AsO4- + H+
H2AsO4- + 3H+ + 2e- H3AsO3 + H2O
!!!!!! arsenite (III) is about 60 times more toxic than arsenate (V)
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Consequences: As Poisoning of
Groundwater
largest ever poisoning - 60 mil. atrisk, 700,000 poisoned (mostly inBangladesh)
1970s: 11m drinking wells
patchy contamination
one theory: As attached to reduced iron
oxides in peat deposits
anoxic microbes release Asduring respiration using Fe-ox
another theory:
As associate with pyrite
groundwater drawdown exposespyrite to O2, weathering releasesAs
UNSOLVED!!
arsenocosis cancer
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Redox Potential
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Redox PotentialAcid Mine Drainage
Sulfate reduction:CH2O + H2O CO2 + 4H
+ + 4e-
SO42- + 10H+ + 8e- HS- + 4H2O
SO42- + 2CH2O + 2H
+ H2S + 2H2O + 2CO2
With the presence of Fe2+
Fe2+ + H2S FeS + 2H+ And FeS + S FeS2
FeS2 + H2O + 7/2O2 Fe2+ + 2SO4
2- + 2H+
And
FeS2 + 14Fe3+ + 8H2O 15Fe
2+ + 8H2SO4
Later
4Fe2+ + O2 + 10H2O 4Fe(OH)3 + 8H+
Pyrite oxidationfrom the action of Acidithiobacillusbacteria
air
organics
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Acid DraiangeTerms
Acid Mine Drainage (AMD)
Water that is polluted fromcontact with mining activity
Acid Rock Drainage (ARD)
Natural rock drainage that isacidic
Both produce acidic waters
R k D i
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Rock Drainage
(general)
acid rock drainage neutral mine drainage
saline drainage
= contaminants are
released from solid toliquid phase by sulphidemineral oxidation
Includes: tailing, waste
rock, underground mineand pit walls, pit lakes,spent ore heaps andlow grade stockpiles
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Surface Water
Ground Water
Sediment/soil
Air
MediaSource
Smelters/ore processing
Tailings
Undergroundworkings
Waste rock
Heap leachpads
Pit lakes
Hg, Pb
As, Se
Cd, Sb,Ag, CNCu, ZnPb, UCr, Fe
Hg
Metals
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Acid Rock Drainage
Dissolution, sorption,nucleation, growth
Oxidation-Reductionreactions
Acid-Base reactions
Isotope exchange reactions
Modeling exercises Chemical Speciation
Saturation /equilibrium
Kinetics
Chemical Processes Physical & BiologicalProcesses
Transport
Water
Sediment Wind
Microbial
S-oxidizers, Fe-
oxidizers
S-reducers, Fe-reducers
Wetland Plants
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Pyrite Oxidation
Pyrite Dissolution/Overall Reaction
FeS2 + 15/4O2 + 7/2H2O = Fe(OH)3 + 2H2SO4
Low pH (high acidity)
Metal rich: As, Sb, Zn, Cu
Fe, Al, Mn rich
Sulfate rich
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Pyrite OxidationFeS2 + 7/2O2 + H2O = Fe
2+ + 2SO42- + 2H+
FeS2 + 14Fe3+ + 8H2O = 15Fe
2+ + 2SO42- +16H+
After Stumm and Singer (1980)
FeS (s) + O2
Fe(II) + S22-
+ O2
Fe (II) + SO42-
+ FeS (s)2
fast
fastmicrobial
+ O2
slowinorg.
Fe(III) = Fe(OH) (s)3
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Pyrite Oxidation
Chemical
oxygen, Fe(III), water, buffering
Physical texture, grain size Ore processing, framboidal pyrite
Biological
Fe- and S-oxidizing bacteria
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Mine Tailings, Sudbury Ontario
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Almau Mare , Mina Hanes
S
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AMD Prediction
Assessment of Acid-generation and Acid-neutralization capacity (acid,
sulfate) Hydrologic Assessment:
Availability of Oxygen andWater (acid, sulfate)
Ore Deposit/Wasterock/TailingsCharacterization (metals)
T f O id i
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Transport of OxidationProducts to Surface Waters
Sorption trend onto Fe pptPb>Hg>Ag>As>Ni>Cu>Cd>Zn
Drainage Surface Water
H, Fe, SO
Mn,
+ 2-
4
M Mixing/dilution
pH increase
ppt of Fe(OH), Al(OH)sulfate diluted/sorbedMetals sorbed/co-ppt with
Fe and Al
3 3
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Wetland Processes
InputFe , SO , H , Me
3+ 2- +
4 Output=Cleaner
Plant uptake
ReducingSO to H S
ppt (Fe,Me)S4 2
2-
Oxidizingppt of Fe-OH-O-SO
adsorption4
[inc. pH]
See Kwong & Stempvoort (1994)
(Mt. Washington, B.C.)
Other ORD work at SPRD: T. Canfield et al.
Constructed Wetlands
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G d W t /A i Li t
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Ground Water/Anoxic LimestoneDrains
High Fe(II)/Fe(III)pH 2-6, low O2Al, Metals
GW
Limestone Drain
Calcite dissolutionAlkalinity production
Retain Anoxic [Fe(II)/Fe(III)]pH increase
High O2Fe(II)=>Fe(III)Fe(OH)3 ppt
alk takes up acid
Surface
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Thank you