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Department of Metallurgical and Materials Engng.
Universidade Federal de Minas Gerais - UFMG
Virginia S.T. CiminelliChem. Eng.; M.Sc; Ph.D.
Professor
Chair,Millennium Institute: Water- a mineral approach
ArsenicArsenic inin MiningMining EffluentsEffluents::RemovalRemoval andand StabilityStability
I CM 3
I q u i q u e ,A u g u s t , 1 8 - 2 0
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ContentsContents
n
Introduction: the problem of Arsenicin mining effluents
n Industrial Practice
Scorodite; Type I e II
Calcium Arsenates
Arsenical Ferrihydrite
n Stability of precipitatesn Mobility of Arsenic
n Final Remarks
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TheTheArsenicArsenic ProblemProblem
n High Toxicity carcinogenic/mutagenic
As(III) is more toxic and mobile thanAs(V)
n Found in Organic and Inorganic FormsThe most common hydrosoluble species are:
As(III) Arsenite - H3AsO3
As(V) Arsenate - H3AsO4
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n Remediation in view of the lack of
applications,
TheTheArsenicArsenic ProblemProblem
n Sources of contaminationanthropogenicnatural (Bangladesh, India, and others)
Immobilization/Inertization
n Recommended level of10 ug As/L toWHO - World Health Organization
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Waste minimization, dense residuesRecovery/recycling value addedStable residues
TreatmentTreatment ofofMiningMining EffluentsEffluents
Aqueous Effluents
Concentrated solutions: precipitation is usualMajor challenge: Trace elements/large volumes
End Product - Solid Residues
Gas emissions production of aqueous effluents
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TreatmentTreatment ofofMiningMining EffluentsEffluentscontainingcontainingAsAs: industrial: industrial
practicepractice
PrecipitationPrecipitation ofof
n Scorodite and Type II (autoclaves)
n Calcium arsenates
n Arsenical Ferrihydrite
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IndustrialIndustrial PracticePractice:: ScoroditeScorodite;;Type I/IIType I/II
Crystalline product; High As content related to other processes
Produced at T > 150 oC Autoclave needed
Stoichiometric consumption of Fe
Easy filtration; Less water retention; less area for disposal
Requires polishing for As
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550.100.231.21
830.090.521.42
780.221.021.00
%Asremoval
[As]f[As]IFe/As(mol/L)
Seeds 200g/L; pH =1.6-2.4; T=95oC
Production ofProduction ofScoroditeScorodite at 95at 95ooCC
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Investigated at McGill University
Key point - controlof supersaturation (pH vs. [As])to avoid homogeneous nucleation
Tested for high [As] :10 0.1 g/LVarious stages needed
[As], ppm
pH
Homogeneousnucleation
Heterogeneousnucleation
C*
Cc= S
cC*
1 2 3
100
10
1
Production ofProduction ofScoroditeScorodite at 95at 95ooCC
Main operational difficulty is
the pH control at high temperatures.
Large amounts of seeds, Peroxide and Iron sources needed.
Real advantages related to the hydrothermal processing?
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Ca3(AsO4).Ca(OH)2
Costly attractive ($) compared with Fe.
Higher residual AspH>12, As < 0.5 mg/L
Major problem: instability by carbonation
IndustrialIndustrial PracticePractice:: PrecipitationPrecipitationofofCaCaArseniteArsenite/Arsenate/Arsenate
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IndustrialIndustrial PracticePractice:: Solubility ofArsenates and Arsenites
Fe
Fe
Ca
Exptal. conditions should be adjusted
for maximum removal
Better removal as Fe compoundsand for As(III)
Ca
Fe
Robins, 1981
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IndustrialIndustrial PracticePractice:: PrecipitationPrecipitationofofArsenicalArsenical FerrihydriteFerrihydrite (FH)(FH)
28.6717.65FeAsO4.2H2O(amorf.)
1.571.78Ca3(AsO4)2
24.1046.53CaSO4.2H2O
7.1110.52Ca5.239.15S
9.906.40As
20.3014.72Fe
Composition (% w/w)H2O2 and Ferrous sulfate
needed for As oxidation
and precipitation.
Polishing
Fe/As~15 e pH 7,5-8,5
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FexOy(H2O)z disordered solid (2L ou 6L DRX= f(T))
Elevated specific surface area, obtained through fast
hydrolysis; pH 3-7; high adsorptive capacity
IndustrialIndustrial PracticePractice:: PrecipitationPrecipitationofofArsenicalArsenical FerrihydriteFerrihydrite (FH)(FH)
Vasconcelos et al., 2003
XRD of the
arsenical FH after
Gypsum leaching
Broadening of FH maindiffraction line w/ Asadsorption/co-pptation
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Most popular process andBest available practice according to the US EPA
[As] may be reduced to < 0.1 mg/L
Consumption of Fe source and oxidation agent
High retention of water (~20% sol.) $ disposal
Stable residue (30 years) for Fe/As> 4
Cations/anions (Si, P, org) increase stability, by slowing
down the rate of conversion FHGoethite/Hematite
IndustrialIndustrial PracticePractice:: PrecipitationPrecipitationofofArsenicalArsenical FerrihydriteFerrihydrite (FH)(FH)
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Short term - leaching std procedures such as
the TCLP - toxicity characteristic leaching procedure
Long-term prediction - ?
Thermodinamics and kinetics
Phases and solubility
Stability of Arsenic compoundsStability of Arsenic compounds
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Relative TCLP solubility of IronRelative TCLP solubility of Ironand Calcium Arsenatesand Calcium Arsenates
Adapted from Swash and Monhemius, 1995;except for *- our study
3000Hadingerite;Pharmacolite**1000Guerinite**
50FH Fe:As 1.5:1
0.4FH Fe:As 9:1
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LongLong--Term StabilityTerm Stability -- ThermodynamicsThermodynamics
Stability of arsenatesand sulfides are
restricted to specificconditions of Eh-pH.
Fe hydroxides/oxidesare the most stablespecies under
circumneutral pH andaerobic conditions !
Zhu e Merkel (2002)
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LongLong--Term StabilityTerm Stability Effect of COEffect of CO22
Fe(II) and Ca arsenates decompose in thepresence of CO2
Robins, 1983
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Robins, 1983
LongLong--Term Stability:Term Stability: Fe(III) and Al hydroxidesare most common end products
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Usual Conditions of PrecipitationUsual Conditions of Precipitation
pH 3pH 3-- 77
Fe/As >4Fe/As >4
favor adsorption of Arsenic by oxihydroxides
of Iron
Adsorption on iron, aluminum and
manganese oxyhydroxides play an importantrole on As mobility in the environment
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Factors controlling As mobilityFactors controlling As mobility
n Soil characteristicsMineralogy/ Specific Surface Area/Loading Capacity/Complex Stability
n Eh-pH; drainage water composition
n As Speciation
Reactions involved:
Precipitation/dissolution
Adsorption/desorption
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n Natural soil samples
n Natural mineral samples:
Al-, Fe- and Fe-Mn oxihydroxides
n Modified samples of Me-oxides
n Synthetic Fe-Mn materials
n Biosorbents
Appr oach: Se lect ion , Molecu la r m ode l ing , Design .
Studied Materials at UFMGStudied Materials at UFMG
To understand and control As mobility in tailings ponds
To select cheap materials for As remediation
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Application: Adsorption on Fe and AlApplication: Adsorption on Fe and Aloxidesoxides -- Irreversible immobilization of AsIrreversible immobilization of As
SorbentSorbent
Filters
ArsenicSolution
SorbentSorbent
TailingsTailings
Tailings DamSolution withoutarsenic
Water
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00 200200 400400 600600 800800 10001000
00
11
22
33
44
AsAsadsorbe
d
adsorbe
d(mg/g)
(mg/g)
As(III)As(III) eqeq. (. (mg/Lmg/L))
SampleSample redred 11SampleSample yellowyellow 22samplsamplSampleSample brownbrown 33SampleSample blueblue 44
QmaxQmax.=3.6.=3.6mg/gmg/g
QmaxQmax.=1.0.=1.0mg/gmg/g
QmaxQmax.=0.7.=0.7mg/gmg/g
QmaxQmax.=0.1.=0.1mg/gmg/g
Factors controlling As mobility:Factors controlling As mobility:Type of SoilType of Soil
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Factors controlling As mobility:Factors controlling As mobility:Soil Composition and SSASoil Composition and SSA
VermelhoVermelho MarromMarrom AmareloAmarelo RejeitoRejeito% % % %
Al2O3 ( tot) 23,25 18,91 17,93 19,35
Al2O3 ( sol ) 17,28 4,90 7,70 0,92
As 0,002 0,002 0,36 0,002
Fe 2O3 (tot) 11,90 8,17 5,52 4,90
Fe2O3 (sol.) 10,56 5,00 4,41 2,33
SiO2 (tot ) 50,48 61,06 63,17 63,74
SiO2 (silica) 44,00 57,70 48,20 41,00
Specific Surface
Area (SSA)35,7 16,2 18,9 5,2
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0
2
4
6
8
10
12
14
goethite gibbsite trop. Soil kaolinite0
2
4
6
8
10
goethite gibbsite trop. Soil kaolinite
As(III)As(V)
Qmax.
(mg/g)
Qmax.
(mg/g
)
Factors controlling As mobility:Factors controlling As mobility:Soil MineralogySoil Mineralogy
Relatively higher As(V) uptake
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Factors controlling As mobility:Factors controlling As mobility:SpeciationSpeciation
121086420
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
As - H2O - System at 25.00 C
C:\HSC4\As25.iep pH
Eh (Volts)
H3AsO3(a)
H3AsO4(a)
HAsO4(-2a)
H2AsO3(-a)
H2AsO4(-a)
ELEMENTS Molality Pressure
As 1.000E+00 1.000E+00
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In addition to a lower uptake, loaded As(III) on (Fe, Al)
oxides is more easily remobilized.
9.9MgCl2 1M
12.0H2O
19.5K2SO4 0.5M
6.6TCLP2/0.8
2.46TCLPOxisol
62.6TCLP8/5.6
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WhatWhat isis thethe reasonreasonforfor thethe differentdifferent behaviourbehaviour
of As(III)of As(III) andandAs(V)?As(V)?
((ImmobilizationImmobilizationMechanismMechanism))
Adsorption/Desorption of AsAdsorption/Desorption of Ason Fe and Al oxideson Fe and Al oxides
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Adsorption on S/L Interfaces
NonNon--specific adsorptionspecific adsorption
Electrostatic attraction; reversibleElectrostatic attraction; reversibleOuterOuter spheresphere coordinationcoordination
SpecificSpecific adsorption
Stronger bonds; may occur regardless theStronger bonds; may occur regardless the
solid chargesolid charge . Inner sphere coordinationInner sphere coordination
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Electrophoretic Mobility and
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Electrophoretic Mobility andElectrostatic Interaction
3 4 5 6 7 8 9 10 11
-0.6
-0.3
0.0
0.3
0.6 Gibbsite
IEP=7.7
IEP=7.6
IEP=7.0
unloadedloaded with As(III)loaded with As(V)
E
lectrophoreticmobility(/sec)/(volt/cm)
-1
pH
-0.6
-0.3
0.0
0.3
0.6
Goethite
IEP=8.1
IEP=7.9IEP=6.3
EM= f(Potential at theshear plane)
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Ad tiAd ti St tSt t l t dl t d bb
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bidentate_binuclear(bb) monodentate_mononuclea
(mm)
monodentate_binuclear(mb) bidentate_mononuclear(bm)
As
As
AsAs
Al AlAl Al
AlAl Al Al
O OO
O OO
O O
O
O
O
O
O
O
O
OH
HH
H
O
H
O O OO
O OO
ooH
H
O
O
O
O
AdsorptionAdsorption StructuresStructures evaluatedevaluated bybyDFTDFT calculationscalculations
Prof. H. A. Duarte, Chemistry Dept. V.V. CiminelliCiminelli, 04, 04
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O O
O
O
O
O
O
O O
O
1.92
1.92
1.68
109.3o
3.19 3.19
AlAl
As
2.95
The distance of 3.19 (EXAFS) was consistent with aBidentateBidentate--Binuclear ModelBinuclear Model. Release of two hydroxyls .
AdsorptionAdsorption StructureStructure byby EXAFSEXAFS andandDFT: As(V)DFT: As(V)
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d iAd i S bb /EXAFS/ DFT
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AdsorptionAdsorption StructureStructure byby EXAFS/ DFT:EXAFS/ DFT:As(III)As(III)
The distance of 3 (EXAFS)was consistent with a
BidentateBidentate--Mononuclear ModelMononuclear Model
And no pH variation.
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Final Considerations
Low temperature As-FH, and High temperature
Scorodite+ Type II (autoclave) are the best established
processes for As removal from mining effluents.
As-FH most popular; product contains typically 3-9%
As (co-pptated/adsorbed); considered stable if molar
Fe/As> 4; slow kinetics for phase transformation.
Short-term Stability can be evaluated by std. leaching
procedures - not always consistent with the conditionsfound in the actual disposal site. The results will dictate
disposal constraints, e.g. as toxic or non-toxic waste.
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Final Considerations
Long term stability Thermodynamics indicates that
usual As compounds are stable under restricted
conditions of pH and Eh. Transformation to more stable
phases will depend on the site conditions.
Arsenic Mobility is mostly controlled by adsorption on
Fe, Al and other metal oxihydroxides.
The relatively high remobilization of As(III) is explained
by a model where the arsenic molecule binds to the oxide
surface without deprotonation; the hydroxyl group, OH,
remains as the bridging unit.
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Acknowledgements
My co-authors
Ana Cludia Q. Ladeira
Hlio A. Duarte
Claudia Lima Caldeira
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