Estab-Arsenic Como Scorodita

7/27/2019 Estab-Arsenic Como Scorodita

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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

V.V. CiminelliCiminelli, 04, 04


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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

V. Ciminelli 04


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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


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




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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

A . C. Q. La d e i r a ( P h .D . T h e s is ) V.V. CiminelliCiminelli, 04, 04


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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




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


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)


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)


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


Estab-Arsenic Como Scorodita

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