METALS TREATMENT AND IN SITUPRECIPITATION

Mike HaySeptember 30, 2016

© Arcadis 2016

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About the Presenter

o 720-409-0684

e michael.hay@arcadis.com

MICHAEL HAY, PHDSenior Geochemist and the Geochemical Modeling focus area lead within Arcadis

© Arcadis 2016

Learning Objectives

After attending this session, participants should be able to:

• Identify challenges unique to in situ metals treatment

• Recognize in situ treatment approaches: injection-based vs. barriers

• Explain the importance of precipitate stability

• Describe geochemical processes/principles that can be used to yield metals precipitation

• Explain the importance of long-term precipitate stability and geochemical parameters that may affect stability

© Arcadis 2016

In Situ Metals Precipitation• In situ treatment of metals vs. organics: What is the fundamental difference?

• Metals cannot (“metals” = metals/metalloids/oxoanions, etc…)o Removed from solution via adsorption, precipitation: Reversible processeso Requires long-term alteration/stabilization of geochemical environment

• Organics can be irreversibly altered, destroyedReductive Dechlorination

Carbon source CO2

e-

TCE Ethene

Hydrocarbon Bio-Oxidation

O2 H2O

e-

C6H6 CO2

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Implications of Fixation

Metals Plume

Organic Plume

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Implications of FixationOriginal Plume Boundary

Mobile Contaminant Mass

Immobilized Contaminant Mass

Metals Plume

Organic Plume

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Implementation Approaches2) Permeable Reactive Barriers1) Soluble Injections

Aqueous phase delivery of treatment reagents through injection wells Excavation and direct emplacement

of solid-phase reagent

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Injection-Based Approaches• Alteration of geochemical environment to yield mineral saturation

o pH Reduce solubility of the metal: Al3+ Al(OH)3 (solid) Pb2+ Pb(OH)2 (solid)

Reduce solubility of other constituents: Fe3+ Fe(OH)3 (solid) HAsO42-

o Redox environment Reduce/oxidize the metal: U(VI) U(IV) UO2 (solid)

Reduce/oxidize other constituents: SO42- S2- Ni2+ + S2- = NiS (solid)

o Dissolved ion concentrations Example: Sulfide Addition Pb2+ + NaHS = PbS (solid) + Na+ + H+

Example: Phosphate Addition UO22+ + PO4

3- + Ca2+ = Ca(UO2)2(PO4)2 (solid)

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Challenges• Long-term effectiveness

• pH rebound?• Ability to maintain redox environment?• Wash-out of added reagents?

• Secondary effects• Geochemical alteration Release of other constituents

• Reagent injectability/radius of influence• Balance between reagent distribution and reagent deposition

Fe2+

CO2

Organic Carbon

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Field Examples

1) Microbial reduction of Cr(VI)• Direct-redox effect

2) Arsenic coprecipitation with iron• Indirect-redox effect, pH adjustment

3) Uranium Precipitation with Phosphate• Precipitation with co-solute addition

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1) In Situ Treatment of Cr(VI)• Organic carbon injection

• Microbial reduction of Cr(VI) to Cr(III)

• Iron reduction enhances Cr precipitation

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In Situ Treatment of Cr(VI)• Organic carbon injection

• Microbial reduction of Cr(VI) to Cr(III)

• Iron reduction enhances Cr precipitation

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In Situ Treatment of Cr(VI)• Reoxidation of Cr(III) by O2

• Thermodynamically favorable• Kinetically limited!• In practice, reoxidation by O2 is not observed

• Reoxidation of Cr(III) by Mn(III/IV) oxyhydroxides

• Primary known/environmentally-relevant oxidant of Cr(III)

• Kinetically limited: Solid-solid interaction

• Cr(III) extremely stable upon redox rebound

O2 H2O

NO3- N2

MnO2 Mn2+

SeO42- HSeO3

-

HCrO4- Cr(OH)3

Fe(OH)3 Fe2+

HAsO42-

SO42- H2S

HCO3- CH4

H3AsO3

UO2UO22+

Oxidized Reducedn O

HSeO3- Se

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

Eh (volts)pH = 7

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In Situ Treatment of Cr(VI)Pilot testing

• Cr(VI) effectively reduced• Reductive dissolution/release of Fe, Mn, As

0

40

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0

1,000

2,000

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1400 To

tal O

rgan

ic C

arbo

n (m

g/L)

Hexa

vale

nt C

hrom

ium

(ppb

)

Elapsed Time (days)

Hexavalent Chromium

Total Organic Carbon

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-06

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6

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-06

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-07

Nitra

te (m

g/L)

Sulfa

te (m

g/L)

Date

Sulfate

Nitrate

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20

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Arse

nic (

ppb)

Elapsed Time (days)

Dissolved Arsenic

PT-1D

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Controlling Byproduct GenerationField-scale predictions: Attenuation of As and Mn before river discharge

Chromium Manganese

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2) Arsenic Removal with IronStrategy:

• Addition of iron as soluble Fe(II)• Addition of chemical oxidant to achieve:

– Fe(II) Fe(III)

– As(III) As(V)

• Alkaline oxidant (e.g., CaO2) balances acidity• Arsenic “co-precipitation” with iron

Challenges:• In situ mixing of soluble iron and oxidant• Retardation/consumption of reagents:

Fe(II), oxidant• Achieving target “ROI”

Sorption,Coprecipitation

As(III) As(V)

CaO2 Ca(OH)2

Arsenic Oxidation

CaO2Ca(OH)2

Fe(III) Fe(II)

Iron Oxidation

Iron oxyhydroxide

As(V)

As(V)

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Field Pilot TestGroundwater arsenic at a coal ash site

View along highway of arsenic “hot spot” area Well installation

Completed well network, with IW-1 in center

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Field Pilot TestGroundwater arsenic at a coal ash site

Reagent tanks Injection well (IW-1) below grade

Ferrous sulfate injection, with tube added to provide additional head for gravity injection

Fluorescein tracer and ferric iron precipitate in observation well

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Coal Ash Site Field Pilot Test

• Specific capacity loss with solid-phase CaO2 injection

• Capacity regained with acidic ferrous sulfate injection

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0.90

1.00

0 1000 2000 3000 4000 5000

Spec

ific

Cap

acity

(gpm

/ft)

Cumulative Volume Injected (gal)

Ferrous sulfate Calcium peroxide

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Coal Ash Site Field Pilot Test• Iron and calcium largely retained in 10-ft ROI

• “Emplaced reactive zone” established

0%

10%

20%

30%

40%

50%

60%

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80%

90%

100%

-30 0 30 60 90 120 150 180 210 240 270

Rea

gent

and

Tra

cer R

ecov

ery

Days Post-injection

OW-3

Calcium - Dissolved Iron - Dissolved Bromide

GW Flow

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Coal Ash Site Field Pilot Test• Arsenic treatment at injection well

• OW-3: As treated to 20 ppb, gradual rebound

• Rebound due to change in flow direction, bypass of narrow treatment zone

GW Flow

0102030405060708090

100110120130140

-30 0 30 60 90 120 150 180 210 240 270

Dis

solv

ed A

rsen

ic C

once

ntra

tion

(µg/

L)

Days Post-injection

GW-41 IW-1 OW-3 OW-5

Method and laboratory reporting limit changed

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6

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14

-30 0 30 60 90 120 150 180 210 240 270

pH

Days Post-injection

GW-41 IW-1 OW-3 OW-5

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3) Uranium Precipitation with Phosphate

Uranium Mill Tailings Site, New Mexico Former Uranium Mine, Colorado

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Uranium Precipitation with Phosphate• U(VI) highly soluble

• U(VI) U(IV) reduction: Effective, but sensitive to reoxidation

• U(VI) insoluble in phosphate precipitates

Autunite: Ca(UO2)2(PO4)2·3H2O

Chernikovite: H3O(UO2)(PO4)·3H2O

MCL

Mehta et al., 2014

© Arcadis 2016

Uranium Precipitation with Phosphate

Injection strategies• Direct injection as orthophosphate

• Injection of polyphosphate, timed release of orthophosphate

P

P

P P

P

P

Tripolyphosphate Orthophosphate

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Uranium Mine ImplementationUnderground Mine Workings Waste Rock Piles

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Underground Mine Workings Implementation“Push-Pull” Test Results

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Conc

entr

atio

n (m

g/L)

Gallons Extracted from P-11

Phase-2b Pull: Uranium

Uranium(dissolved)

Uranium(total)

MixingPrediction

InjectateAverage

P-11Baseline

Theoretical concentration based on mixing of injectate and groundwater (calculated using fluorescein tracer)

Actual measured concentration

Actual < Theoretical = REMOVAL

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Conc

entr

atio

n (m

g/L)

Gallons Extracted from P-11

Phase-2b Pull: Fluorescein

Pull-PhaseMeasured

InjectateAverage

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1800

0 2000 4000 6000 8000 10000

Conc

entr

atio

n (m

g/L)

Gallons Extracted from P-11

Phase-2b Pull: Phosphate

Phosphate(total)

Phosphate(dissolved)

MixingPrediction

InjectateAverage

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Waste Rock Dump Implementation• Sustained uranium removal observed in

downgradient well (~60 ft)

• Additional results pending

GW flow

RD-01Injection Well

RD-02,03Extraction Wells

RD-04Monitoring Well

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/201

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2016

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2016

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Conc

entr

atio

n (m

g/L)

Uranium (dissolved)

RD-01

RD-02

RD-03

RD-04

InjectionsActive

© Arcadis 2016

Learning Objectives

After attending this session, participants should be able to:

• Identify challenges unique to in situ metals treatment

• Recognize in situ treatment approaches: injection-based vs. barriers

• Explain the importance of precipitate stability

• Describe geochemical processes/principles that can be used to yield metals precipitation

• Explain the importance of long-term precipitate stability and geochemical parameters that may affect stability

© Arcadis 2016

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