delivering sustainable solutions in a more competitive world remediation of mixed chromium and tce...

Delivering sustainable solutions in a more competitive world

Remediation of Mixed Chromium and TCE

Releases

Paula R. ChangRichard A. Brown, Ph.D.ERM, Inc.


W.O./Init./Date, 2

Distribution of Cr and TCE in Desert Soils

Vadose Zone: Dry, oxidized environment. Low carbon. Moderate to high calcium, pH 6-8. Retention of Cr VI. Little retention of TCE

Saturated Zone: Oxidized environment. Low Carbon. pH 6 – 8. Some retention of Cr. Little retention of TCE; large dissolved plume

Cr

TCE


W.O./Init./Date, 3

What Controls Cr VI Distribution and Persistence?• Eh-pH

• Mineralogy

• Binding to Calcium/Iron

• Recharge

• FOC

• Quantity spilled

• Depth to Water

• Recharge

What Controls TCE Distribution and Persistence?


W.O./Init./Date, 4

Speciation of Chromium

Cr2O7= + H2O ↔ 2CrO4

= + 2H+

Acid Base

Desert Soils


W.O./Init./Date, 5

Binding to Calcium/Fe• Ca+2 + CrO4

= CaCrO4 Ksp= 7.1 x 10-4

• Na/K+ + 3Fe+3 + 6H2O + CrO4= KFe3(CrO4)(OH)6 +

6H+ (Jarasite)C

a/F

e C

on

ten

t o

f S

oil

Plume length


W.O./Init./Date, 6

Ion Exchange with Sulfate in Minerals

Ca6Al2(SO4)3(OH)12 + CrO4= ↔ Ca6Al2(SO4)2(CrO4)(OH)12 + SO4

=

Ettringite

Chromium(VI) hydrocalumite.

Ca2Al(OH)4.5Cl0.5(CrO4) + SO4= ↔ Ca2Al(OH)4.5Cl0.5(SO4) + CrO4

=

Ion Exchange of Minerals is a significant cause of rebound!


W.O./Init./Date, 7

Ion Exchange with Sulfate in Minerals

Before and after submergence in 3 mM sulfate


W.O./Init./Date, 8

Chromium VI Remediation

• Processes

• Flushing

• Biological

• Chemical

• Permanence

• Re-oxidation?


W.O./Init./Date, 9

Flushing of Chromium VIC

a/F

e C

on

ten

t o

f S

oil

Pore Volumes required for complete removal

Other Factors Affecting Flushing Efficiency

• pH• Lithology• Mineralogy

4


W.O./Init./Date, 10

Biological Reduction of Chromium VI

• Direct Metabolism

• Coincidental Reduction of Cr(III) to Cr(VI)

• Both Require Carbon Addition


W.O./Init./Date, 11

Direct Metabolism

• Cr VI is the Electron Acceptor

• Some Sulfate Reducing Bacteria can utilize chromate as the electron acceptor

• 2CrO4= + -CH2- + 10H+ 2Cr+3 + 6H2O + CO2


W.O./Init./Date, 12

Coincidental Reduction

• Cr(VI) is reduced by products generated by other anaerobic pathways

• Iron reduction

• 6Fe+3 + -CH2- + 2H2O 6Fe+2 + CO2 + 6H+ Iron metabolism

• 6Fe+2 + Cr2O7= + 14H+ 6Fe+3 + 2Cr+3 + 7H2O Cr VI Reduction

• Sulfate reduction

• SO4= + -CH2- + 2H+ S= + CO2 + 2H2O Sulfate metabolism

• 3S= + Cr2O7= + 14H+ 3S0 + 2Cr+3 + 7H2O Cr VI Reduction


W.O./Init./Date, 13

Why Biological Reduction Fails

• Requisite Bacteria not present

• Chromate-utilizing SRBs

• SRBs

• Iron reducing bacteria

• Requisite Electron Acceptor not available

• Ferric iron

• Sulfate

• Chromium levels are toxic to bacteria (inhibition)

• pH too low needs to be between 5.5 - 8

• Cannot maintain reducing conditions (vadose zone)


W.O./Init./Date, 14

Chemical Reduction of Chromium VI

• Cr VI is a strong Oxidant

• Electrode Potential (Eo) = 1.33 V

• It reacts with reductants with a reducing potential > -1.33 V

• Reduced Sulfur Compounds

• Reduced Iron

• Reduced (oxidizable) organics

• The issue is kinetics not reactivity

• Reaction times vary from < 5 min to > 5 days


W.O./Init./Date, 15

Screening of ReductantsCr6+

Crtotal Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) 150 154 150 142 160 142 160 142Soil Concentration (mg/kg) 17 41.1 25 38 24 36.5 26 52.5

Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) ND 1.2 ND 0.724 ND 0.85Soil Concentration (mg/kg) ND 657 0.12 961 0.082 445

Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) ND 80.4 ND 60.3 ND 86.9Soil Concentration (mg/kg) 2.6 130 2.0 229 2.1 249

Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) 110 121 43 103 7.8 106Soil Concentration (mg/kg) 21 223 12 346 2.3 534

Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) ND 1.04 ND 1.89 ND 4.92Soil Concentration (mg/kg) 16 877 3.9 172 11 855

Cr6+Crtotal Cr6+

Crtotal Cr6+Crtotal

Aqueous Concentration (mg/L) 12 3.77 2.6 5.5 0.7 1.42Soil Concentration (mg/kg) 25 343 23 365 19 248

T = 120 Minutes

Ferrous SulfateT = 30 Minutes T = 60 Minutes T = 120 Minutes

Oxalic AcidT = 30 Minutes T = 60 Minutes

T = 60 Minutes T = 120 MinutesSodium Hydrogen Sulfate (Control)

Calcium PolysulfideT = 30 Minutes

T = 0 T = 30 Minutes

T = 60 Minutes T = 120 Minutes

T = 120 Minutes

T = 120 MinutesZero Valent Iron

T = 30 Minutes T = 60 Minutes

Sodium DithionateT = 30 Minutes T = 60 Minutes


W.O./Init./Date, 16

Cr VI Treatment

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

0 5 10 15 20 25 30

Months

Cr

VI,

ug

/L

MW-312 MW-314 MW-320

Comparison of Biological and Chemical Reduction

Injection of Molasses Injection of Polysulfide


W.O./Init./Date, 17

Comparison of Chemical and Biological Chromium VI Treatment

• Chemical• Applicable over wide

pH range• Fast reaction• Able to maintain

residual - No reoxidation

• Biological• Narrow pH range

- 5.5-8

• Slow reaction• Competitive consumption

of carbon• Possible reoxidation

Chemical reduction is simpler, faster and more reliable


W.O./Init./Date, 18

TCE Remediation

• Vadose zone

• SVE

• Chemical Oxidation

• Saturated

• Air sparging

• Pump and treat

• Dual Phase

• Recirculation wells (ART)

• Anaerobic reductive dechlorination

• Zero Valent Iron

• Chemical Oxidation

TCE


W.O./Init./Date, 19

ART Recirculation Well


W.O./Init./Date, 20

Available Oxidants• Ozone

• O3 + 2H+ + 2e- O2 + H2O Eo 2.07v

• Key Issues: Stability, low mass delivery

• Persulfates

• S2O8= + 2e- 2SO4

= Eo 2.01v

• Key Issues: Needs activation, Long term stability, reaction rate

• Hydrogen Peroxide

• H2O2 + 2H+ + 2e- 2H2O Eo 1.77v

• Key Issues: Decomposition, needs activation

• Permanganate

• MnO4- + 4H+ + 3e- MnO2 + 2H2O E

o 1.695

• Key Issues: Soil Demand (low for low FOC)


W.O./Init./Date, 21

Applicability of ISCO

Dissolved

DNAPL

Adsorbed

MnO4-

H2O2

O3

S2O8=


W.O./Init./Date, 22

TCE Remediation

Technology Impact on Chromium VI

Pump and Treat – Captures dissolved TCE. Recovered water is treated by air stripping, carbon, or bioreactors

Chromium VI would also be recovered since it is soluble. It would also require treatment in the recovered water. Air stripping has no effect on Cr VI. Carbon may reduce the Cr VI. Bioreactors would reduce Cr VI.

SVE/Air Sparging, ART – Removes TCE by volatilization

Has no effect on Cr VI.

Reductive Dechlorination – A carbon amendment is added which stimulates the biological reductive dechlorination of the TCE

The carbon amendment and biology will also reduce the Cr VI. High levels of Chromium can be inhibitory to biological processes.

Chemical Reduction – uses zero valent iron or ferrous iron minerals to dechlorinate TCE.

Zero valent iron and ferrous iron will both reduce Cr VI.

Chemical Oxidation – A chemical oxidant - peroxide, permanganate, or persulfate is added to oxidize the TCE to carbon dioxide and chloride ion.

The oxidants can potentially reoxidize chromium III to hexavalent chromium. Sequencing of oxidation with any chromium reduction needs to be considered. Generally chromium reduction would follow TCE oxidation. Excess oxidant consumes reductant


W.O./Init./Date, 23

Chromium VI Remediation

Technology Impact on TCE

Pump and Treat – dissolved chromium VI is recovered and removed from the water by reduction and precipitation

Dissolved TCE would also be recovered and would have to be treated by air stripping and/or carbon. Generally chromate is removed separately by reduction.

Chemical Reduction – a chemical reductant is added to convert Cr VI to Cr III.

Reductants will reduce iron present in soils. The reduced iron minerals can dechlorinate TCE

Biological reduction of Cr VI to Cr III Reductive biological processes are generally compatible with TCE dechlorination. They may need to be done sequentially. Chromate reduction may occur before CE reduction


W.O./Init./Date, 24

Conclusions

• Remediation of mixed spills is achievable

• Remediation of vadose zone and saturated zone may require different packages of technology

• Reductive dechlorination in vadose zone is difficult

delivering sustainable solutions in a more competitive world remediation of mixed chromium and tce...

Documents

o cro

sustainable solutions

competitive world w

o sulfate metabolism

reduction slide

sulfate slide

retention of cr vi

distribution of cr