Remediation of Mixed Chromium and TCE Releases www.erm.com 19-Apr-17 Remediation of Mixed Chromium and TCE Releases Paula R. Chang Richard A. Brown, Ph.D. ERM, Inc.

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

What Controls Cr VI Distribution and Persistence? www.erm.com 19-Apr-17 What Controls Cr VI Distribution and Persistence? Eh-pH Mineralogy Binding to Calcium/Iron Recharge FOC Quantity spilled Depth to Water What Controls TCE Distribution and Persistence? Iron complex- ferric chromate

Speciation of Chromium www.erm.com 19-Apr-17 Speciation of Chromium Cr2O7= + H2O ↔ 2CrO4= + 2H+ Acid Base Desert Soils Chromic acid baths– typical source from metals plating Hydrogen chromate, dichromate – acid conditions - CrVI Chromate – basic conditions Chromic dioxide, insoluble Chromite – under basic conditions, < 11.5, insoluble

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) Ca/Fe Content of Soil Plume length

Ion Exchange with Sulfate in Minerals www.erm.com 19-Apr-17 Ion Exchange with Sulfate in Minerals Ettringite Ca6Al2(SO4)3(OH)12 + CrO4= ↔ Ca6Al2(SO4)2(CrO4)(OH)12 + SO4= Chromium(VI) hydrocalumite. Ca2Al(OH)4.5Cl0.5(CrO4) + SO4= ↔ Ca2Al(OH)4.5Cl0.5(SO4) + CrO4= Chromate ore processed residuals (COPR) Ion Exchange of Minerals is a significant cause of rebound!

Ion Exchange with Sulfate in Minerals www.erm.com 19-Apr-17 Ion Exchange with Sulfate in Minerals SEM – scanning electron microscope Before and after submergence in 3 mM sulfate

Chromium VI Remediation Processes Flushing Biological Chemical Permanence Re-oxidation?

Flushing of Chromium VI www.erm.com 19-Apr-17 Flushing of Chromium VI Other Factors Affecting Flushing Efficiency pH Lithology Mineralogy Ca/Fe Content of Soil Chromium exchange with sulfate in minerals – very slowly, addition of sodium sulfate to replace and mobilize chrome 4 Pore Volumes required for complete removal

Biological Reduction of Chromium VI Direct Metabolism Coincidental Reduction of Cr(III) to Cr(VI) Both Require Carbon Addition

Direct Metabolism Cr VI is the Electron Acceptor www.erm.com 19-Apr-17 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 Microbes not necessarily present

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

Why Biological Reduction Fails www.erm.com 19-Apr-17 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) 10 mg/L or greater – toxicity level Cr6+

Chemical Reduction of Chromium VI www.erm.com 19-Apr-17 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 Typical Eo = .3 to .9 of reductants

Screening of Reductants www.erm.com 19-Apr-17 Screening of Reductants Important for the vadose zone, due to lack of moisture difficult to maintain contact between reductant and Cr. Need faster reactions. CaSx is cheapest and fastest for vadose treatment.

Comparison of Biological and Chemical Reduction www.erm.com 19-Apr-17 Comparison of Biological and Chemical Reduction Injection of Molasses Injection of Polysulfide Likely toxicity issue

Comparison of Chemical and Biological Chromium VI Treatment 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

TCE Remediation Vadose zone SVE Chemical Oxidation Saturated Air sparging Pump and treat Dual Phase Recirculation wells (ART) Anaerobic reductive dechlorination Zero Valent Iron

ART Recirculation Well www.erm.com 19-Apr-17 ART Recirculation Well

Available Oxidants Ozone O3 + 2H+ + 2e-  O2 + H2O Eo 2.07v www.erm.com 19-Apr-17 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 Eo 1.695 Key Issues: Soil Demand (low for low FOC) There are a number of oxidants that are potentially usable. They are, in order of decreasing oxidative strength, ozone, persulfate, hydrogen peroxide, permanganate, oxygen nitrate and iron. Of these, ozone, peroxide and permanganate are the most widely used and are generally commercially available. Ozone is a very strong oxidant. It reacts directly as well as through the generation of hydroxyl radicals. The hydroxyl radical formation can be enhanced by adding hydrogen peroxide. This is known as the “dark” AOP process because it does not require UV light. Persulfates are strong oxidants but generally have a high activation energy. They are typically thermally activated in industrial use. A potential use is to supplement permanganate if there is a lot of reduced metals. Persulfates are less expensive than permanganate. Hydrogen peroxide is

Applicability of ISCO Adsorbed S2O8= O3 MnO4- H2O2 Dissolved DNAPL

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

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

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