Kinetics of Reductive Reactions of Cr(VI) to Cr(III) in Wastes Containing TCE

Ramesh Chawla and Jacob MlusuDepartment of Chemical Engineering

Howard University

Significance of study

Most hazardous waste sites have a cocktail of mixed contaminants

Limited systematic investigation on co-occurrence of mixed contaminants

Need for economical, efficient and eco-friendly remediation strategies

Redox reactions offer potential for hazardous waste site remediation

GOAL: Simultaneous remediation of Cr(VI) and TCE via redox reactions in aqueous and soil systemsRELEVANCE to DOE: Chromium and TCE co-contamination at several hazardous sites (e.g. Hanford, Savannah River)

These maps show chromium in the upper part of the unconfined aquifer in the 100-K Area. Two pump-and-treat systems reduce the amount of chromium entering the Columbia River.

Cr(VI)-Fe(II) Process flow diagram

Cr(VI)

Fe(II)

reactor

ReactorH2SO4

1,5- Diphenylcarbazide

H+

UV-Vis spectrophotometer

TCE-KMnO4 Process flow diagram

KMnO4

Na2SO4

Hexane

QUENCH REACTION

In-Reactor extraction

Gas chromatographVials

containing TCE

Chromium reduction and TCE oxidation in the presence of other contaminant

0 0.5 1 1.5 2 2.5 3 3.50

0.2

0.4

0.6

0.8

1

1.2

CFT1CFT2CFT3

Time (Hrs)

C/Co

Figure 1: Cr(VI) reduction to Cr(III) for molar ratios of Fe(II) to Cr(VI) of 1, 3 and 6 (CFT1, CFT2 and CFT3) in the presence of initial equimolar quantities of Cr(VI) and TCE of 1 mmol.

0 0.5 1 1.5 2 2.5 3 3.50.00

0.20

0.40

0.60

0.80

1.00

1.20

TKC1TKC2TKC3

Time (Hrs)C/

Co

Figure 3: TCE oxidation by KMnO4 in water with Cr(VI) present in the system. Where C= concentration of TCE and Co = initial concentration of TCE. TKC1, TKC2 and TKC3 represent in the presence of initial equimolar quantities of Cr(VI) and TCE of 5 mmol

CONCLUSIONS1. The effect of Cr(VI) on TCE oxidation and effect of TCE on Cr(VI)

reduction are synergistic when used with KMnO4 as oxidant and Fe(II) as reductant, respectively. This finding is useful to develop remediation strategies of hazardous waste sites containing both heavy metals (which may require reduction) and organics (which may require oxidation).

2. Both Cr(VI) reduction by Fe(II) and TCE oxidation by KMnO4 follow second order kinetics, enhanced by the presence of other co-contaminant TCE and Cr (VI), respectively in the system.

3. Rate of chromium reduction increased with the concentration of oxidizing species, closely following second order kinetics. The relative order of rate constants (kCFT > kCF > kCT) followed the relative reduction potentials of the species involved.

Proposed remediation strategy

KMnO4

Cr(III)Fe(III)

CO2Cl-

MnO2

Fe(II)

TCECr(VI) Cr(VI)

Continuing Work:Kinetics of Hexavalent Chromium Removal from Soil by Chelation Agents

(MS Thesis- Jude Ighere)

1. To compare the extraction efficiency of the selected chelants for the removal of chromium(VI) from soil.

2. To determine the effect of pH on the rate and extent of the process.

3. To determine the kinetics for chelation of chromium(VI).

4. To develop a technique for the recovery and re-use of the EDTA from the complex formed between chromium and EDTA.

Hanford Tank Farm Sludge Batch 7 Washing and Settling Experiments

James H. Johnson, Jr.Graduate StudentHoward University

Background – Hanford Tank Waste

• Hanford Site– 177 tanks – 193 Mci in 204,000m3 of waste– Tank closures scheduled for 2014 and 2032

• 2006 National Academies study stated DOE should:– Initiate a research program for tank waste retrieval,

treatment, closure and disposal– Decouple its schedule for waste retrieval and closure

Previous Study – Hanford Tank Waste Retrieval

• Reboul, et al. (2011)– Tank mixture –high sulfur, high oxalate and high

aluminum concentrations– Washing experiments results

• Improved settleability• Reduced sodium concentration• Increased oxalate concentration

– Interfere with acidification and redox adjustment during vitrification

• Decreased pumpability

Proposed Study – Hanford Tank Waste Retrieval

• Use additives in addition to washing to reduce waste volume by at least 25%– Multivalent cations– Polymers– Nanoparticles

• Improve pumpability– Maintain yield stress below 15 Pa

• Improve treatability– Decreases sodium and oxalate concentrations