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

Ramesh Chawla and Jacob Mlusu Department of Chemical Engineering Howard University

Significance of study

GOAL:

Simultaneous remediation of Cr(VI) and TCE via redox reactions in aqueous and soil systems RELEVANCE 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) H + Fe(II) H 2 SO 4 1,5- Diphenyl carbazide reactor Reactor UV-Vis spectrophotometer

TCE-KMnO

4

Process flow diagram

KMnO4 Na2SO4 QUENCH REACTION Hexane In-Reactor extraction Vials containing TCE Gas chromatograph

Chromium reduction and TCE oxidation in the presence of other contaminant 1,4 1,2 1,20 1,00 1 0,8 0,6 0,80 0,60 CFT1 CFT2 CFT3 0,40 TKC1 TKC2 TKC3 0,4 0,2 0,20 0 0 1 2

Time (Hrs)

3 4

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.

0,00 0 -0,20 1 2 3 4

Time (Hrs) Figure 3: TCE oxidation by KMnO 4 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

CONCLUSIONS

1. The effect of Cr(VI) on TCE oxidation and effect of TCE on Cr(VI) reduction are synergistic when used with KMnO 4 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 KMnO 4 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 (k CFT > k CF potentials of the species involved.

> k CT ) followed the relative reduction

Proposed remediation strategy

KMnO4 Fe(II) TCE Cr(VI) Cr(VI) CO2 Cl MnO2 Cr(III) Fe(III)

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 Student Howard University

Background – Hanford Tank Waste

• • Hanford Site – 177 tanks – 193 Mci in 204,000m 3 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