1. 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

2. 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
The significance of this study comes from----- >
Environmental pollution which has become a serious problem
most of the hazardous waste sites have mixed contaminants which presents a complex and challenging problem for remediating them.
So far there has been limited systematic investigation on coexisting metal and hydrocarbon contaminants and their potential effects on the remediation process.
Hence the increase in for demand and development of economical, efficient and eco-friendly environmental remediation techniques
One method of achieving soil remediation that has been studied involves the manipulation reduction and oxidation reactions.

3. 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.

4. Cr(VI)-Fe(II) Process flow diagram
reactor H+
Fe(II)
Reactor
The cr –fe process flow of procedure is shown on this slide: We reacted Cr(VI), Sulfuric acid and ferrous ion in a continuous stirred reactor.
At periodic interval, the samples were treated with sulfuric acid and 1-5 Diphenylcarbazide as part of EPA method 7196a for the analysis of hexavalent chromium.
The treated samples were then the analyzed using a UV spectrophotometer to determine chromium concentration.
H2SO4
1,5- Diphenyl
carbazide
UV-Vis
spectrophotometer

5. TCE-KMnO4 Process flow diagram
Na2SO4
QUENCH
REACTION
The tce – kmno4 process flow of procedure is shown on this slide: We reacted TCE, KMno4 in continuous stirred reactors.
At periodic interval, the samples were treated with sodium thiosulfate to quench the reaction.
Hexane was then added to the reactors for in-vial extraction of TCE mixed vigorously for 1 minute at which point the sample in the hexane phase was injected into the GC for analysis of TCE
Vials containing TCE
Gas chromatograph
Hexane
In-Reactor
extraction

6. Chromium reduction and TCE oxidation in the presence of other contaminant
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
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.

7. CONCLUSIONS
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).
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.
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.

8. Proposed remediation strategy
KMnO4
Fe(II)
TCE
Cr(VI)
Cr(VI)
CO2
Cl-
MnO2
Cr(III)
Fe(III)

9. Continuing Work:
Kinetics of Hexavalent Chromium Removal from Soil by Chelation Agents (MS Thesis- Jude Ighere)
To compare the extraction efficiency of the selected chelants for the removal of chromium(VI) from soil.
To determine the effect of pH on the rate and extent of the process.
To determine the kinetics for chelation of chromium(VI).
To develop a technique for the recovery and re-use of the EDTA from the complex formed between chromium and EDTA.

10. Hanford Tank Farm Sludge Batch 7 Washing and Settling Experiments
James H. Johnson, Jr.
Graduate Student
Howard University

11. 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

12. 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

13. 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