# Presentation: 081202

## Influence of Mass Transfer on U(VI) Microbial Reduction

- Chongxuan Liu1, Zheming Wang1, John M. Zachara1, James K. Fredrickson1, Byong-Hun Jeon1,2, Paul D. Majors1, James P. McKinley1, and Steve M. Heald1,3

***1******Pacific Northwest National Laboratory***

***DOE Environmental Remediation Sciences Division 2006 Meeting***

***3******Argonne National Laboratory***

***2******Yonsei University, Korea***

## Uranium Physical Location in Contaminated Sediments

**Hanford 200 Area Sediments**

**Hanford 300 Area Sediments**

- At Hanford site, sorbed U exists as U(VI) with complex speciation, physical location, and mineral association.

- Sorbed U is commonly associated with intragrain regions.

**U(VI)-**
**silicate**
**precipitates**
**in granitic**
**lithic**
**fragment**

**U(VI) in**
**secondary **
**materials**

## Conceptual Model

**Dissolution**

**   ****Diffusive**
**Mass Transfer**

**e****-**** donor**

**U(VI) **

**U(IV)**

**Porewater/Groundwater**
**   ****Microbial Activity**

## Characterization of Intragrain Diffusion

**Tortuosity factor (D****p****/D****H2O****): **
**            ****0.66        fast region**
**            ****0.006      slow region**

**A**** 1****H nuclear magnetic resonance, pulse-field gradient spin echo (NMR-PGSE) approach was developed to measure intragrain diffusion properties using H****2****O as a tracer.**

## Models of Ion Diffusion Coefficients

**Models of Ion Diffusion Coefficients**

**Coupled electrodynamics-nonequilibrium thermodynamics**** ****  **
**(EDNT) model:****        D****i****p**** = f(********, ********c****, D****j****p****, C****j****)**
-        c: surface charge density, Djp and Cj: diffusivity and concentration of ion j, respectively.

**Geometrical model:****  D****i****p**** = ******** D****i****w**** **
-           **D****i****p**: pore diffusivity, ****: tortuosity; **D****i****w**: diffusivity in water,

## Microbial Reduction of Intragrain U(VI)

-  **Coupling of Biogeochemical Processes: Synthetic System**

**Dissolved U(VI)**

-   Alginate beads with
- synthetic Na-boltwoodite

- +

**Dissolution/**
**  ****diffusion**

**Bioreduction**

- 5x108 cells/mL

- 2x108 cells/mL

*S. Oneidensis*
-     MR-1

- R = 1 mm

## Intrabead U(VI) speciation and Distribution

**LIFS spectra**

**LIFS image**

- No change of intrabead U(VI) speciation;
- U(VI) dissolved/diffused starting from bead edge to center.

## U(IV) Precipitation on Bacterial Surface

**U(IV) Precipitation on Bacterial Surface**

- Time = 2 days

- Time = 7 days

- Time = 100 days

- U(IV) accumulation on bacterial surfaces and periplasm

## Modeling Coupled Biogeochemical Processes

- Intragrain dissolution

- +  Intragrain diffusion

- +  Extragrain bioreduction

## Microbial Reduction of Hanford Intragrain U(VI)

- Synthetic system

- Sediment system

- UO2(CO3)3 4- reference

- Ca2UO2(CO3)3 reference

**Microbial Reduction of Hanford Intragrain U(VI)**

**Effects of Calcite Dissolution**

- Simulation

- Cell spike

**LIFS Spectra of Aqueous Solutions at liquid helium temperature**

## Microbial Reduction of Hanford Intragrain U(VI)

**Microbial Reduction of Hanford Intragrain U(VI)**

**Coupling of Biogeochemical Processes**

**XRM analysis of Residual U**

## Microbial Reduction of Hanford Intragrain U(VI)

**Microbial Reduction of Hanford Intragrain U(VI)**

**SEM analysis showed residual U silicate in occluded regions**

## Microbial Reduction of Hanford Intragrain U(VI)

**     ****XANES analysis found mixed valence distribution of U(IV) and U(VI)  inside grains.**

**Microbial Reduction of Hanford Intragrain U(VI)**

**Other Reduction Process?**

## Conclusions

-  **An NMR-PGSE technique indicated that a dual region diffusion model was required to simulate ion diffusion in the intragrain fractures of the granitic lithic fragment in Hanford sediment.**
**Coupled electrodynamics and nonequilibrium thermodynamics (EDNT) model indicated that macroscopic ion diffusivity is a complex function of microscopic properties of mineral surface charges, ion exchange reactions, electrostatic double layers, and ion charge coupling.**
**Macroscopic aqueous U(VI) concentration was determined by the microscopic coupling of biogeochemical processes of dissolution/desorption, diffusion and microbial activity. **
**Microbial reduction of intragrain U(VI) in the contaminated Hanford sediment was complicated by the dissolution of calcite that released Ca to complex uranyl carbonates, which in turn slowed bioreduction rate and increased dissolution/diffusion rates. Some intragrain U(VI) in the contaminated Hanford 200 Area sediment was in occluded regions and might not be reactive due to mass transfer limitation. **