# Presentation: 657298

**Baolin Deng (PI) and Silvia Jurisson**
**University of Missouri-Columbia, Missouri**
**E.C. Thornton**
**Pacific Northwest National Laboratory (PNNL)**
**Jeff Terry **
**3****Illinois Institute of Technology (IIT), Chicago, IL**

**Reductive Immobilization of Toxic Metals and Radionuclides by Hydrogen Sulfide**

_**Acknowledgments: **_
*** ******Financial  support - DOE Environmental Remediation Science Program/EMSP ***

## 	Reductive Immobilization

**HCrO****4****-**** + 3e****-**** + 4H****+**** =   Cr(OH)****3****(s) + H****2****O**
**UO****2****2+**** + 2e****-****  ****	****= UO****2****(s)**
**TcO****4****- ****+ 3e****-**** + 4H****+****	****= TcO****2**** (s) +  2H****2****O**

_**Reductants for groundwater remediation**_** **
**Dithionite, sulfide**
**Fe****2+****, Fe****0****, FeS**
**Organic compounds**
**	**_**Biotic vs abiotic processes**_
_**Gaseous Reductant for soil remediation**_** **
**Hydrogen Sulfide**

## Creation of Reductive Barriers for Contaminant Interception

**Creation of Reductive Barriers for Contaminant Interception**

**In Situ Gaseous Reduction with H****2****S**
**>70% of Cr(VI) at the demonstration site was immobilized**
**ISGR technology can be safely deployed**

**Reductive Immobilization **
**of Existing Contaminants**

**Notes:**
In laboratory test, the efficiency of H2S gas treatment is higher than 90%, and in a field demonstration at White Sand Missile Range, New Mexico, more than 70% of Cr(VI) was immobilized. The filed demonstration has also shown that H2S gas can be handled safely and no secondary contamination occurs. 

## Aqueous Chromate Reduction by Sulfide in Anoxic Systems

- S0

- Cr(OH)3(s)

**Aqueous Chromate Reduction by Sulfide in Anoxic Systems**

**Reaction Stoichiometry:**
**2****CrO****4****2- ****+ 3HS****-**** + 7H****+**** = 2Cr(OH)****3****(s) +****3****S(s)**** +2H****2****O**

**HRTEM Image**

| Temp.
   (oC) | [SII-]t=0
(M) | [CrVI]t=0
(M) |        SII- oxidized v.s.
         CrVI reduced |     R2 |
| --- | --- | --- | --- | --- |
|          15 |   100 |    40 |       Y = 1.49 x - 0.28 |  0.9625 |
|          15 |   100 |   200 |       Y = 1.60 x + 0.38 |  0.9968 |
|          25 |   100 |   200 |       Y = 1.54 x + 0.72 |  0.9972 |
|          25 |   100 |    40 |       Y = 1.53 x + 2.74 |  0.9894 |
|          25 |   200 |    40 |       Y = 1.44 x + 0.42 |  0.9769 |

**Molar ratios of the consumption of S(-II) vs. Cr(VI)**

**NOT as previously reported**
**(****8****CrO****4****2-**** + ****3****H****2****S + 10H****+**** + 4H****2****O ******** 8Cr(OH)****3****(s) +3SO****4****2-****)**

**Notes:**
To further confirm that elemental sulfur is the reaction product, after the reaction is completed, we analyzed the solid sample with Transmission Electron Microscopy and associated EDS. In this picture, we have the light area, Area A, which is identified as elemental sulfur, and the dark area, area B, which is mainly chromium hydroxides. 

## Kinetics of Chromate Reduction by Sulfide in Anoxic Systems

**(pH=7.80, 25****o****C, [S****II-****]****0**** = 800 ********M)**

- [Cr(VI)]0
-  A. 20M
-  B. 30M
-  C. 40M
-  D. 50M
-  E. 60M

**Notes:**
After the reaction stoichiometry is established, we now look at the reaction kinetics. Some typical kinetics results are illustrated here, with x axis as the time and Y axis as the ln [Cr(VI)]. The results show that at the initial stage of the reaction up to 15 minutes, all curves are linear, with the same slopes. But at the later stage of the reaction, the rate is accelerated when compared to the first-order plot. I will first focus on the initial stage and then discuss the rate acceleration afterwards. 

## Kinetics Determined by the Initial Rate Method 

**Kinetics Determined by the Initial Rate Method**

**Reaction order with respect to [Cr****VI****] = 0.94**

**Reaction order with respect to [H****2****S]****0 ****= 1.1**

**Reaction rate increases as pH is decreased**

## Proposed Reaction Mechanism

**    ****Step 1. Precursor Formation **

**Step 2. Electron Transfer**

**Step 3. Fast reactions involving Cr(IV) and Cr(V)**

**The following derived kinetic equation explains all experimental observations**

**Notes:**
Based on our observed reaction stoichiometry and the kinetics data, we propose that the reaction is taking place following this mechanism. 
The first step is the reaction between chromate and sulfide to form a precursor complex. The second step is the electron transfer process forming elemental sulfur and reactive intermediate chromium species, and the third step is the fast reaction involving Cr(IV) and Cr(V).  

## Reduction of Re-spiked Cr(VI)

**(pH=7.80, 25****o****C, [Cr(VI)]****0**** =40μM, [S(-II)]****0****=800 μM)**

- A. Reaction for the 1st time
- B. Reaction for the 2nd time
-      (with re-spiked Cr(VI))
- C. Reaction for the 2nd time
-      (removed S, then added
-      Cr(VI) and S(-II) again)

**Notes:**
Now let’s take a look at the rate acceleration at the later stage of the reaction we briefly mentioned earlier. A rate acceleration like this is normally resulted from some reaction intermediates or products. Here is a set of experiments designed to see whether this is the case. 

The curve A is the normal concentration profile for Cr(VI) reduction. If additional Cr(VI) is added after the reduction of the originally added Cr(VI), this additional Cr(VI) is reduced much faster than the first batch (curve B). So something left in the solution is apparently catalyzing the reaction. On the other hand, if the solution from the first batch is removed by centrifugation and filtered, both Cr(VI) and S(-II) were added again, the rate of Cr(VI) reduction is very close to the first batch. So the stuff that catalyzing the reaction is removed by this centrifugation and filtration processes. 

## Effect of externally added elemental S 

**(pH=8.30, 25****o****C, [H****2****S]=800mM)**

- A. Control
- B. With  S0 80M
- C.             160M
- D.             240M
- E.             320M

**Elemental sulfur nanoparticles **
**catalyze Cr(VI) reduction by sulfide**

***Environ. Sci. Technol.***** 39, 2087 – 2094 (2005) **

**Notes:**
As we already know, for the chromate/sulfide system, the main products are elemental sulfur and chromium hydroxide. So we conducted experiments in the presence of sulfide and chromium hydroxides. 

With elemental sulfur added into the system at the very beginning of the reaction, as shown here, much faster reaction rates were obtained in the presence of elemental sulfur. The reaction rate increases as elemental sulfur concentration is increased. 

We did parallel experiments by adding chromium hydroxide externally, but we found that chromium hydroxide does not affect Cr(VI) reduction.

These experiments demonstrates that elemental sulfur catalyzes Cr(VI) reduction, which is something that has never been reported before. 

## Cr(III) Reoxidation and Remobilization

**Possible Mechanisms**

## Coupled Biotic/Abiotic Processes for Cr(III) Oxidation 

***Pseudomonas putida***** was obtained from ATCC**
**Microbial experiments were conducted in modified LEP medium with 10mM HEPES buffer (pH 7) in a shaker at 150 rpm at 26****o****C**

**Cell density (OD600):**** using spectrophotometric method at 600 nm**
**Mn oxides:****  modified LBB assay (Boogerd and de Vrind, 1987)**
**Cr(VI):**** ****	****measured using diphenyl carbazide colorimetric ****	****	****method after filtration through 0.2 ********m membrane filters.**
**Total soluble Mn and Cr:****  measured with ICP.**

**H****2****O**

**Mn(II)**

**Mn-Oxides**

**Cr(III)**

**P. putida**

**O****2**

**Cr(VI)**

## Microbial Growth

**A: 0.10 mM Mn(II)**
**B: 0.0    mM Mn(II)**
**C: 0.10 mM Mn(II), sterile **

**Microbial Growth**

**Mn(II) Oxidation**

**Notes:**
Here is shown the generation of manganese oxides in the presence and absence of manganese-oxidizing bacterium, Pseudomonas putida. Three treatments: A: P.putida+Cr(III)+Mn(II); B:P.putida+Cr(III); C:Sterile A (Figs. 1 & 2) autoclaved at 121oC, 1 atm, for 15 minutes. You can see that Mn oxides produced only in Treatment A, where the active cell is present. No Mn oxides were produced in both B and C. Mn(II) initial concentration was 0.1 mM, Cr(III) 0.05 mM.

## Cr(III) Oxidation Coupled with Microbially-mediated Mn-Oxides Production 

**A: 0.10 mM Mn(II)**
**B: 0.0    mM Mn(II)**
**C: 0.10 mM Mn(II), sterile **

**Cr(III) Oxidation Coupled with Microbially-mediated ****Mn-Oxides Production **

***Geomicrobiology Journal, 22:161-170, 2005***

**Notes:**
This slide shows you the results of three treatments about Cr oxidation. In Treatment A, where active cell and Mn(II) present, Cr(III) started decrease at the 24th hour and Cr(VI) produced in the medium. About 80% of Cr(III) decreased and 50% of the added Cr(III) oxidized to Cr(VI). About 30% of the added Cr(III) decreased and 10% of Cr(III) added was oxidized to Cr(VI) in the medium of Treatment B, where active cell present but no Mn(II). In Treatment C, where Mn(II) present but no active cell present, 20% of the added Cr(III) lost, but no Cr(VI) produced.

Summary of the 3 experiments: 1. P. putida enhanced Mn(II) and Cr(III) oxidation; 2. The oxidation of Mn(II) and Cr(III) by oxygen in the medium did not happen; 3. Mn(II) oxidation relies on cell, but Cr(III) oxidation by the bacterium contributed for only a small part, about 20% of the oxidation in the presence of Mn(II) and then formation of Mn oxides.

## Long term stability of Cr in soils following H2S treatment

**Contaminated soil sample from the 100K Area at the Hanford Site was treated with diluted hydrogen sulfide gas. **
**Original Cr(VI) concentration:  110 mg/kg**

## How Important Is U(VI) Reduction by Hydrogen Sulfide in the Aqueous Phase???   --- UNCLEAR --- 

**U(VI) could coexist with high concentration of sulfide (400mM) in anoxic see water (Anderson et al., 1989) **
**U(VI) was not reduced by sulfide in bicarbonate (30 mM) buffered solution (Lovley et al., 1991).**** **
**Uranyl (CO****2****-free) was precipitated by 800 mg/L H****2****S at pH 6.0-6.5 (Kochenov et al., 1977).**
**Uranyl was reduced to uraninite by 500 mM sulfide in the anoxic system with P****CO****2**** = 0.15 atm (Beyenal et al., 2004).**** **
**A better understanding of the effect of carbonate on U(VI) reduction by sulfide is needed. **

**H****2****S**

**UO****2****(CO****3****)****2****2-**

**UO****2****OH****+**

**OH****-**** (pH)**

**CO****3****2-**

**UO****2**** + S**

- Reaction Products

**?**

**Notes:**
If we compare them with the researches about uranium adsorption on ferric iron minerals, the reports on the reaction between uranyl and aqueous sulfide or sulfide minerals are not only limited but also somewhat confusing 
Anderson found U(VI) could coexist with sulfide as high as 400M in anoxic Black Sea
A Russia scientist, Kochenov, observed that Uranyl (CO2-free) could be precipitated by 800 mg/L H2S at pH 6.0-6.5
Recently, Beyenal reported uranyl could be reduced by sulfide in a system with 15 % CO2 in surrounding atomsphere
Wersin noticed that uranyl adsorbed by galena or pyrite was partially reduced  

## Molar ratio of [U(VI)]reduced to [S2-]oxidized (pH = 6.89; [CO32-]T = 4.0 mM)

**Molar ratio of [U(VI)]****reduced ****to [S****2-****]****oxidized**** ****(pH = 6.89; [CO****3****2-****]****T**** = 4.0 mM)**

**Slope = 0.94**

**Electron Diffraction Pattern**

**HRTEM Micrograph**

_**Reaction Stoichiometry**_
**UO****2****2+**** + H****2****S =  UO****2**** + S****0**** + 2H****+**

**Notes:**
Simultaneous monitoring the changes of [U(VI)] and [S2-]  could not only tell us whether the reduction reaction takes place but tells us something about the reaction stoichiometry as well
Our test was conducted @ pH 6.89, [CO32-] 4.0 mM and the results showed that the ratio was 0.94, which suggests us that the reaction does occur and the ratio of the consumption of the two reactants is close to 1:1.  

## Effects of [CO32-]T on [U(VI)]reduced/[U(VI)]0 (%)   ([S2-]0 = 2.0 mM; [U(VI)]0 =168mM; reaction time = 60 min) 

**Effects of pH on U(VI) reduction** **([CO****3****2-****]****T**** = 4.0mM; [S****2-****]****0**** =2.0 mM)**

**Notes:**
In the preliminary experiments, we found pH and CO3 had great influence on the reaction. So, the third experiments was used to test the influence of CO3 at fixed pH. Actually, we designed three pHs. At pH 6.89, when [CO32-]total  was lower than 1.0 mM, over 96% U(VI) was reduced in one hour; while when [CO32-]total  ≥  15.0 mM, less than 10% U(VI) was reduced.   At pH 8.01, when [CO32-]total  was lower than  1.0 mM, 80% U(VI) was reduced in one hour, while when [CO32-]total   ≥  4.0 mM, almost no reaction was observed.   At pH 9.06, when [CO32-]total was set at 1.0 mM, only 18% U(VI) was reduced.   

## Impact of carbonate on U(VI) speciation and reduction (calculated with MINEQL+, 1998)

**Impact of carbonate on U(VI) speciation and reduction**** ****(calculated with ****MINEQL+, 1998)**

**Notes:**
There is the question: which one or ones is/ are major reaction species? MINEQL+ was used to calculate the U speciation. As the [CO32-]total increases from 4.0 mM to 15.0 mM,  the calculated percentage of [UO2(CO3)34-] and [UO2(CO3)22-] increase from 2.2% to 8.7% and 87.9% to 88.8% respectively, however, the experiment results showed that the percentage of U(VI) reduction decreased from 64% to 8%. This comparison suggested that both UO2(CO3)22- and UO2(CO3)34- are not the major species participating in reduction reaction. The same comparison indicated UO2CO3(aq) was not the major species reduced too.  

## Effects of [CO32-]T on [U(VI)-hydroxyl species]0

**Effects of [CO****3****2-****]****T ****on ********[U(VI)-hydroxyl species]****0**

**Notes:**
The figure on the right side is the experiment results we showed before; the figure on the left side is the species got from the speciation calculations
To our surprise, the curves shown in these two Figures are very similar  
Recall the [CO32-] influence, for any of the three pHs, there exists a region of [CO32-]total in which corresponds an abrupt decrease of the rate of U(VI) reduction. For pH 6.89, it is from 4.0 mM to 30.0 mM; for pH 8.01, it is from 1.0 mM to 6.0 mM; and for pH 9.06, it was from 0.2 mM to 2.0 mM
As shown in the left figure, for the corresponding [CO32-]total regions, Σ[hydroxo-U(VI)]inital also has an abrupt decrease. To tell in detail, it deceases from 1.4×10-7 M to 1.3×10-9 M at pH 6.89; from 1.4×10-7 M to 1.3×10-9 M at pH 8.01; and from 1.3×10-8 M to 2.0×10-9 M at pH 9.06. 

## Measured initial rate is directly proportional to the calculated Σ[U(VI)-hydroxyl species]0 

## U(VI) Reduction on FeS  - A Two-step Process 

**FIG 1. Rapidly U(VI) uptake by iron sulfide **
**([U(VI)]****0**** = 168.0 ********M; [FeS] = 0.18 g/L **

**FIG. 2  Release of Fe(II) is proportional to the amount of U(VI) uptake (pH 6.90. ([U(VI)]****0**** = 168.0 ********M)**

**FIG. 3.  Reduction of ****[U(VI)] on the FeS surfaces measured by carbonate extraction of U(VI).  **

## Technetium Sulfide Chemistry

**Investigation the pertechnetate-sulfide chemistry in aqueous solution**
**Controlled pH, O****2****, ionic strength**
**Product determination**
**Tc immobilization (precipitation/colloid formation)**
**Reaction kinetics and stoichiometry determination**

## Pertechnetate-sulfide chemistry in aqueous media

- In acidic solution:
- In basic solution:

- 2 TcO4- + 7 HS- + H2O        Tc2S7 + 9 OH-

- 2 TcO4- + 7 S2- + 8 H2O        Tc2S7 + 16 OH-

- (Equation  1)

- (Equation  3)

- (Equation  4)

## Kinetics studies under pH 9 anaerobic conditions

- ln[TcO4-](μM)

- ln[TcO4-](μM)

**Kinetics studies under**** ****pH 9 anaerobic**** ****conditions**

**With respect**
** ****to [TcO****4****-****]**

**With regard **
**to sulfide**

## Kinetics studies under pH 4 aerobic conditions

- ln[TcO4-](μM)

- ln[TcO4-](μM)

**Kinetics studies under**** ****pH 4 aerobic**** ****conditions**

**With respect**
**to [TcO****4****-****]**

**With regard **
**to sulfide**

## Sample characterization

**●** XANES & EXAFS
**● **Elemental analysis

| Sample | S (%) | Tc (%) | Mole ratio of S/Tc | Suggested formula |
| --- | --- | --- | --- | --- |
| Product from pH 4 aerobic solution | 45.585 | 39.95 | 3.53 | Tc2S7 |
| Product from pH 9 anaerobic solution | 45.88 | 40.5 | 3.54 | Tc2S7 |

**TcO****4****- ****+ 3e****-**** + 4H****+****	****= TcO****2**** (s) +  2H****2****O**

**???**

## SUMMARY

**Reductive Cr(VI) immobilization could be achieved by ISGR treatment. Reoxidation of Cr(III) is possible by biogenic Mn-oxides.  A thorough evaluation under a wide variety of environmental conditions is needed. **
**Reductive immobilization of U(VI) takes place by H****2****S under anoxic environments, and the rate depends on pH and [CO****3****2-****]****total****.  It is Σ[U(VI)-hydroxyl complex], not total [U(VI)], that determines the rate of U(VI) reduction at various pH and [CO****3****2-****]****total****.  Reduction rates could be predicted through speciation calculation. U(VI) reduction on FeS surfaces takes place through a two-step process.  **
**Tc(VII) could be immobilized by H****2****S treatment,  with Tc****2****S****7**** being the product.  Research is under way to understand the reaction mechanism.    **

**Notes:**
To end my presentation, what we found can be summarized as: (i)-(iv).  
Thank you for you attention! I’d like to take some questions.

## QUESTIONS?

- Thank you!