# Presentation: 119293

## In-situ Geochemical Fixation of Chromium in Groundwater in Arid Climates:A Comparison of Chemical Reductant Technologies 

**Peter Storch***** ***P.E.
- Andrew Messer, and Chris Lawrence
**URS Corporation, Arizona, USA**

## In Memory of Roman Pyrih1946-2006

## Objectives

**“****One size does **_**not**_** fit all”**
- Identify key parameters for evaluating chemical reductant technologies
- Compare key parameters
- General recommendations

**Notes:**
You have heard presentations of case studies showing results of specific one technology – ZVI, bioenhancement, etc
What I would like to do is take you back to the beginning and help you get started on the technology selection process
One size does not fit all
Make those of us who use only one technology, consider others

## Chromium Reduction Technologies

**Sodium Dithionite – Na****2****S****2****O****4**
**Zero Valent Iron (ZVI) - Fe****0**
- Micron-scale
- Nano-scale
**Ferrous Sulfate Fe(SO****4****) - Fe(II)**
**Calcium Polysulfide (CPS) - CaS****x**
**Bioreductants**
- Molasses
- Vegetable Oil
- Corn Syrup
**HRC  –  **polylactate /organic sulfur

**Notes:**
Sodium Dithionite – 

Veg Oil – typ. 50% by vol oil/water emulsion with lactate and nonionic surfactants

## General Technology Selection Criteria

**Effectiveness**
**Subsurface Distribution**
**Safety and Handling**
**Regulatory Acceptance**
**Cost**

## Effectiveness

**Cr(VI) reduction per mass reductant**
**Demonstrated in the field**
**Persistence**

- 100 mg/L Cr(VI)

- 0.02 mg/L Cr(VI)

- precipitate
- Cr(III) 9,100 mg/Kg

## Cr(VI) Reduction per Mass Reductant

**ZVI**
**Micron****	****  2  g ****Cr(VI)**** /kg Fe****1**
**Nano****	****	****70  g ****Cr(VI)**** /kg Fe****1**
**Ferrous Sulfate****	****	**
**CPS****	****	****135 g ****Cr(VI)**** /kg CaS****5****2**
**HRC/MRC****	****     ****40 g**** ****Cr(VI)**** /kg HRC**
**1 ****Source:  DOE WMP-28124, Rev , Cao and Zhang**
**2****Source:  URS, 2002**

## Cr(VI) Reduction per Mass Reductant

| Reagent | Theoretical
g Cr(VI)/kg reagent | Actual
g Cr(VI)/kg reagent |
| --- | --- | --- |
| ZVI Micro/Nano | 2 / 70 | 2 / 70 |
| Ferrous Sulfate | 114 | 20-35 |
| CPS | 173 | 40-50 |
| Bio-reductants | 10-50 | - |
| HRC | 52 | 40-50 |

## Slide 9

## Comparison of Persistence

**Comparison of Persistence**

- Greater persistence means fewer injections

## Persistence of Reducing ConditionsOne CPS Injection, Cr(VI) = 50 mg/L

## Factors Affecting Distribution

**Transport Parameters – ****viscosity, density**
**Reaction Mechanism/Kinetics - ****stability**
**Migration – ****will it move with groundwater?**

## Comparison of Distribution Factors

|  | Estimated
Kinematic
Viscosity 
(viscosity/density)
cm2/s | Reaction
Mechanism
(kinetics) | 
Migration w/
Groundwater |
| --- | --- | --- | --- |
| Water | 0.01 | - | - |
| Dithionite
5.0% Sol’n | 0.10 | Highly reactive
Unstable | No |
| Ferrous Sulfate
5% Fe (II) | 0.08 | Fast reaction | No |
| CPS
29% Sol’n
5.0% Sol’n | 0.13
0.04 | Slow decomp. step 
Fast reaction,
Stable | Yes |
| ZVI
Micro-scale
Nano-scale | 1.0
0.05 | Slow, Surface- mediated, 
Fast reaction | No,
possible  |
| Bio-
reductants | 0.02 – 200 | Slow, biologically-mediated | Some do |
| HRC/MRC | 100 | Slow decomp, biologically-mediated | No |

## Atomized Slurry in N2 gas Carrier

- Atomized Slurry in N2 gas Carrier
- (ARS Technologies)

- (Toda America)

## Safety of Reductants

**Risk to human health from direct exposure**
**Risk to groundwater quality from byproducts**

- Add photo

## Typical Regulatory Concerns

**Dilution**
**Degradation of groundwater quality from byproducts ****(**NO3-        NH4+), SO42-
**Pore Space Plugging**
**Displacement**
**Preferential Pathways**

## Preferential Pathways

**Preferential Pathways**
**in fine-med sand**

## Regulatory Acceptance

**Dithionite:**  Hanford, Elizabeth City
**CPS:**  full-scale in 12 US states, Australia
**ZVI:**  numerous PRBs,
**Bio:**  approved by EPA, most US states
**HRC:** approved by EPA, most US states

## Reagent Cost Comparison

|  | Unit Cost
($/kg reagent)  | Cr Reduction Unit Cost
($/kg Cr)  |
| --- | --- | --- |
| Dithionite
powder  | 1.5 - 2.0  | 9.3 – 13 |
| CPS
CaSx  | 0.5 - 0.7  | 3.7 – 5.2 |
| ZVI
  Micro
  Nano (25% slurry)  | 2.2 -3.3
15 - 20  |  1,100 – 1,650
215 – 1,500  |
| Bioreduction
  Molasses
  Corn Syrup
  Veg Oil (emul.)  | 
0.4 - 0.7
2.0 - 3.5  | 
15-30  |
| HRC | 11 – 13 | 1,000 - 2,000  |

## Reagent Cost as Percentage of Total Application Cost

|  | Shallow
 (<60 ft) | Deep 
(>60 ft) |
| --- | --- | --- |
| Dithionite | 15 - 20 | 10 - 15 |
| CPS | 15 – 20 | 10 - 15 |
| ZVI | 40 – 50 | 25 - 35 |
| Bio-reductants | 10 – 15 | 5 - 10 |
| HRC | 30 - 40 | NA |

**Reagent Cost as Percentage of Total Application Cost**

## Desert/Arid Environment

**Deep Groundwater**
**Coarser Grains**
**pH  7-9**
**Low Organic Content**
**Preferential Pathways**

## Arid Environment

**Arid Environment**

- Fracing

- Conc./Fracing

## Arid Environment

**Arid Environment**

## Summary

**Sodium Dithionite**
**highly reactive, low persistence **
**questionable cost-effectiveness**

**Ferrous Sulfate**
**low cost**
**very reactive**
**lower persistence**
**history of lowering permeability**

## Summary

**ZVI**
**cost-effective, deep PRB**
**shallow source area, **
**mixed Cr/VOC,**
**Nano-scale needs development, **
**good choice for arid climates.**

**CPS**
**Low cost**
**well-developed**
**flexible **
**combine with bio-reductants to offer low cost, long life  **
**good choice for arid climates.**

## Summary

**HRC**
**high unit cost **
**good shallow, source-zone treatment,**
**more difficult to distribute in deep aquifer**
**mixed Cr/VOCs**

**Bio-reductants**
**low unit cost, **
**multiple injections for long-term effectiveness and persistence,**
**good for arid environment****.**

## 2 CrO42- + 3 CaS5 + 10H+           2 Cr(OH)3(s) + 15 S(s) + 3Ca2+ + 2H2O

- 2 CrO42- + 3 CaS5 + 10H+           2 Cr(OH)3(s) + 15 S(s) + 3Ca2+ + 2H2O

- In-situ Geochemical Fixation
- Groundwater Pilot Test
- Metal Plating Shop, Arizona

**Notes:**
CPS was selected because polysulfide effectively reduces Cr(VI) to Cr(III), is more stable and persistent in subsurface environments than other reductants, does not form insoluble precipitates when oxidized such as ferrous sulfate, and is a relatively safe chemical to store and handle in the field. 
CPS has been used extensively in agriculture as a soil amendment and has found application in drinking water and wastewater treatment for the removal of heavy metals from solution 
Recently, CPS has been used for the in-situ remediation of Cr(VI) and, among other sites around the country, was selected by the California Regional Water Quality Control Board, North Coast Region, for full-scale application at three sites [Goeble, 2001].
CPS is a solution of many sulfide compounds consisting primarily of CaS2, CaS3, CaS4, and CaS5 and is sold as a 29-percent aqueous solution. The specific gravity of the commercially-available solution is 1.273. Also referred to as “lime sulfur solution”, it is a deep orange-red, alkaline solution with a pH of 11.3 to 11.5 

9000 gallons of CPS was delivered to the site in tanker trucks and stored in two 6500 gallon tanks

## Chromium Non-detect After 600 Days

- Injection Well MW-15

**Notes:**
Pumped samples in the injection well at 3, 9, 30, and 180 days indicated complete reduction of hexavalent chrome from the baseline concentration of about 35 mg/L

## Slide 35

- Application of CaSx through infiltration trenches

## Impact to Groundwater in 160 Days