An Experiment Illustrating How Iron
Metal is Used to Remediate
Contaminated Groundwater
Barbara Balko, Department of Chemistry
Lewis & Clark College
Portland, OR

Discovery
Contaminant hydrologists investigated the effect that materials used in sampling groundwater had on the concentration of halogenated solvents.

Chemistry
Oxidation-Reduction Reaction:
Fe 0

Fe 2+ + 2e E o = 0.44 V
CCl
4
+ 2e + H +

CHCl
3
+ Cl E o = 0.67 V
_____________________________________________
CCl
4
+ H + + Fe 0

CHCl
3
+ Cl + Fe 2+ E o = 1.11 V

Kinetics
Rate = -d[RX]/dt = k[Fe active sites][RX]
-d[RX]/dt = k obs
[RX] where k obs
= k[Fe active sites]
*expect k obs to be proportional to the mass of iron used as well as the iron surface area

Application
Iron Wall versus Pump-and-Treat

Uses

Adapting the Technology to the Lab
•Dyes are used to simulate groundwater contaminants
•Degradation is followed using a visible light source and detector
•Dye-Iron interaction occurs in a sealed cuvet
•k obs is obtained by plotting ln(A/A o
) versus time
•Vary experimental parameters to learn details about the reaction mechanism

t = 0
Experiment

Equipment/Chemicals granular iron (0.25 g/cuvet) polystyrene cuvets with caps
~ 20 ppm dye solution light source/detector
(rotator)
(sieves)

Logistics
•Suitable for freshman undergraduates; can also be used as a demonstration
•Requires two (or more) 3 hour lab periods
•Students are divided into groups of 2 – 4
•1 st week: measure k obs under standard conditions; plan experiment; confirm that Beer’s law is satisfied
•2 nd week: self-designed experiment
•Provide time for inter-group discussion and presentation of results
•Poster presentation

Typical Results
0
0.25 g, coarse grain
0.25 g, unsieved
0.25 g, fine grain
-1
0.50 g, unsieved
0 2 8 10 4 6
Time (minutes)
Results obtained using Fluka iron, indigo carmine (20 ppm), and a rotation rate of 18 rotations/min

Conditions
0.25 g unsieved iron
0.50 g unsieved iron
0.25 g fine grain (< 0.4 mm) iron
0.25 g coarse grain (> 1 mm) iron k obs
(min -1 ) t
1/2
(min)
0.065
11
0.12
0.10
0.056
5.8
6.9
12

Effect of Temperature

Students designed and constructed setup to control temperature

Results: E act
= 64.1 kJ/mole

Unintended result: Rotator position matters

Are Other Metals as Effective?

Search for Metals with Similar
Particle Sizes

Correlation between E o and Metal
Reactivity?
Al 3+ /Al E o = -1.66 V
Zn 2+ /Zn E o = -0.763 V
Fe 2+ /Fe E o = -0.440 V
Sn 2+ /Sn E o = -0.136 V

Does Oxide Coating Slow Reaction?

How to Control for Particle Scattering?

Rusted Iron is Less Reactive

Can Iron be Used to Remove Dye Stains in Cloth?

How to Quantify Stain Removal?

Other Project Ideas
•Effect of Mass of Iron Used
•Effect of Iron Surface Area
•Effect of Dye Concentration
•Source of Iron
•Rotation Rate
•Dye
•pH/buffering

Trouble-Shooting
•Air Bubbles in Cuvets
•Oxygen Leakage into Cuvets
•Light Scattering
•Biased Sampling of Iron
•Adsorption of Dyes to Cuvets, Iron…

J. Chem. Ed. (78 (12), 1661, 2001)

Resources: MERL CD-ROM
Available here or send a request to merl@ese.ogi.edu

Chem. Educator (6, 172-179, 2001)

Acknowledgements
Paul Tratnyek, Dept. of Environmental Science and
Engineering, Oregon Health and Sciences
University
Lewis & Clark College Chemistry Department
Accelerated General Chemistry, Spring 2002

Is k obs linearly proportional to the mass of iron used?

Linear Correlation between k obs
Mass of Fe Used and the

The actual oxidant may be Fe 2+ or H
2 due to the reduction of dissolved oxygen and/or water by Fe 0

How does oxidation of the iron surface affect the reaction long term?
Fresh reagent-grade
Iron
Iron after 12 hrs exposure to aqueous CCl
4

Results Suggest Iron Cannot Remove
Dye Stains from Cloth and that Controls are Important!

Implementation
Installation of an iron wall at a site formerly occupied by a semiconductor manufacturing factory (Sunnyvale,
CA)

Performance
The first field test of an iron wall (Canadian
Forces Base, Borden,
Ontario) showed that halogenated solvents would be degraded. The performance of the wall did not deteriorate in subsequent years.

Locations
The map shows the iron walls installed (or under construction) as of August 1999. There are also iron walls in
Europe, Australia, and
Canada.

Possible Topics for Class Discussion
•Oxidation-Reduction Reactions
•Pseudo First-Order Kinetics
•Environmental Chemistry
•Heterogeneous Reactions
•Corrosion
•Passive Film Growth
•Mass Transport