An Experiment Illustrating How Iron Metal is Used to Remediate

Contaminated Groundwater

Barbara Balko, Department of Chemistry

Lewis & Clark College

Portland, OR

Contaminant hydrologists investigated the effect that materials used in sampling groundwater

had on the concentration of halogenated solvents.

Discovery

Chemistry

Oxidation-Reduction Reaction:

Fe0 Fe2+ + 2e- Eo = 0.44 V

CCl4 + 2e- + H+ CHCl3 + Cl- Eo = 0.67 V

_____________________________________________

CCl4 + H+ + Fe0 CHCl3 + Cl- + Fe2+ Eo = 1.11 V

Kinetics

Rate = -d[RX]/dt = k[Fe active sites][RX]

-d[RX]/dt = kobs[RX]

where kobs = k[Fe active sites]

*expect kobs to be proportional to the mass of iron used as well as the iron surface area

Iron Wall Pump-and-Treatversus

Application

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

•kobs is obtained by plotting ln(A/Ao) versus time

•Vary experimental parameters to learn details about the reaction mechanism

Experiment

t = 0

Equipment/Chemicals

granular iron (0.25 g/cuvet) light source/detectorpolystyrene cuvets with caps (rotator)~ 20 ppm dye solution (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

•1st week: measure kobs under standard conditions; plan experiment; confirm that Beer’s law is satisfied

•2nd week: self-designed experiment

•Provide time for inter-group discussion and presentation of results

•Poster presentation

Typical Results

Time (minutes)

0 2 4 6 8 10

ln(A

t/A0)

-1

0

0.25 g, unsieved

0.50 g, unsieved

0.25 g, fine grain

0.25 g, coarse grain

Results obtained using Fluka iron, indigo carmine (20 ppm), and a rotation rate of 18 rotations/min

Conditions kobs (min-1) t1/2 (min)

0.25 g unsieved iron 0.065 11

0.50 g unsieved iron 0.12 5.8

0.25 g fine grain (< 0.4 mm) iron

0.10 6.9

0.25 g coarse grain (> 1 mm) iron

0.056 12

Examples of Student Projects

Effect of Temperature

Students designed and

constructed set-up to control temperature

Results: Eact = 64.1 kJ/mole

Unintended result: Rotator position matters

Are Other Metals as Effective?

Search for Metals with Similar Particle Sizes

Correlation between Eo and Metal Reactivity?

Al3+/Al Eo = -1.66 V

Zn2+/Zn Eo = -0.763 V

Fe2+/Fe Eo = -0.440 V

Sn2+/Sn Eo = -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 kobs linearly proportional to the mass of iron used?

Linear Correlation between kobs and the Mass of Fe Used

The Real World is More Complex….

The actual oxidant may be Fe2+ or H2

due to the reduction of dissolved oxygen and/or water by Fe0

How does oxidation of the iron surface affect the reaction long term?

Fresh reagent-grade Iron

Iron after 12 hrs exposure to aqueous CCl4

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.

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

Locations

Possible Topics for Class Discussion

•Oxidation-Reduction Reactions

•Pseudo First-Order Kinetics

•Environmental Chemistry

•Heterogeneous Reactions

•Corrosion

•Passive Film Growth

•Mass Transport