Date post: | 30-Dec-2015 |
Category: |
Documents |
Upload: | amaranta-nova |
View: | 36 times |
Download: | 0 times |
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)
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