An Experiment Illustrating How Iron Metal is Used to Remediate Contaminated Groundwater

Transcription

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

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

3 Chemistry Oxidation-Reduction Reaction: Fe 0 Fe e - 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

4 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

5 Application Iron Wall versus Pump-and-Treat

6 Uses

7 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

8 t = 0 Experiment

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

10 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 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

11 Typical Results g, coarse grain 0 ) t /A ln(a 0.25 g, unsieved 0.25 g, fine grain g, unsieved Time (minutes) Results obtained using Fluka iron, indigo carmine (20 ppm), and a rotation rate of 18 rotations/min

12 Conditions k obs (min -1 ) t 1/2 (min) 0.25 g unsieved iron g unsieved iron 0.25 g fine grain (< 0.4 mm) iron 0.25 g coarse grain (> 1 mm) iron

13 Examples of Student Projects

14 Effect of Temperature

15 Students designed and constructed setup to control temperature

16 Results: E act = 64.1 kj/mole

17 Unintended result: Rotator position matters

18 Are Other Metals as Effective?

19 Search for Metals with Similar Particle Sizes

20 Correlation between E o and Metal Reactivity? Al 3+ /Al E o = V Zn 2+ /Zn E o = V Fe 2+ /Fe E o = V Sn 2+ /Sn E o = V

21 Does Oxide Coating Slow Reaction?

22 How to Control for Particle Scattering?

23 Rusted Iron is Less Reactive

24 Can Iron be Used to Remove Dye Stains in Cloth?

25 How to Quantify Stain Removal?

26 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

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

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

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

30 Chem. Educator (6, , 2001)

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

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

33 Linear Correlation between k obs and the Mass of Fe Used

34 The Real World is More Complex. The actual oxidant may be Fe 2+ or H 2 due to the reduction of dissolved oxygen and/or water by Fe 0

35 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

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

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

38 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.

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

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