Martin C. Rabenhorst University of Maryland Environmental Science & Technology

1. Synthetic Iron Oxides: An Indicator of Reduction In Soils (IRIS)What do we now know?2006 NE Cooperative Soil Survey Conference Martin C. Rabenhorst University of Maryland Environmental Science & Technology

2. Why document reduction in soils? • Field Indicators of Hydric Soils • Applying the Technical Standard for Hydric Soils • To test existing Field Indicators • To evaluate proposed Field Indicators • Demonstrate that a soil is “hydric” in the absence of Field Indicators • Also using the Technical Standard for Hydric Soils • Demonstrate occurrence of redox processes • In constructed or restored wetlands • Performance criteria • Other research endeavors

3. Available Technologies for Documenting Reduction in Soils • Eh measurements with Platinum electrodes (and pH) • Plot on Eh-pH diagrams • Time consuming and a bit tedious, equipment issues • Use of alpha-alpha-dipyridyl • Reacts with ferrous Fe+2 • Difficult to obtain (hazardous) • Now available as test papers www.gallard.com • Both provide only a “snapshot in time” of the soil conditions

4. www.gallard.com • Dipyridyl paper • item# 90725Pack Size: box of 200 stripsHazard Class /UN Number: Not RestrictedStorage Temp: Room Temperature • Impregnated with a,a’-dipyridyl (= 2,2’-bipyridine), the paper reacts when Fe 2+ ions are present in mineral acid solutions, yielding a deep red very stable complex cation. Dipyridyl paper is specific for iron(II) (Fe 2+ ) and permits the detection of minute quantities of Fe 2+ , even in the presence of substantial amounts of Fe 3+ . • Limit of sensitivity: 2 mg/L Fe 2+ • $26.90 per box of 200

6. IRIS (Indicator of Reduction in Soils) Tubes • Fe Oxide paint is applied to ½ inch schedule 40 PVC tubing while the tube is on a lathe device to ensure an even distribution of the paint. • Jenkinson, B. 2002. Indicators of Reduction in Soils (IRIS): A visual method for the identification of hydric soils. Ph.D. Diss. Purdue Univ., West Lafayette, IN

7. Pilot hole made for each IRIS tube • IRIS Tubes inserted into the soil • Under anaerobic conditions, microbes oxidize OM utilize Fe oxides on IRIS tubes as e- acceptors • As Fe(III) in paint is reduced to Fe(II), it dissolves • Zones where Fe paint has been removed is visible and can be documented (quantified)

8. Originally we made composite photos of tubes after taking three photos while rotating each tube 120o • Then estimated paint removal • Visually • Using image analysis

9. New Approach to Collecting the Data from IRIS Tubes Can collect data on 0-40 cm soil zone

12. How effective are visual estimates of IRIS paint removal?

14. Visual Estimates – Averages of 12 individuals

17. For more than approximately 20% removal, +/- 10% (absolute) Up to approximately 20% removal, +/- 50% (relative)

18. Guides for estimating percent removal Stoops, G. 2003. Guidelines for Analysis and Description of Soil and Regolith Thin Sections. Soil Sci. Soc. Am.

19. Summary of Initial StudyComparison of Fe removal from IRIS tubes to redox potential measurements (Pt electrodes) based on the NTCHS Tech Std. Where 10% of the iron oxide paint was removed, the soils were reducing in 82% of the sections while in 18% of the cases, the soil was oxidized. Where 20% of the iron oxide paint was removed, the soils were reducing in 89% of the sections while in 11% of the cases, the soil was oxidized. Where 25% of the iron oxide paint was removed, the soils were reducing in 100% of the sections. Percentage of Observations Oxidized or Reduced by the TS

20. When initially synthesized, the paint is primarily ferrihydrite Fe5HO8*4H2O Poorly crystalline – probably similar to newly formed Fe oxides in wet soils

21. Paint of 4 ages were located in the lab left over from previous studies 1 2 3 4 120 d 270 d 625 d 14 d Color changes reflect changes in mineralogy

22. Problem with Newly Synthesized Paint • We began to notice that newly synthesized paint would not adhere well to the PVC, but that sometimes paint 2-3 weeks old would adhere better • Therefore, we postulated that there might be some “aging” (mineralogical alteration) required for the paint to be useable. • We knew from earlier observations that old paint stored in the lab changes to more crystalline forms • Schwertmann and Cornell (2000) indicate that pH away from the ZPC causes more rapid alteration of ferrihydrite to other crystalline phases

23. 12.0 11.0 7.5 4.0 Chloride and nitrate salts behaved identically

24. Newly formed Fe oxides (4 days old) Goethite peaks pH 12 pH 11 pH 7.5 pH 4 0.75 0.6 0.5 0.4 0.35 0.3 0.25 0.2 0.15 d-spacing nm 2 broad peaks for ferrihydrite

25. 28 days pH 12 pH 11 pH 7.5 pH 4 0.75 0.6 0.5 0.4 0.35 0.3 0.25 0.2 0.15 d-spacing nm

26. pH 4 & 7.5 pH 11 pH 12 Feo/Fet = proportion of Ferrihydrite 1-Feo/Fet = proportion of Goethite • Fe oxides formed by titration to pH 4 or 7.5 remain as essentially entirely oxalate extractable phases over time (confirming dominance of ferrihydrite) • When Fe oxides were formed by titration to pH 11 or 12, a substantial portion of the Fe oxides initially were not oxalate extractable (8% and 30% respectively), and they continued to show alteration to more crystalline phases over time

27. Abrasion Resistance and Durability • 1 - paint wipes off when applying very slight pressure • 2 - paint wipes off when applying slight pressure • 3 - paint wipes off when applying moderate pressure • 4 - paint wipes off only when applying firm pressure • 5 - paint does not wipe off when applying firm pressure.

28. Abrasion Resistance

29. Feo/Fet = proportion of Ferrihydrite 1-Feo/Fet = proportion of Goethite pH 4 & 7.5 pH 11 pH 12

30. Newly Synthesized Paint What about storage? Effects of Temperature?

31. Three storage temperatures examined: 6C, 20C, 35C 36.5 20.1 6.7

32. 6C Over 46 days

33. 20C Over 46 days

34. 35C Over 46 days

35. Estimates of Goethite content based upon peak heights

36. Summary • What we now know: • We understand basically how to interpret the tubes • We have developed a more rapid method for scanning and analysis of the tubes • We know how to routinely make paint that will adhere well to the tubes and why • We know that refrigeration will sustain the “pot life” of paint (retard mineralogical change) • What we still don’t know: • How significant are mineralogical differences in paint performance (40% Gt vs 70% Gt)? • What the variations in colors on IRIS tubes represent?

37. Quick (7 day) IRIS Tube Paint Recipe and Construction Procedure 1 page handout available