Synthetic Iron Oxides: An Indicator of Reduction In Soils (IRIS)What do we now know?

2006 NE Cooperative Soil Survey ConferenceMartin C. Rabenhorst

University of Maryland

Environmental Science & Technology

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

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

Available Technologies for Documenting Reduction in Soils

Both provide only a “snapshot in time” of the soil conditions

www.gallard.com Dipyridyl paper item# 90725                                

Pack 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

Eh-pH stability diagram

-300

-100

100

300

500

700

4 5 6 7 8

pH

Eh

Goethite HematitePyrolucite SulfideTechnical Standard FerrihydriteWorld Reference Base rH < 19

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 Jenkinson, B. 2002. Indicators of Reduction in Soils (IRIS): A visual method for the identification of hydric soils. Ph.D. Diss. Purdue Univ., West for the identification of hydric soils. Ph.D. Diss. Purdue Univ., West Lafayette, INLafayette, IN

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)

Originally we made composite photos of tubes after taking three photos while rotating each tube 120o

Then estimated paint removal Visually Using image analysis

New Approach to Collecting the Data from IRIS TubesCan collect data on 0-40 cm soil zone

How effective are visual estimates of IRIS paint removal?

0 10 20 30 40 50 60 70 80 90

% Fe paint removed

0

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Ave

rage

Est

imat

e of

12

indi

vidu

als

Group 1

Group 2

Visual Estimates – Averages of 12 individuals

0 10 20 30 40 50 60 70 80 90

% Fe Paint Removed

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100Vi

sual

Estim

ate

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e Pa

int R

emov

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Group 1

Group 2

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Actual % Paint Removed

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Vis

ual E

stim

ate

of P

aint

Rem

oval

R-square = 0.95 # pts = 20 y = 0.521 + 0.884x

Average of Two Individual Estimates(worst and best)

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% Fe Removed

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me

an

de

via

tion

Up to approximately 20% removal, +/- 50% (relative)

For more than approximately 20% removal, +/- 10% (absolute)

Guides for estimating percent removal

Stoops, G. 2003. Guidelines for Analysis and Description of Soil and Regolith Thin Sections. Soil Sci. Soc. Am.

0 20 40 60 80 100

percentage of iron oxide removed

0%

20%

40%

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100%

perc

enta

ge o

f sec

tions

reduced in, reduced outoxidized in, oxidized out

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.

Perc

en

tag

e o

f O

bserv

ati

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s

Oxid

ized

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Red

uced

by t

he

TS

When initially synthesized, the paint is primarily ferrihydriteFe5HO8*4H2O

Poorly crystalline – probably similar to newly formed Fe oxides in wet soils

14 d 120 d 270 d 625 d

1 2 3 4

Paint of 4 ages were located in the Paint of 4 ages were located in the lab left over from previous studieslab left over from previous studies

Color changes reflect changes in mineralogy

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

Problem with Newly Synthesized Paint

0

2

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12

14

0 1 2 3 4

mol KOH per mol Fe

pH Ferric Chloride

Ferric Nitrate

4.0

7.5

11.012.0

Chloride and nitrate salts behaved identically

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

Goethite peaks

Newly formed Fe oxides (4 days old)

2 broad peaks for ferrihydrite

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

28 days

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0.90

1.00

0 10 20 30 40 50 60

time (d)

Feo

/ F

et

.

Cl pH 4.0

Cl pH 7.5

Cl pH 11.0

Cl pH 12.0

NO3 pH 4.0

NO3 pH 7.5

NO3 pH 11.0

NO3 pH 12.0

•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

pH 4 & 7.5

pH 11

pH 12

Feo/Fet = proportion of Ferrihydrite1-Feo/Fet = proportion of Goethite

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.

Abrasion Resistance and Durability

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0% 20% 40% 60% 80% 100%

Goethite

Dur

abili

ty

.

Ab

rasi

on R

esi

sta

nce

0.00

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0.20

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1.00

0 10 20 30 40 50 60

time (d)

Feo

/ F

et

.

Cl pH 4.0

Cl pH 7.5

Cl pH 11.0

Cl pH 12.0

NO3 pH 4.0

NO3 pH 7.5

NO3 pH 11.0

NO3 pH 12.0

pH 4 & 7.5

pH 11

pH 12

Feo/Fet = proportion of Ferrihydrite1-Feo/Fet = proportion of Goethite

Newly Synthesized Paint

What about storage? Effects of Temperature?

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12/10/05 12/17/05 12/24/05 12/31/05 1/7/06 1/14/06 1/21/06 1/28/06

lab

fridge

incubator

20.1

6.7

36.5

Three storage temperatures examined: 6C, 20C, 35C

6C Over 46 days

20C Over 46 days

35C Over 46 days

0%

20%

40%

60%

80%

100%

0 10 20 30 40 50

days

6C

20C

35C

Estimates of Goethite content based upon peak heights

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?

Quick (7 day) IRIS Tube Paint Recipe and Construction Procedure

1 page handout available