Martin C. RabenhorstMartin C. RabenhorstEnvironmental Science and TechnologyEnvironmental Science and Technology

University of MarylandUniversity of Maryland

Documenting Reducing Conditions in Soils

Significance of documenting Significance of documenting reducing conditions in soils?reducing conditions in soils?

To demonstrate that a soil meets the Technical Standard for Hydric Soils In order to evaluate or test new Field

Indicators (FI) for Hydric soils In order to confirm that a soil is hydric in the

absence of a Field Indicator (disturbed site) To demonstrate that the soil of a recently

created or restored wetland is functioning like a hydric soil

Alternate Technologies

Eh measurements with Platinum electrodes Eh measurements with Platinum electrodes (and pH)(and pH) Time consuming and a bit difficult (especially for the Time consuming and a bit difficult (especially for the

practitioner)practitioner) Plot on Eh – pH diagramsPlot on Eh – pH diagrams Considered as “the standard”Considered as “the standard” But there are some unresolved issues regarding But there are some unresolved issues regarding

exactly what is being measuredexactly what is being measured

Multiple electrodes used to Multiple electrodes used to improve the statistical improve the statistical reliability.reliability.

Redox potential measurement using Pt electrodesRedox potential measurement using Pt electrodes

Use of alpha-alpha-dipyridylUse of alpha-alpha-dipyridyl Reacts with ferrous FeReacts with ferrous Fe+2 +2 forming pink colorforming pink color Difficult to obtain (hazardous)Difficult to obtain (hazardous) Now available as test papersNow available as test papers www.gallard.comwww.gallard.com

Dipyridyl paper, item# 90725                                Dipyridyl paper, item# 90725                                Pack Size: box of 200 strips; Hazard Class /UN Number: Not RestrictedPack Size: box of 200 strips; Hazard Class /UN Number: Not RestrictedStorage Temp: Room TemperatureStorage Temp: Room Temperature

Impregnated with a,a’-dipyridyl (= 2,2’-bipyridine),Impregnated with a,a’-dipyridyl (= 2,2’-bipyridine), Limit of sensitivity: 2 mg/l Fe 2+Limit of sensitivity: 2 mg/l Fe 2+

Alternate Technologies

IRIS (Indicator of Reduction in Soils) Tubes IRIS (Indicator of Reduction in Soils) Tubes

Fe Oxide paint is applied to Fe Oxide paint is applied to ½ inch schedule 40 PVC ½ inch schedule 40 PVC tubing while the tube is on a tubing while the tube is on a lathe device to ensure an lathe device to ensure an even distribution of the even distribution of the paint.paint.

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

Utilization of scanner to collect undistorted images

Images must be composited

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

0 10 20 30 40 50 60 70 80 90

% Fe Paint Removed

0

10

20

30

40

50

60

70

80

90

100Vi

sual

Estim

ate

of F

e Pa

int R

emov

ed

Group 1

Group 2

0 10 20 30 40 50 60 70 80 90

Actual % Paint Removed

0

10

20

30

40

50

60

70

80

90

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)

0 20 40 60 80 100

percentage of iron oxide removed

0%

20%

40%

60%

80%

100%

perc

enta

ge o

f sec

tions

reduced in, reduced outoxidized in, oxidized out

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

on

s

Oxid

ized

or

Red

uced

by t

he T

S

Castenson, K. L. and M. C. Rabenhorst. 2006. Indicator of reduction in soil (IRIS): Evaluation of a new approach for assessing reduced conditions in soil. Soil Sci. Soc. Am. J 70: 1222-1226.

Jenkinson called for synthesis of ferrihydrite Fe5HO8*4H2O

(FeCl3 titrated to pH 7.5 with KOH)

We noticed that newly We noticed that newly synthesized paint would synthesized paint would not adhere well to the not adhere well to the PVC.PVC.

A number of observations A number of observations led us to postulate that led us to postulate that variation in mineralogical variation in mineralogical composition might affect composition might affect behavior of the paint.behavior of the paint.

Problem with Newly Synthesized Problem with Newly Synthesized PaintPaint

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

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

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 DurabilityAbrasion Resistance and Durability

0

1

2

3

4

5

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

Goethite

Dur

abili

ty

.

Ab

rasi

on R

esi

sta

nce

Rabenhorst, M. C. and S. N. Burch. 2006. Synthetic Iron Oxides as an Indicator of Reduction in Soils (IRIS). Soil Sci. Soc. Am. J. 70: 1227-1236.

What does the goethite do?

200 X 1000 X

5000 X5000 X 25000 X25000 X

200 um 20 um

1 um4 um

15% Goethite; 85% Ferrihydrite 1:5.7 Durability Index = 1 to 2

200 X 1000 X

25000 X25000 X

100 um 20 um

1 um8 um 5000 X5000 X

44% Goethite; 54% Ferrihydrite 1:1.3 Durability Index = 5

The lath-shaped goethite crystals are prevalent and The lath-shaped goethite crystals are prevalent and appear to form a reinforcing network. appear to form a reinforcing network. analogous to the common (19th century) practice of analogous to the common (19th century) practice of

masons adding some strong fibrous material to plaster masons adding some strong fibrous material to plaster such as hair or hemp such as hair or hemp

or the ancient practice of adding straw to clay when or the ancient practice of adding straw to clay when making bricks to increase their strength and cohesion. making bricks to increase their strength and cohesion.

Thus, the growth of lath-shaped goethite crystals Thus, the growth of lath-shaped goethite crystals within the Fe oxide mixture appears to contribute within the Fe oxide mixture appears to contribute strength and cohesion of the material.strength and cohesion of the material.

What do the variations in color represent? What do the variations in color represent? Partial removal of Fe OxidesPartial removal of Fe Oxides

-10 -5 0 5 10 velocity (mm/s)

(Tot

al C

ount

s) /

(B

asel

ine

coun

ts)

– 1.

0 Reference Goethite

Blank (peak from Fe in detector window)

Sample from “yellow” area on IRIS tubeRICH IN GOETHITE

Means of four replicate analyses

FeFeoxox Fe Fe 6M HCl6M HCl Fe Fe TotalTotal FhFh GtGt

ug/cmug/cm22 %%

RedRedmeanmean 55.555.5 56.256.2 111.6111.6 49.8%49.8% 50.2%50.2%

SESE 3.03.0 4.14.1 7.07.0 0.7%0.7% 0.7%0.7%

YellowYellowmeanmean 5.15.1 72.272.2 77.377.3 6.7%6.7% 93.3%93.3%

SESE 0.90.9 9.99.9 10.210.2 1.3%1.3% 1.3%1.3%

satu

rate

d so

il

Red

Yellow

Rabenhorst, M. C., D. W. Ming, and R. V. Morris. 2006. Synthesized Iron Oxides Used as a Tool for Documenting Reducing Conditions in Soils. 18th World Congress of Soil Science, Philadelphia, PA. July 9-15, 2006.

GOETHITE

Next QuestionNext Question

Does the mineralogical composition of the Does the mineralogical composition of the paint affect how IRIS tubes will behave in paint affect how IRIS tubes will behave in the soil?the soil?

We know we need 30-40% goethite for We know we need 30-40% goethite for good durability of the paint, but does the good durability of the paint, but does the proportion of goethite relative to proportion of goethite relative to ferrihydrite make any difference in how ferrihydrite make any difference in how they function?they function?

Rabenhorst, M. C., R. R. Blank, and B. R. James. 2006. Reduction of Iron Oxides in Wetland Soils. 18th World Congress of Soil Science, Philadelphia, PA. July 9-15, 2006.

Mineralogical composition* of the Mineralogical composition* of the nine Fe oxide paints examined in this nine Fe oxide paints examined in this study.study.

## % Fh% Fh% % GtGt Fh:GtFh:Gt

11 0520 75d0520 75d 59.559.5 40.540.5 1.471.47

22 0521 22d0521 22d 84.884.8 15.215.2 5.585.58

33 0521 33d0521 33d 80.980.9 19.119.1 4.244.24

44 0523 36d0523 36d 63.363.3 36.736.7 1.721.72

55 0601 10d0601 10d 46.646.6 53.453.4 0.870.87

66 0601 14d0601 14d 35.035.0 65.065.0 0.540.54

77 0602 9d0602 9d 29.129.1 70.970.9 0.410.41

88 0603 4d0603 4d 50.050.0 50.050.0 1.001.00

99 0604 9d0604 9d 56.356.3 43.743.7 1.291.29

* – based upon acid ammonium oxalate * – based upon acid ammonium oxalate and total Fe analyses. Fh – ferrihydrite; and total Fe analyses. Fh – ferrihydrite; Gt – goethite.Gt – goethite.

Mesocosms filled with two different soil materials (Indiantown A horizon and Berryland A horizon) and each containing 36 IRIS tubes representing 4 replicates of 9 different paints

Berryland

0

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60

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100

1 2 3 4 5 6 7 8 9Paint No.

% R

em

ove

dsubstantialintermediatefull

Indiantown

0

20

40

60

80

100

1 2 3 4 5 6 7 8 9

Paint No.

% R

emov

al

.

substantialintermediatefull

y = -110.88x + 69.09R2 = 0.73

0

20

40

60

80

0% 20% 40% 60% 80%% Goethite

% F

ull R

emov

al

.

y = -85.32x + 99.39

R2 = 0.67

0

20

40

60

80

100

0% 20% 40% 60% 80%% Goethite

% F

ull +

Int

erm

edia

te

.

y = -92.25x + 58.62R2 = 0.75

0

20

40

60

0% 20% 40% 60% 80%

% Goethite

% F

ull R

emov

al

.

y = -32.27x + 61.08R2 = 0.81

0

20

40

60

0% 20% 40% 60% 80%

% Goethite

% F

ull +

Inte

rmed

iate

.

Target – 40-60% goethite; 60-40 % ferrihydriteTarget – 40-60% goethite; 60-40 % ferrihydrite

How then to increase “pot life”of the paintHow then to increase “pot life”of the paint

0%

20%

40%

60%

80%

100%

0 10 20 30 40 50

days

6C

20C

35C

Estimates of Goethite content based upon peak heights

ConclusionsConclusions We know how to make IRIS tubesWe know how to make IRIS tubes We know how to synthesize the paint so that it We know how to synthesize the paint so that it

will adhere, and we know why it adhereswill adhere, and we know why it adheres We know what the variations in color representWe know what the variations in color represent We know that mineralogical composition affects We know that mineralogical composition affects

performanceperformance We know how to preserve “pot life” of the paint We know how to preserve “pot life” of the paint

to slow mineralogical changeto slow mineralogical change We know how to interpret removal of the paint We know how to interpret removal of the paint

with respect to the Technical Standard of with respect to the Technical Standard of NTCHS.NTCHS.

We know how to use them – Protocol to be We know how to use them – Protocol to be covered in workshopcovered in workshop

finis