Documenting Reducing
Conditions in Soils
Martin C. Rabenhorst
Environmental Science and Technology
University of Maryland
Significance of documenting
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
(and pH)




Time consuming and a bit difficult (especially for the
practitioner)
Plot on Eh – pH diagrams
Considered as “the standard”
But there are some unresolved issues regarding
exactly what is being measured
Redox potential measurement using Pt electrodes
Multiple electrodes used to
improve the statistical
reliability.
Alternate Technologies

Use of alpha-alpha-dipyridyl
Reacts with ferrous Fe+2 forming pink color
 Difficult to obtain (hazardous)
 Now available as test papers
 www.gallard.com




Dipyridyl paper, item# 90725
Pack Size: box of 200 strips; Hazard Class /UN Number: Not Restricted
Storage Temp: Room Temperature
Impregnated with a,a’-dipyridyl (= 2,2’-bipyridine),
Limit of sensitivity: 2 mg/l Fe 2+
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





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 eacceptors
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
estimates of IRIS paint removal?
Visual Estimate of Fe Paint Removed
100
90
80
70
60
50
Group 1
Group 2
40
30
20
10
0
0
10
20
30
40
50
% Fe Paint Removed
60
70
80
90
Visual Estimate of Paint Removal
Average of Two Individual Estimates
(worst and best)
90
80
R-square = 0.95 # pts = 20
y = 0.521 + 0.884x
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Actual % Paint Removed
70
80
90
Percentage of Observations
of sections
by the TS
or Reduced
Oxidizedpercentage
reduced in, reduced out
oxidized in, oxidized out
100%
80%
Where
Where10%
20%of
ofthe
theiron
ironoxide
oxidepaint
paintwas
was
Where
25%
of
the
iron
oxide
paint
was
removed,
removed,the
thesoils
soilswere
werereducing
reducingin
89%
removed,
the
soils
were
reducing
inin82%
of
while
ofthe
thesections
sections
whileinin18%
11%of
ofthe
the
100%
of
the sections.
cases,
cases,the
thesoil
soilwas
wasoxidized.
oxidized.
60%
40%
20%
0%
0
20
40
60
80
percentage of iron oxide removed
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.
100
Jenkinson called for synthesis of
ferrihydrite Fe5HO8*4H2O
(FeCl3 titrated to pH 7.5 with KOH)
Problem with Newly Synthesized
Paint


We noticed that newly
synthesized paint would
not adhere well to the
PVC.
A number of observations
led us to postulate that
variation in mineralogical
composition might affect
behavior of the paint.
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
2 broad peaks for ferrihydrite
d-spacing nm
1.00
pH 4 & 7.5
Feo / Fet
.
0.90
0.80
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
0.70
pH 11
0.60
0.50
0.40
pH 12
0.30
0.20
0.10
Feo/Fet = proportion of Ferrihydrite
1-Feo/Fet = proportion of Goethite
0.00
0
10
20
30
40
50
60
time (d)
•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
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.
5
Durability
.
Abrasion Resistance
4
3
2
1
0
0%
20%
40%
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.
60%
Goethite
80%
100%
What does the goethite do?
15% Goethite; 85% Ferrihydrite 1:5.7
200 um
4 um
200 X
5000 X
Durability Index = 1 to 2
20 um
1000 X
1 um
25000 X
44% Goethite; 54% Ferrihydrite 1:1.3
100 um
8 um
Durability Index = 5
200 X
20 um
5000 X
1 um
1000 X
25000 X

The lath-shaped goethite crystals are prevalent and
appear to form a reinforcing network.



analogous to the common (19th century) practice of
masons adding some strong fibrous material to plaster
such as hair or hemp
or the ancient practice of adding straw to clay when
making bricks to increase their strength and cohesion.
Thus, the growth of lath-shaped goethite crystals
within the Fe oxide mixture appears to contribute
strength and cohesion of the material.
What do the variations in color represent?
Partial removal of Fe Oxides
(Total Counts) / (Baseline counts) – 1.0
Reference Goethite
-10
Blank (peak from Fe
in detector window)
Sample from “yellow”
area on IRIS tube
RICH IN GOETHITE
-5
0
5
velocity (mm/s)
10
saturated soil
Red
Means of four replicate analyses
Yellow
GOETHITE
Feox
Fe 6M HCl
Fe Total
ug/cm2
Fh
Gt
%
55.5
56.2
111.6
49.8%
50.2%
SE
3.0
4.1
7.0
0.7%
0.7%
mean
5.1
72.2
77.3
6.7%
93.3%
SE
0.9
9.9
10.2
1.3%
1.3%
mean
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.
Next Question


Does the mineralogical composition of the
paint affect how IRIS tubes will behave in
the soil?
We know we need 30-40% goethite for
good durability of the paint, but does the
proportion of goethite relative to
ferrihydrite make any difference in how
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
nine Fe oxide paints examined in this
study.
% Fh
%
Gt
Fh:Gt
1 0520 75d
59.5
40.5
1.47
2 0521 22d
84.8
15.2
5.58
3 0521 33d
80.9
19.1
4.24
4 0523 36d
63.3
36.7
1.72
5 0601 10d
46.6
53.4
0.87
#
6 0601 14d
35.0 65.0 0.54
Mesocosms filled with two
7 0602 9d
29.1 70.9 0.41
different soil materials (Indiantown
A horizon and Berryland A
8 0603 4d
50.0 50.0 1.00
horizon) and each containing 36
9 0604 9d
56.3 43.7 1.29
IRIS tubes representing 4 replicates * – based upon acid ammonium oxalate
of 9 different paints
and total Fe analyses. Fh – ferrihydrite;
Gt – goethite.
substantial
intermediate
full
Indiantown
100
80
80
% Removal
.
100
60
40
20
substantial
intermediate
full
60
40
20
0
0
1
2
3
4
5
6
7
8
9
1
2
3
4
6
7
60
y = -110.88x + 69.09
2
R = 0.73
60
40
20
% Full Removal .
80
% Full Removal .
5
Paint No.
Paint No.
0
y = -92.25x + 58.62
R2 = 0.75
40
20
0
0%
20%
40%
60%
0%
80%
20%
% Goethite
% Full + Intermediate .
80
60
40
y = -85.32x + 99.39
2
R = 0.67
20
0
20%
40%
% Goethite
60%
80%
60
40
20
y = -32.27x + 61.08
2
R = 0.81
0
0%
0%
40%
% Goethite
100
% Full + Intermediate .
% Removed
Berryland
60%
20%
40%
80%
% Goethite
60%
80%
8
9
Target – 40-60% goethite; 60-40 % ferrihydrite
How then to increase “pot life”of the paint
Estimates of Goethite content based upon peak heights
100%
80%
60%
6C
20C
35C
40%
20%
0%
0
10
20
30
days
40
50
Conclusions







We know how to make IRIS tubes
We know how to synthesize the paint so that it
will adhere, and we know why it adheres
We know what the variations in color represent
We know that mineralogical composition affects
performance
We know how to preserve “pot life” of the paint
to slow mineralogical change
We know how to interpret removal of the paint
with respect to the Technical Standard of
NTCHS.
We know how to use them – Protocol to be
covered in workshop
finis