Upload
raina
View
28
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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. Why document reduction in soils?. Field Indicators of Hydric Soils - PowerPoint PPT Presentation
Citation preview
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
10
20
30
40
50
60
70
80
90
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
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 10 20 30 40 50 60 70 80 90
% Fe Removed
0
5
10
15
20
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%
60%
80%
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
on
s
Oxid
ized
or
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
4
6
8
10
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
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 Durability
0
1
2
3
4
5
0% 20% 40% 60% 80% 100%
Goethite
Dur
abili
ty
.
Ab
rasi
on R
esi
sta
nce
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
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?
0
5
10
15
20
25
30
35
40
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