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IRON AND ALUMINUM OXIDE CHARACTERIZATION FORHIGHLY-WEATHERED ALABAMA ULTISOLSJoey N. Shaw aa Department of Agronomy and Soils , Auburn University , 202 Funchess Hall, Auburn, AL,36849, U.S.A.Published online: 05 Feb 2007.

To cite this article: Joey N. Shaw (2001) IRON AND ALUMINUM OXIDE CHARACTERIZATION FOR HIGHLY-WEATHEREDALABAMA ULTISOLS, Communications in Soil Science and Plant Analysis, 32:1-2, 49-64, DOI: 10.1081/CSS-100102992

To link to this article: http://dx.doi.org/10.1081/CSS-100102992

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COMMUN. SOIL SCI. PLANT ANAL., 32(1&2), 49– 64 (2001)

IRON AND ALUMINUM OXIDECHARACTERIZATION FOR HIGHLY-WEATHERED ALABAMA ULTISOLS

Joey N. Shaw

Department of Agronomy and Soils, Auburn University,202 Funchess Hall, Auburn, AL 36849

ABSTRACT

Characterization of oxide and oxyhydroxide iron (Fe) and alumi-num (Al) forms in highly-weathered Ultisols of the southeasternUnited States is necessary to develop further understanding of col-loidal-facilitated transport of pollutants, sorption of contaminants,erosion, and soil genesis. The objective of this study was to exam-ine Fe and Al oxides from several highly-weathered Ultisols andevaluate their relationships with particle size fractions and othersoil chemical and physical properties. Samples contained in theargillic horizon of 13 highly-weathered pedons were examined.These pedons either contained kandic horizons, were in a kaoliniticmineralogical family, or were in a siliceous mineralogical familywith a subactive cation exchange capacity activity (CEC) class.Standard characterization analyses were performed on all pedons.Samples were fractionated into a coarse (2 to 2000 mm) and fine(�2 mm) fraction, and ammonium oxalate (Feo and Alo) and di-thionite-citrate-bicarbonate (Fed and Ald) extractable Fe and Al

49

Copyright � 2001 by Marcel Dekker, Inc. www.dekker.com

Fax: (�1) 334-844-3945; E-mail: jnshaw�acesag.auburn.edu

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were quantified in each. Selective dissolution treatments were con-ducted on �2 mm fractions prior to mineralogical analyses. Amodified differential XRD (DXRD) was used for further Fe oxidecharacterization. Pretreatments worked well for concentratingcrys-talline oxide forms. All Fe and Al extracted forms were higher in thefine than the coarse fraction. The Fe oxides were highly crystalline,but a significantly higher ratio of Feo/Fed was found in the fine ascompared to the coarse fraction (0.041 to 0.020, respectively). Bothgoethite and gibbsite quantities were negatively correlated with ef-fective cation exchange capacity (ECEC) (r � �0.54 and �0.80,respectively). The Feo/ Fed ratio in the �2 mm fraction was posi-tively correlated (r � 0.49) with hematite content, while it wasnega-tively correlated with goethite quantities (r � �0.61), suggesting aferrihydrite precursor is associated with hematite formation.

INTRODUCTION

Characterization of Fe and Al oxides, hydroxides, and oxyhydroxides (re-ferred to as oxides) in low activity (as per cmol of CEC/g of clay), well-drained,Ultisols is important for evaluating studies on colloidal-facilitated transport ofpollutants, sorption of contaminants, erosion, and soil genesis (1,2,3). Highly-weathered Alabama Ultisols possess a kaolinitic or siliceous mineralogy with akandic horizon, or an argillic horizon with a subactive CEC activity class (CEC/total clay ratio �0.24) (if siliceous mineralogy). In these soils, the Fe and Al inthe oxide minerals originates from primary mineral (e.g., biotite) weathering (4).Iron and Al oxides are concentrated in argillic horizons because these horizonstypically exhibit the maximum pedogenic expression and concentration of pedo-genic minerals (5).

Iron oxides occur as a continuum of crystalline, paracrystalline, and amor-phous forms in soils, and are operationally defined by their extracting medium(6). Iron oxide forms that can be extracted by dithionite-citrate-bicarbonate (Fed)are referred to as ‘‘free’’ (nonsilicate bound) Fe oxides, whereas ammonium oxa-late extractable (in the dark) forms (Feo) consist of amorphous and poorly-crystal-line oxides and organically bound Fe (7). Ferrihydrite, a poorly crystalline Feoxide form, is thought to contribute largely to Fe in oxalate extractions of subsoilhorizons, whereas well-drained, highly-weathered soils tend to have high quanti-ties of crystalline goethite (a-FeOOH) and hematite (Fe2O3) (8,9). Many studieshave investigated the role of Fe oxides on soil color, and it has been shown yel-lower to brownish hues indicate appreciable goethite content while redder huesindicate higher hematite contents (6). Iron oxide minerals in highly-weathered en-

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vironments often possess appreciable Al substitution (9–11), which has beenshown to lead to brighter colors in synthesized soil hematites (12).

Extraction of Al oxide forms with ammonium oxalate and dithionite-citrate-bicarbonate has not been shown to be systematic with regard to mineral types, butit is commonly thought both oxalate (Alo) and dithionite-citrate-bicarbonate (Ald)extracts Al from poorly crystalline oxides (13). Because significant Al substitu-tion commonly occurs in Fe oxide minerals, it is difficult to partition the amountof extracted Al that resides in poorly crystalline Al oxide minerals and the amountresiding in substituted Fe oxide minerals.

Many studies have evaluated the influence Fe and Al oxides have on soilproperties. These minerals have been shown to affect soil erodibility (1), soil ag-gregation (14,15), colloid translocation (2) and anion (3,16,17), cation (6), andpesticide (18) adsorption. Several workers have shown that the ability of a soil tosorb charged solutes is often correlated with Feo and Alo quantities (19,20). Sub-surface horizons enriched with oxides can be exposed at the surface under accel-erated erosion and on disturbed or tilled sites, thus the evaluation of these oxideforms as per particle size enrichment is important for evaluating sediment attachedrunoff of pollutants.

Although it has been shown that extractable Al and Fe forms tend to in-crease with decreasing particle size (21), the relationships between physical andchemical soil properties and oxide mineralogy has not been thoroughly evaluatedfor these highly-weathered Alabama Ultisols. Evidence has suggested crystallineFe oxides tend to occur as discrete particles, while amorphous forms are oftenassociated with phyllosilicate surfaces in clay fractions (22). In coarser fractions,Fe oxides have been shown to occur as coatings on quartz grains (15). The objec-tives of this study were 1) to evaluate the size partitioning of extractable Fe andAl forms and 2) to evaluate relationships between soil’s chemical, physical, andmineralogical properties and Fe and Al oxide forms in these soils.

MATERIALS AND METHODS

Samples were collected from portions of the argillic horizon (mainly Bt1)contained in the control section of 13 pedons sampled during Cooperative SoilSurvey activities in Alabama (Tables 1 and 2). These pedons were chosen becausethey typified highly-weathered soils of the region. Pedons were sampled fromeither: 1) the Piedmont physiographic province with metamorphic and igneouscrystalline-rock parent materials, or, 2) the Upper Coastal Plain physiographicprovince with parent materials consisting of highly-weathered transported mate-rial mostly eroded from the Piedmont plateau. All pedons were Ultisols, possessedeither kandic horizons or were in a subactive CEC activity class, and were in

IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 51

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kaolinitic or siliceous mineralogical families (Table 1). Samples from the argillichorizon ranged in color from dark red to yellowish-brown, and ranged from sandyloam to clay textured (Table 2). These pedons were all well-drained and appear tohave not been impacted by contemporary fluctuating water tables that would sig-nificantly alter Fe oxide forms (23).

Samples were crushed to pass a 2-mm sieve, and particle-size analyses wereconducted on all samples by the pipette method (24). Sand grains were separatedinto standard size fractions by sieving. Base cations [calcium (Ca), magnesium(Mg), potassium (K), and sodium (Na)] were extracted with 1 M NH4OAc (pH 7)and measured with atomic absorption spectroscopy (AAS), and cation exchangecapacity (CEC) was obtained by the NH4OAc (pH 7) method (25). Aluminum wasextracted with 1 M KCl, and was measured by titration (25). Effective cation ex-change capacity (ECEC) was calculated by summing the extractable bases andKCl Al.

For each whole soil, acid ammonium oxalate in the dark was used to extractnoncrystalline (poorly crystalline and organically bound) Fe (Feo) and Al (Alo)forms, and dithionite-citrate-bicarbonate (DCB) was used to extract ‘‘free’’ Fe(Fed) forms consisting of both oxide (crystalline and noncrystalline) and organi-cally bound forms (25). Poorly crystalline, organically bound, and Al containedin Fe oxide minerals was extracted with DCB (Ald) (25). Quantities of each ex-tracted form were measured by atomic AAS. The product of Fed–Feo for the wholesoils was interpreted as a measure of Fe in crystalline oxide form.

Samples were fractionated into a coarse fraction consisting of sand � silt(2 to 2000 mm), and a clay fraction (�2 mm) using centrifugation (26). Separatecoarse fraction samples were extracted with acid ammonium oxalate and DCB,

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Table 1. Classification of Pedons from 13 Alabama Locations

Pedon Family

S92AL-123-1 Fine-loamy, kaolinitic, thermic Typic KanhapludultS92AL-123-2 Fine-loamy, kaolinitic, thermic Typic KandiudultS92AL-123-3 Fine, kaolinitic, thermic Typic KanhapludultS92AL-123-5 Fine, kaolinitic, thermic Typic KanhapludultS92AL-123-7 Loamy, siliceous, subactive, thermic Arenic HapludultS93AL-123-1 Fine, kaolinitic, thermic Typic HapludultS95AL-087-1 Fine-loamy, kaolinitic, thermic Rhodic KandiudultS95AL-087-3 Fine-loamy, siliceous, subactive, thermic Typic PaleudultS95AL-087-4 Fine-loamy, siliceous, subactive, thermic Plinthic PaleudultS95AL-123-1 Fine, kaolinitic, thermic Typic HapludultS95AL-123-3 Fine, kaolinitic, thermic Typic HapludultS96AL-123-1 Fine, kaolinitic, thermic Typic KanhapludultS97AL-123-2 Fine, kaolinitic, thermic Rhodic Kandiudult

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IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 53

Tabl

e2.

Mor

phol

ogic

al,P

hysi

cal,

and

Che

mic

alD

ata

for

Port

ions

ofth

eA

rgill

icH

oriz

onof

the

13Pe

dons

Sam

ple

Hor

.M

unse

llSa

ndSi

ltC

lay

Ca

Mg

KN

aA

lE

CE

CC

EC

pHFe

daFe

obA

l dc

Al o

d

Col

or(%

)(c

mol

ckg

�1 )

H2O

(%)

S92A

L-1

23-1

-2B

t110

YR

5/6

52.0

16.9

31.1

0.85

0.55

0.15

0.11

1.00

2.67

4.40

4.90

2.76

0.06

0.38

0.13

S92A

L-1

23-2

-3B

t110

YR

5/6

58.9

13.7

27.4

0.47

1.10

0.21

0.13

0.97

2.89

4.57

5.00

2.00

0.05

0.33

0.10

S92A

L-1

23-3

-3B

t12.

5YR

4/6

41.1

14.5

44.4

0.15

1.10

0.15

0.18

0.23

1.80

5.78

5.60

6.71

0.34

0.40

0.18

S92A

L-1

23-5

-4B

t17.

5YR

5/6

35.4

13.6

50.9

2.16

0.93

0.05

0.04

0.62

3.81

6.33

5.13

3.52

0.07

0.43

0.11

S92A

L-1

23-7

-4B

t17.

5YR

5/6

66.1

14.4

19.5

0.14

0.32

0.28

0.04

1.46

2.25

3.59

4.92

1.37

0.05

0.18

0.11

S93A

L-1

23-1

-2B

t12.

5YR

3/6

37.3

22.5

40.2

0.70

0.61

0.03

0.13

0.04

1.51

7.23

5.39

5.93

0.22

0.45

0.17

S95A

L-0

87-1

-4B

t22.

5YR

3/6

52.1

17.5

30.4

0.94

0.32

0.22

0.00

1.26

2.75

4.68

4.71

2.00

0.18

0.14

0.12

S95A

L-0

87-3

-5B

t25Y

R4

/648

.623

.028

.51.

490.

950.

090.

000.

012.

544.

915.

861.

650.

040.

220.

12S9

5AL

-087

-4-4

Bt

10Y

R5/

850

.521

.827

.71.

700.

530.

140.

000.

743.

115.

965.

291.

470.

030.

250.

15S9

5AL

-123

-1-3

Bt

10Y

R5/

635

.610

.653

.80.

461.

870.

050.

263.

195.

8211

.03

5.41

3.65

0.10

0.43

0.19

S95A

L-1

23-3

-2B

t12.

5YR

4/6

27.8

23.1

49.1

0.42

0.11

0.06

0.04

2.44

3.06

6.70

4.94

6.85

0.20

0.51

0.16

S96A

L-1

23-1

-2B

t12.

5YR

5/6

43.2

20.3

36.5

0.59

0.70

0.06

0.01

0.92

2.27

5.71

5.13

3.41

0.22

0.26

0.20

S97A

L-1

23-2

-2B

t12.

5YR

3/4

37.4

23.0

39.5

2.62

0.91

0.05

0.03

0.01

3.61

5.58

5.95

5.98

0.28

0.43

0.21

aFe

dis

dith

ioni

te-c

itra

te-b

icar

bona

teex

trac

tabl

eFe

.bFe

ois

oxal

ate

extr

acta

ble

Fe.

cA

l dis

dith

ioni

te-c

itra

te-b

icar

bona

teex

trac

tabl

eA

l.dA

l ois

oxal

ate

extr

acta

ble

Al.

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and the quantity of Feo, Alo, Fed, and Ald in the coarse fraction were determinedusing the following techniques. For the oxalate extractions, 60 mL of 0.2 M am-monium oxalate (pH 3) were added to coarse fractions. Because particle size dis-tribution varied, the initial wt. of sand � silt varied between 4.98 and 8.71 g.Samples were shaken for 4 h in the dark at room temperature, and centrifuged at1,800 rpm for 6 min. The supernatant was decanted, filtered, and brought to vol-ume. The Feo and Alo in the extractant was measured with AAS and reported as apercentage of the whole soil and the coarse fraction. For the DCB extractions,50 mL of 0.3 M sodium citrate/1 M sodium bicarbonate solution were added tothe coarse fraction. Initial wt. ranged from 4.99 to 8.60 g. Successive incrementsof sodium dithionite were added (with stirring) to samples placed in a hot waterbath (80�C), and samples were washed four times (decanted at each washing) with0.3 M sodium citrate/1 M sodium bicarbonate solution. The Fed and Ald in thecoarse fraction was measured with AAS and reported as a percentage of the wholesoil and the coarse fraction.

The quantities of Feo, Alo, Fed, and Ald in the clay fraction were measuredwith the same techniques as used for the coarse fraction. Initial weights for theclay samples were calculated by determining the suspension density, then dispens-ing a known volume.

For mineralogical analyses, both treated and untreated samples were ana-lyzed to minimize the confounding effects of goethite on gibbsite quantificationand vice versa during thermal analyses (27). Clay fractions were split into un-treated, DCB treated to remove Fe oxide minerals, and NaOH treated to removegibbsite. The NaOH treatment involved placing clay separates in 100 mL of 0.5 MNaOH and boiling for 2.5 min, then centrifuging (28). Treated and untreatedsamples were K-saturated, 150 mg of clay were suction oriented onto glass slides,and samples were analyzed by X-ray diffraction (XRD) at room temperature and300� (29). The XRD analyses were used as a qualitative assessment of the effec-tiveness of the pretreatments on preferential removal of oxide minerals. Figure 1illustrates that the pretreatments for differential oxide removal were successful.The goethite (110) and hematite (104) peaks are absent in the DCB treated sample,the gibbsite (002) appears to have been selectively removed with the NaOHtreatment.

Treated and untreated samples were analyzed by thermogravimetric analy-ses (TGA). These analyses were conducted from 25�C to 625�C in a N atmo-sphere, and kaolinite was quantified on the untreated sample (using the theoreticalwater loss of 14.0%), goethite was quantified in the NaOH treated sample (theo-retical water loss of 10.1%), and gibbsite was quantified in the DCB treatedsample (theoretical water loss of 31.2%). The goethite and gibbsite quantities ob-tained in treated amounts (slightly higher weight loss due to concentration of re-sistant minerals) were normalized to total sample amounts by calculating a ratio

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of water loss in the untreated to the treated sample, and multiplying the treatedsample quantities by this ratio. Kaolinite was also quantified using differentialscanning calorimetry (DSC). Samples were analyzed in covered Al pans in a Natmosphere, and were heated from 300�C to 625�C. Kaolinite was quantified us-ing integrated endotherms and standards created from a bauxite source consistingof 56.0% kaolinite. For the quantitative mineralogy results, kaolinite quantitiesare averages obtained by the TGA and DSC methods (30).

The amount of hydroxy interlayered vermiculite (HIV) and quartz (Qtz) wasestimated using the techniques of Karathanasis and Hajek (30). This method usesthe proportional intensity of HIV and Qtz XRD peaks to an internal kaolinitestandard (corresponding peaks to d-spacings of 1.40 nm for HIV, 0.72 nm forkaolinite, and 0.425 nm for Qtz were used). In order to estimate hematite content,the quantity of goethite determined with TGA analyses was converted to % Feusing its theoretical formula. The % Fe in FeOOH form was then subtracted from

IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 55

Figure 1. X-ray diffractograms of the clay fraction (� 2 mm) for sample #S93Al-123-1-2. Samples shown are for DCB treated (K saturated) at 25�C and 300�C, NaOH treated (Ksaturated) at 25�C and 300�C, and untreated (K saturated) at 25�C and 300�C.

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the Fed -Feo content of the clay, and the difference was hypothesized to be Fe inFe2O3 form.

A modified differential X-ray diffraction (DXRD) technique was utilized forfurther evaluation of Fe oxide minerals (31) (Fig. 2). The modification to the stan-dard method was that pretreated, suction oriented samples were analyzed. Thedifferential diffractograms were calculated as (31):

A � kB � C (1)i i i

where Ai � number of counts at angle I in the NaOH treated sample; Bi � numberof counts at angle I in the DCB treated sample; Ci � number of counts at angle Iin the differential diffractogram; k � scale factor. The k factor was created byvisually assessing the optimum multiplier such that the created diffractogram in-tensities did not fall substantially below or above the XRD baseline (31).

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Figure 2. Modified differential X-ray diffractograms (DXRD) of the clay fraction (� 2mm) for six samples. Samples are: a) S95-123-1-3, b) S92AL-123-5-4, c) S97AL-123-2-2,d) S96AL-123-1-2, e) S95AL-087-1-4, f) S92AL-123-3-3. Notice samples a, b, and c haverelatively more goethite while samples d, e, and f possess relatively more hematite.

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RESULTS AND DISCUSSION

Whole Soil Analyses

Soils with redder hues tended to have higher amounts of Fed (Table 2), simi-lar to findings from other researchers (9). Overall, these highly-weathered soilstended to have low amounts of Feo, suggesting a highly-crystalline nature to theFe oxides in these soils (Table 2). Analyses of representative XRD patterns fromthe clay fraction generated using DXRD confirmed the crystallinity of the soilgoethite and hematite (Fig. 2). It was hypothesized that most of the Feo existed aspoorly-crystalline oxide minerals because these subsurface environments werethought to contain relatively low quantities of organic matter. Extremely low Feo

and subsequently low quantities of poorly crystalline Fe forms in these soils aresimilar to findings by other researchers on similar soils (7,9). For whole soils,extractable Al forms did not display any noticeable trend (Table 2).

Clay Mineralogy

Quantitative mineralogical results are reported in Table 3. Significant ( p �0.05) correlation (r � 0.86) between the ratio of oriented XRD peak intensities ofthe hematite (110) :goethite (110) as compared to the measured ratio of % hema-tite: % goethite was found (when the two samples that did not contain goethitewere removed from the analyses to avoid division by 0). Similarly, good correla-tion (r � 0.65, p � 0.05) was found between the measured ratio of % hematite:%goethite and the ratio of XRD peak intensities for hematite (104) � goethite (130):goethite (110). At a minimum, this suggested relatively decent Fe oxide quantita-tive estimates were obtained using the techniques described in this study. Similarto other studies (31), it is also believed these XRD peak intensity ratios createdusing a modified DXRD can provide reasonable estimates of Fe oxide quantitiesin similar soils.

As shown in other studies, samples with redder hues possessed relativelyhigher hematite quantities than samples with browner hues (Table 3). Even forthis highly-weathered mineral suite, these soils possessed a fairly wide range inoxide (hematite, goethite, and gibbsite) and phyllosilicate (kaolinite, HIV, andmica) content in the clay fraction.

Comparison of Coarse and Fine Fractions

Significantly ( p � 0.05) higher extractable Fe and Al forms were found inthe clay as compared to the coarse fractions (Table 4). Differences existed when

IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 57

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58 SHAW

Tabl

e3.

Qua

ntita

tive

Min

eral

ogic

alC

ompo

siti

onof

�2m

mFr

acti

on

Sam

ple

Hor

.M

unse

llC

olor

Kln

.(%

)G

i.(%

)H

IV(%

)Q

tz(%

)H

em.

(%)

Goe

.(%

)M

icaa

Sum

Tota

l

Fe2O

3

and

FeO

OH

/Fe

OO

Hb

Fe2O

3/Fe

OO

Hc

S92A

L-1

23-1

-2B

t110

YR

5/6

36.9

38.

4440

.48

3.49

1.14

13.7

1n

104.

20.

520.

39S9

2AL

-123

-2-3

Bt1

10Y

R5/

644

.45

0.65

13.3

10.

260.

0011

.22

x84

.90.

330.

24S9

2AL

-123

-3-3

Bt1

2.5Y

R4

/631

.99

15.1

68.

431.

2611

.41

2.36

x85

.61.

621.

80S9

2AL

-123

-5-4

Bt1

7.5Y

R5/

656

.75

2.07

17.0

81.

060.

0013

.68

n90

.60.

320.

27S9

2AL

-123

-7-4

Bt1

7.5Y

R5/

643

.68

1.59

13.8

80.

960.

963.

42x

79.5

0.54

0.41

S93A

L-1

23-1

-2B

t12.

5YR

3/6

56.9

69.

826.

851.

232.

1611

.28

n88

.30.

680.

54S9

5AL

-087

-1-4

Bt2

2.5Y

R3/

648

.37

0.79

15.3

42.

264.

540.

00x

86.3

––

S95A

L-0

87-3

-5B

t25Y

R4

/646

.84

2.60

22.8

80.

863.

640.

00n

76.8

––

S95A

L-0

87-4

-4B

t10

YR

5/8

43.7

24.

9334

.29

1.14

0.00

8.97

n93

.10.

000.

00S9

5AL

-123

-1-3

Bt

10Y

R5/

659

.28

0.91

12.3

61.

260.

0010

.51

n84

.30.

200.

14S9

5AL

-123

-3-2

Bt1

2.5Y

R4

/629

.60

10.2

417

.64

0.43

3.78

13.0

5x

89.8

0.45

0.37

S96A

L-1

23-1

-2B

t12.

5YR

5/6

42.5

711

.97

19.4

53.

720.

737.

71n

86.5

1.41

1.06

S97A

L-1

23-2

-2B

t12.

5YR

3/4

56.0

91.

1015

.27

1.67

3.00

12.7

4n

89.9

1.12

0.55

aE

stim

ated

from

XR

D,n

�0

%,x

�15

%.

bR

atio

ofX

RD

inte

nsit

ies

for

0.26

9nm

:0.4

15–

0.41

8nm

,cor

resp

ondi

ngto

hem

atite

(104

)(I

�10

)an

dgo

ethi

te(1

30)

(I�

3):g

oeth

ite(1

10)

(I�

10).

cR

atio

ofX

RD

inte

nsit

ies

for

0.25

2nm

:0.4

15–

0.41

8nm

,cor

resp

ondi

ngto

hem

atite

(110

)(I

�5)

:goe

thite

(110

)(I

�10

).

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IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 59

Tabl

e4.

Ext

ract

able

Fean

dA

lFor

ms

inE

ach

Size

Frac

tion

Ave

rage

dfo

rA

llSo

ilsa

Perc

enta

geof

Who

leSo

il

Fed

(%)

Feo

(%)

Al d

(%)

Al o

(%)

Fed–

Feo

b

(%)

Feo/F

e d(%

)

Perc

enta

geof

Part

icle

Size

Frac

tion

Fed

(%)

Feo

(%)

Al d

(%)

Al o

(%)

Fed–

Feo

b

(%)

Cla

y(�

2m

m)

2.68

a0.

12a

0.24

a0.

14a

2.56

a0.

041a

6.15

a0.

35a

0.83

a0.

45a

5.79

aC

oars

e(2

to20

00m

m)

0.96

b0.

02b

0.10

b0.

02b

0.93

b0.

020b

1.65

b0.

04b

0.17

b0.

03b

1.62

bls

d0.

750.

060.

080.

030.

700.

015

1.35

0.13

0.29

0.10

1.27

aM

eans

follo

wed

byth

esa

me

lette

rar

eno

tsig

nific

antly

diff

eren

t(p

�0.

05).

bE

stim

ate

ofth

eam

ount

ofFe

incr

ysta

lline

oxid

efo

rm.

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ORDER REPRINTS

extractable forms were based on both a percentage of the whole soil as well as apercentage of the size fraction. For example, the averaged amount of Fed in theclay fraction was 2.68% of the whole soil and 6.15% of the clay fraction. Thesedifferences are thought to be due to several reasons including: 1) most pedogenicminerals (including Fe and Al oxides) are clay sized, and 2) oxides (particularlypoorly crystalline) have been shown to be associated with phyllosilicate mineralsconcentrated in the clay fraction, and 3) clay fractions have higher surface areas.Undoubtedly, a combination of these and other factors result in these differences.The Feo/Fed ratio, although extremely low, was twice as high in the clay as in thecoarse fraction (0.041 to 0.020, respectively), suggesting a higher concentrationof poorly crystalline Fe forms in the clay fraction. This could be attributed to: 1)the contemporary formation of a pedogenic poorly-crystalline Fe oxide form (e.g.,ferrihydrite), or 2) phyllosilicate–oxide associations inhibiting oxide crystalliza-tion (32), or 3) retardation of Fe oxide crystallization due to the presence of Aland Si in solution (33). Researchers have noted the inhibition of Fe oxide crystal-lization in the presence of clay minerals, which may be the case for these soils(32,34). The Feo/Fed ratio in the clay fraction was significantly positively corre-lated (r � 0.49) with hematite content, while it was negatively correlated withgoethite quantities (r � �0.61). Hematite quantities were also correlated with thelow amounts of %Feo in the clay fraction (r � 0.75). This suggests a ferrihydriteprecursor may favor hematite formation at the expense of goethite, which is simi-lar to what other workers have found (6).

Significant correlations for certain properties are given in Table 5 (reportedif p � 0.10). For these soils, hematite quantities in the clay fraction were nega-tively correlated with goethite amounts (r � �0.51), but were positively corre-lated with gibbsite quantities (r � 0.49). Goethite quantities were correlated withthe %Ald in the clay fraction (r � 0.67), which suggests appreciable Al substitu-tion in this mineral.

Both gibbsite and hematite quantities were negatively correlated with ECEC(r � �0.54 and �0.80, respectively). Because of the pH of these soils (Table 2),these Fe and Al oxides are thought to have significant (�) charge, thus it is notsurprising to see ECEC negatively correlated with hematite and gibbsitequantities.

Interestingly, gibbsite amounts were negatively correlated with kaolinitequantities (r � �0.61), suggesting a possible desilication of kaolinite and trans-formation to gibbsite may be occurring in some of these soils. Alternatively, thismay indicate that preferential leaching of gibbsite as compared to kaolinite maybe occurring (2). It was also interesting to note in the clay fraction, a fairly highcorrelation between %Alo and gibbsite (r � 0.83) was found. Although it isthought ammonium oxalate in the dark is not a good extractant of crystalline Aloxide forms, future work needs to be done to more fully evaluate the forms of Alextracted with this method. The %Fed in the clay was correlated with the same

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IRON AND ALUMINUM OXIDES FROM ALABAMA ULTISOLS 61

Tabl

e5.

Cor

rela

tion

Mat

rix

a

% Hem

%G

oeth

%G

ibb

% HIV

% Kao

%Fe

o

clay

%A

l ocl

ay%

Fed

clay

%A

l dcl

ay%

Feo

co.

%A

l oco

.%

Fed

co.

%A

l dco

.pH

EC

EC

b

KC

lA

l% BS

c

%H

em–

�0.

510.

49n

n0.

75n

nn

nn

nn

n�

0.54

nn

%G

oeth

–n

nn

nn

0.53

0.67

nn

0.59

0.68

nn

nn

%G

ibb

–n

�0.

610.

620.

830.

57n

nn

nn

n�

0.80

n�

0.52

%H

IV–

nn

nn

0.51

nn

nn

nn

nn

%K

ao–

nn

nn

nn

nn

nn

nn

%Fe

ocl

ay–

0.64

0.71

nn

n0.

49n

n�

0.73

nn

%A

l ocl

ay–

0.67

nn

n0.

48n

n�

0.71

nn

%Fe

dcl

ay–

0.64

0.53

0.59

0.76

0.75

n�

0.64

nn

%A

l dcl

ay–

nn

n0.

65n

nn

n%

Feo

co.

–0.

990.

700.

650.

54n

n0.

50%

Al o

co.

–0.

750.

780.

60n

nn

%Fe

dco

.–

0.88

nn

nn

%A

l dco

.–

nn

nn

pH–

n�

0.47

0.50

EC

EC

–n

nK

ClA

l–

�0.

57%

BS

–

ar

�0.

80at

p�

0.00

1,r

�0.

68at

p�

0.01

,r�

0.55

atp

�0.

05.

bE

CE

C(e

ffec

tive

cati

onex

chan

geca

paci

ty)i

sba

sed

ona

per

hund

red

gram

ofcl

aysa

mpl

e.cB

Sis

%ba

sesa

tura

tion

.

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ORDER REPRINTS

measure in the coarse fraction (r � 0.76). Although differences existed in theamounts of DCB extractable forms in the fine and coarse fractions, no trend withregard to particle size and DCB extractable forms was evident.

CONCLUSIONS

The selective pretreatments used in this study allowed for a detailed exami-nation of oxide forms for these highly-weathered Alabama Ultisols. Similar toother studies, Fe oxide minerals in the argillic horizon of these soils were shownto be highly crystalline, with a higher concentration of oxide forms found in theclay versus the coarse (sand � silt) fraction. Furthermore, empirical relationshipsexisted that gave evidence to theories derived from other studies concerning Feoxide genesis and occurrence. The low amounts of oxalate extractable Fe formsin these subsoils coupled with the low activity silicate mineralogies dictate theseargillic horizons have a finite capacity to sorb contaminants. The higher ratio ofoxalate extractable and therefore the more reactive Fe forms concentrated in theclay fraction indicate these colloidal materials are more reactive than the bulksoils, thus possibly enhancing colloidal facilitated transport of pollutants.

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