IRON OXIDE REMOVAL FROM SOILS AND CLAYS BY A
DITHIONITE-CITRATE SYSTEM BUFFERED WITH
SODIUM BICARBONATE
by
O. P. MEdinA A~D M. L. JACKSO~r
D e p a r t m e n t of Soils, U n i v e r s i t y of W i s c o n s i n , Madison, W i s c o n s i n
ABSTRACT
T h e o x i d a t i o n p o t e n t i a l of d i t h i o n i t e (Na~S204) increases f r o m 0.37 V to 0 . 7 3 V w i t h
increase in p H f r o m 6 to 9, because h y d r o x y l is c o n s u m e d d u r i n g o x i d a t i o n of dithionite.
A t tile s a m e t i m e t h e a m o u n t of iron o x i d e dissolved in 15 m i n u t e s falls off (from 100
p e r c e n t to less t h a n 1 p e r c e n t e x t r a c t e d ) w i t h increase in p H f r o m 6 to 12 o w i n g to solubility p r o d u c t relationships of iron oxides. A n o p t i m u m p H for m a x i m u m r e a c t i o n kinetics
ocem~ a t a p p r o x i m a t e l y p H 7.3. A buffer is n e e d e d to hold t h e pl~ a t t h e o p t i m u m
level b e c a u s e 4 moles of O H are u s e d u p in r e a c t i o n w i t h each mole of Na2S204 oxidized. T e s t s s h o w t h a t NaHCO3 effectively serves as a buffer in t h i s application.
Crystalline h e m a t i t e dissolved in a m o u n t s of several h u n d r e d m i l l i g r a m s in 2 m i n .
Crystalline g o e t h i t e dissolved m o r e slowly, b u t dissolved d u r i n g t h e two or t h r e e 15 rain
t r e a t m e n t s n o r m a l l y g i v e n for i r o n oxide r e m o v a l f r o m soils a n d clays.
A series of m e t h o d s for t h e e x t r a c t i o n of iron oxides f r o m soils a n d clays was tested
w i t h soils h i g h in free iron oxides a n d w i t h n o n t r o n i t e a n d o t h e r i r o n - b e a r i n g clays.
I t was f o u n d t h a t t h e bicarbonate-buffere~l Na2S2Oa-citrate s y s t e m was t h e m o s t
effective in r e m o v a l of free i r o n oxides f r o m latosolic soils, a n d t h e least d e s t r u c t i v e of
iron silicate clays as i n d i c a t e d b y least loss in c a t i o n e x c h a n g e c a p a c i t y a f t e r t h e iron
oxide r e m o v a l t r e a t m e n t . W i t h soils t h e decrease w a s v e r y little b u t w i t h t h e v e r y
susceptible W o o d y d i s t r i c t n o n t r o n i t e , t h e decrease was a b o u t 17 p e r c e n t as c o n t r a s t e d
to 35-80 p e r c e n t w i t h o t h e r m e t h o d s .
INTRODUCTION
The removal of" amorphous coatings and crystals of free iron oxides, particularly hematite and goethite, which act as cementing agents, is important
in many types of analysis of softs and clay minerals. The removal of free iron
oxides aids in dispersion of the silicate portion, which is essential for effective
segregation into different particle size fractions. For x-ray diffraction studies
the removal of free iron oxides greatly enhances the parallel orientation of
layer silicate clays and brings out some x-ray diffraction peaks that are
otherwise difficult or impossible to detect. Differential and integral thermal
analysis, electron micrographs and cation exchange capacity are greatly
improved after removal of free iron oxides. Use of citrate chelating agent
(Aguilera and Jackson, 1953) with Na~S204 not only helps with iron extraction but removes some coatings of alumina and thereby assists in the dissolution of free silica cements that are stabilized by alumina coatings, as will
be shown in the present paper.
317
318
SEVENTH I~ATIONAL CONFERENCE ON CLAYS A N D CLAir MINERALS
In 1877 Bemmelen and others (reviewed by Jackson, 1956, p. 47) dissolved
iron oxide and other soil colloids by the use of mineral acids and alkalies.
T a m m (1922) used acid Na2C204to remove iron oxide. In an attempt to
speed up the reaction, Drosdoff and Truog (1935) used H2S and oxalic acid
to reduce and chelate the iron. Truog et al. (1937) modified this method by
the use of Na2S and oxalic acid as a source of nascent H2S. They reported
a considerable decrease in cation exchange capacity of nontronite, Miami
silt loam, and certain other soils.Allison and Scarseth (1942) removed the
iron oxide by microbiological reduction in the presence of sucrose. Jeffries
(1941) used nascent hydrogen produced by Al acted on by oxalic acid. Dion
(1944) made a comparative study of destructive effects of oxalic acid and
tartaric acid (with nascent hydrogen from Al) and showed less destruction
at p H 6 than at a lower pH. However, partial destruction of mineral colloids
stillresulted with this procedure. H e reported up to 45 percent loss in cation
exchange capacity by both montmorillonite and kaolinite clays by this and
several methods with which it was compared. This loss could be attributed
only to the breakdown of mineral structure.Jeffries(1947) employed nascent
hydrogen produced by the action of oxalic acid on magnesium. Haldane
(1956) substituted Zn for M g because iron was deposited on the M g ribbon,
and thus its dissolution was not complete. Deb (1950) proposed the use of
Na2S204 with H20, or with sodium tartrate and sodium acetate in p H ranges
fi-om 2.9 to 6.0 at 40~ Essentially the D e b method (Na2S204 in water solution) was used by Mitchell and Mackenzie (1954) at plI 5.8 to 6.0 and
Mackenzie (1954) at p H 3.5 to 6.5. Removal of iron oxide from latosols by
these methods was very slow and incomplete. Also there was unwanted precipitation of FeS and S, 0.02 to 0.05 N HCI being used to dissolve the FeS
and CS~ being used to dissolve the S. Considerable decrease in cationexchange capacity occurred with some soils (31.8 meg/g decreased to 24.6 at
pH 5.8, Mitchell and Mackenzie, 1954).
Aguftera and Jackson (1953) proposed the use of Na2S~O 4 with sodium
citrate (with or without ferric iron specific Versene) adjusted to pH 7.3 by
10 percent NaOH at 80~ for very complete and rapid removal of iron oxides
from softs. Almost immediately after publication in I953, the more convenient
practice of adding the Na2S204 as 1 g of dry powder was ~dopted (as reported
by Jackson, 1956, p. 57), and this increased difficulty in keeping the pH up
to 7.3 since no NaOH was added with the Na2S204. Careful p H adjustments
were required to keep FeS and elemental S from precipitating which happened
ff the pH were allowed to drop much. To stabilize the p H at 7.3, the method
was modified in the present study by addition of NaHCO 3 buffer (as described
by Jackson, 1956, p. 57) with highly satisfactory results.
MATERIALS
Soils rich in free iron oxides and soils and clay minerals containing ironbearing 2 : 1 layer sihcates were studied. The samples included : Fajardo clay
(8-16 in.), Red-Yellow Podzolic soil from Puerto Rico (courtesy J. A. Bonnet); Bayamon clay (10-28 in.), Reddi,~h Brown lateritie soil from Puerto
DITHIONITE-CITRATE SYSTEM BUFFERED WITH SODIUM BIC~REONATE
319
R i c o ( c o u r t e s y J . A. B o n n e t ) ; M i a m i silt l o a m B2 (14-27 in.), G r a y - B r o w n
P o d z o l i c soils f r o m S u n P r a i r i e , Wis. ( c o u r t e s y R . W . S i m o n s o n ) ; W a i p i a t a
B (6-10 in.), d a r k y e l l o w b r o w n soil d e v e l o p e d f r o m olivine basalt, f r o m n e a r
W a i p i a t a , N e w Z e a l a n d ( c o u r t e s y L. D. Swindale), v e r l n i c u l i t e w e a t h e r e d
f r o m g r a n i t e , M a n i t o u E x p e r i m e n t a l F o r e s t , Colo. ( c o u r t e s y J . L. R e t z e r ) ;
n o n t r o n i t e f r o m W o o d y district, Calif. (from R . M. Wilke, P a l e Alto, Calif.) ;
glauconite from Cambrian sandstone at Shot Tower State Park, Spring
Green, Wis. ; K u r o i s h i b a r n (6-10 in.) a l l o p h a n i c A n d o soil f r o m J a p a n
( c o u r t e s y S. A o m i n e ) .
PROCEDURE
Methods of Iron Oxide Removal
Sodium dithionite-sodium citrate with N a H C 0 8 buffer (proposed method).-T h i s t e c h n i q u e is a m o d i f i c a t i o n of t h e m e t h o d o f A g u i l e r a a n d J a c k s o n
(1953) b y t h e inclusion of N a H C O a as a source of h y d r o x y l , t h e r e b y stabilizing
t h e o x i d a t i o n p o t e n t i a l a n d p H , g i v i n g m o r e effective r e m o v a l o f iron oxides.
T h e p r o c e d u r e has b e e n d e t a i l e d elsewhere ( J a c k s o n , 1956, pp. 57-58), a n d
is m o d i f i e d in p a r t as follows :
A suitable amount of the sample (4 g of many soils or 1 g of clay per 100 ml
tube) containing 0.5 g of extractable Fe203 or less, is placed in a 100 ml centrifuge
tube and 40 ml of 0.3 l~I Na.eitrate solution and 5 ml. of 1 ]~I NaHCOs solution are
added. The temperature is brought to 80~ in a water bath, then 1 g of selid Na~SzO4
(89g suffices for clays low in free iron oxides) is added by means of a spoon, and the
mixture is stirred constantly for 1 rain and then occa~sionally for a total of 15 rain.
At the end of the 15 min digestion period, 10 ml of saturated NaOl solution (and
10 ml of acetone, particularly needed for allophanic soils) is added to the tube to
promote flocculation. The suspension is then mixed, warmed in a water bath, and
centrifuged for 5 min at 1600-2200 rev/min. The clear supornatant is decanted into
a 500 In] volumetric flask (or a 1000 ml flask if the volume exceeds 500 ml) and
the solution is kept for Fe, A1 and Si determinations.
For samples which originally contained more than 5 percent of extractable
Fe203, the treatment in the previous paragraph is repeated once or twice (sample
combined into fewer tubes for the second treatment), with decantation into the
same 500 ml volumetric flask as before. A final washing (two or more for samples
of more than 1 g of residue) is made with the Na-citrate solution (with NaC1 and
acetone if necessary for floceulation), which is combined with the previous docazatates for Fo determination. If the colloid does not flocculate, 10 ml of acetone
is added (HG1 and CaC12 solutions are avoided). The solution is mixed and warmed
in a water bath. Care is taken that solutions containing acetone do not boil.
Centrifugation for 5 rain is repeated. A pure white color of the residue generally
should not be expected, as some soils and colloids contain cream or green colored
colloids and coarse, black mineral particles. The sample is kept in methanol,
acetone or water, without drying. The sample, freed of extractable Fo203 but not
dried at any time during the procedure, is ready for cation exchange capacity
determination or (Jackson, 1956, p. 72) boiling in 2 percent bla2CO8 for dispersion
and segregation for x-ray diffraction analysis, differential and integral thermal
analysis, infrared analysis, elemental analysis, electron microscope examination,
or other procedures.
Sodium sulfide-oxalic acid (comparison method).--This m e t h o d was pro.
p o s e d by T r u o g et al. (1937). [ t is t a k e n as r e p r e s e n t a t i v e of t h e v,~rious
320
SEVENTHNATIONAL CONFERENCE ON CLAYS AND CLAY ~M_INER.4~S
HaS methods. A 4 g soil sample is placed in 650 ml of distilled water, 5 mI
of 20 percent Na2S. 9H~O solution is added, and the sample is boiled for
5 min ; 10 g of 57H4C1 is added and the sample is kept at 80~176 during the
treatments. Oxalic acid is added to bring to p H 6.0 with vigorous stirring.
Then 10 ml more of 20 percent Na2S.9H20 solution is added. Oxalic acid
is added rapidly until p H 7 is reached and then slowly to p H 6 and again
rapidly to p H 3.5. The mixture is stirred and allowed to stand for a few
minutes as the black FeS is dissolving, then is brought back to p H 7 with
2 N NH4OH. The p H is slowly brought down to 6 and then rapidly to 3.5
with 2 N HC1. After digestion for a few minutes, the sample is centrifuged
and washed twice with 0.001 N HC1. The extract is used for Fe, Si and Al
analysis.
Sodium dithionite in acid system (compariso~ method).--Deb (1950) proposed
three methods for the removal of free iron oxide : ammonium oxalate and
oxalic acid in sunlight (pH 3.8); Na2S204 in sodium acetate and sodium
tartarate system at p H 5-6 ; and sodium dithionite in water at p H 3.5. The
last was considered b y Deb to be faster and less destructive than the first
two and is taken as representative of Na2S204 methods previous to that of
Aguilera and Jackson (1953). A 4 g soil sample is dispersed thoroughly in
50 ml of distilled water, 2 g of Na2S204 is added, and the mixture is digested
for 30-50 rain in a water bath at 40~ The sample is centrifuged and treated
twice again with 0.02 N HC1 for ]0-15 min, centrifuged, and the extract is
taken for Fe analysis.
Zinc-ammonium oxalate (compa'risoJ~ ~ethod).--This is a modification
(Haldane, 1956) of the Jeffries nascent H methods, substituting Zn for A1
(Jeffries, 1941) or Mg (Jeffries, 1947). This modification is said to avoid
precipitation of Fe on the metal and avoids introduction of Mg and A1 ions
which are also present in clay. To a 4 g soil sample placed in a 100 ml evaporating dish, 1 ml of 0.5 N N a O H is added and the mixture is ground into a
paste with a rubber pestle (2-3 min). The suspension is neutralized with
1 ml of 0.5 N oxalic acid. Using 40 ml of ammonium oxalate buffer and
continuous grinding, the sample is transferred to a 100 ml volumetric flask.
Then 0.5 g of Zn powder (300-mesh) is added. The sample is allowed to stand
for 1 hr with intermittent shaking, made up to volume, filtered, and the
filtrate is used for Fe and Si analysis.
Determination of Fe, A1 and Si dissolved.--An aliquot of the supernatant
citrate solution was prepared (Jackson, 1958, p. 169; 1956, p. 58) for Fe
analysis by H202 treatment and color development with KSCN. A second
aliquot was similarly prepared, then given a double N a O H separation in a
Ni beaker to obtain a Si and A1 solution (Jackson, 1956, p. 59) from which
aliquots were analyzed colorimetrically, Si by molybdate (Jackson, 1958,
p. 296) and Al by aluminon (Jackson, 1958, p. 300).
Cation exchange capacity n~easurement.--The cation exchange capacity was
determined by Ca with centrifuge method (Jackson, 1958, p. 59). Five
washings are given with CaC]e and Ca0Ac solutions; and five with water,
~dcohol and acetone. The exchangeable Ca is replaced in NaOAc and deter-
DITHIONITE-CITRATE SYSTEM BUFFERED WITtl SODIUM BICARBONATE 321
m i n e d b y t h e B e c k m a n D U flame emission s p e c t r o p h o t o m e t e r using a wave
length of 424 mt~. N a was used to a v o i d any fixation of NH4 or K by vermiculite samples (Sawhney et al., 1959).
RESULTS
AND
DISCUSSION
Oxidation Potential
Prerequisite to a good m e t h o d for free iron oxide r e m o v a l is a high oxidation p o t e n t i a l (high t e n d e n c y to become oxidized, hence a good reducing
agent, J a c k s o n (1956), p. 651). I n each dithionite system compared in this
section, t h e h y d r o x y l source a n d other variables were applied to 40 ml of
N a citrate, w i t h 1 g of dithionite. The o x i d a t i o n p o t e n t i a l of these dithionite
systems increases with t h e increase in p H (Table 1) owing to use of four OH
[|'ABLE
1.--pH
AND OXIDATION POTENTIAL OF DITHIONITE-CITRATE SYSTEMS "WITI-[
VARYING AI~OUNTS OF NaHCO3 OR NaOH
Addition of
After 2 rain
Vol-
After 2 rain
After 15 rain
--
- -
Ox. pot. ume
(V) /(ml)
(roll
pH
Ox. pot.
(V)
pH
0.0
2.5
3.7
5.0
10.0
6.50
7.10
7.50
7.75
8.35
0.37
0.65
0.68
0.72
0.73
6.00
6.70
6.90
7.25
8.15
ume
Addition of 10 percent NaOH
M NaHCOa
0.36
0.61
0.65
0.69
0.73
After 15 min
t Vot-
/ O.O
1.0
1.5
2.0
4.0
pH
Ox. pot.
(v)
6.50
11.30
11.70
11.90
12.15
0.37
0.51
0.58
0.58
0.59
p~
5.95
6.45
6.75
7.65
12.10
Ox. pot.
(v)
0.34
0.50
0.56
0.56
0.61
groups in o x i d a t i o n of each d i t h i o n i t e molecule. The earlier dithionite
m e t h o d s , which used a low p H , did n o t give effective r e m o v a l of iron oxides
because of t h e low o x i d a t i o n p o t e n t i a l of t h e system.
T h e required OH m a y be o b t a i n e d either from N a O H or N a H C 0 s. I n t h e
N a O H system, t h e r e were s u d d e n increases a n d decreases in p H (Table 1,
column 7) a n d o x i d a t i o n p o t e n t i a l (Table 1, column 8), whereas in N a H C 0 s
s y s t e m (highly buffered) t h e r e were g r a d u a l changes in p H (Table 1, columns
2 a n d 4) a n d a well regulated, high o x i d a t i o n p o t e n t i a l of 0.7 V (Table 1,
columns 3 a n d 5). The theoretical value of 1.12 V for t h e alkaline reaction
(Latimer, 1952; Jackson, 1956, p. 48) was n o t reached in the presently
proposed m e t h o d .
Correlation of Oxidation Potential with Solubility
of Iron Oxides and Reaction Products
The order in which t h e reagents were m i x e d a n d t h e resulting p H g r e a t l y
influenced t h e r a t e of dissolution o f iron oxides a n d t h e p r e c i p i t a t i o n or
n o n p r e c i p i t a t i o n of S a n d FeS. I n one m e t h o d (Table 2, procedure 1), 1)oth
322
SEVENTHNATIONAL CONFERENCE ON CLAYS AND CLAY ~IINERALS
T A B L E 2.--SOI~I.TBI~ITY OF Fe2Oa W I T H D I F F E R E N T PROCEDURES AND
pH LEVELS,
DURING A 15 ]~IINREACTION TIME
i
10%1
NaOH
No. (ml)
1.8
nil
nil
4.
0.5
5.
1.0
6.
1.0
7.
2.0
2.5
3.5
4.0
8.0
16.0
32.0
8.
9.
10.
11.
12.
13.
Procedure
pI-[ : Fe dissolved
Initial1 after
pH 15 minI (p.p.m.) (%)
NaCt2 (pH 7.3) +
Na2S204 solution
(pH 7.3) + sample + heat
NaCt (pH 7.3) +
sample + heat +
N32S304 solid
NaCt -}- sample +
heat -}- Na2S204
solid
NaCt + NaOH -Isample + heat
Na2S2Oa solid
NaCt -5 NaOH
+
Na2S~O4 solid +
sample + heat
NaCt + Na0H +
sample + N32S204
solid + heat
Sall]e
Same
Same
Same
~ame
Same
~ame
1 All reagents added and mixed.
2 Ct represents citrate.
Precipitation
S
FeS
Rate
7.20
6.90
163.5
84.1
No
Little
Fast
6.30
6.25
180.73 92.7
Yes
Yes
Fast
6.35
6.20
197.53 I01.0
Yes
Yes V. fast 4
6.40
6.25
195.03 100.0
Yes
Yes V. fast *
6.80
6.45
162.5
83.6
No
No
Fast
6.80
6.35
177.03
90.8
No
No
Fast
7.40
8.20
10.50
12.08
12.15
12.17
12.16
6.50
6.80
6.95
8.15
8.40
8.45
8.55
127.0
31.5
9.0
0.2
0.2
0.2
0.2
65.1
16,4
4.7
0.1
0.1
No
No
No
No
No
No
No
No
No
No
No
No
No
Slow
Slow
Slow
V. slow
V. slow
V. slow
V. slow
,
0.1
0.1
No
s Average of duplicates.
4 Time given -~ 10 rain.
the N a citrate a n d the Na2S204 solutions were a d j u s t e d to p H 7.3, yet F e
dissolution was n o t complete in 15 min. I n a n o t h e r system (Table 2, procedure 2) when only the N a citrate solution was adjusted to p H 7.3, the recovery
w a s more nearly complete b u t S a n d FeS were precipitated.
As the p H was increased to 6.4, the Fe208 dissolved completely (Table 2,
column 7), b u t FeS a n d S precipitation occurred. As the p H was increased
further the percentage of Fe dissolved went down steadily. This relationship is
brought out clearly b y the curves of Fe208 solubility and oxidation potential
against 9 H (Fig. 1). The oxidation potential increases sharply up to p H 8
a n d t h e n levels off. T h e solubility of Fe203 decreases r a p i d l y above p H 7.
The two curves intersect a t a b o u t p H 7.3, suggesting this as the o p t i m u m
p H for the most effective a n d rapid r e m o v a l of free iroa oxides. The order
of addition of reagents i n the adopted procedure was based on procedures
3 a n d 4 (Table 2) because t h e y were the most effective. S u b s t i t u t i o n of 5 ml
DITHIONITE-CITRATE SYSTEM BUFFERED WITH SODIUM BICARBONATE
323
of 1 M NaHCOa buffer for NaOH in the adjustment of the usual 40 ml of Na
citrate solution to p H 7.3 overcame the difficulty of S and FeS precipitation,
maintained a high oxidation potential (Table 1, column 5), and was completely effective for dissolution of free iron oxides (Table 3).
Solubility of Hematite and Goethite in Proposed Method
A 0.2 g sample of finely ground crystalline hematite was dissolved completely (Table 3, column 3) in 2 min, whereas with an excess of NaHC03
or N a O H (i.e. at higher p H values), a longer treatment was required. Moreover, the rate of dissolution of goethite decreased even more sharply (Table 3,
column 4) with increase in pH. The recovery was complete in two or three
treatments of 15 min each. Therefore, for soils with a high content of free
iron oxides, two or three treatments of 15 min each are given.
-x--x-
0.8
After 15 min.
After 2mln.
I00
9o
8O
0.7
q~
70
6o~
0.6
5o ~
~5
WO.5
2o
io
0.4
0.7
6
7
8
9
pH
I0
II
12
FI~URE l.--Curves of p H against oxidation potential of •a2S2Od-Na citrate systems
buffered with NaHCOa, and associated solubility of Fe2Os. NaOH was added to
obtain the three solutions above pH 9.
Comparative Study of Iron Oxide Removal Methods
Since 1877, when Bemmelen and others used mineral acids and alkalies
to dissolve iron oxides (and clays as well) in order to clean the sand and silt
for petrographic study, various methods have been developed which are
more selective. Most of them lack some of the essential requirements of a
desirable iron oxide removal method; namely, the method should be (a)
effective, (b) rapid, (c) free of analytical difficulties, and above all (d) should
not attack iron silicate minerals in clays. Three types of methods are compared with the proposed buffered dithionite method in the present study,
namely one using H2S, one using unbuffered dithionite and one using nascent
hydrogen, as described in the procedure section.
Fe203, Al~Os, and Si02 dissolved by various methods.--In the lat0sohc
soils, Fajardo and Bayamon from Puerto Rico, the proposed method removes
about 6 and 8 percent of free iron oxides (Table 4, columns 2 and 5) as contrasted to less than 2 percent by the H~S method and somewhat higher by
324
SEVENTH NATIONAL CONFERENCE ON CLAYS AND
CLAYMINERALS
TABLE 3,--Fe203 DISSOLVED BY CITRATE--DITHIOI~ITE SYSTEMS
Goethite (0.2 g)
Hematite
(0.2 g)
Fe
Time Recovered
(rain)
(%)
H y d r o x y l Source
2.5
5
10
1
ml
ml
ml
ml
Fe recovered (%)
1 M Na~ICO3
NaHCOa
NaHCOa
10% N a O H
2
2
15
15
100
100
99.1
98.0
15-rain t r e a t m e n t
1st
2nd
3rd
Total
83.4
81.7
80.0
67.6
13.4
14.6
15.4
26.2
3.0
3.3
4.0
5.4
99.8
99,6
99.4
99.2
TABLE 4.--PERCENT Fe~O3, A1203 a n d SiO2 DISSOLVED ]~ROM SOILS BY DIFFERENT
METHODS OF IRON OXIDE REMOVAL
Method Fe2Oa AlaOa SiO2 F e t e 3 A1203 Si02 Fo203 A1203 SiO2 Fo202 AlcOa SiOa
-i"
1.
2.
3.
9 4.
I.
2.
3
4.
1.
2.
6 hr.
3.
4.
3 hr.
7.2
1.0
-3.6
Bayam0n,P.R,
7.9
1.8
8.6
1.8
2.1
0.8
5.3
--6.6
-3.5
MiamiBu,
Wis,
1.0
1.8
2.5
2.3
1.5
-1.3
--
Vermiculite,
Colo.
4.5
1.6
8.4
6.0
3 . 8 10.2
4.2
--5.9
--2.1
Nontronite,
Woody, Calif.
0.5
2.0
8.4
2.9
1.5
1.1
2.8
4.1
-4.0
Glauconite,
Wis.
0.8
1.0
8.6
0.6
0.5
2.3
0.7
--0.6
-2.6
5.5
1.5
4.8
4.0
Fajardo,
P.Ro
2.2
1.6
---
!
-
-
-
-
"[I
7.5
7.7
-4,7
Waipiata,
N.Z.
1.8
2.2
2.1
1.4
2.6
-3.4
--
8.0
1.5
-3.1
Kuroishibaru,
Japan
5.0
5.6 15.0
6.1 10.3 10.2
4.7
-5.9
-8.6
Proposed : Na2S204-Na e i t r a t e - N a H C O a ; p H 7.3 ; temp. 80~ ; elapsed time, 1 hr.
T r u o g (1937) : Na~S-NH4Cl-oxalie acid ; p H 3 . 5 - 1 0 ; temp. 95~ ; elapsed time,
Deb (1950) : Na2SuO4-HuO ; p H 3.5 ; temp. 40~ ; elapsed time, 3 hr.
H a l d a n e (1956) : Zn-oxalic a e i d - N H 4 oxalate ; p H 3.6 ; temp. 20~ ; elapsed time,
other methods. The greater effectiveness of the proposed method for removing
free Fe203 from iron oxide rich soils is thereby shown. For Miami (Wis.)
and Waipiata (New Zealand) nontronite-bearing soils, the H2S, acid nascent
H, and acid Na2S204 methods attack and dissolve more of the combined iron
than did the proposed method (Table 4). This effect is even more pronounced
for nontronite and iron-rich vermiculite (Table 4).
The various methods dissolved a considerable amount of Al20a and Si02,
especially from the allophane of the Japanese Ando (Kuroishibaru) soil. Tile
DITHIONITE-CITRATE SYSTEM BUFFERED WITH SODIUM BICARBONATE 305
d e t e r m i n a t i o n of A1 a n d Si is v e r y difficult in t h e D e b or H a l d a n e e x t r a c t s
owing to t h e excess of Na2S204 a n d oxalate, respectively. The t i m e elapsed
for r e m o v a l of iron oxides from a set of eight samples is m u c h higher (3-6 hr)
for t h e comparison m e t h o d s as c o m p a r e d to less t h a n 1 hr for t h e p r o p o s e d
m e t h o d (Table 4, footnote).
Effect of iron oxide removal on cation exchange capacity of soils.--To measure
t h e d e s t r u c t i v e effect of various m e t h o d s of iron oxide r e m o v a l on other iron
silicate minerals in soils, their cation exchange c a p a c i t y was d e t e r m i n e d
before a n d after iron oxide r e m o v a l t r e a t m e n t . A f t e r t h e t r e a t m e n t s of Truog,
Deb, a n d H a l d a n e , t h e r e is a m a r k e d decrease in t h e cation exchange c a p a c i t y
of Miami a n d W a i p i a t a soils (Table 5, columns 4 a n d 5). The decrease is even
more p r o n o u n c e d for vermiculite a n d n o n t r o n i t e (Table 5, columns 6 and 7),
TABLE ~ . - - ~ F F E C T OF DIFFERENT IRON OXIDE ~EMOVAL METHODS ON CATION E x C~IANGE ~APACITY OF ~u
SOILS AND CLAYEY I~OCKS ( -- 2 ram) ttic~ I ~ IRON SILICATES
Cation Exchange Capacity
(moq/100 g)
Treatment 1
~Tone
1
2
3
4
None
1
2
3
4
Fajardo
Soil
Bayamon
Soil
Miami
Soil
Waipiata
Soil
37
58
59
49
45
33
59
69
35
48
42
53
32
41
50
49
52
31
39
45
Vermiculite-rich
Rock
Nontronite.rich
Rock
Glauconite-rich
Rock
Kuroishibaru
Soil
108
107
54
55
73
44
50
23
47
43
10
18
11
10
37
83
93
25
65
125
1 Details given in footnote of Table 4.
v a r y i n g from 25 to 50 percent. This decrease is due to t h e b r e a k d o w n of iron
silicate minerals. F o r t h e J a p a n e s e A n d o soft t h e r e is a b o u t 75 p e r c e n t decrease after Truog's t r e a t m e n t a n d a b o u t 30 percent decrease after D e b ' s
t r e a t m e n t , owing to t h e dissolution of allophane (Table 5, column 5). There
is a slight increase in t h e exchange capacity after t h e proposed t r e a t m e n t ,
a t t r i b u t a b l e to t h e r e m o v a l of iron, a l u m i n u m a n d silica complexes which
m a y h a v e blocked some exchange sites. Also, possibly, t h e r e m a y h a v e been
reduction of some l~ttice iron which is n o t clearly shown up b y earlier m e t h o d s
because of their d e s t r u c t i v e effect on iron silicate clays. There is a large
a p p a r e n t increase in the exchange c a p a c i t y of glauconite a n d Ando soil after
326
SEVENTH I~ATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS
the Haldane t r e a t m e n t (Table 5, columns 4 and 5), possibly owing to the
precipitation of a voluminous zinc oxalate phase (quite noticeable to casual
observation) and its exchange (precipitation) with Ca salts.
Iron oxide dissolved and its effect on cation exchange capacity of clays.--The
effect of iron oxide removal is b r o u g h t out more clearly with the clay size
fraction (--2~). The proposed m e t h o d removes free iron oxides much more
effectively from the latosolic soil clays as contrasted to other methods (Table 6,
column 2). Methods 2, 3 and 4 a t t a c k the iron-rich silicate minerals (vermiculite and nontronite) and extract very large amounts of iron (Table 6,
TABLE 6.--Fe2Oa DISSOLVED AND THE EFFECT ON CATION EXCItANGE CAPACITk" OF
CLAYS (--2/l) BY VARIOUS METHODS OF IRON OXIDE ]~EMOVAL
Bayamon
1.
2.
3.
4.
Method I
Fe208
] (%)
No treatment
Proposed
Truog
Deb
I-Ialdane
12.5
6.5
10.5
2.2
-
-
Vermiculite
CEC2
Fe203
(%)
CEC
42
43
37
41
47
6.3
0.3
7.8
8.7
130
136
81
99
120
Nontronite s
Allophane
(Kuroishibaru)
~%
(%)
CEC Fe203
CEC (%)
0.2
7.9
8.0
2.3
64
54
25
46
5O
Fe2{
Fe203
7.9
6.2
5.9
7.1
CEC
128
109
53
79
69
1 Details given in footnote of Table 4.
2 Cation exchange capacity, meq/100 g of original --2/t clay sample.
a Contains some quartz in this --2~ fraction.
columns 4 and 6) from within the lattice of these minerals as shown b y the
sharp decrease in cation exchange capacity (Table 6, columns 5 and 7) after
the treatment. There is also considerable decrease in the exchange capacity
of Japanese Ando soil with the three comparison methods (Table 6, column
9).
CONCLUSIONS
The proposed method of buffering the dithiouite-citrate system with
NaHCOs gives freedom from S, FeS, zinc oxalate and other u n w a n t e d precipitates. I t is very rapid, taking only 15-60 min as compared to several
hours in earlier methods. I t is highly effective in the dissolution of free iron
oxides owing to maintenance of a slightly ~dkaline p H which keeps a high
oxidation potential in the Na2S204 system, in marked contrast to earlier
acid or less buffered dithionite systems. The determination of Fe, A1 and Si
in the extract is much easier and simpler. Above all, the tr(,atment has
almost no destructive effect on iron silicate clay minerals.
DITHIONITE-CITRATE SYSTEI~I BUFFERED WITH SODIUM BICARBONATE
327
ACKNOWLEDGMENTS
T h i s c o n t r i b u t i o n f r o m t h e D e p a r t m e n t o f Soils, U n i v e r s i t y o f W i s c o n s i n ,
was s u p p o r t e d in p a r t b y t h e U n i v e r s i t y R e s e a r c h C o m m i t t e e t h r o u g h a
grant of funds from Wisconsin Alumni Research Foundation.
REFERENGES
Aguilera, hi. H. and Jackson, M. L. (1953) Iron oxide removal from soils and clays :
Soil Sci. Soc. Amer. Proc., v. 17, pp. 359-364.
Allison, L. E. and Scarseth, G. D. (1942) A biological reduction method for removing
free iron oxides from soils and colloidal clays : J. Amer. Soc. Agron., v. 34, pp. 616-623.
Deb, B. C. (1950) The estimations of free iron oxide in soils and clays and their removal :
J. Soil ~ci., v. l, pp. 212-220.
Dion, H. G. (1944) Iron oxide removal from clays and its influence on base exchange
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Drosdoff, M. and Truog, E. (1955) A method for removing and determining the free
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Haldane, A. D. (1956) Determination of free iron oxide in soils : Soil Sci., v. 82, pp.
483-489.
Jackson, M. L. (1956) Soil Chemical Ar~alysis--Advanced Course: Published by the
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Jcffries, C. D. (1947) A rapid method for the removal of free iron oxides in soils prior
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Latimer, W. M. (1952) Oxidation Potential8 : Prentice.Hall, :New York, 392 pp.
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Tamm, O. (1922) Eine Method zur Bestimmung der anorganischen Komponenten des
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22