Applied Clay Science, 1 ( 1985 ) 103 --114 103 Elsevier Science Publishers B.V., Amsterdam --Pr inted in The Netherlands
THE FORMATION AND PERSISTENCE OF VARIOUS ALUMINUM OXY-HYDROXY-SULFATE SOLID PHASES UNDER DIFFERENT EXPERIMENTAL CONDITIONS* 1
S, SHAH SINGH
Chemistry and Biology Research Institute, Research Branch, Agriculture Canada, Ottawa, Ont. KIA 0C6 (Canada)
(Accepted for publication March 11, 1985)
ABSTRACT
Singh, S.S., 1985. The formation and persistence of various aluminum oxy-hydroxy- sulfate solid phases under different experimental conditions. Appl. Clay Sci., 1: 103-- 114.
The nature and type of interlayered A1 formed in Wyoming bentonite clay in the presence of C1 and SO4 anions is discussed. The ion activity product (Al) (OH) 3, the exchange characteristics and solid phases formed in C1 and SO4 systems were quite different. The cation exchange capacity (C.E.C.) of the Wyoming bentonite clay was permanently reduced in the C! system when the aluminum precipitated was less than 800 meq./100 g clay. In the SO4 system, however, such permanent reduction of C.E.C. did not occur irrespective of the amount of A1 precipitated. The aluminum hydroxy sulfate solid phase formed in the presence or in the absence of Wyoming bentonite clay is metastable with respect to gibbsite. However, this aluminum hydroxy sulfate solid phase is stable with respect to the aluminum interlayered Wyoming bentonite formed in the presence of C1 when the precipitated A1 is less than 800 meq./100 g of Wyoming bentonite. This information has been employed in removing the interlayered aluminum from Wyoming bentonite by changing the chemical environment of the system which resulted in the regaining of the blocked cation exchange capacity.
INTRODUCTION
Aluminum is the most abundant metal element in the earth's crust. During weathering aluminum is released from primary minerals and is precipitated as secondary minerals. The solid phases and aqueous chemistry of aluminum are of interest to many scientists from various disciplines (corrosion chemistry, metallurgy, geochemistry, geology, soil science, etc.). Amorphous compounds of aluminum are formed in most soils due to dissolution of secondary minerals, largely aluminosilicates, in acidic environments. Under normal processes of weathering, the leaching action of CO2-charged water percolating through the profile removes free salts and exchangeable basic
cations. In this process certain forms of aluminum can be fixed in the inter- layer space of expandable clay minerals. Since interlayer aluminum may be responsible for low cation exchange capacity and contribute to A1 toxicity and fixation of plant nutrient ions, many workers have at tempted to characterize the nature of interlayer A1 in order to assess its significance to soil properties (Jackson, 1960, 1963; Ragland and Coleman, 1960; Rich, 1960; Shen and Rich, 1962; Barnhisel and Rich, 1963, 1965; Frink and Peech, 1963; Coleman and Thomas, 1964; Hsu and Bates, 1964; Turner, 1965; Turner and Brydon, 1965, 1967; Brydon and Kodama, 1966; Singh and Brydon, 1967, 1970; Singh and Miles, 1978; Keren, 1980).
Turner and Brydon (1965) and Singh and Brydon (1967, 1970) demon- strated that the nature and stability of interlayered aluminum and hence its effect on soil properties were dependent upon the amount of aluminum precipitated and the types of anion present in the system. This manuscript describes the nature and type of interlayered A1 formed in Wyoming bentonite in the presence of different anions, the ionic activity product (A1) (OH) 3 as determined by the nature of amorphous and crystalline aluminum hydroxy compounds, and the stability relationship of aluminum oxy-hydroxy-sulfate solid phases.
EFFECT OF ANIONS ON THE NATURE OF I N T E R L A Y E R E D A1 A N D IONIC ACTIVITY PRODUCT (A1) (OH) 3
The properties of the interlayered A1 in Wyoming bentonite depend on the type of anion present, the amount of aluminum precipitated and the time of reaction (Figs. 1, 2). In the C1 system (Fig. 1), when no clay was present the (A1)(OH) 3 ion product decreased with time for the first four months and then remained essentially constant with -- log (A1)(OH) 3 ~ 33.75 for the next four months. When the amount of Al(OH) 3 precipitated was 440 meq. per 100g clay, - - log (A1)(OH) 3 was between 33.0 and 33.1 for the entire eight months. With 880meq. Al(OH)3 per 100g the value of (A1)(OH) 3 decreased with time for the first month after which -- log (A1)(OH) 3 stayed between 33.0 and 33.1 until the end of the experiment. When the precipitated A1 was greater than l l 0 0 m e q , per 100g clay, the ionic product (AI)(OH) 3 decreased with time and was --~ 10 -33.60 at the end of eight months. For the SO4 system (Fig. 2), the shapes of curves representing the relationship of ionic activity product (A1)(OH) 3 with time look similar to those for the C1 system. Close examination of these curves, however, reveals that the products formed and nature of reactions in SO4 systems, which determine the ion activity product (A1)(OH} 3, are quite different. The SO4 ions form soluble complexes with A1 to form a monovalent cation A1SO~ in addition to divalent A1OH 2÷. The values of (A1)(OH} 3 without correcting for sulfate complexes (not reported in figure} were much greater than corrected values which is an indication that a substantial amount of aluminum is present in the mono- valent A1SO~ form in SO4 systems. It is therefore imperative to make
105
32.0
32.2~
32.4 ~. .
32.6 1 ~ ,l......
} 'q" 33.2 \
33:4
33.6
33.8
34.0
B
i I , i | I I I
60 120 180 240 DAYS
Fig. 1. Effect of time of reaction on the ion product (AI)(OH) 3 when A1 was precipitated in the presence of Cl ions: (A) no clay and (B) 440, (C) 880, (D) 1100, and (E) 1600meq. A1 per 100 g clay (Turner and Brydon, 1965).
32.6 I
33.0 ~.,.., = . . . . . '~.~
33.2 ~ •, ~ "~Q
33.6
33 I 1 I i I I
'80 40 80 120 160 200 240 DAYS
I I
280 320
Fig. 2. Effect of time of reaction on the ion product (AI)(OH) 3 when AI was precipitated in the presence of SO4 ions: (A) no clay and (B) 387, (C) 765, (D) 1165 and (E) 1516 meq. A1 per 100 g clay.
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corrections for complexing of A1 ions to obtain a true ionic activity product of the aqueous system.
When no clay was present, (A1)(OH) 3 increased for a short time and then decreased slowly to a value of 10-32.9 and remained at this value for the final five months of the experiment (curve A). These results are entirely different from those obtained by Turner and Brydon (1965) when Ca{OH)2 was added to the A1C13 solution. In the presence of clay, the effect of time on the magnitude of (A1)(OH) 3 depended on the amount of Al(OH)x pre- cipitated per unit weight of clay. However, in all cases, in the presence of SO4, the ion product (A1)(OH) 3 decreased eventually to 1 0 - 3 3 " 6 , but the time required to reach this value was directly related to the amount of A1 (OH)~ per 100 g clay. With C1-present, the (A1)(OH) 3 was equal to about 10 -33'° , regardless of the length of time of reaction, providing the amount of AI(OH)x was not greater than 850meq . per 100g of Wyoming bentonite. With larger amounts of AI(OH)x, the (A1)(OH) 3 in the presence of C1- decreased gradually and approached the solubility product of gibbsite.
I N T E R L A Y E R E D A1 A N D C.E.C. O F T H E W Y O M I N G B E N T O N I T E
The cation exchange capacity of soils, which is derived from clay minerals, amorphous materials and organic matter, is an important soil property. Evidence has been accumulated showing that hydrous A1 oxides may react with clays to reduce the cation exchange capacity. The cation exchange capacity of aluminum-saturated Wyoming bentonite prepared in the presence of C1 ions was 79.7 meq. /100 g at the start of equilibration, and remained the same throughout 2 years of equilibration (Table I). In the aluminum-
T A B L E I
C h e m i c a l c h a r a c t e r i s t i c s o f a l u m i n u m - s a t u r a t e d a n d a l u m i n u m - i n t e r l a y e r e d W y o m i n g
b e n t o n i t e
T i m e C h e m i c a l p r o p e r t y : C a t i o n e x c h a n g e c a p a c i t y - - l og ( A 1 ) ( O H ) 3
( d a y s ) S y s t e m : Al- A l - i n t e r l a y e r e d A l - i n t e r l a y e r e d
s a t u r a t e d
0 5 5 0 7 5 0 A l - p r e c i p i t a t e d : 4 0 0 7 5 0
( m e q . / 1 0 0 g c l a y )
I n i t i a l 7 9 . 7 3 1 . 4 - - 3 3 . 0 9 3 3 . 0 4
interlayered Wyoming bentonite sample, containing 550 meq. and 750 meq. precipitated AI per 100 g clay, the cation exchange capacity was 31.4 meq./ 100 g at the beginning of the equilibration, and had changed little after 754 and 618 days, respectively. The ion activity product (A1)(OH) 3 remained relatively constant at a value of 10 -33"1° in both the interlayered samples over a long period of equilibration. These results are in agreement with the findings reported in the literature by Shen and Rich (1962), Turner and Brydon (1967) and Keren et al. (1977).
Precipitation of 400, 800 and 1200 meq. AI per 100 g Wyoming bentonite in the presence of SO4 ions resulted after five days in cation exchange capacities of 69, 74 and 73 meq. per 100 g clay, respectively (Table II). After a one-year reaction time the C.E.C.'s were respectively, 85, 82, and 88 meq. per 100 g clay with 387, 765 and 1165 meq. A1 per 100 g clay. The ionic activity product (A1)(OH) 3 was 10 -33"°s after five days of reaction and it decreased to 10 -33"s8 in all samples in the presence of SO4 ions after one year irrespective of the amount of A1 precipitated in Wyoming bentonite clay. There was a smaller reduction (6 to 11 meq. per 100 g) in the C.E.C. in the presence of SO4 as compared to that reported for C1 (Barnhisel and Rich, 1963; Coleman and Thomas, 1964; Turner, 1965). The charge on the clay competed more successfully with C1 than with SO4 for the positive charge on Al(OH)x precipitate, resulting in much greater reduction in C.E.C. with Cl than with SO4 present. Under many conditions with C1 there was no further change after a few days, the interlayer material persisted, (A1)(OH) 3 in solution did not change and C.E.C. was permanently reduced. With SO4 present there was a continual change until the crystalline material in the form of basic aluminum sulfate was formed.
TABLE II
Effects of different amounts of aluminum precipitated and reaction time on the cation exchange capacity of Wyoming bentonite in the presence of SO4 ions
AI(OH)x per pH -- log (A1)(OH) s C.E.C. 100 g clay (meq./100 g clay) (meq.)
5 days 400 4.39 33.03 68.7 800 4.37 33.04 73.9
1200 4.35 33.01 72.9
One year 387 4.36 33.58 85.3 765 4.30 33.58 81.8
1165 4.35 33.58 88.4
Chemical analysis of the amorphous precipitate and of the solution in contact with the precipitate showed that the amorphous precipitate was an aluminum hydroxy sulfate. On an average, the precipitate contained
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approximately 0.40 SO~- radicals for every A1 atom. Hsu and Bates (1964) obtained an amorphous precipitate of similar composition. Under the present experimental conditions, the sulfate content of the solution in contact with precipitate increased with time and after several months became constant when a crystalline basic aluminum sulfate was formed. The crystalline material contained 0.25 SOl- radical for every A1 atom. The following schematic reactions are most likely involved in the above process.
A12(SO4) 3 + Ca(OH)2 - - ~_ AI(OH)3_2x(SO4) x
A14(OH)IoSO a + SO 4
where x is greater than 0.4 and the precipitates are underlined. The process of crystallization of basic aluminum sulfate was the result of reactions between the solution phase and the precipitate phase, the whole or part of which existed as interlayered material. The presence of Wyoming bentonite in some manner facilitated the formation of the crystalline phase, but the exact nature of its role was not clear.
BASIC ALUMINUM SULFATE AND ITS STABILITY RELATIONSHIPS
When a solution of aluminum sulfate is partially neutralized with a base in the presence of montmoril lonite and maintained at 25°C, a crystalline basic aluminum sulfate is formed. The time required to reach equilibrium is directly related to the amount of A1 precipitated per 100g of clay. The crystalline basic aluminum sulfate formed in the presence or in the absence of clay is represented by the formula A14(OH)10SO4, and its solubility ion activity product (A1) 4 (OH) '° (SO4) is equal to 10- 1~7.30 ~ 0.1 at 25°C (Singh and Brydon, 1969).
The ion product (AI)(OH) 3 of solutions in equilibrium with crystalline basic aluminum sulfate in the presence as well as in the absence of clay is greater than the ion product (A1)(OH) 3 of gibbsite. Since gibbsite was not found in either these experiments or those of Hsu and Bates (1964) carried out under similar conditions, the crystalline aluminum hydroxy sulfate seems to be a metastable phase with respect to gibbsite. In the two-dimensional diagram {Fig. 3) represented by the following chemical equation:
4AI(OH)3c + H2SO4s = A14(OH)10SO4c + 2H20
where S denotes solution and C indicates crystalline material, the line B is the relationship between the 2 pH + p SO4 and p A1 + 3 p OH for basalumi- nite and with (A1)4(OH)I°(SO4) = 10 -117"3° at 25°C. The vertical line D represents the solubility isotherm of gibbsite at 25°C. This relationship reveals that the metastable nature of the basic aluminum sulfate with respect to gibbsite is relative. When the ionic activity product (H) 2 (SO4) is less than 10-9.3 at 25°C, basic aluminum sulfate is metastable with respect to gibbsite.
1 0 9
o ~
4,- 1 -
12.2
1 1.8
11.4
11.0
10.6
10.2
9.8
9.4
B D
I I i
9'02.4 32.8 33.2 33.6 34.0 34,4
pal + 3pOH
Fig. 3. Solubility isotherm of crystalline basic aluminum sulfate AI(OH)IoSO4 (line B) and gibbsite-Al(OH)3 (line D).
At about ion-activity product (H)2(SO4)= 10 -9.3 basic aluminum sulfate and gibbsite should coexist and when the ion activity product (H) 2 (SO4)is greater than 10 -9"3, gibbsite becomes metastable with respect to basic aluminum sulfate. The solid circles represent the experimental equilibrated samples.
To shift the equilibration from basic aluminite toward gibbsite, one set of samples was seeded with gibbsite and the changes in the ion activity products are presented by a broken line. The seeded samples gradually progressed to approach equilibrium with gibbsite, that is, the experimental points, over two months of time, moved from the basic aluminum sulfate isotherm to that for gibbsite (Fig. 3).
These studies were complemented by X-ray analysis of oriented samples of the solid phases. Before gibbsite was added the patterns showed only crystalline basic aluminum sulfate. Immediately after the addition of gibbsite the bands for both basic aluminum sulfate and gibbsite were present. As reactions progressed the gibbsite lines increased in intensity while those for basaluminite decreased, finally leading to the disappearance of basaluminite. The possibility that basic aluminum sulfate would be hydrolysed to gibbsite if the SO4 concentration in solution was very low, that is, the reaction
llt~
AI41OH}~0SO 4 + 2 H 2 0 - - I~AI(OH)3 + H 2 S O 4 ( A ) w o u l d occu r , was investigated by suspending basaluminite in H20. It was found that the SO4 concentration in solution did not increase over a period of one year, showing that reaction (A) does not occur. Since seeding with gibbsite resulted in the disappearance of basic aluminum sulfate with an increase in the amount of gibbsite present, it was concluded that the mechanisms involved were:
Ala(OHIIoSO 4 ~ 4 A1 ~ ~ 1 0 O H - + SO4
followed by :
4 A13+ + 10 OH- + 2 t ! ,O ....... . . 4 AI(OH)3 + 2 H +
where A1 (OH)3 forms on the surface of gibbsite crystals already present.
STABILITY OF AN ALUMINUM-INTERLAYERED WYOMING BENTONITE IN THE PRESENCE OF SULFATE IONS
Turner and Brydon (1965) found that the amount of aluminum precipitated per unit of clay and the time of reaction were important in determining the stability of interlayered aluminum and permanency of the reduced cation exchange capacity. For example, when the amount of aluminum precipitated was about 16 meq. per gram of clay, there was a decrease in the exchange capacity of Wyoming bentonite, but after ageing, the original C.E.C. was regained, and a separate phase of gibbsite was formed. When the amount of aluminum precipitated was 8 meq. or less per gram of clay, however, there was a decrease in the exchange capacity and C.E.C. was not regained on ageing. The ionic activity product (A1)(OH) 3 was 10 -33.oo and remained so during the two years of aging time. Singh and Brydon (1967, 1970) demonstrated that the nature and the stability of interlayered aluminum were dependent on the type of anion present in the system. It was shown that in the presence of SO4 ions, the interlayered material formed was aluminum hydroxy sulfate and the interlayering did not reduce the exchange capacity of clay. The ion activity product (A1)(OH) 3 of the equilibrated 804 system was lower than that of the C1 systems when the A1 precipitated was less than 8 meq. per gram of clay. From solubility considerations aluminum interlayers of clay in the presence of C1 ions are metastable with respect to aluminum precipitated in the presence of sulfate ions in Wyoming bentonite.
Table III shows that on equilibration with SO4 solutions of 10, 20 and 30meq./1, respectively, the exchange capacity of three samples each containing 550 meq. of precipitated A1/100 g clay increased from the initial value of 31 .4meq. /100g of the interlayered clay. The regaining of lost exchange capacity was complete in samples 2 and 3, and partial in sample 1. In sample 1, the C.E.C. increased from 31.4 to 53.8 meq./100 g after 40 days of equilibration. Over the next 100 days of equilibration, the increase was only 2 meq./100 g clay. Analysis of the equilibrating solution in
111
sample 1 showed that the amount of SO4 taken out of solution to react with the aluminum hydroxy precipitate was not sufficient to form a neutral precipitate of A14 (OH)10 SO4. The ion activity product (A1)(OH) 3 decreased with the passage of reaction time from the constant ion activity product of
10-33.0
T A B L E III
Ca t ion exchange capac i ty and ionic ac t iv i ty p r o d u c t of i n t e r l aye red Wyoming b e n t o n i t e c o n t a i n i n g 550 meq. p r ec i p i t a t ed A1/100 g clay a f te r equ i l i b ra t ion wi th SO4 so lu t ions
Time - - log (A1)(OH) 3 (days)
1"1
C.E.C. ( m e q . / 1 0 0 g clay)
2 3 1 2 3
Ini t ia l 32 .96 32 .96 32 .96 31.4 31.4 31.4 40 33 .10 " 33 .17 33 .22 53.8 72.8 74 .0
• 1 1 = 1 0 m e q . SO4/1; 2 = 2 0 m e q . SO4/1; and 3 -- 3 0 m e q . SO4/1.
I N C R E A S E IN C.E.C. OF SOILS ON E Q U I L I B R A T I O N WITH SO4 S O L U T I O N S
Treatment of four acid soil samples with SO4 solutions resulted in an increase of exchangeable cations extracted by 2 N CaC1 (Table IV). After three equilibrations with CaSO4 solutions the sum of NaC1 extractable cations were 6.56, 11.99, 5.62 and 4.31 meq. /100 g soil, respectively, for soil samples CSSC-2 (Sombric Ferro-Humic Podzol), CSSC-19 (Orthic Ferro- Humic Podzol), SSD-330 and SSD-331 (Dystric Brunisol), respectively. On further equilibration there was no increase in total extractable cations for soil sample CSSC-2; for the other samples, however, there were progressively smaller increases. For CSSC-19, the total extracted cations were 16.04 and 1 7 . 4 3 m e q . / 1 0 0 g after the 6th and 9th equilibrations, respectively, for a total increase of 9.94 meq. /100 g over the initial C.E.C. of 7.49 meq. /100 g. For soil sample SSD-330, the sums of cations were 6.65 and 7.25 meq. /100 g after the 6th and 9th equilibrations for a total increase of 2.95 meq. /100 g over the directly extractable C.E.C. of 4 .30meq . /100g . For soil sample SSD-331 the sums of cations were 5.13 and 5 . 4 5 m e q . / 1 0 0 g after 6th and 9th equilibrations for a total increase of 2 . 9 5 m e q . / 1 0 0 g (which by coincidence was the same as for SSD-330 which was a surface sample from the same soil site). The accumulated incremental increases in the cation exchange capacity after nine equilibrations ranged from 27 to 132% of the cation exchange capacity values determined in unequilibrated samples.
112
The greatest increases in the C.E.C. and OH ions were fo u n d in the Orthic Fe r ro -Humic Podzo l soil. Almost all a luminum was c o m p l e x e d with organic mat te r , which demons t r a t e s the impor t ance o f a luminum-~organic-clay com- plexes in acid soils. Our knowledge a b o u t the la t ter complexes is very l imited.
TABLE IV
Effect of successive CaSOa equilibrations on the NaC1 extractable cation exchange capacities of soils (in meq./100 g)
• 1 a--m = Values listed horizontally followed by the same letter are not significantly different by Duncan's multiple range test at P = 0.05.
The equi l ib ra t ion data on these soils show tha t there are at least two reac t ions which occur red on the addi t ion o f CaSO4 solut ions, i.e. increase in soil pH (release o f OH ions) and some negat ively charged sites on the surfaces o f the clay were made available for ca t ionic exchange. It is d i f f icul t to say with ce r t a in ty if there is any pr ior i ty be tween these two react ions or if t he y oc c u r s imul taneous ly , bu t the increase in the neut ra l sa l t -extractable ca t ions on ly became substant ial a f te r a few t r ea tmen t s with CaSO4 solut ions. Schemat ica l ly the above m e n t i o n e d reac t ions m a y be represen ted as:
AI(OH + clay surface) ° + AI(OH) ° + x SO~-
AI (OH + clay surface3_x(SO4)x/2) ° + Al(OH)3_x(SO4)°j2
+ x (clay surface)-1 + x OH-1
The above reac t ions show the release of OH ions (increase in soil pH) as well as the increase o f negative sites on the clays. The schemat ic representa- t ion is in c o n f o r m i t y wi th the findings o f Rajan (1978) and Singh (1982) .
GENERALCONCLUSIONS
A s tudy o f the aqueous and solid phase reac t ions o f a luminum, in the presence o f clay and soils, reveals t ha t the so lu t ion chemis t ry , solid phase and exchange reac t ions o f a luminum are great ly in f luenced by the presence o f associated anions. Sulfate ions have a significant e f fec t on d is t r ibu t ion of a luminum species in solut ion. It is known tha t SO4 ions fo rm comPlexes wi th A13÷ to fo rm m o n o v a l e n t A1SO~. It is necessary to evaluate the
113
d i s t r ibu t ion o f d i f f e ren t species o f ca t ions to u n d e r s t a n d the exchange r eac t ions in c lays and soils.
The ra te and the na t u r e o f the r eac t ions t h a t o c c u r w h e n A1 is p r ec ip i t a t ed in t he p resence o f W y o m i n g b e n t o n i t e and SO4 ions are d i f f e ren t f r o m those w h e n C1 is the an ion . T h e p rec ip i t a t i on o f d i f f e ren t a m o u n t s o f a l u m i n u m in the p resence o f SO4 on ly r educed the exchange capac i t y by 6 to 11 meq . pe r 100 g o f c lay and this r e d u c t i o n was very small as c o m p a r e d wi th t h a t resul t ing f r o m the p rec ip i t a t i on o f A1 unde r s imilar cond i t i ons in the p resence o f C1 as an anion. The a l u m i n u m h y d r o x y sul fa te solid phase f o r m e d in the p resence or in the absence o f W y o m i n g b e n t o n i t e clay is m e t a s t a b l e wi th r e spec t t o gibbsi te (AI(OH-)3). Howeve r , this a l u m i n u m h y d r o x y sul fa te solid phase is s table wi th r e spec t to the a l u m i n u m in te r l aye red W y o m i n g b e n t o n i t e f o r m e d in the p resence o f C1 w h e n the p rec ip i t a t ed A1 is less t han 800 m e q . / 100 g o f W y o m i n g b e n t o n i t e . This i n f o r m a t i o n is very useful in the r emova l o f i n t e r l aye red A1 (non-des i rab le phase) b y changing the chemica l environ- m e n t o f the sys tem. This t e c h n o l o g y has been successful ly appl ied in the l a b o r a t o r y in regaining the r educed exchange capac i ty o f clay and soils on equ i l ib ra t ion wi th SO4 solut ions .
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Singh, S.S. and Miles, N.M., 1978. Effect of sulfate ions on the stability and exchange characteristics of aluminum interlayered Wyoming bentonite. Soil Sci., 126:323--329.
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