chemical dissolution techniques in the study of soil...

Clay Minerals (1965) 6, 23.

C H E M I C A L D I S S O L U T I O N T E C H N I Q U E S IN THE STUDY OF SOIL CLAYS: PART I

E. A. C. F O L L E T T , W. J. M c H A R D Y , B. D. M I T C H E L L AND

B. F. L. S M I T H

The Macaulay Institute [or Soil Research, Aberdeen

(Received 25 November 1964)

ABSTRACT: The mineralogy of the clay fractions of two soil profiles repre- sentin~ the end-members of a catena developed on a glacial till derived from basic lavas has been determined. Particular attention has been given to the assessment of the nature of the amorphous inorganic material in the clay fraction of these soils. Chemical dissolution techniques were used and their effects on the clay fraction were followed by X-ray diffraction, differential thermal, infrared absorption, electron-optical and surface area measurements. The principal conclusion is that the soil clays are a continuum from completely disordered, through poorly ordered to well crystallized material.

The clay fraction of soil contains the finest and therefore the most surface-reactive particles. Consequently, many of the properties of the soil are determined by the nature of this fraction even when the amount present may be merely a few per cent. Although it is inevitable that chemical pretreatment will result in some alteration of these properties (Harward & Tlieisen, 1962; Farmer & Mitchell, 1963), this need not necessarily be disadvantageous and indeed careful and controlled degradation of the clay fraction should provide valuable information concerning its composition and properties. Neither clay deposits nor soil clays have been found to be mono- mineralic and free from accessory minerals and while the stability of clay constituents cannot be equated directly to the inherent degree of order, because other factors such as particle size are involved, nevertheless, studies based upon the relative stability of a component or group of components in clays to specific chemical reagents provide a useful approach to the identification and estimation of the constituents of such complex systems.

For some time it has been customary to determine soluble silica in soils and minerals by treatment with hot Na2COa and N a O H solutions, and, in general, alkali reagents have been used to dissolve X-ray-amorphous aluminosilicates, free silica and alumina, whereas acids have been employed for the selective dissolution of crystalline material. Digestion with dilute Na2CO~ solution has been considered

24 E. A. C. Follett et aL

to remove the amorphous siliceous material (Jackson, 1956), although Hashimoto & Jackson (1960) have stated that frequently amorphous inorganic material was not dissolved completely with this reagent. They consequently studied the differential dissolution of clays with NaOH and found that substantial amounts of aUophane, free silica, and alumina were brought into solution by boiling for 2�89 min with 0"5 r~ NaOH solution. Mitchell & Farmer (1962), however, successfully removed highly hydrated amorphous inorganic material from the clay fraction of certain brown forest soils and grey-brown podzolic soils by successive digestions on a steam bath with 5 % Na2COa solution and a New Zealand allophanic soil clay was also largely dissolved by this treatment. In view of these observations it has been considered desirable to carry out a detailed study of the graded dissolution of particular soil clays, pure layer lattice silicates and accessory minerals with dilute Na2COa solution.

M A T E R I A L S A N D M E T H O D S

The clay fractions (less than 2/~ equivalent spherical diameter) from the morpho- logically differentiated horizons of two soil profiles from North Ayrshire (Mitchell & Jarvis, 1956) were used. These soil clays were selected because unlike those studied by Mitchell & Farmer (1962) they did not, from differential thermal analysis evidence, contain appreciable amounts of highly hydrated inorganic material (Fig. la and c). The soil profiles, a podzol with thin iron pan (South Drumboy No. 4) and a non- calcareous humic gley (South Drumboy No. 3), represent the end-members of a hydrologic sequence developed on a glacial till which was derived from intermediate lavas of Calciferous Sandstone Age (Mitchell & Jarvis, 1956).

The clay fraction was obtained by dispersing the soil with ammonia according to the method of Mackenzie (1956) and the bulk of the organic matter removed by treatment with 6 % hydrogen peroxide on a steam bath. To prevent the formation of calcium oxalate during peroxide treatment the clays were first saturated with ammonia using neutral normal ammonium acetate; although this treatment leads to the formation of complex ammonium oxalates in amounts depending upon the orginal organic matter content of the clay, these can be readily removed by hot water washing (Farmer & Mitchell, 1963). Peroxide treatment did not remove the organic matter completely in every instance but that which remained did not interfere significantly with the differential thermal and infrared absorption measurements made on the clays. After water-washing, the clays were ammonium-saturated and then equilibrated at 55% relative humidity for 4 days. Furthermore, at each stage of a chemical dissolution sequence the clays were subjected to the procedure of ammonium saturation and equilibration before physical and chemical tests were carried out to assess the effect of the dissolution treatment.

Soil clays may contain crystalline clay minerals, accessory minerals and amorphous material. Consequently, it is important to assess the effect of dilute alkali treatment on pure samples of the commonly occurring components : kaolinite, illite, montmorillonite, vermiculite, chlorite, goethite, gibbsite, quartz, silica gel, and allophane.

Chemical dissolution techniques 25

The following Na2CO~ dissolution procedure was adopted. A 100 mg sample of clay was placed in a 100 ml polypropylene centrifuge tube (squat pattern) fitted with a screw cap, extracted with 80 ml of 5 % Na~CO~ solution on an end-over-end shaker for 16 hr, centrifuged at 2500 rev/min, and the silica and alumina content of the supernatant liquid determined. The leaching with cold Na2COz solution was con- tinued until the amounts of silica and alumina extracted were minimal (about 0.2 % of the sample); normally three or four extractions were found to be sufficient. The residue from the cold carbonate treatment was retained in the polypropylene centrifuge tube and subjected to a 2-hr digestion with 80 ml of 5% Na2CO~ on a steam bath, centrifuged at 2500 rev/min, and the silica and alumina content of the supernatant liquid again determined. The digestions were repeated until the amounts of silica and alumina extracted were low and constant (about 0-6%); three treatments were usually required. Aliquots of the soil clays and some of the minerals were treated by the alkali dissolution technique of Hashimoto & Jackson (1960), which involves boiling the sample for 2�89 min with 0"5 N NaOH solution, and silica and alumina were again determined in the extract.

The weight losses at 105 ~ C were determined on the peroxidized NH4-saturated clay and on the residues after alkali dissolution. The silica and alumina contents at these different stages were determined colorimetricaUy after fusion with Na2COs; silica was analysed by the ammonium molybdate method of Jeffery & Wilson (1960) and alumina by the aluminon method (Robertson, 1950). The same colorimetric methods were employed to determine the constituents in the dilute alkali extracts.

The differential thermal curves of the clays were determined in nitrogen with a controlled-atmosphere differential thermal apparatus similar to that described by Mitchell & McKenzie (1959). The infrared absorption spectra were obtained from potassium bromide pressed discs using a Grubb Parsons $4 double-beam spectro- meter equipped with a grating for the 3 t~ region and a sodium chloride prism for the 5-16/~ region. X-ray diffraction patterns of the clays were obtained by a modified Debye-Scherrer powder technique, and the electron micrographs with an A.E.I. E.M, 6 instrument. The clays, after being degassed at 90 ~ C, had their specific surface areas measured by nitrogen adsorption according to the method of Brunauer, Emmett & Teller (1938).

RESULTS AND I N T E R P R E T A T I O N

Clay and accessory minerals

The effect of Na2CO3 dissolution technique on pure clay minerals and accessory minerals was assessed by silica and alumina determinations on the extracts. The results are shown in Table 1 and are expressed as percentages of the silica and alumina content of the untreated mineral. As far as the crystalline minerals are con- cerned successive treatments with cold and hot dilute carbonate solution do not result in appreciable dissolution; the largest amount of silica was removed (11.3 %) from the illite and most alumina (16.7%) from the montmorillonite. These figures represent the sum of the silica and alumina removed by the number of extractions which were

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required to reduce the silica and alumina removed to a small and virtually constant level of about 0.2-0.5 % of the sample weight. The results, thus, show that most of the crystalline clay mineral samples examined are relatively unaffected by this alkali dissolution technique. As to the material removed, it is noteworthy that cold Na2CO3 solution dissolved, in general, significantly larger amounts of silica than alumina, while the material removed by hot Na2CO~ digestion contained, except that from the chlorite, a larger proportion of alumina.

The results obtained for carbonate treatment of the allophane indicate almost complete disruption and dissolution. Almost 80% of the alumina was removed, the majority of it by cold alkali, and more than half the silica. Of the remaining silica some must be attributed to the quartz contaminant observed by X-rays as it was found that highly crystalline quartz was unaffected by cold alkali and only very slightly by hot; the amorphous counterpart, silica gel, was readily soluble in cold carbonate solution. The pronounced dissolution of poorly-organized material in comparison with the insolubility of crystalline minerals is again illustrated by the effect on a crystalline sample of gibbsite which was unattacked by either cold or hot carbonate solution.

The silica and alumina removed from the crystalline clay minerals by the Na2CO3 dissolution technique might well be attributed to the presence of X-ray amorphous material in these minerals. Although the rapid fall-off of dissolution with three or four extractions would indicate removal of a separate phase it cannot be decided from the available results whether this disorganized material is closely associated with the crystalline minerals or exists as a separate phase. In only one mineral, kaolinite, was the presence of separate, siliceous, alkali-soluble material established by electron microscopy. There was no evidence from X-ray, infrared, or electron microscope examination that there was any marked dissolution of the crystalline clay minerals although the slight amounts of silica and alumina dissolved on con- tinued extraction must signify a gradual attack on the mineral lattice.

Kaolinite, iUite, and montmorillonite were the only clay minerals subjected to the 2�89 min treatment with 0"5 N NaOH recommended by Hashimoto & Jackson (1960). This method extracted, in general, considerably less silica and alumina than the successsive cold and hot Na2CO3 treatments. Only one treatment was given as it was known (Foster, 1953) that prolonged boiling with 0"5 N NaOH resulted in partial dissolution of clay minerals--in contrast to the Na2CO3 method. The sample of allophane was also subjected to NaOH treatment. Nearly 80% of the sample was removed: apparently NaOH is a more effective extractant of silica than 5 % Na2CO3 under the conditions used. Because of the differences in the effects of the two extractants on these pure soil clay components it was thought desirable to subject the soil clays to both procedures.

Soil clays Peroxide treatment

During the removal of organic matter from soil clays with hydrogen peroxide, soluble and insoluble chelated oxalates of aluminium and iron are formed (Farmer

28 E. A. C. Follett et al.

& Mitchell, 1963); the latter, however, is produced in much smaller amounts. Most of the iron and aluminium in these complex oxalates is undoubtedly associated originally with organic matter, probably as humate complexes (Williams, Scott & McDonald, 1958), but some could arise from the clay minerals. The soil clays used in this investigation were pretreated with peroxide and, in order to detect possible effects of this treatment, the hot water extracts of the peroxide-treated clays were analysed for silica, alumina, and iron. The extracts contained only a trace of silica, less than 1% of alumina, and no iron; this indicated that peroxide treatment had not affected the inorganic soil fraction to any extent.

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120 527 870

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FIG. 1.

Temperature (~

Differential thermal curves for soil days: (a) humic gley B3g horizon, (b) podzol B 3 horizon, (c) highly hydrated soil clay.

Alkali extraction The total amounts of silica and ahmfina extracted by seven cold Na2CO3 treat-

ments and seven subsequent carbonate digestions are given in Table 2 for the clays separated from the various horizons of the podzolic and humic gley soils. The results of six treatments with boiling 0-5 N NaOH are also given. The amounts of silica and alumina removed with each extraction by these various reagents is given in Fig. 2 (a and b) where it is apparent that a rapid decrease occurs with number of extractions, the effect being most marked with 0"5 N NaOH. These curves refer to the clays from the B~g horizon of the humic gley profile and the C1 horizon of the podzol but they are typical of the clays of the two profiles examined. They do not indicate an extensive systematic dissolution of crystalline material but as they level off without reaching zero, this could be taken to indicate a very limited attack

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Chemical dissolution techniques

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29

(b)

2. 3 4 5 6 7 2 3 4 5 6 7 1 2 3 4 5 6 7 2 3 4 5 6 7 Numbel- of extractions

FIG. 2. Extraction curves for: (a) clay from B~g horizon gley, (b) clay from C horizon podzol. (3, 0"5 N NaOH; D, hot 5% Na2COs; • cold 5% Na2CO s.

on highly crystalline material. The small constant values attained differ for each reagent, 0-5 N NaOH > hot Na2CO3 > cold Na2COs, and it is noteworthy that the total amounts of silica and alumina removed also vary with the reagent and in exactly the same sequence. This sequence is identical to the expected chemical reactivity of the reagents and these results thus support the concept of soil clays as a continuum from completely disordered, through poorly ordered, to well crystallized material; the range of this continuum which can be extracted depends upon the reagent employed.

Physical examination o[ residues alter alkali treatment

X-ray examination of the clays of the podzolic soil showed them to be a mixture of kaolinite, illite, vermiculite and a material giving rise to diffuse scattering be- tween 10 and 14 A. Apart from a small amount of hematite found in the Bz, B3 and C horizons no significant differences were observed throughout the profile. The clays of the humic gley profile, which also exhibited a uniform mineralogy, contained 10-20% kaolinite, 5% quartz, and less than 5% hematite; strong, diffuse scattering was again noted from 10 to 14 A and partial collapse of these spacings occurred on heating to 600 ~ C. A randomly interstratified illite-vermiculite may account for these spacings but no specific component could be identified in the 10-14 A range although vermiculite was identified in the clay of Cg horizon.

As far as could be ascertained from X-ray diffraction, dilute Na~CO3 and caustic

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soda treatments of the clays produced similar effects. The background of the diffraction patterns of the treated clays was considerably reduced indicating that disordered material had been removed, but" no measurable change was noted in the ratios of the crystalline components before and after treatment.

The infrared absorption spectra of the clays from both profiles were somewhat similar, a marked kaolinite group pattern predominating. Because of the general similarity only three clays were examined in detail, those from the Ba and C1 horizons of the podzolic soil and that from the Bag horizon of the humic gley. Treatment with cold Na2CO~ solution had the same overall effect on the spectra of the three clays namely: a sharpening of the kaolinite minerals and quartz patterns and a reduction in the absorption in the 2-79-2"93 t~ region, both effects being attributable to the removal of hydrous oxides. There was no evidence of attack on the crystalline material. However, after hot Na2COa treatment some reduction in the intensity of the kaolinite minerals absorption was noted even when allowance had been made for the dilution effect caused by the relative concentration of iron oxide. The reduction in the kaolinite minerals absorption was most marked on the spectrum of the C horizon clay and further evidence that hot carbonate had attacked the kaolinite minerals in this clay was obtained from electron-optical examination, no hexagonal platelets being observed in the clay treated with hot Na2COa. Dissolution of the kaolinite minerals could also account for the somewhat larger amounts of silica (20%) and alumina (22%) found in the hot carbonate extract (Table 2). The effect of treating the clays with 0-5 r~ NaOH was, from infrared absorption evidence, to remove hydrous oxides, and the attack on the kaolinite minerals in the C horizon clay appeared to be less.

Infrared absorption studies also indicated the presence of a gibbsite-like material in the Ba horizon clay of the podzol. Absorption in the 2-79-2.93 t~ region due to this material was observed only after washing the peroxidized clay and was absent in the spectrum of the clay after cold carbonate treatment. Gibbsite was not detected on the X-ray diffraction pattern of the clay: however, the differential thermal curve of the peroxide-treated and water-washed clay showed a small well-defined endotherm about 280 ~ C consistent with gibbsite (Fig. lb). This peak was absent on the curve of the material treated with cold carbonate and relatively large amounts of alumina, 25.3%, were found in the cold carbonate extract; these facts are consistent with the removal of the gibbsitic material. The results suggest that a disordered aluminium hydroxide, in the peroxidized clay, partially recrystallizes during washing with hot water. Because digestion with 5% NaOH solution is required to dissolve highly crystalline gibbsite :from soil clays (Taboadela, 1953), and the free aluminous material in this soil clay was removed by a comparatively mild alkaline reagent, one might perhaps postulate a pseudo-gibbsitic or poorly crystalline gibbsitic structure.

Detailed electron-optical studies were also limited to the Bag horizon of the humic gley soil and the B3 and C1 horizons of the podzol. These showed the peroxide- treated and washed clay of the humic gley (Plate la) was composed of clumps of material with very few translucent flakes. Some very small electron-dense granules,


probably of iron oxide, both free and associated with the larger flakes or clumps were evident. Most of the clumps were too dense for diffraction but the lessdense clumps gave a pattern of diffuse tings denoting that they consist either of masses of small flakes or of large flakes with c-axis rotational disorder. After successive treatment with cold and hot Na~CO~ solutions the material was seen to be much more efficiently dispersed (Plate lb), the specimen being composed principally of a mixture of small flakes, areas of which gave a diffuse ring pattern, and large flakes which gave a spot and ring pattern. The original clumps described above are, therefore, probably composed of masses of fine flakes. Electron-dense granules were again evident, but their number greatly increased. In the original material they must thus have been contained within the dense aggregates and become dispersed by Na.~CO~ treatment. Substitution of 5% NaCI for 5 % NazCO3 gave markedly poorer dispersed specimen and hence dissolution must be a governing factor in providing the improved dispersion after Na2CO~ treatment. Digestion of the clay with 0-5 N NaOH resulted in even better dispersion the bulk of the specimen being composed of very fine, thin flakes. Dense granules were still evident and usually in association with the flakes. Large flakes were also observed and again they showed a spot and ring pattern.

Electron micrographs of the peroxide-treated and washed clays from the Bz and C1 horizons showed that dispersion was poor (Plate 2a). However, the aggregates appeared to be more crystalline than those of the humic gley clay in that distinct edges and corners were evident and also there was a greater number of large flakes. Some electron-dense granules were observed on the aggregates. After treatment with Na~CO3, dispersion was much improved and numerous electron-dense granules were revealed associated with the fine flakes produced by this treatment. As with the humic gley soil these granules must in the original samples have been bound up in the aggregates (Plate 2b).

The specific surface areas of the clays were determined initially and after each stage in the chemical treatment. The results are shown in Table 3. In general, there is an increase in area as the treatment progresses. The initial increase brought about by washing is thought to be associated with the removal of water-soluble oxalates produced by the peroxide treatment, this being most marked in the surface horizons where the organic content is highest. Extraction with cold Na2CO3 produces a significant increase in area, more especially in the podzolic soil, whereas subsequent extraction with hot Na2COz does not increase the values to the same extent. These results can be adequately explained by the increased dispersion, after alkali treatment, already noted in the electron-optical results although the increase is less than would be expected. This suggests that a high proportion of the interior surfaces of the aggregates in the untreated, water-washed clays is accessible to nitrogen.

In this study the specific surface areas of a silica gel, an alumina gel, an alumino- silica gel (Sio~/A1203 = 0"3) and a naturally occurring allophane were found to be 500, 200, 150 and 200 m2/g, respectively, compared to average values of 20 m2/g for kaolinites, 90 m2/g for illites and 80 m2/g for montmorillonites. It would, therefore, be expected that removal of any highly disordered material of this type would produce a sharp decrease in surface area. The converse effect observed in these

PLATE 1

Electronmicrographs of humic gley B3g horizon clay: (a) before treatment with 5% Na2CO3, (b) after treatment.

(Facing p. 32)

PLATE 2

Electronmicrographs of clay from C 1 horizon of podzol: (a) before treatment with 5% Na2CO 3, (b) after treatment.

Chemical dissolution techniques TABLE 3. Specific surface area (mS/g) of soil clays after various treatments

33

Profile Cold 5% Successive

Horizon Peroxidized HaO washed Na~CO3 cold and hot 0"5 N NaOH 5% Na~CO3

Podzol As 4 58 87 91 96 with~on pan B~ 48 98 104 104 98

B3 49 73 96 91 92 C1 39 55 75 89 87 C~ 43 56 75 87 91

Humicgley Ag 36 82 80 85 99 B~g 57 78 86 86 99 B3g 73 89 86 97 100 Cg 67 64 75 72 88

results would indicate that the mater ial being removed, if highly disordered, cannot be present as a finely divided, porous, separate phase.

DISCUSSION AND CONCLUSIONS

The improved dispersion of the clays which result from alkali dissolution was accompanied, from X-ray evidence, by a reduction in the amorphous material content of the clays, from infrared absorption, by a slight reduction in the amount of hydrous oxides, and from nitrogen absorption, by an increase in specific surface area. Chemical evidence points to the dissolution of appreciable quantities of poorly organized aluminosilicates of variable composition, the results of cold and hot NazCO~ treatments, in particular, emphasizing the heterogeneity of the disordered material. Moreover, because electron-optical examination did not reveal in the untreated clays any evidence of gel or other disordered material but rather dense masses of fine flakes, it must be concluded that most of the poorly ordered aluminosilicates removed by alkali are present in the original material as coatings on the surfaces of the flakes, serving to cement these primary particles into ill-defined aggregates.

The results given in Table 2 show that up to 25% of the silica and 38% of the alumina can be removed from a soil clay by successive extraction with cold and hot NazCO3 solution. Similar extractions of the pure clay minerals and accessory minerals, identified in the soil clays, indicate resistance to the reagent and the only aluminosilicates found to have any appreciable solubility are silica gel and allophane. Although the inherent complexity and heterogeneity of soil clays makes direct cor- relation of results determined on pure minerals and on soil clays extremely difficult, the above chemical results at ]east indicate the presence in soil clays of a poorly ordered component (or components) reactive to mild alkali. The physical techniques c


show that the well crystallized components are not generally the source of alkali- extractable silica and alumina.

The surface area measurements and the electron-optical studies both indicate that, in physical nature, this alkali-soluble material differs from normal finely particulate allophane. As it was noted in the electron-optical studies that no separate, disordered particulate phase was present in any of the soils, the only explanation consistent with this observation and the low specific surface area of the extracted material is that the readily soluble aluminosilicate extracted by alkali is closely associated with the clay particles as a film or coating. The observation that Na +- saturated clays were less well dispersed than Na2CO~ extracted clays indicates these alkali-soluble aluminosilicate coatings serve to bind the crystallites and flakes into aggregates. These aggregates must only be loosely bound together with a large proportion of the surface coatings accessible to nitrogen as the electron-optical results would predict a very much larger increase in surface area after alkali treatment than was measured. I t could be inferred from the greater increase in observed area, after cold carbonate treatment, of the podzolic soil compared to the humic gley that a higher proportion of the surface coating in the podzol is involved in binding and this may be attributable, at least in part, to the influence of the iron present in the clay. The removal and nature of the iron oxides in these soil clays forms the second part of this investigation.

A C K N O W L E D G M E N T S

The authors wish to thank Dr V. C. Farmer for the infrared absorption determinations and Mr A. P. Thomson for supplying the X-ray information.

R E F E R E N C E S

BRUNAUER S., EMMETT P.H. & TELLER E. (1938) J. Am. chem. Soc. 60, 309. FARMER V.C. & MITCHELL B.D. (1963) Soil Sci. 96, 221. FOSTER M.D. (1953) Geochim. cosmochim. Acta 3, 143. HARWARD M.E. & THEISEN A.A. (1962) Proc. Soil Sci. Soc. Am. 26, 335. HASHIMOTO 1. & JACKSON M.L. (1960) Clays and Clay Minerals. Proc. 7th Con/., p. 102. Pergamon

Press, Oxford. JACKSON M.L. (1956) Soil Chemical Analysis--Advanced Course. Published by Professor Jackson,

Madison, Wisconsin. JEFFERY P.G. & WmSON A.D. (1960) Analyst, Lond. 85, 478. MACKENZIE R.C. (1956) Clay Miner. Bull. 3, 4. MITCHELL B.D. & FARMER V.C. (1962) Clay Miner. Bull. 5, 128. MITCHELL B.D. & JARVlS R.A. (1956) The Soils of the Country around Kilmarnock. Mem. Soil

Surv. Scot., H.M.S.O., Edinburgh. MITCHELL B.D. & MACKENZIE R.C. (1959) Clay Miner. Bull. 4, 31. ROBERTSON G. (1950) J. Sci. Fd Agric. 1, 59. TABOADELA M.M. (1953) J. Soil Sci. 4, 48. WILLL~MS E.G., ScoTr N.M. & MCDONALD M.J. (1958) J. Sci. Fd Agrie. 9, 551.

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