ANOMALOUS FLOCCULATIOX I N COLLOIDAL CLAYS AND SOILS BY F. HARDY
Introductory Anomalous flocculation is the term applied by Comber‘ to denote the particular behavior of aqueous suspensions of colloidal clay towards lime. According to the isoelectric point theory, aqueous disperse systems of electronegative colloids should be stabilized, not flocculated, by small additions of alkali. Clay suspensions, however, are rapidly flocculated by calcium hydroxide, though markedly stabilized by hydroxides of alkali metals and ammonium. By a series of critical experiments with soils, Comber was able to demonstrate that fine silt belongs to the suspensoid class of colloids, whilst clay, though not necessarily composed entirely of emulsoid matter, consists of particles so heavily protected by emulsoid matter, that it behaves as an emulsoid colloid2. In order to explain anomalous flocculation, Comber has suggested that a chemical interaction takes place between the emulsoid material and its flocculants. The fact that silicic acid in aqueous dispersion reacts with calcium hydroxide, or with neutral calcium salt in the presence of soluble alkali hydroxide, to form a flocculant precipitate of calcium silicate, is adduced as evidence that a similar reaction may account for the anomalous flocculation of siliceous clay by lime. Hydroxyl-ion peptizes the emulsoid protection of the clay particles; calcium-ion unites with, or is “adsorbed” by the highly dispersed silica; the emulsoid loses its intimate relationship with the dispersion medium, and the particles settle, entrained in the voluminous coagulum. Anomalous flocculation by lime is apparently not confined to emulsoid clay suspensions. Comber’s further investigations4 have shown that aqueous suspensions of aluminium phosphate, ferric phosphate, basic slag, rock phosphate and powdered bones display similar anomality, provided they are initially free from uncombined lime. In certain instances, ignition does not impair the reactivity of the substance sufficiently to prevent its exhibiting anomalous behaviour in flocculation experiments. It therefore appears that emulsoid soil clay possesses some constitutionzl feature in common with many phosphatic materials. Recently the writer has extended the scope of Comber’s suggestive researches by demonstrating that certain tropical lateritic soils, examined by the methods elaborated by Comber, exhibit marked anomalous flocculation by lime. The lateritic soils studied have been fully described elsewhere6. They were two in number. The first is an orange-red subsoil occurring in the J. Agr. Sci. 10, 425 (1920); Trans. Faraday SOC., 17, 349 (1922). Sei. 10, 426-430 (1920). J. Agr. Sei. 11, 450 (1921); Trans. Faraday Soc., 17, 349 (1922). J. Agr. Sei. 12, 372 (1922). F. Hardy: J . Agr. Sei 13, 243, 340 (1923).
* J. Agr.
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upland regions of Barbados. It is a residual clay derived from coral limestone containing andesitic volcanic ash. The second is a dark-red subsoil occurring in the rainy mountainous regions of Dominica, where andesite has been intensely weathered. Both soils are acidic (pH 6.7 and pH j . 9 in I to 3 aqueous suspension) ; they contain traces only of organic matter and lime, but are rich in alumina and ferric oxide. They also contain appreciable amounts of silica. Their aqueous suspensions in all concentrations settle rapidly; they are stabilized markedly, however, by the addition of relatively small but different amoun.ts of soluble alkali, such as sodium hydroxide and sodium carbonate. In the field, the soils, in coinmon with lateritic soils in general, occur in a highly aggregated condition, so that water percolates readily through them. The chemical constitution of the colloidal matter of clays and soils is uncertain. conventional chemical analysis merely expresses Lhe fact that inorganic soil colloids are mainly composed of varying proportions of water, silica, alumina and ferric oxide. Whether these components are simply mixed, or are combined to form colloidal complexes or true compounds, cannot be decided by precise experimental methods. Nevertheless, there is sufficient evidence to prove that many highly siliceous clays and soils contain free hydrous silica, and that many laterites contain free hydrous alumina and free hydrous ferric oxide. A comparison of the physico-chemical features and constitution of these individual substances may therefore lead to a clearer understanding of the properties and behaviour of clays and soils, and in particular to a more exact elucidation of their flocculation phenomena.
I. Hydrous Alumina Alumina, in aqueous dispersion, behaves as a weak amphoteric electrolyte. The basic and acidic dissociation equations are usually written thus1:A1(OH)3+A103’” 3H’ and AI(OH)3--tAl“‘ 3(OH)’. The isoelectric point of alumina has been determined by von Euler and Nilsson2, who assign to it the pH value 6.50. This implies that the basic and acidic dissociation constants are numerically equal3when the reaction of the aqueous dispersion medium is pH 6.50. The aluminates are generally regarded as salts of the tribasic acid, Heyrovsky4 suggests, however, that they are more probably salts of the monobasic acid, HA1(OH)4, or A1(OH)3.Hz0,and that they are really “additive compounds”. Sodium aluminate, for example, according to tlhis authority, should be represented by the formula Al(OH)3. XaOH. The composition and properties of several hydrogels of alumina, prepared under different conditions, have recently been investigated by Willstatter and Kraut5, who obtained evidence that the different dry hydrogels are true
+
+
Lewis: “A System of Physical Chemistry”, 1, 26 (1921). Ber. 57, 217; Chem. hbs. 18, I 2 5 4 (1924). 3 Clark: “The Determination of Hydrogen-ions”, 25 (1920). J. Chem. SOC., 117, 1013 (1920). 5Ber. 56, 149, 1 1 1 7 (1923); 57, 58 (1924). 1
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hydrates, containing various molecular proportions of water, and exhibiting diminishing basic and acidic properties with diminishing degree of hydration. Weiser‘ has adduced corroborative evidence of the fact that the reactivity of hydrogels of alumina depends on the degree of hydration, but states that only one true hydrate, namely the trihydrate of alumina, exists. Pascdz, basing his conclusions on the results of certain magnetometric experiments, maintains that neither the hydrates nor the hydrogels of alumina are to be regarded as aluminium hydroxide, but as various hydrous aluminium oxides, and that, even in solutions of aluminates, aluminium occurs as colloidal oxide. This view is upheld by earlier results obtained by Chatterji and Dhar3, who found tha,t the electrical conductivity of a solution of sodium hydroxide is not appreciably altered when hydrous alumina is dissolved in it, proving that true chemical combination does not occur, but that the alumina is merely peptized by the alkali. The aluminates thus appear to be adsorption complexes rather than the additive compounds of Heyrovsky. I n accordance with these findings, it would perhaps be more exact to express, in its simplest form, the dissociation of hydrous alumina thus: A1203 . 3H20-+A1203. 3OH’ 3 H ‘ , and &OS. 3Hz0--t&O3. 3H’ 3(OH)’ +2Al‘ ’ ’ 3(OH)’ 3(OH)’. Since the chemical activity of hydrous alumina dispersed in water appears to be mainly decided by the degree of colloidal hydration, it follows that the primary cause of reactivity is the ionic dissociation of molecules of water that are held at the surface of the solid phase, The relationship be tween these bound water molecules and the molecules or atoms comprizing the rest of the colloid is obscure. Evidently few analyses of the structure of hydrous inorganic compounds by X-ray methods have so far been attempted4. It appears probable, however, that the elements of water are held by residual valency forces of atoms that form the surface layer of the crystal lattices composing the colloid particles5. Irregular orientation of these atoms, resulting from the action of agents of disintegration6, which produce higher degrees of dispersion at the solid crystal surface, may conceivably increase the hydration capacity, and bring about in the particles of the dispersed phase’, “a gradual transition from the solid nucleus through stages of gel material to the water”. Conversely, alumina hydrogels are known to undergo spontaneous change on standing, becoming gradually crystalline, but not necessarily losing all their waters. This process, usually termed “ageing”, appears to be mainly a manifestation of atomic rearrangement, whereby the
+
+ +
+
J. Phys. Chem. 24, 505 (1920). Compt. rend. 178, 481 (1924). Discussion Rept. “Physics and Chemistry of Colloids”, Dept. Sci. Indus. Res. London, 1921, 123. Bragg: “X-rays and Crystal Structure”, 303 (1924). 5 Ruff: Kolloid-Z. 30,356 (1922); Mukherjpe: Discussion Rept. “Physics and Chemistry of Colloids”, 1921, 103. Bassett: “Fourth Report on Colloid Chemistry”, 8 (1922). Donnan: Presidential Address. Sect. B, British Ass. Adv. Sci., (1923); J. SOC.ChemTnd. 42, 900 (1923) ; Harrison: Discussion Rept. “Physics and Chemistry of Colloids”, 1921, 116. Comber: J. Agr. Sci. 11, 450 (1921). 8Fricke and Wever: Z. anorg. Chem. 136, 321 (1924). 1
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elements of water come to occupy a more stable spacial relationship to the other atoms of the system. We are thus led to regard a particle of hydrous alumina,suspended in an aqueous medium, as a complex colloidal electrolyte, whose reactivity or strength depends on the degree of hydration a t the boundary surface. The total area of surface, and consequently the hydration capacity, will depend on the degree of dispersion,or,if we assume that the hydrogel possesses a reticulate structure,l on the conformation and porousness of the hydrogel skeleton. Recent research on emulsoid colloids such as soaps2,proteinss, and starches4, has revealed the existence of complex colloidal ions (the ionic micelles of McBain), which originate through the electrolytic dissociation of neutral colloidal systems dispersed in aqueous media. Such ions, in certain instances, niay apparently occur in association with un-ionized colloid. At the isoelectric point, the colloid exists as a non-ionogenic substance. Khen the hydrogen-ion concentration of the medium is decreased, the colloid behaves as an acid; when increased, it behaves as a base. In the case of hydrous alumina, whose isoelectric point is pH 6.5, the colloid, when placed in pure neutral water (pH 7 . 0 ) , should therefore exhibit weak acidic properties, because ionization now tends to proceed in the direction of greater production of hydrogen-ion and complex aluminate-ion. Further increase in hydroxyl-ion concentration, brought about for example, by adding a soluble hydroxide, should enhance this tendency. If the added soluble hydroxide be calcium hydroxide, calcium-ion may simultaneously combine with aluminate-ion to form a calcium-aluminate complex. The electric charges on the colloidal ion mill consequently be more or less neutralized5; the system will be “weighted”; its stability will be reduced; its relationship to the enveloping liquid will be altered through diminished hydration, and precipitation will tend to occur. By this sequence of events is the term anomalous flocculation defined. Should the added hydroxide be the hydroxide of an alkali metal, accompanied or followed by a neutral calcium salt, calcium-ion may combine in preference to alkali metal ion, because of its lower electroaffinity.6 The final result nevertheless, in effect, should he the same. If neutral calcium salt be added before alkali hydroxide, the reaction may be impeded, because the first-formed calcium-aluminate complex may conceivably hinder the further and continued dissociation of the colloidal electrolyte.
* Wilsdon: Mem. Dept. Agr. India, Agr. Res. Inst. Pusa, Chem. Ser.6 (1921);F. Hardy:
J. Agr. Sci.
’
13, 243 (1923). McBain: J. Am. Chem. SOC.,43, 426 (1920); “Third Report on Colloid Chemistry”, 2 (1920); J. SOC.Chem. Ind. 42, 615 (1923). 3 Pauli: “Colloid Chemistry of the Proteins” (1922); Loeb: “Proteins and the Theory of Colloidal Behavior” (1922) ; Discussion Rept. “Physics and Chemistry of Colloids”, 153 (1921 ) . Samec: ref-Pauli: Discussion Rept. “Physics and Chemistry of Colloids”, 1921, 16. Donnan has pointed out that coagulation may occur as soon as the potential difference a t the interface of a suspended colloid is brought within a certain critical zone which may extend considerably on either side of the jsoelectjric point, (Presidential Address, Sect. B, British Ass. Adv. Sci. (1923); J. Soc. Chem. Ind. 42, 898 (1923). Bassett : “Fourth Report on Colloid Chemistry”, 9 (1922); Mukherjee: Discussion Rept. “Physics and Chemistry of Colloids”, 1921, I I I .
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On the acid side of the isoelectric point, complex aluminous cations may combine with acid anions to form salts. According to Paulil, a series of oxysalts of aluminium, such as Al(OH)CL, Al(OH)&l and z A ~ ( O H. )Al(OH)LY, ~ is formed when hydrous alumina is peptized by acids. If the process is further continued, a hydrosol results. Pauli regards this sol as an aqueous suspension of a mixture of oxysalts, ionizing to give a complex cation, together with acid anions, as for example, 3A1(OH)3.A1(OH+)z/CH3.COO-, but so far his conclusions have not been experimentally substantiated, I n the parallel case of ferric oxide sols, Smith and Giesy2 have recently shown by the use of the oxygen electrode, that oxysalts have no real existence in such sols. Most authorities3 prefer to regard the sols of hydrous metallic oxides as composed of particles of oxide suspended in an aqueous phase, and adsorbing greater or lesser amounts of molecules or ions (water, metallic chloride, hydrogen chloride, hydrogen-ion, chloride-ion) , depending on their nature and concentration. This view is based on the widely accepted physical theory of adsorption, which Mukherjeed, has attempted to place on a fundamental basis by suggesting that adsorption may be an actual attachment of ions to a colloidal surface by chemical forces due to residual valences. Whilst it is highly probable that direct chemical combination often occurs at surfaces, it seems more reasonable to assume in the case of a hydrated colloidal electrolyte such as hydrous alumina, that peptization by acids is the result of the enhancement of its acidic dissociation through the effect of hydrogen-ion, which, when introduced in excess, combines with hydroxyl-ion, produced by the ionizing colloid, to form non-ionizing water molecules, The colloid is thereby removed further from its isoelectric point, the potential difference between its surface and the dispersion medium is increased, and the system becomes increasingly stable. The concurrent effect of the anion of the added acid is probably mainly decided by its specific nature and by its relative electroaffinity. Thus, in particular, if the anion be phosphate-ion, silicate-ion, or aluminosilicate-ion, insoluble complexes may result, and precipitation may occur. Each of these particular cases is entirely analogous to the anomalous flocculation of the negatively-charged colloidal aluminous-ion by calcium-ion, which has already been discussed, If the anion have other identity, its preferential combination and consequent precipitating power, may depend on its relative electroaffinity. Only in dispersions of high acidity may simple aluminium salts, such as the metallic chloride, be produced, giving aluminium-ion, Thus McGeorgeEwas able to demonstrate that free aluminium-ion is present in extracts of acid soils only when the reaction is more acidic than that represented by pH 5.8. Finally, before the flocculation of positively charged ions of alumina, contained in sols or their transition systems, can be effected by the addition of 1 Discussion Rept. “Physics and Chemistry of Colloids”, 1921, 1 4 ; Adolf and Pauli: Kolloid-Z. 29, 281 (1921). 1 J. Am. Pharm. Ass. 12, 855 (1923). 3Bancroft: J. Phys. Chem. 19, 232 ( 1 9 1 5 ) ;Browne: J . Am. Chem. Soc., 45, 297 (1923). Discussion Rept. “Physics and Chemistry of Colloids”, 1921, 103. 5 Soil Science, 18, I (1924).
ANOMALOCS FLOCCULATION IN CLAYS
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lime, sufficient flocculant must be added, not only to change the reaction of the dispersion medium to the pH value representing the approximate isoelect,ric point, but also to reverse acidic dissociation into basic dissociation. This necessitates the satisfying of a definite “lime requirement”, the magnitude of which will depend mainly on the initial acidity of the system, on the presence of ions already adsorbed, and on the extent of its active surface, that is, on the degree of dispersion and hydration. That such conditions control the successful liming of soils, is well known to soil investigators. 11. Hydrous Ferric Oxide Like hydrous alumina, hydrous ferric oxide is an amphot’eric electrolyte. Whilst its basic properties are prominent, it’sacidic properties are not so pronounced as those of aluminal. The “ferrites” are less easily prepared than their analogues, the “a1uminaOes”. As far as the writ’er is aware, t’he isoelectric point of hydrous ferric oxide has not yet been determined. The conditions for precipitation of hydrous ferric oxide and of hydrous alumina by alkali, however, are almost identical. Thus, Carr and Brewer2 found that, in the one case, precipitation commences at pH 5.5 and is complete a t pH 8.6, and in the other, at pH 5.5 and pH 7.9. This suggest’s t’hat t’he isoelectric points of the two ampholytes are approximately the same. The state of occurrence of water in hydrous ferric oxide appears to be similar to that in hydrous alumina3, but only one definit’e hydrate of ferric oxide has so far been described4. This is found in crystalline form as the mineral goethite, F e z 0 3 .Hz05, whose amorphous form is limonite6, and whose anhydrous form is haematite. Other hydrated ferric oxide minerals, according to Weiser, are probably hydrous oxides, containing variable and indefinit’e amounts of water’. Ferric oxide hydrogels possess the characteristic property of instability, being easily dehydrated to the monohydrate, which exhibits no tendency to reabsorb waters. This property, which may be termed rapid ageing, finds expression in t’hewell-known method for the quant’itativeseparation of iron from aluminium, in which the hydrogels of t’he oxides are first precipitated and then boiled wit’h excess of caustic alkali. Alumina, hydrogel is thereby peptized, whereas ferric oxide hydrogel is dehydrated and remains in suspeiisiong. The yellow, red, and brown colours of hydrous ferric oxide are believed to be due to diminishing degrees of hydrat’ionlo. M7eiser11,however, was able to Caven and Lander: “Systemat,ic Inorganic Chemistry”, 316 (1921). (16,233 (1924)) gives narrower limits for the stability of the solid phase of hydrous alumina, namely pH 5.7 and pH 7.3. 3Weiser: J. Phys. Chem. 24, 277 (1920). Posnjak and Merwin: Am. J. Sci. 47, 311 (1917). Spencer: Min. Mag. 18, 339 (1919). 6 Fischer: abdracted in J. Chem. SOC. 96, 241 ( 1 9 ~ 9 ) . 7 J. Phys. Chem. 24, 277 (1920). * Zsigmondy-Spear: “Chemistry of Colloids”, 169 (1917). Bassetl: “Fourth Report on Colloid Chemistry”, 18 (1922). 10 Fischer: Ioc. cit. l1 See also Bancroft: “Applied Colloid Chemistry”, 205 (1921); Bradfield: 3. Am. Chem. soc., 45, 965 (1923).
* Ind. Eng. Chem. 15,634 (1923); Hatfield
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prepare transition forms of the substance, exhibiting colours ranging from yellow to red, by merely increasing the size of the particles, and Yoel found that red coloured forms could be produced from finely divided yellow particles by agglomeration. There is evidently some close relationship between colour and degree of colloidality in the various types of hydrous ferric oxide, and this should be reflected in the physical and chemical properties of laterites. Because of their amphoterism, and the close resemblance between hydrous ferric oxide and hydrous alumina as regards the state of occurrence of water in the hydrogel, the general properties and structure of disperse systems of these emulsoid colloids are markedly similar. Thus, the much-studied ferric oxide sols, in which the dispersed phase is positively charged, conform closely in character to the sols of alumina, and have recently been investigated from similar standpoints*. The ampholytic nature of hydrous ferric oxide has been established beyond doubt by the researches of Fische~-3,Powis4 and Dhar and Sen6,who have demonstrated that a reversal of sign of charge can be effected in disperse systems of this colloid by simply reversing the reaction of the dispersion medium. It therefore follows that the particular mechanism which has tentatively been propounded to explain anomalous flocculation of hydrous alumina, might equally well apply to the anomalous flocculation of hydrous ferric oxide. In actuality, the process is certainly not so simple as a consideration of the ideal cases suggests, for subsidiary factors, introduced by the presence of extraneous salts and colloidal matter, doubtless modify the sequence of changes that has been described. The precise manner in which such subsidiary factors may affect the flocculation of disperse systems of hydrous ferric oxide, (and of hydrous alumina), has received considerable attention by recent investigators6. Although mainly viewed from other aspects, the results obtained are susceptible of explanation in terms of ordinary ionic interaction and mass influence, regard being paid to the relative electroaffinities of the reacting units’. In parallel cases, certain irregular results have been shown to be due to an actual entrapping of solutions of unionized electrolytes in the intermicellar spaces of the colloid*. 111. Hydrous Silica
Hydrous silica differs chemically from hydrous alumina and hydrous ferric oxide in that its acidic properties are much more pronounced than theirs, whilst its basic properties, in most of its reactions, are in entire abeyance. J. Phys. Chem. 25, 196 (1921). Malfitano: Z. physik. Chem; 68,232 (1909); Neidle: J.Am. Chem. SOC.,39,2334 (1917); Pauli and Matula: Kolloid-Z. 21, 49 (1917); Pauli: 28,49 (1921);Weiser: J. Phys. Chem. 24, 277 (1920);Browne: J. Am. Chem. SOC.,45, 297 (1923). Biochem. Z. 27, 223 (1910). J. Chem. SOC., 107, 818 (1915). 6 J. Phys. Chem. 27, 376 (1923); Ganguly and Dhar: 28, 313 (1924). 8 Seidle, Pauli, Dhsr, Browne: loc. cit.; Weiser et al.: J. Phys. Chem. 23, 205 (1919); 24, 30, 630 (1920); 25, 399 (1921); 26, 66.5 (1922). 7 Bassett: “Fourth Report on Colloid Chemistry”, 9 (1922). * Wintgen: Z.physik. Chem. 103, 238 (1922).
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Hence hydrous silica is usually not regarded as an amphoteric electrolyte, but as a true acid. Xevertheless, the similarity that exists between compounds of silicon and corresponding compounds of germanium, tin and lead, (especially the halidesl), suggests that silicon may be considered as a basigenic element of very low electroaffinity. Furthermore, there is some direct evidence in support of the view that hydrous silica is an ampholyte possessing an isoelectric point well on the acid side of neutrality. Thus Losenbeck2 has demonstrated by cataphoretic methods that the addition of hydrochloric acid to a silica sol brings about a gradual decrease in the magnitude of the negative charge on the silica particles, and that eventually a stage which appears to be coincident with the isoelectric point of the colloid, is reached. Further additions of hydrochloric acid are stated to cause the particles to assume a positive charge. The pH value of the isoelectric point is apparently not given by Losenbeck. Schwarz3 found no evidence, however, of the formation of a definite chemical compound be tween colloidal silica and hydrochloric acid, but llTerner4noted that acids exert a powerful peptizing effect on silica hydrogel. RecentlyMichaelis6has suggested that colloidal electrolytes might be classified into “ampholytoids”, “acidoids” and “basoids”. He places hydrous alumina and hydrous ferric oxide in the first category, hydrous silica in the second, but can find no examples of the third class If we regard amphoterism as a property possessed in different degree by certain colloidal electrolytes, and manifesting itself in different pH values for their isoelectric points, there would appear to be no particular advantage in this classification, at least insofar as hydrous oxides are concerned. The state of occurrence of water in hydrous silica, as in the case of the other hydrogels considered, has been the subject of much experimentation and speculation. The characteristic changes in moisture content, vapour pressure, volume and physical properties which silica hydrogel undergoes when dried, are too well known to require repetition6. They may perhaps most clearly be explained on the assumption that silica hydrogel possesses a reticulate structure’, which, when saturated, contains water in two phases, the one adsorbed at the surface of the material comprising the gel framework; the other filling the intermicellar spaces, It is generally conceded that no true hydrates of silica exist*. Nevertheless, many authorities favour the view that definite silicic acids, such as orthosilicic acid (H4SiOI), are present in silica hydrosols and hydrogelsg. The solubility ‘Huddleston and Bassett: J. Chem. Soc., 119, 403 (1921). (1922). Kolloid-Z. 34, 2 3 (1924). J. Am. Pharm. Ass. 17, go1 (1920). Kolloid-Z. 31, 256 (1922). Zsigmondy-Spear: “Chemistry of Colloids”, 137-1 j o (1917). Van Bemmelen, Zsigmondy, Butschli, Bachmann, Anderson and Patrick, VC’illiams: J. SOC.Chem. Ind. 43. 97 T (1924). . _ ,, Hutchinson: Ann: Rept. Chem. Soc. London, 9, 259 (1922); LeChatelier: Compt. rend. 147, 660 (1908). Norton and Roth: J. Am. Chem. SOC.19, 832 (1897); Schwartz and Sonard: Ber. 53, I (1920).
* Kolloidchem. Beihefte, 16, 27
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of silica in water has been measured‘; the dissociation constants estimated2. and the reaction of a silica sol freed from electrolytes has been examined3, and found to be acidic (pH 6. j ) . Other authorities deny the existence of definite chemical compounds of silica and water, and prefer to regard all associations between them as adsorption complexes4. The question, however, appears to be mainly one of nomenclature. As in the case of other hydrous oxides, hydrous silica may contain water molecules united with silica in stoichiometric proportions through the agency of “chemical” adsorption forces. These complexes may exhibit acidic properties because of the ability of certain of the combined water molecules to ionize. The splitting off of small unitsof the adsorption complex (superficial dispersion) may account for the slight solubility of the substance in water. The reactivity of hydrogels of silica is believed to depend mainly on the degree of hydration. Thus Tschermak, may years ago, demonstrated that the various silicic acids decolourize solutions of methylene blue to an extent varying directly with the number of hydroxyl groups presents. Furthermore, it has been proved that the reactivity of quartz is increased by long grinding with watexO. On the other hand, silica hydrogels rapidly age into hydrophane’ or into quartz8. These many considerations indicate that hydrous silica conforms closely to the type of emulsoid colloid exemplified by hydrous alumina and hydrous ferric oxide. It may therefore also be classified as a colloidal ampholytic hydrous oxide, differing from the others chiefly jn that its isoelectric point is of very much lower pH value than theirs. According to Comber, hydrous silica probably occurs in siliceous soils and clays as a gelatinous matrix binding together discrete mineral grains to form separate compound particles. The grains themselves may exhibit zonal differentiation as regards degree of weathering and hydration; their cores are presumably composed of anhydrous quartz or of siliceous minerals such as felspar. In aqueous suspension, the compound particles may behave as electronegative colloidal ions, because they will assume the properties of the emulsoid material that coats and protects them.
IV. Hydrous Aluminosilicates and Ferrosilicates During the processes of weathering, the mineral fragments of soils are hydrolysed and hydrated; the soluble bases that are liberated are simultan‘Huddleston and Bassett: J. Chem. SOC., 19, 832 (1897); Joseph and Hancock: 123, (1923). Grunhut: Chem. Abs. 8, 3829 (1914). Rradfield: J. Am. Chem. Soc., 44, 965 (1922). Lenher: J. Am. Chem. Soc., 43, 391 (1921); Pascal: Compt. rend. 175, 814 (1922). 5 Z. physik. Chem. 53, 349 (1905). Bancroft: “Applied Colloid Chemistry”, 97, 179 (1921); Lenher: J. Am. Chem. Soc. 43, 391 (1921). Bachmann: Chem. Abs. 12, 1610 (1918); Schwarz and Stomener: Kolloidchem. Beihcfte, 19, 171 (1924). Zsigmondy-Spear : “Chemistry of Colloids”, 137 (1917). 2022
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eously removed by leaching. Hydrous aluminosilicates and hydrous ferrosilicates are frequently present in the residues. A conspicuous end-product of weathering is amorphous kaolinite (clayite, lithomarge, halloysite) , which has long been considered by some authorities to be an aluminosilicic acid, forming salts of zeolitic type1. The researches of Gedroiz2and others suggest, however that zeolites are adsorption compounds, rather than true salts, for they exhibit the phenomena of basic exchange in marked degree. Highly leached clays, such as laterites, contain only traces of alkali and alkaline earth metals, and in extreme cases, appear to consist almost entirely of hydrous alumina and hydrous ferric oxide in a more or less “aged” condition. The majority of lateritic clays also contain silica. Certain investigators have recently suggested that aluminosiliceous clays are colloidal ampholytes, and, to support this view, have adduced experimental evidence, based mainly on measurements of hygroscopicity and of settling rates of clay suspensions at different hydrogen-ion concen t r a t i o d . From their results, values for the isoelectric points as diverse as pH 9.7 (brick clay, Arrhenius) and pH 2 . 7 8 (ball clay, Hall), have been derived. Although the validity of conclusions based on such experiments may be questioned, it is now a t least generally admitted that hydrous aluminosilicates, (and hydrous ferrosilicates), react as true acids when dispersed in water, and that acidity in mineral soils is mainly due to their presence4. If hydrous aluminosilicates be ampholytes, possessing basigenic as well as acidic properties, their aqueous suspensions ought to be increasingly stabilized by strong acids, added in sufficient amount to lower the pH value of the system to the acid side of the isoelectric point or region. Durham’s discovery (1874), recorded by Wolkoffj, is therefore of significance in this connection; he found that “although it requires a very small amount of sulphuric acid to flocculate a suspension of white clay (? kaolin), on further additions of sulphuric acid he reached a point when the suspension did not clarify for a long time”. Possibly also, certain results obtained by Bradfield, who examined the relation of hydrogen-ion concentration to the flocculation of a colloidal clay6,may partly be interpreted in the light of these considerations. The whole question of the possible amphoterism of hydrous aluminosilicates might profitably be re-examined with the aid of physico-chemical instruments of high precision. Certain aluminosiliceous clays are probably composed mainly of adsorption complexes of hydrous alumina and hydrous silica, rather than true hydrous aluminosilicates in which alumina and silica are present in molecular or atomic combination. It has already been pointed out that hydrous alumina, when Mellor and Holdcroft: Trans. Eng. Ceramic SOC.10, I O (1911); Scurti: J. Chem. SOC. 124 I, I047 (1923). See Joffe and McLean: Soil Science, 18, I (1924). a Arrhenius: J. Am. Chem. Soc. 44, 521 (1922); Hall: J. Am. Ceramic Soc. 6, 989 (1923). Crowther: Trans. Faraday SOC.,17, 317 (1922); Page: Ann. Rept. Chem. SOC.London, 9, 19 (1922); 10,20 (1923); Bradfield: J. Am. Chem. SOC.45, 2667 (1923). Soil Science, 1, 585 (1916). 6 J. Am. Chem. SOC.45, 1243 (1923).
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dispersed in an aqueous medium of pH value less than that representing the isoelectric point of the colloid, may suffer “anomalous flocculation” by silicateion. Mutual precipitation of hydrous alumina and hydrous silica ought therefore, in theory, to occur in an acid medium of pH value lying somewhere between the pH values representing their isoelectric points, and would be complete if the colloids were present in equivalent amount Under such circumstances, the one colloid occurs as a complex cation, the other, as a complex anion. Similarly, mutual precipitation may occur, under appropriate reaction conditions, in dispersions containing hydrous ferric oxide and hydrous silica, and, indeed, in dispersions containing any assortment of these various hydrous oxides and hydrous aluminosilicates and ferrosilicates, granting that each of these colloidal substances possesses amphoteric properties. The facts that most lateritic clays and soils containing alumina, ferric oxide and silica occur in a highly aggregated condition in the field, and yield aqueous dispersions that rapidly settle, even in the presence of acid in excess, lend support to these conclusions. It ought to be possible to test the various suggestions made in the foregoing discussions by a detailed examination of selected soil and clay samples. In particular, laterites seem to offer peculiar advantages for study, and their flocculation phenomena are worthy of further investigation. Note on Phosphatic Compounds Comber’s discovery that certain phosphatic materials exhibit anomalous flocculation in aqueous suspension, indicates that phosphates may exist as colloidal electrolytes, and may perhaps be amphoteric. The researches of Bassett2 demonstrate that rock phosphate and bone consist mainly of either true compounds or adsorption complexes of tricalcium phosphate, cslcium hydroxide and water. The water, at least, appears to be adsorbed, and this suggests that the phosphatic compounds may be of the hydrous oxide type, although some other relationship may account for their peculiar properties. Meig’s observation that calcium phosphate can form a semi-permeable membrane, is ascribed t o its hydrated colloidal nature3. Phosphates present peculiar features as components of soil and as soil amendments. They appear to be particularly strongly adsorbed, and to occur in a state of low availability in soils of lateritic type4. This is probably due to the formation of insoluble complexes between phosphate-ion (whether crystalloidal or colloidal), and the cations of hydrous alumina and ferric oxide, under suitable conditions of acidity. The recent researches of Gordon and Starkeyj, who found that colloidal alumina and ferric oxide possess a very much greater adsorptive power than colloidal silica for phosphate, furnish Splichal: Chem. Abs. 15, 3012 (1921); Smith: J. Am. Chem. SOC.42, 460 (1920). J. Chem. SOC.111, 620 (1917). 8 Am. J. Physiol. 38, 465 (1915). McGeorge: Agr. Exp. Sta. Hawaii, Bull. Xo. 41 (1916). Soil Science, 14, I , 449 (1922); Lichtenwalner, Flenner and Gordon: 16, 157 (1923).
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direct evidence of this. In many instances, the beneficial effect of phosphatic manures may actually be due t o their flocculating action on those colloids, and may be of considerable significance in the amelioration of tropical laterites1.
Summary I. The flocculation phenomena of lateritic clays and soils are discussed on the basis of Comber’s theory, which ascribes the anomalous flocculation of clay by lime to the formation of insoluble colloidal complexes. Reasons are presented in support of the view that hydrous alumina 2. and hydrous ferric oxide, which probably occur in laterites, are colloidal ampholytes, possessing isoelectric points not far removed from neutrality. On this assumption, the mechanism of their anomalous flocculation by lime, and by certain anions, such as silicate-ion and phosphate-ion, may satisfactorily be explained. 3. An attempt is made to extend the theory so as to embrace hydrous silica and hydrous aluminosilicates and hydrous ferrosilicates, and it is submitted that these colloids also may be regarded as ampholytes. 4. Mutual precipitation of colloidal ions is discussed in its relation to flocculation in certain soils. The writer wishes to express indibtedness to his colleague, Mr. P. E. Turner, for many helpful suggestions. Imperial College of Tropical Agriculture Trinidad, B. E7. I December 24, 1924. 1 Howard and Howard: J. Agr. India, 18 11, 148 (1923); Iyengar: J. Mysore Agr. Exp. Union, Bangalore, 4, KO.3 (1923).
