ADSORPTION BY SOlLS' BY J.
E. HARRIS
In a former paper,2 the writer published the results of some experiments undertaken to throw some light on the cause of acidity in soils. The results of those experiments supported in every way the theory that acidity in soils is due to selective adsorption rather than to the presence of true acids. The experiments described in this paper have been undertaken to obtain more evidence concerning the phenomena of soil acidity and of adsorption by soils. Before taking up a discussion of the experimental work it will be well to describe briefly the properties of acid soils and to review the various theories concerning the cause of acidity. Acid soils may be divided into two types. The first, which may be truly called an acid soil, will give an extract with water which is acid to litmus, indicating that there is present in the soil a soluble acid. This type of acid soil is comparatively rare. The second type, which is the type considered in this paper, gives a water extract that is neutral to litmus, but gives a sharp acid action when a piece of sensitive litmus paper is brought into direct contact with the moistened soil particles themselves. These soils, although giving a neutral extract with water, will give a strongly acid solution when shaken with a neutral salt solution. Concerning the cause of acidity in the first type of soil there can be no question. It is undoubtedly due to the presence of true acids, these acids manifesting themselves by giving an acid solution when the soil is extracted with water. Occasionally appreciable quantities of a mineral acid, such as sulphuric acid, are found in the soil extract, but usually the 1 The experiments described in this paper were carried out while the writer was temporarily employed a t the Michigan Agricultural College Experiment Station. The paper is published by permission of the Director of the Station. *Jour. Phys. Chem., 18, 355 (1914)~
AdsorptioN by Soils
45 5
acid found is of organic nature. Formerly these organic compounds were given names such as humic, ulmic, crenic, and apocrenic acids. These compounds are described in the literature between 1825 and 1850.l Later work by Eggertz,2 van Bemmelen3and others has shown that the so-called “acids” described by the earlier writers are not definite compounds a t all, but complex mixtures of colloidal nature exceedingly variable in composition. However, Schreiner and Shorey, O h 5 and others have demonstrated beyond a question of doubt that definite compounds, some of them of an acid character, can be extracted from the soil. As noted above, however, the cases in which these compounds are present in sufficient quantities to give an acid extract are rare. Acid soils of the second type are exceedingly common in occurrence as is indicated by the statement of Whitson and Weir6 that two-thirds of the soils of Wisconsin are acid in character. Undoubtedly soil acidity is as common in Michigan and other middle western states as in Wisconsin. Formerly the acidity of these soils as well as those of the first type was explained by the presence of acids of the type of humic, ulmic, and crenic acids. This theory concerning the cause of acidity has gradually been abandoned for two reasons. The first is that, as noted above, it has been found that humic, ulmic, and crenic acids are not definite compounds a t all. The second is that a very large part of the acid soils of the second type are upland sandy soils so deficient in organic matter that the acidity could not be ascribed to organic matter. In fact, Daikuhara7 has found soils in Japan which, although devoid of organic matter, are acid in character. The writers has 1 For the literature on this subject see Technical Bulletin 19,Michigan Experiment Station (1914). Biedermann’s, Zent. Agr. Chem., 18, 75 (1888). 3 Land. Vers. Sta., 26, 113 (1888). 4 U. S. Bureau of Soils, Bulletins 47,70,74,77,80,83,87,88 and go. Ber. deutsch. chem. Ges., 45, 651 (1912). 6 Univ. of Wis. Exp. Sta. Bull. No. 230. Chem. Zeit., 32, 1187(1908). 8 LOC.cit.
I
456
J . E . Harris
shown that soils from which all organic matter had been removed were more acid in character than in their original state. The acidity in such cases cannot be ascribed to the presence of organic acids whether they be of the nature of humic acids or definite compounds of the type extracted by Schreiner and his co-workers. There are two hypotheses offered to explain the acidity of soils in such cases. According to one, the acidity is due to the presence of true acids, in some cases to organic acids and in others to silicates of an acid nature. According to this explanation, the fact that test-papers are not affected by water extracts of the soil is accounted for on the assumption that the acids are so insoluble that the litmus must be brought into direct contact with the soil particles to be affected. The fact that quite appreciable quantities of acid are liberated when solutions of soluble salts are shaken with the soil is explained on the assumption that the insoluble acids enter into double decomposition with the salt liberating the corresponding acid. That is to say, if the soil is shaken with a solution of potassium chloride for example, the insoluble acid would in part be changed over to the potassium salt and an equivalent quantity of hydrochloric acid would be set free. There are certain fundamental objections to this theory. In the first place, if the acidity is due to the presence of a true acid, the water extract of the soil should give just as strong an acid action toward litmus as the moistened soil particles themselves. According to the generally accepted idea, an acid owes its properties to the presence of the hydrogen ion. Now it is known that if a piece of sensitive litmus paper be drawn carefully through the supernatant liquid so as not to touch the soil particles in the bottom of the receptacle, the paper will be unaffected. If, on the other hand, the paper be dragged over the surface of the soil particles or the mixture be agitated in such a way that the soil particles come into direct contact with the test-paper, the blue litmus is instantly reddened. If this effect is due to the presence of a true acid, in other words to the presence of the hydrogen ion, we must as-
Adsorption by Soils
45 7
sume that the hydrogen ions are held in some mysterious way to the soil particles themselves and are not free to migrate through the solution in the usual manner. If we cannot assume that the hydrogen ions are bound in this manner by the soil particles, then it is necessary to assume that the reddening of the litmus is due to something other than the hydrogen ions, in which case this something cannot be a true acid. Again, as mentioned above, these acid soils have the property of liberating free acids when shaken with solutions of neutral salts. If this is brought about by a double decomposition with the insoluble acids it means that an insoluble and slightly dissociated acid is reacting to liberate quite appreciable quantities of a soluble and highly dissociated acid, a phenomenon that is hardly conceivable in the light of the law of mass action. These objections to the theory concerning the presence of true acids in those acid soils which give neutral water extracts have given rise to a second theory concerning the cause of the peculiar behavior of such soils. This theory is based on the assumption that the colloidal matter of the soil has the power of adsorbing selectively the cation from a neutral salt solution leaving an equivalent quantity of soluble acid in solution. If the soil is shaken with a solution of potassium chloride for example, it is assumed that the negatively charged colloidal matter of the soil will adsorb a certain amount of the positively charged potassium ions from the solution, setting free a corresponding amount of hydrochloric acid. This theory furnishes a plausible explanation for the action of soil toward litmus paper. The soil will adsorb the base of the blue litmus salt when the test-paper is brought into direct contact with the soil particles leaving the red acid dye on the paper. Since this theory does not assume the presence of a true acid, the water extract would be found to be neutral toward litmus, and this is found to be the case in a great majority of cases of soil acidity. More will be said concerning the mechanism of selective adsorption later. In the paper mentioned above, the writer described some
458
J . E. Harris
experiments which were undertaken to secure some positive evidence in favor of the adsorption theory. It was shown that, when samples of a soil were shaken with solutions of different salts such as sodium nitrate, and potassium chloride, different quantities of acid were set free, indicating that different quantities of the cation were taken up by the soil in the two cases. It might be argued that the difference in quantity of acid set free is a function of the strength of the two acids involved. However, if this were the case, we would find that with successive applications of the salt solution to the soil, the total quantity of acid set free in the successive applications of the two salts would approach the same limit if the change were brought about by a double decomposition with any free acid that is present in the soil; this limit wouldbe measured by the quantity of free acid in the soil. However, it was found that the total amount of acid obtained in the successive applications of potassium chloride in the one case and of sodium nitrate in the other did not approach the same limit. Instead, with each additional application, the ratio of the total quantity of acid set free in the case of potassium chloride to that i.n the case of sodium nitrate became greater. This indicated that some change other than double decomposition with a fixed quantity of some insoluble acid was responsible for the liberation of the acid. Again when a soil was treated with barium chloride solution, it was shown that the barium taken up by the soil could be removed almost quantitatively by a single application of N / 2 o hydrochloric acid, although it could not be washed out by persistent washing. If the action between the barium chloride and the soil is one of double decomposition, we must conclude that the acid present in the soil forms a salt with the barium that is more insoluble than the acid which is itself so insoluble that it cannot be washed out in measurable quantities even with several days of continuous washing. It would be hard to conceive of anything more insoluble than this barium salt. We would not then expect this action to be reversible to any appreciable extent. It was found, however,
Adsorption by Soils
459
as stated above, that the barium could be changed back to the soluble chloride by treating the soil with dilute hydrochloric acid. This fact would indicate that the barium is held by the soil in some other form than an insoluble barium salt. E. Truogl has criticized the conclusions drawn from the above experiments. Concerning the first experiment, Truog asserts that the fact that different quantities of sodium and potassium were taken up by the soil can be accounted for by various “side reactions.” He suggests as some of these side reactions, different rates of hydrolysis of the salts formed by different bases, action of organic matter, “latent acidity,” etc. Wherever his own results or those of other experimenters do not agree with his theory, he dismisses the nonagreement on the ground of side reactions. He does not show that these side reactions actually take place. In another experiment, he attempts to avoid these side reactions by using small quantities of soil with large volumes of salt solutions. He finds that using sodium chloride, potassium chloride, barium chloride, and calcium chloride, the quantities of acid liberated measured in terms of cc of N/25 NaOH, were, respectively, 1 . 2 , 1.4, 1.4, and 2 . 0 in the case of a silt loam soil, and 1.9, 1.9, 1.9,and 2 . 7 in the case of a peat soil. He concludes from this that “the reactions due to soil acidity take place according to chemical equivalence and exhibit all the properties of true chemical reactions.” However, an examination of his figures shows that 67 percent more acid is liberated in the calcium chloride solution than in the sodium chloride in the case of the loam and 43 percent more in the case of the peat soil. This, however, has very little to do with the question a t issue. Even though a particular soil does take up nearly equivalent quantities of different bases, this fact does not show that all soils will behave in the same manner nor does it show that the action is not one of adsorption. Certainly in the soils investigated by the writer much greater quantities Jour. Phys. Chem.,
20,
457 (1916).
460
J . E. Harris
of acid were liberated from potassium salts than from sodium salts. Of course, these results can be dismissed with the magic phrase “side reactions,” but until these side reactions. are actually shown to take place and that these can be avoided in the way Truog suggests, about the only conclusion that can be drawn is that different types of soil vary in their power of liberating acids from various salts. With regard to the experiment in which the writer has shown that the adsorbed barium may be recovered by the addition of a dilute acid, Truog says “Harris, apparently, forgets that in the first reaction an overwhelming mass of barium chloride was used and the reaction forced as a consequence to the right. In the second reaction the excess of barium chloride had previously been removed and now a large excess of hydrochloric acid is added and, to be sure, the reaction is reversed and forced to the right. The results are entirely in accord with the law of mass action and serve as evidence in favor of the existence of true acids as the cause of soil acidity.” In order to determine the validity of Truog’s conclusion, it will be well to consider once more the nature of the substances involved. If there is an acid present in the soil, its insolubility is indicated by the fact that when the writer subjected a sample of acid soil to continuous washing for a period of two weeks, its power for liberating an acid from a neutral salt was just as great as before the washing, thus showing that none of the “acid” had been removed by the long-continued washing. The fact that barium chloride reacts with this very weak and insoluble acid liberating a very large quantity of a strong acid, makes it necessary that the barium salt be even more insoluble than the acid itself. This would make the barium salt about as insoluble a product as any that can be imagined. This being the case, we would not expect to render appreciable quantities of the barium soluble by treatment with hydrochloric acid, if the barium compound were formed by double decomposition, whereas the writer found that with N/20 hydrochloric acid the barium could be recovered almost quantitatively. The fallacy of Truog’s conclusion concerning
Adsorptiox by Soils
46 1
the experiment may be seen by considering a similar instance with substances with which we are somewhat more familiar. Starting with barium oxalate and adding sulphuric acid, we would get of course barium sulphate and oxalic acid, the barium oxalate disappearing. Now according to Truog’s reasoning, if we remove the sulphuric acid completely and add a slight excess of oxalic acid, the barium sulphate should be changed back almost completely to barium oxalate. Of course this could not happen. It is impossible to believe that any acid of the character that might be present in an almost absolutely insoluble condition in the soil can react with a soluble salt and liberate large quantities of a strong acid. And even if this were possible (as it might be if the barium salt were many times as insoluble as the corresponding acid) it would certainly be impossible to carry the barium back into solution by removing the soluble salt and somewhat increasing the concentration of hydrochloric acid above that liberated by the action of the soil on the barium chloride. Truog goes farther, and states that the “selective adsorption from the common, simple, stable, neutral salts has never been demonstrated conclusively.” Also that “various properties having no existence in either pure chemistry or physics are ascribed to colloids in order that certain phenomena may be explained without going to the trouble of finding the real cause.” Since the only property of colloids under discussion is their power for selectively adsorbing ions from solutions of electrolytes, he would thus place in this category such chemists as Whitney and Ober, Cameron, Billitzer, Freundlich, Michaelis and many others who have worked on colloidal precipitation. In the paper above mentioned, the writer likened the action of the soil toward solutions of salts to that of colloidal arsenic trisulphide. Picton and Linderl first observed that in the coagulation of arsenic trisulphide by barium chloride solution, the precipitate carries down with it a certain amount of barium, an equivalent amount of hydrochloric acid being liberated in the solution. Truog calls attention to the fact Jour. Chem. SOC.,67, 63 (1895).
J . E . Harris
462
that Picton and Linderl explain this action on the basis of a double decomposition. While it is true that Picton and Linder do offer such an explanation it is also true that their explanation has not been generally accepted. I n fact, Whitney and Ober2, F r e ~ n d l i c h Taylor4 ,~ and practically all who have written on this subject regard the action of electrolytes with arsenic trisulphide as a case of adsorption. But granting that the interpretation in this particular experiment is doubtful, another example may be cited concerning which there can be no doubt. Freundlich and Losev6 have shown that when charcoal is treated with methyl violet (an organic chloride) the dye is adsorbed in the form of the organic base, and an equivalent quantity of hydrochloric acid is set free in the solution. The base thus adsorbed is not soluble in water, but may be easily removed by acids, alcohol, pyridine and other organic solvents. Now methyl violet is a strongly dissociated electrolyte and is not hydrolyzed in water solution, and in this regard is comparable to barium chloride. The action is in every way analogous to the action of arsenic trisulphide, soils and other colloids toward electrolytes. There is the difference that the methyl violethas an organic cation instead of a metal cation, but it is not possible t o endow it with special properties for this reason. Freundlich6 has found that arsenic trisulphide has the same effect on dyes as has the charcoal, and finds that the adsorption of metallic ions follows the same law as does the adsorption of the base from such dyes as methyl blue. The question next arises as to why any substance should selectively adsorb any ion. I n seeking an explanation for the selective adsorption of ions, we must keep in mind certain well-known properties of colloids. With very few exceptions colloidal substances (either in solution or in suspension) Jour. Chem. Soc., 87, 1914 (Igoj). Jour. Am. Chem. SOC.,67, 63 (1901).
6
Kapillarchemie. Chemistry of Colloids, p. 104, Longmans, Green and Co. Zeit. phys. Chem., 59, 284 (1907). LOC.cit.
Adsorption by Soils
463
carry electric charges, in a few cases positive but in most cases negative. Soils, kaolin, and practically all suspensions are negatively charged. The presence of this charge is shown by the migration of the particles in suspension under an applied electromotive force. Again this colloidal condition seems to depend upon this charge because as soon as the charge is neutralized by any method, and the colloid becomes electrically neutral (the isoelectric point), it is coagulated. This precipitation by neutralization of the charge carried may be brought about ( I ) by the addition of electrolytes, in which case it is always the ion carrying an opposite charge to that of the colloid that causes the precipitation, ( 2 ) by the addition of a colloid of opposite charge in which case both colloids are precipitated, (3) by the action of negatively charged prays in the case of positively charged colloids. In this latter connection,l it has been shown that the negatively charged rays will precipitate certain positively charged colloids, and will increase the mobility of certain negatively charged substances. The facts concerning the precipitation of colloids by electrolytes, such as the influence of valence, etc., are too well known t o need further discussion here, but the question as to whether that portion of the precipitating ion that is almost invariably carried down with the colloidal precipitate, is held enmeshed by the colloid through adsorption or through a chemical process is the important question in connection with the present problem. There are two facts that tend to support the adsorption hypothesis. First, the fact that the precipitation of one colloid by another of opposite charge is accompanied also by the precipitation of the latter, would lead us to believe that the precipitation of a colloid by an oppositely charged ion should be accompanied by the enmeshing of a sufficient quantity of that ion to neutralize the charge on the colloid. Second, in the case mentioned above, of the action of charcoal, silk, wool, and cotton on dyes such asmethyl kiolet we have undoubted cases of adsorption of the cation, Hardy: Jour. Physiology, 29, z g (1903).
J . E. Harris
464
this adsorption being accompanied by the liberation of an equivalent quantity of acid. Freundlichl has suggested an explanation of these facts based on Perrin’s2 work. This explanation accounts for the charge carried by the colloid, the adsorption of the cation, and the liberation of an equivalent quantity of an acid. Perrin has shown that the presence of the hydrogen or hydroxyl ions has an important influence on the charges carried by solid particles in contact with water. In contact with acids and high concentrations of the hydrogen ion, the solid particles become positively charged, while in contact with bases and high concentrations of the hydroxyl ions the particles become negatively charged. Freundlich explains this on the supposition that when a body in suspension is positively charged it has adsorbed hydrogen ions, these hydrogen ions forming one layer in a system of Helmholtz double layers, the other layer being formed by the acid ions, these two layers, of course, being an infinitesimal distance apart. In case the substance carries a negative charge, it is the hydroxyl ions that will form the layer on the surface of the substance thus giving it its charge. In case the substance is charged in contact with water and in the absence of bases and acids, it is the hydrogen and hydroxyl ions of the water that are responsible for the charges. In the case of a negatively charged body the hydroxyl ions form the layer next to the solid and the hydrogen ions the other. In the case of the positively charged body, the conditions are reversed. To explain this preferential adsorption, it is only necessary t o recall that any substance which will lower the surface tension of a liquid in a system consisting of a solid and a liquid will be mechanically a d ~ o r b e d . ~If the hydrogen ion lowers the surface tension of the liquid in contact with the solid it will be adsorbed and, of course, impart to the adsorbing surface its charge. If the hydroxyl ion lowers the surface tension it will be adsorbed and the charge
*
Zeit. phys. Chem., 59, 284 (1907). Comptes rendus, 136, 1288, 1440;137,513, 564 (1903). See Michaelis: Dynamics of Surfaces, p. 22, E. and F. N. Spon.
Adsorption by Soils
465
will be negative. Since most suspensions are negatively charged, the hydroxyl ion must, in most cases, form that part of the double layer next to the particles. If the charge on the suspended particles is in any way neutralized, the suspension is flocculated. If the suspended particles are brought into contact with an electrolyte, that ion again which has the greatest effect in lowering the surface tension will tend to pass into the inner layer. In most cases apparently, when the electrolyte is a salt, i t is the metal ion that is adsorbed. Of course, upon passing to the inner layer it will counterbalance the charge within this layer and the suspension will become electrically neutral, permitting the particles to coalesce and settle out, the precipitate carrying down with it the cation and the hydroxyl ion. The balancing of the charge due to the hydroxyl ions in the ‘inner layer will set free the hydrogen ions of the outer layer. These will pass into the solution forming with the acid ions of the electrolyte a free acid. This hypothesis of Freundlich’s furnishes a better explanation for the facts concerning selective adsorption than any of the others that have been advanced. To suppose that the colloid in being coagulated simply carries down with it the ion of charge opposite to its own, would leave free ions with unbalanced charges in solution. To suppose as does Parker1 that the colloid hydrolyzes the salt, and carries down the base leaving the acid in solution, would not account for the neutralization of the charge on the colloid. Also it would give to the colloid a power of decomposing salts, for which we would have no adequate explanation. Freundlich’s hypothesis has the advantage that it furnishes an explanation for the charge on the colloid, for its precipitation by an electrolyte, and for the carrying down by the precipitate of the ion carrying the opposite charge. Another hypothesis to explain the action of soils toward salt solutions has recently been advanced by Frank E. Rice,2 who suggests that the acid set free when a soil isshaken with Jour. Ag. Research, I, 179 (1913). Jour. Phys. Chem., 20, 214(1916).
J . E. Harris
466
a neutral salt is a restilt of the hydrolysis of aluminium salts formed by the displacement of aluminium ions by the cation of the salt. This phenomenon undoubtedly does account for a large part of the acid set free in the salt solution, for it has been noticed by all who have experimented with soils of this type that large quantities of aluminium salts are present in the solution after a soil has been shaken with a neutral salt solution. However, this hypothesis cannot explain all the properties of such soils. For example, the writer1 has shown that if a soil be treated with a dilute acid, and this dilute acid washed out, the soil has a very much greater power for liberating acids from neutral salt solutions than before. The effect of the acid would be to remove any adsorbed bases, including aluminium so that if Rice’s hypothesis were correct, their power for liberating acid in a salt solution should be decreased by such treatment rather than increased. This hypothesis also fails to account for the power of peat soils of liberating an acid from a salt solution. Sharp and Hoagland2 have recently used the hydrogen electrode for the determination of hydrogen ion concentrations in soil solutions. They found that soil solutions from acid soils showed higher hydrogen ion concentrations than did those from neutral soils. They also found that the hydrogen ion concentration was materially increased by the addition of neutral salts in the case of acid soils. They conclude that acidity in soils is due to the presence of true acids. The writer wishes to call attention to the fact that in acid soils (meaning by this, soils deficient in basic material), the conditions are especially good for the accumulation of small quantities of acid due t o the deficiency of basic material for them t o combine with. Under ordinary conditions of drainage there might be sufficient quantities of acid accumulated to be detected by the hydrogen electrode but not by litmus. It cannot be concluded, however, that this small quantity of acid can be the cause of the liberation of large quantities of acid when treated 1
LOC.cit. Jour. Ag. Research, 7, 123 (1916).
Adsorption by Soils
467
with a salt, nor can those experiments explain the fact that, although litmus is not affected by the soil solution, it is sharply acted upon by the soil particles themselves. The experiments described in this paper were undertaken with a two-fold purpose; first, to secure additional evidence as to whether the cause of acidity (where such acidity is characterized by a neutral soil solution, but acid action when testpapers are brought into direct contact with the soil particles) is due to colloidal adsorption or to the presence of true acids; second, to secure data that might throw some light on the action of fertilizer salts. It is known that in cases of adsorption by colloids, whether from solutions of dyes or from solutions of electrolytes, the quantitative relations are expressed by the equation x/un = a ~ ' where / ~ x is the mass of the material adsorbed, un the mass of the adsorbing substance, c the concentration of the solution with respect to the material adsorbed, and a and n are constants characteristic of the substances used. This formula may be written in the form cl/czlln = k , in which form it is an empirical modification of Nernst's partition law, which is formulated thus, Cl/Cz = K. This latter equation gives the equilibrium conditions when a substance is shaken up with . two immiscible liquids in each of which it is soluble. There is a second adsorption formula that has been developed by Freundlichl partly on theoretical and partly on empirical a x considerations. This formula is expressed thus : V log r where represents the volume of solution, (E)the mass of adsorbing material, a the total quantity of adZJ
rrt
a
sorbable material, x the quantity adsorbed, and a and n are constants. However, by the expansion of the terms involved, Freundlich showed that the first adsorption equation expressed above could be derived from his. The first equation is the one most often used in work on adsorption. The value2 Zeit. phys. Chem., 57, 385 (1906). Freundlich: Kapillarchemie, p. 150.
J . E . Harris
468
of
I/% usually lies between 0.1and 0.5. This formula gives us a good method for testing the action in the case of the soil to determine whether we have a case of adsorption or one of
chemical reaction. The test was applied to a soil of the sandy loam type. In preparing the soil samples, the soil was air-dried and put through a 20-mesh sieve, every precaution being taken to secure as nearly uniform samples of the soil as possible. 50gram samples of the soil were treated with solutions of barium chloride varying in concentration from 0.8 N to 0.01 N , the mixtures being shaken a t intervals for a period of twentyfour hours. The volume of solution used in each case was 125 cc. The solution was analyzed before and after application to the soil, the quantity of barium adsorbed being determined by difference. In calculating the results, the equation used was x = U G ' / ~ . The use of this simpler form of the equation given above is made possible by the fact that the mass, m, of the adsorbing substance was kept constant. The elimination of this quantity merely changed the value of the constant a. The values of I/% were calculated and the results are shown in the following table. The concentrations of the barium ion were calculated from the values given in conductivity tables for barium chloride solutions.,
TABLE I Normality of BaClz sol.
0.8 0.4 0.2 0. I
0.07 0.04 0.02 0.01
-
Conc. of Ba ions. Grams per IOO cc
Total quantity of Ba adsorbed
3.3175 I . 8133 0.9849 0.5233 0.3778 0.2253 0.1175 0.0611
0.2375 0.1983 0 . I550 0.1125 0.1109 0.0819 0.0580 0.0413
A similar experiment was performed with kaolin which had been treated with N / 2 0 hydrochloric acid, the excess acid being
Adsorpiout by Soils
469
then removed by careful washing. This treatment is the same as that applied to the kaolin used in the experiments described in a previous paperel The kaolin before treatment with the acid was perfectly neutral in its behavior, but after the treatment with the acid, although washed until the wash water was found t o be neutral, had the same power as the acid soils of liberating a soluble acid when shaken with a solution of a neutral salt. The results are given in Table 11.
TABLE I1 Normality of BaClz sol. 0.I
0.07 0.04 0.02 0.01
Conc. of Ba ions. Gms. per IOO cc
3uantity of Ba ions adsorbed
0.5233 0.3778 0.2253 0.1175 0.0611
0.1065 0.0982 0.0885 0.0714 0.0567
0.275 0.274 0.262 0.274 0.284
The above results show values for I / % that agree remarkably well especially when i t is remembered that it is extremely difficult to get exactly uniform samples, particularly in the case of the soil. The fact that the values of I / % are so nearly constant, indicates that the action is one of adsorption in both cases. It was next undertaken to determine the power of adsorption on the part of the soil for various cations. As a preliminary to this experiment, samples of various soils were treated with barium chloride solution to determine whether or not the acid radical is adsorbed. In no case was there any evidence of adsorption of the chloride ion, indicating that the soil adsorbs only the positive ion. The salts used in this experiment were sodium chloride, potassium chloride, calcium chloride, barium chloride, manganese chloride , magnesium chloride, and aluminium chloride. To determine the quantities of the cations adsorbed, 5o-gram samples of soil were treated with 125 cc of N/IO solutions of each of the above 1
LOC.cit.
J . E. Harris
470
salts. The solutions were left in contact with the soil for a period of twenty-four hours, being frequently shaken during that period. The solutions were analyzed before and after treatment of the soil, and the quantities adsorbed determined by difference. TABLEI11 Solution
KC1 KaC1 CaClz MnClz MgCh AlC13
Quantity of cation adsorbed
No. of equivalents adsorbed
0.0395 0.0041 0.0134 0.0177 0.0057 0.0113
0.00101
0.00013 0.00067 0.00064 0.00047 0.00125
From the above tables, i t may be seen that the calcium, manganese and magnesium are adsorbed in almost equivalent quantities, while the potassium and aluminium are adsorbed in larger quantities and the sodium in much smaller quantities. It is of interest to note that the quantities of the ions adsorbed follow closely the valence rule for the precipitation of colloids, the potassium being excepted. The writer can offer no explanation for the anomalous behavior of the potassium. Whether there is any connection between the high adsorptive power of the soil for this element and the important role played by this element in influencing the fertility of the soil is hard to say. Presumably the potassium would have a greater effect on the physical properties of the soil than would the sodium or the divalent ions Ca, Mn or Mg. On the other hand the AI would be more effective so far as its flocculating power is concerned but the beneficial effect of this would be destroyed through the hydrolysis of aluminium salts and the consequent setting free of soluble acids. To determine the extent to which the various ions m replace adsorbed potassium ions, a large quantity of soil w treated with normal potassium chloride solution, the two being shaken together for several hours. The solution was then re-
Adsorption by Soils
471
moved by centrifuging. This was repeated six times to insure th/e adsorption of as large a quantity of potassium as possible. The soil was thoroughly washed to remove the greater part of the soluble potassium. After drying, nine 50-gram samples were treated as follows: No.
I
Treated with
distilled water N/IO NaCl N/IO NHdC1 N/IO CaClz N/IO MnClz N/IO MgClz N/IO AlCI3 zoo cc water with 1.72 g CaS04.2H~0 2 0 0 cc water and I g CaC03
2 0 0 cc 2 0 0 cc 200 cc 2 0 0 cc 2 0 0 cc 200 cc 200 cc
I 2
7 1
9 8
The samples of soil were left in contact with the solutions, with occasional shaking, for a period of seventy-two hours, after which the solutions were drawn off and the quantity of potassium determined. The results are shown in the following table: TABLE IV I
Sample No.
7
8 9
I
Treated with
K20 found in solution
0,oogz 0.0393 0.0568 0.0511 0.0514 0.0413 0.0617 O.OjI0
0.0322
&O liberated by action of solution
0.0301 0.0476 0.0419 0.0422 0.0321 0.0525 0.0418 0.0230
From the above table i t may be seen that large quantities of the potash held adsorbed by the soil are liberated upon treatment with various salt solutions, and that the quantity varies with the salt solution used, Aluminium chloride is
J . E . Harris
472
found to liberate the greatest quantity followed in order by ammonium chloride, manganese chloride, calcium chloride, calcium sulphate, magnesium chloride, sodium chloride and the suspension of calcium carbonate in water. The order for the ions Al, Mn, Ca and Na is the same as that found in Table 111. This was to be expected because the abilityto liberate potassium held adsorbed by the soil should depend directly upon the relative tendencies of the different ions for being adsorbed. It is possible that there is some connection between this * displacement of potassium, held adsorbed by the soil, by other ions, and the beneficial effect noted by a number of experimenters upon the addition of gypsum, sodium chloride, and manganese salts to the soil. If such additions are beneficial because of the power of these substances to displace adsorbed potassium, the beneficial effect should be a temporary one, and such has usually been found to be the case. To determine the effect of the presence of one ion upon the adsorption of another, three samples of soil were treated as follows: No. I with 2 0 0 cc of N/IOKN03, No. 2 with 2 0 0 cc of N / I O CaClz and No. 3 with IOO cc of N / 5 K N 0 3 and IOO cc of N / j CaClz. In No. 3 after mixing the solutions of K N 0 3 and CaC12,the soil was, of course, in contact with a solution that was tenth normal with regard to both salts. The quantities of the ions adsorbed are shown in the following table:
TABLE V ~~
Quantity of KnO adsorbed
No. I No. 2 No. 3
0.0471
-
Quantity of CaO adsorbed
-
No. of equivalents adsorbed K~O
j
CaO
-
0.0010
0.00087
0.0245 0.;07~+
~
0.00067
It is seen from these results that the quantity of one ion adsorbed is materially decreased by the presence of another ion, but the amount of decrease does not correspond to the
Adsorptioa by Soils
473
amount of the second ion adsorbed. In other words the sum of the equivalents adsorbed from the mixture is greater than the number of equivalents adsorbed from either solution alone. This would indicate that a soil that had adsorbed all that i t could of one ion, would still retain some power of adsorption for another ion, over and above that adsorbed by the displacement of the first ion. Summary It has been shown that, when a soil or kaolin is treated with salt solutions of varying concentrations, the quantities of the cation adsorbed follow very closely the adsorption isotherm represented by the equation x/m =ac '/%, indicating that the action is one of adsorption and not of double decomposition. When the soil was treated with different salt solutions i t was found that the number of equivalents of the different cations adsorbed was not the same. The cations with reference to their tendency for being adsorbed occurred in the order : Al, K, Ca, Mn, Mg and Na. The numbers for the ions Ca, Mn and Mg were very nearly the same. It is observed that the metals with the exception of the potassium occur in the order of their valence and that metals of the same valence give practically the same values. It was found that a soil that had adsorbed large quantities of potassium, would give part of this up when treated with various salt solutions. In the case of the solutions tried i t was found that, with reference to their ability to set free adsorbed potassium, the salts occurred in the following order : A1C13, NH4C1, MnCL, CaC12, CaS04, MgCL, IV'aC1, CaC03. When the soil was treated with a mixture of salts it was found that the amount of each ion adsorbed was cut down by the presence of the other. The total number of equivalents adsorbed from the mixture was greater, however, than from either of the salts alone. Ann Arbor, Michigan
'
