ZINC MONTMORILLONITE CLAYS
399
CATION AND ANlON INTERCHANGE WITH ZINC MONTMORILLONITE CLAYS M. ill.ELGABALY AND H. JENNY Division of Soils, University of California, Berkelev, CaliJornia Received April 7, 1943
.
INTRODUCTION
The discovery that zinc will cure certain diseases of fruit trees and field crops has centered interest on the fixing power of soils for zinc. In practice,Xtrees must be treated by the cumbersome method of spraying the foliage with zinc compounds, because soil treatments prove relatively ineffective. Apparently zinc is fixed in the soil in a form not readily available to growing plants. The writers have investigated the mechanism of zinc fixation on various types of colloidal clays of known crystal structure. In this paper are reported the relationships found with montmorillonitic clays, which are the dominant clay minerals of bentonites and of many agricultural soils. EXCHANGE ADSORPTION O F ZINC
A Wyoming type of bentonite was converted into hydrogen bentonite by electrodialysis. Homoionic sodium and calcium bentonites were prepared by leaching hydrogen bentonite with neutral normal solutions of sodium and calcium acetates. Excess salt was displaced with neutralized 95 per cent methyl alcohol. The cation-adsorption capacity (exchange capacity) of these clays, as determined by the ammonium acetate method (2), was found to be 92 milliequivalents per 100 g. of oven-dry material. As is well established, the cations sodium, calcium, and ammonium replace each other in stoichiometric proportions. To portions of these homoionic bentonites were added increasing amounts of zinc chloride solutions. Usually about 0.5 g. of bentonite, containing 0.46 milliequivalent of exchangeable cations, was suspended in 100 cc. of solution. After 48 hr., when equilibrium had been reached, the suspensions were centrifuged in a McBain-type spinning-top centrifuge. The clear supernatant liquid was then analyzed for cations and anions. Zinc was determined by the polarographic method (6). The results are shown in figure 1. On the abscissa are plotted the amounts of zinc chloride added, expressed in terms of the saturation capacity of the clay suspension. The value IS corresponds to 0.46 milliequivalent of zinc chloride in 100 cc. On the basis of 100 g. of bentonite, 1S is equal to 92 milliequivalents of zinc chloride. On the ordinate, above the zero line, is plotted the uptake of ions, expressed as milliequivalents per 100 g. of bentonite. Below the zero line the outgo or release of ions is indicated. It should be noted that the zinc data always refer to divalent zinc having an equivalent weight of 32.69. Comparison of the uptake and the release curves clearly indicates that the adsorption of zinc is governed by an exchange process. More specifically, the uptake of zinc is accompanied by a release of sodium and calcium. However, in
400
M. M. ELGABALY A N D H. JENKY
contrast to the ordinary cation-exchange reactions involving potassium, sodium, calcium, and ammonium ions, the reactions with zinc exhibit two novel features.
,*1 I//
Cl- KDtUke
FIG.1. Exchange-adsorption isotherms of zinc chloride by homoionic bentonite clays. On the abscissa are plotted the amounts of zinc chloride added, expressed as multiples of the ammonium-adsorption capacity of the clays (92 milliequivalents per 100 g. of clay). On the ordinates are given the adsorption and release of cations and anions as milliequivalents per 100 g . of clay.
First, the milliequivalents of zinc taken up by the bentonite exceeds the milliequivalents of sodium or calcium released. Second, the process involves an adsorption of chloride anions.
ZINC MONTMORILLONITE CLAYS
40 1
This behavior may be explained by assuming that part of the zinc is adsorbed as monovalent complex zinc ion. Solutions of zinc chloride contain, besides Zn++ and C1-, small amounts of (ZnOH)+ and, according to Scatchard and Tefft (5), also (ZnCl)+. The proportion of these complex ions depends upon the concentration and on the pH of the solution. Accordingly, calcium clay in equilibrium with a solution of zinc chloride may have the following ions in the surface layer: Ca++, Zn++, (ZnCl)+, and (ZnOH)+. With t,he aid of the analytical data a t hand it is possible to calculate with a reasonable degree of'certainty the amount of these ions on the clay. For example, a t 4s concentration the experimental data obtained (figure 1) are as follows: Initial Ca++ on clay.. . . . . . . . . . . . . . 92.0 milliequivalents per 100 g. . . . . . . . . . . . . . 81.5 milliequivalents per 100 g. Ca++ released Zn++ adsorbed.. . . . . . . . . . . . . . . . . . . 110.5 inilliequivalents per 100 g. C1- adsorbed. . . . . . . . . . . . . . . . . . . . . 17.5 milliequivalents per 100 g. We may write the following two equations containing the unknown milliequivalents of Zn++ (2) and of complex Zn+(g): 81.5 = z 110.5 = z
+g + 2g
Solving for z and u, the amount of Zn++ is found to be 52.5 milliequivalents and the combined number of milliequivalents of (Zn0H)f and (ZnCl)+ is 29.0. Taking into consideration the chloride adsorption measured, the coating of the bentonite particles may thus be represented as follows:
r1 Ca++ (10.5)
Zn++ (52.5)
J particle
(ZnCl)+
(17.5)
(ZnOH)+ (11.5)
The numbers in parentheses denote the number of milliequivalents of each ion species adsorbed on 100 g. of clay. Similar patterns may be computed for any other equilibrium concentration. It should be noted, however, that the above picture merely portrays the amounts of the ion species removed from the solution by the clay particles. It does not necessarily follow that the complex ions remain in that form on the clay surface. RELEASE O F ADSORBED ZINC
Zinc clay was prepared by leaching the aforementioned sodium bentonite with normal zinc acetate solution having a pH of 6.4. The excess acetate was removed with 50 per cent solution of methyl alcohol. Zinc clay so prepared contained 129 milliequivalents of total zinc per 100 g., as determined by fusion analysis. Leaching of 1g. of zinc clay with 400 cc. of neutral ammonium acetate removed 120 milliequivalents of Zn++ per 100 g., which may be designated as exchangeable zinc or, more specifically, zinc exchangeable with ammonium
402
ill. hf. ELGABALT
A K D R. 3ENXY
acetate. X siinilar increase in exchangeable zinc as compared 154th the adsorption capacity of the clay determined with the ammonium acetate method has been noted by Ron-er and Tiuog (1). After all exchangeable zinc had been removed, thc clay possessed an ammonium-exchange capacity of only 84.0 milliequivalents per 100 g., as compared with the value of 92.0 of the original sodium clay. A 1 per cent aqueous suspension of zinc clay had a pII of 13.70, and only 3.5 milliequivalente of hydroxide per 100 g. of clay was required to bring the sol to pH 7.0, This value probably represents the amount of exchangeable hydrogen ions on the zinc clay. Thermal decomposition curyes (4) of the zinc clay suggested the presence of adsorbed acetate. Quantitative determination \vas accomplished by leaching zinc clay with normal pot,assiuin sulf a k , and by steam distillation of the leachate in presence of phosphoric acid. The amount of acet,ate was found to be 10.1 milliequi\.alents per 100 g. of clay. These data, pexnit enhiation of the probable ion coating of the zinc bentonite with the aid of the follo\ving pair of equations: (84.0
- 3.5)
=
TC
120 =
5
+y + 2y
The results of the calculation may be depicted as follows:
1 particle
(ZnOH)+ (29.4)
(Zn-a&)+ (10.1) I t is of interest to note that the proportion of monovalent complex zinc to diTalent zinc is much higher on the clay than in the zinc acetate solution. This is indicative of strong selective adsorption of complex zinc ions. I n order to ascertain the mode of release of adsorbed zinc, variable amounts of soclium chloride, or calcium chloride, or hydrochloric acid (IS = 120 milliecluir-alents per 100 9.) were added to zinc bentonite suspensions containing 0.G0 milliequivalent of exchangeable zinc (always expressed as divalent' zinc) in 100 cc. After equililsrium had been reached (48 hr.) the suspensions mere centrifuged and the clear supernatant liquids were analyzed for cations and anions. The results are shon-n in figure 2. Compared with the curves in figure 1, the magnitudes of cation interchange are considerably smaller. Even hydrochloric acid, a t 4s concentration, was iinable to replace more than TO per cent of the exchangeable zinc. Again we note that the replacement of zinc is not stoichiometric, for the amounts of zinc (calculated as divalent zinc) released are greater than the amounts of calcium and sodium &ken up. As in the study on uptake of zinc, the deviation from stoichiometric proportions may be explained by the assumption that' calcium chloride and sodium chloride replace both divalent zinc and monovalent coinpler zinc ions. Data indicate that the latter is mainly zinc acetate ion. Probably the most surprising feature in figure 2 is t.he concurrent adsorption of
ZINC MONTMORILLONITE CLAYS
403
FIG.2. Isotherms depicting the interchange of ions with zinc bentonite clay and chlorides. Amount of chlorides added is plotted on the abscissa (1s = 120 niilliequivalents per 100 g. of clay). Adsorption and release of cations and anions are shown on the ordinate as milliequivalent8 per 100 g. of clay.
chloride ions. Bentonites near the neutral point do not possess, as far BS is known, the power of chloride-ion adsorption from neutral salt solutions such as sodium chloride and calcium chloride.
404
M.
III. ELGABALY AKD H. J E N N Y
ANIOK EXCHANGE O F ZINC BENTONITES
The adsorption of chloride ion by zinc clay may be harmonized by attributing to zinc clay the property of anion exchange. In order to account for such behavior the aforementioned pictures of ion coatings of the negative zinc clay particles must be modified to include a mosaic surface capable of both cation and anion exchange The possible assembly of zinc ions on an entirely negative surface (oxygen ions) and on a mosaic type of surface possessing positive arid negative charges simultaneously may be visualized as follows:
i
I
Inner layer
, Outer layer
Negative surface
Inner layer
1
1
Outer layer
Mosaic surface
The right-hand picture satisfies the requirements of simultaneous cation and anion exchange. X similar mosaic t>>-peof surface had been previously suggested for magncsium pcrmutite by one of the authors (3). It may he well t o point out, that, in contrast to proteins n.hich exhibit cation exchange on the alkaline side of the isoelectric, point and anion exchange on the acid side, the mosaic surface permits concurrent adsorption of cations and anions. Data indicate that this mosaic picture prevails at reactions below pH 7, whereas the left-hand picture seems to be favored in alkaline systems. For example, from zinc clays, prepared by leaching of clay with zinc chloride, sodium hydroxide will replace all of the zinc in the adsorption layer, whereas sodium chloride will displace only divalent zinc. This indicates that zinc in the inner layer is not readily replaceable near and below the neutral point. A number of experiments were performed to explore the anion exchange properties of zinc bentonite. The results are given below.
Interchange of 61 and OH Zinc bentonite was prepared by leaching hydrogen clay (cap. = 87.0 milliequivalents) with filtered, ,normal zinc chloride solution (pH 5.2) and subsequent leaching with 95 per cent methyl alcohol. Titration t o pH 7.0 of a 1 per cent suspension having a p1-I of 6.5 revealed a content of exchangeable hydrogen ions amounting to 1.4 milliequivalents per 100 g. of oven-dry clay. Zinc exchangeable by normal, neutral ammonium acetatr was found to be 113.0 milliequivalents per 100 g. of oven-dry bentonite. Fusion analysis yielded 122.0 milliequivalents of total zinc and 7.2 milliequivalents of chloride. The adsorpt'ion capacity of zinc clay for ammonium, as determined by leaching with ammonium acetate, was only 80.1 milliequivalents per 100 I$., as compared with 87.0 milli-
405
ZINC MONTMORILLONITE CLAYS
equivalents of the original hydrogen clay. With the aid of these analytical data, the following picture of the mosaic surface may be formulated: H+ (1.4)
l----l
ILA' I
Zn++ (44.4)
particle
C1-
(7.2)
OH- (27.1) For the system under consideration, the active adsorption centers (inner layer) on the surfaces would consist of 57 per cent negative charges and 43 per cent positive charges. In order to study both cation and anion exchange, various amounts of sodium chloride mere added to this zinc clay. The results are recorded in table 1. TABLE 1 Cation and anion exchange in a 1 per cent suspension of zinc bentonite plus s o d i u m chloride (-411 values are expressed as milliequivalents per 100 g . of oven-dry clay)
* Calculated as divalent zinc It is clearly seen that the cation interchange involves primarily divalent zinc, for the values of sodium uptake and zinc release are nearly identical. I n the last row, for example, it may be calculated that the 34.2 milliequivalents of sodium adsorbed by the clay replaced 31.8 milliequivalents of divalent zinc and only 2.4 milliequivalents of monovalent complex zinc ions, either as (ZnCl)+, (ZnOH)+, or both. In this system the chloride uptake reached a value of 13.4 milliequivalents and it may be ascribed to an interchange with hydroxide ions. Assuming the adsorption of chloride to be the result of an interchange with hydroxide, the reaction should be more pronounced for hydrochloric acid than for sodium chloride, as may be seen from the following two equations: / l O H
i&-]OH
+ NaCl + HC1
S
/IC1
$
[
q
C
+ NaOH 1 + HOH
In the case of hydrochloric acid one of the resultants is water, the formation of which favors the reaction to proceed from left to right. These trends were verified by adding 348 milliequivalents of sodium chloride or hydrochloric acid to 100 g. of the abovementioned zinc clay, which resulted in a chloride uptake of 10.0 milliequivalents from sodium chloride and of 27.1 milliequivalents from hydrochloric acid. Calcium chloride was found to occupy an intermediate position. The amount of chloride taken up was 20.4 milliequivalents. The
406
M. M. CLGABALI ARD E. JENNY
addition of zinc chloride to zinc clay produced a chloride adsorption of only 1.3 milliequivalents per 100 g. of clay, which is within experimental error, a? it should be. Interchange of C1, OH, and S O , An attempt was made t o convert the aforementioned zinc clay containing chloride and hydroxide ions into a system possessing replaceable nitrate ions. This was successfully accomplished by additions of sodium nitrate and nitric acid to the zinc clay. The addition of sodium nitrate in the proportion of 87.0 milliequivalent; to 100 g. of oven-dry clay released 22.3 milliequivalents of zinc and 2.0 milliequivalents of chloride. In turn, 21.4 milliequivalents of sodium and 6.3 milliequivalents of nitrate n-ere taken up by the clay. The resulting changes in the composition of the exchange layer on the clay may be approximately illustrated as follows:
€I+
H' Clay particle
(1.4)
Zn++ (44.4)
c1-
(1.4)
Zn++ (23.9)
--+
(7.2)
Clay particlc
OH- (27.1)
Naf
(21.4)
C1-
(5.2)
OH- (22.8)
XO3- (0.3) This presentation neglects the possible release of some of t,he hydrogen by sodium. Furthermore, it does not take into account t,he release of 0.9 millieyuivalent of monovalent complex zinc ion as computed from the difference betneen sodium uptake and zinc outgo. To ascertain the reversibility of nitrate interchange, this altered zinc clay mas leached with 95 per cent methyl alcohol and, subsequently, 348 milliequkalents of sodium chloride per 100 g. of clay was added t,o the system. The eiectrolyte released from the clay 20.2 milliequivalents of zinc and 3.1 milliequivalents of nitrate. Simultaneously an adsorption of 19.0 milliequivalents of sodium and 11.9 milliequivalents of chloride took place. Seglecting again the possible small replacements of exchangeable hydrogen and the computed release of 1.2 niilliequivalents of monovalent complex zinc ions (zinc - sodium), the major changes in the composit>ionof the exchange layer may be indicated as follows: H+ (1.4) H+ (1.4)
r I
Clay particle
Zn++ (6.1)
Zn++ (23.9)
Naf
(40.4)
c1-
(17.1)
OH- (22.8)
OH-
(14.0)
NO,'- (6.3)
NO,- (3.2)
Xaf
(21.4) -4
C1-
(5.2)
Clay particle
407
ZINC MONTMORILLOXITE CLAYS
Comparison of the initial and the final clays clearly indicates that the cation exchange resulted mainly between sodium and divalent zinc. The two treatments reduced the content in exchangeable hydroxide ion by substitution of chloride ion and nitrate ion. The anions C1- and NO; are thus mutually interchangeable. The addition of nitric acid in place of sodium nitrate to the original zinc clay gave rise to the following displacements: 49.0 milliequivalents of zinc and 3.3 milliequivalents of chloride were released, and 19.0 milliequivalents of nitrate were adsorbed. After the material had been leached with 95 per cent methyl alcohol, 348 milliequivalents of sodium chloride (per 100 g. of clay) were added. Sodium chloride brought about a release of 3.4 milliequivalents of zinc and 8.4 milliequivalents of nitrate. The uptake involved 2.3 milliequivalents of sodium and 13.6 milliequivalents of chloride. A 1 per cent aqueous suspension of4his zinc clay was very acid, having a p H of 2.5. Titration with hydroxide to the pH of the original zinc clay (pH 8.5) indicated the presence of 45.4 milliequivalents of exchangeable hydrogen ion on the clay. This value is in fairagreement with the amount of Zn++ (44.4) initially on the zinc clay. I t may be concluded that the cation- and anion-exchange studies strongly support the concept of the mosaic surface of the zinc clay capable of independent cation and anion exchange. KATURE O F FIXATION OF NON-REPLACEABLE ZINC
1
A careful examination of the experimental data reported will show that, in addition to the difficultly replaceable zinc of the inner layer, there exists another form of non-replaceable zinc. This type is revealed by comparing the zinc found in the ammonium acetate extract of the zinc clay with that obtained by its fusion analysis. For the two zinc clays studied the amount of zinc not replaceable with ammonium acetate was 9.0 milliequivalents per 100 g. in each case. Conceivably this form of zinc has become part of the clay lattice and therefore is not accessible to the large ammonium ions. Cavities of suitable size are furnished by the unoccupied oxygen and hydroxyl octahedra of the brucite layer of the montmorillonite crystal. The ionic size of Zn++ (0.78 is identical with that of RIg++ (according to Goldschmidt), which is a common constituent of the brucite layer. In order to satisfy the electrostatic neutrality of the clay crystal, the introduction of 1 millimole of divalent zinc into the lattice must be accompanied either by the adsorption of 2 milliequivalents of anions or by a reduction of 2 milliequivalents of the cation-exchange capacity. Inasmuch as ammonium bentonite prepared by leaching of zinc clay with ammonium acetate did not exhibit any power of chloride adsorption, the first alternative must be ruled out. The second alternative is supported by the obscrvation that the transformation of sodium clay into zinc clay by leaching with zinc acetate reduced the ammonium-ion capacity by 8.0 milliequivalents per 100 g. Likewise, the conversion of hydrogen clay to zinc clay by leaching with zinc chloride lowered the ammonium-ion capacity by 8.9 milliequivalents per 100 g. In both clays fusion analysis indicated the presence of 9.0 milliequivalents of fixed zinc. A more
w.)
408
11. 11. ELGbBALY .4XD K. J E N N Y
detailed discussion of this phase of zinc fixation will be presented by one of the authors (I1.M.E.). SUhIhldRY
1. Cation and anion adsorption was studied for the systems sodium bentonite plus zinc chloride, calcium bentonite plus zinc chloride, hydrogen bentonite plus zinc chloride, and for the systems zinc bentonite plus sodium chloride, zinc bentonite plus calcium chloride, and zinc bentonite plus hydrochloric acid. 2. The uptake of zinc from zinc chloride solutions involves t,he ions Zn++, (ZnCl)+, and (ZnOH)'. The release of zinc from zinc clap by sodium chloride and calcium chloride is restricted mainly to divalent zinc. 3. Zinc clays possess pronounced anion-exchange properties. The anions C1-, OH-, and KO$- are mutually replaceablc. On the surface OH- is held more tightly than either xO3- or Cl-. 4. It is postulated that zinc clay has a mosaic surface capable of independent cation and ,anion exchange. The approximate constitution of the mosaic adsorption layer is indicated. 5 . Some of the zinc adsorbed exists in non-replaceable form. It is suggested that it is inside of empty oxygen and hydroxyl octahedra of the brucite layer of the montmorillonite crystal. REFERENCES (1) ROWER,C. A , , A N D TRUOG, E . : Proc. Soil Sci. Soc. Am. 6, 58 (1940). (2) HIBBARD, P. L.: ,Vethods of Chemical Analysis (mimeographed). University of California (1939). (3) JESKY, €1.:Kolloidchem. Beihefte 23, 428 (1927). (4) NORTON, E. F . : J. Am. Ceram. Sac. 22, 54 (1939). (5) SCATCHARD, G., AND T I F F T , R . F . : J. Am. Chem. Soc. 62, 2272 (1930). (6) STOUT,P., ASD LEVY,J.: Collection Czechoslov. Chem. Commun. 2-3, 136 (1938).
