CATION EXCHANGE PROPERTIES OF MICAS

Clay Minerals (1970) 8, 267. CATION EXCHANGE PROPERTIES OF MICAS II. HYSTERESIS AND IRREVERSIBILITY POTASSIUM EXCHANGE DURING A. C. D. NEWMAN Rothamsted Experimental Station, Harpenden, Herts. (Received 11 July 1969) ABSTRACT : The sorption of potassium by mica partially depleted of potassium was studied by adding potassium to a solution of sodium chloride in quasi-equilibrium with the mica. The concentration of potassium in the solution was increased 1 "5 times before reverse exchange was initiated. Potassium resorbed by the depleted mica was extracted more readily than potassium from the original mica. Potassium exchange from mica is not reversible; there is irreversible change in the mica when potassium is exchanged and also an additional hysteresis in the forward and reverse exchange. The replacement of potassium in trioctahedral micas by a hydrated cation follows a simple pattern at neutral ph, and a graph of equivalent fraction of potassium in solution (XK) against equivalent fraction in the solid (Xr:) is sigmoid with the middle part of the curve parallel to the XK axis for a wide range of Xx. This indicates that the free energy change when mica K is replaced by a hydrated cation is independent of the proportion exchanged (an unusual condition in cation exchange) and that the stationary state solution activity ratio may be used as a relative measure of the free energy of exchange (Newman, 1969; hereafter referred to as Part I). In Part I, the stationary state was used as the criterion of equilibrium, that is, the state of equilibrium was assumed to be attained when the reaction rate was sensibly zero, although a condition slightly short of this ideal was used for some of the experiments because the reaction rate approached zero very slowly. A more rigorous demonstration of whether the stationary state represents reversible equilibrium can be made by determining whether the same stationary state is approached from either direction. In terms of the practical variables in the exchange of potassium from mica, suppose that a mica and an aqueous solution react until a stationary state is reached in the region where XK is independent of XK. If a small amount of K is added to the solution, K should be taken up by the mica until the stationary state value of XK is regained, and as further additions are made the K-desorption exchange curve should be retraced. This paper reports the results of testing for reversible equilibrium in this way. EXPERIMENTAL When K was exchanged from micas by batch extraction, the solution concentration of K after 3 days at 25~ was 0"9 times the concentration after 1 month, whereas at 60~ the concentration of potassium was the same after 16 hr, and 2 and 3 days

268 A. C. D. Newman (Part I); further experiments showed that a stationary state is attained even sooner at 100~ and that mica does not decompose at the higher temperature. So, to diminish ambiguity caused by not attaining the stationary state, the reversal experiment was done in boiling solutions, i.e. at approximately 100~ Mica P5 (300 mg each of size fractions 30-100 and 100-300 mesh; Part I) was boiled under reflux with 500 ml 0"25 M NaC1 in 1 litre borosilicate flasks; other experiments showed that detectable amounts of K were not extracted from the flasks. Flasks and contents were weighed before the start so that the initial proportion of solution/solid could be maintained through the experiment. After 17 hr the supernatant solution was removed (this extract contained surface K exposed during comminution of the mica) and was replaced with fresh boiling 0.25 M NaCI solution. Aliquots (25 ml) were removed from this second addition at intervals ranging from 6 min to 3 days, and after removing each aliquot, each flask was replenished with 25 ml of fresh NaC1 solution. On sampling after 3 days, most of the supernatant fluid (450 ml) was removed and replaced with fresh NaC1 solution; this replacement was repeated three times at intervals of 3 or 4 days. About 45% of the mica K had been exchanged after the fifth replacement. At this stage, the exchange of K was reversed by adding NaC1 solution containing a greater concentration of K than the previous three extracts (mean concentration in three extracts was 0.16 me K per litre). 500 ml of hot 0'25 M NaC1 containing 0.212 me K per litre (0.37 S* of the exchanged sites) was added to each flask, and after refluxing for 17 hr, the supernatant fluid was sampled (25 ml); the aliquot volume was replaced with 0.25 M NaC1 (25 ml) containing 0"04 me K, bringing the total amount of K added to 0.52 S. Sampling and replacement were continued with similar increments of K until none of the added K was taken up by the solid after an interval of 2 days. Added K was then desorbed by extraction with K-free NaC1 solutions, as in the first part of the experiment; extractions were continued until no further K was removed from the mica. Each extract or aliquot was analysed for K using an EEL flame photometer, standardized with K solutions containing the same concentration of NaC1 as the extracts; portions of the solutions containing added K were also analysed at the same time as the extracts to determine the starting concentrations of K. RESULTS After the preliminary removal of surface K, the approach to a stationary solution K concentration was measured after adding the second batch of solution. This was initially faster with the smaller size fraction of the mica (50-150 /~) than with the 150-500 /~ fraction, but after three days the solution concentrations were similar for both (Fig. 1). For each treatment, K released to, or sorbed from, solutions was calculated from the difference between the concentration of K in solution before and after *A symmetry (S) is an amount of K equivalent to the exchange capacity.

Cation exchange properties of micas. H 269 0"8-- 0"6 _. ~ I ~--'--- o 8~ 0"2 I I I I I I0 20 50 40 50 60 (Time, min) I/z FIG. 1. Rate of K release from mica P5 during the second extract: 9 = 30-100 mesh fraction; 9 = 100-300 mesh fraction. reaction; the apparent total of K removed was the sum of these values. Errors accumulate when total K is estimated in this way, and the apparent total was about 15% more than the amount originally present in the mica. This discrepancy is caused mainly by the imprecision of sampling near-boiling solutions that quickly lose solvent by evaporation, and by loss of solvent during refluxing. This error does not affect the values of Xr:, which are proportions of K to Na in solution. Exchange curves were drawn by plotting Xr: against (I--XD, the proportion at each stage of the experiment of the apparent total of K exchanged. The exchange curves for the two size fractions were so similar that only one is given (Fig. 2). The exchange curves are divided into sections corresponding to the successive stages in the experiments. In section AB (Fig. 2), K was exchanged from the mica by NaC1 solution that contained only impurity levels of K, i.e. less than I ppm (the exact amount could not be determined with the filter flame photometer used). In section BC, adding K increased the solution concentration but K was not sorbed by the solid; in section CD further additions of K were sorbed by the mica without an increase in solution concentration. After D, both solution and solid K increased when K was added. At E, the supernatant fluid was removed, the solid washed once to remove solution K and the desorption of K was resumed by extraction with NaC1 that did not contain added K (EFGH). In this section, X~: was larger for the first three extracts than Xr: during the first section AB, but after G, the path of the initial exchange curve AB was resumed.

270 A. C. D. Newman 20-- 1.6 I-2-- x o o_ 0"8-04- I I i J I 0-2 04 06 OB I'0 (I--XK) FIG. 2. Exchange curves for forward and reverse exchange in mica P5 size fraction 30-100 mesh. See text for explanation of lettering. DISCUSSION The exchange curves show that the almost stationary K concentration attained after 3 days' reaction of the mica with 0.25 M NaC1 at 100~ was not a reversible equilibrium state, and K was taken up by the partly-exchanged mica only when the solid concentration was increased by a factor of about 1"5. This may indicate that equilibrium was not reached after 3 days, or it may show that the exchange of K is intrinsically non-reversible; these alternatives correspond respectively to a metastable condition, and a process exhibiting hysteresis, as defined by Everett & Whitton (1952). Because micas have a layer structure, interlayer cations exchange mainly by migration between the layers and through the edges into solution, and a partly exchanged mica particle has a core of unaltered mica surrounded by a rim of material almost entirely depleted of K (Rausell Colom et al., 1965). The particles in a mica preparation are irregular in shape, but the width of the alteration rim can be calculated to a near approximation by assuming the mica particles to be discs of mean radius R and thickness T. If there are n particles in the sample weight, the total amount of potassium is np 7r R 2 TX/100, where p is the density and X is percentage of K in the mica. After the K has been partly exchanged, the unaltered core of each particle is surrounded by a rim of K-depleted mica of width 8, and the

Cation exchange properties of micas. H 271 total amount of K in the mica now np ~r (R-3) 2 TX/IO0. The proportion of K remaining in the mica (XK) is therefore (R--~)~/R 2 and by rearrangement, 3 = R (1 --~/kx~0. (1) For the same fractional exchange, therefore, the exchanged region is narrower in the smaller size fraction than in the larger. If diffusion within the exchanged region is limiting the rate of attainment of a stationary concentration of K in solution, this rate will be faster with a smaller size fraction, as was observed in the initial stages of the rate experiments (Fig. 1). Later, however, there was no distinguishable difference between the concentrations of K in solution from the two size fractions, so that after three days, diffusion across the exchanged region could no longer be limiting solution K concentration. From this argument, it is clear that a stationary condition was close after 3 days, and that the 50% increase of solution K necessary to drive back into the structure must result from non-reversibility at the stationary condition. In discussing this irreversibility, a distinction is made between hysteresis because of the presence in a microscopic system of two sets of microscopic states ('domains'), and 'absolute' irreversibility in which, because of an irreversible change in the whole system, the original state can never be recovered by reversing the direction of reaction. Evidence in the exchange curve suggests both types of irreversibility. X~ in the first three extracts in the final desorpfion section (FGH, Fig. 2) was larger for corresponding values of X~z than in the initial desorption section AB, though smaller than in the sorption section CDE. The difference between FG and AB is probably a symptom of an irreversible change in the system that prevents the original mica state being recovered, whereas the area enclosed by BCDEFG is a true hysteresis loop. In the following paper (Brown & Newman, 1970, Part III) further evidence is given that micas are not reconstituted when their K-depleted alteration products are potassium-saturated, and possible reasons for this are discussed, so that discussion here is confined to the hysteresis obervation. Hysteresis is not uncommon in clay mineral properties although it has not often been recognized in cation exchange. Keay & Wild (1964) found that exchange isotherms for the Na-Mg pair in vermiculite differed little on the forward and reverse directions, but Tabikh, Barshad & Overstreet (1960) observed that in bentonites, the equilibrium quotients for the pair K-Na depended on the direction of exchange and hydration state; in soil clays, Deist & Talibudeen (1967) found hysteresis in K-Ca exchange. Tamers & Thomas (1960) concluded that changes in aggregation in kaolinite were responsible for slow equilibrium in Cs-Na exchange; van Bladel & Laudelout (1967) suggested that hysteresis in the exchange of NH4 in montmorillonite depends on the ionic strength of the solution and disappears at infinite dilution. Olsen & Sherry (1968) noted that hysteresis of Na-Sr exchange in the zeolite Linde X is accompanied by a change in unit cell dimensions, and according to Wilklander (1964) hysteresis in layer silicates is often associated with cation fixation, a process in which hydration water is eliminated with concomitant decrease in basal spacing. Hysteresis is usual in adsorption/desorption of hydration C

272 A. C. D. Newman water in montmorillonite and vermiculite, both from the vapour (van Olphen, 1965; Kittrick, 1966, 1969) and the solution phases (Laffer, Posner & Quirk, 1966). It seems reasonable to generalize that hysteresis may be expected whenever there is a change in the number of layers in interlamellar water, and that exchange reactions between the weakly hydrated cations K, NH4, Rb and Cs and the more strongly hydrated cations Li, Na, Mg, Ca, Sr and Ba will all exhibit hysteresis. This work shows that K is resorbed by a partly K-depleted mica only when the solution K concentration is increased above the maximum that can be attained when K is desorbed from the mica by an aqueous electrolyte containing a strongly hydrating cation. The resorbed K is exchanged more readily than K from the original mica. REFERENCES BROWN G. & NEWMAN A.C.D. (1970) Clay Miner. 8, 273~ DEIST J. & TALIBUDEEN O. (1967) J. Soil Sci. 18, 125. EVERETT D.H. & WnrrxoN W.I. (1952) Trans. Faraday Soc. 48, 749. KEAY J. t~ WILD A. (1964) J. Soil Sci. 15, 135. KIT~ICK J.A.(1966) Proc. Soil Sci. Soc. Am. 30, 801. KITTRICK J.A. (1969) Proc. Soil Sci. Soc. Am. 33, 217. LAFFER B.G., POSNER A.M. & QLrmK J.P. (1966) Clay Miner. 6, 311. NEWMAN A.C.D. (1969) J. Soil. Sci. 20, 357. OLSEN D.H. & SHERRY H.S. (1968) J. phys. Chem. 72, 4095. RAUSELL COLOM J.A., SWEATMAN T.R., WELLS C.B. & NORmSH K. (1965) Experimental Pedology. Proc. 1 lth School Agric. Sci. Nottingham. (E. G. Hallsworth and D. V. Crawford, editors) p. 40, Butterworths. TABIKH A.A., BARSHAD I. & OVERSTREET R. (1960) Soil ScL 90, 219. TAMERS M.A. & THOMAS H.C. (1960). J. phys. Chem. 64, 29. VAN BLADEL R. & LAtJDELOUT H. (1967) Soil Sci. 104, 134 VAN OLPHEN H. (1965) J. Colloid Sci. 20, 822. WIKLArqDER L. (1964) Chemistry of the Soil (ed. by F. E. Bear), p. 163, Reinhold Publ. Corp., New York.