BEHAVIOUR OF POTASSIUM IN SOME WEST INDIAN SOIL CLAYS

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1 Clay Minerals (1972) 9, 287. BEHAVIOUR OF POTASSIUM IN SOME WEST INDIAN SOIL CLAYS C. E. DAVIS Department of Mines, Hope, Kingston 6, Jamaica (Received 25 January 1972) ABSTRACT: Fixation of K by some West Indian soil clays was proportional to their montmorillonite contents. Reductions in c.e.c, and surface areas occurred after fixation. The reductions in c.e.c, were equivalent to the amounts of K fixed, suggesting that an ion exchange mechanism was operative. Reductions in c.e.c, were proportionately greater than reductions in surface area. Release of native K from the clays was greater from the < 0"2 tzm fractions than those from 0"2--2"0/zm. Appreciable amounts of'recently fixed' K were not released after extraction with 0"03 N NaC1 solution. Removal of 'free' iron increased the release of: (i) native K from the 0"2--2"0/~m fractions; and (ii) 'recently fixed' K from both fractions. The presence of NH4 ions decreased the release of'recently fixed' K. The release pattern of the clays indicates that intra-particle diffusion rather than film diffusion or mass action exchange, is the rate-limiting mechanism. INTRODUCTION Although much has been written on the phenomenon of K fixation in soils, the subject remains open, mainly because several of the experimental results published so far have been indefinite and in some cases the conclusions drawn have been contradictory. Numerous reports have also appeared on the release of K from soils and clays. But like fixation, the actual mechanism(s) operating and the various factors affecting this phenomenon are still in doubt. Hence in either case further research is necessary before more definite conclusions can be drawn. An earlier paper (Davis, Ahmad & Jones, 1971) discussed hydrothermal and dry heat (high temperature) fixation of K by some West Indian soil clays. The purpose of this paper is to present another phase of this work: (i) the fixation of K by the clays at the more conventional dry heat temperature of 105~ and the effects of fixation on the c.e.c., surface area and mineralogy of the clays; and (ii) the release of 'native' and 'recently-fixed' K from the clays.

288 C. E. Davis Sources of samples EXPERIMENTAL The sources of the clays were described earlier (Davis et al., 1971). Mineralogical compositions were determined by methods described in that paper.

2 288 C. E. Davis Sources of samples EXPERIMENTAL The sources of the clays were described earlier (Davis et al., 1971). Mineralogical compositions were determined by methods described in that paper. Fixation of K K-fixation was induced by first shaking the clays with N KC1, then removing the 'excess' KC1 with a water-methanol solvent (60:40 ratio was used to minimize dispersion). They were then dried in an oven at 105~ for about 12 h. 'Exchangeable' K was then removed by washing thrice with N SrC12 solution. The 'excess' SrC/2 was also removed with water-methanol solvent; and then Sr in the clays was determined by X-ray fluorescence (Davis et al., 1971). Release of K Approximately 0.5 g of clay was shaken in 50 ml of 0"03 N NaC1 on a reciprocal shaker. This concentration of NaC1 was chosen because it represented the maximum concentration at which the Na interference effect on K in solution--as measured by the EEL flame photometer--was minimal. Due to known temperature effects on the rates of K release, shaking took place in a room at fairly even temperature (20 + I~ After shaking for a prescribed period of time, the clays--in particular the <0.2 t~m fractions--were sedimented in a high-speed centrifuge. The clear filtrate was poured off into volumetric flasks for subsequent determination of K by flame photometry. When a longer period of shaking was desired, the sedimented clays were returned to the flasks with the aid of a rubber policeman, and up to 50 ml of 0"03 N NaC1 solution added. C.E.C., sur[ace area and mineralogy C.E.C. (Sr by X-ray fluorescence), surface area (EGME method of Carter, Heilman & Gonzalez, 1965) and mineralogy (X-ray diffraction and other methods) were carried out as before (Davis et al., 1971). Results Because of their special relevance to this paper, the mineralogical compositions of the clays previously determined (Davis et al., 1971) are reproduced in Table 1. The relationships between the K contents, c.e.c, and surface area before and after fixation, are indicated in Table 2. The total K released by the days, i.e. after a period when virtually no further release occurred (this varied with treatments and samples), are indicated for various treatments in Table 3. 'Untreated' refers to the control samples, hence the K released can be assumed to be 'native' K. '+K' means clays to which K-fixation was induced by first shaking the clays in ~ KCI solution as described above. '+K+NH~' refers to clays in which fixation was induced by first shaking the clays in a solution 0"5 N with respect to KC1 and 0.5 N with respect to NH4C1. '-Fe' indicates clays

3 K in West Indian clays TABLE 1. Calculated mineralogy of the soil clays 289 Free Quartz and/ Soil Clays Amorphous Fe203 Kaolinite Montmor. Mica or others <0.2 ~,m St John's Valley #m St John's Valley < 0.2/zm Cunupia I /zm Cunupia < 0.2/zm Mon tserrat tzm Montserrat < 0-2/~m Princes Town "2-2-0/,m Princes Town l < 0-2/zm River Estate /zm River Estate < 0.2 ~m Talparo t~m Talparo in which non-lattice iron was removed by citrate-dithionite procedure. '-Fe+K' indicates clays in which K-fixation was induced after removing non-lattice iron. Fig. 1 indicates the release curves of some of the clays, based on diffusion assumptions. DISCUSSION The data (Table 2) support the view that 2:1 clay minerals are primarily responsible for K-fixation in clays. A majority of the clays fixed K in proportion to their montmorillonite contents. However the two St John's Valley clays and the <0-2 t~m River Estate Clay behaved anomalously. Despite having estimated values of 22, 25 and 30% montmorillonite respectively, they fixed appreciably less K than those clays which had slightly higher or even less montmorillonite. This suggests the possible role of other factors such as the blocking effect of Fe and A1 'islands' (Rich & Lutz, 1964). The presence of these interlayer materials could retard the movement of the K ions through the interlayers, and later prevent complete collapse of the lattice during drying. The data in Table 1 show that the 'free' Fe contents of these clays are fairly high with values of 13, 11 and 18% respectively. Table 2 shows that reductions in c.e.c, accompanied fixation. A 't' test indicated that the difference between the means of c.e.c, reductions and K fixed is not significant. This suggests that fixation is by an ion exchange mechanism, whereby the K ion replaces an equivalent amount of some other cation. This observation of equivalence between K fixed and c.e.c, reductions, supports the view of Truog &

4 290 0'225 C. E. Davis 0,200 0'175 0'150 ~ ~li;l Princes Town (control) Cunupio j ~ ~ Tolporo (control) I--I Princes Town(-I-K) A,5 Montserrat (-~ K) O~O Cunupia Tolparo (-I-K) ~k----a Montserrat (control) [] 0"i25-0'I00 0'075 0"050 0' "60 0' " HO FIG. 1. Potassium release from four West Indian soil clays. TABLE 2. K, C.E.C., and Surface area data of clays before and after K fixation Before Fixation After Fixation Surface Surface Soil clays C.E.C. K area C.E.C. K area A C.E.C. (me/100g) (mz/g) (me/100g) (ms/g) (me/100g) < 0.2/~m St John's Valley " /~m St John's Valley 27'5 5' " "20 6"34 <0"2 t~m Cunupia "5 52" " '2-2-0/~m Ctmupia " " <0"2 t~m Montserrat " " " "2-2"0 #m Montserrat 43" <0"2/~m Princes Town 77"1 21" "0 47" "2-2'0 t~m Princes Town " <0-2 ~m River Estate " " '80 7"30 0"2-2"0 t~m River Estate <0.2/zm Talparo t~m Talparo " "30 t (AC.E.C.; AK) = 0.32; t.0~ (for 11 d.f.) = 2.2. AK

5 K in West Indian clays 291 Jones (1938), Bailey (in Reitemier, 1951), Wear & White (1951), Barshad & Kishk (1969), and others. However it is contrary to the conclusions of Joffe & Levine (1940), that although reductions in c.e.c, accompany fixation, there is no equivalence between amounts fixed and c.e.c, reductions and those of Mathews & Beckett (1964) that there is no change of c.e.c, as a result of K fixation. It is interesting to note however, that Joffe & Levine (1940) found the magnitude of fixation was in the range me/100 g to me/100 g; while Mathews & Beckett (1964) found the amounts fixed were as low as 0"1 me/100 g. On the other hand Truog & Jones (1938) showed fixation values between 1.5 me/100 g and 4"9 me/100 g. The present data (Table 2) show the amounts fixed ranged from approximately 4 me/100 g to 25"6 me/100 g. Because fixation is not necessarily due to an ion exchange mechanism, it is possible that the non-equivalence of K fixed and c.e.c, reductions observed by these workers is quite valid. On the other hand it is conceivable that non-observance of equivalence could have been due to errors involved in measuring small magnitudes of fixation. The data in Table 2 also show that reductions in surface areas accompanied fixation. However, the values for different clay fractions indicate that the reductions in c.e.c, were proportionately greater than those in surface areas. The reason for this disparity in reductions is not clear, but it means that the c.e.c.-surface area relationship in clays is affected by the K-fixation history of the clays. Hence in extrapolating c.e.c, from surface area or vice versa, this factor should be taken into consideration. Mineralogical changes were evident for the <0.2 t~m clays but because they were essentially the same as reported earlier (Davis et al., 1971), the diffractograms are not reproduced here. K release K release was affected by : (i) particle size; (ii) type of K-bearing minerals present; (if) K fixation; (iv) NH4 fixation; and (v) presence of interlayer Fe~O3. The effects of these factors were generally consistent with those observed by other workers. As the data in Table 3 show, in general considerably more 'native' K was released from the finer (<0.2 tzm) fractions than from the coarser. This was partly the effect of surface area per se, and that the coarser particles have relatively more of the less-easily weatherable K-minerals (e.g. feldspars) than the finer particles. The importance of the type of K-bearing mineral present is illustrated by the behaviour of the t~m River Estate clay, which despite having 52~ mica (Table 1), released less K (0.16 me/100 g) than the other clays. The mica of the River Estate clay is thought to be muscovite (Ahmad, Jones & Beavers, 1966). As is well known the trioctahedral micas, of which biotite is a prominent member, weather more easily than a dioctahedral member like muscovite. Bassett (1960) explains this difference in behaviour in terms of the orientation of the hydroxyl ions in the respective micas. The dipole moments of the hydroxyl ions in trioctahedral micas are perpendicular to the cleavage planes, whereas those in the dioctahedral micas are obliquely oriented. As such, the proton is further from the ion, and

292 C. E. Davis TABLE 3. Total K released (me/10og) by soil clays Treatments Soil clays Untreated ( + K) (+ K + NH4) ( - Fe) (- Fe + K) < 0.2/zm St John's Valley 1 "04 3-25 1.37 0-47 4-64 0.

6 292 C. E. Davis TABLE 3. Total K released (me/10og) by soil clays Treatments Soil clays Untreated ( + K) (+ K + NH4) ( - Fe) (- Fe + K) < 0.2/zm St John's Valley 1 " t~m St John's Valley < 0.2 tzm Cunupia " " t~m Cunupia < 0-2 t~m Montserrat "32 17" "0/~m Montserrat < 0.2/zm Princes Town 11' ' "2-2"0 ~m Princes Town <0-2 ~m River Estate t~rn River Estate < 0-2 tzm Talparo ' " am Talparo places the negative or oxygen end of the dipole closer to the K, with a resulting decrease in binding energy. The release data support the suggestion (Ahmad et al., 1966) that the mica of the River Estate clay is muscovite. On the other hand, the micas of the other clays are likely to be mainly trioctahedral. The inducing of K-fixation by the clays resulted in four of the six samples releasing more K. However, the two very similar clays--cunupia and Talparo-- behaved anomalously by releasing les~ K than from the control samples. This possibly suggests that when K was added and fixation induced, the last-named sealed off areas that were hitherto accessible to the exchanging Na and K ions. These K ions, though potentially exchangeable, would not be exchanged due to the closure of the passages, As a result, the Na ions could not reach them to effect the exchange. Comparison of the actual amounts of K fixed by the clays (Table 2) and the amounts released (Table 3), shows that an appreciable amount of the fixed K was not released even after 14 days of nearly continuous extraction. This is of some significance for it raises the question of whether or not K fixation is 'sufficiently irreversible' to be considered of importance in diagenesis. Keller (1964) points out that many geologists restrict the term 'diagenesis' to a more fundamental and relatively more permanent change in the crystal structure, than cation exchange. However, the holding of K ions by a clay against extraction by 0"03 N NaCI (0-5 g clay to 50 ml NaC1) over nearly 14 days of continuous shaking may well indicate a fairly irreversible change. The numerous observations that there is some competition between K and NH, ions for specific adsorption sites in the clay structure are supported by the data. All the samples (Table 3) showed an appreciable decrease (relative to the +K

K in West Indian clays 293 samples) in the amounts of K released. However, but for one exception, the reductions in K release were less than 50% of the +K samples.

7 K in West Indian clays 293 samples) in the amounts of K released. However, but for one exception, the reductions in K release were less than 50% of the +K samples. This means in effect that less NH~ was fixed than K, supporting the concept that the configuration of the K ions in the hexagonal voids (Wear & White, 1951) is more stable than that of the NH4 ions in the voids. The removal of non-lattice iron did not have any consistent effect with respect to release by the fine days. In three cases a decrease (over the control) of release occurred, while in one case a very significant increase occurred. The effect however on the 0"2-2.0 t~m clays was quite appreciable, resulting in much more K being released than from the control samples. A possible explanation for this is that by removing the iron, aggregated particles were dispersed into their ultimate sizes. This means that despite the ultrasonic and other dispersion treatments that were used, the bonding of the clay particles by iron may have been so strong, that particles centrifuged as 0.2 ~m were in fact finer. Hence because of reduced surface area in an aggregated state (as Table 2 shows) they released less K. Release of K from the 'iron-free' samples to which fixation was induced was greater than all other treatments. This could have been due to a combination of two factors: (i) the particle size effect discussed above; and (ii) the fact that 'the iron islands' were removed from the interlayers thus enabling both the exchanging Na ion and the exchanged K ion to move more freely in and out of the particle, respectively. Mechanisms of K release Meller & Bright (1957) in reviewing some of the relationships between the rates of reaction such as the exchange of K by Na pointed out that the following relationship holds" dx dt= ake- k, (1) dx log ~- --- log ak - kt (2) where dx/dt = rate of K release; k ---- reaction rate constant; and t = time. Hence for mass action exchange a plot of the log of the rate of release against time should yield a linear curve. Boyd, Adamson & Myers (1947) and Mortland & Ellis (1959) pointed out that a linear curve could also be indicative of film diffusion--i.e, diffusion across a thin film in and out of the particle--because: dx qdt dt= B-~- (3) qdt or In d~ = In B- v~- (4)

8 294 C. E. Davis where x is the amount of K remaining in the sample at time t, q is the cross-sectional area of the diffusion film, v is the volume of the diffusion chamber, l is the thickness of the diffusion chamber, D is the diffusion coefficient. Because both equations (2) and (4) can lead to a linear curve, it is necessary, in order to determine whether mass action or film diffusion is the rate limiting factor in a reaction, to consider other distinguishing criteria, such as the activation energies of the reactions and the effects of leaching, and particle size. Chute & Quirk (1967) assumed a model based on radial diffusion in a cylinder to explain the diffusion of, for example, a Na ion through a mica particle to the exchange site, and then the movement of the exchanged K ion through the particle to the solution. They proposed the equation: 1Mte(D)89 I.D tmoo=~ 9 ~ 9 t~ a-- ~ (5) where Mt is the amount of K which has diffused in time t, Moo is the amount which has diffused in infinite time, a is the radius of the cylinder, D is the diffusion coefficient. Mt l Thus a plot Moo vs~ (6) t should give a straight line, if diffusion through the particle is operative. Plots (not shown) of the log of the~ate of release against time were made for the Cunupia, Montserrat, Princes Town and Talparo clays. In no case was a linear curve obtained. Hence on the basis of equations 2 and 4, both mass action and film diffusion were eliminated as the rate-limiting processes affecting the release of K from these clays. The data (Fig. 1) were plotted according to equation 6. Moo was based on the total K value for each clay (Table 2). As the figure shows all the curves are linear, indicating that diffusion through the particle is the rate-limiting process. As also observed by Chute & Quirk (1967) the diffusion equation was not strictly obeyed during the initial period of release, because of mass action exchange between Na and K at sites on the external surface of the particles. ACKNOWLEDGMENTS The author wishes to acknowledge the help of Professor N. Abroad (Department of Soil Science, U.W.I., St. Augustine, Trinidad) and Dr Robert L. Jones of the University of Illinois, Urbana, Illinois.

9 K in West Indian clays 295 REFERENCES AHMAD N., JONES R.L. & BEAVERS A.H. (1966)J. Soil ScL 19, 9. BARSHAD I. & KaSnK F. (1969) Clays Clay Miner. 18, 127. BASSETT W.A. (1960) Bull. geol Soc. Am. 71, 449. BOYD G.E., ADAMSON A.W. & MYERS L.S. (1947) J. Am. chem. Soc. 69, CARTER D.L., HEILMAN M.D. & GOtqZALEZ C.L. (1965) SoilSci. 100, 356. CHUTE J.H. & QUIRK J.P. (1967) Nature, Lond., 213, DAvis C.E., AHMAD N. & JONES R.L. (1971) Clay Miner. 9, 219. JoFr~ J.S. & LEVINE A.K. (1940) Proc. Soil Sci. Soc. Am. 4, 157. KELLER W.D. (1964) Soil-Clay Mineralogy, pp Univ. of North Carolina Press, Chapel Hill, North Carolina. MATHEWS B.C. & BECKEZ~r P.H.T. (1964)./. agric. ScL, Camb. 58, 59. MELLER A. & BRIGHT J.E. (1957) J. phys. Chem., Ithaca, 62, 395. MORTLANO M.M. & ELLIS B. (1959) Proc. SoilSci. Soc. Am. 23, 363. REITEMIER R.F. (1951) Advances in Agronomy 3, p Academic Press, New York. RICH C.I. & LUTZ J.A., JR. (1964)Proc. Soil. Sci. Soe. Am. 29, 167. TRUOO E. & JONES R.J. (1938) Ind. Engng. Chem. ind. (int.) Edn. 30, 882. WEAR J.J. & WHITE J.L. (1951) Soil Sci. 71, 1.

HETEROGENEITY IN MONTMORILLONITE. JAMES L. MCATEE, JR. Baroid Division, National Lead Co., Houston, Texas

HETEROGENEITY IN MONTMORILLONITE By JAMES L. MCATEE, JR. Baroid Division, National Lead Co., Houston, Texas ABSTRACT X-ray diffraction patterns and cation-exchange data are presented for centrifuged Wyoming