Potassium sorption by calcium

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Potassium sorption by calcium‐bentonite (Ca‐b) a

M. Doula , A. Ioannou Mitsios

a b

a

, A. Dimirkou & J.

a

a

National Research Agricultural Foundation of Greece , Soil Science Institute of Athens , Sof. Venizelou 1, Lycovrissi, Attiki, 14123, Greece b

14 Thermopillon St., Pallini, 15344, Greece Published online: 11 Nov 2008.

To cite this article: M. Doula , A. Ioannou , A. Dimirkou & J. Mitsios (1994) Potassium sorption by calcium‐bentonite (Ca‐b), Communications in Soil Science and Plant Analysis, 25:9-10, 1387-1400, DOI: 10.1080/00103629409369122 To link to this article: http://dx.doi.org/10.1080/00103629409369122

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COMMUN. SOIL SCI. PLANT ANAL., 25(9&10), 1387-1400 (1994)

POTASSIUM SORPTION BY CALCIUM-BENTONITE (Ca-b)


M. Doula, A. Ioannou,1 A. Dimirkou, and J. Mitsios2 National Research Agricultural Foundation of Greece, Soil Science Institute of Athens, Sof. Venizelou 1, Lycovrissi 14123, Attiki, Greece ABSTRACT: The kinetics of potassium adsorption from solution to exchangeable phases was investigated on bentonite samples which were first saturated with calcium in the form of CaCl2. Potassium adsorption time was evaluated on Casaturated samples using 125, 150, 200 and 250 µgK/ml solutions, equilibrated for 10, 15, 20, 30, 35, 45, 60, 75, 80 and 120 minutes. Sample pH levels varied between 4.0 and 9.0. Equilibrium in potassium exchange was reached faster in lower concentrations and higher pH values. Four mathematical models (first-order rate, parabolic diffusion, Elovich and modified Freundlich equation) were used to describe potassium sorption. Comparison of coefficients of determinations (r2), and plots indicated that modified Freundlich and parabolic diffusion models provided the best fits of the adsorption data (r2 > 0.979). Constants for the Freundlich equation were estimated and the model was expressed as a function of pH. INTRODUCTION Montmorillonite [Al4(SiOi0)2(OH)4] is the major component of bentonite (> 60%), which consists of one octahedral sheet [A1O6 unit] sandwiched between

1

University of Athens, Department of Chemistry, Panepistimiopolis-Zografou, Athens 15771, Greece. Postal address of the corresponding author: A. Ioannou, 14 Thermopillon St., 15344, Pallini, Greece. 2

Thessalia University, Pedio Areos, Volos 38334, Greece.

1387 Copyright © 1994 by Marcel Dekker, Inc.

1388

DOULAETAL.


two tetrahedral sheets [SiO4 units]. The oxygens of one 2:1 layer always face the oxygens of the next layer; therefore, no hydrogen bond can take place and the layers are not strongly held together. Two interesting characteristics of montmorillonite are its capacity to expand by absorbing water or other liquids and its potential for ion exchange when, under certain conditions, the cations sodium, potassium and calcium present between layers in the structure are replaced by other cations present in a solution (O'Neil, 1985). Potassium exchange in soils and clay has been investigated by numerous researchers, most of whom have centered on the specific adsorption sites for K on clays (Goulding and Talibudeen, 1980; Talibudeen and Goulding, 1983) and soils (Bolt et al., 1963; Singh et al., 1981). Kinetic reactions are thought to exist between the various phases of K. The reaction between the soil-solution and exchangeable phases of K is proposed to be almost instantaneous (Malcom and Kennedy, 1969). Potassium exchange on pure montmorillonite, "illite" and kaolinite was found to be rapid, with 75% of the total exchange occurring within 3 sec. However, the rate of potassium exchange on vermiculitic materials was slower with 50 and 97% of the exchange reached after 10 and 720 sec, respectively (Malcom and Kennedy, 1969). This slower rate of exchange in vermiculite was attributed to slow diffusion into interlayers. Several model equations have been used to study the adsorption of potassium by soil materials. Selim et al. (1976) proposed that a kinetic reaction existed between soil-solution and exchangeable K with an adsorption rate coefficient (ka) governing the forward reaction and a résorption rate coefficient (kd) governing the reverse reaction. They proposed that the adsorption reaction (ka) was the nth order while the desorption reaction (kd) was first order. Conversely, Sparks et al. (1981) claimed that potassium adsorption follows the first-order kinetic model. In a previous study, Sparks et al. (1980) studied the kinetics of potassium adsorption from solution to exchangeable phases for two Dothan soils. The adsorption rate coefficients ranged from about 0.7 to 22.0 hours"1 and generally decreased at higher initial concentrations of K-solution. They observed that the sorption of potassium follows a modification of the Freundlich equation, which has been proposed by Kuo and Lotse (1974). Barshad (1954) reported that diffusion-controlled exchange is characterized by a linear relationship between


K SORPTION BY Ca-b

1389

potassium adsorbed and (time)"2 for sand and gravel-sized mica and for vermiculite. The same model has been used to describe the kinetics of potassium release (Feigenbaum et al., 1981; Havlin et al., 1985). Simple first-order rate functions have also been used to describe potassium adsorption-desorption (Sparks and Jardine, 1981) and K release over short (< 1000 h) time periods (Munn et al., 1976; Martin and Sparks, 1983). The exponential Elovich equation, which has general application to chemisorption kinetics (Low, 1960), has been applied to the dissolution of hydroxyapatite (Smith et al., 1977), kinetics of phosphate release and sorption (Chien and Clayton, 1980), dissolution of phosphate rocks in soils (Chien et al., 1980), and kinetics of potassium release (Havlin et al., 1985). The purpose of the study was to investigate the kinetics of reaction occurring between the solution and exchangeable phases of K in calcium-bentonite samples and to analyse how the process is influenced by different pH values of the samples and initial added potassium. A second objective was to determine the applicability of the four mathematical models. MATERIALS AND METHODS The clay samples (< 2u) used for this study were bentonite (Argilometalic Lot No 7063). Calcium-bentonite was prepared by treating bentonite with 1 N CaCl2 solution until all exchangeable potassium was replaced by calcium. The clay was then washed until free of chloride. Adsorption studies were carried out using triplicate samples (0.25 g) of calcium-bentonite. KC1 solution (12.5 ml) containing 125, 150, 200 and 250 ppm of potassium and 10.0 ml of buffer solution at pH levels of 4.0, 5.0, 7.0, 8.0 and 9.0 was added to samples of calcium-bentonite. The samples were shaken at 25°C for 10, 15, 20, 30, 35, 45, 60, 75, 80 and 120 minutes and centrifuged. Potassium was determined in the liquid phase. The amount of potassium sorbed was calculated by subtracting the final from the initial potassium concentration. Cation exchange capacity (C.E.C.) was determined by a MgCl2 saturation with subsequent displacement by CaCl2 (Okazaki et al., 1963; Rich, 1962). The exchangeable cations, Na and K, were determined following extraction by 1 N ammonium acetate, pH 7. The CaCO3 equivalent was determined by treatment with dilute acid and the volume of released CO2 was measured by the Bernard calcimeter. The chemical properties of calcium-bentonite are shown in Table I.

1390

DOULA ET AL.


Table I. Some chemical properties of calcium-bentonite. Exchangeable Na CaCO3 equiv C.E.C meq/100g % meq/100g 81.4 6.7 2.56

Exchangeable K meq/100g 0.64

The modified Freundlich equation, as proposed by Kuo and Lotse (1974), is: X = kCjVm 1 W log X = [logK + logCJ + —logt tn where X is the amount of K adsorbed (ngK/g of Ca-b), Co the initial K concentration (ngK/g Ca-b), t the reaction time (min), K a kinetic factor (min1) and m a constant. The parameters 1/m and K were calculated from the slope and the intersection with y-axis of the linear portion of the plots, respectively. The equation describing the first-order rate model was derived by Sparks and Jardine (1981). From the equation: log(l -Fa) = kat

(2)

the apparent adsorption rate coefficient, ka', is the slope obtained by plotting log (1-Fa) against t where, Fa fraction sorbed (Xt/Xeq), Xt total K adsorbed on the soil at time t (ngK/g Ca-b), Xeq total K adsorbed on the soil at equilibrium (ugK/g Ca-b), ka absolute velocity coefficient of the adsorption process and ka = kaXeq/2.303. The Elovich equation, as modified by Chien and Clayton (1980) to study the kinetics of phosphate adsorption and release, is expressed as Xf =

í

(3)

/n(l + act) where Xt is the adsorbed amount of potassium (ugK/g Ca-b) at time t, and a and c are constants. If we assume that a c t » l , then the equation can be written as Xt = -In(ac) + -Int c c

(4)

K SORPTION BY Ca-b

1391

A plot of Xt vs. Int should yield a linear relationship with a slope 1/c and an intersection with the y-axis of (l/c)ln(ac). The chemical significance of these constants is not clearly resolved (Chien and Clayton, 1980). The parabolic rate equation, as given by Sparks and Carski (1985), is:


Fa = Rtxri + constant

^

where R is an overall diffusion constant. Although initially used as an empirical equation, it can be derived from Fick's diffusion laws for the case of uncharged particles (Fromhold, 1976). Plots of Fa against time"2 are often used to test a diffusion controlled reaction rate (Cooke, 1966; Vaidyanathan and Talibudeen, 1968; Sparks and Jardine, 1981). RESULTS AND DISCUSSION Differences in the potassium adsorption process are due to different pH values and initial potassium concentrations. Experimental data show that pH is a significant factor influencing the rate of potassium sorption. An increase in pH resulted in faster sorption and in greater amounts of potassium being sorbed at a given solution concentration. The authors indicated that this probably was caused by the formation of new sorption sites, together with a decrease of competition between H+ and K+ for these sites (Garcia-Miragaya and Page, 1987). Initial potassium concentration also influences the adsorption rate coefficient and time until equilibrium. The experiment indicated a faster exchange rate for the lower concentrations of added K, a finding which is in agreement with Bronsted's activity rate theory (Moore, 1972). For example, when initial potassium concentration (Co) was 125 ppm and pH 9.0, the equilibrium was reached in 45 minutes, but when Co was 250 ppm and the pH value was the same, the equilibrium was reached in 60 min. Conformity of the data to the linear form of each rate equation was tested using linear regression analysis. The resulting coefficients of determination (r2, Tables II and III) indicate that the data can be best fit by the modified Freundlich equation [1] (Figures la and lb). The parabolic diffusion model [5] also describes successfully the adsorption data by giving a curvilinear relationship (Figure 4). Adsorption data plotted according to the first-order rate equation [2] (Figures 2a

1392 Table IL


Co ugK/g 6250

7500

10000

12500

DOULA ET AL. Regression equations and coefficients of determinations (r2) of adsorption for first-order rate and Elovich models. First-order rate pH equation r2 Elovich equation r2 4.0 -0.858-0.160t 849.7+59.771nt 0.668 0.996 5.0 -0.799-0.017t 731.3+63.761nt 0.999 0.993 7.0 -0.665-0.022t 1054.8+106.91nt 0.998 0.989 8.0 -0.562-0.021t 1118.0+167.61nt 0.999 0.996 9.0 0.471-0.028t 1200.2+227.61nt 0.992 0.986 4.0 -0.597-0.024t 995.5+67.991nt 0.736 0.916 5.0 1040.4+85.721nt -0.709-0.018t 0.990 0.679 7.0 -0.595-0.015t 1192.1+144.61nt 0.986 0.669 8.0 -O.515-O.O18t 1188.3+239.31nt 0.994 0.998 9.0 -0.400-0.020t 1227.1+369.91nt 0.987 0.995 4.0 -0.684-0.010t 1136.9+114.91nt 0.999 0.997 5.0 -0.448-0.022t 1217.8+163.61nt 0.930 0.999 7.0 -0.463-0.017t 1313.9+288.01nt 0.989 0.996 8.0 -0.373-0.020t 0.992 1355.9+472.71nt 0.999 9.0 -0.265-0.020t 0.994 1096.5+882.61nt 0.997 4.0 -0.580-0.010t 1067.0+139.21nt 0.998 0.808 5.0 -0.310-0.022t 1156.1+243.61nt 0.854 0.930 7.0 0.924 1286.3+482.61nt -0.197-0.021t 0.724 8.0 1020.2+962.41nt 0.634 -0.210-0.017t 0.985 9.0 -0.110-0.019t -1037.3+2391.11nt 0.995 0.993

and 2b) and according to the Elovich equation [4] (Figures 3a and 3b) showed considerable deviation from linearity. Figures la and lb present the plotting of adsorption data according to the modified Freundlich equation for the same initial concentration (la) and for the same pH value (lb). The adsorbed time increased as the initial concentration increased. This resulted in the appearance of a greater number of points in the plots logX vs. logt. Although not shown, a similar behaviour occurred in the other examined experimental conditions. Linearity coefficients (r2) of the Freundlich model varied between 0.994 and 0.999. The estimated K and m values (Table IV) were conformed to a linear

1393

K SORPTION BY Ca-b

Table III.


Co ugK/g 6250

7500

10000

12500

Regression equations, coefficients of determination (r2) and K values of potassium adsorption for Freundlich equations. K r2 pH Modified Freundlich -2.947+0.048t 0.159 4.0 0.999 0.178 -2.995+0.054t 5.0 0.999 0.196 7.0 -3.038+0.077t 0.998 0.212 -3.077+O.lOlt 8.0 0.999 0.234 9.0 3.118+0.120t 0.995 4.0 5.0 7.0 8.0 9.0 4.0 5.0 7.0 8.0 9.0 4.0 5.0 7.0 8.0 9.0

-2.998+0.059t -3.021+0.067t 3.080+0.094t -3.120+0.122t -3.167+0.154t -3.074+0.074t -3.112+0.092t -3.169+0.124t -3.226+0.165t -3.275+0.226t -3.039+0.096t -3.092+0.133t -3.186+0.18H -3.243+0.25U -3.299+0.368t

0.999 0.999 0.993 0.998 0.998 0.998 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999 0.999

0.142 0.150 0.173 0.190 0.210 0.118 0.130 0.148 0.169 0.190 0.089 0.100 0.124 0.142 0.370

function of pH. Substituting K and m values for their pH function at equation [1] resulted in the modified Freundlich equation as a function of pH. Figures 2a and 2b present the plotting of experimental data according to the first-order rate model for the same initial concentration (Figure 2a) and the same pH value (Figure 2b). The linearity coefficients (r2) varied between 0.916 and 0.998. Estimated ka values are not in agreement with the general result obtained from the present and previous studies, that the absolute velocity coefficient increased with increased pH and decreased initial concentration of added K (Table

1394

DOULA ET AL.


Co=6250 pgK/g

bgt Figure la. Freundlich equation plot for potassium adsorption on calciumbentonite for the pH values 4.0, 5.0, 7.0, 8.0, 9.0 and 6250 ugK/g Ca-b initial concentration.

3.8 -, 3,7 •

• 6250|i3K/3

3,6 -

A 10000|igK/g

pH8.0

O12500MSK/S

3,5 • |> 3.43.3 • 3.2 •

3,1 3-

•+-

0,5

1

1,5

logt Figure Ib. Freundlich equation plot for potassium adsorption on calciumbentonite for 6250, 7500,10000 and 12500 |igK/g Ca-b initial concentrations and pH 8.O.


K SORPTION BY Ca-b

1395

20

50

40

30

Time , minutes

Figure 2a. First-order rate equation plot for potassium adsorption on calciumbentonite for the pH values 4.0, 5.0, 7.0, 8.0, 9.0 and 6250 ngK/g Ca-b, initial concentration.

• 625OM9K/g

pH8.0

O7500,igK/g AlOOOOpsK/g A12500 pgK/g

20

30

40

50

60

Time , minutes

Figure 2b. First-order rate equation plot for potassium adsorption on calciumbentonite for 6250, 7500, 10000 and 12500 HgK/g Ca-b initial concentrations and pH 8.O.

DOULA ET AL.

1396 2500

3

a

2000 •

Co = 6250 ugK/g

OPH9

ô

-2?

• pH 4 OpH5 ApH7

1500 ••


"O Ja

1000 •

(0

500-

Int

Figure 3a. Elovich equation plot for potassium adsorption on calcium-bentonite for the pH values 4.0, 5.0,7.0, 8.0,9.0 and 6250 ugK/g Ca-b initial concentration.

5000 T 4 5 0 0 ••

• 6250pgK/g O 7500 iigK/g A10000 pgK/g

3 ¿ •§ ^ •£ | X

4000 •• 3500-3000 •• 2500 2000 •• 1500.. íooo .

Ol2500|i 9 K/g

500 • •

0•0

tnt

Figure 3b. Elovich equation plot for potassium adsorption on calcium-bentonite for 6250, 7500, 10000 and 12500 ugK/g Ca-b initial concentrations and pH 8.O.

K SORPTION BY Ca-b

Table IV.

1397

pH-dependent forms of Freundlich equations.

Co ugK/g

Modified Freundlich as function of PH

K

x

Co

x

Xt = (0.104+0.014pH) r2 = 0.976

x

6250

x

Xt = (0.085+0.014pH) r2 = 0.977

x

Xt = (0.060+0.014pH) r2 = 0.965

x

Xt = (0.030+0.014pH) r2 = 0.988

x

Xt = Downloaded by [Technical University of Crete] at 02:12 15 April 2014

6250 7500 10000 12500

0,9 0,8 ••

0,7 0,6 -•

A O O

l/(31.0-2.58pH)

7500

x

t r2 = 0.999

10000 x

t

l/(19.8-1.70pH)

r2 = 0.996 12500 x

t

l/(15.7-1.46pH)

r2 = 0.966

O

o

o o

O

o

t

r2 = 0.995

ss

pH 8.0

j l/m

o

0,5 0,4 -

• 6250 (jgK/g AV5O0|igK/g 010000 MK/g Ol2500pgK/s

0.3 0,2 - •

0,1 0 --

—I 10

\¡ Time.minutes Figure 4. Parabolic diffusion equation plot for potassium adsorption on calciumbentonite for 6250, 7500, 10000 and 12500 ngK/g Ca-b initial concentrations and pH 8.O.


1398

DOULA ET AL.

II). The fact that first-order rate model is not applicable to potassium adsorption is expected, since several separate equations taking place do not have similar reaction rates. The plot of the Elovich equation [3] (Figures 3a and 3b) shows an unsuccessful description of the process. Linearity coefficients ranged from 0.634 to 0.999. It should be mentioned that although the Elovich equation has been used primarily to describe the chemisorption of gases on metal surfaces (Atkinson et al., 1971), it has also been applied to many different processes, including bulk and surface diffusion, as an empirical tool (Sposito, 1984). Plots of Fa vs. the square root of time (Figure 4) gave a nonlinear relationship, suggesting that reaction rate is controlled by the diffusion of ions to the reactive sites, either through a stagnant water film surrounding the soil particle or through the particle itself. However, the actual mechanism involved can be determined only by experiments designed to evaluate chemical and physical factors affecting the rate constant.

REFERENCES: Atkinson, R.J., J.F. Hingston, A.M. Posner, and J.P. Quirk. 1971. Kinetics of heterogeneous isotopic exchange reactions: Derivation of an Elovich equation. Proc. Royal Soc. A. 324:247-255. Barshad, I. 1954. Cation exchange in micaceous minerals: II. Replaceability of ammonium and potassium from vermiculite, biotite and montmorillonite. Soil Sci. 78:57-76. Bolt, G.H., M.E. Summer, and A. Kamphorst. 1963. A study of the equilibria between three categories of potassium in an illitic soil. Soil Sci. Soc. Am. Proc. 27:294-299. Chien, S.H. and W.R. Clayton. 1980. Application of Elovich equation to the kinetics of phosphate release and sorption in soils. Soil Sci. Soc. Am. J. 44:265-268. Cooke, I.J. 1966. A kinetic approach to the description of soil phosphate status. J. Soil Sci. 17:56-64. Feigenbaum, S., R. Edelstein, and I. Shainberg. 1981. Release rate of potassium and structural cations from micas to ion exchangers in dilute solutions. Soil Sci. Soc. Am. J. 45:501-506.

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Fromhold, A.T. 1976. Theory of Metal Oxidation, Vol. I: Fundamentals. North Holland Publishing Co., New York, NY.


Garcia-Miragaya, J. and A.L. Page. 1976. Influence of ionic strength and inorganic complex formation on the sorption of trace amounts of Cd by montmorillonite. Soil Sci. Soc. Am. J. 40:658-663. Goulding, K.W.T. and O. Talibudeen. 1980. Heterogeneity of cation exchange sites for K-Ca exchange in aluminosilicates. J. Colloid Interface Sci. 78:1524. Havlin, J.L. and D.G. Westfall. 1985. Potassium release kinetics and plant response in calcareous soils. Soil Sci. Soc. Am. J. 49:366-370. Havlin, J.L., D.G. Westfall, and R.S. Olsen. 1985. Mathematical models for potassium release kinetics in calcareous soils. Soil Sci. Soc. Am. J. 9:371377. Kuo, S. and E.G. Lotse. 1974 Kinetics of phosphate adsorption and desorption by lake sediments. Soil Sci. Soc. Am. Proc. 38:50-54. Low, M.J.D. 1960. Kinetics of chemisorption of gases on solids. Chem. Rev. 60:267-312. Malcom, R.L. and V.C. Kennedy. 1969. Rate of cation exchange on clay minerals as determined by specific-ion electrode techniques. Soil Sci. Soc. Am. Proc. 33:247-253. Martin, H.W. and D.L. Sparks. 1983. Kinetics of nonexchangeable potassium release from two Coastal Plain soils. Soil Sci. Soc. Am. J. 47:883-887. Moore, W.J. 1972. Physical Chemistry, 4th ed. Prentice-Hall, Englewood Cliffs, NJ. Munn, D.A., L.P. Wilding, and E.O. McLean. 1976. Potassium release from sand, silt and clay soil separates. Soil Sci. Soc. Am. J. 40:364-366. O'Neil, P. 1985. Environmental Chemistry. G. Allen and Unwin, London. Okazaki, R., H.W. Smith, and C.D. Moodie. 1963. Hydrolysis and salt-retention errors in conventional cation exchange capacity procedures. Soil Sci. 96:205209. Rich, C.I. 1962. Removal of excess salt in cation-exchange capacity determinations. Soil Sci. 93:87-94.

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Selim, H.M., R.S. Mansell, and L.W. Zelazny. 1976. Modeling reactions and transport of potassium in soils. Soil Sci. 122:77-84.


Singh, D., R. Pal, and S.R. Poonia. 1981. Exchange equilibria of potassium versus calcium plus magnesium in soils of arid and semiarid region, India. Geoderma 25:55-62. Smith, A.N., A.M. Posner, and J.P. Quirk. 1977. A model describing the kinetics of dissolution of hydroxyapatite. J. Colloid Interface Sci. 62:475-494. Sparks, D.L. and T.H. Carski. 1985. Kinetics of potassium exchange in heterogeneous systems. Appl. Clay. Sci. 1:89-101. Sparks, D.L. and P.M. Jardine. 1981. Thermodynamics of potassium exchange in soil using a kinetics approach. Soil Sci. Soc. Am. J. 45:1094-1099. Sparks, D.L., D.C. Martens, and L.W. Zelazny. 1980. Plant uptake and leaching of applied and indigenous potassium in Dothan soils. Agron. J. 72:551-555. Sparks, D.L., L.W. Zelazny, and D.C. Martens. 1980. Kinetics of potassium exchange in a Paleudult from the Coastal Plain of Virginia. Soil Sci. Soc. Am. J. 44:37-40. Talibudeen, O., and K. W. T. Goulding. 1983. Charge heterogeneity in the smectities. Clays Clay Miner. 31:37-41.