Illinois
Fertilizer Conference Proceedings
January 25-27, 1999
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J.W. Stucki and K. Lee2
The goal of this work is to develop an improved soil test for plant-available potassium (K). The focus has been on the application of an electrodialysis method in which an electrode is used to stimulate the plant root in extracting K from the soil. The amount of K released from the soil is measured as a function of the voltage or electrical potential on the electrode. These relationships are then correlated with field-tested results on the same soils exhibiting various levels of K unavailability. With this technique, we hope to establish the correct electrical potentials on the electrode that correspond to the different pools of K-namely, plant-available and plantunavailable. Collaborators at Purdue University are conducting the field experiments. The underlying principles being tested in these experiments are founded in the fundamentals of chemical thermodynamics and kinetics. While the theory is rather straightforward, the experimental design has been challenging because of ancillary reactions occurring at the electrode and in the electrochemical cell.
Initial studies were unsuccessful in producing a stable, equilibrium concentration of K near the electrode, so experiments were undertaken in the beginning of the second year to identify the relationship between the applied voltage and (1) the amount of K extracted from the sample, and (2) the amount of time required to reach an equilibrium point. K ions were extracted from KCl solution with different voltages (ranging from 50 V to 400 V) and with different concentrations of initial K in solution. Results from this voltage range revealed that the K concentration near the electrode steadily increased, passed through a maximum, then continuously declined after reaching the maximum concentration value. Hence, no equilibrium point could be readily established. This was attributed to hydrolysis of water and other compounds at the electrodes. When lower voltages (1 to 10 V) were used in the eletrodialysis cell, an apparent equilibrium concentration was found. The effect of diffusion of K with no external electric force (i.e., with zero voltage on the electrode) was also investigated.
The first electrodialysis cell constructed thus demonstrated the possibility that this method can be employed to yield extractable quantities of K, provided that the applied voltage is relatively small (< 10 V). But a problem was found in the choice of material in the electrode. The stainless steel electrodes were found to be highly susceptible to corrosion. Thus, a new cell was designed and constructed, consisting of a Plexiglas body and graphite electrodes, which resist corrosion. Other needed improvements involved the gasket used to seal the various compartments together and the sampling system. For the latter, a peristaltic pumping system was installed. Other improvements in gauges, meters for measuring electrical current and voltage, and power supplies were also introduced. As a result, this reconstruction enhanced the electrodialysis cell so it could obtain reliable data for K desorption from soil and clay samples.
The electrodialysis method differs from other soil-testing methods in that it not only yields extractable quantities but also provides information on the rates with which these quantities are desorbed when an external electrical force is applied. The rate of equilibration reactions between solution and soil K-exchangeable phases determines whether applied K will be taken up by plants, converted into insoluble forms, or released into soluble forms. A knowledge of the kinetics of K desorption in the soil system is necessary in order to predict the fate of added K fertilizer in soils and to properly make K fertilizer recommendations.
Talibudeen et al. (1978) observed that the rate of desorption of soil K was linearly proportional to the square root of time (t1/2). Feigenbaum et al. (1981) described K release kinetics by a parabolic diffusion rate equation. A simple first-order rate function has also been used to describe K adsorption -desorption (Sparks and Jardine, 1981). Havlin et al. (1985) concluded that three mathematical models (Elovich, power function, and parabolic diffusion equations) adequately described cumulative K release. These authors used extractants such as CaCl2, NaCl, and deionized H20, or used saturated resins containing either Ca or Na, to investigate desorption of K, whereas the electrodialysis method employs H+ (produced as a result of oxidation at the anode) as the exchanging canon.
The rate at which elements are released from clay minerals in the soil is important
in plant nutrition. Many studies reported that clay fractions greatly affect
the rate of K desorption and correlate with the amounts of K available in soils
(Sparks et al., 1980; Jardine and Sparks, 1984; Havlin et al., 1985). The desorption
rate also depends on the temperature, which is related to the activation energy.
Activation energy for K desorption is determined from the desorption rate at
a given temperature using the Arrhenius equation, and reveals the magnitude
of binding force to be overcome during the process of the ionic exchange (Spark
and Jardin, 1981). The kinetics of K desorption of a ferruginous smectite, SWa-1,
was determined using the electrodialysis method in this study.
This study was performed to investigate desorption of K from K-saturated ferruginous smectite (SWa-1) by the electrodialysis method. Smectite SWa-1 was prepared initially by saturating its ion exchange complex with K by adding 1 N KCl. The suspension was shaken for 24 hours on a reciprocating shaker, then centrifuged. The supernatant was removed, and the clay sample resuspended in 1 N KCl. This procedure was repeated three times. The clay suspension was washed with deionized H20 and with 5x10-4 N KCl using a centrifuge. The sample was dialyzed to remove excess electrolyte until a negative test for Cl- was obtained using AgNO3, then freezedried. Weighed portions of this stock material were then resuspended in H20 as needed for the experiments. The canon exchange capacity of this clay is 81.4 meq/100 g.
The electrodialysis apparatus consisted of three compartments, arranged laterally. K-saturated SWa-1 suspension of 0.4% (w/w) was placed into the center compartment, and the left and right compartments were filled with deionized water. The respective compartments were separated from one another by a 0.22-mm Millipore filter which permitted H20 and ions, but no clay particles, to pass. The anode (positive electrode) was attached to the end of the left compartment, and the cathode (negative electrode) was placed at the end of the right compartment. The total length of this apparatus was 13.7 cm, and the distance between the center compartment and the cathode was 4.3 cm. The K migrating from the clay solution in the center compartment, due to electrical and chemical potential gradients, through the membrane toward the cathode in the right compartment was monitored to determine the rate of desorption and equilibrium concentration of K. Three different voltages (1, 5, and 10 V) were applied for 134, 136, and 60 hours, respectively. The concentration of desorbed K was measured with a K-selective electrode.
Desorption values (mg/L) for each treatment were plotted against reaction time (minutes) on four different scales. Conformity of the K-extraction data to different kinetics models was tested by comparing their respective coefficients of determination (r2), as determined by least squares regression analysis.
The kinetics experiments for the activation energy for desorption were conducted
at 10 V and at 25, 40, 55 ± 1.0°C. In order to obtain more powerful ion exchange
by hydrogen ion, the electrodialysis apparatus consisted of two compartments
(the anode and cathode compartments). The potassium-saturated clay sample was
placed in the anode compartment. The pH of solution was monitored at each chamber.
The electrodialysis method provided information on the rates with which these quantities are desorbed at the respective applied voltages (Figure 1). When a larger voltage was applied to the electrodialysis cell, the extracted amount of K was larger and the migration rate of K ion from the center cell toward the cathode was faster. At 10 V, K ions reached the maximum concentration faster than any other voltage applied. Four mathematical models were used to describe the kinetics of K desorption on SWa-1 at the three different voltages. The release of K from smectite SWa-1 by ultimate exchange with H+, which developed from the increasing pH at the anode, may be considered as a simple exchange reaction,
H+ - K-clay = H-clay + K+ (1)
The Elovich equation applied to K desorption is expressed as
dy/dt = ae-by (2)
where y is the amount of K desorbed at time t , and a and b are
constants (Havlin at al., 1985).
The integrated form of Eq.(2) is
y = 1/b ln (1 + abt) (3)
If abt>> 1, then Eq. (3) can be simplified to
y = 1/b ln (ab) + 1/b ln (t) (4)
Thus, a plot of y vs. ln (t), according to Eq. (4), should be linear with slope l/b and intercept 1/b ln (ab), where b is the rate constant.
The Parabolic diffusion equation for K desorption may be described as follows (Jardine and Sparks, 1984):
y = (4/pi1/2)(Dt/a2)1/2 - (Dt/a2) (5)
where a is a radius parameter and D is the diffusion coefficient.
The linear form of the first-order, power function, and parabolic diffusion equations are given in Table 1. A linear plot of K desorbed vs. t1/2 (Figure 2) indicates that a parabolic diffusion equation adequately describes the K desorption from smectite SWa-1 and, more generally, the diffusion of K out of the interlayer sites of 2:1 clay minerals (Reed and Scott, 1962; Havlin et al., 1985). A plot of the Elovich equation (4) also shows a good description of K desorption kinetics from SWa-1 clay (Figure 3). Both parabolic diffusion and Elovich equations suggest a diffusioncontrolled rate process. Results from Table 1 indicate that first-order (Figure 4) reaction kinetics fail to describe K desorption well. Sparks and co-workers (Sparks et al., 1980; Jardine and Sparks, 1984) suggested that K desorption follows first-order kinetics, but our results contradict that suggestion. The power function equation also provides a very poor description of K desorption kinetics in smectite SWa-1, as revealed by the nonlinear form of the desorption line of K (Figure 5). In all cases, the rate constant values decreased in the order 10 V > 5 V > 1 V.
In order to determine the activation energy of K desorption in SWa-1, kinetic desorptions were measured at three different temperatures (Figure 6). Four different kinetic equations also were applied to these data. Most of the kinetic equations for K desorption in SWa-1 exhibited good linear relationships among the respective parameters. A plot of the parabolic diffusion equation (Figure 7) revealed the reaction rates of K desorption at 25°C and 40°C well, but showed a low correlation coefficient (r2) at 55°C. The Elovich equation was the best of the various kinetic equations applied to describe the desorption rate in SWa-1 (Figure 8). The first-order equation produced relatively low correlation coefficients (Figure 9). However, the first-order plots clearly described the slope differences between different temperatures applied. The slope of the firstorder plots was highest for 55°C, indicating the highest rate of K desorption, and the lowest rate of K desorption occurred at 25°C. Thus, the rate values increased with increasing temperature. The power function equation also described K desorption well in SWa-1 (Figure 10). The desorption rate coefficient (kd') were determined using the first-order equation (Sparks and Jardine, 1981):
kd' = ln (Kt/K0)/2.303t (6)
where Kt is the amount of K on the exchange sites of the soil at time t and K0 is the amount of K on the exchange sites of the soil at zero time. The kd' values increased with increasing temperature (Table 2). The activation energy of K desorption (Ed) was determined using the Arrhenius equation:
ln kd' = ln A - Ed/RT (7)
where A is a constant, R is the gas constant, and T is the absolute temperature. When ln kd' is plotted vs. 1/T (Figure 11), the slope of the line through these points is equal to -Ed/R, from which Ed can be obtained.
These desorption results are attributed to the proton being generated at the anode and the electromigration of K+ due to the electrical field. When a do potential is applied to graphite electrodes, the primary electrode reactions of electrolysis at the anode will be the generation of hydrogen ions. Figures 12 and 13 reveal very low pH at the anode and high pH at the cathode, respectively. At 10 V, the effect of temperature on pH was minimal, except for a slight elevation of the pH at 55°C in the cathode compartment. Thus, the electro-migration of K+ is expected to contribute greatly to its desorption from the clay or soil sample, facilitated by H+ ion exchange at the clay surfaces.
Another interesting result is that the extracted concentration of K correlates well with the current (Figure 14), showing a direct relationship. At 10 V, K desorption generally increased with increasing current, but again the effect of temperature on such relationships was minimal (Figure 15).
The rates and extent of K desorption were directly proportional to the applied voltage. Four mathematical models were applied to describe the kinetics of K desorption from smectite clay SWa-1 at three different voltages and temperatures. The first-order and power function equations were much less satisfactory than the Elovich and parabolic diffusion equations, as determined by the relatively high r2 values. All plots showed that the highest applied voltage had the largest rate constant. This study showed that a stable K desorption curve was attained by using electrodialysis.
Table 2. The apparent rate constants and activation energy for K desorption frm smectite clay SWa-1.
Figure 1. K desorption curves in the electrodialysis cell at 1, 5, and 10 V.
Figure 6. K desorption curves in the electrodialysis cell at 10 V and 25, 40, and 55°C.
Figure 11. Relation of ln kd' to 1/T for the first-order reaction for SWa-1.
Figure 12. pH measured in the electrodialysis cell at 10 V and 25°C.
Figure 13. pH measured in the electrodialysis cell at 10 V and 55°C.
Figure 14. Relation of electrical current to K desorption at different voltages.
Figure 15. Relation of electrical current to K desorption at different temperatures
1 Presented at the Illinois Fertilizer and Chemical Association Conference, Peoria,IL Jan. 25, 1999.
2 R.G. Hoeft and E.D. Nafziger are Professors, Dept. of Crop Sciences; R.L. Mulvaney is Professor, Dept. of Natural Resources and Environmental Sciences, and L.C. Gonzini and J..1. Warren are Senior Research Specialists, Dept. of Crop Sciences, Univ. of IL.
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.
Havlin, J.L., D.G. Westfall, and S.R. Olsen. 1985. Mathematical models for potassium release kinetics in calcareous soils. Soil Sci. Sec. Am. J. 49:371-376.
Jardine, P.M. and D.L. Sparks. 1984. Potassium-calcium exchange in a multireactive soil system: 1. Kinetics. Soil Sci. See. Am. J. 48:39-45.
Reed, M.G. and A.D. Scott. 1962. Kinetics of potassium release from biotite and muscovite in sodium tetraphenylboron solutions. Soil Sci. Sec. Am. Proc. 26:437-440.
Sparks, D.L., L.W. Zelazny, and D.C. Martens. 1980. Kinetics of potassium desorption in soil using miscible displacement. Soil Sci. Sec. Am. J. 44:1205-1208.
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Sparks, D.L. and P.M. Jardine. 1984. Comparison of kinetic equations to describe potassiumcalcium exchange in pure and in mixed systems. Soil Sci. 138:115-122.
Talibudeen, O., J.D. Beasley, P. Lane, and N. Rajendran. 1978. Assessment of soil potassium reserves available to plant roots. J. Soil Sci. 29:207-218.
