Effects of organic matter on the rate of potassium adsorption by soils

Transcription

1 Effects of organic matter on the rate of potassium adsorption by soils F. L. Wang and P. M. Huang 1 Department of Soil Science, University of Saskatchewn, Saskatoon, Saskatchewan, Canada S7N 5A8. Received Wang, F. L. and Huang, P. M Effects of organic matter on the rate of potassium adsorption by soils. Can. J. Soil Sci. 81: Soil organic constituents may strongly affect the kinetics of soil chemical processes, including K exchange reactions. We investigated the influence of organic matter on the rate of K adsorption by selected soils (Ferric Ultisol, Orthic Ultisol and Vertisol) using a H 2 treatment and a K ion-selective electrode technique. In the reaction period of 0 30 s, in which the adsorption was too fast for one to determine rate coefficients of K adsorption, the amount of K adsorbed by the untreated soils was mg kg 1, compared with mg kg 1 for the treated soils. In the reaction period of s, K adsorption data based on the first-order kinetics show that rate coefficients of K adsorption by the untreated soils were s 1 (Ferric Ultisol), s 1 (Orthic Ultisol) and s 1 (Vertisol); by contrast, after H 2 treatment, the rate coefficients were s 1 (Ferric Ultisol), s 1 (Orthic Ultisol) and s 1 (Vertisol). Similar treatment effects were observed for the reaction period of s, though the difference in the rate coefficients between the treatments was not as great as that for the reaction period of s. These results indicate that organic matter considerably promotes the initial fast rate of K adsorption and has more easily accessible adsorption sites for K compared with mineral constituents of the soils. Key words: Organic matter, kinetics, potassium adsorption, adsorption site, accessibility Wang, F. L. et Huang, P. M Incidence de la matière organique sur le taux d adsorption du potassium dans le sol. Can. J. Soil Sci. 81: Les composants organiques modifient parfois considérablement la cinétique des réactions chimiques dans le sol, notamment l échange de potassium (K). Les auteurs ont examiné l influence de la matière organique sur le taux d adsorption du K dans différents sols (ultisol ferrique, ultisol orthique et vertisol) en traitant les échantillons au H 2 et en recourant à une électrode à membrane sélective pour le K. Pendant les trente premières secondes de la réaction, où l adsorption est trop rapide pour qu on mesure le coefficient du taux d adsorption, la quantité de K adsorbée par les sols non traités variait de 158 à 363 mg kg 1, contre 0,5 à 47 mg kg 1 pour les sols traités. Pendant la période de réaction de 30 à 120 secondes, les données sur l adsorption du K reposant sur la cinétique du premier degré révèlent un coefficient de s 1 (ultisol ferrique), de s 1 (ultisol orthique) et de s 1 (vertisol) pour les sols non traités. Les sols traités au H 2, en revanche, présentaient un coefficient du taux d adsorption de s 1 (ultisol ferrique), de s 1 (ultisol orthique) et de s 1 (vertisol). Le traitement a des effets similaires au cours des 120 à 600 secondes de la réaction, mais l écart entre le coefficient du taux d adsorption des deux traitements n est pas aussi important que pour la période de réaction de 30 à 120 secondes. Ces résultats indiquent que la matière organique favorise considérablement une adsorption initiale très rapide du K et présente des sites d adsorption du K beaucoup plus accessibles que les composants minéraux du sol. Mots clés: Matière organique, cinétique, adsorption du potassium, site d adsorption, accessibilité Soil organic matter is one of the most important cation exchange capacity (CEC) contributors in soil systems. Carboxyl and phenolic hydroxyl functional groups contribute most to the CEC of soil organic matter (Posner 1966; Van Dijk 1971; Hayes and Swift 1978). The contribution of organic matter to CEC of soil has been well recognized. Its average contribution to the CEC of A horizons ranges from 14 to 56% (Thompson et al. 1989). Thus, it is an important factor affecting the kinetics of cation exchange reaction. Though, generally, selectivity of the organic matter for divalent cations is higher than that for monovalent ones, unusual selectivity is displayed by certain organic compounds (e.g., valinomycin) for monovalent cations (Talibudeen 1 To whom correspondence should be addressed ). One of the well-known examples is the complex of the K + ion formed with valinomycin. The extreme selectivity of valinomycin for K + even enables its use in ion-selective electrodes. In contrast to the existence of extensive literature on the role of soil organic matter in transformations of soil constituents and plant nutrients, little is known about its influence on kinetics and mechanisms of cation exchange reactions in soils. Bunzl (1974) investigated the lead-hydrogen exchange reaction in a purified humic acid system, and reported that film diffusion was the rate-determining step. Jardine and Sparks (1984) speculated that the fast K exchange rate observed during the early reaction period at 298 K might be related to the behavior of organic polymers of the soil. The previous study on K adsorption by the soils (Wang and Huang 1990a) suggested that organic matter might play a role in increasing the adsorption rate. Though it has already been

2 326 CANADIAN JOURNAL OF SOIL SCIENCE stated that organic matter is one of the soil constituents affecting K reactions in soils because of its cation exchange capacity (Sparks and Huang 1985), the evidence of the role of soil organic matter in modifying the kinetics of K exchange reactions has not been provided to date. The objective of this study was, therefore, to investigate the effect of soil organic matter on the rate of K adsorption by selected soils. The K adsorption study was conducted on the soils before and after removal of the majority of organic matter by hydrogen peroxide treatment. MATERIALS AND METHODS Samples of the Ap horizons of Jinghua (Ferric Ultisol, silty clay), Fuyang (Orthic Ultisol, silt loam), and Saoxin (Vertisol, silty clay loam) soils from Zhejiang Province, China, were used in the present study. These soils are representative Chinese paddy soils, which account for 70% of the agricultural land in the Province. The soil samples were airdried and crushed to pass a 2-mm sieve. Selected chemical and mineralogical properties of the soils were determined. The soil ph was measured in a 2:1 water/soil (ml g 1 ) mixture. The soils were pretreated and their particle size fractions were separated by the method of Jackson (1979). The study was then followed by X-ray diffraction analysis (Whittig and Allardice 1986) and quantification of vermiculite and smectite in these fractions (Jackson et al. 1986). The X-ray analysis was conducted on Rigaku D/Max-RBX (Cu Kα) X-ray diffractometer with a monochromator (Rigaku Co., Tokyo), using parallel-oriented samples on glass slides. The quantitative mineralogical analysis was only conducted for vermiculite and smectite since they are the primary inorganic soil constituents that contribute to the CEC and K adsorption of soil. The ph and selected mineralogical analysis are presented in Table 1. Detailed qualitative mineralogical analysis was reported by Wang (1991). Table 1. ph and mineralogical properties of the soils studied Soils Selected Jinghua Fuyang Saoxin properties z (Ferric Ultisol) (Orthic Ultisol) (Vertisol) ph (water/soil 2:1) y Smectites (g kg 1 ) x Vermiculites (g kg 1 ) x z The soils contained mica, quartz, kaolinite, feldspars, and interlayered silicates. In addition, the Fuyang and Saoxin soils contained chlorite. y The error range of ph determination for the soils was ± 0.03 to ± x Sum of smectites or vermiculites in different particle fractions of the soil. The error ranges of the determination of smectites and vermiculites for the soils were ± 0.7 to ± 1.0 (g kg 1 ) and ± 0.4 to ± 1.2 (g kg 1 ), respectively. Table 2. The organic matter content of the soils before and after the H 2 treatment Soil organic matter (g kg 1 ) Soil Before the treatment After the treatment Jinghua 26.0 ± ± 0.01 Fuyang 21.6 ± ± 0.05 Saoxin 49.2 ± ± 0.03 To remove the organic matter of the soil samples, hydrogen peroxide (H 2 ) was used (Kunze and Dixon 1986). The organic matter content of both untreated and H 2 - treated soil samples were determined according to the procedures of Nelson and Sommers (1982). The results are presented in Table 2. The CEC of the soils before and after the H 2 treatment was determined by the method of Jackson (1979) and the results are shown in Fig. 1. Calcium is the most common exchangeable cation in productive agricultural soils. For a well-controlled kinetic study of K adsorption, Ca-saturated soils are needed. Prior to the Fig. 1. The CEC of the soils before and after the H 2 treatment.

3 WANG AND HUANG ORGANIC MATTER AND RATE OF K ADSORPTION 327 initiation of the adsorption studies, subsamples from the untreated soil and H 2 -treated soil were Ca-saturated by washing with 0.5 mol L 1 CaCl 2. The soils were subsequently washed with deionized distilled water to remove excess Ca 2+ ions. After each washing, the soil suspension was centrifuged at g at 293 K for 20 min; the supernatant was filtered through a millipore filter with a pore size of µm to collect the fine soil particles that might still remain suspended in the supernatant. The washing was continued until a negative test for Cl was obtained. No organic solvent was used in washing in order to minimize the possible complications caused by the organic solvent on surface properties of the samples. The method of Wang and Huang (1990b) was used in the kinetic study. The method combines batch and electrode techniques. A 100-mL beaker, which contained 50 ml KCl solution ( mol L 1 ), was placed in a shaker bath (Blue M) and maintained at 298 ± 0.5 K. The soil (1 g, ovendry basis, of Ca-saturated sample of the untreated or the Fig. 2. Time function of K adsorption by the soils in the s reaction period at 298 K as influenced by the removal of organic matter from the soils. H 2 treated soil) was suspended in the KCl solution in the beaker under a shaking condition of 159 ± 6 strokes min 1. Changes in concentration of solution K were monitored by measuring the electrical potential (emf) of the KCl-soil suspension over the reaction period of 10 min. The emf measurement was made by connecting a K ion-selective electrode (ORION Model 93-19) and a reference electrode (ORION Model 90-01) to a ph/mv-meter (Fisher 825MP), and by fixing the electrodes in the reaction beaker using a holder. The emf values were read from the ph/mv-meter as frequently as every 10 s. The values were then converted to K concentrations by a standard curve, which was established by plotting the negative logarithm of the K concentration (mol L 1 ) of the standard solution vs. its emf (mv) measured under the same conditions as those for soil suspensions. All the determinations were conducted in triplicate. The previous studies have shown that the first-order model is one of the best kinetic models to describe K adsorption by soils (Sparks 1989). The second-order, para-

4 328 CANADIAN JOURNAL OF SOIL SCIENCE bolic and Freundlich models had little superiority over the first-order model in fitting the data of K adsorption by the same soils used in the present study (Wang 1991). Hence the effect of organic matter on the kinetics of K adsorption by the soils was investigated based upon the first-order kinetic model. RESULTS AND DISCUSSION The K adsorption by the soils during the first 10-min reaction period was studied. On the same weight basis, amounts of K adsorbed by the H 2 treated soils in the 10-min reaction period were substantially lower ( mg kg 1 ) than those for the untreated soils ( mg kg 1 ). In the earlier reaction period ( s), K adsorption was faster in the untreated soils than in the treated soils, which was suggested by the steeper slopes of the curves of K remaining in the solution (mol L 1 ) vs. time (s) (Fig. 2). The high degree of fit of the first-order model to K adsorption data in the reaction period of s is shown in Fig. 3. When the organic matter of the soils was not removed, rate coefficients of K adsorption in the reaction period of s were s 1 (Jinghua), s 1 (Fuyang), and Fig. 3. First-order plots for adsorption of K by the soils during the s reaction period at 298 K. *** Significant at P < s 1 (Saoxin). In contrast, the rate coefficients were s 1 (Jinghua), s 1 (Fuyang) and, s 1 (Saoxin) after the removal of the organic matter (Table 3). Hence, for the Jinghua and Fuyang soils, the rate of K adsorption in the reaction period of s decreased by two to more than three times after the removal of the organic matter. Similar observations were made for the later reaction period ( s), although the treatment effect was not as pronounced as that for the early reaction period ( s); the decrease in magnitude of the rate coefficient was by 36% at most (Table 4). The evidence that the organic matter substantially affected the reaction rate of the initial rapid reaction period was further strengthened by the amount of K adsorbed in the first 30-s reaction. At the end of the 30-s reaction, in which the adsorption was too fast for one to determine the rate coefficients with the K electrode technique, the amount of K adsorbed by the treated soils was mg kg 1, compared with mg kg 1 for the untreated soils (Table 5). The data, thus, indicate that the removal of organic matter resulted in the decrease of K adsorption by these soils by 6 to 316 times in the first 30-s reaction.

5 WANG AND HUANG ORGANIC MATTER AND RATE OF K ADSORPTION 329 Table 3. The influence of soil organic matter on the rate coefficient of K adsorption by the soils in the reaction period of s based on the first-order kinetics (298 K) Rate coefficient ( s 1 ) 10 5 Before removal of After removal of Soil organic matter organic matter z Jinghua 47 ± 1 23 ± 1 Fuyang 59 ± 2 17 ± 1 Saoxin 61 ± 2 42 ± 0.1 z Based on the wight after the H 2 treatment. Table 4. Rate coefficients of K adsorption by the soils in the reaction period of s based on the first-order kinetics (298 K) z Rate coefficient ( s 1 ) 10 5 Before removal of After removal of Soil organic matter organic matter y Jinghua 14 ± 1 9 ± 0.4 Fuyang 15 ± 1 11 ± 1 Saoxin 18 ± ± 1 z The r values of the first-order rate plots for the soils before the H 2 treatement were (Jinghua), (Fuyang) and (Saoxin). The r values for the soils after the H 2 treatment were (Jinghua), (Fuyang) and (Saoxin). The r values of all the rate plots tested were significant at P < y Based on the wight after the H 2 treatment. Table 5. The influence of soil organic matter on the amounts of K adsorbed by the soils in the reaction period of 0 30 s Amount of K adsobed (mg kg 1 ) Before removal of After removal of Soil organic matter organic matter z Jinghua 210 ± 5 35 ± 1 Fuyang 158 ± ± 0.04 Saoxin 363 ± 6 47 ± 2 z Based on the weight after the H 2 treatment. Though more organic matter was removed from the Saoxin soil (Table 2), the extent of the decrease in the rate of K adsorption by the soil was not as high as those of the other two soils in the reaction period of s (Table 3). The data of the Saoxin soil indicate that the degree of the effect of the organic matter on K adsorption rate appeared to depend not only on its amount, but also on its nature and association with minerals. The nature of the organic matter and organo-mineral complexes of soils merits attention in the future studies of kinetics of K adsorption in soils. Further, the Saoxin soil had the highest amount of smectites and vermiculites among the three soils studied (Table 1). These clay minerals have the high CEC to adsorb K. The dispersion effect, which is due to the decrease in size of the soil aggregates caused by the removal of the organic matter as cementing agents, also should be taken into account in explaining the data of the Saoxin soil. This effect is a rate-increasing process, because it leads to a decrease in tortuosity of the system and consequently, an increase in the rate of a cation exchange reaction. The higher the organic matter content a system has, the stronger the effect should be. Consequently, the rate-retarding effect caused by the removal of the organic matter, which possibly acted as a contributor of easily accessible exchange sites, would be offset more by the rate-promoting effect caused by the dispersion for the Saoxin soil than for the other two soils. The H 2 treatment could alter the minerals and thus cause an increase in edge exchange sites on the minerals. This is indicated by the CEC values of the soils before and after the H 2 treatment (Fig. 1). Because of the high oxidizing power of H 2, the treatment could alter surface properties of 2:1 layer silicates in the soils by modifying octahedral population of the minerals. As reported by Juo and White (1969), in the process of the oxidation, the octahedral Fe 2+ would become Fe 3+ under the oxidation condition and cause a change in OH orientation in the mineral structure, i.e., the H portion of the OH no longer orients toward the K + adsorbed. Consequently, attraction between K + and adsorption sites would increase, because of the asymmetry of the positive charges; selectivity of the system for K + would increase (Juo and White 1969; Barshad and Kishk 1970). Furthermore, as previously mentioned, the removal of the organic matter as cementing agents could considerably decrease the size of the soil aggregates in the system. The diffusion path for the ion to the exchange sites would become more straight forward. Thus, a significant increase in the rate of the K adsorption by the soils should be expected. All these possible side effects brought about by the H 2 treatment of the soils as discussed in the foregoing discussions tend to increase the rate of K adsorption. The data obtained in the present study show the opposite trend, however. The rate-promoting effect of the organic matter in the systems studied was so strong that the net effect of the H 2 treatment resulted in a substantial decrease in the K adsorption rate in the initial rapid reaction period due to the removal of the organic matter. The rate of a cation exchange reaction in soil should be in part related to the number of exchange sites as represented by CEC, since an electrostatic attraction in the system will enhance oppositely charged objects toward each other (Adamson 1979). However, unlike the difference in the amount of K adsorbed (or in the rate of K adsorption) in the first 10-min reaction period, the difference in CEC before and after the H 2 treatment is not substantial (Fig. 1). These observations are not contradictory. The obtained CEC values are the result of an equilibrium study. In contrast, the substantial differences in the amount of K adsorbed (or in the rate of K adsorption) in the first 10-min reaction period are the result of a kinetic study, which was obtained from a system where K adsorption was still going on at the sampling time and the reaction equilibrium was far from being reached. Further, the substantial difference in the amount of K adsorbed is attributed to a change in the accessibility of cation exchange sites in the system after the removal of organic matter. The removal of the organic matter apparently decreased the fast accessibility of the exchange sites in

6 330 CANADIAN JOURNAL OF SOIL SCIENCE the soil system. This led to the adsorption of a much smaller amount of K in the H 2 -treated system in the first 10- min reaction period as compared with the system without H 2 treatment, even thought the difference in CEC is not substantial for both two systems. The accessibility for K ions is expected to decrease after the removal of organic matter since the majority of the exchange sites of the soils after the removal of organic matter were located in the internal surface of smectites and vermiculites in the system and relatively difficult for K ions to reach. Rich (1964) indicated that the diffusion path was tortuous for monovalent ions towards the exchange sites in the internal surface of these minerals. In contrast, the organic matter model of Stevenson (1994) suggests that ion exchange sites on the surface of organic matter are relatively easy to access because they are basically external sites. The fact that the organic matter affected the reaction rate of K adsorption in the earlier reaction period more than that in the later period further strengthens the above reasoning, since initial K adsorption should take place on those sites which are relatively more accessible to K. The data obtained in this study, thus, indicate that the accessibility of the exchange sites of the organic matter to K ions was substantially higher than that of the minerals in the soils studied. CONCLUSIONS Based on per-unit weight of the untreated and H 2 -treated soils, K adsorption by the treated soils in the initial rapid reaction period was substantially slower than that by the untreated soils, suggesting the rate-promoting effect of the organic matter on the reaction. Because the data obtained in the present study were opposite to the possible side effects of the H 2 treatment of the soils on the K adsorption, it is further evident that, compared with mineral components, organic matter in the soils had a faster rate of K adsorption. This is attributed to the presence of more accessible exchange sites of soil organic matter for K. ACKNOWLEDGMENTS This study was supported by Potash and Phosphate Institute of Canada and Research Grant GP 2383-Huang of Natural Sciences and Engineering Research Council of Canada. Publication No. R865, Saskatchewan Centre for Soil Research, the University of Saskatchewan, Saskatoon, SK, Canada. Adamson, A. W Physical chemistry. 2nd ed. Academic Press Inc., New York, NY. Barshad, I. and Kishk, F. M Factors affecting potassium fixation and cation exchange capacities of soil vermiculite clays. Clays Clay Miner. 18: Bunzl, K Kinetics of ion exchange in soil organic matter: II. 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