- Email: [email protected]
Isoelectric point of soils determined by the diffusion potential method
Geoderma 85 Ž1998. 371–380
Isoelectric point of soils determined by the diffusion potential method M.C. Lamas a
a,)
, R.M. Torres Sanchez ´
b
FAUBA, Edafologıa, ´ AÕ. San Martın ´ 4453, (1417) Cap. Fed. Buenos Aires, Argentina b CETMIC, Cno. Centenario y 506, CC 49 (1897) M.B. Gonnet, Argentina Received 11 March 1997; accepted 11 February 1998
Abstract The diffusion potential method was used to determine the Isoelectric Point ŽIEP. of the surface horizons of five soils ŽTypic Hapludoll, Typic Argiudoll, Vertic Argiudoll, Rhodic Kandiudalf and Lithic Kandihumult. in the following conditions: without treatment Žnatural., homoionized with Kq ŽKq-soil. or Ca2q ŽCa2q-soil. and after organic matter ŽOM. removal. Soil samples were characterized through their mineralogy, total organic carbon ŽTOC., clay percentage, cation exchange capacity ŽCEC., basic cations in the exchange complex, pH in water, Fe 2 O 3 content and zero point of charge ŽZPC.. The IEPpH of soils gave the following order: Ca2q-soil ) natural-soil ) Kq-soil. These results were explained in terms of the higher affinity of bivalent over monovalent cations for negative surfaces. IEPpH of K-soils was dissimilar to ZPC pH , indicating that Kq did not behave as an indifferent cation. Data of IEPpH after OM removal of Anguil ŽTH., Cepeda ŽVA. and Cerro Azul ŽLK. soils were lower than those IEPpH of the same natural samples and higher for Castelar ŽTA. and Virasoro ŽRK. soils. Results could not be explained in a simple way. The IEPpH of natural soils correlated with ZPC pH , pH in water and Fe 2 O 3 and clay contents. The diffusion potential method showed to be rapid, simple and adequate to measure IEPpH of natural or treated soils. q 1998 Elsevier Science B.V. All rights reserved. Keywords: IEPpH ; diffusion potential methods; soils; homoionized soils
)
Corresponding author. E-mail: [email protected]
0016-7061r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 1 6 - 7 0 6 1 Ž 9 8 . 0 0 0 2 5 - 1
372
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
1. Introduction One of the most relevant parameters to characterize the electrical state of soil surface particles in solution is the zero point of charge Ž ZPC. which is defined as: the pH where the net surface charge, resulting of the adsorption of the potential determining ions, Hq and OHy, is zero. Generally the ZPC is obtained by the titration curve method in different electrolyte concentrations and it is determined as the cross point of these curves Ž Blok and De Bruyn, 1970.. The change of measurement conditions produces different cross points, defined as follows: point of zero net charge ŽZPNC. is the pH where the cationic exchange capacity equals the anionic exchange capacity; point of zero salt effect ŽZPSE. is the pH of intersection of the potentiometric titration curves at different electrolyte concentration, etc. Ž Gillman and Uehara, 1980; Sposito, 1984.. They are used in the literature to understand different surface soil conditions. Another parameter utilized to characterize the surface status of soils is the isoelectric point Ž IEPpH . which is the pH where the charge at the slipping plane of the Stern Layer is zero. Generally it is obtained when an electric field Ža pressure gradient, etc.. is used to move the charged particles of colloids in solution Ž microelectrophoresis, electroosmosis methods, etc.. . The IEPpH of oxides and minerals is usually measured by microelectrophoresis. The IEPpH of whole soils is not measured by this method because of the large soil particles distribution size and its complex composition. The soil IEPpH was measured by the diffusion potential method after Tschapek et al. Ž1989.. This method measures the transference number of cations Ž tq. of an electrolyte that moves within the double electric layer of a solid created by a surface charge. The method was proved to be sensitive enough to distinguish IEPpH values of kaolins and smectites homoionized with different cations ŽTschapek et al., 1991; Torres Sanchez et al., 1992. . Values of IEPpH and ZPC pH ´ only coincide when no specific adsorption exists Ž Lyklema, 1984. . Ions that are specifically adsorbed Ž Gillman and Uehara, 1980. may be displaced by titration during the ZPC pH measurement, and this fact does not happen when the IEPpH is determined. In the above mentioned works ŽTschapek et al., 1991; Torres Sanchez et al., ´ q . 1992 the IEPpH values of homoionized K samples determined by the diffusion potential method coincided with the ZPC pH obtained by the titration method ŽTschapek et al., 1989. and by this reason it was accepted that Kq behaved as an indifferent ion. The aim of this work is to extend the diffusion potential method to the IEPpH of soil determination. The sensitivity of the method was evaluated by measuring the IEPpH of surface samples of five soils with different characteristics. All of them were measured in natural conditions, after different homoionization treatments ŽKq and Ca2q . and after organic matter ŽOM. removal.
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
373
2. Materials and methods 2.1. Samples Samples of surface horizons Ž A 1 . of five soils were used. Three of them from a temperate region: Pampean Prairie ŽTypic Hapludoll, Vertic Argiudoll and Typic Argiudoll, named Anguil, Cepeda and Castelar, respectively. and two soils from a subtropical and tropical zone Ž Rhodic Kandiudalf and Lithic Kandihumult, identified as Virasoro and Cerro Azul, respectively. . The classification, texture, crystalline species, iron content, ZPC pH and pH of these soils are shown in Table 1. 2.2. Treatments Soils were treated with different solutions to replace their saturating cations. All samples, after treatment, were water washed, centrifugated several times until negative Cly reaction was verified by ŽAgNO 3 .. Ža. Nat-soils: samples were washed by percolation with distilled water. Žb. Ca2q-soils: nat-soils were percolated with 0.3 N Cl 2 Ca. Žc. Kq-soil: two different homoionized procedures were compared: Ž1. soil was Kq saturated with 1 N KCl, Ž2. soils were at first percolated with 0.05 N HCl to displace exchangeable cations and then neutralized with 0.25 N KOH. For OM removal: nat soils were treated with H 2 O 2 following the method of Black et al. Ž1965.. 2.3. Methods Crystalline species were qualitatively identified by X-ray diffraction, in a Philips equipment 1140, using Cu K a radiation at 20 mA and 40 kV. The ZPC pH was determined by classical fast potentiometric titration method on 2 g sample to which 30 ml of 10y1 and 10y3 M KCl was added as supporting electrolyte. The samples thus prepared were titrated under N2 atmosphere with additions of 0.1 N HCl each 10 min ŽBlok and De Bruyn, 1970.. Total organic carbon ŽTOC. was determined by the Walkley Black technique ŽNelson and Sommers, 1982. . Cation exchange capacity Ž CEC. was determined by the NH 4OAc 1 N method at pH s 7, saturating cations were determined by spectrophotometry and Kq by flame photometry treatment. The IEPpH is measured by the diffusion potential method. When putting in contact two concentrations of the same solution Ž i.e., KCl. a diffusion of ions from the most to the least concentrated is produced. The ion that moves more rapidly will give the charge to the diluted side of the solution, creating a potential difference which can be measured in an electrochemical cell and it is called liquid junction potential.
374
Soil sample Soil classification
Texture
Crystalline species
ZPC ŽpH. Fe 2 O 3Ž%. pH Žwater.
Bentonite Diaspore Bentonite Pyrophyllyte Bentonite Diaspore Pyrophyllyte Goehite Diaspore Goehite Diaspore Pyrohyllyte Magnetite
3.5 2.8 3.0 3.5 5.7
2000–50 m m 50–2 m m - 2 m m Anguil Cepeda Castelar Virasoro C. Azul
Typic Hapludoll Vertic Arguidoll Typic Arguidoll Rhodic Kandiudalf Lithic Kandihumult
67 11 10 5 13
19 68 64 37 18
14 21 26 57 69
3.6 3.0 1.8 10.0 14.0
Besides the showed crystalline species: quartz, illite, kaolinite, feldspar, halloysite and hematite were found in all the samples.
6.2 5.8 5.5 5.5 6.9
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
Table 1 Soil classification, clay content, crystalline species, ZPC pH , Fe 2 O 3 content and pH Ž1:2.5, soil:water. of the studied soils
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
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Fig. 1. Electrochemical cell to measure EMF s e1 q e 2 q e 3 q e 4 .
If we interpose a charged membrane between the two solutions, this acts accelerating the ions of opposite charge and hindering those of the same charge. A soil with surface charge behaves like a membrane which alters the mobility of ions, generating a new potential difference which can be measured in the electrochemical cell ŽFig. 1. based on Hendershot equation ŽTschapek et al., 1976.. Because e1 s e 4 , the EMF of this cell is s e 2 q e 3. Changing the pH we can change the soil charge. In the pH of the IEPpH the net surface charge of the soil is zero and the EMF of the cell is the liquid junction potential of the KCl. Diffusion potential was measured with a Keithley 616 electrometer Ž with an input resistance ) 2.10 14 V and a sensitivity of 10y15 A. and calomel electrodes. The apparatus used for the IEPpH measures of soils is shown in Fig. 2. The liquid junction potentials were measured when the plug was empty and when the paste soil was in the plug. Measures were performed in the following way: E1 is the diffusion potential of KCl electrolyte Ž E1 s y0.35 mV obtained by Henderson equation when the concentration electrolyte ratio C 2rC 1 s 2.. When the plug was filled with a soil paste Žprepared with soil plus solutions at different pH., E2 was measured. The difference E s E2 y E1 is the potential
Fig. 2. Apparatus used to measure IEPpH of soils by the diffusion potential method. The stop cock has a diameter of 1 cm and 1.5 cm length. The left compartment contained KCl 2=10y3 M and the right one KCl 1=10y3 M.
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M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
generated by the charge of soil paste, being E s 0 when the material has zero net charge at the IEPpH E1 s E2 s y0.35 mV.. When E ) 0 or - 0, the soil is negatively or positively charged, respectively. Tschapek et al. Ž 1976. observed that the IEPpH should be extrapolated from an E vs. pH curve at pH - 3.5 because Hq activities interfere in the E measures. Solutions with different concentrations of HCl or KOH were added to the dried soil to obtain the soil paste. It was divided in two fractions: one of them was used for pH determinations and the other one for E determinations. The paste moisture modify E but not IEPpH values as was demonstrated by Torres Sanchez et al. Ž1992.. ´ 3. Results and discussion Characterization of the studied soil samples are shown in Table 1. Among the XRD identified crystalline species: quartz, feldspar, illite, kaolinite, halloysite and hematite were found in all samples. The ZPC pH values obtained in Pampean Prairie soils Ž Cepeda, Castelar and Anguil. were generally lower than those obtained for tropical soils and seemed to be determined by the presence of clay mineral type 2:1 Ž bentonite. and TOC content Ž Table 2. . The ZPC pH values of Cerro Azul was higher than ZPC pH of other soils and this is in agreement with the high iron oxide contents Ž Escudey and Galindo, 1983. . Furthermore, Virasoro sample with similar iron content has a lower ZPC pH than Cerro Azul sample, attributed to the higher proportion of TOC Ž3.5% of TOC.. Organic carbon shifts the ZPC pH into opposite direction than that observed for iron oxide ŽEscudey and Galindo, 1983.. For all samples, the ZPC pH was lower than the pH in water, indicating that these soils had negative charge in natural conditions. Table 2 TOC content, CEC, saturating cations ŽCa2q, Kq, Mg 2q . and IEPpH of the soil samples before and after OM removal by H 2 O 2 treatment Soil samples
OM removal
TOC Ž%.
CEC ŽCmol c rkg.
Ca2q ŽCmol c rkg.
Kq ŽCmol c rkg.
Mg 2q ŽCmol c rkg.
IEP ŽpH.
Anguil
before after before after before after before after before after
1.4 0.2 1.3 0.1 1.8 0.2 3.5 0.7 2.3 1.2
15.8 11.9 13.2 11.7 19.6 20.3 15.4 18.2 13.9 12.0
10.2 3.8 7.4 3.6 10.5 3.1 5.2 0.3 9.1 4.0
3.1 2.8 1.1 1.3 1.3 1.2 0.5 0.2 0.8 0.8
3.1 2.7 1.8 1.7 1.2 1.0 1.7 1.1 1.5 1.3
5.2 4.8 5.2 4.1 4.9 5.1 5.0 5.3 6.7 4.7
Cepeda Castelar Virasoro C. Azul
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
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3.1. IEPpH of soils saturated with different cations Curves to obtain the IEPpH of soils saturated with different cations by the diffusion potential method are reported in Fig. 3. As expected the IEPpH values presented the following order: Ca2q-soil ) natsoil ) Kq-soil for all samples analyzed. This can be explained in the following way: calcium is specifically adsorbed and decreases the net negative charge. On the contrary Kq can be displaced by Hq during the pH adjustment and liberates negative surface charges, showing IEPpH lower than Ca2q-soils. Intermediate values for nat-soils are attributed to the different amounts of saturating cations Žshown in Table 2.. Different authors ŽCurtin et al., 1992; Pardo et al., 1992; Ceppi et al., 1993. have mentioned that Ca2q decreased the electrostatic repulsion between negative
Fig. 3. IEPpH of natural Ž Ø ., Ca2q Ž'. and Kq ŽB. homoionized samples of Anguil ŽA., Cepeda ŽB., Castelar ŽC., Virasoro ŽD. and Cerro Azul ŽE. soils.
378
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
surfaces and the anions of the solution Ž phosphates, humic acids, etc.. . This may be understood from our data ŽFig. 3. where for same pH, charge and surface potentials are different, depending on the saturating cation. The IEPpH of Ca2q or nat-soil may be of interest to know the charge status of soil surfaces and a tool to explain the electrostatic component in the interactions of soil particles. 3.2. IEPpH of K q-soils No differences were found on the IEPpH of Kq soil values obtained in samples homoionized by different methods. IEPpH values - 4.5 were extrapolated from E vs. pH curves ŽFig. 3. and almost all IEPpH of Kq soils were near to this value. Tschapek et al. Ž 1976. found that the IEPpH values of potassium saturated materials were similar to the ZPC pH obtained by titration. In our study the IEPpH of Kq soil was higher than ZPC pH in four soils: Anguil, Castelar, Cepeda and Virasoro and it was lower than ZPC pH in Cerro Azul Ž Fig. 3 and Table 1.. Differences might be due to any of the following reasons: 1. deficient elimination of saturating cations, 2. Kq does not behave like an indifferent ion or, 3. comparison with ZPC pH is not adequate because sometimes ZPC pH is very low and dissolution of soil material can occur. More studies should be made to consider IEPpH of Kq-soil as a particular IEPpH measure. 3.3. IEPpH of soil after OM remoÕal In this experiment the IEPpH of natural soils and after a treatment with H 2 O 2 was measured. After OM removal data of the IEPpH of Anguil, Cepeda and Cerro Azul were lower than the IEPpH of the same natural soils and higher than those for Castelar and Virasoro. Data are shown in Table 2. According to the results shown above Ž soils saturated with different cations. a relation between the IEPpH and the proportion of basic cations adsorbed on the exchange complex was expected. TOC, CEC and basic cations were determined ŽTable 2. and the sum of basic cations and the percentage of basic saturation were calculated. Levels of TOC and Ca2q after the H 2 O 2 treatment were variables. Magnesium and Kq were probably not eliminated because they are associated to the mineral fraction, in coincidence with Heil and Sposito Ž 1993. . CEC values increased in some samples after H 2 O 2 treatment because of liberation of mineral surface charge. Torres Sanchez and Falasca Ž1997. found that this treatment did ´ not affect the crystal but artifacts between the surface and ions or molecules from the solution could be formed ŽSequi et al., 1980. . Statistical associations between IEPpH and the mentioned studied variables, considering the treated and the test soils, alone or together, were not found.
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
379
However, positive lineal correlations among IEPpH of natural soils and ZPC pH Ž r s 0.97, P - 0.001., pH Ž r s 0.88, P - 0.01. , percentage of Fe 2 O 3 Ž r s 0.87, P - 0.01. and clay content Ž r s 0.77, P - 0.05. were found. More data are needed to confirm these results and find new relations between IEPpH and other variables. 4. Conclusions The diffusion potential method showed that the IEPpH changed, owning to the saturation with different cations in the following order: Ca2q soil ) nat-soil) Kq-soil. IEPpH of Kq-soils did not coincide with ZPC pH , showing that more studies should be done to consider it as a special measure of IEPpH . After OM removal the IEPpH of Anguil, Cepeda and Cerro Azul were lower than those of natural soils, and higher for Castelar and Virasoro soils. Results could not be explained in a simple way. The IEPpH of natural soils was positively correlated with ZPC pH , pH in water, and Fe 2 O 3 and clay content. The diffusion potential method showed to be a simple, quick and adequate method to measure IEPpH of natural soils, but pH must not attain extreme acid pH values. References Black, C.A., Evans, D.D., White, J.L., Ermsminger, L.G., Clark, F.E., 1965. Methods of Soil Analysis. Am. Soc. of Agric. Madison, WI. Blok, L., De Bruyn, P.L., 1970. The ionic double layer at the ZnOrsolution interface. J. Coll. Interf. Sci. 32, 518–525. Ceppi, S.B., Velasco, M.I., De Pauli, C.P., 1993. Influencia del Naq y el Ca2q en la adsorcion ´ de acidos humicos sobre partıculas de suelo bajo diferentes sistemas de manejo. Agrochimica ´ ´ ´ XXVII Ž1–2., 134–146. Curtin, D., Syers, J.K., Bolan, N.S., 1992. Phosphate sorption by soil in relation to exchangeable cation composition and pH. 31, 137–149. Escudey, M., Galindo, G., 1983. Effect of iron oxide coating on electrophoretic mobility and dispersion of allophane. J. Coll. Interf. Sci. 93, 78–83. Gillman, G.P., Uehara, G., 1980. Charge characteristics of soils with variable and permanent charge minerals: II. Experimental. Soil Sci. Soc. Am. J. 44, 252–255. Heil, D., Sposito, G., 1993. Organic matter role in silisic colloids flocculation: I. Commuter ions and pH. Soil Sci. Soc. Am. J. 57, 1241–1246. Lyklema, J., 1984. Interfacial electrochemistry of disperse systems. Agricultural University, 211–234. Nelson, D.W., Sommers, I., 1982. Total carbon, organic carbon and organic matter. Methods of soil analysis: Part 2. American Society of Agronomy, USA. Agronomy, 9, pp. 539–579. Pardo, M.T., Guadalix, M.E., Garcia-Gonzalez, M.T., 1992. Effect of pH and background ´ electrolyte on P sorption by variable charge soils. Geoderma 54, 275–284. Sequi, P., Arighieri, R., Pardin, G., 1980. Effect of peroxidation on soil electrochemical properties. Z. Pflanzenernaehr. Bodenk. 143, 298–305.
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Sposito, G., 1984. The surface chemistry of soils, Chap. 3, Oxford Univ. Press, New York, pp. 81–88. Torres Sanchez, R.M., Volzone, C., Curt, E.M., 1992. Zero point of charge determination of ´ monoionic montmorillonite by transport number method. Z. Pflanzenernaehr. Bodenk. 155, 77–80. Torres Sanchez, R.M., Falasca, S., 1997. Specific surface and surface charges of some Argentian ´ soils. Z. Pflanzenernaehr. Bodenk. 160, 223–226. Tschapek, M., Torres Sanchez, R.M., Wasowski, C., 1976. Determination of PZC by EMF ´ measurements of cell with two junctions. Colloid Polym. Sci. 254, 516–521. Tschapek, M., Torres Sanchez, R.M., Wasowski, C., 1989. Handy methods for determining the ´ isoelectric point of soils. Z. Pflanzenernaehr. Bodenk. 152, 73–76. Tschapek, M., Wasowski, C., Falasca, S., 1991. K-montmorillonite: its electric double layer extension and point of zero charge by diffusion potential. Colloid Polym. Sci. 269, 1190–1195.
Isoelectric point of soils determined by the diffusion potential method M.C. Lamas a
a,)
, R.M. Torres Sanchez ´
b
FAUBA, Edafologıa, ´ AÕ. San Martın ´ 4453, (1417) Cap. Fed. Buenos Aires, Argentina b CETMIC, Cno. Centenario y 506, CC 49 (1897) M.B. Gonnet, Argentina Received 11 March 1997; accepted 11 February 1998
Abstract The diffusion potential method was used to determine the Isoelectric Point ŽIEP. of the surface horizons of five soils ŽTypic Hapludoll, Typic Argiudoll, Vertic Argiudoll, Rhodic Kandiudalf and Lithic Kandihumult. in the following conditions: without treatment Žnatural., homoionized with Kq ŽKq-soil. or Ca2q ŽCa2q-soil. and after organic matter ŽOM. removal. Soil samples were characterized through their mineralogy, total organic carbon ŽTOC., clay percentage, cation exchange capacity ŽCEC., basic cations in the exchange complex, pH in water, Fe 2 O 3 content and zero point of charge ŽZPC.. The IEPpH of soils gave the following order: Ca2q-soil ) natural-soil ) Kq-soil. These results were explained in terms of the higher affinity of bivalent over monovalent cations for negative surfaces. IEPpH of K-soils was dissimilar to ZPC pH , indicating that Kq did not behave as an indifferent cation. Data of IEPpH after OM removal of Anguil ŽTH., Cepeda ŽVA. and Cerro Azul ŽLK. soils were lower than those IEPpH of the same natural samples and higher for Castelar ŽTA. and Virasoro ŽRK. soils. Results could not be explained in a simple way. The IEPpH of natural soils correlated with ZPC pH , pH in water and Fe 2 O 3 and clay contents. The diffusion potential method showed to be rapid, simple and adequate to measure IEPpH of natural or treated soils. q 1998 Elsevier Science B.V. All rights reserved. Keywords: IEPpH ; diffusion potential methods; soils; homoionized soils
)
Corresponding author. E-mail: [email protected]
0016-7061r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 1 6 - 7 0 6 1 Ž 9 8 . 0 0 0 2 5 - 1
372
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
1. Introduction One of the most relevant parameters to characterize the electrical state of soil surface particles in solution is the zero point of charge Ž ZPC. which is defined as: the pH where the net surface charge, resulting of the adsorption of the potential determining ions, Hq and OHy, is zero. Generally the ZPC is obtained by the titration curve method in different electrolyte concentrations and it is determined as the cross point of these curves Ž Blok and De Bruyn, 1970.. The change of measurement conditions produces different cross points, defined as follows: point of zero net charge ŽZPNC. is the pH where the cationic exchange capacity equals the anionic exchange capacity; point of zero salt effect ŽZPSE. is the pH of intersection of the potentiometric titration curves at different electrolyte concentration, etc. Ž Gillman and Uehara, 1980; Sposito, 1984.. They are used in the literature to understand different surface soil conditions. Another parameter utilized to characterize the surface status of soils is the isoelectric point Ž IEPpH . which is the pH where the charge at the slipping plane of the Stern Layer is zero. Generally it is obtained when an electric field Ža pressure gradient, etc.. is used to move the charged particles of colloids in solution Ž microelectrophoresis, electroosmosis methods, etc.. . The IEPpH of oxides and minerals is usually measured by microelectrophoresis. The IEPpH of whole soils is not measured by this method because of the large soil particles distribution size and its complex composition. The soil IEPpH was measured by the diffusion potential method after Tschapek et al. Ž1989.. This method measures the transference number of cations Ž tq. of an electrolyte that moves within the double electric layer of a solid created by a surface charge. The method was proved to be sensitive enough to distinguish IEPpH values of kaolins and smectites homoionized with different cations ŽTschapek et al., 1991; Torres Sanchez et al., 1992. . Values of IEPpH and ZPC pH ´ only coincide when no specific adsorption exists Ž Lyklema, 1984. . Ions that are specifically adsorbed Ž Gillman and Uehara, 1980. may be displaced by titration during the ZPC pH measurement, and this fact does not happen when the IEPpH is determined. In the above mentioned works ŽTschapek et al., 1991; Torres Sanchez et al., ´ q . 1992 the IEPpH values of homoionized K samples determined by the diffusion potential method coincided with the ZPC pH obtained by the titration method ŽTschapek et al., 1989. and by this reason it was accepted that Kq behaved as an indifferent ion. The aim of this work is to extend the diffusion potential method to the IEPpH of soil determination. The sensitivity of the method was evaluated by measuring the IEPpH of surface samples of five soils with different characteristics. All of them were measured in natural conditions, after different homoionization treatments ŽKq and Ca2q . and after organic matter ŽOM. removal.
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
373
2. Materials and methods 2.1. Samples Samples of surface horizons Ž A 1 . of five soils were used. Three of them from a temperate region: Pampean Prairie ŽTypic Hapludoll, Vertic Argiudoll and Typic Argiudoll, named Anguil, Cepeda and Castelar, respectively. and two soils from a subtropical and tropical zone Ž Rhodic Kandiudalf and Lithic Kandihumult, identified as Virasoro and Cerro Azul, respectively. . The classification, texture, crystalline species, iron content, ZPC pH and pH of these soils are shown in Table 1. 2.2. Treatments Soils were treated with different solutions to replace their saturating cations. All samples, after treatment, were water washed, centrifugated several times until negative Cly reaction was verified by ŽAgNO 3 .. Ža. Nat-soils: samples were washed by percolation with distilled water. Žb. Ca2q-soils: nat-soils were percolated with 0.3 N Cl 2 Ca. Žc. Kq-soil: two different homoionized procedures were compared: Ž1. soil was Kq saturated with 1 N KCl, Ž2. soils were at first percolated with 0.05 N HCl to displace exchangeable cations and then neutralized with 0.25 N KOH. For OM removal: nat soils were treated with H 2 O 2 following the method of Black et al. Ž1965.. 2.3. Methods Crystalline species were qualitatively identified by X-ray diffraction, in a Philips equipment 1140, using Cu K a radiation at 20 mA and 40 kV. The ZPC pH was determined by classical fast potentiometric titration method on 2 g sample to which 30 ml of 10y1 and 10y3 M KCl was added as supporting electrolyte. The samples thus prepared were titrated under N2 atmosphere with additions of 0.1 N HCl each 10 min ŽBlok and De Bruyn, 1970.. Total organic carbon ŽTOC. was determined by the Walkley Black technique ŽNelson and Sommers, 1982. . Cation exchange capacity Ž CEC. was determined by the NH 4OAc 1 N method at pH s 7, saturating cations were determined by spectrophotometry and Kq by flame photometry treatment. The IEPpH is measured by the diffusion potential method. When putting in contact two concentrations of the same solution Ž i.e., KCl. a diffusion of ions from the most to the least concentrated is produced. The ion that moves more rapidly will give the charge to the diluted side of the solution, creating a potential difference which can be measured in an electrochemical cell and it is called liquid junction potential.
374
Soil sample Soil classification
Texture
Crystalline species
ZPC ŽpH. Fe 2 O 3Ž%. pH Žwater.
Bentonite Diaspore Bentonite Pyrophyllyte Bentonite Diaspore Pyrophyllyte Goehite Diaspore Goehite Diaspore Pyrohyllyte Magnetite
3.5 2.8 3.0 3.5 5.7
2000–50 m m 50–2 m m - 2 m m Anguil Cepeda Castelar Virasoro C. Azul
Typic Hapludoll Vertic Arguidoll Typic Arguidoll Rhodic Kandiudalf Lithic Kandihumult
67 11 10 5 13
19 68 64 37 18
14 21 26 57 69
3.6 3.0 1.8 10.0 14.0
Besides the showed crystalline species: quartz, illite, kaolinite, feldspar, halloysite and hematite were found in all the samples.
6.2 5.8 5.5 5.5 6.9
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
Table 1 Soil classification, clay content, crystalline species, ZPC pH , Fe 2 O 3 content and pH Ž1:2.5, soil:water. of the studied soils
M.C. Lamas, R.M. Torres Sanchezr Geoderma 85 (1998) 371–380 ´
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Fig. 1. Electrochemical cell to measure EMF s e1 q e 2 q e 3 q e 4 .
If we interpose a charged membrane between the two solutions, this acts accelerating the ions of opposite charge and hindering those of the same charge. A soil with surface charge behaves like a membrane which alters the mobility of ions, generating a new potential difference which can be measured in the electrochemical cell ŽFig. 1. based on Hendershot equation ŽTschapek et al., 1976.. Because e1 s e 4 , the EMF of this cell is s e 2 q e 3. Changing the pH we can change the soil charge. In the pH of the IEPpH the net surface charge of the soil is zero and the EMF of the cell is the liquid junction potential of the KCl. Diffusion potential was measured with a Keithley 616 electrometer Ž with an input resistance ) 2.10 14 V and a sensitivity of 10y15 A. and calomel electrodes. The apparatus used for the IEPpH measures of soils is shown in Fig. 2. The liquid junction potentials were measured when the plug was empty and when the paste soil was in the plug. Measures were performed in the following way: E1 is the diffusion potential of KCl electrolyte Ž E1 s y0.35 mV obtained by Henderson equation when the concentration electrolyte ratio C 2rC 1 s 2.. When the plug was filled with a soil paste Žprepared with soil plus solutions at different pH., E2 was measured. The difference E s E2 y E1 is the potential
Fig. 2. Apparatus used to measure IEPpH of soils by the diffusion potential method. The stop cock has a diameter of 1 cm and 1.5 cm length. The left compartment contained KCl 2=10y3 M and the right one KCl 1=10y3 M.
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generated by the charge of soil paste, being E s 0 when the material has zero net charge at the IEPpH E1 s E2 s y0.35 mV.. When E ) 0 or - 0, the soil is negatively or positively charged, respectively. Tschapek et al. Ž 1976. observed that the IEPpH should be extrapolated from an E vs. pH curve at pH - 3.5 because Hq activities interfere in the E measures. Solutions with different concentrations of HCl or KOH were added to the dried soil to obtain the soil paste. It was divided in two fractions: one of them was used for pH determinations and the other one for E determinations. The paste moisture modify E but not IEPpH values as was demonstrated by Torres Sanchez et al. Ž1992.. ´ 3. Results and discussion Characterization of the studied soil samples are shown in Table 1. Among the XRD identified crystalline species: quartz, feldspar, illite, kaolinite, halloysite and hematite were found in all samples. The ZPC pH values obtained in Pampean Prairie soils Ž Cepeda, Castelar and Anguil. were generally lower than those obtained for tropical soils and seemed to be determined by the presence of clay mineral type 2:1 Ž bentonite. and TOC content Ž Table 2. . The ZPC pH values of Cerro Azul was higher than ZPC pH of other soils and this is in agreement with the high iron oxide contents Ž Escudey and Galindo, 1983. . Furthermore, Virasoro sample with similar iron content has a lower ZPC pH than Cerro Azul sample, attributed to the higher proportion of TOC Ž3.5% of TOC.. Organic carbon shifts the ZPC pH into opposite direction than that observed for iron oxide ŽEscudey and Galindo, 1983.. For all samples, the ZPC pH was lower than the pH in water, indicating that these soils had negative charge in natural conditions. Table 2 TOC content, CEC, saturating cations ŽCa2q, Kq, Mg 2q . and IEPpH of the soil samples before and after OM removal by H 2 O 2 treatment Soil samples
OM removal
TOC Ž%.
CEC ŽCmol c rkg.
Ca2q ŽCmol c rkg.
Kq ŽCmol c rkg.
Mg 2q ŽCmol c rkg.
IEP ŽpH.
Anguil
before after before after before after before after before after
1.4 0.2 1.3 0.1 1.8 0.2 3.5 0.7 2.3 1.2
15.8 11.9 13.2 11.7 19.6 20.3 15.4 18.2 13.9 12.0
10.2 3.8 7.4 3.6 10.5 3.1 5.2 0.3 9.1 4.0
3.1 2.8 1.1 1.3 1.3 1.2 0.5 0.2 0.8 0.8
3.1 2.7 1.8 1.7 1.2 1.0 1.7 1.1 1.5 1.3
5.2 4.8 5.2 4.1 4.9 5.1 5.0 5.3 6.7 4.7
Cepeda Castelar Virasoro C. Azul
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3.1. IEPpH of soils saturated with different cations Curves to obtain the IEPpH of soils saturated with different cations by the diffusion potential method are reported in Fig. 3. As expected the IEPpH values presented the following order: Ca2q-soil ) natsoil ) Kq-soil for all samples analyzed. This can be explained in the following way: calcium is specifically adsorbed and decreases the net negative charge. On the contrary Kq can be displaced by Hq during the pH adjustment and liberates negative surface charges, showing IEPpH lower than Ca2q-soils. Intermediate values for nat-soils are attributed to the different amounts of saturating cations Žshown in Table 2.. Different authors ŽCurtin et al., 1992; Pardo et al., 1992; Ceppi et al., 1993. have mentioned that Ca2q decreased the electrostatic repulsion between negative
Fig. 3. IEPpH of natural Ž Ø ., Ca2q Ž'. and Kq ŽB. homoionized samples of Anguil ŽA., Cepeda ŽB., Castelar ŽC., Virasoro ŽD. and Cerro Azul ŽE. soils.
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surfaces and the anions of the solution Ž phosphates, humic acids, etc.. . This may be understood from our data ŽFig. 3. where for same pH, charge and surface potentials are different, depending on the saturating cation. The IEPpH of Ca2q or nat-soil may be of interest to know the charge status of soil surfaces and a tool to explain the electrostatic component in the interactions of soil particles. 3.2. IEPpH of K q-soils No differences were found on the IEPpH of Kq soil values obtained in samples homoionized by different methods. IEPpH values - 4.5 were extrapolated from E vs. pH curves ŽFig. 3. and almost all IEPpH of Kq soils were near to this value. Tschapek et al. Ž 1976. found that the IEPpH values of potassium saturated materials were similar to the ZPC pH obtained by titration. In our study the IEPpH of Kq soil was higher than ZPC pH in four soils: Anguil, Castelar, Cepeda and Virasoro and it was lower than ZPC pH in Cerro Azul Ž Fig. 3 and Table 1.. Differences might be due to any of the following reasons: 1. deficient elimination of saturating cations, 2. Kq does not behave like an indifferent ion or, 3. comparison with ZPC pH is not adequate because sometimes ZPC pH is very low and dissolution of soil material can occur. More studies should be made to consider IEPpH of Kq-soil as a particular IEPpH measure. 3.3. IEPpH of soil after OM remoÕal In this experiment the IEPpH of natural soils and after a treatment with H 2 O 2 was measured. After OM removal data of the IEPpH of Anguil, Cepeda and Cerro Azul were lower than the IEPpH of the same natural soils and higher than those for Castelar and Virasoro. Data are shown in Table 2. According to the results shown above Ž soils saturated with different cations. a relation between the IEPpH and the proportion of basic cations adsorbed on the exchange complex was expected. TOC, CEC and basic cations were determined ŽTable 2. and the sum of basic cations and the percentage of basic saturation were calculated. Levels of TOC and Ca2q after the H 2 O 2 treatment were variables. Magnesium and Kq were probably not eliminated because they are associated to the mineral fraction, in coincidence with Heil and Sposito Ž 1993. . CEC values increased in some samples after H 2 O 2 treatment because of liberation of mineral surface charge. Torres Sanchez and Falasca Ž1997. found that this treatment did ´ not affect the crystal but artifacts between the surface and ions or molecules from the solution could be formed ŽSequi et al., 1980. . Statistical associations between IEPpH and the mentioned studied variables, considering the treated and the test soils, alone or together, were not found.
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However, positive lineal correlations among IEPpH of natural soils and ZPC pH Ž r s 0.97, P - 0.001., pH Ž r s 0.88, P - 0.01. , percentage of Fe 2 O 3 Ž r s 0.87, P - 0.01. and clay content Ž r s 0.77, P - 0.05. were found. More data are needed to confirm these results and find new relations between IEPpH and other variables. 4. Conclusions The diffusion potential method showed that the IEPpH changed, owning to the saturation with different cations in the following order: Ca2q soil ) nat-soil) Kq-soil. IEPpH of Kq-soils did not coincide with ZPC pH , showing that more studies should be done to consider it as a special measure of IEPpH . After OM removal the IEPpH of Anguil, Cepeda and Cerro Azul were lower than those of natural soils, and higher for Castelar and Virasoro soils. Results could not be explained in a simple way. The IEPpH of natural soils was positively correlated with ZPC pH , pH in water, and Fe 2 O 3 and clay content. The diffusion potential method showed to be a simple, quick and adequate method to measure IEPpH of natural soils, but pH must not attain extreme acid pH values. References Black, C.A., Evans, D.D., White, J.L., Ermsminger, L.G., Clark, F.E., 1965. Methods of Soil Analysis. Am. Soc. of Agric. Madison, WI. Blok, L., De Bruyn, P.L., 1970. The ionic double layer at the ZnOrsolution interface. J. Coll. Interf. Sci. 32, 518–525. Ceppi, S.B., Velasco, M.I., De Pauli, C.P., 1993. Influencia del Naq y el Ca2q en la adsorcion ´ de acidos humicos sobre partıculas de suelo bajo diferentes sistemas de manejo. Agrochimica ´ ´ ´ XXVII Ž1–2., 134–146. Curtin, D., Syers, J.K., Bolan, N.S., 1992. Phosphate sorption by soil in relation to exchangeable cation composition and pH. 31, 137–149. Escudey, M., Galindo, G., 1983. Effect of iron oxide coating on electrophoretic mobility and dispersion of allophane. J. Coll. Interf. Sci. 93, 78–83. Gillman, G.P., Uehara, G., 1980. Charge characteristics of soils with variable and permanent charge minerals: II. Experimental. Soil Sci. Soc. Am. J. 44, 252–255. Heil, D., Sposito, G., 1993. Organic matter role in silisic colloids flocculation: I. Commuter ions and pH. Soil Sci. Soc. Am. J. 57, 1241–1246. Lyklema, J., 1984. Interfacial electrochemistry of disperse systems. Agricultural University, 211–234. Nelson, D.W., Sommers, I., 1982. Total carbon, organic carbon and organic matter. Methods of soil analysis: Part 2. American Society of Agronomy, USA. Agronomy, 9, pp. 539–579. Pardo, M.T., Guadalix, M.E., Garcia-Gonzalez, M.T., 1992. Effect of pH and background ´ electrolyte on P sorption by variable charge soils. Geoderma 54, 275–284. Sequi, P., Arighieri, R., Pardin, G., 1980. Effect of peroxidation on soil electrochemical properties. Z. Pflanzenernaehr. Bodenk. 143, 298–305.
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Sposito, G., 1984. The surface chemistry of soils, Chap. 3, Oxford Univ. Press, New York, pp. 81–88. Torres Sanchez, R.M., Volzone, C., Curt, E.M., 1992. Zero point of charge determination of ´ monoionic montmorillonite by transport number method. Z. Pflanzenernaehr. Bodenk. 155, 77–80. Torres Sanchez, R.M., Falasca, S., 1997. Specific surface and surface charges of some Argentian ´ soils. Z. Pflanzenernaehr. Bodenk. 160, 223–226. Tschapek, M., Torres Sanchez, R.M., Wasowski, C., 1976. Determination of PZC by EMF ´ measurements of cell with two junctions. Colloid Polym. Sci. 254, 516–521. Tschapek, M., Torres Sanchez, R.M., Wasowski, C., 1989. Handy methods for determining the ´ isoelectric point of soils. Z. Pflanzenernaehr. Bodenk. 152, 73–76. Tschapek, M., Wasowski, C., Falasca, S., 1991. K-montmorillonite: its electric double layer extension and point of zero charge by diffusion potential. Colloid Polym. Sci. 269, 1190–1195.