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DIVISION S-2—"PARTICLE INTERACTIONS IN COLLOIDAL SYSTEMS"

Adsorption and Desorption of Indifferent Ions in Variable Charge Subsoils

The Possible Effect of Particle Interactions on the Counter-Ion Charge Density

Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602

* Corresponding author (nik.qafoku{at}pnl.gov)

	ABSTRACT

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The relationship between soil solution ionic strength (IS) and the surface or counter-ion charge density (CICD) is well described by the double layer theory. However, the magnitude of the counter-ion charge may be affected by phenomena that are likely to occur in variable charge subsoils, and research to investigate the extent of indifferent ion sorption as a function of IS is certainly needed. The objective was to study adsorption and desorption of ions in batch and column experiments, and to propose a mechanism that describes best the experimental observations in subsoils from the southeastern USA. Results showed that the cation and anion of an electrolyte were simultaneously adsorbed in approximately equivalent amounts with no net release of other ions into the soil solution. Ions were adsorbed in the Stern and diffuse layers of oppositely charged colloids since the subsoil was treated with dilute solutions. They were immediately displaced when the subsoil was washed with distilled water (dH₂O), confirming the reversible nature of this phenomenon. Three different magnitudes of the CICD were observed at similar IS values. The surface charge density on negatively and positively charged soil particles may not be equal to the CICD in dilute soil solutions, and other mechanisms may operate to balance the surface particle charges. It is possible that a portion of the surface charge may be balanced as a result of the interactions among oppositely charged soil particles when their diffuse layers extend and overlap, or compress and separate in response to IS changes.

Abbreviations: BEC, background electrolyte concentration • BTC, breakthrough curve • CICD, counter-ion charge density • dH₂O, distilled water • EC, electrical conductivity • ICP-MS, inductively coupled plasma mass spectroscopy • IS, ionic strength • PV, pore volumes

	INTRODUCTION

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THE TRANSPORT OF INDIFFERENT ANIONS through some acid tropical and subtropical soils classified as Alfisols, Ultisols, and Oxisols (Soil Survey Staff, 1975) is retarded because these variable charge soils develop appreciable positive charge on amphoteric minerals surfaces (Bellini et al., 1996; Qafoku et al., 2000a). The positive surface charge is balanced by an equivalent amount of indifferent anions adsorbed in the outer-sphere and diffuse layers on soil particles, and their sum is defined as the counter-ion charge, or the anion exchange capacity of the soil. The magnitude of the surface charge density on amphoteric surfaces is a function of pH and IS in the soil solution that encompasses both the effects of the background electrolyte concentration (BEC) and ionic composition of the soil solution. Double layer theory (van Olphen, 1977) predicts that the counter-ion positive charge in soils change in direct proportion to the surface charge in response to IS changes in the soil solution. However, Barber and Rowell (1972) reported that this was not the case in a variable charge subsoil collected in Tanzania. They found that the change in the magnitude of the surface charge in response to the change in Cl concentration in the soil solution predicted by the double layer theory was different from the change in the magnitude of the counter-ion charge measured experimentally and expressed in terms of the total amount of Cl adsorbed. Since the effect of the soil solution IS on the magnitude of the counter-ion charge is not well understood in the variable charge soils and subsoils (Ji, 1997; Ji and Li, 1997; Zhang and Zhao, 1997), research to investigate phenomena related to the indifferent anion (e.g., nitrate and chloride) adsorption and desorption as a function of IS is certainly needed.

Salt adsorption, a common phenomenon in variable charge soils and subsoils, is defined as the simultaneous adsorption in equivalent amounts of the cation and anion of an electrolyte with no net release of other ions into the soil solution. Even though this phenomenon has been observed and reported in many studies with variable charge soils and subsoils, and some mechanisms to describe it have been already advanced (Thomas, 1960; Wada, 1984; Marcano-Martinez and McBride, 1989; Alva et al., 1991; Bolan et al., 1993; Pearse, 1994; Bellini et al., 1996; Pearse and Sumner, 1997), a unified and clear explanation has yet to be established (Qafoku, 1998). Quite recently, Qafoku and Sumner (2001) reported that in packed columns with the Cecil (Ultisol) subsoil from the southeastern USA, Ca²⁺ and NO^-₃ appeared at the same time in the leachate after >2.5 pore volumes (PV) of the leaching solution [5 mmol_c L^-1 Ca(NO₃)₂] were passed through the column. They suggested that salt adsorption caused the simultaneous depletion of Ca²⁺ and NO^-₃ from the leaching solution. Because the mineralogy of the Cecil subsoil is dominated by kaolinite and Al and Fe oxides, one may use the multiple-site adsorption model (Schindler, 1987; Zachara and McKinley, 1993) to describe the adsorption of Ca²⁺ and NO^-₃ via surface complex formation with the following surface reactive sites in the subsoil: hydroxylated sites on Al oxides surfaces (AlOH), hydroxylated sites on Fe oxides surfaces (FeOH), hydroxylated sites on kaolinite edges (AlOH and SiOH), and fixed-charge sites on kaolinite surfaces (X^-). The possible chemical reactions should be the following: SOH + Ca²⁺ = SOCa⁺ + H⁺ (inner sphere) SOH + Ca²⁺ = SO^--Ca⁺ + H⁺ (outer sphere) SOH + H⁺ + NO^-₃ = SOH⁺₂ - NO^-₃ (outer sphere) XMg + Ca²⁺ = XCa + Mg²⁺ (ion exchange to fixed charge sites X^- on kaolinite), and XK + Ca²⁺ = XCa⁺ + K⁺.

These chemical reactions predict that (i) pH of the soil solution (or pH in leachate coming out of the column outlet) should either remain unchanged or become more acidic as a result of release of the protons into the soil solution, and (ii) other exchangeable ions that were present on the permanent and variable charge surfaces in the subsoil should be exchanged for Ca²⁺ and NO^-₃ and appear in the leachate. However, Qafoku and Sumner (2001) reported that pH in the leachate remained well above the pH value in the subsoil, and no other ions were released in the leachate in the first 2.5 PV, that is, when salt adsorption occurred. Since the multiple-site adsorption model did not predict these experimental observations, it is quite likely that other mechanism(s) may operate to cause the simultaneous adsorption of Ca²⁺ and NO^-₃ with no net release of other ions into the soil solution.

The objective of this paper was to study adsorption and desorption of indifferent ions in variable charge subsoils from southeastern USA. In order to present evidence that salt adsorption of an indifferent background electrolyte occurs even when the concentration of this electrolyte in the soil solution is maintained low, column experiments with a very dilute leaching solution were conducted and the results are presented in this paper. The magnitude of the counter-ion charge was also studied in subsoil under different leaching conditions. In addition, since there is no evidence in the literature to show that salt adsorption of an indifferent electrolyte is a reversible phenomenon and the adsorbed salt is displaced when soil is washed with dH₂O, data on desorption of indifferent ions in these soils are also presented. A mechanism that describes best the experimental observations on adsorption and desorption of indifferent ions in variable charge subsoils is also proposed.

	MATERIALS AND METHODS

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Leaching Experiments with Very Dilute Cesium Chloride Solution
The Cecil (fine, kaolinitic, thermic Typic Kanhapludults) and Faceville (fine, kaolinitic, thermic, Typic Kandiudults) subsoils (Table 1) collected in Georgia were used in the first experiment that was designed to provide evidence on salt adsorption when subsoil was leached with a very dilute CsCl solution (0.5 mM). Twenty-centimeter-long Plexiglas columns with an interior diameter of 5 cm were used in this experiment. The air-dried subsoil was packed very carefully into the column to a uniform bulk density using a powder funnel with a plastic extension tube. Washed sand layers (0.01-m thick) were used at the top and the bottom of the column to disperse flow throughout the entire cross-sectional area and to reduce the impact of the flow on colloid movement which can result in a progressive reduction in the hydraulic conductivity (inlet sand layer), and to assure that saturation is reached in the columns before the first drop of leachate emerges (outlet sand layer). Columns were oriented horizontally and very slowly saturated and leached with 0.5 mmol L^-1 CsCl solution at a constant flow rate using a peristaltic pump. Electrical conductivity (EC) and pH in the leachate were measured continuously. Thirty-two-milliliter leachate aliquots were stored in plastic vials at 0 to 5°C prior to analysis. Volumetric water content in the columns was calculated from the difference in column mass before and after saturation. The pump flow rate was 5.56 x 10^-9 m³ s^-1. A Dionex (Sunnyvale, CA) model DX500 HPLC System with an AS3500 Dionex autosampler equipped with a Rheodyne (Cotati, CA) 9010 injection valve, and a 5 x 10^-8 m³ injection loop was used to analyze the samples of leachate. A Dionex IonPac AS4A guard column (0.004 by 0.050 m) and anion exchange column (0.250 by 0.004 m) were used for NO₃, Cl, PO₄, SO₄, and F measurements, and a carbonate and bicarbonate solution was used as eluent. Cesium, Na, K, Ca, Mg, and Al were determined by inductively coupled plasma mass spectroscopy (ICP-MS). Flow and other column parameters are presented in Table 2.

Batch Experiments with Distilled Water Washes
Batch experiments were conducted to study adsorption and desorption of indifferent ions as well as the reversible nature of salt adsorption by washing the Cecil subsoil with dH₂O after it was saturated first with a NaCl solution. The batch experiments were conducted as follows: 5 g (383 K oven dry-weight basis) of air-dry soil sample were placed into each of 50-mL prelabeled and preweighed centrifuge tubes to 0.0001 g. Twenty-five milliliters of 1 M NaCl were added in each centrifuge tube. The tubes were shaken at room temperature for 1 h in a horizontal reciprocating shaker, centrifuged, and the supernatant discarded. Twenty milliliters of 100 mmol L^-1 NaCl solution were added into the centrifuge tubes afterwards. The tubes were shaken for 2 h, centrifuged, and the supernatant was discarded. They were washed four more times with the same solutions. The supernatants of the final washing were placed in 28-mL scintillation vials and analyzed for Cl and Na. The centrifuge tubes were weighed to obtain the volume of entrained solution in order to estimate the entrained Cl and Na. The concentrations of the entrained Cl and Na were assumed to be equal to that in the supernatant. The tubes were then washed five times with dH₂O and supernatants were collected in 28-mL scintillation vials and analyzed for Cl and Na. All treatments were washed four times at the end with 20 mL of a 0.5 M KNO₃ solution each to displace all the Cl and Na remaining in the subsoil.

	RESULTS

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Leaching Experiments with a Dilute Cesium Solution
Cesium and Cl concentrations were nearly constant in the first 10 PV, which indicated that salt adsorption occurred when the Cecil subsoil was leached with a very dilute (0.5 mM) CsCl solution (Fig. 1) . Cesium and Cl concentrations in the effluent (0 and 0.113 mM L^-1, respectively) were much lower than those in the input solution (0.5 mM L^-1). Because Cs has different adsorption properties from Cl, its concentration in the effluent remained close to zero for 30 PV. The EC remained approximately constant, well below the EC in the input solution, and was similar to the soil solution EC of the Cecil subsoil before leaching. This indicated that during salt adsorption the soil solution IS did not change. The average pH value in the leachate in the first 10 PV was greater than the pH value found in the subsoil (5.90 vs. 4.21) (Fig. 1), which suggests that no inner-sphere complexes of cations were formed when salt adsorption occurred in the first 10 PV.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Changes in leachate pH, electrical conductivity (EC), and Cs and Cl concentrations during leaching with 0.5 mmol L-1 CsCl solution (Cecil subsoil).

View larger version (20K): [in this window] [in a new window]   Fig. 2. Changes in leachate pH, electrical conductivity (EC), and Cs and Cl concentrations during leaching with 0.5 mmol L-1 CsCl solution (Faceville subsoil).

View larger version (18K): [in this window] [in a new window]   Fig. 3. Changes in the concentration of cations in leachate during leaching with 0.5 mmol L-1 CsCl solution (Cecil subsoil).

View larger version (22K): [in this window] [in a new window]   Fig. 4. Changes in the concentration of anions in leachate during leaching with 0.5 mmol L-1 CsCl solution (Cecil subsoil).

Reversibility of Salt Adsorption
Experiments with Packed Columns
The plateau of approximately constant values of Cs and Li concentrations in the leaching solution (the chemical steady-state) was reached after 3.5 and 2 PV, in the respective columns packed with the Cecil subsoil and leached with the 30 mM L^-1 CsCl or LiNO₃ solutions (Fig. 5) . Subsequently, a good portion of the Cs and almost all of the Li adsorbed in the subsoil were displaced and leached by dH₂O (Table 4). If one compares the cation exchange capacity in the subsoil (2.61 cmol_c kg^-1) measured by the ion adsorption method at IS = 0.01 (Zelazny et al., 1996), with the counter-ion positive charge after leaching the Li-saturated soil with dH₂O, the difference is 2 cmol_c kg^-1. Even though leaching with dH₂O continued in both columns until the IS in the leachate reached the value of the IS in subsoil under natural conditions, the counter-ion positive charge was much smaller in the case when the LiNO₃ solution was used to saturate the subsoil. The counter-ion positive charge in this case was even smaller than that found in the Cecil subsoil under natural conditions, and three different values of the positive counter-ion charge were observed when the same IS was maintained in the subsoil solution.

View larger version (19K): [in this window] [in a new window]   Fig. 5. Cs and Li breakthrough curves in the Cecil subsoil leached with 30 mmol-1 CsCl and LiNO3 solutions followed by distilled water.

	DISCUSSION

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View larger version (60K): [in this window] [in a new window]   Fig. 6. The mechanism of indifferent ion adsorption and desorption as a function of ionic strength in the soil solution of variable charge subsoils.

The inter-penetration or overlapping of the double layers around the positively charged Fe and Al oxides and negatively charged silicate minerals is not a new concept in the literature. It has been successfully used to interpret experimental data from studies conducted during the last 37 yr with variable charge soils and subsoils (Sumner, 1963a,b; Sumner and Davidtz, 1965; Singh and Kanehiro, 1969; Reeve and Sumner, 1971; Barber and Rowell, 1972; Ji, 1997; Ji and Li, 1997; Qafoku et al., 2000a; Zhang and Zhao, 1997). However, no one has related the overlapping of diffuse layers on oppositely charged soil particles with the salt adsorption phenomenon observed frequently in variable charge soils and subsoils. Since the magnitude of salt adsorption was greater in acid subsoils with an extremely low IS in the soil solution (Qafoku et al., 2000b), it is quite possible that overlapping and salt adsorption are indeed related to one another, and salt adsorption of indifferent electrolytes is caused by the simultaneous adsorption of their ions in oppositely charged diffuse layers, when they compress in response to the increase in IS in the soil solution.

Much evidence from the literature supports that idea. Thomas (1960) reported that the concentration of salts in the soil solution remained rather low even when salt was added because the soil acted both as a cation-exchanger and as a salt-sorber. Sumner (1963a) observed that even two alcohol washings (10 mL) were sufficient to cause a severe drop in the measured positive and negative charges in equivalent amounts. Allophanic soils from Japan adsorbed significant quantities of Na⁺ and Cl^-, but neither aluminum hydroxide nor silica alone exhibited any salt sorption behavior (Wada, 1984). This strongly suggests that salt adsorption occurs only when two oppositely charged solid phases are present. Wada (1984) and later Pearse (1994) reported that salt adsorption was more frequently observed in soils with a low or very low soil solution concentration and the magnitude depends strongly on the initial IS in the soil or subsoil solution. The depletion of the added salt from the soil solution was observed in many subsoils collected from Georgia and other tropical and subtropical regions (Qafoku and Sumner, 2001). The magnitude of salt adsorption was larger in subsoils where both kaolinite and Al and Fe oxides were present in appreciable amounts (Qafoku and Sumner, 2001), which indicates that this phenomenon occurs when two types of oppositely charged colloids are present. An L-type NO₃ isotherm was observed in the Cecil subsoil at low IS, which suggests that more than one mechanisms may be responsible for NO₃ sorption at low concentrations in variable charge subsoils (Qafoku et al., 2000a).

	CONCLUSIONS

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The results from this study showed that the simultaneous adsorption in approximately equivalent amounts of cation and anion of the electrolyte with no net release of other cations or anions into the soil solution occurred when a variable charge subsoil was leached with a very dilute solution. Because the concentrations of Cs and Cl in the leaching solution were very low during salt adsorption, and the adsorption of ions in the inner Stern layer was not promoted under this experiment's conditions, both Cs and Cl were most probably simultaneously adsorbed in the outer-spheres or diffuse layers on oppositely charged soil particles. The fact that three different values of the counter-ion charge were observed for the same value of the soil solution IS suggested that the surface charge was not balanced only by the counter-ions adsorbed as outer-sphere complexes and in the respective diffuse layers. Evidence from column and batch experiments was also presented to prove the reversibility of salt adsorption. It is quite likely that in the acid variable charge subsoil from the humid regions of the southeastern USA, the surface charge on negatively and positively charged soil surfaces and particles is not equal to the counter-ion charge, that is, the sum of the charges of individual ions adsorbed as outer-sphere complexes or in the diffuse layers, under conditions of very low BECs in the soil solution. In order to describe the experimental observations presented in this paper and to explain how the particle charges on colloid surfaces would be balanced under these conditions, a mechanism that considers particle-particle interactions and their effects on the magnitude of the counter-ion charge was proposed.

Received for publication June 20, 2001.

	REFERENCES

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• Alva, A.K., M.E. Sumner, and W.P. Miller. 1991. Salt adsorption in gypsum amended acid soils. p. 93–97. In R.J. Wright et al. (ed.) Plant-soil interaction at low pH. Kluwer Academic Press, Dordrecht, The Netherlands.
• Barber, R.G., and D.L. Rowell. 1972. Charge distribution and the cation exchange capacity of an iron-rich kaolinitic soil. J. Soil Sci. 23:135–146.
• Bellini, G., M.E. Sumner, D.E. Radcliffe, and N.P. Qafoku. 1996. Anion transport through columns of highly weathered acid soils: adsorption and retardation. Soil Sci. Soc. Am. J. 60:132–137.[Abstract/Free Full Text]
• Bolan, N.S., J.K. Syers, and M.E. Sumner. 1993. Calcium-induced sulfate adsorption by soils. Soil Sci. Soc. Am. J. 57:691–696.[Abstract/Free Full Text]
• Gillman, G.P., and L.C. Bell. 1978. Soil solution studies on weathered soils from tropical North Queensland. Aust. J. Soil Res. 16:67–77.
• Hunter, R.J. 1993. Introduction to modern colloid science. Oxford Science Publ., New York.
• Jackson, M.L., and G.D. Sherman. 1953. Chemical weathering of minerals in soils. Adv. Agron. 5:219–318.
• Jackson, M.L. 1965. Clay transformation in soil genesis during the quaternary. Soil Sci. 99:15–22.
• Ji, G.L. 1997. Electrostatic adsorption of anions. p. 112–139. In T.R. Yu (ed.) Chemistry of variable charge soils. Oxford Univ. Press, New York.
• Ji, G.L., and H.Y. Li. 1997. Electrostatic adsorption of cations. p. 64–111. In T.R. Yu (ed.) Chemistry of variable charge soils. Oxford Univ. Press, New York.
• Marcano-Martinez, E., and M.B. McBride. 1989. Comparison of the titration and ion adsorption methods for surface charge measurement in Oxisols. Soil Sci. Soc. Am. J. 53:1040–1045.[Abstract/Free Full Text]
• Pearse, R.C. 1994. Investigation of salt adsorption behavior by acid subsoils and synthetic goethite. Ph.D. diss. Univ. of Georgia, Athens, GA.
• Pearse, R.C., and M.E. Sumner. 1997. Apparent salt sorption reactions in an unfertilized acid subsoil. Soil Sci. Soc. Am. J. 61:765–772.[Abstract/Free Full Text]
• Qafoku, N.P. 1998. The effects of mineralogy and chemistry on ionic retention, displacement, and transport in variable charge subsoils. Ph.D. diss. Univ. of Georgia, Athens, GA.
• Qafoku, N.P., M.E. Sumner, and D.E. Radcliffe. 2000a. Anion transport in columns of variable charge subsoils: Nitrate and Chloride. J. Environ. Qual. 29:484–493.[Abstract/Free Full Text]
• Qafoku, N.P., M.E. Sumner, and L.T. West. 2000b. The mineralogy and chemistry of some variable charge subsoils. Commun. Soil Sci. Plant Anal. 31:1051–1070.
• Qafoku, N.P., and M.E. Sumner. 2001. Retention and transport of calcium nitrate in variable charge subsoils. Soil Sci. 166:297–307.
• Reeve, N.C., and M.E. Sumner. 1971. Cation exchange capacity and exchangeable aluminum in Natal Oxisols. Soil Sci. Soc. Am. J. 35:38–42.
• Schindler, P.W., P. Liechti, and J.C. Westall. 1987. Adsorption of copper, cadmium, and lead from aqueous solution to the kaolinite/water interface. Neth. J. Agric. Sci. 35:219–230.
• Schulthess, S.P., and D.L. Sparks. 1988. A critical assessment of surface adsorption models. Soil Sci. Soc. Am. J. 52:92–97.[Abstract/Free Full Text]
• Seaman, J.C., P.M. Bertsch, and W.P. Miller. 1995. Chemical controls on colloid generation and transport in a sandy aquifer. Environ. Sci. Technol. 29:1808–1815.
• Singh, B.P., and Y. Kanehiro. 1969. Adsorption of nitrate in amorphous and kaolinitic Hawaiian soils. Soil. Sci. Soc. Am. Proc. 33:681–683.
• Soil Survey Staff. 1975. Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agic. Handb. 436. U.S. Gov. Print. Office, Washington, DC.
• Sposito, G. 1989. The chemistry of soils. Oxford Univ. Press, New York.
• Sumner, M.E. 1963a. Effect of alcohol washing and pH value of leaching solution on positive and negative charges in ferruginous soils. Nature (London) 198:1018–1019.
• Sumner, M.E. 1963b. Effect of iron oxides on positive and negative charges in clays and soils. Clay Miner. Bull. 5:218–226.
• Sumner, M.E., and J.C. Davidtz. 1965. Positive and negative charges in some Natal soils. S. Afr. J. Agric. Sci. 8:1045–1050.
• Thomas, G.W. 1960. Effects of electrolyte imbibition upon cation exchange behavior of soils. Soil Sci. Soc. Am. J. 24:329–332.[Abstract/Free Full Text]
• van Olphen, H. 1977. An introduction to clay colloid chemistry. John Wiley & Sons, New York.
• Wada, Shin-ichiro. 1984. Mechanism of apparent salt absorption in ando soils. Soil Sci. Plant Nutr. 30:77–83.
• Zachara, J.M., and J.P. McKinley. 1993. Influence of hydrolysis on the sorption of metal cations by smectites: Importance of edge coordination reactions. Aquat. Sci. 55(4):250–261.
• Zelazny, L.W., L. He, and A.M. Vanwormhoudt. 1996. Charge analyses of soils and anion exchange. p. 1231–1254. In D.L. Sparks et al. (ed.) Methods of soil analysis. Part 3. SSSA Book Ser. 5. SSSA, Madison, WI.
• Zhang, X.N., and A.Z. Zhao. 1997. Surface charge. p. 17–63. In T.R. Yu (ed.) Chemistry of variable charge soils. Oxford Univ. Press, New York.

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