Constant Capacitance Model - ACS Publications - American Chemical

Chapter 14

Constant Capacitance Model Chemical Surface Complexation Model for Describing Adsorption of Toxic Trace Elements on Soil Minerals

Downloaded by UNIV QUEENSLAND on June 7, 2013 | http://pubs.acs.org Publication Date: February 22, 1993 | doi: 10.1021/bk-1993-0518.ch014

Sabine Goldberg U.S. Salinity Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Riverside, CA 92501

The constant capacitance model provides a molecular description of adsorption phenomena using an equilibrium approach. Unlike empirical models, the constant capacitance model defines surface species, chemical reactions, equilibrium constant expressions, surface activity coefficients and mass and charge balance. The model was able to describe cadmium, lead, copper, aluminum, selenite, arsenate, and boron adsorption on oxide minerals, cadmium, lead, copper, selenite, arsenate, and boron adsorption on clay minerals, and selenite, arsenate, and boron adsorption on soils. Additional research into model application to heterogeneous natural systems such as clay minerals and soils is needed. The constant capacitance model has been incorporated into transport models. Validation of these models is necessary.

Specific ion adsorption produces surface complexes which contain no water between the surface functional group and the adsorbing ion. Such surface complexes are quite stable and are referred to as inner-sphere (7). Aluminum and iron oxide surfaces and clay mineral edges often constitute the primary sinks for retention of specifically adsorbed ions in soil systems. The present study will describe the behavior of the following specifically adsorbing and toxic trace elements: cadmium, lead, copper, aluminum, selenium, arseniC., and boron. Trace element adsorption reactions on soil minerals have historically been described using the Freundlich and Langmuir adsorption isotherm equations. Although these equations are often very good at describing adsorption, they are strictly, empirical numerical relationships (2). Chemical meaning cannot be assigned to Langmuir and Freundlich isotherm parameters without independent experimental evidence for adsorption (1). Since the use of the Langmuir and Freundlich adsorption isotherm equations constitutes This chapter not subject to U.S. copyright Published 1993 American Chemical Society In Emerging Technologies in Hazardous Waste Management III; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Constant Capacitance Model

essentially a curve-fitting procedure, the equation parameters are only valid for the conditions under which the experiment was carried out. Chemical modeling of ion adsorption at the solid-solution interface has been successful using surface complexation models. These models use an equilibrium approach with mass-action and mass-balance equations to describe the formation of surface complexes. Unlike the empirical Langmuir and Freundlich equations, these models attempt to provide a general molecular description of adsorption reactions. Because surface complexation models take into account the charge on both the adsorbate ion and the adsorbent surface, they can have wide applicability and predictive power over changing conditions of solution pH, ionic strength, and ion concentration.


The Constant Capacitance Model The constant capacitance model is a chemical surface complexation model developed by the research groups of Stumm and Schindler (5-5). The constant capacitance model explicitly defines surface species, chemical reactions, equilibrium constant expressions, and surface activity coefficients. The surface functional group is defined as SOH, an average reactive surface hydroxyl ion bound to a metal ion, S (Al or Fe), in the oxide mineral or an aluminol group on the clay mineral edge. The adsorbed ions are considered to form inner-sphere surface complexes, located in a surface plane with the protons and hydroxyl ions. A diagram of the surface-solution interface in the constant capacitance model is provided in Figure 1. Assumptions. The constant capacitance model is characterized by four essential assumptions: (1) all surface complexes are inner-sphere complexes, (2) anion adsorption occurs via a ligand exchange mechanism, (3) the Constant Ionic Medium Reference State determines the aqueous species activity coefficients in the conditional equilibrium constants and therefore no surface complexes are formed with ions in the background electrolyte, (4) the relationship between surface charge, σ (mol L" ), and surface potential, φ (V), is linear and given by: 1

c

2

2

1

where C (F m" ) is the capacitance density, S (m g") is the specific surface area, a (g L" ) is the suspension density of the solid, and F (C mol ) is the Faraday constant. 1

_1

c

Reactions. In the constant capacitance model the protonation and dissociation reactions of the surface functional group are:

In Emerging Technologies in Hazardous Waste Management III; Tedder, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


280

EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT III

Figure 1. Placement of ions, potential, charge, and capacitance density for the constant capacitance model. (Adapted from ref. 6.).


14.

GOLDBERG


281 (2)

SOH + H* «SOH * 2

3

SOH-SO+H*

()

The surface complexation reactions for specific ion adsorption are defined as: m

(m l)

4

SOH + M * - SOM ' +r


2SOH + Ai

im

()

2)

« (SO) M -

(5)

2

= S f f ^ O - » + ff 0 + ( t - l ) J T 1

SOW +

2

(6)

2SOH + Hji= SflÂ }[H+Y 2

2

iM

( 3 6 )

l)+

[SOH] [ML ~ ] Table VII provides values for type A ternary surface complexation constants. The ability of the constant capacitance model to describe copper adsorption on silicon oxide by considering a ternary surface complex is indicated in Figure 12 for ethylenediamine. The model describes the adsorption data very well. Incorporation of the Constant Capacitance Model Into Computer Codes The constant capacitance model has been incorporated into various chemical speciation models. The computer program MINTEQ (48) combines the mathematical framework of MINEQL (49) with the thermodynamic data base of WATEQ3 (50) and contains constant capacitance modeling. The constant capacitance model is incorporated into the computer speciation program SOILCHEM (57). The computer program H Y D R A Q L (52) was developed




302

Figure 11. Fit of the constant capacitance model to competitive phosphate and arsenate adsorption on gibbsite (b). Prediction of competitive adsorption (a) and (c) using the surface complexation constants obtained from fit (b). Model results are represented by solid lines. L o g K ^ i n t ) = 6.59, logK (int) = 0.96, logK (int) = -5.66, logKînt) = 7.69, logK (int) = 1.92, logK (int) = -4.68. 2

3

As

2

Ae

3

P

P

(Reproduced with permission from ref. 28. Copyright 1986.)



2

Si0

2

Ti0 , rutile

2

Si0

Solid

Co

Cu

Cu

1 M NaC10

ethylenediamine glycine glycine

2+

2+

2+

-4.40

4

-5.64

1 M NaC10

(46) (16)

-10.94

(47)

Reference

-12.29

-12.57

ML

-5.22

2

logK

1

logK ,^

IMKNO3

4

Ionic Medium

Ligand

Metal

Table VII. Values of type A ternary surface complexation constants for oxide minerals



304


Figure 12. Fit of the constant capacitance model to copper adsorption on silicon oxide using a type A ternary copperethylenediamine surface complex. Model results are represented by solid lines. Model parameters are provided in Table VII. (Reproduced with permission from ref. 47. Copyright 1978.)

S c 8 C Ο Ο

Figure 13. TRANQL simulation of cadmium transport in a solution of 1 χ 10^ M Cd containing 3 χ 10 M C1 and 1 χ 10" M Br . A step input increases C1 to 3 χ ΙΟ" M and Br to 1 χ 10" M . LogK'înt) = -7.0, pH = 7.0. (Adapted from ref. 54.). 3

T

3

T

T

2

T

2

T


14. GOLDBERG


305


from the computer program MINEQL (49) and includes the constant capacitance model. Unlike the above mentioned computer programs, the programs MICROQL (44) and FITEQL (23) do not contain thermodynamic data files. Instead, the user enters the values of the equilibrium constants for the problem of interest. Transport Models. A few studies have incorporated the constant capacitance model into transport models (53, 54). Jennings et al. (53) linked the constant capacitance model into a transport model and simulated competitive sorption of two hypothetical ligands at constant pH. The computer program TRANQL (54) links the program MICROQL (44) with the transport program ISOQUAD (Pinder, 1976, unpublished manuscript). The TRANQL program was used to simulate cadmium transport in the presence of chloride and bromide in a one-dimensional laboratory column. Figure 13 presents TRANQL simulations for cadmium transport where total cadmium concentration is held constant and the total concentrations of chloride and bromide experience a step increase. Validation of these predictions has not yet been carried out. Acknowledgment Gratitude is expressed to Dr. Samuel J. Traîna, Department of Agronomy, Ohio State University for presenting this paper at the Emerging Technologies in Hazardous Waste Management III Symposium in Atlanta, GA, October 1991. Literature Cited

(1) Sposito, G. The surface chemistry of soils. Oxford Univ. Press, Oxford, UK, 1984. (2) Harter, R.D., Smith, G. In Chemistry in the soil environment; Dowdy, R.H et al., Ed.; Soil Science Society of America. Madison, WL, 1981, pp. 167-181. (3) Schindler, P.W., Gamsjäger, H. Kolloid-Z. Z. Polymere, 1972, 250, 759-763. (4) Stumm, W., Kummert, R., Sigg, L. Croatica Chem. Acta, 1980, 53, 291-312. (5) Schindler, P.W. In Adsorption of inorganics and organic ligands at so liquid interfaces; Anderson, M.A., Rubin, A.J., Ed.; Ann Arbor Science. Ann Arbor, MI., 1981, pp. 1-49. (6) Westall, J.C.Am. Chem. Soc. Symp. Ser. 1986, 323, 54-78. (7) Sposito, G. J. Colloid Interface Sci., 1983, 91, 329-340. (8) Westall, J.Am. Chem. Soc. Adv. Chem. Ser., 1980,189,33-44. (9) Hohl, H., Stumm, W.J.Colloid Interface Sci., 1976, 55, 281-288. (10) Kummert, R., Stumm, W.J.Colloid Interface Sci., 1980, 75, 373-385. (11) Pulfer, K., Schindler, P.W., Westall, J.C., Grauer, R.J. Colloid Interface Sci., 1984, 101, 554-564. f


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(12) Bleam, W.F., Pfeffer, P.E., Goldberg, S., Taylor, R.W., Dudley, R. Langmuir, 1991, 7, 1702-1712. (13) Sigg, L. UntersuchungenüberProtolyse und Komplexbildung zweiwertigen Kationen von Sïlikageloberflächen, M.Sc. Thesis, University Bern, Bern, Switzerland, 1973. (14) Sigg, L.M. Die Wechselwirkung von Anionen und schwachen Säuren αFeOOH (Goethit) in wässriger Losung Ph.D. Thesis. Swiss Federal Institute of Technology, Zurich, Switzerland, 1979. (15) Fürst, B. Das Koordinationschemische Adsorptionsmodel: Oberflächenkomplexbildungvon Cu(II), Cd(II) undPb(II) anSi0 (Aero Ti0 (Rutil). Ph.D. Thesis. University of Bern, Bern, Switzerland, 1976. (16) Gisler, A. Die Adsorption von Aminosäuren an Grenzflàchen Oxid-Wa Ph.D. Thesis. University of Bern, Bern, Switzerland, 1980. (17) Regazzoni, A.E., Blesa, M.A., Maroto, A.J.G.J.Colloid Interface Sci., 1983, 91, 560-570. (18) Farley, K.J., Dzombak, D.A., Morel, F.M.M. J. Colloid Interface Sci., 1985, 106, 226-242. (19) Lövgren, L., Sjöberg, S., Schindler, P.W. Geochim. Cosmochim. Acta, 1990, 54, 1301-1306. (20) Schindler, P.W, Kamber, H.R. Helv. Chim. Acta, 1968, 51, 1781-1786. (21) Osaki, S, Miyoshi, T, Sugihara, S., Takashima, Y.Sci.Total Environ., 1990, 99, 105-114. (22) Schindler, P.W, Stumm, W. In Aquatic Surface Chemistry; Stumm, W, Ed.; Wiley-Interscience, New York, NY, 1987, pp. 83-110. (23) Westall, J.C. FITEQL: A computer program for determination of equilibrium constants from experimental data. Rep. 82-01. Department Chemistry, Oregon State University, Corvallis, OR, 1982. (24) Goldberg, S, Sposito, G. SoilSci.Soc. Am.J.,1984, 48, 772-778. (25) Westall, J, Hohl, H. Adv. Colloid Interface Sci., 1980, 12, 265-294. (26) Goldberg, S. In ENVIROSOFT 86 Zanetti, P. Ed.; CML Publications, Ashurst, Southampton, UK, 1986, pp. 671-688. (27) Goldberg, S. SoilSci.Soc. Am.J.,1985, 49, 851-856. (28) Goldberg, S. SoilSci.Soc. Am.J.,1986, 50, 1154-1157. (29) Goldberg, S.J.Colloid InterfaceSci.,1991,145,1-9. (30) Goldberg, S, Sposito, G. SoilSci.Soc. Am.J.,1984, 48, 779-783. (31) Benjamin, M.M, Leckie, J.O. In Contaminants and sediments; Baker, R.A, Ed.; Ann Arbor Science. Ann Arbor, MI, 1980, Vol. 2. pp. 305-322. (32) Benjamin, M.M, Leckie, J.O.J. Colloid InterfaceSci.,1981, 79, 209-221. (33) Schindler, P.W, Fürst, B, Dick, R, Wolf, P.U.J.Colloid Interface Sci., 1976, 55, 469-475. (34) Schindler, P.W, Liechti, P, Westall, J.C. Netherlands J. Agric.Sci.,1987, 35, 219-230. (35) Tamura, H , Matijevic., E, Meites, L.J.Colloid InterfaceSci.,1983, 92, 303-314. (36) Benjamin, M.M. Effects of competing metals and complexing ligand 2


2



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trace metal adsorption at the oxide/solution interface. Ph.D. Thesis, Stan University, Stanford, CA., 1978. (37) Goldberg, S., Glaubig, R.A. SoilSci.Soc. Am. J., 1985, 49, 1374-1379. (38) Goldberg, S., Glaubig, R.A. Soil Sci. Soc. Am. J., 1988, 52, 87-91. (39) Goldberg, S., Glaubig, R.A. SoilSci.Soc. Am. J., 1986, 50, 1442-1448. (40) Goldberg, S., Glaubig, R.A. Soil Sci. Soc. Am. J., 1988, 52, 954-958. (41) Goldberg, S., Glaubig, R.A. Soil Sci. Soc. Am. J., 1988, 52, 1297-1300. (42) Goldberg, S., Glaubig, R.A. Soil Sci. Soc. Am. J., 1986, 50, 1173-1176. (43) Sposito, G, deWit, J.C.M., Neal, R.H. Soil Sci. Soc. Am. J. 1988, 52, 947-950. (44) Westall, J.C. MICROQL. I. A chemical equilibrium program in BASIC II. Computation of adsorption equilibria in BASIC. Technical Report, Swi Federal Institute of Technology, EAWAG, Dubendorf, Switzerland, 1979. (45) Goldberg, S., Traina, S.J. SoilSci.Soc. Am. J., 1987, 51, 929-932. (46) Schindler, P.W. Rev. in Mineralogy, 1990, 23, 281-307. (47) Bourg, A.C.M., Schindler, P.W. Chimia, 1978 32, 166-168. (48) Felmy, A.R., Girvin, D.C., Jenne, E.A. MINTEQ: A computer program for calculating aqueous geochemical equilibria EPA-600/3-84-032. Office of Research and Development. U.S. EPA, Athens, GA., 1984. (49) Westall, J.C., Zachary, J.L., Morel, F.M.M. MINEQL: A computer program for the calculation of chemical equilibrium composition of aq systems. Technical Note 18, Ralph M. Parsons Laboratory, Department of Civil Engineering, Massachusetts Institute of Technology, Cambridge, MA, 1976. (50) Ball, J.W., Jenne, E.A., Cantrell, M.W. WATEQ3: A geochemical model with uranium added. USGS, Washington, D.C. Open File Report 81-1183, 1981. (51) Sposito, G., Coves, J. SOILCHEM: A computer program for the calculation of chemical speciation in soils. Kearney Foundation of Soil Science, University of California, Riverside, CA., 1988. (52) Papelis, C., Hayes, K.F., Leckie, J.O. HYDRAQL: A program for the computation of chemical equilibrium composition of aqueous batch s including surface-complexation modeling of ion adsorption at the oxide interface. Technical Report No. 306. Dept. of Civil Engineering, Stanford University, Stanford, CA., 1988. (53) Jennings, A.A., Kirkner, D.J., Theis, T.L. Water Resour. Res., 1982, 18, 1089-1096. (54) Cederberg, G.A., Street, R.L., Leckie, J.O. Water Resour. Res. 1985, 21, 1095-1104. RECEIVED

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Constant Capacitance Model - ACS Publications - American Chemical

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