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VI. Application of Models to Organic Ligand Adsorption Reactions on oxides

VI. Application of Models to Organic Ligand Adsorption Reactions on oxides

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SURFACE COMPLEXATION MODELS



3 09



Figure 35. Fit of the constant capacitance model to phthalate adsorption on aluminum

oxide. Model results are represented by solid lines. Model parameters are provided in

Table XVII.After Kummert and Stumm (1980).



concentrations. Table XVII provides values for intrinsic organic ligand

surface complexation constants obtained with the constant capacitance

model for oxide minerals.



B. TRIPLE-LAYER

MODEL

The triple-layer model has been used to describe organic ligand adsorption envelopes on the iron oxide, goethite (Balistrieri and Murray, 1987)

and on amorphous iron oxide (Davis and Leckie, 1979). In the application

of the triple-layer model to organic anion adsorption on goethite, the reactions Eqs. (24) and (25) and the equilibrium constants Eqs. (30) and (31)

are considered. For adsorption of glutamate on amorphous iron oxide,

reaction Eq. (24) is replaced by the formation of a neutral surface complex, SOH-H2L (Davis and Leckie, 1979). Figure 36 presents the ability of the triple-layer model to describe glutamate adsorption on amorphous iron oxide. The model describes the data well for three different

total organic ligand concentrations. Unfortunately, values for the intrinsic

surface complexation constants were not provided by the authors. Table

XVIII provides values for intrinsic organic ligand surface complexation

constants obtained with the triple-layer model for iron oxides.



Table X V n



Values of Intrinsic Organic Ligand Surface Complexation Constants Obtained with the Constant Capacitance Model



Solid



Amino acids

Si02(am)

Si02(am)

Si02(am)

Si02(am)

TiOz, rutile

TiOz, rutile



Ligand



Ionic medium



Benzoate

Catechol

Phthalate

Salicylate

Acetate



0.1 M NaC10,

0.1 M NaClO,

0.1 M NaClO,

0.1 M NaClO,

0.1 M NaClO,



Glycine

a-Alanine

P- Alanine

y-Aminobutyric acid

Glycine

Glycine



1.0 M NaClO,

1.O M NaClO,

1.0 M NaClO,

1.0 M NaCIO,

1.0 M NaC10,

1.0 M NaClO,



log K:(int)

3.1



3.7

1.3



6.0

2.9



3.0

3.3



log Kt(int)



log K:(int)



<-5

2.4

-0.6

-



4.9



5.5



Reference

Kummert and Stumm (1980)

Kummert and Stumm (1980)

Kummert and Stumm (1980)

Kummert and Stumm (1980)

Sigg and Stumm (1981)



0.04

-0.14

-0.26

-0.68

2.9

3.3



Gisler (1980)

Gisler (1980)

Gisler (1980)

Gisler (1980)

Gisler (1980)

Gisler (1980)



311



SURFACE COMPLEXATION MODELS



PH

Figure 36. Fit of the triple-layer model to glutamate adsorption on amorphous iron

oxide. Model results are represented by solid lines. Model parameters were not provided by

the authors. From Davis and Leckie (1979), reproduced with permission from the American

Chemical Society.

Table XVIII

Values of Intrinsic Organic Ligand Surface Complexation Constants Obtained

with the Triple-LayerModel



Solid



Ligand



Ionic medium



log KL(int)



a-FeOOH

a-FeOOH

a-FeOOH

u-FeOOH



Oxalate

Phthalate

Salicylate

Lactate



NaCl

NaCl

NaCl

NaCl



10.8

9.7

9.2



Y



0



b



I



200



1



log K:(int)



Reference



15.5

15.7

22.9



Balistrieri

Balistrieri

Balistrieri

Balistrieri



-



c



I



400



Citrate concentration (



600



and

and

and

and



Murray

Murray

Murray

Murray



(1987)

(1987)

(1987)

(1987)



a



800



p o l liter ')



Figure 37. Fit of the Stern VSC-VSP model to citrate adsorption on goethite. Model

results are represented by solid lines. Model parameters are provided in Table XV. From

Bowden et al. (1980).



3 12



SABINE GOLDBERG



C. STERN

VSC-VSP MODEL

Application of the Stern VSC-VSP model to organic anion adsorption

has been restricted to citrate adsorption on goethite (Bowden et al., 1980).

As for inorganic anion adsorption, values of surface site density, maximum

adsorption, binding constants, and capacitances are optimized to fit charge

and adsorption data. Table XV presents parameter values obtained by

computer optimization for citrate adsorption on goethite. The adsorption

of the trivalent ion L3- is postulated. The ability of the Stern VSC-VSP

model to describe citrate adsorption is good and is presented in Fig. 37.



VII. APPLICATION OF MODELS T O

COMPETITIVE ADSORPTION REACTIONS ON OXIDES

A. METAL-METAL

COMPETITION

1. Constant Capacitance Model



The effects of metal-metal competition on the description of adsorption

with the constant capacitance model have been investigated only preliminarily. The constant capacitance model containing the surface precipitation

model was used to describe the effect of copper on cadmium adsorption by

amorphous iron hydroxide (Farley et al., 1985). These authors overpredicted the competitive effect for the data of Benjamin (1978) obtained after

4 hr of reaction time. Using one competitive data point after 30 hr of

reaction time, the authors concluded that the surface precipitation model is

capable of predicting competitive adsorption of metal ions if slow kinetics

are considered. Additional research is needed to substantiate the conclusion of Farley et al. (1985).

2. Triple-Layer Model



The competitive adsorption of alkaline earth cations (Balistrieri and

Murray, 1981) and trace metal cations has been investigated on goethite

(Balistrieri and Murray, 1982b), manganese oxide (Catts and Langmuir,

1986), and amorphous iron oxide (Cowan etal., 1991) using the triple-layer

model. Using intrinsic surface complexation constants from single-cation

systems, Balistrieri and Murray (1981) were able to quantitatively predict

calcium and magnesium adsorption on goethite in a synthetic major-ion

seawater solution. Using this same approach, Balistrieri and Murray

(1982b) were able to predict decreases in lead, zinc, and cadmium adsorp-



SURFACE COMPLEXATION MODELS



313



tion on goethite in the presence of magnesium in major-ion seawater. The

competitive effect of magnesium was quantitatively described for zinc

adsorption over the entire pH range and for lead adsorption above pH 5 .

Deviations from the experimental data occurred for lead adsorption below

pH 5 and for cadmium adsorption over the entire pH range (Balistrieri and

Murray, 1982b). Competitive adsorption of the trace metal ions copper,

lead, and zinc on manganese oxide from a solution containing all three ions

was predicted from single-ion systems (Catts and Langmuir, 1986). The

adsorption of lead was predicted quantitatively, whereas the description of

copper and zinc adsorption was qualitatively correct (see Fig. 38).

In order to describe cadmium and calcium adsorption on amorphous

iron oxides in single-ion systems, as well as to predict competitive adsorption, Cowan et al. (1991) hypothesized inner-sphere surface complexes for

cadmium and a combination of inner- and outer-sphere surface complexes

for calcium. This study represents the first time that both inner- and

outer-sphere complexes have been postulated for a single adsorbing ion.

Cowan er al. (1991) were able to describe competitive adsorption of cadmium in the presence of calcium, qualitatively. However, because a better fit was obtained using a nonelectrostatic model with fewer adjustable

parameters, these authors suggested that competitive adsorption of cadmium and calcium on amorphous iron oxide is due to a mass-action effect.



Figure 38. Prediction of competitive trace metal adsorption from single-ion systems on

manganese oxide using the triple-layer model. Model results are represented by solid lines.

Model parameters are provided in Table X. From Catts and Langmuir (1986).



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