Evaluation of Data

For evaluation of equilibrium parameters the generalized separation factors have to be determined from equilibrium concentrations and resin loadings. These are then plotted against the respective dimensionless resin loading or the (algebraic) sum of resin loadings. In the case of binary systems with pure cation or anion exchange, the respective graphical representations directly yield the equilibrium parameters. The intersection of the linear relationship leads to log K and the slope of the straight line yields m(i, j). Figures 2-5 show examples for different types of exchangers.

The results show that linear relationships are obtained in most cases. Systematic deviations are found for small loadings. This is attributed to the neglect of counterions in the diffuse layer. As has been demonstrated by Horst for simple systems, when this layer is considered, excellent agreement is obtained. For strongly acidic exchangers this region is compar-

jtlk^ ■

10 mmol L"1

NaCI

^jflp A

25 mmol L"1

NaCI

50 mmol L"1

NaCI

0

100 mmol L"1

Jstf^ ' ■

10 mmol L"1

NaCI

A

25 mmol L"1

NaCI

f

50 mmol L"1

NaCI

O

100 mmol L"1

Figure 4 Development of the generalized separation factor for the exchange of calcium for sodium ions on two different strongly acidic resins. (A) Dowex HCRS; (B) LEWATIT S100.

atively larger than for other resin types. Evaluation of data for the exchange of sulfate for chloride on strongly basic exchangers shows that there are two different straight lines. There are clearly two different layers that are subsequently 'filled'. Evaluation of data for a broad range of total concentrations up to 100 mmol in the liquid phase reveals identical development of the linear relationships. For the entire range of ionic strength investigated the parameters can, therefore, be considered as 'constant' equilibrium constants.

Evaluation of amphoteric data requires a more sophisticated approach. The first step is the assumption of a certain sequence of layers. Evaluation of data starts with the evaluation of the equilibrium for the uptake of protons and ions in the first layer. The respective parameters are required for evaluation of the sorption of species in the next layer. Determination of the parameters for the third layer requires the sets of parameters for both the inner layers. This process has to be continued until all sets of binary parameters are derived. The sequence of a layers is assumed correctly if both the log K and m(H, i) values show a steady increase with increasing distance. If this conditions is not fulfilled the calculation must be repeated with a modified sequence. Considering a system of layers H +, CP, NO3, Na + on activated alumina as an example, the equilibrium

■ 5 mmol L"1 NaN03 • 50 mmol L"1 NaNO,

Z 10

■ 5 mmol L"1 NaHC03 • 50 mmol L"1 NaHCO,

sof + NO3"

a 5 mmol L_1

NaN03

A 10 mmol L"1

NaN03

▼ 20 mmol L"1

NaN03

AJr

/AA AA

JJV N03" + so42"

X7 ■

2.5 mmol L

Na2S04

r O

5 mmol L-

Na2S04

C

10 mmol L~

Na2S04

a

25 mmol L~1

Na2S04

Figure 5 Development of the generalized spearation factor for the strongly basic ion exchange resin AMBERLITE IRA 410 (Reprinted from Holl, Horst, Franzreb and Eberle (1993) by courtesy of Marcel Dekker Inc.).

parameters can be evaluated from the following relationships:

log Qno3 — m(H, Cl) • { — y(Cl)} = log KH03 + m(H, NO3) • {y(Na) — y(N03)} [38] log QHa — m(H, Cl) • { — y(Cl)} — m(H, NO3) • { — y(N03)} = log KHa # m(H, Na) • y(Na) [39]

Figure 6 Development of the generalized separation factor for the uptake of nitrate by activated alumina (COMPALOX AN/V800) from a multicomponent system (cation = Na # or K#). The symbols represent different experiments.
Figure 7 Development of the generalized separation factor for the sorption of sodium on COMPALOX AN/V800.

-0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 y(C a) + y(Mg) + y( Na /K)-/(C03)-k(CI/N03/S04)

Figure 8 Development of the generalized separation factor for the sorption of carbonate species by activated carbon (NORIT ROW 0.8 Supra) from a multicomponent system.

Results of the evaluation of equilibrium data for the sorption of CP, NO3" and Na + on activated alumina (COMPALOX AN/V800) are plotted in Figures 6 and 7. Although the results for the uptake of sodium ions are not satisfactory, the data demonstrate that linear relationships are obtained.

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