The objective was to establish the mechanism by which modified clinoptilolite (in Na form) adsorbed Pb ions and to assess the extent of the influence of pH upon the adsorption capacity. The experimental data were fitted by adsorption isotherms, thermodynamic and kinetic models. Based upon the standard errors during experiments, it was deduced that the accuracy of prediction of the isotherm models considered for adsorption decreased in the order: Dubinin-Raduschkevich > Langmuir > Freundlich. For kinetic models, the accuracy of prediction decreased in the order: intra-particle > Weber-Morris > heterogeneous diffusion > pseudo-second order kinetic model > diffusion through the particle surface > pseudo-first kinetic model. The mechanism of adsorption of lead ions by Na-clinoptilolite occurred in a monolayer heterogeneous surface. The pH of contact solutions played an important role owing to competition by hydrogen ions. As the pH of the solution decreased, the maximum monolayer adsorption capacity established theoretically, based upon the Langmuir isotherm, also decreased. If the pH decreased from 4 to 1, the maximum adsorption capacity decreased from 0.3569 to 0.1604mol/kg. At high pH values of the contact solution, adsorption occurred by ion exchange and, at low pH it was physical. The variation of the Gibbs free energy demonstrated that adsorption occurred spontaneously. The process occurred at a higher rate at low acidity. Diffusion through the internal structure of macro- and micropores was the slowest stage during the adsorption process and played an important role in the mechanism of adsorption. The intra-particle diffusion coefficient depended upon pH, which could modify the shape and concentration of the hydrated metal complexes in solutions; thus affecting the adsorption process. A decrease of pH from 4 to 1 resulted in a decrease of the intra-particle diffusion coefficient from 4.06·x 10-11 to 1.96·x 10-11m2/min. The film diffusion coefficients were 10 times larger than the intra-particle coefficients; suggesting that diffusion to the external surface could not be the rate-limiting step in the adsorption mechanism.

The Influence of pH on the Adsorption of Lead by Na-Clinoptilolite: Kinetic and Equilibrium Studies. C.L.Mihaly, C.A.Mihaly, A.Peter, C.Nicula, N.E.Bakatula, H.Tutu: Water SA, 2012, 38[2], 269-78

 

Table 16

Self-Diffusion Parameters for Hector Clinoptilolites

Ion

Temperature(K)

Do(m2/s)

E(kJ/mol)

Na

253-283

5 x 10-8

13.0

Rb

243-273

1 x 10-7

14.7

Cs

253-273

2 x 10-7

27.6

Ca

303-345

3 x 10-4

85.2

Sr

303-345

6 x 10-4

79.2

Ba

293-333

4 x 10-5

63.2

Table 17

Interdiffusion in Clinoptilolite at 298K

 

Ion Exchange

D(m2/s)

NH4↔Na

2.71 x 10-11

NH4↔K

2.58 x 10-11

NH4↔Ca

1.31 x 10-12

NH4↔Mg

2.60 x 10-13

Na↔NH4

1.53 x 10-11

Na↔K

2.07 x 10-11

Na↔Ca

6.10 x 10-13

K↔Na

1.24 x 10-12

K↔NH4

1.60 x 10-12

K↔Ca

1.91 x 10-12

Mg↔Na

9.80 x 10-13

Mg↔K

5.70 x 10-13

Na↔NH4

1.06 x 10-12

Mg↔Ca

1.00 x 10-12