Results and Discussion

The characteristics of adsorbents surface detected are given in the Table 1.

TABLE 1. Characteristics of adsorbents surfaces.

Sorbent

Specific surface area (m2/g)

Zeta potential (mV)

Cellulose

2.0

-25

Kaolin

15.1

-30

Calcium carbonate

7.1

-16

Titanium dioxide

9.2

+67

The solids used in the present work as adsorbents, excluding TiO2, have the surface charge of the same sign as LS polyanions. Therefore, it could be waited that electrostatic interactions between the negative charges of the sulphonic, carboxylic and phenolic groups of the macromolecule and the positively charges at the surface of the adsorbent will be the driving force of adsorption for TiO2 only. In this case, the regularities of LS adsorption can be described in accordance with Fleer and Scheutjens theory15 as polyelec-trolyte adsorption on a charged surface. The maximum on the experimental curve of the relationship "LS adsorption on TiO2 plateau value - solution pH" was observed at pH 4-5, that corresponded to the effective pK for lignosulphonic acids. The shape of the experimental relationship corresponded to the theoretical one calculated according the self consistent field (SCF) model15 for polyelectrolyte adsorption on an oppositely charged surface. The amount of LS adsorbed is larger at the low values of pH than at high pH due to a decrease of intersegmental repulsion. The presence of a maximum can be explained by the fact, that LS macroions with non-zero degree of dissociation, thus having some charge, more effectively compete with counterions to compensate the positive charge on the TiO2 surface. At these values of pH electrostatic forces between adsorbate and adsorbent are higher than intersegmental repulsion.

The adsorption isotherms obtained for LS and Si-LS adsorption on all solids investigated are shown in the Figure 2a and b, respectively. It has been shown7 by the authors of the present paper that the adsorption isotherms of LS on the interfaces are described satisfactory by the Aranovich model of polylayer adsorption16:

where A - current adsorption value; Am - adsorption plateau value; Nœ -current equilibrium adsorptive concentration in the solution; Nœ* = N for the saturated solution; C - energetic constant of adsorption equilibrium.

The good fit of the Aranovich model of polylayer adsorption not only for LS but also for Si-LS experimental isotherms was observed (Figure 3).

On the basis of the experimental adsorption isotherms the value of differential adsorption heat at infinitesimal coverage, AqA, can be calculated by application of the Aranovich adsorption model:

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