Fig. 5 shows that lignosulfonate displayed the lowest affinity
toward kaolinite in deionised water. While there was a slight
increase in adsorption density when lignosulfonate concentration
was increased to 50 mg/L, the adsorption density levelled off at
about 0.3 mg/g when lignosulfonate concentration was further
increased to about 400 mg/L. In contrast, there is a significant
increase in adsorption density of lignosulfonate on kaolinite surface in saline water. A steep increase in adsorption density can
be found when lignosulfonate concentration was increased to
100 mg/L, followed by a slower growth rate. The maximum
amount of the adsorbed lignosulfonate reached 2.8 mg/g at
500 mg/L lignosulfonate in saline water. As a result, the adsorption
density of lignosulfonate on kaolinite increases with increasing the
ionic strength, suggesting that electrostatic forces largely control
the adsorption process [29–32]. The low adsorption affinity in
deionised water may arise from the electrostatic repulsion
between kaolinite and anionic lignosulfonate. In the presence of
salts, however, charges are screened, and hence electrostatic repulsion between kaolinite and lignosulfonate is reduced. At the same
time, the attractive forces, such as van der Waals force, hydrogen
bonding attraction and metal cation- p interactions start to play a
dominant role and consequently result in an increase of adsorption
density on the kaolinite surface [33]. The growth of the adsorption
density in saline water may contribute to the reduced ash recovery
in problematic coal flotation in process water