Adsorption of sodium lignosulfonates on hematite

The adsorption of three sodium lignosulfonates on hematite was studied as a function of pH in the range from 4 to 10.5, and in the presence or absence of calcium ions (10^?3 mol/L calcium choride). Intrinsic viscosity measurements demonstrated that the effective size of the macromolecules was not significantly affected under the experimental conditions. The adsorption results showed that electrostatic forces between the anionic polyelectrolytes and the hematite surface controlled the adsorption process although substantial non-electrostatic interactions were also clearly observed. The adsorption density inversely correlated with the anionicity of the tested lignosulfonates, especially with the content of sulfonic groups, with the least anionic polyelectrolyte producing the highest adsorption level. It was suggested that lateral repulsive electrostatic forces between the adsorbed macromolecules prevented dense adsorption of more anionic lignosulfonates. From this point of view, the effect of calcium on adsorption was attributed not only to its specific adsorption and surface charge reversal, but also to screening the anionic groups of lignosulfonates and minimizing the lateral repulsive interactions especially at high pH.