Specific Anion Effects for Aggregation of Colloidal Minerals: A Joint Experimental and Theoretical Study

In this work, dynamic light scattering experiments and density functional calculations were combined to demonstrate the specific anion effects for the aggregation of negatively charged colloidal minerals. Although the aggregation kinetics is dominated by electrolyte cations, anions also play a significant role. The critical coagulation concentrations and activation energies indicated a clear Hofmeister series for the various anions as H2PO4- < Cl- < NO3- < SO42- < HPO42- < PO43-. Moreover, interaction energies of anions with cations and proton affinities of anions were explored as the influencing factors for anion specificities, which were corroborated by measurement of surface charge densities. Owing to the largest interactions with cations, PO43- causes the most pronounced inhibition effect to the aggregation kinetics and corresponds to the strongest anion specificity. Proton exchange from H2PO4- reduces the negative charges of minerals and accelerates the aggregation process, thus resulting in an inferior anion specificity than NO3-. Density functional calculations indicated that proton transfer from minerals to OH- can occur facilely and increase the negative charges of minerals, as confirmed by charge density measurements and dilution experiments. This further adds to the aggregation difficulty and causes OH- to show distinctly stronger anion specificity than other univalent anions.