Phosphate-induced aggregation kinetics of hematite and goethite nanoparticles

Abstract

The purpose of the present study is to examine the effect of phosphate on the aggregation kinetics of hematite and goethite nanoparticles. The dynamic light scattering method was used to study the aggregation kinetics of hematite and goethite nanoparticles. Specific adsorption of phosphates could promote aggregation through charge neutralization at low P concentrations, stabilize the nanoparticle suspensions at medium P concentrations, and induce aggregation through charge screening by accompanying cations at high P concentrations. Two critical coagulation concentration (CCC) values were obtained in each system. In NaH2PO4, the goethite CCC at low phosphate concentrations was smaller than hematite and vice versa at high phosphate concentrations. Stronger phosphate adsorption by goethite rapidly changed the zeta potential from positive to negative at low phosphate concentrations, and the zeta potential became more negative at high phosphate concentrations. The clusters of hematite nanoparticles induced by phosphate adsorption had an open and looser structure. Solution pH and the phosphate adsorption mechanisms in NaH2PO4, KH2PO4, and Na3PO4 solutions affected zeta potential values and controlled the stability of hematite suspensions during aggregation. High pH and preference for non-protonated inner-sphere complexes in Na3PO4 solution decreased the zeta potential of positively charged hematite and promoted aggregation. Activation energies followed the order NaH2PO4 > KH2PO4 > Na3PO4 at low P concentrations. K+ was more effective than Na+ in promoting hematite aggregation due to the non-classical polarization of cations. Phosphate can enhance or inhibit the aggregation of hematite and goethite nanoparticles in suspensions by changing surface charge due to specific adsorption onto the particles. The phosphate-induced aggregation of the nanoparticles mainly depended on the initial concentration of phosphate.