Specific ion effects of divalent cations on the aggregation of positively charged goethite nanoparticles in aqueous suspension

Nanoparticles, especially natural nanoparticles, are closely related to many environmental processes. In this study, the aggregation kinetics of goethite nanoparticles (GNPs) in the presence of Mg(NO3)(2),Ca(NO3)(2) and Cu(NO3)(2) were investigated by dynamic light scattering. The specific cation effects of Cu2+, Ca2+ and Mg2+ on the aggregation of the positively charged GNPs were observed. The results showed that: (1) the mean aggregation rate followed the order of Cu2+ < Ca2+ < Mg2+, e.g. under an electrolyte concentration of 50 mM, the mean aggregation rate in the presence of Cu2+, Ca2+ and Mg2+ were 0, 38.3 and 49.5 nm min(-1), respectively; (2) the critical coagulation concentration (CCC) for the presence of Cu2+, Ca2+ and Mg2+ of GNPs were respectively 117, 65.3 and 37.9 mM, which was consistent with the orders of aggregation rate; and (3) correspondingly, the activation energy was the highest for the presence of Cu2+, followed by Ca2+ and then by Mg2+, e.g. under an electrolyte concentration of 5 mM, activation energy for the presence of Cu2+, Ca2+ and Mg2+ approached infinity, 1.1 RT and 0.88 RT, respectively. We conclude that, the difference in surface reactions of the three cations with the surface functional group of - OH resulted in the difference in particle aggregation: Cu2+ produced the strongest covalent bonding, and followed by Ca2+ and Mg2+; the covalent bonding in forming - OCu+ could be explained by the classic theory, but the covalent bonding in forming -OCa' or -OMg' might be a polarization induced one.