Effect of monovalent and divalent cations, anions and fulvic acid on aggregation of citrate-coated silver nanoparticles.

The dynamic nature of nanoparticle (NP) aggregation behavior is of paramount interest to many current studies in environmental and toxicological nanoscience. The present study seeks to elucidate the influence that different electrolytes have on the aggregation of citrate-coated silver NPs (cit-AgNPs). The use of both UV-vis spectroscopy and dynamic light scattering (DLS, both z-average hydrodynamic diameter (z-dh) and size distribution analysis data) allowed improvement in the data quality and interpretation as compared to other studies using only DLS and reporting solely the z-dh, as the change in the z-dh can be related to analytical errors and uncertainties rather than only aggregation or dissolution of NPs. Divalent cations (CaCl2, Ca(NO3)2, CaSO4, MgCl2 and MgSO4) have stronger influence (ca. 50-65 fold) on aggregation of cit-AgNPs as compared to monovalent cations (NaCl, NaNO3, Na2SO4), as expected. For electrolytes with monovalent cations, there was no specific ion effect of nitrate and sulfate anions. However, the addition of chloride anions resulted in enhanced apparent aggregation, possibly due to the formation of AgCl NPs that sorb/attach to the surface of cit-AgNPs. Suwannee River fulvic acid enhances the stability of cit-AgNPs and shifts the critical coagulation concentrations to higher electrolyte concentrations for all types of electrolytes. Aggregation kinetics in the presence of mixture of monovalent and divalent cations is additive and controlled by the dominant cations. An empirical formula (αmixture=αNa+(50 to 65)Ca) is proposed that reproduces the effect of mixtures of electrolytes in the presence of humic substances and cations that can be used to help predict the aggregation behavior of cit-AgNPs in environmental and ecotoxicological media.