The application of nanoparticles is increasing in commercial products due to their special properties in electric, thermal, optical, and magnetic aspects. As the commercial usage of nanoparticles is increasing, the fate and transformation of commercial nanoparticles in environmental is of significant interest to ecosystem and human health. This research investigated the stability and morphology change of three metal oxide nanoparticles in aqueous solutions. Three commercial nanoparticles, TiO2, ZnO, SiO2, were received in powder form, and in water they were aggregated quickly into micro-sized particles due to electric double layer compression. The morphology and characteristic changes were also examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV/Vis absorption spectroscopy (UV/Vis), and transmission electron microscopy (TEM).
Due to their fast aggregation behaviors, a series of separation methods are performed to disperse these three nanoparticle aggregates to their primary particle size. The direct ultrasonic probe method can well disperse these three nanoparticles. The stability of TiO2 and SiO2 can maintain one day; however, it is not for ZnO. This direct ultrasonic method is better than other physical dispersion method.
As compared to nanoscale TiO2 and SiO2 particles, the aggregation rate of ZnO nanoparticles is the fastest. It could be result form the aggregation enhancement of ZnO nanoparticles through the ion bridge by zinc ions and zinc chemicals derived from the dissolution of ZnO nanoparticles in water. The temperature in the range of 15~35 oC did not affect the stability of these nanoparticles a lot. With the close of pH to the pHzpc of nanoparticles, the obvious sedimentation behaviors were found for TiO2 and SiO2 particles. Due to the high pH buffer capacity of ZnO nanoparticles, the final pH changed a lot from their initial pH to around pH7 so dissolved zinc ion species enhance the aggregation process.
The ionic composition and strength can strongly affect the aggregation and sedimentation of these nanoscale materials in the aqueous environment. The nanoparticles aggregated more quickly in a higher concentration of cation. The increase in ionic strength resulted in compression of the electrical double layer (EDL), and therefore a decrease in the EDL repulsive energy such that the flocculation can be predicted. Therefore, the presence of either sodium or calcium ions can compress the EDL of these nanoparticles. Multivalent metal cations could form bridges with nanoscale particles or neutralize their surface charges to induce a quick aggregation. The critical coagulation concentration (CCC) values for nanoscale SiO2 particles were estimated as 220meq/L NaCl, 100meq/L KCl, and 90 meq/L CaCl2. The critical coagulation concentration (CCC) values for nanoscale TiO2 particles were estimated as 90meq/L NaCl, 10meq/L KCl, and 20meq/L CaCl2. The critical coagulation concentration (CCC) values for nanoscale ZnO particles were estimated as 5meq/L NaCl, 0.25meq/L KCl, and less than 0.25meq/L CaCl2. and The DLVO analysis can explain the aggregation behaviors of nanoparticles in presence due to the smaller interaction forces between particles with the increase of cation concentration. On the other hand, the energy gap of ZnO decreased with the increase of cation concentrations due to the larger particle size as the higher cation content in aqueous solutions.
Surfactants can disperse and aggregate these nanoparticles, which depends on the properties of surfactant and the nature of nanoparticles. For SiO2 nanoparticles, the coagulation occurs in the presence of SDS with 10 critical micelle concentration (CMC); however, SiO2 nanoparticle size only slightly increased in presence of 0.1 CMC of SDS and then SiO2 nanoparticles keep stable for several days. In the presence of CTAB and TX-100, SiO2 nanoparticle size slightly increased and also SiO2 nanoparticles keep stable for several days. For TiO2 nanoparticles, TiO2 nanoparticles maintain stable for several days in presence of SDS and CTAB surfactants although the particle size of TiO2 nanoparticles increased a little. For TX-100, a neutral surfactant, the well dispersion of TiO2 nanoparticles only occurred when its concentration in its CMC. As compared to the aggregation of ZnO in absence of surfactants, these three surfactants can maintain the stability of ZnO nanoparticle except the 10CMC SDS, an anionic surfactant. The low pH values of 3.83 was observed for the rapid aggregation of ZnO in the presence of 10CMC SDS, indicating that could be result from the neutralization of this high concentration of SDS with dissolved zinc ion species.
In presence of humic acid, the stability of these three nanoparticles can keep several days although their size may increase a little. With the increase of humic acid in aqueous solutions, the increase of these nanoparticle stability. It could result from the steric repulsion caused by humic acid molecule structure and electric repulsion of functional groups in humic acid.
The fate of metal oxide nanoparticles in water would significantly depend on pH, ionic strength, ionic composition, humic substance and chemical dispersants in the aqueous environment. Our results provide important insights into the ways in which nanoparticle change under different aqueous conditions that may be generally relevant to the nanoparticle fate in diverse natural environment.