The fate and transformation of engineered nanoparticles in environmental is of significant interest to ecosystem and human health due to the large application and development of engineered nanoparticles in recent years. Stable TiO2 and ZnO nanoparticles in suspensions were synthesized with benzyl alcohol and diethylene glycol as capping agents, respectively. This research investigated the stability and morphology change of two stable metal oxide nanoparticles in a variety of aqueous conditions. The sizes of these two synthesized particles of TiO2 and ZnO were identified as in nanoscale by a transmission electron microscopy (TEM). Nano-sized particles of TiO2 were stable more than 25 days and ZnO nanoparticles were stable more than 1 day due to their surface modification to create high zeta-potentials. The particle size change with time in the aqueous phase was monitored by a dynamic light scattering (DLS). The morphology and characteristic of these two nanoparticles were also examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and TEM.
The particle concentration did not significantly affect the particle size of the stable nanoscale TiO2 suspensions but the particle size of TiO2 nanoparticles increased when the particle concentration less than 10 mg/L. The particle size of zinc oxide maintained approximately 110 nm when the range of particle concentration between 50 mg/L to 500 mg/L. The particle intensity of ZnO was too low to detect when the concentration less than 50 mg/L. For the ZnO concentration higher than 500 mg/L, the particle size increased due to the more collision of particles. The temperature in the range of 15~35 oC also did not significantly affect the stability of TiO2 and ZnO nanoparticles. With the aqueous pH close to the pHzpc of TiO2 nanoparticles (pH 7.2) and ZnO nanoparticles (pH 9.7), the obvious coagulation behaviors were observed. These two stable nanoparticles could suspend when aqueous pH out of 1 and 1.8 units of their pHzpc for TiO2 and ZnO, respectively.
The ionic composition and strength can strongly affect the aggregation and sedimentation of these two stable nanoscale materials in the aqueous environment. For the stable TiO2 nanoparticles, the particle size increased more quickly in a high concentration of salts. The critical coagulation concentration (CCC) values for nanoscale TiO2 particles were estimated as 340 meq/L NaCl, 290 meq/L CaCl2, and 1.4 meq/L SO42- respectively. The results indicated that the divalent anion than monovalent is more easily to neutralize the surface charge of particles and increase aggregation in the same ion strength. The second affective factor is the ion 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. Multivalent anions could form bridges with nanoscale particles or neutralize their positive surface charges to induce a quick aggregation. Furthermore, these CCC values are higher than those of TiO2 nanoparticle powders in previous study, indicating the strong stability of the synthesized TiO2 nanoparticles. For stable zinc oxide, the CCC values for sodium chloride, calcium chloride and sodium sulfate are 30 meq/L, 30 meq/L and 0.25 meq/L, respectively. The interaction energies of nanoparticles in water can be analyzed using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. The decreasing energy barriers of nanoparticle with the increase of ion strength increasing are consistent with the observation of the particle size change. Furthermore, the energy barriers decreased to the maximum value as the ion concentration of CCC.
In the presence of a low humic acid concentration at low pH, the stability of these TiO2 nanoparticle suspensions can keep several days. When the humic acid concentration was higher than 50 mg/L, TiO2 nanoparticles aggregated because humic acid could bridge these nanoparticles. In different pH conditions, the addition of humic acid cause the pHzpc of TiO2 shift to a low value. The increase of the concentration of humic acid enhanced the pHzpc shift. At pH 7, the quick aggregation of TiO2 nanoparticles was observed even in the presence of humic acid, indicating that humic acid can not maintain the TiO2 nanoparticles stable when aqueous pH close to pHzpc of TiO2. However, humic acid can decrease the aggregation process of ZnO nanoparticles in the presence of humic acid higher than 10 mg/L when the aqueous pH close to pHzpc (pH 9.7) of ZnO. At pH 7 and 10, humic acid can maintain the ZnO nanoparticles stable. In the presence of fulvic aicd, the zeta potentials of TiO2 and ZnO also decreased. With the increase of fulvic acid concentrations, the stability of these two nanoparticles increased. Humic and fulvic acids seem to facilitate the stability of ZnO nanoparticles, which could result from the steric repulsion caused by humic molecule structure of functional groups in humic and fulvic acids.
Once these two stable nanomaterials into four environmental water samples, they aggregated because aqueous pH close to the pHzpc of TiO2 nanoparticles and ion strength in water samples enhanced the coagulation process. Hence, the particles are more easily to aggregate in the environment. Especially both effluents from industry wastewater and sewage wastewater contained a lot of ions more than the CCC values of these two stable nanoparticles, they easily aggregated and precipitated in wastewater effluents. These results suggest that these two stable nanoparticles could not cause the nanotoxicity effect in aqueous environment due to the easy aggregation process of them in aqueous phase..
The fate of these stable metal oxide nanoparticles in water would depend on pH, ionic strength, ionic composition and humic substance in the aqueous environment. The aqueous pH and pHzpc play key roles in the stability. These findings provide important insights into the ways in which stable nanoparticle change under different aqueous conditions that may be generally relevant to the nanoparticle fate in diverse natural environment.