After the studies about aggregation of commercial nanopowders in 2008 and self-synthsized suspension nanoparticles in 2009, two commercial nanoparticle suspensions were chosen in this year (2010). The two commercial metal oxide nanoparticle suspensions, TiO2 and ZnO, in a variety of aqueous conditions were investigated. These two commercial suspensions, TiO2 and ZnO, were identified as nanoscale particles by a transmission electron microscopy (TEM) and dynamic light scattering (DLS). At 25℃, the particle concentration of nanoparticles between 20 mg/L and 30 g/L did not significantly affect the particle size of these commercial nanoscale TiO2 (pH 3~4) and ZnO (pH 8~9) suspensions. The temperature in the range of 15~35℃ also did not significantly affect the stability of 1000 ppm TiO2 (pH 3~4) and ZnO (pH 8~9). When the pH value closes to pHpzc of TiO2 nanoparticles (pH 6.5) at 25℃, the obvious sedimentation behaviors were found for 1000 ppm TiO2 nanoparticle suspension. Far from the pHpzc of TiO2 nanoparticles, the TiO2 nanoparticles keep stable in nanoscale. ZnO nanoparticles (1000 ppm) keep stable in the nanoscale under alkaline condition (pH 8~12) at 25℃. Under neutral and acid conditions, the ZnO nanoparticles aggregated quickly, which could result from its dissolved zinc species.
The ionic composition and strength can strongly affect the aggregation of nanoscale materials in the aquatic environment. For the 1000 ppm commercial TiO2 suspensions (pH 3~4) at 25℃, the particle size increased more quickly in a higher concentration of salts. The critical coagulation concentration (CCC) values for nanoscale TiO2 particles with positive surface were estimated as 290 meq/L and 2.3 meq/L for Cl- and SO42-, respectively. 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. Multivalent ions could form bridges with nanoscale particles or neutralize their surface charges to induce a quick aggregation. Furthermore, these CCC values are higher than those of TiO2 nanoparticle powders in previous study (2008), indicating the strong stability of the commercial TiO2 nanoparticles. Besides, these CCC values are similar to those of manufacturing TiO¬2 suspensions in previous study (2009), indicating the similar behavior of two kinds of modified-TiO2 nanoparticles.
The CCC value for 1000 ppm commercial ZnO suspension particles with negative surface (pH 8~9) was estimated as 40 meq/L for Ca2+, which is much higher than ZnO nanoparticle powder and commercial TiO2 suspensions. In addition, the ZnO nanoparticles were not aggregated obviously in the presence of NaCl. The results indicated that the divalent cation than monovalent is more easily to neutralize the surface charge of particles and increase aggregation in the same ion strength. This phenomenon is consistent with Schulze-Hardy rule. The interaction energies of nanoparticles in the presence of electrolytes were also evaluated by using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory. Besides, compared with manufacturing ZnO suspensions, the CCC values of commercial ZnO suspensions are obviously higher. We need to pay attention on these highly stable commercial nanoparticle suspensions.
In the presence of a low humic acid (HA) concentration, the 1000 ppm commercial TiO2 suspension particles can keep stable several days at pH 3~4 and 25℃. When Suwannee river humic acid (SRHA) concentration was higher than 30 mg/L, TiO2 nanoparticles aggregated. Humic acid could enhance the aggregation process possibly through the cross-linking of nanoparticles with SRHA in aqueous solution. For the co-effect of ions and HA, the CCC values decreased to 100 meq/L and 1.8 meq/L for Cl- and SO42-, respectively, as compared to those values in the absence of humic acid. Because unchanged surface potentials were observed under these CCC values, ions seem to enhance the aggregation effect in the presence of SRHA molecules.
1000 ppm ZnO suspension particles still maintained stable as HA concentration in the range of 0~50 mg/L at pH 8~9 and 25℃. The commercial ZnO suspension particles did not affect by NaCl in the presence of humic acid. In the presence of HA, the CCC value of ZnO nanoparticles slightly reduced to 30 meq/L for Ca2+, which is very high as compared to nanoparticle powders. Consequently, these commercial suspension nanoparticles could have hazardous potential in aquatic environment due to their high stability.
In this study, the DLS technology is also cooperated with sample pretreatment process including filtration and centrifugation to develop a process for the analysis of nanoparticles in the aquatic environment. The centrifugation process was efficient to removal the interference effect of large particles on DLS analysis. In the artifical environmental waters, centrifugation is also more suitable than filtration to be used as a pretreatment process of DLS measurement for nanoparticles. The combination of centrifugation process into the DLS analysis and confirmation by SEM or TEM is suggested to detect nanoparticles in the environment. The cooperation of other instruments is recommended for the precise analysis of nanomaterials in the environment in near future.