The results in Table 6 showed that low recoveries of AgNPs and AuNPs were obtained with these five types of filters. It indicated that the NPs could not pass through these filters, although the NPs diameter was smaller than the pore size of the nominal filter membrane. The NPs were blocked to the membrane surfaces due to electrostatic attraction
(Hassellov et al., 2008; Mitrano et al., 2012). The nanoparticles which were smaller than the pore size transported through the membrane more slowly than the liquid did. This was because that the electrostatic repulsion could lead to higher collision rates between particles and aggregation.As a result high-efficiency trapping of nanoparticles and aggregates occurred on the membranes (Hassellov et al., 2008). This study indicated that filtration might be a problematic pre-treatment method and it can't be applied in the sample preparation for the detection of nanoparticle in natural water.