any nano particles like semiconductors oxides that used to remove dues from water will escape in pouted water and that cant overcomes due to not easy to collect that nano particles from the water
The simplest solution: try to deposit the nanoparticles on solid phase, maybe any filter or membrane (of course with limited solubility in liquid phase). The escape of particles will be certainly less and their remouving from water will be easly.
We have used this product (Amicon Ultra-15 Centrifugal Filter Units from EMD Millipore) to separate ZnO agglomerates from dispersion with good success. After separation, we normally analyze the supernatent with UVVis to confirm the absence of the band edge peak on the spectra. Good luck!