A colloidal solution can be purified following the method of

Ions are eliminated from solutions during dialysis by the diffusion phenomenon across semi-permeable membranes. Colloidal solution is dialyzed in electrodialysis under the influence of an electric field.
Colloid and crystalloid particles are separated during dialysis by the phenomenon of diffusion via semi-permeable membranes.
The idea behind it is that whereas ions of an electrolyte can travel through a membrane made of parchment or cellophane, colloidal particles cannot.
Impurities diffuse slowly, yet pure colloidal solution is left behind.