PURPOSE: To efficiently remove colloidal substance and trace heavy metals from a water solution containing the metallic elements and the colloidal substance such as elements which are likely to form multibranched bodies with a cation (s), by bringing the water solution into contact with powdery active carbon.

CONSTITUTION: Powdery active carbon is added to a water solution containing metallic ions, e.g. iron, aluminum and mercury, and colloidal substance, having a size of 10-7W10-4cm, such as elements, e.g. silicon which are likly to form multibranched bodies with a hydroxyl radical or other cations, to adsorb the colloidal substance with the active carbon. The water solution is then filtered. As the powdery active carbon may be any type of coal, coconut husk, conifer or pitch. The powdery active carbon has a particle size of about 300μ or less, and preferably contains about 50% or more of a part having a particle size less than 300μ but above 44μ. The powdery active carbon is added to the colloidal substance-contng. water solution at a ratio of 0.1W0.001wt%.