Coprecipitation process by several hydroxides has been usually used for the treatment to a large amount of wastewater contaminated by inorganic toxic elements. Investigation to know the uptake mechanism of toxic elements at solid/liquid interface is very important because not only wastewater purification but also volume reduction of sludge formation and immobilization of harmful elements into the sludge are desired after wastewater treatment. In this paper, immobilization mechanism of inorganic toxic elements at solid/liquid interface of hydroxides was categorized into surface complexation and surface precipitation, and explained their concept with introducing experimental methods to know their mechanism as example of As（V） coprecipitation with ferrihydrite.