The use of insoluble reagents in micro particle form has been extensively investigated in the search for new and simpler processes for purifying water. Very finely divided particles allow rapid rates of reaction to be attained. However, they pose a severe handling problem. To overcome this difficulty, the microparticles may be bound together to form a composite particle of conventional size, held together by a polymeric binder which is permeable to water and the impurities to be removed. Another technique involves incorporating small amounts of a magnetic material such as iron oxide within the micro particles of reagent. When magnetized, the particles flocculate strongly and quickly settle out. Variation of the nature of the reagent allows a wide variety of impurities to be adsorbed. The feasibility of using microparticles suitably modified has been demonstrated for a number of ion-exchange reactions, for a new clarification and decolorization process involving alkali-treated magnetite, and for a selective adsorption process based on a magnetic activated carbon which can adsorb only small molecules.