Effect of an alternating current for crystallization of CaCO3 on a porous membrane.

Calcium carbonate formation on a porous membrane was accelerated by using an alternating current system. A hydrophilic-treated polytetrafluoroethylene (PTFE) porous membrane was utilized for the calcium carbonate formation. The PTFE membrane was interposed with two pairs of glass cells. Calcium chloride and sodium carbonate solutions were filled in each glass cell, and an alternating current of 10 Hz was applied for 30 min. The resulting membrane was characterized in terms of crystal morphology and polymorph using scanning electron microscopy and X-ray diffraction. The major component of the resulting calcium carbonate was spheroidal vaterite, along with rhombohedral calcite. The composition of polymorphs was slight different on the two sides of the membrane. In the absence of the alternating current, only calcite was formed on the membrane. It is considered that the alteration of calcium carbonate formation is regulated by supersaturation, including electric migration, diffusion and electrolysis, during application of the alternating current. The resulting supersaturation was different on the two sides of the membrane, and different quantities of the polymorphs were formed. Taking this result into account, the alternating current catalyzed the development of supersaturation around the membrane, enabling calcium carbonate crystal to be formed.