Abstract
Uphill transport of boric acids in aqueous solutions through PVA membrane was investigated. The PVA membranes were prepared by casting aqueous solution of PVA, subsequent drying at 294 K for 1 day, and treated at 383 K for 15 min to form cross-link in membrane. The permeation experiments were carried out under pH control, and the concentrations of boron were analyzed by the ICP-AES technique. When both sides were kept at an equal pH, a normal permeation due to the concentration gradient of boron was observed, but the flux of boron was higher in acid solutions than in alkaline solutions. For the condition where one side was kept at pH = 5.0 (acid side) and other side at pH = 10.0 (alkaline side), permeation of boron from the acid side to the alkaline side was observed under equal initial concentration of both sides at 100 ppm. Such an uphill transport was observed even under the condition of which the initial concentration of the alkaline side (200 ppm) was higher than that of the acid side (100 ppm). The uphill transport could be explained in terms of the pH sensitivity of a complex formation equilibria of boron with PVA; B(OH)3, the dominant form under lower pH could form the complex with PVA more easily than [B(OH)4]-, the dominant form under higher pH. As a result, transport of the complex due to their concentration gradient would take place against the concentration gradient of boron between the acid side and the alkaline side. We also investigated uphill transport of boric acids through modified PVA membrane, but we couldn't observe uphill transport with these membranes.
