폴리올과 붕산의 착화합물 형성 원리를 이용한 수용액 중의 보론 제거에 관한 연구

Abstract

Boron is difficult to be removed from seawater by simple RO (reverse osmosis) membrane process, because the size of boric acid (B(OH)3), the major form of aqueous boron, is as small as the typical pore size of RO membrane. Thus, a 2-pass RO membrane process has been used where a part of stream is by-passed to increase the efficiency of boron removal by converting boric acid into borate ion (B(OH)4-) at high pH. The essence of this process is to increase the size of aqueous boron compound for better rejection in RO membrane. In this study, the complexation of boric acid with polyols at moderate pH has been introduced as an alternative way to increase the size of aqueous boron compound and the complexation has been investigated with Raman spectroscopy and computational chemistry. As a reference, the Raman peak for B-O stretching molecular vibrational mode both in boric acid and borate ion had been selected, and a shift (877 cm-1 → 730 cm-1) was observed, confirming that boric acid in water is converted to borate as the pH increases. The Raman peak positions were also very correctly predicted by computation chemistry. Meanwhile, the development of the Raman peak of borate ion (730 cm-1) did not appear as the pH increased when polyols had been applied into aqueous solution of boric acid. Although it is indirect, it suggested that boric acid forms complexing compounds by combining with polyols. In actual RO experiments to verify the favorable effect of the complexation on the removal of aqueous boron, the boron removal efficiency increased from ~50% (no polyols) to ~80% (with polyols). And interestingly enough, xylitol turned out to be the most effective as the complexing agent compared to glycerol and mannitol. This was also predicted by computational chemistry. As an extended research, PVA (polyvinyl alcohol) mat was synthesized by using electrospinning. The PVA mat has many hydroxyl (–OH) groups as does polyols, and it could be used as a simple and cheap membrane for aqueous boron removal. The adsorption of boric acid on PVA mat by the same complexation mechanism was investigated with Raman spectroscopy. A peculiar change of Raman spectrum was obtained for the PVA mat immersed into aqueous solution of boric acid, but the interpretation of Raman spectrum has still been under way.