해수 중에 보론은 주로 붕산(boric acid, B(OH)3)으로 존재하며 그 크기가 역삼투압(RO, reverse osmosis) 공정의 막의 공칭공경(nominal pore size)만큼 작아 이를 제거하기가 쉽지 않다. 이에 본 연구에서는 수용액 중 보론 화합물의 크기를 역삼투압 공정에서 제거가 용이하도록 키우기 위한 방법으로 B(OH)3가 폴리올과 착화합물을 형성하는 원리를 이용하고자 하며 착화합물의 형성 여부를 라만 분광학 연구를 통해 확인하고자 하였다. 먼저 B(OH)3와 붕산염(borate ion, B(OH)4 -) 이온의 대칭(symmetric) B-O 신축운동 모드(stretching vibrational mode)의 진동수를 확인하기 위하여 pH를 증가시키며 라만 스펙트럼을 측정한 결과 877cm-1에서 730 cm-1으로 피크의 이동이 관찰되었다. 이는 수용액 중의 B(OH)3가 pH가 증가하면서 B(OH)4 -로 전환됨을 나타내고 이러한 피크의 이동은 진동수 계산(frequency calculation)에 의한 예측과 정확하게 일치하였다. 반면 B(OH)3 수용액에 폴리올을 첨가한 경우 pH가 증가하여도 B(OH)4 -의 특성 진동수인 730 cm-1가 나타 나지 않았으며 이는 B(OH)3가 B(OH)4 -로 전환되는 것이 아니라 폴리올과 착화합물을 형성함을 간접적으로 보여주고 있다.

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 nominal pore size of RO membrane. Thus, the complexation of boric acid with polyols was suggested as an alternative way to increase the size of aqueous boron compounds and the complexation behavior was investigated with Raman spectroscopy. As a reference, the Raman peak for symmetric B-O stretching vibrational mode both in boric acid and borate ion (B(OH)4 -) was selected. A Raman peak shift (877 cm-1 →730 cm-1) was observed to confirm that boric acid in water is converted to borate ion as the pH increases, which is also correctly predicted by frequency calculation. Meanwhile, the Raman peak of borate ion (730 cm-1) did not appear as the pH increased when polyols were applied into aqueous solution of boric acid, suggesting that the boric acid forms complexing compounds by combining with polyols.

Boric Acid Polyol Complexation Raman Spectroscopy Frequency Calculation

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