Detection of free primary amines in TFC membranes by microscopic fluorescence imaging

A microscopic imaging method using the fluorescent binding agent, fluorescamine, has been applied to determine the presence and location of primary amines in the selective layer of TFC polyamide RO membranes. The presence of such amines is attributed to incomplete condensation reactions in the preparation of the membrane's active layer for fully aromatic TFC membranes. The concentration of free amines is greatest in the section of the active layer most distant from the external membrane surface. The free amines are effectively removed by combining rinsing with application of oxidation agents. As delivered in spiral wound modules they have already been removed to a large extent. In flat sheet membrane they still contain various extents of free amine depending on the membrane type and the supplier.

► The primary unreacted amines from fully aromatic polyamide membranes are imaged by fluorescamine. ► Rinsing or reacting with oxidants removes the amines from roll sheet samples. ► There are more primary amines below the surface than on it. ► Oxidants had difficulty reaching the amines below the surface. ► Polyamide membrane from spiral elements had much less primary amines.