(144g) Fundamental Characterization of Surface Mineral Crystallization on Aromatic Polyamide Ro Membrane Surfaces

Reverse osmosis membranes used for surface and groundwater desalination are prone to mineral salt scaling. In order to develop advanced scaling resistant membranes, there is a need to first understand why membranes scale and what factors contribute to surface mineral scaling. When mineral salt scaling occurs, water flux gradually declines, which result from the decreasing of water permeability. Previous studies have used flux decline monitoring as a major indication for mineral salt scaling. However, it is well known that surface crystallization (and nucleation) of mineral salts takes place prior to the detection of any measurable flux decline. The primary mineral salt scalants that are typically of concern in inland water desalination are calcite, gypsum and barite.

In order to directly quantify the impact of surface functionality and topology on the kinetics of surface mineral salt crystallization, a systematic study was carried out for gypsum crystallization on surrogate polymer surfaces and in diagnostic RO membrane plate-and-frame system. Polymer surfaces were formed by spin-coating on a Quartz crystal microbalance (QCM) which served to monitor the surface accumulation of mineral salt precipitate. Surrogate polymer surfaces that resemble the topology and chemical functionality of commercial RO membrane characteristics were prepared by solution spin-coating of aromatic polyamide (AP) on the surface of quartz crystal sensor. Surface crystallization experiments were subsequently conducted with model solution at a range of saturation indices with respect to the mineral salt scalant. The impact of surface chemistry on the severity of surface scaling was also investigated by post-modification of the polyamide surface by a surface graft polymerization technique that relies on surface activation by plasma treatment. Feed solutions of the precursor anion and cation of the mineral salt were pumped from separate feed reservoirs and mixed just prior to entering the QCM crystallizer flow cell. Given the relatively low level of supersaturation index (~1.5 or lower) of the fed solution and the short convective residence time of the mixed solution, bulk crystallization was avoided and crystallization was essentially restricted to the QCM surface. In order to evaluate the impact of temperature on the onset and kinetics of surface crystallization on the polyamide surface studies were carried out over a temperature range of 15 °C to 40 °C which covers the range of interest in membrane desalination processes. Crystallization induction time were compared with those obtained from direct visual observation of membrane scaling tests in a high pressure transparent RO cell in order to provide a comparison of scale formation in the presence of permeate flow. The study results demonstrated that surface topology and chemistry can have a measurable impact on surface crystallization suggesting that the optimization of surface structure of RO membranes can be beneficial for the development of effective scale mitigation strategies.