Concentration polarization in membrane distillation: I. Development of a laser-based spectrophotometric method for in-situ characterization

Abstract

Membrane-based processes have gained prominence for environmental and industrial separations, especially in water purification and desalination. In spite of these developments, a fundamental knowledge gap exists with regards to fully understanding the processes that occur at or near the membrane surface. Concentration polarization is a phenomenon that may have a significant impact on mass transfer and fouling of the membrane. While attempts have been made to experimentally characterize this phenomenon for pressure-driven filtration processes, most studies employ semi-empirical methods to estimate the extent of concentration polarization. This study presents a novel non-intrusive spectrophotometric method developed to measure the concentration profile of solute near the membrane surface in a direct contact membrane distillation system. A custom membrane cell was designed and fabricated for this study to allow accurate in-situ characterization of the concentration polarization layer with spatial resolution of 4.5 μm. The spectrophotometric method was developed and validated using transition metal salt (i.e., nickel chloride) in the feed solution to probe the concentration profile near the membrane surface. The method developed in this study can be extended to other membrane-based separation processes and other solutes with known absorption spectra.