Design and characterization of macroporous alumina membranes for passive samplers of water contaminants

Macroporous ceramic membranes with hierarchical pore structure were developed and characterized for the time-integrated analysis of water contaminants. The alumina membranes were boosted to allow a suitable diffusion rate for envisaged applications. Sacrificial fugitives as cornstarch and graphite and partial sintering technique were utilized for the control of pore structure and morphology. The present study includes: (i) Development of ceramic membrane preparation methods based on wet forming techniques and partial sintering in order to control of membrane parameters such as porosity, tortuosity factor and thickness; (ii) Microstructural characterization of the ceramic membranes using two complementary techniques: Hg-porosimetry and SEM; (iii) the diffusion tests performed under laboratory conditions by an innovative, easy and accurate method based in the use of methylene blue (C16H18ClN3S) water solutions as test compound to study the characteristics of ceramic membranes. Using slip casting and partial sintering processing three different Al2O3 membrane configurations with high values 34.5, 36.9 and 39.6 vol% of interconnected porosity and 1.75, 1.7 and 1.3 mm thickness were prepared. The pore morphology consisted of spherical cavities of 5-10 mu m diameter and small pores of 70-200 nm. This study also gathers the basis for the calculation of diffusion coefficients, membrane geometrical factor and sampling rates using methylene blue water solutions whose concentration can be accurately determined by optical absorption measurements at 664 nm. Diffusion coefficients for methylene blue range from 2.3 +/- 0.5E-8 to 3.0 +/- 0.5E-8 cm(2) s(-1) increasing as connected porosity increases and membrane thickness decreases. The membrane geometrical factors are low, between 0.6 and 1E-02 and Archie's constants large 4.5-5.5. This has been associated to the bimodal pore structure with coexistence of large and small pores that may increase the tortuosity factors in membranes. The macroporous ceramic membranes herein developed are suitable for the fabrication of passive samplers aimed at preconcentrating organic contaminants in water.