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SynopsisThe determination of the diffusion coefficient for water in various porosity cellulose acetate membranes by a gravimetric method, using a humidified carrier gas, is described. It was found to be impossible to obtain meaningful results for very porous membranes, although dense membranes gave limiting values of diffusion coefficient a t high carrier gas velocities. This phenomenon is explained in terms of the dissipation of the heat of sorption by the forced convection provided by the carrier gas. The variation of diffusion coefficient with concentration of water in dense cellulose acetate is explained in terms of clustering of water molecules in the polymer a t high concentration. INTROnDUCTIONA common method for the determination of diffusion coefficients in polymers utilizes a vacuum technique in which the sample, initially evacuated, is exposed to a controlled vapor pressure source on sorption and to a vacuum again on desorption. ' We have used a carrier gas flow technique to study the interaction between water and cellulose acetate reverse osmosis membranes and have measured the effects of humidity, flow rate, membrane structure, and equilibrium concentration. EXPERIMENTAL MaterialsThe preparation and morphology of the cellulose acetate (39.8% acetyl) membranes used are described by Gittens et a1.2 The membranes were cast from acetone-formamide dope, allowing various evaporation times, then immersed in iced water (Table I). The membrane with shorter evaporation times showed a heterogeneous structure, the 30-sec evaporation membrane having a three-layer structure dominated by a porous lower layer. The 48-hr evaporation membranes were essentially dense homogeneous films.

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Plant nutrients such as potassium (K) may be limited in soil systems and additions (i.e., fertilizer) are commonly required. Glaucony is a widely distributed and abundant marine-derived clay mineral present in soils worldwide which may serve as a source of potassium. The South Island of New Zealand contains numerous deposits of glaucony-rich rocks and related soils providing an opportunity to explore how glaucony might be a beneficial source of potassium. Here, the geochemistry of glaucony and its suitability as a mineral source of soil K from four deposits in New Zealand was examined using spatially resolved chemical analyses and dissolution experiments. Geochemical and morphological analyses revealed that glaucony from all deposits were K-enriched and were of the evolved (6%–8% K2O) to highly evolved type (>8% K2O). Glaucony derived from growth inside pellets contain elevated K and Fe concentrations compared to bioclast-hosted glaucony. Solubility analysis showed that K was released from glaucony at rates higher than any other metal present in the mineral. Additionally, decreasing the pH and introducing an oxidizing agent (i.e., birnessite which is ubiquitous in soil environments) appeared to accelerate K release. Trace metals including Cr, Zn, Cu, and Ni were present in the solid phase analysis; however, further investigation with a focus on Cr revealed that these elements were released into solution at low concentrations and may present a source of soil micronutrients. These results suggest that glaucony may offer a source of slow releasing K into soils, and so could be used as a locally sourced environmentally sustainable K resource for agriculture, whether in New Zealand or worldwide