A comparison of bulk, supported, and contained liquid membranes (BLM, SLM, and CLM) for metal separations with respect to permeability, stability, and selectivity is presented. The overall mass-transfer coefficient of magnitude order of 10-6 and 10-7 m s-1 was typically found for SLM/BLM and CLM, respectively. The SLM has the highest organic utilizing efficiency and the poorest membrane stability. Osmotic pressure across the membrane proves to be one of the major causes of SLM instability, and membrane pore elongation due to morphology changes reduces the SLM lifetime. The separation efficiency of liquid membranes can be enhanced by minimizing the organic inventory and maximizing the contact area between the aqueous phase and the organic membrane phase. However, this needs to be balanced against the improvement in stability of various liquid membrane configurations, which can be achieved by employing a significant organic inventory. The selectivity of liquid membranes mainly depends on the extractant used in the membrane phase and is similar to that of one-stage solvent extraction. The paper concludes with a technical comparison of the various liquid-membrane techniques.
Hematite (10 mg of Fe/L) floc-humic acid assemblages have been formed at pH 4 either by first aggregating hematite particles with salt (100 mM KCl) and then adding humic acid (salt-particle-organic or SPO assemblages) or by suspending the hematite particles in humic acid solutions and then adding salt to induce aggregation (organic-particle-salt or OPS assemblages). The behavior of these assemblages upon deposition on microfiltration (MF) membranes has then been investigated. In the OPS case, the fractal dimension (dF) of the assemblages formed varied dramatically depending upon the extent of charge neutralization by added fulvic acid with dF values typical of diffusion-limited cluster aggregates at low (0.1-0.2 mg/L) humic acid concentrations and dF values typical of reaction-limited cluster aggregates either in the absence of humic acid or concentrations greater than 0.4-0.6 mg/L. In the SPO case, dF values on the order of 2.1 were initially observed and were found to decrease to around 1.8-1.9 for humic acid concentrations greater than 0.6-0.8 mg/L. OPS assemblages with low fractal dimensions were found to be highly compressible once deposited on MF membranes with significantly higher specific cake resistances than was the case for SPO assemblages at transmembrane pressures of 50 kPa and above. These results highlight the importance of both the choice of coagulant (e.g., preformed vs formed in situ) and the transmembrane pressure to which a membrane filtration process might be allowed to rise prior to removal of the fouling layer.
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