Desalination using supported liquid membranes

In the present work, the supported liquid membrane (SLM) technique has been applied to the desalination of saline water. A simple apparatus designed and constructed in our lab was used to conduct the experiments. Various factors that would affect the degree of desalination were studied and these were: type of organic membrane liquid (ML), quantity of ML (i.e. its thickness), presence of emulsifier or mobile carrier (MC) in the ML, concentration of MC in ML, presence of polyelectrolyte (sequestrant) in the receptor phase and presence or absence of magnetic stirring. The volume ratio of donor phase to receptor phase was kept constant at 4:1 and the concentration of sodium chloride solution in the donor phase (simulated seawater) ranged between 36 and 39 g/L. Cellophane constituted the support for the ML. A previous work on desalination by emulsion liquid membranes (ELMs) has proven to be a successful one stage operation with a percent water recovery of 98% and in which >99% of the salt (equivalent to seawater concentration) is removed in only a few minutes. Accordingly, a preliminary work on desalination using SLM technique was attempted and evaluated and compared to the aforementioned ELM technique. The most important findings emphasized the importance of MC in the ML due to the enhancement of mass transfer through the liquid membrane (LM) and the importance of stirring in promoting mass transfer by minimizing the boundary layer adjacent to the cellophane support. In addition, an optimum concentration of MC existed in the LM. However, desalination using SLMs is much slower than ELMs due to a much lower surface mass transfer area of the former compared with the ELMs. On the other hand, SLMs enable various factors that would affect the process of desalination to be conducted by simple means with a target of optimizing the process.

Several conclusions have been deduced from the present work. First it has been shown that the SLM technique is a very time-consuming method by which desalination can take place, compared with the very rapid ELM technique [22]. However, it is a suitable method for further lab investigations with other complexation agents in the DP, other MCs, other membrane liquids, etc. It has also been shown that the emulsifier alone did not affect mass transfer, but that the presence of an optimum quantity of MC is of primary importance in affecting desalination. The presence of a PE in the AP is essential in sequestering the NaCl, although its effect is not pronounced as in the ELM technique applied to desalination [22]. Stirring should have assisted in minimizing the resistance at the AP–LM interface and should have promoted mass transfer towards the AP. On the other hand, continuous magnetic stirring of the DP could have produced even better results, only this was hard to fulfil for various reasons. As to the LM thickness, its effect is less significant than the MC, but comes second in importance. Lastly, it is noticed that different organic LMs lead to different permeabilities. To this end, it could be added that testing other supporting membranes too could lead to minimizing the membrane resistance to diffusion of NaCl. Further work remains to be carried out to optimize the extent of desalination and make it more rapid, and this is going on at present in our lab. Furthermore, a flowing LM technique is currently investigated for desalination in our lab as well, with the DP and AP flowing con-currently to each other and counter-currently to the LM.