Boron removal from seawater by supported liquid membranes

Abstract

Water covers 70% of the Earth’s surface but, even so water is becoming a scarce resource. In order to achieve the target 7.c of the Millennium Development Goals, to assure potable water for the people without sustainable access to it, alternative sources must be found. Considering that the oceans contain 97.5% of the earth’s water, seawater (SW) can be use as a source of safe drinking water and for irrigation; the salts contained are usually removed by membrane techniques, especially reverse osmosis (RO). By using this alternative source more traces of contaminants start to appear in the final product, that is the case of the boron. The average boron concentration in the seawater is 5 mg/L, if the rejection values for the new SWRO membranes are between 80-87%, the concentration of boron in the permeate is more than 0.5 mg/L, exceeding the WHO Guidelines for Drinking-water Quality. To reduce this limit value is necessary put one additional step of separation. As well as know, the supported liquid membranes (SLM) are very effective in removing and recovering metals from liquid effluents at low concentration, since they combine extraction and stripping processes in one step. For that, this technique must be suitable to extract boron from seawater. The main problem of the conventional SLM is the limited time of performance associated to the stability of the membrane. To avoid it, hollow fiber strip dispersion technique (HFSD) has been used in this work. The 1,3 diols, as 2,2,4-trimethyl-1,3-pentanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-ethtyl- 1,3-hexanediol, 2-methyl-2,4-pentanediol, are appropriate to extract boron selectively. The liquid liquid extraction experimental trials have allowed to choose the 0,3 M BEPD (2-butyl- 2-ethyl-1,3-propanediol) with 20% decanol in kerosene as the best extractant for the boron separation from neutral or acidic aqueous media. The extraction yield is well and this value can be maintained after successive extractions, this behaviour is very important for its application to hollow fiber membrane module by strip dispersion. The results obtained with flat supported liquid membranes have given permeability coefficient values witch corroborate the possibility to use this extractant on the hollow fiber strip dispersion technology.

Peer Reviewed

Postprint (published version)