New ceramic nanofiltration membranes

Nanofiltration is becoming more and more important in wastewater treatment as a pressure-driven membrane separation process. The limitation in polymer membranes operation occurs when the chemical, thermal and mechanical stability of the membrane is exceeded by the medium to be treated. The long term resistance of even the polymer membranes with the greatest chemical and mechanical resistance, often proved inadequate for separation problems at industrial level.

Ceramic membranes have the combined advantage of high chemical, mechanical and thermal resistance and for this reason many companies and research institutes have been working on the development of ceramic membranes for several years. 

In the article 'Characteristics and application of new ceramic nanofiltration membranes', Weber at Al. describe some tests conducted with ceramic single or multi-channel membranes with an active layer made of TiO₂ or ZrO₂.
Zeta potential extent and polarity sign allows to make conclusions about the charge on the membrane surface. Zeta potentials of the ceramic nanofiltration membranes were determined by the streaming potential method, the only method suitable to characterize the flat surfaces of membranes. Ultrafiltration and microfiltration are in fact characterised by transmembrane determination of the streaming potential which is induced when an electrolyte passes through the membrane pores. The streaming potential in this study was measured when the electrolyte was pumped tangentially across the surface.
A membrane test unit was used for the experiments on the filtration properties of the ceramic nanofiltration membranes. Separation behaviour and the charge properties of five commercial ceramic membranes as well as the newly developed ceramic nanofiltration membrane made of TiO₂ were characterised.

The results of pure water permeability showed that the permeability rates of all ceramic membranes under study were considerably higher than the rates of currently available polymer membranes. The test to determine molecular weight cut off showed that the new ceramic membrane in TiO₂ could clearly be defined as a nanofiltration membrane. In contrast, the membrane retention determined for all the other commercial ceramic membranes was distinctly poorer, while permeability was similar or even lower.
Salt retention was controlled by the charge on the membrane. This charge was determined by measuring the zeta potential at different pH. Only negligible salt retention values were recorded for the ceramic nanofiltration membrane at pH 6.5 (isoelettric point) whereas retention in higher charge ranges, i.e. at high and low pH values, increased greatly.
Electrolyte retention increases when pressure rises from 6 to 15 bar. Besides dependence on pH, membrane retention was also strongly influenced by electrolyte concentrations in all the electrolyte solutions under study. When concentration was increased in the NaCl solution from 0.01 to 0.1 mol/l membrane retention decreased noticeably.
The tests proved that it was possible to develop a ceramic membrane which can be classified as a nanofiltration membrane because of its retention properties of organics. In contrast, the comparative tests on all other commercial membranes showed that they fall into an intermediate area between ultrafiltration and nanofiltration, considerably higher than 1000 g/mol. The salt retention is controlled by the membrane charge and depends, on a great extent, on the type of salt, salt concentration as well as the pH value of the solution.

As ceramic nanofiltration membranes are generally more expensive than standard commercial polymer membranes, their use should focus on fields application which demand grater thermal or chemical resistance. The use of new ceramic nanofiltration membranes was investigated for several applications of real media, focussing on the decolourisation of textile wastewater, the treatment of hot alkaline solutions from bottle washing machines and the treatment of pickling bath solutions from the metal-working industry.
High permeability rates, good retention of organics as well as low fouling tendency were confirmed particularly in the case of the newly developed ceramic membranes. The advantages of this membrane over commercial polymer and commercial ceramic membranes provide a good argument in favour of the industrial application of the newly developed ceramic membrane, particularly for the treatment of textile wastewater containing dyes as well as for the treatment of alkaline solutions from bottle washing machines. However, in contrast, salt retention decreases strongly as electrolyte concentrations rise during the treatment of pickling bath solutions so that the use of ceramic nanofiltration membranes in this field is not a viable option.

Use of ultra and nanofiltration ceramic membranes for desalination

Freshwater is very important for all aspects of life. Wastewater, brackish water and seawater treatment are good solution as a source of freshwater. Among all the techniques used for desalination, reverse osmosis is known as a classical process, and nanofiltration and ultrafiltration can also be used.
Desalination performances depend on the concentration of ions and on the complexity of the medium to be filtered, but also on the type of membrane material.
In the scientific article 'Use of ultra and nanofiltration ceramic membranes for desalination' Condom, Larbot et Al. discuss the effects from the results obtained for the filtration of different saline solutions using several ceramic membranes prepared by the sol gel route. The membrane tested were made of g alumina, CoAl₂O₄ and TiO₂/ZnAl₂O₄. The filtration experiments were carried out on a laboratory pilot scale.

The results obtained for the g-alumina membrane show that the rejection rates depend strongly on the nature on the filtered salt. The best rejection was observed for the MA₂ salt, bad rejections with M₂A salts. The surface charge of the material, which depends on the pH of the filtering material, is then an important parameter which governs the efficiency of a membrane process, especially for removing ionic species.
The variation of the rejection rates of the different salts depends on the pH of the filtered solution, as a consequence of the electrical interactions developed between the ions and the surface charge of the membrane. When the pH of the solution reaches the pH value where the charge of the membrane is zero the rejection becomes very low and then increases for the pH values where the membrane is negatively charged.

Generally, the streaming potential measurement across the membrane seems to be a good parameter to predict the rejection rates of the salt instead of the electrophoretic powder mobility, which sometimes is not in agreement with salt rejection.

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