(628j) Comparative Assessment of Fouling of Iron Oxide Ceramic Membranes by Organic Compounds

Membrane treatment has been applied increasingly in the last decade driven by the advances that made possible significant improvements in efficiency while making the cost competitive alongside other technologies.  

Ultrafiltration has proven to be an effective process from the removal of natural organic matter and more generally of organic macromolecules, such as proteins, sugars and humic substances. However, these molecules may adsorb to the membrane surfaces and therefore produce strong fouling. This is a condition that needs to be avoided, since it creates the need for physical and/or chemical cleaning, lowering the volume of water produced and shortening membrane life.

There exist numerous studies in the literature that investigated the fouling of polymeric membranes by organic compounds, mainly because polymer is the material of choice for almost all water treatment applications. Developments in new materials for water treatment have shifted the attention to ceramics, motivated by nanotechnology innovations and the search for catalytic surfaces. Although ceramic materials such as titania, alumina and iron oxides have been introduced in the water treatment industry, very little is known about the fouling characteristics of ceramic membranes by organic macromolecules. Therefore in order to advance the use of ceramics membranes, it is necessary to understand the chemical and physical phenomena involved.

We fabricated and characterized an iron oxide ultrafiltration ceramic membrane. Due to the chemical characteristics of the iron oxides, the material can be used as an adsorbent for the removal of a variety of contaminants, a semipermeable barrier and a reactive porous material. The fabrication approach consists in the top-down synthesis of organic-coated iron oxides nanoparticles (ferroxanes) and their subsequent use as precursors for the formation of a porous film. 

 In this work, ferroxane nanoparticles were obtained by lepidocrocite (FeOOH) reaction with acetic acid and then deposited onto alumina supports via a dip-coating process. The dried layer of iron oxide nanoparticle was finally sintered to decompose the organic coating and form a porous ceramic layer. For example, the membrane sintered at 450ºC is composed exclusively by hematite, had surface area of 30 sq.m/g and an average pore size of 75 nm.

The membranes were fouled with three different model organic compounds: humic acids (HA) (natural organic matter), bovine serum albumin (BSA)(protein), and sodium alginate (SA) (polysaccharide) to investigate the relative fouling capacity of each one.

Flux decline, permeability and cleaning techniques were investigated. Comparing with the value measured for clean water permeability, an assessment of the fouling potential of each type of compound was obtained. In all cases there was a rapid decline in flux, indicating the onset of fouling was occurring.

After the initial fouling, the membranes were rinsed thoroughly with ultrapure water to simulate a hydrodynamic cleaning step followed by a second measurement of the filter permeability. 

The decrease in permeability was recorded as a function of concentration and pH of the feed solution for each one of the model organic compounds. Two pH values were investigated, above and below the point of zero charge of the membrane.   

In the case of HA and BSA, a positive correlation was observed between fouling and concentration increase in the feed. For SA, the flux dropped sharply in all concentrations tested, due to the formation of a gel like layer on the membrane surface. After rinsing with ultrapure water, recovery was approximately 50% of the initial flux. This is attributable mainly to the high solubility of polysaccharides in water. The use of hydrogen peroxide as a cleaning agent on the iron oxide surface was successfully tested, with no detectable damage to the membrane.

The pH affects fouling due to changes in the surface electrostatic of membrane. More important fouling is observed when pH < 6, due to electrostatic interaction between membrane and foulant substances.

The concomitant effect of humic acids and serum albumin was also studied. A significant fouling increase was observed when using HA and BSA were filtered simultaneously, hinting to a synergistic interaction between these compounds.

SEM and AFM images of clean and fouled membranes were collected to thoroughly characterize the process. Even in the cases when practically complete flux recovery was achieved, the surfaces evidenced differences with the new material, suggesting that a memory effect could be present during the consequent cycles of operation and cleaning.

The results provide evidence to support the use of ceramic ultrafiltration membranes for the treatment of water with high organic matter content. Although significant fouling was observed, we were able to achieve successful cleaning of the membrane with unnoticeable damage to the filter surface.