Preparation of mixed matrix membranes for water treatment

Abstract

The treatment of wastewater and its reuse is a very important topic in industrial processes. This because not only avoids drawing on natural resources, but also enables a significant reduction in the amount of wastewater discharged into the natural environment. Wastewater can also be used for various purposes where drinking water quality is not mandatory, including agricultural irrigation, the cleaning of industrial equipment, the watering of green spaces, and street maintenance, etc. In fact, the water reuse has become essential in all areas in the world that suffer from water shortages [1]. Different methods are used for wastewater treatment. These processes can be to divide in: primary, secondary and tertiary treatment. Primary treatment (screening, filtration, centrifugation, sedimentation, coagulation, gravity and flotation method) includes preliminary purification processes of a physical and chemical nature while secondary treatment deals with the biological treatment of wastewater. In tertiary treatment process wastewater is converted into good quality water that can be used for different types of purpose, i.e. drinking, industrial, medicinal etc. supplies [2]. The complexity of industrial processes, the variety of pollutants and the limitation of a single operation, has led to the need for more complex processes and especially to a combination of processes. Membranes technologies falls on the tertiary water treatment technologies and are actually the most effective separation processes and they are still in rapid development creating new prospects of their applications in clean technologies [3]. The utilization of membrane operations as hybrid systems, i.e. in combination with other conventional techniques or integrated with different membrane operations is considered the way forward for more rationale applications [4]. The possibilities of redesigning various industrial cycles by combining various membrane operations have been studies and in some case realized with a low environmental impact and a low energy consumption [5]. Different processes can be used in various steps of a hybrid system, depending from the size of the pollutants to be removed. Microfiltration (MF) and ultrafiltration (UF) membrane processes, can be used as pre-treatment, while nanofiltration (NF) and reverse osmosis (RO) can used in the final step of the integrated system to remove particles with smaller dimensions (Chapter 1) The membranes have different morphological characteristics that affect their performance. The study of all the conditions which modulate these characteristics is a crucial point in the choice of membranes to be used in the various separation processes. Therefore, it is important to investigate about new materials and new types of membranes, like as mixed matrix membrane (MMM). MMM is a heterogeneous membrane consisting of inorganic fillers embedded in a polymeric matrix and can be made into flat sheets and hollow-fiber. Nevertheless, the selection of membrane configuration is greatly dependent on the application and therefore the production of MMMs in useful configuration is undoubtedly a crucial point in membrane development [6]. Also, the selection of inorganic fillers depends of desired membrane performance and their use. More attention was focus on the interesting characteristic of carbonanotubes (CNTs) (chapter 2). CNTs themselves have remarkable electrical, thermal, and mechanical properties. These nanotubes have the structure of a rolled-up graphene sheet with smaller diameter. Multiwalled carbonanotube (MWCNTs) were used to prepare MMMs for wastewater treatment. Different compounds, as additives in the polymeric membranes were used in high percentage; in the case of MWCNTs was observed as a low amount can change the membrane morphologies, mechanical and transport properties. A crucial point was the choice of membrane materials. Two type, hydrophilic poly(imide) (PI) and hydrophobic poly(vinylidenfluoride) (PVDF) were choose for membrane materials to produce MMMs. Another important point in this study was the use of functionalized MWCNTs that provide a good dispersion in the casting solution first, and in the polymeric matrix after phase separation. The main limitation in the use of CNTs is their poor dispersion in the main solvents used for the preparation of membranes. The functionalization has been proven an efficient method to overcome this limitation improving the compatibility with the polymer matrix. The presence of polar groups on the carbon nanotubes can reduces their tendency to aggregate by van der Waals interactions, while forming hydrogen bonds and electron donor/acceptor interactions with the polymer. Low percentages of CNTs were used for the preparation of membranes. These percentages were sufficient to improve better performance to modified membranes. PI was select as polymeric materials because combine easy processability in the form of membranes, with a high chemical and thermal stability, over a wide range of operative conditions. Three different PI polymers were used to prepared porous asymmetric membrane by non-solvent induced phase separation (NIPS): a homopolymer (Matrimid) and two co-polymers (Lenzing P84 and Torlon). The effect of membrane preparation conditions on the membrane morphology and transport properties, were investigate. Moreover, mixed matrix based on co-polyimide P84 and functionalized multiwalled carbon nanotubes (oxidized and aminated MWCNTs) were prepared. The different polymeric membranes were compared in the rejection of organic dyes, as model of organic pollutant present in wastewater (chapter 3). To investigate about the influence of functional groups on the MWCNTs for their interaction with polymeric matrix, three different type of functionalized MWCNTs (oxidized, amined and aminated) were dispersed also in polymeric hydrophobic PVDF membranes. PVDF was selected as polymeric materials of its outstanding properties: excellent chemical resistance and hydrolytic stability; high mechanical strength and stability over a broad pH range; polymorphism (main crystalline phases are: α, β, γ, δ and ε) [7]. The aim was to tailor the interactions with the polymeric matrix in order to realize high performing composite film with improved performance. Bovine serum albumin (BSA) protein was select as compound to evaluate the membrane performance. In particular, the antifouling properties and the permeation flux of mixed matrix membranes, were evaluate as well as thermal and structural and mechanical properties (chapter 4).