Fouling and biofouling resistant membranes for water treatment processes

Abstract

The demand for new water resources has increased worldwide due to the rapid growth population, socio-economic development, and changing consumption patterns. This situation, coupled with rising water scarcity, generates a need for improved techniques to purify contaminated waters. Membrane technology plays an important role in water purification processes due to its efficient and versatility separation properties. However, most commercial membranes are prepared from hydrophobic materials, which makes them more susceptible to suffer the adsorption or deposition of molecules over their surface or inside their pores. This phenomenon, commonly termed as fouling, can be classified in organic, inorganic or biological fouling depending on the nature of the components. Organic fouling which is caused by the presence of organic compounds, such as polysaccharides or proteins. Inorganic fouling refers to the deposition of inorganic materials like salts or metal oxides and biofouling designates the formation of biofilms due to the attachment and growth of microorganisms on the membrane surface. Foulants can deposit within membrane pores or form a cake layer on the surface. Bacterial biofilms are complex microbial communities, embedded in a self-produced polymer matrix of extracellular polymer substances (EPS) mainly composed of water, polysaccharides, proteins and nucleic acids aimed to protect bacteria in adverse conditions. Membrane (bio)fouling is one of the major operational problems in membrane processes because it causes a decrease in permeation flux, increases energy consumption and operational costs, and reduces membrane lifespan. Due to the adverse impact of fouling, different physical and chemical cleaning processes have been proposed to prevent or reduce membrane fouling. However, these methods are not sufficiently effective and, new strategies need to be investigated for the purpose of effectively mitigating membrane fouling. The aim of this Doctoral Thesis was to investigate membranes modifications techniques to enhance permeability, reduce fouling and the accumulation of microorganisms on the membrane surface. To achieve this goal, membranes were prepared by the non-solvent induced phase inversion method, followed by a physical and chemical characterization and then, membranes were tested with different biofilm-forming bacterial strains to assess the anti-biofouling behaviour of the newly developed materials. Several techniques are used for this purpose, including blending hydrophilic additives or surface coating by an electrospun nanofiber layer to produce new composite ultrafiltration membranes with enhanced anti-(bio)fouling behaviour. Blending organic or inorganic additives into the casting solution is an important approach to reduce membrane hydrophobicity and improve water filtration performance. Electrospun nanofibers are produced by the electrospinning system which is a versatile technique that utilizes a high voltage electric field to produce polymer fibres below the nanoscale from a polymer solution. Nanofibers showed several advantages such as a high surface area to volume ratio or tuneable porosity, contributing to enhance membrane fouling resistance. The new composite ultrafiltration membranes were characterized using the following microscopy techniques: membranes morphology by Scanning Electron Microscopy (SEM), surface porosity using a Field Emission Scanning Electron Microscopy (FE-SEM), and elemental analyses using SEM combined with Energy Dispersive X-ray (SEM-EDX). The chemical composition was analysed using Attenuated Total Reflectance Fourier Transform Infrared (ATRFTIR) spectroscopy. The hydrophilicity of membranes was determined by measuring water contact angles and surface charge by surface ζ-potential measurements. ICP-MS analyses from metal-loaded membranes were performed to assess the possible release of nanoforms during membrane use. Membrane fouling was studied using bovine serum albumin (BSA) as a model of protein organic foulant. The intrinsic, reversible and irreversible fouling resistances, as well as the flux recovery ratio and solute rejection, were analysed to explore the effect of fouling on the membrane permeation performance. The anti-biofouling behaviour of the prepared membranes was tested against two different bacterial strains Escherichia coli (CECT 516, strain designation ATCC 8739) and Staphylococcus aureus (CECT 240, strain designation ATCC 6538P). The antimicrobial activity was assessed by counting colony-forming units. Biofilm formation was studied using SEM and confocal micrographs, biofilms were stained with FilmTracer FM 1-43 to visualize the surface of colonized membranes. Finally, bacterial viability was examined using the nucleic-acid stains SYTO 9 and propidium iodide (PI), detecting cell wall damage. The effects of adding different hydrophilic additives were evaluated in this Thesis. Nanoparticles supported in sepiolite fibres or mesoporous silica displayed a good dispersion in casting solutions and, hence, in the polymer matrix. The results showed that the membranes functionalized with metal nanoparticles exhibited higher porosity and better pore interconnectivity. Membrane permeability was significantly enhanced with improved antifouling properties without compromising organic rejection. No leaching of metal particles was observed during use, confirming the stability of composite membranes. Metal-loaded membranes exhibited high antimicrobial activity against gram-positive and gram-negative bacteria due to the oligodynamic action of silver and copper ions. Alternatively, the addition of hyperbranched polyamidoamine nanomaterial, Helux-3316, generate a high density of positively charged functional groups at the membrane fluid-interface, increasing membrane hydrophilicity and water permeability. Functionalized membranes displayed antifouling behaviour revealed after filtering BSA solutions, with reduced irreversible fouling. Moreover, membranes showed an important anti-biofouling functionality due to antimicrobial activity explained by the interaction of positively charged moieties with negatively charged cell envelopes. Other technique used in this work was the coating of membrane surface by on-top electrospinning a layer of nanofibers made by a blend of poly (acrylic acid) and poly (vinyl alcohol) onto polysulfone membranes. The results showed that electrospun layers increased membrane hydrophilicity and reduced organic fouling without affecting permeability and protein rejection performance. Moreover, the nanofibers coating showed a considerable antimicrobial activity, particularly for the bacterium S. aureus, attributed to the chelating effect of PAA on the divalent cations stabilizing bacterial cell envelopes. The results are relevant to demonstrate that the previously described modification techniques effectively improve the performances of ultrafiltration membranes.