Fouling and biofouling resistant membranes for water treatment processes

Abstract

La tecnología de membrana desempeña un papel esencial en los procesos de purificación del agua debido a sus propiedades de separación eficientes y versátiles. Sin embargo, la mayoría de las membranas comerciales se fabrican a partir de materiales hidrófobos, lo que las hace más susceptibles de sufrir la adsorción de moléculas (fouling) o microorganismos (biofouling) sobre su superficie y/o dentro de sus poros. El ensuciamiento de la membrana constituye uno de los principales problemas operativos, ya que causa el taponamiento de los poros, la reducción del flujo de agua, el aumento del consumo de energía, lo cual reduce la vida útil de la membrana. Así, el objetivo general de esta tesis doctoral es el desarrollo y/o modificación de membranas de ultrafiltración, para minimizar su tendencia a acumular materia orgánica o microrganismos sobre su superficie. Para ello, se utilizaron varias técnicas, como la incorporación de diferentes aditivos hidrófilos orgánicos e inorgánicos o el recubrimiento superficial mediante una capa de nanofibras electrohiladas, evaluándose los efectos que producen en el rendimiento y en las propiedades de las membranas. Los resultados mostraron que las membranas funcionalizadas con nanopartículas metálicas soportadas en fibras de sepiolita o sílice mesoporosa, presentaron una porosidad más elevada y una mayor permeabilidad, evitando la agregación de las nanopartículas en la solución polimérica. La permeabilidad de la membrana mejoró significativamente, presentando mejores propiedades anti-ensuciamiento, todo ello sin comprometer el rechazo orgánico. No se observó lixiviación de partículas metálicas durante su uso, lo que confirma la estabilidad del material. Además, dichas membranas exhibieron una alta actividad antimicrobiana contra bacterias gram-positivas y gram-negativas debido a la acción oligodinámica de los iones de plata y cobre. Alternativamente, la adición del Helux-3316, una poliamidoamina hiperramificada comercial, generó una alta densidad de grupos funcionales cargados positivamente sobre la superficie de las membranas, aumentando su hidrofilicidad y la permeabilidad al agua. Las membranas también mostraron una mejora en su comportamiento anti-ensuciamiento, manifestándose después de filtrar soluciones de suero bovino de albúmina (BSA). Además, las membranas funcionalizadas también mostraron una importante actividad antimicrobiana, atribuida a la interacción de los grupos cargados positivamente de la poliamidoamina con las membranas bacterianas cargadas negativamente, lo que conduce a la desestabilización de su pared celular. Otra técnica utilizada en este trabajo consistió en el recubrimiento de la superficie de las membranas mediante fibras electrohiladas, compuestas por una mezcla del ácido poli(acrílico) (PAA) y alcohol de polivinilo (PVA). Los resultados mostraron que las nanofibras, aumentaron la hidrofilicidad de la membrana, reduciendo la adherencia de materia orgánica sobre su superficie, todo ello sin afectar la permeabilidad y el rechazo de proteínas. Además, el recubrimiento de las nanofibras mostró una considerable actividad antimicrobiana, particularmente para la bacteria S. aureus, atribuida al efecto quelante de PAA con los cationes divalentes, especialmente el Ca2+ que estabilizan la pared celular bacteriana. Los resultados de este trabajo son relevantes para demostrar que las técnicas de modificación descritas anteriormente mejoran con eficacia el rendimiento de las membranas de ultrafiltración, reduciendo la adhesión de materia orgánica y microorganismos sobre su superficie.

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 Abstract 2 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 (ATR-FTIR) spectroscopy. The hydrophilicity of membranes was determined by measuring water contact angles and surface charge by surface ζ-potential Abstract 3 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 Abstract 4 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.