Ana Sofia Figueiredo

Membrane processes are a reality in a wide range of industrial applications, and efforts to continuously enhance their performance are being pursued. The major drawbacks encountered are related to the minimization of polarization concentration, fouling, and biofouling formation. In this study, silver nanoparticles were added to the casting solutions of cellulose acetate membranes in order to obtain new hybrid membranes that present characteristics inherent to the silver nanoparticles, namely antibacterial behavior that leads to biofouling reduction. A systematic study was developed to assess the effect of ionic strength, membrane polymeric structure, and silver nanoparticle incorporation on the cellulose acetate (CA) membrane surface charge. Surface charge was quantified by streaming potential measurements and it was correlated with BSA permeation performance. CA membranes were prepared by the phase-inversion method using three casting-solution compositions, to obtain membranes with different polymeric structures (CA400-22, CA400-30, CA400-34). The nanocomposite CA/silver membranes (CA/Ag) were prepared through the incorporation of silver nanoparticles (0.1 and 0.4 wt% Ag) in the casting solutions of the membranes. To evaluate the electrolyte concentration effect on the membranes zeta potential and surface charge, two potassium chloride solutions of 1 mM and 5 mM were used, in the pH range between 4 and 9. The results show that the zeta-potential values of CA/Ag membranes were less negative when compared to the silver-free membranes, and almost independent of the silver content and the pH of the solution. The influence of the protein solution pH and the protein charge in the BSA solutions permeation was studied. The pH conditions that led to the lower permeate fluxes were observed at the isoelectric point of BSA, pH = 4.8.

The antibacterial properties of cellulose acetate/silver nanoparticles (AgNP) ultrafiltration membranes were correlated with their integral asymmetric porous structures, emphasizing the distinct features of each side of the membranes, that is, the active and porous layers surfaces. Composite membranes were prepared from casting solutions incorporating polyvinylpyrrolidone-covered AgNP using the phase inversion technique. The variation of the ratio acetone/formamide and the AgNP content resulted in a wide range of asymmetric porous structures with different hydraulic permeabilities. Comprehensive studies assessing the antibacterial activity against Escherichia coli (cell death and growth inhibition of bacteria in water) were performed on both membrane surfaces and in E. coli suspensions. The results were correlated with the surface chemical composition assessed by XPS. The silver-free membranes presented a generalized growth of E. coli, which is in contrast with the inhibition patterns displayed by the membranes containing AgNP. For the surface bactericide test, the growth inhibition depends on the accessibility of E. coli to the silver present in the membrane; as the XPS results show, the more permeable membranes (CA30 and CA34 series) have higher silver signal detected by XPS, which is correlated with a higher growth inhibition. On the other hand, the inhibition action is independent of the membrane porous structure when the membrane is deeply immersed in an E. coli inoculated suspension, presenting almost complete growth inhibition.

The winemaking process generates a large amount of residues such as vine shots, stalks, grape pomace, and wine lees, which were only recently considered for exploitation of their valuable compounds. The purpose of this work was to investigate the performance of nanofiltration for the recovery of phenolic compounds, with bioactive capacity like antioxidant, from red grape pomace extract. Four membranes were compared in this study—three cellulose acetate (CA series: lab-prepared by phase inversion) and one commercial (NF90). All membranes were characterized for their hydraulic permeability and rejection coefficients to reference solutes like saccharose, glucose, raffinose, polyethylene glycol, sodium chloride, and sodium sulfate. Permeation flowrates and rejection coefficients towards total phenolics content, antioxidant activity, proanthocyanidins, glucose and fructose were measured in the nanofiltration of grape pomace extract using selected operating conditions. Among the investigated membranes, the CA400-22 exhibited the highest permeate flux (50.58 L/m² h at 20 bar and 25 °C), low fouling index (of about 23%), the lowest rejection coefficients towards the reference solutes and the best performance in terms of separation between sugars and phenolic compounds. Indeed, the observed rejections for glucose and fructose were 19% and 12%, respectively. On the other hand, total phenolics content and proanthocyanidins were rejected for 73% and 92%, respectively.

Mixed matrix asymmetric membranes were prepared by the addition of silver nanoparticles to cellulose acetate/acetone/formamide casting solutions with ratios acetone/formamide varying from 1.44 to 2.77 to prepare ultrafiltration/nanofiltration membranes covering a wide range of hydraulic permeabilities. Binding of the silver nanoparticles to the polymer matrix is revealed through comparison of the FTIR spectra of the cellulose acetate and the Ag/cellulose acetate membranes. In the later, there is a decrease of the ratio between the bands intensities at 2,000–2,500 cm⁻¹. Membrane surface charge of the mixed matrix membranes varies with the pore size and pH, and when compared with cellulose acetate membranes there is a decrease of the negative surface charge densities. The silver nanoparticles in all mixed matrix membranes results in an enhancement of the hydraulic permeabilities, ranging from 10.8 kg m⁻² h⁻¹ bar⁻¹ to 67.1 kg m⁻² h⁻¹ bar⁻¹. POLYM. COMPOS., 38:32–39, 2017. © 2015 Society of Plastics Engineers

Cellulose acetate (CA)–silver (Ag) nanocomposite asymmetric membranes were prepared via the wet‐phase inversion method by dispersing polyvinylpirrolydone‐protected Ag nanoparticles in the membrane casting solutions of different compositions. Silver nanoparticles were synthesized ex situ and added to the casting solution as a concentrated aqueous colloidal dispersion. The effects of the dispersion addition on the structure and on the selective permeation properties of the membranes were studied by comparing the nanocomposites with the silver‐free materials. The casting solution composition played an important role in the adequate dispersion of the silver nanoparticles in the membrane. Incorporation of nanoscale silver and the final silver content resulted in structural changes leading to an increase in the hydraulic permeability and molecular weight cut‐off of the nanocomposite membranes. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41796.