Gábor Veréb

The present study aimed to investigate the interaction between strontium titanate photocatalysts and alkaline soil (solonetz) soil solutions. For this purpose, one commercially available and several synthesized strontium titanates were considered. The photocatalytic activity and material characteristics were assessed before and after immersion in the soil solutions. All samples were characterized by X-ray diffractometry (XRD), infrared spectroscopy (IR), diffuse reflectance spectroscopy (DRS), and scanning electron microscopy (SEM). After interaction with the soil solution, most of the samples became more active for phenol degradation. It was found that the crystalline structure of each sample was preserved, while the primary crystallite sizes decreased after both phenol degradation and immersion in solonetz soil solutions. Moreover, the surface of all synthesized nanostructures was covered by organic residues from either the soil solution or the by-products of phenol degradation. This was also visible from the DR spectra, as an intensive color change was observed. The bandgaps of most samples were also changed, except for the commercial material. The results imply that it is important to investigate the ecofriendly nature of any photocatalytic material, as it tends to influence the surrounding environment. This is important, as solar photocatalysis is rising among the possible methods for water purification and disinfection.

Ultrafiltration is a key technology for treating dairy wastewater; however, its efficiency is often hindered by critical challenges such as membrane fouling and concentration polarization. To mitigate these issues, one effective approach involves optimizing operational parameters to enhance system performance and longevity. In this study the influence of key operational parameters on the performance of a lab-scale low-pressure ultrafiltration membrane system for treating dairy wastewater model were investigated. The optimization focused on three critical factors: transmembrane pressure (TMP), stirring speed, and membrane molecular weight cut-off (MWCO). Key performance metrics, including permeate flux, membrane retention efficiency, and total, reversible, and irreversible membrane resistances, were analyzed. The results showed that the optimal conditions were identified using a 20 kDa polyethersulfone membrane, with a TMP of 0.3 MPa and a stirring speed of 400 rpm. Statistical analysis was conducted to further refine and validate the optimization of these parameters.

This comparative study investigates the modification of polyvinylidene fluoride (PVDF) membranes with different nanoparticles (TiO₂ or TiO₂-based composites containing BiVO₄ and/or CNT), using three distinct methods (blending, coating, and grafting) and polyvinylpyrrolidone (PVP). The objective was to enhance the photocatalytic and filtration performance for the separation of oil-in-water emulsions. Regarding the UV activity, the PVDF-TiO₂/CNT/PVP-coated membrane presented the best performance. Overall, the addition of 2 wt.% CNT to the TiO₂ notably enhanced the photocatalytic activity of the membranes for both UV and visible irradiations. Meanwhile, the presence of 2 wt.% BiVO₄ was beneficial only for photocatalysis under visible light irradiation. Regarding the filtration of the oil-in-water emulsions, 2 wt.% CNT or BiVO₄ addition resulted in the highest fluxes in the series of the PVDF-TiO₂-grafted membranes. The presence of pore former PVP led to relatively high fluxes and photocatalytic activities for all series. Regarding the modification methods, coated membranes showed the highest photocatalytic efficiency and lowest fluxes. Grafted membranes showed relatively high photocatalytic efficiencies and the best filtration performances.

Polyvinylidene fluoride (PVDF) membranes were coated with TiO₂ and TiO₂-Ag to enhance their efficiency for oil-in-water emulsion separation. The photocatalytic activities of the two modified membranes and their filtration performances were compared in detail. The significantly enhanced photocatalytic activity of the TiO₂-Ag composite was proved using a methyl orange (MO) solution (c = 10⁻⁵ M) and a crude oil emulsion (c = 50 mg·L⁻¹). The TiO₂-Ag-coated membrane reduced the MO concentration by 87%, whereas the TiO₂-modified membrane reached only a 46% decomposition. The photocatalytic reduction in the chemical oxygen demand of the emulsion was also ~50% higher using the TiO₂-Ag-coated membrane compared to that of the TiO₂-coated membrane. The photoluminescence measurements demonstrated a reduced electron/hole recombination, achieved by the Ag nanoparticle addition (TiO₂-Ag), which also explained the enhanced photocatalytic activity. A significant improvement in the oil separation performance with the TiO₂-Ag-coated membrane was also demonstrated: a substantial increase in the flux and flux recovery ratio (up to 92.4%) was achieved, together with a notable reduction in the flux decay ratio and the irreversible filtration resistance. Furthermore, the purification efficiency was also enhanced (achieving 98.5% and 99.9% COD and turbidity reductions, respectively). Contact angle, zeta potential, scanning electron microscopy (SEM), and atomic force microscopy (AFM) measurements were carried out to explain the results. SEM and AFM images revealed that on the TiO₂-Ag-coated membrane, a less aggregated, more continuous, homogeneous, and smoother nanolayer was formed due to the ~50% more negative zeta potential of the TiO₂-Ag nanocomposite compared to that of the TiO₂. In summary, via Ag addition, a sufficiently hydrophilic, beneficially negatively charged, and homogeneous TiO₂-Ag-coated PVDF membrane surface was achieved, which resulted in the presented advantageous filtration properties beyond the photocatalytic activity enhancement.

Six different TiO₂/CNT nanocomposite-coated polyvinylidene-fluoride (PVDF) microfilter membranes (including –OH or/and –COOH functionalized CNTs) were evaluated in terms of their performance in filtering oil-in-water emulsions. In the early stages of filtration, until reaching a volume reduction ratio (VRR) of ~1.5, the membranes coated with functionalized CNT-containing composites provided significantly higher fluxes than the non-functionalized ones, proving the beneficial effect of the surface modifications of the CNTs. Additionally, until the end of the filtration experiments (VRR = 5), notable flux enhancements were achieved with both TiO₂ (~50%) and TiO₂/CNT-coated membranes (up to ~300%), compared to the uncoated membrane. The irreversible filtration resistances of the membranes indicated that both the hydrophilicity and surface charge (zeta potential) played a crucial role in membrane fouling. However, a sharp and significant flux decrease (~90% flux reduction ratio) was observed for all membranes until reaching a VRR of 1.1–1.8, which could be attributed to the chemical composition of the oil. Gas chromatography measurements revealed a lack of hydrocarbon derivatives with polar molecular fractions (which can act as natural emulsifiers), resulting in significant coalescent ability (and less stable emulsion). Therefore, this led to a more compact cake layer formation on the surface of the membranes (compared to a previous study). It was also demonstrated that all membranes had excellent purification efficiency (97–99.8%) regarding the turbidity, but the effectiveness of the chemical oxygen demand reduction was slightly lower, ranging from 93.7% to 98%.

Bismuth oxybromides were prepared via a solvothermal method by applying different urea amounts during synthesis. The effects of the urea ratio on the morpho–structural properties and photocatalytic activity of the samples were investigated. X-ray diffraction, diffuse reflectance spectroscopy, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and surface tension measurements were carried out to characterize the samples. Their photoactivity was evaluated by the photocatalytic degradation of rhodamine B and ibuprofen under UV and visible light irradiations. The urea ratio notably influenced morphology, particle size distribution, and photoactivity. However, it only had a limited effect on the crystalline composition, primary crystallite size, and band gap of bismuth oxybromides. The formation of Bi-based complexes and degraded urea-based products were observed, which were deduced to influence band gap energies and hence, photoactivity. Predominantly, samples prepared at low urea ratios proved to be the best for both rhodamine B and ibuprofen degradations under both irradiations.

Membrane filtration is an effective technique for separating micro- and nano-sized oil droplets from harmful oil-contaminated waters produced by numerous industrial activities. However, significant flux reduction discourages the extensive application of this technology; therefore, developing antifouling membranes is necessary. For this purpose, various titanium dioxide/carbon nanotube (TiO₂/CNT) nanocomposites (containing 1, 2, and 5 wt.% multi-walled CNTs) were used for the modification of polyvinylidene fluoride (PVDF) ultrafilter (250 kDa) membrane surfaces. The effects of surface modifications were compared in relation to the flux, the filtration resistance, the flux recovery ratio, and the purification efficiency. TiO₂/CNT_2% composite modification reduced both irreversible and total filtration resistances the most during the filtration of 100 ppm oil emulsions. The fluxes were approximately 4–7 times higher compared to the unmodified PVDF membrane, depending on the used transmembrane pressure (510, 900, and 1340 L/m²h fluxes were measured at 0.1, 0.2, and 0.3 MPa pressures, respectively). Moreover, the flux recovery ratio (up to 68%) and the purification efficiency (95.1–99.8%) were also significantly higher because of the surface modification, and the beneficial effects were more dominant at higher transmembrane pressures. TiO₂/CNT_2% nanocomposites are promising to be applied to modify membranes used for oil–water separation and achieve outstanding flux, cleanability, and purification efficiency.

This study aimed to investigate the performance of composite photocatalytic membranes fabricated by incorporating multiple nanoparticles (TiO₂, carbon nanotubes, BiVO₄) into polyvinylidene fluoride membrane material for real dairy wastewater treatment. The composite photocatalytic membranes exhibited superior antifouling behavior, lower filtration resistance, better flux, and higher flux recovery ratio than the pristine membrane. Salinity, pH, and lactose concentration are determinant factors that affect filtration resistance and rejection performance during the ultrafiltration of dairy wastewater. Generally, higher irreversible and total resistances and slightly lower chemical oxygen demand (COD) rejections were found at higher salinity (expressed by electric conductivity values of >4 mS/cm) than lower salinity (2, carbon nanotubes, and BiVO₄) showed the best regeneration performance, exceeding that of the pristine membrane by 30%.

The photocatalytic performances of doped and non-doped TiO₂ photocatalysts (TiO₂-s) were compared under solar and various types of artificial irradiation using phenol as a model contaminant. Non-doped (mainly anatase phase) TiO₂-s had significantly higher photocatalytic efficiency than highly visible-light-active TiO₂-s under natural solar irradiation. To explain these unexpected results, we measured the wavelength dependence of photocatalytic efficiency at six different wavelength ranges (λ = 300–650 nm). For this purpose, UV fluorescence tubes and five LED lights of different colors (violet, blue, green, yellow, and red) were used to activate the photocatalysts. The photon fluxes of the irradiation were measured, and apparent quantum yields were calculated for all irradiation conditions. The highest apparent quantum yield was 1.43% for our own TiO₂ (prepared via flame hydrolysis) under UV irradiation. However, apparent quantum yields were significantly lower (by 1–2 orders of magnitude) in the visible range, even for the most visible-light-active TiO₂.