Persistent organic pollutants (POPs) are carbon-based chemical substances that are resistant to environmental degradation and may not be completely removed through treatment processes. Their ...persistence can contribute to adverse health impacts on wild-life and human beings. Thus, the solar photocatalysis process has received increasing attention due to its great potential as a green and eco-friendly process for the elimination of POPs to increase the security of clean water. In this context, ZnO nanostructures have been shown to be prominent photocatalyst candidates to be used in photodegradation owing to the facts that they are low-cost, non-toxic and more efficient in the absorption across a large fraction of the solar spectrum compared to TiO2. There are several aspects, however, need to be taken into consideration for further development. The purpose of this paper is to review the photo-degradation mechanisms of POPs and the recent progress in ZnO nanostructured fabrication methods including doping, heterojunction and modification techniques as well as improvements of ZnO as a photocatalyst. The second objective of this review is to evaluate the immobilization of photocatalyst and suspension systems while looking into their future challenges and prospects.
The performance of nanofiltration (NF) processes is mainly governed by factors such as the sieving effect (also known as size exclusion) and the Donnan effect (which depends on membrane surface ...charges). This has encouraged the development of new types of NF membranes using various kinds of polyelectrolytes as they have good pore-sealing effects and are able to improve the membrane surface charge density. Manipulation of the pH, supporting electrolyte concentration, type and concentration of polyelectrolyte solutions can significantly vary the characteristics of polyelectrolyte molecules thus improving their electrostatic interactions with the surrounding compounds. This is highly desired and useful when polyelectrolytes are to be incorporated in membrane surface modification as the charges formed can increase the membrane surface charge density, membrane surface coating stability and membrane selectivity. Most of the research discussed in this paper employed the special features of polyelectrolyte molecules to improve the performance of NF membranes in various applications. Various methods have been used to incorporate polyelectrolytes in order to improve NF membrane performance, such as static deposition, dynamic deposition, single layer coating, layer-by-layer (LbL) coating, and so forth. Some of the suitable devices or instruments used for polyelectrolyte-modified membranes are recommended and evaluated. In conclusion, polyelectrolyte-modified membranes offer significant improvements, can be produced in a short period of time, require less energy during membrane modification or fabrication and incur lower production costs. Thus, a full understanding of the factors affecting polyelectrolyte-modified membranes is very much desired and worth further detailed investigation in the near future.
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•Polyelectrolytes provide thin but effective barriers in membrane applications.•Polyelectrolytes can modify membrane surface through UV-initiated modification.•Layer-by-layer adsorption method produced membranes with charged surfaces.•Layer-by-layer adsorption method can be divided into static or dynamic methods.•Polyelectrolytes offer faster modification and better membrane performances.
Polyaniline membrane tends to swell and brittle when it undergoes excessive acid doping upon increasing its conductivity and hydrophilicity properties. Therefore, modification was done by introducing ...polyelectrolyte layers on polyaniline membrane surface with the aim to produce nanofiltration range membrane. Modification of PANI membrane with layer-by-layer polyelectrolyte is a new method for modifying PANI membrane; thus, the effect of doping time with number of polyelectrolyte bilayers with the membrane properties was investigated. From the results, the optimum doping time was 120 min to maximally increase the membrane’s hydrophilicity and conductivity. Next, introduction of polyelectrolyte layers on the membranes’ surface via layer-by-layer coating has found to decrease the porosity of the membranes top layer up to 10 times from ultrafiltration to nanofiltration ranges. Their surface hydrophilicity and negativity has also increased significantly while having a similar conductivity increment trend upon measured in wet condition (similar to water filtration condition). Furthermore, the permeability of the modified PANI membranes before and after back-flushing was performed for membranes cleaning process was seen to have no significant difference. Thus, it can be concluded that the presence of polyelectrolyte multilayers on the membrane surface has successfully formed a stable nano-range membrane with the same desired permeability even after back-flushing.
•Graphene oxide (GO) as nanoplates synthesised using natural graphite powder.•Polysulfone membranes have been embedded with silver nanoparticles on GO nanoplates.•Improved hydrophilicity and ...permeability for all Silver-decorated GOwt% additions.•Membranes exhibit superior antibacterial properties.
Combining silver nanoparticles with graphene oxide (GO) nanoplates may provide a more uniform distribution of silver in membranes. GO nanoplates were synthesised using natural graphite powder according to Hummers method. Silver-decorated GO was prepared by reducing silver nitrate in the presence of aqueous sodium borohydride solution. X-ray diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared (FTIR) spectroscopy were used to investigate the silver-decorated GO. The membranes were fabricated by the wet-phase inversion method and were mixed with different amounts (0.00–1.00wt%) of silver-decorated GO. Membrane properties such as hydrophilicity, pure water flux, and rejection were enhanced for all of the nanohybrid membranes. The optimum amounts of silver-decorated GO for optimum membrane properties was 0.5wt%, which resulted in a lower contact angle as well as higher flux and porosity. The nanohybrid membranes also showed excellent antibacterial properties, which could delay or prevent the formation of biofouling on the membrane surface and provide an opportunity for new applications in the future.
In this work, synergistic effect of solar photocatalysis integrated with adsorption process towards the degradation of Congo red (CR) was investigated via two different approaches using a ...photocatalytic membrane reactor. In the first approach, sequential treatments were conducted through the adsorption by graphene oxide (GO) and then followed by photocatalytic oxidation using Fe-doped ZnO nanocomposites (NCs). In the second approach, however, CR solution was treated by photocatalytic oxidation using Fe-doped ZnO/rGO NCs. These nanocomposites were synthesized by a sol-gel method. The NCs were characterized by X-ray diffraction (XRD), photoluminescence (PL), Fourier transmission infrared (FTIR), ultraviolet–visible (UV-vis) spectroscopy, and field emission scanning electron microscopy (FESEM). It was observed that Fe-doped ZnO could enhance the photoactivity of ZnO under solar light. When Fe-doped ZnO were decorated on GO sheets, however, this provided a surface enhancement for adsorption of organic pollutants. The photocatalytic performances using both approaches were evaluated based on the degradation of CR molecules in aqueous solution under solar irradiation. Nanofiltration (NF) performance in terms of CR residual removal from water and their fouling behavior during post-separation of photocatalysts was studied. Serious flux declined and thicker fouling layer on membrane were found in photocatalytic membrane reactor using Fe-doped ZnO/rGO NCs which could be attributed to the stronger π–π interaction between rGO and CR solution.
A selective nanofiltration membrane was engineered layer-by-layer (LBL) from polydiallyldimethylammonium chloride (PDADMAC) and polysodium-4-styrenesulfoate (PSS) polyelectrolytes. Thickness for 5 ...bilayers of PDADMAC/PSS was 321.00±11nm and showed 73.87±7% rejection of MgSO4, 22.52±2% rejection of NaCl and satisfactory flux of 12.33L.m−2.h−1 at 5bar. Formation of PDADMAC/PSS multilayers using 1.5M of supporting electrolyte produced nanofiltration membrane with the best NaCl/MgSO4 selectivity of 2.97. In addition, the stability of polyelectrolyte multilayers was evaluated through membrane back-flushing. Modified membrane using polyelectrolyte solutions of moderate ionic strength (by adding 1.5M NaCl) displayed the best performance stability in which the MgSO4 retention capability reduced for only 4%.
•Polyelectrolyte solution ionic strength can improve membrane salt selectivity.•Polyelectrolyte layers stability can be affect by solution ionic strength.•PDADMAC and PSS possessed hydrophobic and hydrophilic properties, respectively.•Polyelectrolyte layers stability can be studied by back-flushing the membrane.•Polyelectrolyte layers can smoothen the membrane surface.
Nanomaterials can be incorporated in the synthesis of membrane to obtain mixed-matrix membrane with marked improvement in properties and performance. However, stability and dispersion of the ...nanomaterials in the membrane matrix, as well as the need to use high ratio of nanomaterials for obvious improvement of membrane properties, remain a major hurdle for commercialization. Hence, this study aims to investigate the improvement of polyamide 6,6 membrane properties with the incorporation of silver nanoparticles decorated on graphene oxide (Ag-GO) nanoplates and at the same time focus is given to the issues above. Graphene oxide nanoplates were synthesized using the modified Hummers' method and decorated with silver before embedded into the polyamide 6,6 matrix. Physicochemical characterizations were conducted on both nanoplates and the mixed-matrix Ag-GO polyamide 6,6 membrane. The issues of Ag agglomeration and leaching were not observed, which could be attributed to the decoration of Ag on GO that helped to disperse the nanomaterials and provided a better anchor point for the attachment of Ag nanoparticles. The synthesized membrane showed marked improvement regarding flux (135% increment) and antifouling (40% lower irreversible fouling), which could be ascribed to the more negative charge of membrane surface (-14 ± 6 to -31 ± 3.8 mV) and hydrophilicity (46% enhancement) of the membranes. With minimal embedment of Ag nanoparticles, the membrane showed superior antibacterial property where the E. coli bacteria could not form a single colony on the membrane surface. Overall, the decoration of Ag on GO nanoplates could be a promising approach to resolve the agglomeration and leaching issues as well as reduce the amount of precious Ag in the synthesis of Ag-GO polyamide 6,6 membrane.
Cellulose nanocrystals with various functionalities have received significant interest in recent years due to their wide applications. Elaeis guineensis empty fruit bunches (EFB) have been explored ...by researchers as one of the potential sources for cellulose nanocrystals extraction in recent years. However, cellulose nanocrystals extraction methods using EFB as raw materials are yet to be evaluated based on technical, economic and environmental aspects. Thus, this study aims to assess the effect of EFB pretreatment methods, cellulose nanocrystals isolation methods, with or without any post-treatment, towards the final properties of cellulose nanocrystals. Characterization methods suitable for evaluating the properties of cellulose nanocrystals extracted from EFB are suggested and supported with data from other similar studies. In brief, sulphuric acid hydrolysis exhibits a more significant advantage in cellulose nanocrystals conversion yield and stable suspension than other treatments when empty fruit bunch fibres are used as the cellulose source. By evaluating the benefits or limitations of extraction and characterization methods, future studies on the cellulose nanocrystals extraction from EFB could be further enhanced to prepare a more efficient cellulose nanocrystals extraction from EFB at a commercial scale.
Ultrafiltration is a promising technique to produce value-added products from skimmed coconut milk. Unavoidably, membrane fouling always hinders the membrane performance. In this work, Hermia's ...models were used to investigate the fouling mechanisms. Effects of the molecular weight cut-off of the polysulfone membrane (10, 20 kDa), feed solution temperature (50, 55, 60 degree C) and operating pressure (1.8, 2.0, 2.2, 2.4 bar) towards the membrane fouling were analysed. The results showed that the best fit (R super(2) >= 0.98) of the experimental data to all fitted fouling mechanisms (complete blocking, standard blocking, intermediate blocking and cake formation) occurred for experiments using a 20 kDa polysulfone and 60 degree C feed temperature. All fouling mechanisms were present during the ultrafiltration but dominated by complete blocking, followed by standard, intermediate blocking and cake layer formation. The characteristics of the membrane and feed solution were found to be highly influential on the membrane fouling mechanisms in this study.
Polyethersulphone (PES) membranes have been widely applied in various separation applications such as microfiltration, ultrafiltration and nanofiltration. This has occurred as these membranes are ...easy to form, have good mechanical strength and good chemical stability (resistant to acidic or alkaline conditions) due to the presence of aromatic hydrocarbon groups in the structure. PES membranes are commonly fabricated through the phase inversion method due to the simplicity of the process. However, PES membranes are generally hydrophobic, which usually requires them to be modified before application. In most cases, these methods can reduce the hydrophobicity of the membrane surface and thus reduce membrane fouling during application. This review will further discuss the recently developed UV-induced modifications of PES membranes. The UV-induced grafting method is easy to apply to existing PES membranes, with or without the need for a photo-initiator. Additionally, nanoparticles entrapped in PES membranes subsequently exposed to UV-irradiation have been reported to possess photo-catalytic activity. However, UV-irradiation methods still require special care in order to produce membranes with the best performance.
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