Multifunctional fly ash-based GO/geopolymer composite membrane for efficient oil-water separation and dye degradation

Membrane fouling and separation materials with low cost and high efficiency are challenges for membrane separation technology in wastewater treatment. Superhydrophilic and underwater superoleophobic membranes show broad application prospects in oily wastewater treatment because of their high permeability, selectivity, and antifouling performance; however, they are generally ineffective for organic pollutant molecules. In this study, a novel graphene oxide (GO)/geopolymer composite membrane with superhydrophilic and underwater superoleophobic characteristics was prepared by dipping a mixed slurry of GO and fly ash-based geopolymer onto a stainless steel mesh via a facile self-assembly process. The results show that GO could adjust the hydrophilicity and water flux of composite membranes. The composite membrane containing 0.4 wt% GO (4GO/GCM) had the best hydrophilic, water flux of 1363 kg/(m²·h), and high separation efficiencies (≥98.2%) for oil-water mixtures and oil-in-water emulsions under gravity-driven. In addition, the 4GO/GCM sample exhibited excellent stability under harsh conditions, including hot water and strong acid, alkali, and salt solutions. Importantly, the sample derived from fly ash exhibited unique photocatalytic degradation performance for organic dye molecules under simulated solar-light irradiation. Thus, it is believed to this strategy has substantial potential for high-value utilization of fly ash and the sustainable treatment of oily and dye wastewater.     

A TiO2 modified whisker mullite hollow fiber ceramic membrane for high-efficiency oil/water emulsions separation

Design and preparation of membranes with ultrahigh separation performance and antifouling property for oil-in-water (O/W) emulsions remains challenging. In this study, a high flux mullite/TiO₂ ceramic composite membrane was prepared via multi-precipitation of TiO₂ on a whisker mullite hollow fiber support synthesized by combining phase inversion and high-temperature sintering techniques. The results showed that the generated whisker mullite structure improved the permeation flux, and the micro-nano structured TiO₂ functional layer endowed the membrane surface with superhydrophility and stability. The retention of the optimal composite membrane (M20T13) that was soaked in the titanium solution 20 times for 13 min each time for the O/W emulsions like n-hexane, toluene and engine oil maintained over 98 %, and the flux after 6 h filtration was 668.34 L·m⁻²·h⁻¹, 487.25 L·m⁻²·h⁻¹ and 258.66 L·m⁻²·h⁻¹, respectively, much higher than that of the optimal substrate (F3A1, mass ratio of fly ash: Al₂O₃ = 3:1). Moreover, the flux recovery rate of M20T13 was much higher than that of F3A1 after chemical backwashing. This work manifests great potential in O/W treatment fields.     

Enhancement of super-hydrophilic/underwater super-oleophobic performance of ceramic membrane with TiO2 nanowire array prepared via low temperature oxidation

A gradient porous ceramic membrane with surface super-hydrophilic and underwater super-oleophobic performance was prepared by combining hydrogel directional freezing method and low temperature oxidation process. The effects of solid contents and sintering temperature on the ceramic membrane matrix were examined. The reaction time and synthesis temperature on the TiO₂ nanowire array were also evaluated. In addition, the related effects on pore size distribution, permeation flux, contact angle, and oil-in-water emulsion separation were systematically investigated. The ceramic membrane matrix pore size changed from 0.5 μm to 25 μm gradually, indicating the gradient structure controlled by the growth of ice. The super-hydrophilic and underwater super-oleophobic performance of ceramic membrane surface was obtained with surface modification by TiO₂ nanowire array, and the surface water contact angle and underwater oil contact angle were less than 5° and over 158°, respectively. The bonding strength between TiO₂ nanowire and ceramic membrane matrix was high enough to withstand ultrasonic waves. The ceramic membrane modified with TiO₂ nanowire array was used for 1000 ppm diesel oil-in-water emulsion separation, and the stable separation efficiency and flux were about 97% and 100–200 L/(m² h bar) even after 10 filtration cycles.     

Preparation and properties of anorthite-based ceramics by using metallurgical solid waste and fly ash

A large amount of metallurgical solid waste accumulation poses a serious threat to the environment. Study on synergistic reinforcement of synthetic process of metallurgical solid waste-based ceramics with fly ash is of great significance in reducing environmental pollution and resource utilization. A metallurgical solid waste-based ceramic used as building ceramic was developed with the erosion part of used MgO–C bricks and fly ash as main raw materials, and the amount of solid waste added to the prepared ceramics was at least 60 wt% and up to 90 wt%. The effects of fly ash content and sintering temperature on the crystalline phase transitions, morphologies, and the main physical and mechanical properties of ceramics were investigated by X-ray diffraction, scanning electron microscopy, and mechanical testing. The results show that the obtained ceramics presented maximum bending strength and minimum water absorption, 80.14 MPa and 5.04%, respectively, when the raw material proportions were the erosion part of used refractories accounted for 60 wt%, fly ash 20 wt%, pyrophyllite 10 wt%, and quartz sands 10 wt%, and the process parameters were the sintering temperature 1150°C, sintering time 120 min, and molding pressure 15 MPa.     

Preparation and characterization of the one-piece wall ceramic board by using solid wastes

For the purpose of building energy-saving, a novel one-piece wall ceramic board was prepared by using fly ash and ceramic waste as the main raw materials for its matrix part and foam part, respectively. The effects of raw material composition, sintering temperature on the macro and micro properties were systematically investigated. The optimum parameter for the matrix part was obtained at 1220 °C with 70 wt% fly ash and 4 wt% quartz, while that for the foam part was 1220 °C with 97 wt% ceramic waste and 3 wt% silicon carbide. For the matrix sample, the highest rupture modulus reaches 53.97 MPa, and the corresponding water absorption capacity and thermal conductivity are 1.08% and 0.54396 W/(m K), respectively. For the foam part, the best bulk density and thermal conductivity are 443 kg/m³ and 0.10528 W/(m K), respectively. Subsequently, the optimal matrix and foam samples were introduced into the co-fired process (1220 °C), and the results show that the new method for the preparation of one-piece wall ceramic board was fully acceptable. Furthermore, the simulated results indicate that the proposed one-piece wall ceramic board can efficiently reduce the thermal bridges and exerts excellent energy conservation effect.     

One-step preparation of asymmetric ceramic membranes based on sea urchin-like mullite whisker skeletons and their oil-water separation properties

In this work, novel sandwich-type asymmetric ceramic microfiltration membranes with a sea urchin-like mullite whisker skeleton were prepared one step. Their structural properties and oil-water separation performance were investigated. The results show that after sintering at 1400 °C, the prepared membrane possesses good hydrophilic, underwater oleophobic, and anti-fouling properties. During the continuous separation of a 300 mg/L oil-in-water emulsion, a maximum stable flux of 267 L·m⁻²·h⁻¹ was achieved without membrane cleaning. After chemical cleaning and simple physical cleaning, the membranes recovered to a steady flux of 397 L·m⁻²·h⁻¹ and 305 L·m⁻²·h⁻¹, respectively, and maintained a 95% oil rejection. The good underwater oleophobicity and selective permeability brought about by the flat-lying whiskers on the top surface, coupled with the efficient water channels between the sea urchin-like structures inside the membrane, are considered to be the main reasons for its improved separation characteristics over conventional low-cost ceramic membranes.     

High-flux whisker layer ceramic membrane prepared by gel spin-coating method for low-pressure oil/water emulsion filtration

Insufficient permeability and membrane fouling significantly influence the efficiency of ceramic microfiltration (MF) membranes in oil/water emulsion treatment. In this study, a high-flux whisker layer ceramic MF membrane with super-hydrophilicity was successfully fabricated through gel-spin coating method and a low-temperature oxidation method, which was used to separate oil/water emulsion. The effects of the whisker layer and surface wettability were systematically investigated, and the mechanism of in-situ gelling and pore size distribution was proposed. The super-hydrophilic ceramic MF membrane with an average pore size of 250 nm exhibited a high gas flux of 934 m³/(m²·h·bar) and excellent pure water flux of 9754 L/(m² h bar). Even after a long-term circulating filtration process, the super-hydrophilic ceramic MF membrane still maintained a high water flux of over 50 L/(m²·h) at a transmembrane pressure of 5 KPa during the treatment of oil-in-water emulsion with a concentration of 1000 mg/L. Overall, the developed ceramic MF membrane demonstrated high permeability and excellent anti-fouling performance, making it a promising candidate for oil/water emulsion wastewater treatment.     

Treatment of oily wastewater using low cost ceramic membrane: Comparative assessment of pore blocking and artificial neural network models

This work addresses the performance and modeling of the separation of oil-in-water (o/w) emulsions using low cost ceramic membrane that was prepared from inorganic precursors such as kaolin, quartz, feldspar, sodium carbonate, boric acid and sodium metasilicate. Synthetic o/w emulsions constituting 125 and 250 mg/L oil concentrations were subjected to microfiltration (MF) using this membrane in batch mode of operation with varying trans-membrane pressure differentials (ΔP) ranging from 68.95 to 275.8 kPa. The membrane exhibited 98.8% oil rejection efficiency and 5.36 × 10⁻⁶ m³/m² s permeate flux after 60 min of experimental run at 68.95 kPa trans-membrane pressure and 250 mg/L initial oil concentration. These experimental investigations confirmed the applicability of the prepared membrane in the treatment of o/w emulsions to yield permeate streams that can meet stricter environmental legislations (<10 mg/L). Subsequently, the experimental flux data has been subjected to modeling study using both conventional pore blocking models as well as back propagation-based multi-layer feed forward artificial neural network (ANN) model. Amongst several pore blocking models, the cake filtration model has been evaluated to be the best to represent the fouling phenomena. ANN has been found to perform better than the cake filtration model for the permeate flux prediction with marginally lower error values.     

Recycling of lime mud and fly ash for fabrication of anorthite ceramic at low sintering temperature

Lime mud is a kind of waste generated during causticization reaction of an alkali recycling process in paper industry. Lime mud and fly ash were reused as raw materials to fabricate anorthite ceramics through solid state reactions. Both sintering temperature and lime mud content influenced the crystalline phases in the prepared ceramics. Anorthite was the major phase in all samples (samples L36, L40, L50 and L60) and it was prominent in sample L36 (containing 36 wt% lime mud). The results also showed that anorthite ceramic can be synthesized at low sintering temperature (1100 °C). Gehlenite and wollastonite were formed in the samples possessing higher calcium (above 40 wt% lime mud) or at lower sintering temperatures. Bulk density, water absorption and compressive strength were measured. These ceramics were of light weight and had high water absorption. Recycling of lime mud and fly ash as raw materials of anorthite ceramic is a feasible approach to solve the solid wastes.     

Preparation and characterization of porous anorthite ceramics from red mud and fly ash

The porous anorthite ceramics with high porosity, good mechanical strength and low heat conductivity were prepared using red mud and fly ash as raw materials via the pore forming method. The effects of sintering temperature and fly ash on phase evolution, densification, compressive strength, thermal conductivity and microstructure of the ceramic materials were investigated. The results showed that the compressive strength of the porous ceramics had an obvious improvement with the increase in fly ash, and the densification and heat conductivity decreased firstly and then increased. In particular, specimen S2 containing 30 wt% red mud and 40 wt% fly ash sintered at 1150°C had the better performances. It had the water absorption of 18.18%, open porosity of 38.52%, bulk density of 1.29 g/cm³, compressive strength of 42.46 MPa, and heat conductivity of 1.24 W/m·K. X-ray diffraction analysis indicated that mullite, anorthite, α-quartz, and diopside ferrian were the dominant phases in the specimens. Scanning electron microscopy micrographs illustrated that plenty of open pores with strip shape and closed pores with axiolitic shape existed in the specimens. Furthermore, the existence of mullite could prevent crack propagation to enhance the energy of inter-granular fracture. It endowed the porous anorthite ceramics with high porosity, good compressive strength, and low heat conductivity.     

Recycling of harmful waste lead-zinc mine tailings and fly ash for preparation of inorganic porous ceramics

The porous ceramics were prepared by directly sintering of lead-zinc mine tailings and fly ash as the raw materials without any additional sintering and foaming agent. The effects of fly ash addition on the crystalline phases, pore structure, physical–chemical porosities and mechanical strength were investigated. The results showed that the bulk density decreased firstly and then increased while the porosity and water absorption presented the opposite tendency with the increase of fly ash content. Meanwhile, the chemical stability improved and the flexural strength had the same variation tendency of the bulk density. The phase evolution of sample with 60 wt% fly ash addition indicated that anorthite phase was formed at low temperature (1000 °C). The thermal behavior illustrated that the foaming process was initiated by the reaction of internal constituents in the lead-zinc mine tailings. Different pore structures indicated different foaming mechanisms that probably occurred at different temperatures. The porous ceramics with 60 wt% fly ash addition exhibited excellent properties, including bulk density of 0.93 g/cm³, porosity of 65.6%, and flexural strength of 11.9 MPa.     

Substitution of quartz and clay with fly ash in the production of architectural ceramics: A mechanistic study

Quartz and clay are substituted gradually by fly ash using a triaxial ceramic formulation under simulated industrial conditions and the effects of fly ash substitution on the macroscopic properties and microstructures of the sintered ceramics are evaluated systematically. With the substitution of 35 wt% (1250 °C), the ceramic sample exhibited optimal properties, including linear shrinkage of 15.61%, bulk density of 2.39 g cm^-3, water absorption of 0.62% and flexural strength of 41.70 MPa, due to the accelerated densification and fly ash-spurred needle-shaped mullite. The microstructure analysis shows that the sintered matrix consists of three types of particles, quartz-, clay- and feldspar-like particles showing sintering behavior with respect to filling the glassy matrix with preserved morphology, precipitating mullite crystals, and fusing with the surrounding glassy matrix, respectively. The strength of the fly ash - containing ceramics is analyzed by the dispersion-strengthening mechanism and porosity and the results indicate that the fly ash particles affect the mechanical strength due to Griffith flaws when the total porosity is less than 25% and pores at higher total porosity. This study provides a viable strategy to recycle industrial fly ash in the production of architectural ceramics.     

Preparation of fused silica membrane with high performance for high temperature gas filtration by sealing and spray coating

In order to improve the gas permeability and thermal shock resistance of the ceramic membranes applied in high temperature gas-solid separation techniques, fused silica and graphite particles were used as the primary raw material and pore-former agent, and the spray coating based-on PVA sealing was applied to prepare the separation membrane. These approaches remarkably decreases filtration resistance by increasing support permeability and reducing the intrusion of ceramic membrane forming particles into the support as well as the thickness of the membrane. The fabricated membrane had an average pore diameter of 9.85?μm and a gas permeability value of 8.2?×?10⁴?m³/(m² h bar), its dust removal efficiency reached 98.6%.     

Influence of additive contents on the properties of SiC ceramic membranes and their performance in oil-water separation

The high processing cost is the key challenge for the economic industrial use of SiC membrane The aim of this research was to fabricate mullite bonded porous SiC ceramic membrane at low temperature from novel combinations of fly ash and alumina (FA) in weight ratio 44.5:55.5 as sintering additives. The influences of FA and pore former content on the porosity, morphology, crystalline phase composition, mechanical performance, permeability properties were investigated. The membrane prepared at 1300°C using 20 wt% FA showed pure water permeability 3690 Lm⁻²h⁻¹bar⁻¹ and exhibited high oil removal efficiency of ∼ 98% from the synthetic oil-water emulsion having oil concentration of 1000 mg/L. The corrosion behaviour of silicon carbide membrane in the strong acid and alkali solution and its mechanism were investigated. The utilization of fly ash successfully reduced the raw material cost and sintering temperature and the use of alumina reduced the amount of oxidation of SiC as well as increased the amount of mullite bond phase which resulted excellent mechanical strength to the final ceramics.     

Ceramic membrane synthesis based on alkali activated blast furnace slag for separation of water from ethanol

The main purpose of this research is synthesis of zeolite ceramic membranes based on alkali activated blast furnace slag for pervaporation separation of ethanol/water mixture (90 wt%). A new and simple method was applied to fabricate these ceramic membranes. In addition, gross waste of steel industry (blast furnace slag) was firstly used as the main starting material for making the membranes. In this study, for making the zeolite ceramic membranes, some experiments were conducted with water levels of 38, 40, 42 and 44 wt% of the blast furnace slag and NaOH levels of 4, 4.2, 4.4 and 4.6 wt% of the blast furnace slag. At first, for making the membranes, a primary geopolymer gel was prepared. Afterward the membranes were cast at 25 °C for 24 h. In order to form the zeolite layer, the membranes after geopolymerization process were kept at 90 °C for 24 h. The maximum value of selectivity (2579.48) was obtained for separation of water from ethanol using the synthesized membrane with 42 wt% water and 4 wt% NaOH.     

Recycling of waste attapulgite to prepare ceramic membranes for efficient oil-in-water emulsion separation

Large-scale application of ceramic membranes is restricted by high cost resulting from raw materials and sintering process. In this study, low-cost ceramic membranes were prepared with waste attapulgite (WAT) and α-Al₂O₃ as starting materials and used for oily wastewater treatment. The optimal membrane sintered at 1100 °C possessed excellent properties, with open porosity of 41.6%, flexural strength of 37.2 MPa and average pore size of 0.40 μm. The membrane also displayed outstanding permeability and chemical stability. The hydrophilicity and underwater oleophobicity were enhanced after surface modification. When used for oil-in-water emulsion filtration, the permeate flux reached 236.8 L m^?2 h^?2 bar^-1 under a low transmembrane pressure of 0.2 bar and the oil rejection exceeded 99%. Membrane cleaning with a simple ultrasonic treatment could easily achieve flux recovery. This study proposed a feasible strategy for both solid waste utilization and oily wastewater treatment.     

Separation of oil from oily wastewater using low cost ceramic membrane

Safe disposal of oily wastewater is a global issue across the industrial world. Stable oil-in-water emulsion has been separated by dead end filtration using low cost ceramic membrane. The efficiency of separation at different oil-water emulsion concentrations was evaluated at different trans-membrane pressures. Maximum rejection of oil 95.4% was observed for membrane sintered at 850 °C for oil concentration of 250 mg/L at 137.89 kPa. The permeate oil concentration was within the permissible range of environmental tolerance (<12 mg/L). The flux decline data was compared with various pore blocking models and it was appraised that cake filtration model best represents the fouling mechanism within the experimental range of pressure and oil-in-water concentration. Solvent permeation studies revealed that nonpolar solvents were more permeable than the polar solvents. The selection parameter of 14.78×10^?6m³/m²·s indicates a good combination of flux permeation, declination and rejection for the membrane sintered at 900 °C.     

Effect of phase composition of natural quartz raw material on characterization of microfiltration ceramic membranes

The results of development of multi-layer ceramic membranes on the basis of natural quartz raw material from Mongolia are presented. The influence of the phase composition and temperature of calcination on the porosity, morphology and mechanical strength of large-porous ceramic support obtained by the method of isostatic pressing was studied. It was established that multi-layer ceramic membranes obtained by the application of water suspension of high-disperse quartz sand of Mongolia and alumosilicate binder with the addition of 15–35 wt% of quartz are characterized by optimal properties. The developed tubular ceramic membranes with the average pore size 5.3 µm, coefficient of air permeability (4.17–4.41)×10⁻¹³ m², productivity by water 46.3–48.0 m³/(h×m²×bar) and mechanical strength 2.27–2.53 MPa are perspective for wide use in microfiltration processes.     

Preparation and characterization of mullite whisker reinforced ceramics made from coal fly ash

Ceramics with mullite whiskers were prepared from coal fly ash and Al₂O₃ raw materials, with AlF₃ used as an additive. The phase structures and microstructures of the ceramics were identified via X-ray diffraction and scanning electron microscopy, respectively. The results show that pickling of coal fly ash is an effective method for enhancing the flexural strength of ceramics. Sintering temperature and AlF₃ addition were also key factors influencing the creation of ideal ceramics. The ceramic made from pickled coal fly ash, 6?wt% AlF₃, and sintered at 1200?°C, exhibited the highest flexural strength of 59.1?MPa, and had a bulk density of 1.32?g/cm³ and porosity of 26.8%. The results show that ceramic materials made under these conditions are ideal candidates for manufacturing ceramic proppants for the exploitation of unconventional oil and gas resources.     

Experimental study on foam glass prepared by hydrothermal hot pressing-calcination technique using waste glass and fly ash

Foam glass is a lightweight and high-strength building and decoration material with superior performance in heat insulation, sound absorption, moisture resistance and fire protection. The use of waste glass powder and fly ash to prepare foam glass is one of the most important ways to utilize solid waste as a resource. In this study, waste glass powder and fly ash were used as raw materials to prepare foam glass by a hydrothermal hot pressing–calcination method. The effects of fly ash content (0 wt%, 10 wt%, 20 wt%, 30 wt%), heating rate (1 °C/min, 3 °C/min, 5 °C/min, 8 °C/min, 10 °C/min) and calcination temperature (600 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C) on the microscopic morphology, density, compressive strength, porosity and other properties of the foam glass samples were studied. Their microstructure and morphology were analyzed by thermogravimetric analysis–mass spectrometry, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. At a fly ash content of 10 wt%, the heating rate was 5 °C/min, the calcination temperature was 800 °C, the foam glass density was 0.3 g/cm³, the compressive strength was 1.65 MPa, the total porosity was 75.5%, and the effective thermal conductivity was 0.206 W/m·K. The effective thermal conductivity models of the composite materials were used to verify the experimental data. The relationship between the thermal conductivity of foam glass materials and the related influencing factors was investigated.