A novel electro-Fenton hybrid system for enhancing the interception of volatile organic compounds in membrane distillation desalination.

Membrane distillation (MD) is a promising alternative desalination technology, but the hydrophobic membrane cannot intercept volatile organic compounds (VOCs), resulting in aggravation in the quality of permeate. In term of this, electro-Fenton (EF) was coupled with sweeping gas membrane distillation (SGMD) in a more efficient way to construct an advanced oxidation barrier at the gas-liquid interface, so that the VOCs could be trapped in this layer to guarantee the water quality of the distillate. During the so-called EF-MD process, an interfacial interception barrier containing hydroxyl radical formed on the hydrophobic membrane surface. It contributed to the high phenol rejection of 90.2% with the permeate phenol concentration lower than 1.50 mg/L. Effective interceptions can be achieved in a wide temperature range, even though the permeate flux of phenol was also intensified. The EF-MD system was robust to high salinity and could electrochemically regenerate ferrous ions, which endowed the long-term stability of the system. This novel EF-MD configuration proposed a valuable strategy to intercept VOCs in MD and will broaden the application of MD in hypersaline wastewater treatment.

Fenton pretreatment to mitigate membrane distillation fouling during treatment of landfill leachate membrane concentrate: Performance and mechanism.

Membrane distillation (MD) is regarded as a promising technology for treatment of landfill leachate membrane concentrate (LLMC) due to its merits of low cost and high rejection of non-volatile components. However, the high concentration of pollutants in the wastewater will cause severe membrane fouling, resulting in costly cleaning and maintenance. In this study, Fenton pretreatment was applied to alleviate membrane fouling during MD treatment of LLMC. Compared to rapid flux decline of 88.2% at concentration factor (CF) of 3 for raw LLMC, MD flux only decreased by 17.4% at CF = 6 for treating acidic Fenton effluent without subsequent pH adjustment (Fe2+ and H2O2 concentration were 600 mg/L and 1457 mg/L, respectively). The pH neutralization of Fenton effluent or merely acidification of LLMC could not achieve such excellent fouling mitigation. It was concluded that both oxidation and acidification were critical and the collaboration mechanism was revealed to explain low membrane fouling. Firstly, Fenton oxidation removed organic contaminants, reduced the hydrophobicity of organic substances and increased the percentage of carboxylic group within LLMC. Thus, hydrophobic (HP) attraction was weakened but multivalent cation bridging became dominant fouling mechanism for neutral Fenton effluent. Then, acidification weakened multivalent cation bridging by inhibiting the deprotonation of carboxylic group, further mitigating membrane fouling. However, acidification of LLMC caused more severe organic fouling due to decrease in electrostatic (EL) repulsion. In addition to low membrane fouling, satisfactory total organic carbon (TOC) rejection rate of 96.23% was achieved during combined Fenton-MD process. This study demonstrated that Fenton pretreatment without pH neutralization could effectively alleviate MD fouling and elucidated the synergistic mechanism between oxidation and acidification for fouling mitigation.

Simultaneous Desalination and Glyphosate Degradation by a Novel Electro-Fenton Membrane Distillation Process.

The industrial effluent from glyphosate production has high salinity and refractory organic contaminants. The removal of organics and the recycling of inorganic salts from this kind of water are challenging issues. In this study, electro-Fenton (EF) and membrane distillation (MD) were coupled in a single reactor utilizing a membrane-based electrode (Mem-GDE) with the ability to bidirectionally transfer vapor and oxygen and electrochemically synthesize H2O2. The operating thermal conditions for MD significantly promoted Fenton reactions and, thus, the removal of glyphosate. During operation, Fe species deposited on the Mem-GDE and enhanced its catalytic activity and adsorptive capacity, which markedly increased the apparent reaction rate constant of glyphosate by 6 times. This novel EF-MD process simultaneously removed organics and concentrated the inorganics, which is very meaningful for decreasing the costs for subsequent crystallization and achieving high-quality crystal salts. This study provides an efficient method for the treatment of organic-inorganic hybrid wastewater.

A new insight into the low membrane fouling tendency of liquid-liquid hollow fiber membrane contactor capturing ammonia from human urine.

To unravel the low membrane fouling tendency and underlying membrane fouling mechanism of liquid-liquid hollow fiber membrane contactor (LL-HFMC) capturing ammonia from human urine, the ammonia flux decline trend, membrane fouling propensity, foulant-membrane thermodynamic interaction energy and microscale force analysis at different feed urine pH were comprehensively investigated. The 21-d continuous experiments showed that the ammonia flux decline trend and membrane fouling propensity significantly strengthened with the decrease of feed urine pH. The calculated foulant-membrane thermodynamic interaction energy decreased with the decreasing feed urine pH and agreed with the ammonia flux decline trend and membrane fouling propensity. The microscale force analysis showed that the absence of hydrodynamic water permeate drag force resulted in the foulant located at long distance from the membrane were difficult to approach the membrane surface, thus considerably alleviating membrane fouling. Additionally, the vital thermodynamic attractive force near the membrane surface increased with the decrease of feed urine pH, which made the membrane fouling further relieved at high pH condition. Therefore, the absence of water permeate drag force and operating at high pH condition minimized the membrane fouling during the LL-HFMC ammonia capture process. The obtained results provide a new insight into the low membrane tendency mechanism of LL-HFMC.

Enhanced Mass Transfer of Ozone and Emerging Pollutants through a Gas-Solid-Liquid Reaction Interface for Efficient Water Decontamination.

Ozone (O3), as an environmentally friendly oxidant, is widely used to remove emerging pollutants and ensure the safety of the water supply, whereas the restricted accessibility of O3 and limited collision frequency between pollutants and O3 will inevitably reduce the ozonation efficiency. To promote the chemical reactions between O3 and target pollutants, here we developed a novel gas-solid-liquid reaction interface dominated triphase ozonation system using a functional hydrophobic membrane with an adsorption layer as the O3 distributor and place where chemical reactions occurred. In the triphase system, the functional hydrophobic membrane simultaneously improved the interface adsorption performance of emerging pollutants and the access pathway of O3, leading to a marked enhancement of interfacial pollutant concentration and O3 levels. These synergistic qualities result in high ciprofloxacin (CIP) removal efficiency (94.39%) and fast apparent reaction rate constant (kapp, 2.75 × 10-2 min-1) versus a traditional O3 process (41.82% and 0.48 × 10-2 min-1, respectively). In addition, this triphase system was an advanced oxidation process involving radical participation and showed excellent degradation performance of multiple emerging pollutants. Our findings highlight the importance of gas-solid-liquid triphase reaction interface design and provide new insight into the efficient removal of emerging pollutants by the ozonation process.

Membrane cleaning in membrane distillation of reverse osmosis concentrate generated in landfill leachate treatment.

As a thermally induced membrane separation process, membrane distillation (MD) has drawn more and more attention to the advantages of treating hypersaline wastewaters, especially the concentrate from the reverse osmosis (RO) process. One of the major obstacles in widespread MD application is the membrane fouling. We investigated the feasibility of direct contact membrane distillation (DCMD) for landfill leachate reverse osmosis concentrate (LFLRO) brine treatment and systematically assessed the efficiency of chemical cleaning for DCMD after processing LFLRO brine. The results showed that 80% water recovery rate was achieved when processing the LFLRO brine by DCMD, but membrane fouling occurred during the DCMD process, and manifested as the decreasing of permeate flux and the increasing of permeate conductivity. Analysis revealed that the serious flux reduction was primarily caused by the fouling layer, which consisted of organic matter and inorganic salts. Five cleaning methods were investigated for membrane cleaning, including hydrogen chloride (HCl)-sodium hydroxide (NaOH), ethylene diamine tetraacetic acid (EDTA)-NaOH, citric acid, sodium hypochlorite (NaClO) and sodium dodecyl sulphate (SDS) cleaning. Among the chemical cleaning methods investigated, the 3 wt.% SDS cleaning showed the best efficiency at recovering the performance of fouled membranes.

Titanium nitride nanoparticle embedded membrane for photothermal membrane distillation.

To meet the increasing worldwide need for freshwater, it has become critical to exploit non-potable saline water. Solar membrane distillation (MD) is a promising desalination technique, which does not require conventional energy and can reduce the cost of water production. We developed a cost-effective and high-efficiency photothermal membrane that employs TiN nanoparticles as an absorber of sunlight and energy converter. Due to a strong photothermal effect, the solar energy efficiency significantly improved. With optimal membrane and MD operating conditions, we obtained an MD flux of 0.940 kg/m2∙h and a solar efficiency of 64.1% under 1.0 kW/m2 solar irradiation. Compared with a bare poly(vinylidene fluoride) (PVDF) membrane, 65.8% more pure water was produced. Furthermore, the temperature polarization encountered in the conventional MD process was relieved on account of the unique interfacial heating of the photothermal coating, which also contributed to the high solar efficiency. In addition, the membrane was quite stable and the permeate water was of a high, potable quality. The as-prepared photothermal membrane demonstrated a good performance and application prospects for solar MD.

Enhanced mineralization of reactive brilliant red X-3B by UV driven photocatalytic membrane contact ozonation.

The partial oxidation on refractory organics in ozonation process and the poor performance of mass transfer between ozone (O3) phase and liquid phase by common O3 distribution techniques inhibit the practical application of O3. To overcome these defects, hollow fiber membrane was applied in membrane contact ozonation (MCO)-UV process for the reactive brilliant red X-3B (RBRX-3B) degradation. The efficiency of mass transfer was guaranteed due to the enormous gas/liquid contact area supplied in this bubble-less O3 transfer process. UV photolysis not only significantly improved the O3 utilization efficiency but also accelerated the mineralization of RBRX-3B by promoting O3 to decompose to hydroxyl radicals (OH). When 15 mg/L of O3 was supplied at flow rate of 0.2 L/min, and a liquid velocity of 0.453 m/s, the chemical oxygen demand (COD) removal and total organic carbon (TOC) removal reached 90 % and 77 %, respectively. The rate constant for TOC removal in the MCO-UV process (7.89 × 10-3 min-1) was 3.08 and 6.12 times higher than that in MCO and UV photolysis processes, respectively. Furthermore, the mineralization efficiency (ΔCOD/ΔO3 = 0.84 mg/mg) and electrical energy per mass (EEM = 4.7 kW h/kg) were calculated and these results indicated a promising future for the MCO-UV process.

Electrothermally Driven Membrane Distillation for Low-Energy Consumption and Wetting Mitigation.

Membrane distillation (MD) is a promising alternative approach for desalination, especially for high-salinity brines. Its application has been limited by its high operational cost because of the energy consumption required for hydraulic circulation and heating the entire circulating feed. Localized heating of the feed by Joule heating diminishes energy consumption, but the potential charging on the electrothermal material surface causes water splitting and membrane degradation in high-salinity environments. Herein, a novel reverse Joule-heating air gap MD method was designed in which an electrothermal material was placed at the air gap, isolating itself from saline water. Even though the Joule-heating layer was at the air gap side, 90.56% of heat flowed into the saline water for heating the feed. The opposite temperature gradient in the membrane matrix as opposed to conventional MD-mitigated membrane wetting was caused by capillary condensation. This novel electrothermal-driven MD configuration is worthy to be introduced into applications.

Fabrication of PVDF nanofibrous hydrophobic composite membranes reinforced with fabric substrates via electrospinning for membrane distillation desalination.

To improve the mechanical properties of the electrospun nanofibrous membrane, the nonwoven fabrics and spacer fabrics were employed as support substrates to fabricate polyvinylidene fluoride (PVDF) nanofibrous composite membranes. The influences of the substrate on membrane morphology, hydrophobicity, pore size and pore size distribution, porosity, mechanical strength and permeability were comprehensive evaluated. The electrospun composite membranes had a three dimension bead-fiber interconnected open structure and a rough membrane surface. The membrane surface presented a multilevel re-entrant structure and all the water contact angles were above 140°. In contrast with the pure PVDF nanofibrous membrane, the stress at break and the elastic modulus of the composite membranes increased by 4.5-16 times and 17.5-37 times, respectively. Since the spacer fabrics had less resistance to mass transfer, the membranes composited with spacer fabrics exhibited greater permeate fluxes compared with the composite membranes with the nonwoven fabrics as substrates. During the membrane distillation test, the highest permeate flux was up to 49.3kg/m2/hr at the feed temperature of 80°C. The long-time and repeat operation of membrane distillation desalination indicated the fabricated membrane with a good resistance to scaling and wetting. The results suggested the potential of the electrospun composite membrane for membrane distillation application.

Development of a composite membrane with underwater-oleophobic fibrous surface for robust anti-oil-fouling membrane distillation.

Membrane fouling caused by non-polar foulants is a challenging problem for hydrophobic membranes, which hinders the industrial implementation of membrane distillation (MD). The hydrophilic coating can create a hydration layer at solid-water interface, thereby the hydrophilic surfaces are expected to supply a barrier inhibiting adhesion of hydrophobic foulants. Hence, it should be possible to develop anti-fouling composite membranes through constructing a hydrophilic skin layer onto hydrophobic MD membranes. Herein, we fabricated a novel composite membrane for excellent anti-oil-fouling performance in MD process by electrospinning polyetherimide (PEI) nanofibers on the hydrophobic polyvinylidene fluoride (PVDF) membrane surface, followed by cross-linking with ethanediamine (EDA). The membrane morphology and structure properties, surface zeta potential and wettability, thermal stability were all systematically characterized, and force spectroscopy was used to quasi-quantitatively evaluate oil-membrane adhesion force. Compared with the PVDF membrane, the PVDF/PEI-EDA composite membrane exhibited strong resistance to crude oil with underwater oil contact angle of about 145° and low oil-membrane adhesion force, which contributed to the stable performance during MD desalinating an oily and saline solution. The fabricated composite membrane with underwater-oleophobic fibrous surface can effectively mitigate oil-fouling in MD and promote MD to treat highly saline wastewater with high concentration of hydrophobic foulants.

Enhancement of energy utilization using nanofluid in solar powered membrane distillation.

Nanofluids have excellent solar energy utilization efficiency due to the localized surface plasmon resonance phenomenon. In this study, photothermal nanofluids were employed as the feed solution for energy harvesting in solar powered membrane distillation. Ten different nanofluids were compared and TiN (titanium nitride) was chosen following UV-Vis-NIR-waveband (ultraviolet-visible-near-infrared) optical absorption analysis, zeta potential measurement, and membrane distillation flux testing. Desalination experiments were conducted using a range of TiN concentrations and solar radiation powers. The results showed that water flux and solar energy utilization efficiency increased with increasing TiN content. Compared to the base fluid (35 g/L NaCl aqueous solution), flux increased from 0.47 to 0.74 kg/(m2∙h), while energy utilization efficiency improved from 32.1% to 50.5% for 100 mg/L TiN nanofluid. Flux also increased with the increasing of solar radiation power markedly. With 5 kW/m2 solar radiation power, the flux reached 2.77 kg/(m2∙h). Furthermore, the permeate water produced was of excellent quality contained less than 10 mg/L salinity when using 35 g/L NaCl feed solution. And no nanoparticles were detected transport through the membrane during the process. The nanofluid enhanced solar powered membrane distillation represents a promising perspective for better solar energy utilization.

Domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system.

In this study, real domestic wastewater treatment by forward osmosis-membrane distillation (FO-MD) integrated system was investigated in laboratory scale. The integrated membrane system presented a good separation performance and the removal efficiency of most contaminants in the domestic wastewater was higher than 90%. High molecular weight contaminants were completely removed, while a few low molecular weight contaminants permeated through the membrane. The FO membrane fouling layer mainly consisted of organic substances like polysaccharides and proteins, and was very loose and could be effectively removed by rinsing the membrane surface with tap water. By comparison, the MD membrane fouling was mainly induced by inorganic salts and was not as severe as that of the FO membrane. During 120 h continuous operation, the FO-MD integrated system exhibited satisfying performance stability and maintained a high water yield and high product water quality. The results indicated the potential of the FO-MD integrated system for municipal wastewater treatment in coastal cities, water purification and desalination.

Ozone mass transfer behaviors on physical and chemical absorption for hollow fiber membrane contactors.

To understand the mass transfer behaviors in hollow fiber membrane contactors, ozone fluxes affected by various conditions and membranes were investigated. For physical absorption, mass transfer rate increased with liquid velocity and the ozone concentration in the gas. Gas flow rate was little affected when the velocity was larger than the critical value, which was 6.1 × 10-3m/s in this study. For chemical absorption, the flux was determined by the reaction rate between ozone and the absorbent. Therefore, concentration, species, and pH affected the mass transfer process markedly. For different absorbents, the order of mass transfer rate was the same as the reaction rate constant, which was phenol, sodium nitrite, hydrogen peroxide, and oxalate. Five hydrophobic membranes with various properties were employed and the mass transfer behavior can be described by the Graetz-Lévèque equation for the physical absorption process. The results showed the process was controlled by liquid film and the gas phase conditions, and membrane properties did not affect the ozone flux. For the chemical absorption, gas film, membrane and liquid film affected the mass transfer together, and none of them were negligible.