Desalination Experiments Research Articles

A novel dual-layer composite membrane with underwater-superoleophobic/hydrophobic asymmetric wettability for robust oil-fouling resistance in membrane distillation desalination

In this study, we developed a new type of composite membrane to mitigate oil fouling in membrane distillation (MD). The composite membrane consists of a polytetrafluoroethylene (PTFE) hydrophobic substrate and a hydrophilic poly(vinyl alcohol)/silica nanoparticles (PVA-Si) hybrid fibrous coating prepared via sol-gel and electrospinning. We characterized the pristine PTFE membrane and the modified membrane using contact angle measurements and tensiometer-based oil probe force spectroscopy. While the hydrophilic coating presented excellent oil fouling resistance, the glutaraldehyde cross-linking treatment augmented the in-air hydrophilicity and underwater superoleophobicity of the electrospun coating surface. The cross-linking treatment also prevented the formation of PVA-Si hydrogels for the electrospun hybrid fibers, which made availability to utilize the water-soluble polymer to modify hydrophobic membrane for anti-oil-fouling MD. By comparing the performance of the composite membrane and the pristine PTFE membrane in MD desalination experiments using a saline emulsion with 1000mg/L crude oil, it showed that the fabricated composite membrane was significantly more resistant to oil-fouling compared to the PTFE membrane. The results from this study suggest that underwater superoleophobic coating can effectively mitigate oil fouling in MD, and that the fabricated composite membrane can enable MD to desalinate hypersaline wastewater with high concentrations of hydrophobic contaminants.

Microporous polypropylene membrane prepared via TIPS using environment-friendly binary diluents and its VMD performance

Growing attention given to non-toxic solvents for membrane preparation has been the motivation for membrane scientists to develop future generation membranes due to increasing concerns for environmental impacts and strict rules. In this study, a novel preparation approach for polypropylene (PP) membranes via TIPS using non-toxic binary diluents consisting of carnauba wax as latent solvent and soybean oil as good solvent was developed successfully. Water and ethanol were used as a quenching bath and extractant of diluents in the membrane, respectively. Impacts of both polymer weight fraction and carnauba wax weight fraction on the phase diagram (phase separation behaviors) of the PP/binary diluents system were investigated firstly. The liquid-liquid (L-L) phase separation region can be enlarged with the continuously increasing of carnauba wax. The effect of adding the second latent diluent on the structures and properties of PP membranes was also studied. The morphology of membranes was further controlled by adjusting the composition of binary diluents. The inter-connected spongelike structure was achieved successfully, which greatly increased the membrane elongation at break. Moreover, vacuum membrane distillation (VMD) desalination experiments showed that the PP membranes produced have the potentiality in VMD desalination application. The carnauba wax after extraction by boiling ethanol could be easily crystallized, separated and recycled by cooling.

Effects of watering parameters in a combined seawater desalination process

Seawater desalination experiments using patented experimental platform that could recycle cold energy released from LNG (liquefied natural gas) re-gasification process were conducted in laboratory. Combined freezing, watering and centrifugal desalination (FWCD) process was employed. The effects of the key watering parameters, including proportion, temperature and pure water or seawater, on removal efficiencies of salt and measured ions and on ice yield rate were investigated. Results showed that watering process benefits the brine-ice separation through centrifugation and therefore the ice purity can be enhanced. To ameliorate ice product purity, increasing the mass proportion of watering raw seawater is a more feasible method than watering pure water. The desalination effect of adding raw seawater would be better in summer than in winter when other parameters constant. Comparing with combined freezing, gravity-induced and centrifugal desalination (FGCD) method, FWCD is a continuous process that can accelerate the desalination duration, and more ice with the same purity can be yielded within the salt removal efficiency range of FWCD method. Whereas more energy has to be consumed using FWCD process because higher centrifugation rotation rate is also necessary to achieve the same salt removal efficiency. Whereas the total energy consumption for unit ice product is acceptable.

Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water

Using porous electrodes containing redox-active nickel hexacyanoferrate (NiHCF) nanoparticles, we construct and test a device for capacitive deionization in a two flow-channel device where the intercalation electrodes are in direct contact with an anion-exchange membrane. Upon reduction of NiHCF, cations intercalate into it and the water in its vicinity is desalinated; at the same time water in the opposing electrode becomes more saline upon oxidation of NiHCF in that electrode. In a cyclic process of charge and discharge, fresh water is continuously produced, alternating between the two channels in sync with the direction of applied current. We present proof-of-principle experiments of this technology for single salt solutions, where we analyze various levels of current and cycle durations. We analyze salt removal rate and energy consumption. In desalination experiments with salt mixtures we find a threefold enhancement for K+ over Na+-adsorption, which shows the potential of NiHCF intercalation electrodes for selective ion separation from mixed ionic solutions.

Preparation and characterization of novel poly (vinylidene fluoride) membranes using flower-like Bi 2 WO 6 for membrane distillation

Abstract In this study, hydrophobic flat-sheet composite membranes of polyvinylidene fluoride (PVDF) were fabricated using non-woven fabrics as a support for direct contact membrane distillation (DCMD) through non-solvent induced phase separation (NIPS). The effects of three-dimensional (3D) Bi 2 WO 6 on membrance morphology, porosity, pore size, pore size distribution, hydrophobicity, thermal properties and permeability were investigated. The flower-like Bi 2 WO 6 could optimize membrane structure and improve the membrane porosity by its unique 3D hollow structure. During the desalination process of 3.5% sodium chloride solution, the salt rejection with nano-Bi 2 WO 6 membranes was higher than the pristine PVDF membrane. The maximum transmembrane permeate flux (about 13.12 kg/m 2 h) was obtained with the feed solution at 60 °C and the cold distillate water at 20.0 °C, which was higher than the commercial PVDF membrane (9.43 kg/m 2 h) and the pure PVDF membrane (12.06 kg/m 2 h). Furthermore, the hybrid membrane exhibited outstanding stability in the 100 h continuous desalination experiments. Our results demonstrated that the prepared PVDF flat-sheet composite membrane with 0.5% flower-like Bi 2 WO 6 has a very good prospect to be utilized in membrane distillation process for desalination.

Improved hydrophilicity and anti-fouling properties of polyamide TFN membrane comprising carbide derived carbon

Improving the fouling resistance of reverse osmosis (RO) membranes is still needed in the desalination industry. In this paper, we report for the first time the preparation of Carbide Derived Carbon (CDC)/polyamide hybrid membrane to improve the fouling-resistance of RO membranes. The merits of the prepared membranes were validated with permeation experiments and a series of characterization methods. TEM images of the synthesized CDC particles showed a mixture of amorphous and ordered graphitized carbon and the degree of ordering was directly proportional to the chlorination temperature while SEM images of the prepared membranes showed the ridge and valley structure of aromatic polyamide. Significant improvement of the surface hydrophilicity of the nanocomposite membrane (M3) was observed compared to that of the control polyamide membrane (M0). Desalination experiments showed that the permeate flux increased 51%, 45% and 38% under pure water, brackish water, and fouling conditions; respectively. M0 membrane lost 26% of its initial flux while M3 membrane lost only 11.5% after 2h of continuous operation. This study showed that the CDC can significantly enhance the fouling resistance and permeability of the polyamide RO-membranes.

Carboxyl-functionalized graphene oxide polyamide nanofiltration membrane for desalination of dye solutions containing monovalent salt

Novel carboxyl-functionalized graphene oxide (CFGO)/polyamide (PA) nanofiltration (NF) membranes were prepared via interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC). CFGO was fabricated by a chemical modification (ring opening followed by esterification) to the epoxide ring of GO. CFGO was then introduced in the PIP aqueous phase as an additive in the IP reaction. Compared with a pristine PA reference membrane, both the GO/PA and CFGO/PA membrane have an enhanced permeability (the optimum concentration of GO and CFGO in the membrane was 0.05% and 0.07%, and the corresponding water flux is 96.5 and 112.1L/m2/h, respectively),with a slight decrease of the salt rejection. In dye desalination experiment, the permeation flux of 0.05%GO/PA membrane is only 75.5L/m2/h, with 98.1% rejection of New Coccine (a dye with negative charge) and 28.7% retention for NaCl. For 0.07%CFGO/PA membrane, it can cut off 25.0% NaCl and 95.1% New Coccine, and its permeation flux can reach 110.4L/m2/h, which shows that the CFGO/PA membrane could be potentially applied to the dye desalination and concentration process. The CFGO/PA membrane shows better performances in permeability and dye desalination than GO/PA membrane, due to the significantly increased hydrophilicity and enhanced surface charge density.

Forward osmosis as an approach to manage oil sands tailings water and on-site basal depressurization water

As the volume of oil sands process-affected water (OSPW) stored in tailings ponds increases, it is urgent to seek for water management approaches to alleviate the environmental impact caused by large quantity of toxic water. Forward osmosis (FO) utilizes osmotic pressure difference between two solutions, thereby giving a potential to manage two wastewaters. In this study, FO was proposed to manage OSPW, using on-site waste basal depressurization water (BDW) as draw solution. To investigate its feasibility, both short and long-term OSPW desalination experiments were carried out. By applying this process, the volume of OSPW was decreased>40% and high rejections were achieved, especially, the major organic toxicity source – naphthenic acids (NAs). Although comparative low water flux (≤3L/m2h) was obtained, water flux caused by membrane fouling can be completely recovered using water physical cleaning. Moreover, calcium carbonate precipitation was observed on the OSPW-oriented membrane side. With respect to flux decline, the active layer facing the feed solution (FO mode) and active layer facing draw solution (PRO mode) did not demonstrate a significant difference on anti-fouling performance. The advantages provided by this approach include zero draw solution cost, less reversible membrane fouling and beneficial reuse/recycle of diluted BDW.

Design and test of a reverse osmosis pressure cell for in-situ small-angle neutron scattering studies

We introduce a new method for real-time studies of membrane scaling and biofouling on thin film composite membranes (TFC) in reverse osmosis and nanofiltration water treatment using in-situ small-angle neutron scattering (SANS). SANS delivers information on nano and microscopic structures that support the interpretation of relevant engineering parameters such as membrane permeability and water flux. A flow cell high pressure SANS is described, followed by SANS characterization of TFC membranes finding ~0.5μm large cavities and ~300Å diameter large rod-like cavities inside the non-woven polyester and micro-porous polysulfone layer, respectively. In-situ desalination experiments in cross-flow mode at an applied pressure of 6bars and feed flow velocity of 0.2cm/s are followed. The scattering cross-section times sample thickness (μt=Σt×DS) derived from the transmission coefficient shows an overall enhancement due to newly formed scattering centers which is accompanied by a reduced membrane permeability measured simultaneously. This observation is supported by enhanced scattering of the membrane due to μm large domains of mass fractal structure. The addition of the protein BSA to the feed after desalination of 30h effectuates strong enhancement of the permeability accompanied by a about a 50% decline of μm large scattering centers.

Effects of Natural Organic Matter on Desalination of Coal Mine Wastewater Using a Capacitive Deionization System

Secure access to sufficient and clean water is imperative to sustainable mining production. Water pollution from mining activities raised public concern on the negative impacts of coal mining on water quality and quantity. Therefore, coal mine industry increased water recycling to protect their access to water resources while expanding their operations. However, water recycling without control of salinity can generate elevated salt concentrations in feed water streams resulting in deterioration of product quality. In this study, we developed a coagulation/sedimentation (C/S) – microfiltration (MF) - capacitive deionisation (CDI) system and evaluate its performance as an alternative desalination technique for recycling of coal mine wastewater. Desalination experiments were performed by using synthetic coal mine wastewater (CMW) and a bench-scale CDI system equipped with activated carbon (AC) electrodes. The effects of ion-exchange polymer coated on AC electrodes, turbidity and natural organic matter (NOM) on desalination performance of the hybrid system were investigated. As a result, pre-treatment of turbidity by the coagulation and MF membrane filtration increased desalination performance of the CDI. However, natural organic matter (NOM) affected adversely on salt removal efficiency of the CDI.

Calcium carbonate scaling of desalination membranes: Assessment of scaling parameters from dead-end filtration experiments

Membrane desalination is associated with significantly increased ion concentration of rejected sparingly soluble salts (commonly of calcium, magnesium, barium, etc) right at the membrane surface, leading to undesirable membrane scaling. Optimization of desalination membrane-plant design and its operation strategy requires quantification of scaling evolution phenomena, through appropriate mathematical models involving experimentally determined system parameters. In recent years, a modeling approach based on heterogeneous nucleation-growth theory was developed for this purpose, and proven to be successful in the case of non-stirred dead-end desalination experiments but not in the case of cross-flow and stirred dead-end desalination. To resolve this inconsistency, new results from numerous sets of improved dead-end non-stirred experiments (some with controlled changes of imposed pressure difference during testing) are reported that provide useful quantitative information and insights regarding incipient scaling. The previous modeling approach is generalized considering non-classical nucleation. Moreover, the likely scatter of local membrane-surface properties is considered in a parameter estimation approach. The new modeling approach leads to estimation of a range of scaling-model parameters consistent with the experimental data. Such parameters can be employed in comprehensive modeling tools of membrane-modules, to simulate their operation for the particular type of membrane for which parameters are extracted.

Saline Groundwater from Coastal Aquifers As a Source for Desalination.

Reverse osmosis (RO) seawater desalination is currently a widespread means of closing the gap between supply and demand for potable water in arid regions. Currently, one of the main setbacks of RO operation is fouling, which hinders membrane performance and induces pressure loss, thereby reducing system efficiency. An alternative water source is saline groundwater with salinity close to seawater, pumped from beach wells in coastal aquifers which penetrate beneath the freshwater-seawater interface. In this research, we studied the potential use of saline groundwater of the coastal aquifer as feedwater for desalination in comparison to seawater using fieldwork and laboratory approaches. The chemistry, microbiology and physical properties of saline groundwater were characterized and compared with seawater. Additionally, reverse osmosis desalination experiments in a cross-flow system were performed, evaluating the permeate flux, salt rejection and fouling propensities of the different water types. Our results indicated that saline groundwater was significantly favored over seawater as a feed source in terms of chemical composition, microorganism content, silt density, and fouling potential, and exhibited better desalination performance with less flux decline. Saline groundwater may be a better water source for desalination by RO due to lower fouling potential, and reduced pretreatment costs.

Flux enhancement in membrane distillation by incorporating AC particles into PVDF polymer matrix

In this paper, a novel method of preparing porous hydrophobic membrane was proposed and further experiments had been carried out for verification. A kind of hydrophobic porous inorganic particles, macroporous structure activated carbon (AC), was introduced into polyvinylidene fluoride (PVDF) polymer matrix to fabricate porous membrane for vacuum membrane distillation (VMD) application. The incorporation of AC particles in the polymer matrix can efficiently enhance the membrane permeability because of the intrinsic porous structure of AC particles could directly provide water vapor mass transfer channels in the VMD process. Results showed that: the VMD flux of PVDF hydrophobic membranes was obviously improved by the addition of AC particles. When 0.09wt% AC particles were added in membrane casting dope, a VMD flux of 54.9kg/m2h was obtained. The flux is 30% higher than that of pure PVDF membrane (42.0kg/m2h). Furthermore, the conductivity of the M-3 membrane was kept lower than that of pure PVDF membrane in the 16h continuous desalination experiments. All of the results demonstrated that the prepared PVDF/AC hollow fiber membrane not only has higher permeability but also exhibited satisfying performance stability. It performed well in the VMD program and is expected to be applied in the field of MD to improve the MD membrane performance.

Optimal design of experiments for parameter identification in electrodialysis models

The Nernst–Planck approach, previously used to model the electrodialytic recovery of uni-, di or tri-valent electrolytes, was used to accomplish the desalination of concentrated brines with an initial NaCl concentration up to 4.6kmolm−3. The complexity of the proposed model is such that an extensive experimentation is required for a statistically sound estimation of the relevant model parameters, including solute (tB) and water (tW) transport numbers through the ion-selective membranes; solute (LB) and water (LW) transport rate by diffusion; average electro-membrane resistance (R). A model-based design of experiments (MBDoE) approach is proposed in this paper to minimise the number of trials and resources required for model identification. The use of this approach in an experimental case study allowed for a dramatic reduction of the experimentation time from 1080min (corresponding to a classical experimentation with multiple batch desalination trials) to 30–60min corresponding to a single optimal batch desalination experiment. The results obtained show the potential of MBDoE for quick development and assessment of electrodialysis models, where highly predictive capability can be achieved with the minimum experimental time and waste of resources.

Carbon electrode modified by KOH solution to improve performance of capacitive desalination

A carbon electrode was modified by different concentrations of KOH solution to enhance its capacitive deionization performance. The hydrophilicity and electrochemical properties of the carbon electrode were characterized by FTIR analysis, contact angle test, cyclic voltammetry, and impedance spectroscopy analysis. The results show that contact angles of the carbon electrode decrease from 115° to 62° with an increase in the concentrations of KOH, indicating an increase in the hydrophilicity. The specific capacitance of carbon electrode achieves the maximum of 108.8 Fg−1 when modified by 5% KOH solution. Furthermore, desalination experiments were carried out in a lab-scale experimental system. The salt-removal efficiency of the modified carbon electrode increases by 23% under the conditions of 2.5 V applied voltage, initial concentration of 1,500 μs/cm, and flow rate of 20 ml/min. Thus, the capacitive deionization performance of the carbon electrode is significantly improved by the modification with KOH. This enhanced performance can be attributed to the hydroxyl functional groups introduced on to the surface of the carbon electrode and an increased wettability of the surface. Our results indicate that the KOH-modified carbon electrode will have great potential in the capacitive deionization technology.

Influence of Activated Carbon Film Thickness on the Electrode Performance in Capacitive Deionization

Problems associated with water-scarcity, drought and water pollution are already a global problem and are expected to grow worse in the coming decades, even in regions currently considered to be rich with respect to water resources. That is the case of Brazil, where the drought in the northeast and southeast regions has become a great challenge. In order to address this situation different measures such as developing “greener” methods of water desalination that are energy and cost efficient while minimizing the use of chemicals should be investigated. Capacitive Deionization (CDI) is a promising technology that can be used to remove ions based on the concept of electric double layer (EDL). When a low potential is applied to a CDI cell, the ions are attracted and stored in the EDL formed at the electrode/solution interface. When the removal cycle is complete, the electrodes are mostly saturated with anions and cations. Electrode regeneration is subsequently conducted by switching the voltage or by shorting the system letting, in that way, the adsorbed ions come back to the bulk. Different carbon materials and composites have been investigated as electrodes for CDI. A good candidate for CDI electrode must have a high specific surface area, high conductivity and wettability, and high capacity to remove ions. Since activated carbon (AC) fits all of these characteristics and also presents a low price it has been considered as one the best candidates for being used as electrosorption material. Is this work it was evaluated the effect on CDI performance of the AC films thickness deposited on graphite substrate. Electrodes characterization was conducted by measuring physical properties such as surface resistivity or wettability. In addition, cyclic voltammetry and electrochemical impedance spectroscopy experiments were performed in order to determine the capacitance and the different components of the resistance in the system. Moreover, desalination experiments were conducted in a batch operational mode (Figure 1). The electrode performance was analyzed then in terms of desalination rate, regeneration kinetics, capacitance, coulombic efficiency and specific energetic consumption. As expected, the thicker the electrode the higher the total amount of ions removed (R). However, the study of the electrosorption capacity normalized by mass of electrode material (q) showed that the increase of the electrode thickness led to lower values of q. This result indicated that part of AC electrode material was not being effectively used for removing ions from the solution. On the other hand, it was observed that the variation of the electrode thickness did not have any impact on the specific energy consumption (in kJ per gram of electrode). Thusly, depending on what is aimed and considering the low cost of the AC electrode, the thicker electrode might represent an interesting alternative to thin film electrodes due its higher capacity of storing charge. As the main disadvantage of the thickest electrodes, it was found that desorption kinetics became slower when the electrode thickness increases, thus making the electrosorption/desorption cycles longer. These results suggest that the electrode thickness must be optimized in order to maximize the electrosorption capacity and the desorption kinetics. Figure 1. Schematic representation of the CDI system. Figure 1

Exploring an innovative surfactant and phosphate-based draw solution for forward osmosis desalination

The reverse salt flux phenomenon of forward osmosis affects the quality of the feed water, reduces water flux, and increases the cost for replenishing lost draw solute. In this study, a novel draw solution comprising a mixture of Triton X100 and Na3PO4 for minimizing the reverse salt flux during forward osmosis (FO) was explored. The results indicated that the reverse salt flux caused by coupling 0.5mM Triton X100 to 0.55M Na3PO4 draw solution was only 0.13g/m2h, and the specific reverse salt flux was 0.03g/L using DI water as the feed solution, which are the lowest recorded values among all forward osmosis studies. Hydrophobic attractive interactions between tail groups of Triton X100 with the FO membrane are believed to be the main mechanism for minimizing salt leakage. Results from desalination experiments demonstrated that using 0.55M Na3PO4 coupled with 0.5mM Triton X100 as the draw solution and brackish water and seawater as the feed solution with total dissolved solids of 4090 and 36,800ppm achieved water fluxes of 4.89L/m2h and 1.15L/m2h, respectively. Furthermore, using a two-stage ultrafiltration–nanofiltration system for the draw solution recovery enabled 98% recovery of solutes.

Improved performance in capacitive deionization of activated carbon electrodes with a tunable mesopore and micropore ratio

To improve the desalination performance, the coconut shell-based activated carbon electrodes were prepared by using a two-stage activation method. Both the specific surface area and the ratio of mesopore to micropore can be successfully manipulated by the activation process of KOH etching plus CO2 gasification. The results showed that the activated carbon electrodes with controlled mesoporosity exhibited higher specific capacitance and better rate capability as compared to the commercial one. The coexistence of mesopores and micropores can provide large surface area for ions to form an electrical double layer, while the enlarged mesoporosity can not only facilitate the ion transport but also improve the accessible surface area, suggesting the improved capacity of capacitive ion storage. From the desalination experiments at 1.0V, the activated carbon electrode, associated with a specific surface area of 2105m2g−1 and a 70.7% ratio of mesopore to total pore volume, presented an electrosorption capacity of 9.72mgg−1 and electrosorption rate constant of 0.060min−1, which were considerably higher than the micropore-dominant carbon electrodes. Therefore, the significantly improved desalination performance can be ascribed to the high surface area and the high ratio of mesoporosity in the activated carbon-based capacitive deionization.

Predicting Permeate Fluxes and Rejection Rates in Reverse Osmosis and Tight‐Nanofiltration Processes

AbstractThe performance of reverse osmosis and tight nanofiltration with flat‐sheet membranes can be predicted accurately. The proposed numerical model solves the local momentum and mass conservation equations in the module's feed channel with solution‐diffusion boundary conditions. Both qualitative and quantitative predictions of the permeate flux and of the rejection rate are obtained with an accuracy depending on the limitations of the solution‐diffusion model for describing membrane mass transport and on the value of solute permeability. As an extension of the applications to plate‐and‐frame modules, the ability to describe the performance of processes carried out with spiral‐wound modules is also tested with own desalination experiments and with data from the literature.

Study on the Treatment of Circulating Water: PPy/PAN-based Activated Carbon Fiber Felt Composite as Electro-adsorption Electrode

This study adopted PPy/PAN-based carbon fiber felt composite as a new electrode for desalination experiment. The result shows that this kind of composite electrode can significantly reduce the outlet conductivity when inlet chemical oxygen demand (COD) is over 300 mg/l. It additionally reduces COD to the value obtained with other electrodes.