Direct contact membrane distillation for effective concentration of perfluoroalkyl substances - Impact of surface fouling and material stability.

Polyfluoroalkyl and perfluoroalkyl substances (PFAS) are ecotoxic amphiphilic compounds containing alkyl-fluorinated chains terminated with weak acid moieties, and hence difficult to be degraded or removed from water sources. Direct contact membrane distillation (DCMD) was used for concentrating and removing of perfluoropentanoic acid (PFPeA) compounds from model contaminated water using commercially available poly (tetrafluoroethylene) (PTFE) membranes. The membranes were characterised for surface morphology, roughness, contact angle and pore size distribution before and after the DCMD test to investigate and evaluate membrane fouling. During the DCMD test performed for 6 h using 10 ppm PFPeA solution, the membrane exhibited progressive increased flux (from 17 to 43 kg m-2 h-1) and decreased PFPeA rejection (from 85 to 58%), as the feed temperature was increased from 50 to 70 °C. Further, the feed/retentate side showed a 1.8, 2.1 and 2.8-fold increase in PFPeA concentration tested at feed temperatures 50, 60, and 70 °C, respectively. The permeate side contained less than 1 ppm of PFPeA revealing that the PFPeA moved across the PTFE membrane during DCMD, which is attributed to progressive surface diffusion over time. This study opens a new route to concentrate and remove amphiphilic molecules, such as PFAS, from source points, relevant to landfill leachates or surface waters. The study also points at gaps in materials science and surface engineering to be tackled to deal with PFAS compounds efficiently.

Current and Emerging Techniques for High-Pressure Membrane Integrity Testing.

Ideally, pressure driven membrane processes used in wastewater treatment such as reverse osmosis and nanofiltration should provide a complete physical barrier to the passage of pathogens such as enteric viruses. In reality, manufacturing imperfections combined with membrane ageing and damage can result in breaches as small as 20 to 30 nm in diameter, sufficient to allow enteric viruses to contaminate the treated water and compromise public health. In addition to continuous monitoring, frequent demonstration of the integrity of membranes is required to provide assurance that the barrier to the passage of such contaminants is intact. Existing membrane integrity monitoring systems, however, are limited and health regulators typically credit high-pressure membrane systems with only 2 log10 virus rejection, well below their capability. A reliable real-time method that can recognize the true rejection potential of membrane systems greater than 4 log10 has not yet been established. This review provides a critical evaluation of the current methods of integrity monitoring and identifies novel approaches that have the potential to provide accurate, representative virus removal efficiency estimates.

Seeing is believing: Insights from synchrotron infrared mapping for membrane fouling in osmotic membrane bioreactors.

We employed synchrotron infrared (IR) mapping to resolve forward osmosis (FO) membrane fouling in osmotic membrane bioreactor (OMBR). Synchrotron IR mapping offers a unique perspective to elucidate the fouling mechanisms and associated consequences in OMBR operation. We demonstrated the spatial distribution and relative intensity of carbohydrate and protein longitudinally along of the fouled FO membrane at the conclusion of OMBR operation. Both transmission and attenuated total reflection (ATR) modes were used to map the cross-section and surface of the fouled FO membrane. Micro X-ray computed tomography revealed patchy, "sand-dune" features on the membrane surface at the conclusion of OMBR operation. Synchrotron IR-ATR mapping demonstrated that the development of membrane fouling layer in OMBR operation was initiated by polysaccharide-like carbohydrate, followed by layering with protein-like substance, resulting in a characteristic "sand-dune" three dimensional feature. Synchrotron FTIR mapping shed light on foulant occurrence and accumulation in the draw solution. Strong penetration of protein-like substance into membrane matrix was visualised, resulting the detection of protein adsorption in the region of membrane supporting layer.

Trace organic contaminant rejection by aquaporin forward osmosis membrane: Transport mechanisms and membrane stability.

We investigated transport mechanisms of trace organic contaminants (TrOCs) through aquaporin thin-film composite forward osmosis (FO) membrane, and membrane stability under extreme conditions with respect to TrOC rejections. Morphology and surface chemistry of the aquaporin membrane were characterised to identify the incorporation of aquaporin vesicles into membrane active layer. Pore hindrance model was used to estimate aquaporin membrane pore size as well as to describe TrOC transport. TrOC transport mechanisms were revealed by varying concentration and type of draw solutions. Experimental results showed that mechanism of TrOC transport through aquaporin-embedded FO membrane was dominated by solution-diffusion mechanism. Non-ionic TrOC rejections were molecular-weight dependent, suggesting steric hindrance mechanisms. On the other hand, ionic TrOC rejections were less sensitive to molecular size, indicating electrostatic interaction. TrOC transport through aquaporin membrane was also subjected to retarded forward diffusion where reverse draw solute flux could hinder the forward diffusion of feed TrOC solutes, reducing their permeation through the FO membrane. Aquaporin membrane stability was demonstrated by either heat treatment or ethanol solvent challenges. Thermal stability of the aquaporin membrane was manifested as a relatively unchanged TrOC rejection before and after the heat treatment challenge test. By contrast, ethanol solvent challenge resulted in a decrease in TrOC rejection, which was evident by the disappearance of the lipid tail of the aquaporin vesicles from infrared spectrum and a notable decrease in the membrane pore size.

Surface pattern by nanoimprint for membrane fouling mitigation: Design, performance and mechanisms.

Imparting water treatment membrane with surface pattern by nanoimprint offered a novel approach to fouling resistance. We employed nanoimprint to fabricate line-shape nanostructure on membrane distillation (MD) membrane surface. Patterned MD membrane exhibited strong antifouling property to Bovine Serum Albumin (BSA) protein during MD separation. Water flux decline and protein deposition were substantially minimized on the patterned MD membrane in comparison with the pristine one. Such lower fouling propensity on the patterned MD membrane was mainly driven by the weak hydrophobic interaction between BSA protein and patterned MD membrane surface. Weaker adhesion force mapping of the patterned MD membrane was quantified. Representative force-distance curve of pristine MD membrane showed a strong attractive depletion force comparing with that of patterned one. The simple, chemical-free, and scalable nanofabrication approach enables varying designs on membrane surface for special membrane properties.

A structural basis for the amphiphilic character of alginates - Implications for membrane fouling.

Ostensibly hydrophilic alginates are known to foul hydrophobic membranes, under various conditions. Here, controlled experiments have been conducted at high and low pH on the fouling of a polypropylene membrane by alginate and the results suggest that the observed fouling is due to an intrinsic property of the alginate. Thus quantum chemical calculations on the M and G monomers of alginate reveal that M adopts an equilibrium geometry that is hydrophilic on one face and hydrophobic on the other, i.e. is potentially amphiphilic. Molecular dynamics simulations on short alginate chains of different sequences interacting with a modelled polypropylene surface, show that this characteristic is carried over to the polymer and results in hydrophobic patches along the chain that facilitate attractive interactions with the polypropylene surface. This concept is buttressed by an analysis of the binding characteristics of a previously reported X-ray structure of the mannuronan C-5 epimerase AlgE4 enzyme.

Synchrotron Fourier transform infrared mapping: A novel approach for membrane fouling characterization.

We described a synchrotron-based infrared (IR) microscopic method to characterize fouling layer induced by organic foulants and colloidal silica in membrane distillation (MD). This technique, utilizing the ultrahigh brightness of synchrotron infrared source, enables spectra with high signal-to-noise ratio that was obtained from micrometer-sized samples. Our results showed that synchrotron IR mapping was able to resolve the foulant spatial distribution in combined fouling in MD. Synchrotron IR mapping showed the spatial distribution of binary foulant (i.e., colloidal silica with alginate, bovine serum albumin (BSA) or humic acid, respectively) of the cross-section of MD membrane fouling layer. The well-resolved synchrotron IR mapping is also able to quantify the foulant distribution along the cross-section of the fouled MD membrane, providing detailed information regarding the transport and accumulation of specific foulant, which is of paramount importance to elucidate fouling mechanisms. Our results demonstrated that the synchrotron IR mapping method was a powerful method and had significant potential for both qualitative and quantitative characterization of membrane fouling layer.

Silica scaling in forward osmosis: From solution to membrane interface.

Membrane silica scaling hinders sustainable water production. Understanding silica scaling mechanisms provides options for better membrane process management. In this study, we elucidated silica scaling mechanisms on an asymmetric cellulose triacetate (CTA) membrane and polyamide thin-film composite (TFC) membrane. Scaling filtration showed that TFC membrane was subjected to more severe water flux decline in comparison with the CTA membrane, together with different scaling layer morphology. To elucidate the silica scaling mechanisms, silica species in the aqueous solution were characterised by mass spectrometry as well as light scattering. Key thermodynamic parameters of silica surface nucleation on the CTA and TFC membranes were estimated to compare the surface nucleation energy barrier. In addition, high resolution X-ray photoelectron spectroscopy resolved the chemical origin of the silica-membrane interaction via identifying the specific silicon bonds. These results strongly support that silica scaling in the CTA membrane was driven by the aggregation of mono-silicic acid into large silica aggregates, followed by the deposition from bulk solution onto the membrane surface; by contrast, silica polymerised on the TFC membrane surface where mono-silicic acid interacted with TFC membrane surface, which was followed by silica surface polymerisation.

The effect of electromagnetic fields, from two commercially available water treatment devices, on bacterial culturability.

Commercially available pulsed-electromagnetic field (PEMF) devices are currently being marketed and employed to ostensibly manage biofouling. The reliable application and industry acceptance of such technologies require thorough scientific validation - and this is currently lacking. We have initiated proof-of-principle research in an effort to investigate whether such commercially available PEMF devices can influence the viability (culturability) of planktonic bacteria in an aqueous environment. Thus two different commercial PEMF devices were investigated via a static (i.e. non-flowing) treatment system. 'Healthy' Escherichia coli cells, as well as cultures that were physiologically compromised by silver nano-particles, were exposed to the PEMFs from both devices under controlled conditions. Although relatively minor, the observed effects were nevertheless statistically significant and consistent with the hypothesis that PEMF exposure under controlled conditions may result in a decrease in cellular viability and culturability. It has also been observed that under certain conditions bacterial growth is actually stimulated.

Membrane-based processes for wastewater nutrient recovery: Technology, challenges, and future direction.

Wastewater nutrient recovery holds promise for more sustainable water and agricultural industries. We critically review three emerging membrane processes - forward osmosis (FO), membrane distillation (MD) and electrodialysis (ED) - that can advance wastewater nutrient recovery. Challenges associated with wastewater nutrient recovery were identified. The advantages and challenges of applying FO, MD, and ED technologies to wastewater nutrient recovery are discussed, and directions for future research and development are identified. Emphasis is given to exploration of the unique mass transfer properties of these membrane processes in the context of wastewater nutrient recovery. We highlight that hybridising these membrane processes with existing nutrient precipitation process will lead to better management of and more diverse pathways for near complete nutrient recovery in wastewater treatment facilities.

Properties of acid whey as a function of pH and temperature.

Compositional differences of acid whey (AW) in comparison with other whey types limit its processability and application of conventional membrane processing. Hence, the present study aimed to identify chemical and physical properties of AW solutions as a function of pH (3 to 10.5) at 4 different temperatures (15, 25, 40, or 90°C) to propose appropriate membrane-processing conditions for efficient use of AW streams. The concentration of minerals, mainly calcium and phosphate, and proteins in centrifuged supernatants was significantly lowered with increase in either pH or temperature. Lactic acid content decreased with pH decline and rose at higher temperatures. Calcium appeared to form complexes with phosphates and lactates mainly, which in turn may have induced molecular attractions with the proteins. An increase in pH led to more soluble protein aggregates with large particle sizes. Surface hydrophobicity of these particles increased significantly with temperature up to 40°C and decreased with further heating to 90°C. Surface charge was clearly pH dependent. High lactic acid concentrations appeared to hinder protein aggregation by hydrophobic interactions and may also indirectly influence protein denaturation. Processing conditions such as pH and temperature need to be optimized to manipulate composition, state, and surface characteristics of components of AW systems to achieve an efficient separation and concentration of lactic acid and lactose.

Chemistry of silica scale mitigation for RO desalination with particular reference to remote operations.

Silica scaling in reverse osmosis of groundwater is a significant issue in water stressed areas due to the limitations that scaling imposes on water recovery. While calcium and magnesium scaling potential can be significantly reduced by the use of ion exchange or other softening processes, the silica scaling potential typically remains. Improving the recovery of reverse osmosis by limiting the potential for silica scale is important in ensuring maximum water recovery. This is particularly important for mining and natural gas industries that are located in remote regions. The remote nature of these sites imposes three major restrictions on the silica scale mitigation process. Firstly, the generation of poorly dewaterable sludges must be avoided. Also, the quality of any reverse osmosis (RO) permeate must be able to meet the end use requirements, particularly for boilers. Finally, silica removal should not impact upon other potentially useful or valuable components within the brine, and should not make the disposal of the unusable waste brine components more difficult. Reduction of scaling potential can be achieved in three main ways: operating RO at high pH after hardness has been removed, operating at low pH, and reducing the silica concentration either in pretreatment or by using an interstage technique. Operating at high pH has the initial requirement of hardness removal to prevent scaling and this could be an issue on some sites. Hardness removal operations that use ion exchange resins may be challenged by water chemistry and the operational costs associated with high chemical regeneration costs. Operating at low pH may be more desirable than high pH operation as this can help to reduce the risk of scale formation from calcium or magnesium salts. The drawback comes from the cost of acid, particularly for high-alkalinity waters. There are numerous silica removal techniques including chemical dosing of lime, or aluminium or iron salts, electrocoagulation, adsorption, ion exchange and seeded precipitation. Of these, adsorption onto aluminium compounds appears to give the best results and have received the most attention where restrictions on sludge production and brine disposal common to operations in remote locations are in place. Adsorption onto iron compounds appears to occur more quickly, but leads to the formation of a hard, glass-like scale that may be more difficult to remove, making this process unattractive from the point of view of sorbent regeneration.

The role of poly-M and poly-GM sequences in the metal-mediated assembly of alginate gels.

Whilst the involvement of poly-G sequences in the formation of metal-mediated alginate gels has been previously studied in some detail, investigations into the role of poly-M and poly-GM sequences has been relatively neglected. In this regard, the binding of sodium and calcium ions to poly-M and poly-GM decamers, and their influence on chain aggregation, has been modelled by conducting a series of molecular dynamics simulations. This work complements a previous analogous study carried out for the poly-G decamer, whereby up to three strands are systematically introduced into each simulation. As in the previous study, this method allows intrinsic binding modes and interchain structural motifs to be revealed, that are consistent with those observed in available AFM images of consolidated 3-D networks. It is apparent from these studies that different sequences have different structural implications for metal-mediated chain association.

Exploring the molecular basis for the metal-mediated assembly of alginate gels.

The binding of sodium and calcium ions to single and multiple poly-G decamer strands has been modelled by conducting a series of molecular dynamics simulations. Implications for metal mediated inter-strand interactions and gel assembly have been explored by systematically introducing up to three strands into each of these simulations. A particular emphasis has been placed on revealing intrinsic binding modes by an unbiased initial positioning of the metal ions. The results have revealed binding modes that provide a rationale for the observed gelling of alginate by calcium rather than sodium ions. A number of junction zones involving calcium ions have been identified that result in chain aggregation. This includes a distinctive perpendicular motif that appears to be ubiquitous in previously reported AFM images of open 3-D alginate networks. The coordination geometries of the metal ions have been characterised and the metal-mediated junctions between associated strands are described in detail.