Rejection Of Trace Organic Contaminants Research Articles

Understanding Rejection Mechanisms of Trace Organic Contaminants by Polyamide Membranes via Data-Knowledge Codriven Machine Learning.

Data-driven machine learning (ML) provides a promising approach to understanding and predicting the rejection of trace organic contaminants (TrOCs) by polyamide (PA). However, various confounding variables, coupled with data scarcity, restrict the direct application of data-driven ML. In this study, we developed a data-knowledge codriven ML model via domain-knowledge embedding and explored its application in comprehending TrOC rejection by PA membranes. Domain-knowledge embedding enhanced both the predictive performance and the interpretability of the ML model. The contribution of key mechanisms, including size exclusion, charge effect, hydrophobic interaction, etc., that dominate the rejections of the three TrOC categories (neutral hydrophilic, neutral hydrophobic, and charged TrOCs) was quantified. Log D and molecular charge emerge as key factors contributing to the discernible variations in the rejection among the three TrOC categories. Furthermore, we quantitatively compared the TrOC rejection mechanisms between nanofiltration (NF) and reverse osmosis (RO) PA membranes. The charge effect and hydrophobic interactions possessed higher weights for NF to reject TrOCs, while the size exclusion in RO played a more important role. This study demonstrated the effectiveness of the data-knowledge codriven ML method in understanding TrOC rejection by PA membranes, providing a methodology to formulate a strategy for targeted TrOC removal.

Ultra-permeable polyamide nanofiltration membrane modified by hydrophilic-hydrophobic alternated lignocellulosic nanofibrils for efficient water reuse

Polyamide nanofiltration (NF) membranes are instrumental in water reuse due to their capability to efficiently remove trace organic contaminants (TrOCs), which pose potential health risks. However, while the current NF membranes offer acceptable TrOC rejection, their water permeance, which is highly related to the process energy consumption in water reuse, is relatively low. To address this limitation, we introduced a novel NF membrane by incorporating a lignocellulosic nanofibril interlayer—specifically, nanowires with alternating hydrophilic-hydrophobic regions (denoted as HONW). The resultant NF membrane achieved remarkable water permeance (52.4 L m−2 h−1 bar−1) while still ensuring high TrOC retention (including endocrine-disrupting compounds, pharmaceuticals, and antibiotics). A detailed mechanistic investigation revealed that the eliminated funnel effect or introduced gutter effect induced by the HONW interlayer, improved polyamide layer structure characterized by lower cross-linking degree and higher surface wettability, and reduced interlayer resistance (due to a unique water transport pattern: hydrophilic regions absorbed water molecules while hydrophobic regions strengthened water penetration) were significantly responsible for the enhanced membrane permeance. Moreover, the membrane also demonstrated low specific energy consumption as well as outstanding stability. This investigation unveils an appealing approach for constructing the ultra-permeable NF membrane for efficient water reuse.

Thin film composite reverse osmosis membranes with metal-organic coordination complexes stabilized CNTs interlayer for enhanced removal of trace organic contaminants

Reverse osmosis (RO) has been recognized as a reliable technique for wastewater reclamation. To enhance its rejection of trace organic contaminants (TrOCs), a novel strategy was proposed to improve reverse osmosis (RO) membranes by fabricating thin-film nanocomposite membrane (TFNi) with a TA-Fe3+/CNTs interlayer. The TFNi-30 membrane was obtained by optimizing the loading capacity. This optimized membrane gave rise to an enhanced water permeance of 3.2 ± 0.2 L m-2 h-1 bar-1, which was two times higher than that of the control membrane (1.1 ± 0.2 L m-2 h-1 bar-1), while achieving a considerably higher rejection of inorganic salts and trace organic contaminants (TrOCs) thanks to the formation of a denser polyamide rejection layer. In addition, the specific roles of the interlayer structure in determining the separation performance of RO membranes were investigated by tailoring the support-free RO membranes and comparing them with the conventional membranes. This study would provide a peer reference for designing high performance RO membranes for wastewater reclamation via the TFNi strategy.

Molecular Design of the Polyamide Layer Structure of Nanofiltration Membranes by Sacrificing Hydrolyzable Groups toward Enhanced Separation Performance.

Nanofiltration (NF) is an effective technology for removing trace organic contaminants (TrOCs), while the inherent trade-off effect between water permeance and solute rejections hinders its widespread application in water treatment. Herein, we propose a novel scheme of "monomers with sacrificial groups" to regulate the microstructure of the polyamide active layer via introducing a hydrolyzable ester group onto piperazine to control the diffusion and interfacial polymerization process. The achieved benefits include narrowing the pore size, improving the interpore connectivity, enhancing the microporosity, and reducing the active layer thickness, which collectively realized the simultaneous improvement of water permeance and enhancement of TrOCs rejection performance. The resulting membranes were superior to both the control and commercial membranes, especially in water-TrOCs selectivity. The effects of using the new monomers on the membrane physicochemical properties were systematically studied, and underlying mechanisms for the enhanced separation performance were further revealed by simulating the polymerization process through density functional theory calculation and measuring the trans-interface diffusion rate of monomers. This study demonstrates a novel promising NF membrane synthesis strategy by designing the structure of reaction monomers for achieving excellent rejection of TrOCs with a low energy input in water treatment.

In-situ assembled amino-quinone network of nanofiltration membrane for simultaneously enhanced trace organic contaminants separation and antifouling properties

Nanofiltration has attracted extensive attention in surface water purification and wastewater reclamation with its high permeance and effective interception of dissolved organic matter. However, the poor removal of trace organic contaminants and membrane fouling still limit its application, resulting in the potential health risk and excess energy consumption, respectively. Herein, an amine-quinone networks (AQN) coating coordinated by the 5-hydroxy-1,4-naphthoquinone (HNQ) and polyethyleneimine (PEI) was simply and facilely assembled on a commercial nanofiltration membrane. The optimum modified membrane conferred a significant increase in the rejection of trace organic contaminants (e.g., over 99% for ofloxacin). Simultaneously, the permeance of the modified membrane exhibited a slight enhancement (10%) over that of the pristine membrane thanks to the greater hydrophilicity of AQN coating without blocking pores of the polyamide layer. Static adsorption of BSA indicated that the NF-AQN membrane had excellent fouling resistance against proteins. In addition, after several cycles of dynamic fouling experiment, the higher flux recovery after physical cleaning and lower flux decline during filtration further demonstrated the outstanding antifouling property of the modified membrane. This study presents a low-cost, facile and scalable method using AQN coating for improving the performance of existing commercial nanofiltration membranes and alleviating their fouling in operation.

Forward osmosis membrane developed from the chelation of Fe3+ and carboxylate for trace organic contaminants removal

Wastewater containing trace organic contaminants (TrOCs) is usually poorly purified by unmodified polyamide (PA) membrane due to its poor selectivity and low water permeability. Herein we design a series of carboxylate-functionalized PA membranes via the chelation of Fe3+ and carboxylate for efficient TrOCs removal through forward osmosis (FO). Citric acid, oxalic acid and their sodium salts are individually tethered to the PA membrane in water under ambient conditions. Factors influencing membrane performance are systematically investigated. The carboxylate based modification greatly improves the membrane properties that substantially enhance its separation performance. The sodium citrate (CA-Na) modified membrane produces water fluxes of up to 106% (PRO mode) and 144% (FO mode) higher than those of the PA membrane. This membrane also increases the TrOCs rejection from 75% to over 92%, irrespective of the TrOCs properties. Membrane fouling experiments reveal that the water flux of the CA-Na membrane declines by 26%, while that of the PA membrane decreases by 38% after 10-h tests. The flux recovery ratio of the CA-Na membrane achieves over 95% after physical cleaning. This study demonstrates an eco-friendly protocol to construct PA-based membrane for efficient TrOCs removal.

New insights into the relationship between draw solution chemistry and trace organic rejection by forward osmosis

This study elucidates the impact of draw solution chemistry (in terms of pH and draw solute species) and membrane fouling on water flux and the rejection of trace organic contaminants by forward osmosis. The results show that draw solution chemistry could induce a notable impact on both water flux and TrOCs rejection. In addition, the impact was further influenced by membrane fouling. The reverse flux of proton (or hydroxyl) could alter the feed solution pH, which governed the separation of ionisable TrOCs. In addition, charged compounds generally exhibited higher rejections than neutral ones by the clean membrane. Electrostatic interaction, rather than size exclusion, was therefore the dominant rejection mechanism for most compounds. There was also a weak correlation between rejection and molecular sizes of the 43 TrOCs. Compared with Na+, Li+ with a larger hydrated radius showed a significantly lower reverse salt flux, resulting in a lower ionic strength and therefore a stronger electrostatic interaction. A fouling cake layer consisted of low molecular weight neutral organics could also affect TrOC rejection due to pore blockage and cake-enhanced concentration polarisation.

Retrofitting membrane bioreactor (MBR) into osmotic membrane bioreactor (OMBR): A pilot scale study

Forward osmosis (FO) technology that has been applied to a membrane bioreactor (MBR) is referred to as osmotic MBR (OMBR). The new concept developed in this study aims at retrofitting (partly or fully) existing MBR into OMBR, thus limiting the investment costs, allowing for more flexible and combined operation of MBR and OMBR and to fulfil water quality needs. Submerged FO modules were developed by modifying Kubota microfiltration (MF) modules commonly used in MBR and fitting them with new generation thin-film composite FO membranes. The similar design and water fluxes of both MF and FO modules allowed for an unbiased comparison of OMBR and MBR. For the first time, stable OMBR operation with water fluxes above 10 L.m−2.h−1 was achieved using synthetic seawater as draw solution. The proof of concept of retrofitting of MBR and OMBR was demonstrated using a 50 L reactor under varying operating conditions. Findings indicated that standalone OMBR operation is challenging due to salinity build-up which impacts bacterial activity and permeation flux. Fouling occurred in the FO modules to the same degree as MF but osmotic backwashing proved to be an efficient cleaning solution for OMBR. The energy reduction benefit of using an osmotic gradient instead of hydraulic pressure to drive permeation was outweighed by the energy required for draw circulation in OMBR. Loss of selectivity of the FO membranes was observed due to superficial (active layer) physical damage. However, the overall high rejection of trace organic contaminants by OMBR (>90%), due to high FO membrane rejection and biological degradation, is a great benefit for OMBR implementation in water reuse schemes.

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.

Impact of hydraulic pressure on membrane deformation and trace organic contaminants rejection in pressure assisted osmosis (PAO)

This study provides for the first time an extensive comparison of trace organic contaminants (TrOCs) rejection by commercial cellulose tri-acetate (CTA) and thin film composite (TFC) forward osmosis (FO) membranes from HTI and Porifera operated under pressure assisted osmosis (PAO) conditions. Commercial TFC membranes allowed for higher water permeabilities, higher selectivities and higher water fluxes in FO and PAO operation, compared to the HTI CTA benchmark. As for HTI CTA, TFC membranes suffered from deformation due to the hydraulic pressure applied in the PAO process. However, not only deformation by stretching of the active layer, but also compaction of the support layer was observed, reducing internal concentration polarisation (ICP) and allowing for flux enhancement. In FO operation, the TFC membranes demonstrated a high rejection (>80% for HTI TFC and >90% for Porifera) of the whole range of tested TrOCs due to steric hindrance. It was also noticed that, being more negatively charged, the TFC membranes allowed for very high rejection of negatively charged compounds, but lower rejection of positively charged molecules, as a consequence of electrostatic interactions. In PAO operation, a general decrease of TrOCs rejection was observed. This could possibly be a consequence of decreasing selectivity (due to membrane deformation), increased TrOCs external concentration polarisation and/or lower reverse salt diffusion (less hindrance of forward TrOCs diffusion).

Impact of organic and colloidal fouling on trace organic contaminant rejection by forward osmosis: Role of initial permeate flux

Fouling behaviour and its impact on the rejection of trace organic contaminants (TrOCs) by forward osmosis (FO) were investigated. Membrane fouling was simulated using humic acid and colloidal particles as model foulants at different initial permeate water fluxes. Water flux decline was insignificant at an initial permeate flux of 9L/m2h and the fouling layer was loose and fluid-like. By contrast, the water flux decline was substantial at an initial permeate flux of 20L/m2h, resulting in the formation of a compact fouling layer. Water flux recovery after physical cleaning for both humic acid and colloidal particle fouled membranes was consistently higher at an initial permeate flux of 9L/m2h compared to 20L/m2h. The results suggest that the fouling layer structure varied from a fluid-like loose layer at low initial permeate flux to a more cohesive and compact layer at high initial permeate flux. We surmise that the fluid-like loose layer formed at low initial permeate flux contributed to pore blockage and thus enhanced steric hindrance, thereby leading to an increase in TrOC rejection. By contrast, the cohesive and compact fouling layer formed at high initial permeate flux exacerbated cake-enhanced concentration polarisation, resulting in a decrease in TrOC rejection.

The effects of feed solution temperature on pore size and trace organic contaminant rejection by the nanofiltration membrane NF270

This study investigated the effect of feed temperature on membrane pore size and the rejection of trace organic contaminants (TrOCs) by the nanofiltration (NF) membrane NF270. Filtration experiments were conducted using a cross flow membrane system at 20, 30 and 40°C. The membrane pore radius was estimated using the pore hindrance transport model at each temperature and the rejection data of three reference organic solutes (i.e. erythritol, xylose and glucose) experimentally obtained in this study. The results suggest that the pore size of an NF membrane is dependent on the feed solution temperature. An increase in the feed temperature from 20 to 40°C led to an increase in the effective pore radius from 0.39 to 0.44nm. Consequently, the increase in the feed temperature also caused a considerable drop in the rejection of all TrOCs investigated in this study. The decrease in rejection observed here could be attributed to not only the increase in the solute diffusivity but also the enlargement of the membrane pore size. As the feed temperature increased, the decrease in rejection of neutral TrOCs was more severe than that of negatively charged compounds. This is because in addition to size exclusion (or steric hindrance) the rejection of negatively charged TrOCs is also governed by electrostatic interaction given that the membrane surface is also negatively charged.

Relating rejection of trace organic contaminants to membrane properties in forward osmosis: Measurements, modelling and implications

This study elucidates the relationship between membrane properties and the rejection of trace organic contaminants (TrOCs) in forward osmosis (FO). An asymmetric cellulose triacetate (CTA) and a thin-film composite (TFC) polyamide FO membrane were used for this investigation. The effective average pore radius (rp), selective barrier thickness over porosity parameter (l/ε), surface charge, support layer structural parameter (S), pure water permeability coefficient (A) and salt (NaCl) permeability coefficient (B) of the two membranes were systematically characterised. Results show that measured rejection of TrOCs as a function of permeate water flux can be well described by the pore hindrance transport model. This observation represents the first successful application of this model, which was developed for pressure-driven nanofiltration, to an osmotically-driven membrane process. The rejection of charged TrOCs by the CTA and TFC membranes was high and was governed by both electrostatic repulsion and steric hindrance. The TFC membrane exhibited higher rejection of neutral TrOCs with low molecular weight than the CTA membrane, although the estimated pore size of the TFC membrane (0.42 nm) was slightly larger than that of the CTA membrane (0.37 nm). This higher rejection of neutral TrOCs by the TFC membrane is likely attributed to its active layer properties, namely a more effective active layer structure, as indicated by a larger l/ε parameter, and pore hydration induced by the negative surface charge.

Factors governing the rejection of trace organic contaminants by nanofiltration and reverse osmosis membranes

This study aimed to elucidate key factors governing the rejection of trace organic contaminants (TrOCs) by nanofiltration (NF) and reverse osmosis (RO) membranes. The rejection of 16 selected hydrophilic and hydrophobic TrOCs by an NF and an RO membranes was evaluated at different solution pH values using a cross-flow NF/RO filtration system. An analytical technique consisting of solid phase extraction followed by gas chromatography and mass spectrometry detection was used for the analysis of the TrOCs. In general, rejection increased in the order of decreasing membrane permeability, increasing molecular weight (or equivalent molecular width) of the TrOCs, and increasing hydrophilicity. Adsorption of hydrophobic TrOCs to the membrane could be observed based on a mass balance calculation. However, the correlation between adsorption and log D value (the logarithm of the octanol-water distribution coefficient) of the TrOCs (which indicates their hydrophobicity) observed in this study was rather weak. This is due to the adsorption being not only dependent on hydrophobicity, but also on other physicochemical aspects of TrOCs and the membrane material, such as molecular size, charge of the compounds, pore size, charge, and surface roughness properties of the membranes. Therefore, the results suggest that these factors may also govern the adsorption (and subsequently rejection) of TrOCs to NF/RO membranes.

Effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of trace organic contaminants by forward osmosis

The effects of feed and draw solution temperature and transmembrane temperature difference on the rejection of 12 trace organic contaminants (TrOCs) by two forward osmosis (FO) membranes were investigated. The membrane structure parameter (S) and the reverse salt (NaCl) flux selectivity (RSFS) were constant over the temperature range of 20–40°C, suggesting that within this range, the solution temperature did not significantly influence the membrane polymeric structure. Draw solution properties, including diffusivity, viscosity, and osmotic pressure, played an important role in water and reverse salt (NaCl) flux behaviour and TrOC rejection. Pure water and salt (NaCl) permeability coefficients of the two forward osmosis membranes increased as both the feed and draw solution temperatures increased from 20 to 40°C due to an increase in solute diffusivity and a decrease in water viscosity. Rejection of charged TrOCs was higher than that of neutral TrOCs and their rejection was insensitive to temperature variation. On the other hand, rejection of neutral TrOCs decreased significantly when the feed and draw solution temperatures were 40 and 20°C, respectively, due to the increase in their diffusivity at an elevated temperature. By contrast, rejection of neutral TrOCs increased when the feed and draw solution temperatures were 20 and 40°C, respectively. The reverse salt (NaCl) flux increased due to an increase in the draw solute diffusivity. In addition, at a higher draw solution temperature, the dilution effect induced by higher water flux and the hindrance effect enhanced by a higher reverse salt (NaCl) flux led to the increase in the rejection of neutral TrOCs.

Removal of trace organic contaminants by the forward osmosis process

The rejection of trace organic contaminants (TrOCs) by an osmotically driven membrane filtration process was investigated. A set of 40 compounds representing major groups of TrOCs of concern was selected for this study. The rejection of the TrOCs by a commercial cellulose acetate asymmetric forward osmosis membrane, as well as a “tight” commercial thin-film composite nanofiltration (NF) membrane, was systematically investigated and compared under three different operating modes: forward osmosis (FO), pressure retarded osmosis (PRO) and reverse osmosis (RO). Results revealed that the cellulose acetate membrane had considerably smaller water and salt permeabilities as well as less negative surface charge compared to the NF membrane. However, the cellulose acetate membrane resulted in considerably higher water flux than the NF when operated in FO and PRO modes. Nevertheless, the NF membrane displayed consistently better TrOC rejection than the HTI membrane. In RO mode, electrostatic interactions played a dominant role in governing the rejection of charged TrOCs. In FO and PRO modes, the rejection of charged TrOCs was governed by both electrostatic interaction and size exclusion, while rejection of neutral compounds was dominated by size exclusion, with rejection increasing with TrOC molecular weight. Operating in PRO mode resulted in a higher water flux but a notably lower TrOC rejection as compared with FO mode, because of more severe internal concentration polarization (ICP) phenomenon. Another important observation from this study is that rejection of neutral TrOCs in FO mode was higher than that in RO mode. This could be attributed to the retarded forward diffusion of TrOCs resulting from reverse salt flux of the NaCl draw solution, a phenomenon that takes place in FO mode but is not possible in RO mode.

Effects of Sorption on the Rejection of Trace Organic Contaminants During Nanofiltration

Understanding the removal of trace organic contaminants is critical for membrane applications in water recycling. This study investigates the relationship between trace contaminant sorption and their rejection by nanofiltration (NF) membranes. A mass balance is developed that quantitatively links the rejection decline over time seen with some sorbing compounds to the total mass found sorbed on the membrane. The sorbed mass of perfluorooctane sulfonamide (FOSA) and fluoxetine evaluated from the mass balance agreed to within approximately 30% of the quantity analytically determined via extraction. Static sorption experiments show that sorption takes place predominantly within the polyamide separating layer of the membrane. Finally, the relationship between the steady-state rejection and sorption tendency of ten trace organic compounds is elucidated. A greater tendency to sorb results in lower steady-state rejection, both when comparing compounds of similar size, as well as when comparing the same compound under different conditions. As a result, a major finding is that in the presence of competitive sorption, that is, the presence of other trace organic compounds in the membrane matrix, some compounds sorb less and are therefore rejected more than when these compounds are alone in the feed. At no point during experimentation was any effect on the water flux observed.

Influence of colloidal fouling on rejection of trace organic contaminants by reverse osmosis

A systematic investigation on the influence of colloidal fouling on removal of trace organics (steroid hormones) and inert organics by reverse osmosis (RO) membranes is reported. Results of laboratory-scale crossflow membrane filtration experiments showed that colloidal fouling caused a marked decrease in rejection of inert organic molecules with molecular weight smaller than about 100 g/mol. While the removal of inert organics was mainly due to steric (size) exclusion, adsorption and diffusion across the membrane polymer played a prominent role in the removal of the steroid hormones by the RO membrane. During membrane fouling, rejection of the inert organics and salt declined to a minimum, but then improved slowly and eventually stabilized at a fixed value. Hormone rejection, on the other hand, decreased rapidly initially, and then gradually slowed down regardless of fouling. This rejection behavior is associated with the decrease in feed hormone concentration due to adsorption and the increase in permeate concentration due to diffusion across the membrane skin layer. The decline in hormone rejection with time was much more severe when colloidal fouling took place. The solute rejection behavior is markedly influenced by the cake-enhanced concentration polarization, whereby the colloidal cake layer hinders back diffusion of solutes from the membrane surface to the bulk solution. The buildup of solutes at the membrane surface results in a higher solute concentration gradient across the membrane and, thus, a greater solute transport through the membrane and a lower observed solute rejection. The rate of decline in permeate flux and solute (hormones, inert organics, and salt) rejection correlated inversely with channel wall shear rate.