Rejection mechanisms for contaminants in polyamide reverse osmosis membranes

Machine learning models for predicting the rejection of organic pollutants by forward osmosis and reverse osmosis membranes and unveiling the rejection mechanisms

While forward osmosis (FO) and reverse osmosis (RO) processes have been proven effective in rejecting organic pollutants, the rejection rate is highly dependent on compound and membrane characteristics, as well as operating conditions. This study aims to establish machine learning (ML) models for predicting the rejection of organic pollutants by FO and RO and providing insights into the underlying rejection mechanisms. Among the 14 ML models established, the random forest model (R2 = 0.85) and extreme gradient boosting model (R2 = 0.92) emerged as the best-performing models for FO and RO, respectively. Shapley additive explanations (SHAP) analysis identified the length of the compound, water flux, and hydrophobicity as the top three variables contributing to the FO model. For RO, in addition to the length of the compound and operating pressure, advanced variables including four molecular descriptors (e.g., ATSC2m and Balaban J) and three fingerprints (e.g., C=C double bond and carbonyl group) significantly contributed to the prediction. Besides, the associations between these highly ranked variables and their SHAP values shed light on the rejection mechanisms, such as size exclusion, adsorption, hydrophobic interaction, and electrostatic interaction, and illustrate the role of the operating parameters, such as the FO permeate water flux and RO operating pressure, in the rejection process. These findings provide interpretable predictive models for the removal of organic pollutants and advance the mechanistic understanding of the rejection mechanisms in the FO and RO processes.

Relationships between Water's Structure and Solute Affinity at Polypeptoid Brush Surfaces

Excellent antifouling surfaces are generally thought to create a tightly bound layer of water that resists solute adsorption, and highly hydrophilic surfaces such as those with zwitterionic functionalities are of significant current interest as antifoulant strategies. However, despite significant proofs-of-concept, we still lack a fundamental understanding of how the nanoscopic structure of this hydration layer translates to reduced fouling, how surface chemistry can be tuned to achieve antifouling through hydration water, and why, in particular, zwitterionic surfaces seem so promising. Here, we use molecular dynamics simulations and free energy calculations to investigate the molecular relationships among surface chemistry, hydration water structure, and surface-solute affinity across a variety of surface-decorated chemistries. Specifically, we consider polypeptoid-decorated surfaces that display well-known experimental antifouling capabilities and that can be synthesized sequence specifically, with precise backbone positioning of, e.g., charged groups. Through simulations, we calculate the affinities of a range of small solutes to polypeptoid brush surfaces of varied side-chain chemistries. We then demonstrate that measures of the structure of surface hydration water in response to a particular surface chemistry signal solute-surface affinity; specifically, we find that zwitterionic chemistries produce solute-surface repulsion through highly coordinated hydration water while suppressing tetrahedral structuring around the solute, in contrast to uncharged surfaces that show solute-surface affinity. Based on the relationship of this structural perturbation to the affinity of small-molecule solutes, we propose a molecular mechanism by which zwitterionic surface chemistries enhance solute repulsion, with broader implications for the design of antifouling surfaces.

A Systematic Study on the Role of Hydrogen Bond Donors in Dictating the Dynamics of Choline-Based Deep Eutectic Solvents

Deep eutectic solvents, promising green alternatives to conventional solvents, consist of a hydrogen bond donor and a hydrogen bond acceptor. The hydrogen bonding components in deep eutectic solvents form an extended hydrogen bonding network, which can be tuned to specific applications by changing the hydrogen bond donors. In this work, we have changed the hydrogen bond donor from a diol to a dicarboxylic acid by systematically replacing a hydroxyl group with an acid group one at a time to investigate the solvation structure and dynamics of the deep eutectic systems. Using a combination of ultrafast vibrational spectroscopy and molecular dynamics simulations, we compared the spectral diffusion and orientational relaxation dynamics of three deep eutectic systems using the vibrational responses of a dissolved anion. Our results indicate that although the solvation structures are marginally different across the systems, distinct differences are present in the solvent fluctuation and solute reorientation dynamics. This work provides a detailed molecular understanding of carboxylic-acid-based deep eutectic systems and how they differ from alcohol-based deep eutectic systems.

Does variation in composition affect dynamics when approaching the eutectic composition?

Deep eutectic solvent is a mixture of two or more components, mixed in a certain molar ratio, such that the mixture melts at a temperature lower than individual substances. In this work, we have used a combination of ultrafast vibrational spectroscopy and molecular dynamics simulations to investigate the microscopic structure and dynamics of a deep eutectic solvent (1:2 choline chloride: ethylene glycol) at and around the eutectic composition. In particular, we have compared the spectral diffusion and orientational relaxation dynamics of these systems with varying compositions. Our results show that although the time-averaged solvent structures around a dissolved solute are comparable across compositions, both the solvent fluctuations and solute reorientation dynamics show distinct differences. We show that these subtle changes in solute and solvent dynamics with changing compositions arise from the variations in the fluctuations of the different intercomponent hydrogen bonds.

Intrinsic Dependence of Groundwater Cation Hydraulic and Concentration Features on Negatively Charged Thin Composite Nanofiltration Membrane Rejection and Permeation Behavior

Commercial nanofiltration membranes of different molecular weight cut-offs were tested on a pilot plant for the exploration of permeation nature of Ca, Mg, Mn, Fe, Na and ammonium ions. Correlation of transmembrane pressure and rejection quotient versus volumetric flux efficiency on nanofiltration membrane rejection and permeability behavior toward hydrated divalent and monovalent ions separation from the natural groundwater was observed. Membrane ion rejection affinity (MIRA) dimension was established as normalized TMP with regard to permeate solute moiety representing pressure value necessary for solute rejection change of 1%. Ion rejection coefficient (IRC) was introduced to evaluate the membrane rejection capability, and to indicate the prevailed nanofiltration partitioning mechanism near the membrane surface. Positive values of the IRC indicated satisfactory rejection efficiency of the membrane process and its negative values ensigned very low rejection affinity and high permeability of the membranes for the individual solutes. The TMP quotient and the efficiency of rejection for individual cations showed upward and downward trends along with flux utilization increase. Nanofiltration process was observed as an equilibrium. The higher the Gibbs free energy was, cation rejection was more exothermic and valuably enlarged. Low Gibbs free energy values circumferentially closer to endothermic zone indicated expressed ions permeation.

Molecular dynamics study on the elucidation of polyamide membrane fouling by nonionic surfactants and disaccharides

Reverse osmosis (RO) is a widely used energy-efficient separation technology for water treatment. Polyamide (PA) membranes are the conventional choice for this process. Fouling is a serious problem for RO separation. This issue leads to significant decreases in the water permeability of PA membranes, and it has yet to be fully elucidated. In particular, the fouling behavior of a nonionic substance on the negatively charged surface of a PA membrane in an aqueous environment has not been previously studied. In this work, the mechanisms of nonionic substances such as polyoxyethylene octyl ether (PE5) and maltose (Mal) were investigated using molecular dynamics (MD) simulations. In a PA membrane in which the carboxyl group was not dissociated, the hydrophobic portion of the membrane was exposed due to the localization of water molecules around the carboxyl groups in the PA membrane. This caused hydrophobic interaction with the hydrophobic groups of PE5. In the case of an amine-modified PA membrane containing no carboxyl groups, water was not localized around the functional group, and the water orientation of the polyamide surface was also low. Due to this membrane property, the presence of stabilized water around PE5 reduced the number of hydrophobic interactions. In similar manner, a PA membrane with a slightly dissociated carboxyl group was hydrophilic, which reduced the PE5 adsorption. The presence of many dissociated carboxyl groups, however, enhanced the adsorption of PE5 due to the increase in interactions between the dissociated carboxyl groups and the hydrophilic groups of PE5. Therefore, PE5 exhibited an amphipathic adsorption wherein both hydrophilic and hydrophobic groups contributed to adsorption onto the PA membrane. Mal, on the other hand, was highly stable in every aqueous environment independent of the state of the functional groups of the PA membrane, and was not easily affected by the properties of the PA membrane.

Atomistic Simulations of Biofouling and Molecular Transfer of a Cross-linked Aromatic Polyamide Membrane for Desalination

Reverse osmosis through a polyamide (PA) membrane is an important technique for water desalination and purification. In this study, molecular dynamics simulations were performed to study the biofouling mechanism (i.e., protein adsorption) and nonequilibrium steady-state water transfer of a cross-linked PA membrane. Our results demonstrated that the PA membrane surface's roughness is a key factor of surface's biofouling, as the lysozyme protein adsorbed on the surface's cavity site displays extremely low surface diffusivity, blocking water passage, and decreasing water flux. The adsorbed protein undergoes secondary structural changes, particularly in the pressure-driven flowing conditions, leading to strong protein-surface interactions. Our simulations were able to present water permeation close to the experimental conditions with a pressure difference as low as 5 MPa, while all the electrolytes, which are tightly surrounded by hydration water, were effectively rejected at the membrane surfaces. The analysis of the self-intermediate scattering function demonstrates that the dynamics of water molecules coordinated with hydrogen bonds is faster inside the pores than during the translation across the pores. The pressure difference applied shows a negligible effect on the water structure and content inside the membrane but facilitates the transportation of hydrogen-bonded water molecules through the membrane's sub-nanopores with a reduced coordination number. The linear relationship between the water flux and the pressure difference demonstrates the applicability of continuum hydrodynamic principles and thus the stability of the membrane structure.

Characterizing salt permeability in polyamide desalination membranes using electrochemical impedance spectroscopy

Improving the performance of desalination membranes requires better measurements of salt permeability in the polyamide separating layer to elucidate the thermodynamic and kinetic components of membrane permselectivity. In this work, electrochemical impedance spectroscopy (EIS) is introduced as a technique to measure the salt permeability and estimate the salt partition coefficient in thin polyamide films created using molecular layer-by-layer deposition. The impedance of supported polyamide films ranging in thickness from 3.5 nm to 28.5 nm were measured in different electrolyte solutions. Impedance spectra were modeled with equivalent circuits containing resistive and capacitive elements associated with the EIS measurement system as well as characteristic low-frequency parallel resistive and capacitive elements that are associated with the polyamide film. The characteristic polyamide membrane resistance increases with film thickness, decreases with solution concentration, and is an order of magnitude greater for a divalent cationic solution than for a monovalent cationic solution. For each polyamide film, salt permeability is calculated from the membrane resistance, and a salt partition coefficient is estimated. At the highest solution concentration measured, which is representative of brackish water desalination conditions, the calculated salt permeabilities range from P _s = 1.3 × 10^-16 m s^-1 to 3.9 × 10^-16 m s^-1, and the estimated salt partition coefficients range from K _s = 0.008 to 0.016. These measurements demonstrate that EIS is a powerful tool for studying membrane permselectivity through the measurement of salt permeability in thin polyamide films.

Monte Carlo Simulations of Framework Defects in Layered Two-Dimensional Nanomaterial Desalination Membranes: Implications for Permeability and Selectivity

Two-dimensional nanomaterial (2-D NM) frameworks, especially those comprising graphene oxide, have received extensive research interest for membrane-based separation processes and desalination. However, the impact of horizontal defects in 2-D NM frameworks, which stem from nonuniform deposition of 2-D NM flakes during layer build-up, has been almost entirely overlooked. In this work, we apply Monte Carlo simulations, under idealized conditions wherein the vertical interlayer spacing allows for water permeation while perfectly excluding salt, on both the formation of the laminate structure and molecular transport through the laminate. Our simulations show that 2-D NM frameworks are extremely tortuous (tortuosity ≈10³), with water permeability decreasing from 20 to <1 L m^-2 h^-1 bar^-1 as thickness increased from 8 to 167 nm. Additionally, we find that framework defects allow salt to percolate through the framework, hindering water-salt selectivity. 2-D NM frameworks with a packing density of 75%, representative of most 2-D NM membranes, are projected to achieve <92% NaCl rejection at a water permeability of <1 L m^-2 h^-1 bar^-1, even with ideal interlayer spacing. A high packing density of 90%, which to our knowledge has yet to be achieved, could yield comparable performance to current desalination membranes. Maximizing packing density is therefore a critical technical challenge, in addition to the already daunting challenge of optimizing interlayer spacing, for the development of 2-D NM membranes.

Role of Operating Conditions in a Pilot Scale Investigation of Hollow Fiber Forward Osmosis Membrane Modules

Although forward osmosis (FO) membranes have shown great promise for many applications, there are few studies attempting to create a systematization of the testing conditions at a pilot scale for FO membrane modules. To address this issue, hollow fiber forward osmosis (HFFO) membrane modules with different performances (water flux and solute rejection) have been investigated at different operating conditions. Various draw and feed flow rates, draw solute types and concentrations, transmembrane pressures, temperatures, and operation modes have been studied using two model feed solutions—deionized water and artificial seawater. The significance of the operational conditions in the FO process was attributed to a dominant role of concentration polarization (CP) effects, where the selected draw solute and draw concentration had the biggest impact on membrane performance due to internal CP. Additionally, the rejection of the HFFO membranes using three model solutes (caffeine, niacin, and urea) were determined under both FO and reverse osmosis (RO) conditions with the same process recovery. FO rejections had an increase of 2% for caffeine, 19% for niacin, and 740% for urea compared to the RO rejections. Overall, this is the first extensive study of commercially available inside-out HFFO membrane modules.

Membranes with Functionalized Nanopores for Aromaticity-Based Separation of Small Molecules

Membranes that can separate molecules of similar size based on chemical features could transform chemical manufacturing. We demonstrate membranes with functional, 1-3 nm pores prepared using a simple and scalable approach: coating a porous support with random copolymer micelles in alcohol, followed by precipitation in water and functionalization of pore surfaces. This approach was used to prepare membranes that can separate two hormones of similar size and charge, differentiated by aromaticity, mediated through π-π interactions between the aromatic solute and pore walls functionalized with phenol groups. The aromatic molecule permeates more slowly in single-solute experiments. In competitive diffusion experiments, however, it permeates 7.1 times faster than its nonaromatic analogue. This approach can be used to manufacture membranes for complex separations based on various intermolecular interactions.

Molecular dynamics simulation studies of the structure and antifouling performance of a gradient polyamide membrane

The structure and the antifouling performance of the first gradient polyamide layer model are systematically disclosed using molecular dynamics simulations.

Permselectivity limits of biomimetic desalination membranes

Water scarcity and inadequate membrane selectivity have spurred interest in biomimetic desalination membranes, in which biological or synthetic water channels are incorporated in an amphiphilic bilayer. As low channel densities (0.1 to 10%) are required for sufficient water permeability, the amphiphilic bilayer matrix will play a critical role in separation performance. We determine selectivity limits for biomimetic membranes by studying the transport behavior of water, neutral solutes, and ions through the bilayers of lipid and block-copolymer vesicles and projecting performance for varying water channel densities. We report that defect-free biomimetic membranes would have water/salt permselectivities ~108-fold greater than current desalination membranes. In contrast, the solubility-based permeability of lipid and block-copolymer bilayers (extending Overton's rule) will result in poor rejection of hydrophobic solutes. Defect-free biomimetic membranes thus offer great potential for seawater desalination and ultrapure water production, but would perform poorly in wastewater reuse. Potential strategies to limit neutral solute permeation are discussed.

Advances in coagulation technique for treatment of fluoride-contaminated water: a critical review

Fluoride contamination of groundwater has become a major concern worldwide, resulting in serious medical conditions such as dental and skeletal fluorosis. Consequently, the WHO recommends that drinking water should not contain more than 1.5 mg/l of fluoride. Various defluoridation techniques such as coagulation, reverse osmosis, activated alumina adsorption, and biosorbent adsorption have been developed. Adsorption through the activated alumina and biosorbent process is not cost effective and has regeneration problems, and the reverse osmosis process has the high initial cost which makes it unacceptable for developing countries. Coagulation is a commonly employed field technology for defluoridation, which involves the addition of aluminum salts, lime, and bleaching powder followed by rapid mixing, flocculation, sedimentation, and filtration but suffers from a limitation of high residual aluminum in treated water. This paper critically reviews the recent developments in the coagulation technique for defluoridation along with its comparison to other defluoridation techniques. The review describes the pertinent gaps in the process and throws open suggestions for extending research by citing the recent studies which may lead to the revival of the process. The description about the suspension of alumino-fluoro complexes that constitute a substantial part of the residual aluminum after alum treatment has been narrated in the paper that helps in a deeper understanding of the defluoridation mechanism. To make the process highly suitable for communities, appropriate technological interventions, such as converting it to a continuous mode of operation, replacing alum with poly-aluminum chloride (PAC), and attaching a micro-filtration unit in series of the existing process, can be done. Also, using PAC as a coagulant with sand filtration has to be considered for making the process more efficient.

Understanding the temperature effect on transport dynamics and structures in polyamide reverse osmosis system via molecular dynamics simulations

Molecular simulations could disclose the transport dynamics, membrane structures and temperature effect on reverse osmosis process.

Biguanidine functional chitooligosaccharide modified reverse osmosis membrane with improved anti-biofouling property

The COSG-modified RO membrane with excellent anti-adhesive and antimicrobial properties was successfully fabricated by second interfacial polymerization.

Modeling and simulation of membrane process

The article presents the different approaches to polymer membrane mathematical modeling. Traditional models based on experimental physicochemical correlations and balance models are presented in the first part. Quantum and molecular mechanics models are presented as they are more popular for polymer membranes in fuel cells. The initial part is enclosed by neural network models which found their use for different types of processes in polymer membranes. The second part is devoted to models of fluid dynamics. The computational fluid dynamics technique can be divided into solving of Navier-Stokes equations and into Boltzmann lattice models. Both approaches are presented focusing on membrane processes.

Production of demineralized water for use in thermal power stations by advanced treatment of secondary wastewater effluent

The operation and efficiency of a modern, high-tech industrial full-scale water treatment plant was investigated in the present study. The treated water was used for the supply of the boilers, producing steam to feed the steam turbine of the power station. The inlet water was the effluent of municipal wastewater treatment plant of the city of Bari (Italy). The treatment stages comprised (1) coagulation, using ferric chloride, (2) lime softening, (3) powdered activated carbon, all dosed in a sedimentation tank. The treated water was thereafter subjected to dual-media filtration, followed by ultra-filtration (UF). The outlet of UF was subsequently treated by reverse osmosis (RO) and finally by ion exchange (IX). The inlet water had total organic carbon (TOC) concentration 10-12 mg/L, turbidity 10-15 NTU and conductivity 3500-4500 μS/cm. The final demineralized water had TOC less than 0.2 mg/L, turbidity less than 0.1 NTU and conductivity 0.055-0.070 μS/cm. Organic matter fractionation showed that most of the final DOC concentration consisted of low molecular weight neutral compounds, while other compounds such as humic acids or building blocks were completely removed. It is notable that this plant was operating under "Zero Liquid Discharge" conditions, implementing treatment of any generated liquid waste.