New insights into the definition of membrane cleaning strategies to diminish the fouling impact in ion exchange membrane separation processes

Long-Term Robustness and Failure Mechanisms of Electrochemical Stripping for Wastewater Ammonia Recovery

Nitrogen in wastewater has negative environmental, human health, and economic impacts but can be recovered to reduce the costs and environmental impacts of wastewater treatment and chemical production. To recover ammonia/ammonium (total ammonia nitrogen, TAN) from urine, we operated electrochemical stripping (ECS) for over a month, achieving 83.4 ± 1.5% TAN removal and 73.0 ± 2.9% TAN recovery. With two reactors, we recovered sixteen 500-mL batches (8 L total) of ammonium sulfate (20.9 g/L TAN) approaching commercial fertilizer concentrations (28.4 g/L TAN) and often having >95% purity. While evaluating the operation and maintenance needs, we identified pH, full-cell voltage, product volume, and water flux into the product as informative process monitoring parameters that can be inexpensively and rapidly measured. Characterization of fouled cation exchange and omniphobic membranes informs cleaning and reactor modifications to reduce fouling with organics and calcium/magnesium salts. To evaluate the impact of urine collection and storage on ECS, we conducted experiments with urine at different levels of dilution with flush water, extents of divalent cation precipitation, and degrees of hydrolysis. ECS effectively treated urine under all conditions, but minimizing flush water and ensuring storage until complete hydrolysis would enable energy-efficient TAN recovery. Our experimental results and cost analysis motivate a multifaceted approach to improving ECS's technical and economic viability by extending component lifetimes, decreasing component costs, and reducing energy consumption through material, reactor, and process engineering. In summary, we demonstrated urine treatment as a foothold for electrochemical nutrient recovery from wastewater while supporting the applicability of ECS to seven other wastewaters with widely varying characteristics. Our findings will facilitate the scale-up and deployment of electrochemical nutrient recovery technologies, enabling a circular nitrogen economy that fosters sanitation provision, efficient chemical production, and water resource protection.

Recent advances on micro/nanoplastic pollution and membrane fouling during water treatment: A review

Effluent from sewage treatment plant, as an important source of microplastics (MPs) in receiving water, has attracted extensive attention. Membrane separation process shows good microplastic removal performance in the existing tertiary water treatment process. Problematically, membrane fouling and insufficient removal of small organic molecules are still the key obstacles to its further extensive application. Dissolved organics, extracellular polymers and suspended particles in the influent are deposited on the membrane surface and internal structure, reducing the number and pore diameter of effective membrane aperture, and increasing the resistance of membrane filtration. Exploring the mechanism and approach of membrane fouling caused by micro/nanoplastics is the key to alleviate fouling and allow membranes to operate longer. In this paper, removal performance of micro/nanoplastics by current membrane filtration and the contribution to membrane fouling during water treatment are thoroughly reviewed. The coupling mechanisms between micro/nanoplastics and other pollutants and mechanism of membrane fouling caused by composite micro/nanoplastics are discussed. Additionally, on this basis, the prospect of combined process for micro/nanoplastic removal and membrane fouling prevention is also proposed and discussed, which provides a valuable reference for the preferential removal of micro/nanoplastics and development of antifouling membrane.

Impact of Hierarchical Cation-Exchange Membranes' Chemistry and Crosslinking Level on Electrodialysis Demineralization Performances of a Complex Food Solution

Hierarchical cation-exchange membranes (hCEMs) fabricated by blade coating and UV crosslinking of ionomer on top of a porous substrate demonstrated promising results in performing NaCl demineralization. In the food industry, complex solutions are used and hCEMs were never investigated before for these food applications. The performances of two different coating chemistries (urethane acrylate based: UL, and acrylic acid based: EbS) and three crosslinking degrees (UL5, UL6, UL7 for UL formulations, and EbS-1, EbS-2, EbS-3 for EbS formulations) were formulated. The impacts of hCEMs properties and crosslinking density on whey demineralization performances by electrodialysis (ED) were evaluated and compared to CMX, a high performing CEM for whey demineralization by ED. The crosslinking density had an impact on the hCEMs area specific resistance, and on the ionic conductance for EbS membrane. However, 70% demineralization of 18% whey solution was reached for the first time for hCEMs without any fouling observed, and with comparable performances to the CMX benchmark. Although some properties were impacted by the crosslinking density, the global performances in ED (limiting current, demineralization duration, global system resistance, energy consumption, current efficiency) for EbS and UL6 membranes were similar to the CMX benchmark. These promising results suggest the possible application of these hCEMs (UL6, EbS-2, and EbS-3) for whey demineralization by ED and more generally complex products as an alternative in the food industry.

Fouling and Mitigation Behavior of Foulants on Ion Exchange Membranes with Surface Property in Reverse Electrodialysis

In this study, two different types of ion exchange membranes are used to investigate the tendency of membrane fouling with respect to surface roughness and hydrophilicity. Commercially available membranes reinforced by electrospun nanofiber have rough and hydrophilic surfaces, and lab-made pore-filling membranes exhibit a smooth and hydrophobic surface. Three different organic surfactants (i.e., cationic, anionic and non-ionic surfactants) are chosen as foulants with similar molecular weights. It is confirmed that membrane fouling by electrical attraction mainly occurs, in which anionic and cationic foulants influence anion and cation exchange membranes, respectively. Thus, less fouling is obtained on both membranes for the non-charged foulant. The membranes with a rough surface show a higher fouling tendency than those with a smooth surface in the short-term continuous fouling tests. However, during the cyclic operations of fouling and mitigation of the commercially available membranes, the irregularities of a rough membrane surface cause a rapid increase in electrical resistance from the beginning of fouling due to excessive adsorption on the surface, but the fouling is easily mitigated due to the hydrophilic surface. On the other hand, the membranes with a smooth surface show alleviated fouling from the beginning of fouling, but the irreversible fouling occurs as foulants accumulate on the hydrophobic surface which causes membrane fouling to be favorable.

Tuning cationic/anionic dyes sorption from aqueous solution onto green algal biomass for biohydrogen production

Global water security and energy demands associated with uncontrollable population growth and rapid industrial progress are one of the utmost serious needs dangerously confronting humanity. On account of waste as a wealth strategy; a multifunctional eco-friendly sorbent (MGAP) from green alga was prepared successfully for remediation of cationic/anionic organic dyes and biohydrogen production. The structural and morphological properties of sorbent were systematically scrutinized by a variety of spectral analyses. The loading capacity of MGAP towards rhodamine B (RhB) and methyl orange (MO) dyes was inclusivity inspected under variable experimental conditions. The adsorption kinetics of both dyes onto MGAP was in good agreement with pseudo-second-order theory, whereas adsorption isotherms could fit well with the Langmuir model, with satisfactory loading capacities of 144.92 and 196.04 mg g^-1 for RhB and MO molecules, respectively. Moreover, ultra-sonication treatment admirably decreased the sorption equilibrium time from 180.0 min to 30.0 min. Furthermore, spent sorbent was managed particularly for biohydrogen production with a measured yield of 112.89, 116.59, and 128.17 mL-H₂/gVS for MGAP, MGAP-MO, and MGAP-RhB, respectively. Overall, the produced MGAP can potentially be offered up as a promising dye scavenger for wastewater remediation and biohydrogen production, thereby fulfilling waste management and circular economy.

A Study on Biofouling and Cleaning of Anion Exchange Membranes for Reverse Electrodialysis

This study covers the modification, (bio)fouling characterization, use, and cleaning of commercial heterogeneous anion exchange membranes (AEMs) to evaluate their feasibility for reverse electrodialysis (RED) applications. A surface modification with poly (acrylic) acid resulted in an improved monovalent perm-selectivity (decreased sulfate membrane transport rate). Moreover, we evaluated the (bio)fouling potential of the membrane using sodium dodecyl sulfate (SDS), sodium dodecyl benzenesulfonate (SDBS), and Aeromonas hydrophila as model organic foulants and a biofoulant, respectively. A detailed characterization of the AEMs (water contact angle, ion exchange capacity (IEC), scanning electron microscopy (SEM), cyclic voltammetry (CV), and Fourier Transform Infrared (FTIR) spectra) was carried out, verifying that the presence of such foulants reduces IEC and the maximum current obtained by CV. However, only SDS and SDBS affected the contact angle values. Cleaning of the biofouled membranes using a sodium hypochlorite aqueous solution allows for (partially) recovering their initial properties. Furthermore, this work includes a fouling characterization using real surface and sea water matrixes, confirming the presence of several types of fouling microorganisms in natural streams. A lower adhesion of microorganisms (measured in terms of total bacteria counts) was observed for the modified membranes compared to the unmodified ones. Finally, we propose a cleaning strategy to mitigate biofouling in AEMs that could be easily applied in RED systems for an enhanced long-term process performance.

Strategically Altered Fluorinated Polymer at Nanoscale for Enhancing Proton Conduction and Power Generation from Salinity Gradient

Reverse electrodialysis (RED) generates power directly by transforming salinity gradient into electrical energy. The ion transport properties of the ion-exchange membranes need to be investigated deeply to improve the limiting efficiencies of the RED. The interaction between “counterions” and “ionic species” in the membrane requires a fundamental understanding of the phase separation process. Here, we report on sulfonated poly(vinylidene fluoride-co-hexafluoropropylene)/graphitic carbon nitride nanocomposites for RED application. We demonstrate that the rearrangement of the hydrophilic and hydrophobic domains in the semicrystalline polymer at a nanoscale level improves ion conduction. The rearrangement of the ionic species in polymer and “the functionalized nanosheet with ionic species” enhances the proton conduction in the hybrid membrane without a change in the structural integrity of the membrane. A detailed discussion has been provided on the membrane nanostructure, chemical configuration, structural robustness, surface morphology, and ion transport properties of the prepared hybrid membrane. Furthermore, the RED device was fabricated by combining synthesized cation exchange membrane with commercially available anion exchange membrane, NEOSEPTA, and a maximum power density of 0.2 W m⁻² was successfully achieved under varying flow rates at the ambient condition.

Evaluation of Nanofiltration Membranes for Pure Lactic Acid Permeability

Lactic acid (LA) is an organic acid produced by fermentation or chemical synthesis. It plays a crucial role in the pharmaceutical, food and plastic industries. In the fermentation of, for example, grass silage, LA and different compounds are produced. To purify lactic acid, researchers have tried to investigate membrane technology to achieve a high yield of lactic acid permeance. This study tested four commercially available nanofiltration membranes (NF270, MPF-36, Toray NF, and Alfa Laval NF). Nanofiltration experiments were performed to investigate the rejection levels of lactic acid from a binary solution by using distinct molecular weight cut off membranes. All of the experiments were conducted with a lab-scale cross-flow membrane unit. Different operating conditions (pH, temperature) were studied for each membrane; the optimal process condition was found at 25 °C and pH 2.8. With higher temperatures and pH, an increase in LA rejection was observed. The MPF-36 membrane shows the lowest lactic acid rejection yield of 7%, while NF270 has the highest rejection yield of 71% at 25 °C and pH 2.8. These results will be helpful in the future to understand both the interaction of lactic acid permeance through nanofiltration membranes and process scale-up.

Mathematical Description of the Increase in Selectivity of an Anion-Exchange Membrane Due to Its Modification with a Perfluorosulfonated Ionomer

In this paper, we simulate the changes in the structure and transport properties of an anion-exchange membrane (CJMA-7, Hefei Chemjoy Polymer Materials Co. Ltd., China) caused by its modification with a perfluorosulfonated ionomer (PFSI). The modification was made in several stages and included keeping the membrane at a low temperature, applying a PFSI solution on its surface, and, subsequently, drying it at an elevated temperature. We applied the known microheterogeneous model with some new amendments to simulate each stage of the membrane modification. It has been shown that the PFSI film formed on the membrane-substrate does not affect significantly its properties due to the small thickness of the film (≈4 µm) and similar properties of the film and substrate. The main effect is caused by the fact that PFSI material “clogs” the macropores of the CJMA-7 membrane, thereby, blocking the transport of coions through the membrane. In this case, the membrane microporous gel phase, which exhibits a high selectivity to counterions, remains the primary pathway for both counterions and coions. Due to the above modification of the CJMA-7 membrane, the coion (Na⁺) transport number in the membrane equilibrated with 1 M NaCl solution decreased from 0.11 to 0.03. Thus, the modified membrane became comparable in its transport characteristics with more expensive IEMs available on the market.

A Review on Ion-Exchange Membranes Fouling during Electrodialysis Process in Food Industry, Part 2: Influence on Transport Properties and Electrochemical Characteristics, Cleaning and Its Consequences

Ion-exchange membranes (IEMs) are increasingly used in dialysis and electrodialysis processes for the extraction, fractionation and concentration of valuable components, as well as reagent-free control of liquid media pH in the food industry. Fouling of IEMs is specific compared to that observed in the case of reverse or direct osmosis, ultrafiltration, microfiltration, and other membrane processes. This specificity is determined by the high concentration of fixed groups in IEMs, as well as by the phenomena inherent only in electromembrane processes, i.e., induced by an electric field. This review analyzes modern scientific publications on the effect of foulants (mainly typical for the dairy, wine and fruit juice industries) on the structural, transport, mass transfer, and electrochemical characteristics of cation-exchange and anion-exchange membranes. The relationship between the nature of the foulant and the structure, physicochemical, transport properties and behavior of ion-exchange membranes in an electric field is analyzed using experimental data (ion exchange capacity, water content, conductivity, diffusion permeability, limiting current density, water splitting, electroconvection, etc.) and modern mathematical models. The implications of traditional chemical cleaning are taken into account in this analysis and modern non-destructive membrane cleaning methods are discussed. Finally, challenges for the near future were identified.