Electrochemical removal of hexavalent chromium using electrically conducting carbon nanotube/polymer composite ultrafiltration membranes

Efficient adsorption of hexavalent chromium in water by torrefaction biochar from lignin-rich kiwifruit branches: The combination of experiment, 2D-COS and DFT calculation

A biochar (KBC) enriched with O functional groups was prepared by torrefaction using lignin-rich kiwifruit branches (KBM) as a raw material, which was characterized, and then KBC was used to adsorb hexavalent chromium (Cr6+) from water. The results showed that KBC contained more functional groups compared to KBM. The maximum adsorption of Cr6+ by KBC could reach 143.64 mg·g-1 and also had better adsorption performance than other adsorbents reported in some other reports. Cr6+ absorption by KBC was mainly a mechanism of electrostatic interaction and adsorption-reduction coupling. FTIR and XPS revealed that -OH, -COOH, CO and CC on KBC participated in Cr6+ adsorption and new groups (C=O) were generated during the process of adsorption, which implied that a redox reaction occurred. 2D-COS and DFT calculations showed that the order of functional groups on KBC interacting with Cr6+ was -OCH3 > -COOH > -OH > phenolic hydroxyl, and the binding tightness of the different functional groups to Cr6+ was -OCH3 (the shortest displacement of both groups after the adsorption) > -COOH > -OH > phenolic hydroxyl. KBC has good regeneration performance, and it is a good adsorbent for Cr6+.

Electrochemical filtration for drinking water purification: A review on membrane materials, mechanisms and roles

Electrochemical filtration can not only enrich low concentrations of pollutants but also produce reactive oxygen species to interact with toxic pollutants with the assistance of a power supply, making it an effective strategy for drinking water purification. In addition, the application of electrochemical filtration facilitates the reduction of pretreatment procedures and the use of chemicals, which has outstanding potential for maximizing process simplicity and reducing operating costs, enabling the production of safe drinking water in smaller installations. In recent years, the research on electrochemical filtration has gradually increased, but there has been a lack of attention on its application in the removal of low concentrations of pollutants from low conductivity water. In this review, membrane substrates and electrocatalysts used to improve the performance of electrochemical membranes are briefly summarized. Meanwhile, the application prospects of emerging single-atom catalysts in electrochemical filtration are also presented. Thereafter, several electrochemical advanced oxidation processes coupled with membrane filtration are described, and the related working mechanisms and their advantages and shortcomings used in drinking water purification are illustrated. Finally, the roles of electrochemical filtration in drinking water purification are presented, and the main problems and future perspectives of electrochemical filtration in the removal of low concentration pollutants are discussed.

A review of remediation technologies for uranium-contaminated water

Uranium is a naturally existing radioactive element present in the Earth's crust. It exhibits lithophilic characteristics, indicating its tendency to be located near the surface of the Earth and tightly bound to oxygen. It is ecotoxic, hence the need for its removal from the aqueous environment. This paper focuses on the variety of water treatment processes for the removal of uranium from water and this includes physical (membrane separation, adsorption and electrocoagulation), chemical (ion exchange, photocatalysis and persulfate reduction), and biological (bio-reduction and biosorption) approaches. It was observed that membrane filtration and ion exchange are the most popular and promising processes for this application. Membrane processes have high throughput but with the challenge of high power requirements and fouling. Besides high pH sensitivity, ion exchange does not have any major challenges related to its application. Several other unique observations were derived from this review. Chitosan/Chlorella pyrenoidosa composite adsorbent bearing phosphate ligand, hydroxyapatite aerogel and MXene/graphene oxide composite has shown super-adsorbent performance (>1000 mg/g uptake capacity) for uranium. Ultrafiltration (UF) membranes, reverse osmosis (RO) membranes and electrocoagulation have been observed not to go below 97% uranium removal/conversion efficiency for most cases reported in the literature. Heat persulfate reduction has been explored quite recently and shown to achieve as high as 86% uranium reduction efficiency. We anticipate that future studies would explore hybrid processes (which are any combinations of multiple conventional techniques) to solve various aspects of the process design and performance challenges.

Effective elimination of hexavalent chromium and lead from solution by the modified biochar with MgMn2O4 nanoparticles: adsorption performance and mechanism

Heavy metal pollution is one of the environmental problems that need to be solved urgently. The adsorption method is thought as the most effective and economical treatment technology. Nature biochar usually showed unsatisfactory adsorption capacity due to its relatively small adsorption capacity and slow adsorption rate. The metal of Mn has been widely applied in the modification of biochar, which could effectively improve the adsorption capacity of biochar. However, leaching of Mn2+ on the adsorbent materials would appear during the adsorption process. And it would increase the risk of secondary pollution. The multifunctional binary modified biochar could improve the adsorption capacity of environmental pollutant removal. In addition, it could also act as a metal support carrier, reducing the risk of secondary pollution. A novel effective biochar loaded by Mg-Mn binary oxide nanoparticles (MgMn2O4@Biochar) was prepared and applied for the Cr(VI) and Pb(II) removal in aqueous solution. The characteristic of MgMn2O4@Biochar was analyzed by SEM, TEM, FTIR, and XRD. The irregular and somewhat flaky shaped particles of different shape and sizes clustered on the surface of MgMn2O4@Biochar appeared. Abundant functional groups of O-H, -C-OH, C-O, and C-OOH could be observed on the surface of MgMn2O4@Biochar. The elements of Mg and Mn elements besides of C, O, and Si elements were presented on the surface of MgMn2O4@Biochar. The wt% of C, O, Mg, Mn, and Si were 42.82%, 48.99%, 2.83%, 4.44%, and 0.93%, respectively. The operational parameters had an important influence on adsorption capacity of Cr(VI) and Pb(II) removal. The results showed that the adsorption capacity of MgMn2O4@Biochar for Cr(VI) and Pb(II) would reach 33.5 mg/g and 536 mg/g, respectively, within 360 min. Additionally, the adsorption processes of Cr(VI) and Pb(II) in solution could be described with pseudo-second-order. For Cr(VI), the Langmuir model was suitable to the adsorption process. However, the adsorption process of Pb(II) in solution could be described with Freundlich model. Furthermore, it could be concluded that the possible mechanism of Cr(VI) and Pb(II) removal by MgMn2O4@Biochar was physical adsorption, surface complexation reaction, and electrostatic adsorption.

Insight into the photocatalytic reduction of hexavalent chromium using photodeposited metal nanoparticle-TiO2 photocatalysis

Hexavalent chromium (Cr(VI)) is carcinogenic to organisms. It is widely used in several industries. In this work, we investigated the Cr(VI) photocatalytic reaction with a scavenger on Pt and Cu-TiO2 photocatalysts. Metal-deposited TiO2 was successfully synthesized by a photodeposition method. TEM-EDX, XRD, and UV-DR were analyzed to study the changes in morphology, crystallinity, and the electronic properties of photocatalysts. The rate of charge recombination during reduction and photoluminescence (PL) spectroscopy was used to examine the catalysts in depth. Cu-TiO2 demonstrates the highest photocatalytic activity for 63.74% of Cr(VI) removal. To understand the photoreduction of Cr(VI), the fate transformation of Cr species during the adsorption and reaction was investigated using in situ XANES. The results demonstrated that the Cr(III) was noticeably main component adsorbed over the catalyst, particularly in Cu-TiO2. The presence of humic acid can boost the Cr(VI) removal efficiency and enhanced the Cr(VI) reduction to Cr(III). We believe that the extensive research on Cr(VI) photoreduction on metal-TiO2 heterojunction will provide a comprehensive understanding of catalytic behaviors, paving the way for rationally designed novel Cr reduction catalysts.

Characterization and synthesis of new adsorbents with some natural waste materials for the purification of aqueous solutions

In this study, hexavalent chromium Removal from aqueous environments was investigated by using polyaniline composites with some natural waste materials. Batch experiments were used, and some parameters such as contact time, pH and adsorption isotherms were determined for the best composite with the highest removal efficiency. Scanning Electron Microscope (SEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray Diffraction (XRD) were used to characterize the composites. According to the results, the polyaniline/walnut shell charcoal/PEG composite outperformed other composites and showed the highest chromium removal efficiency of 79.22%. Polyaniline/walnut shell charcoal/PEG has a larger specific surface area of 9.291 (m²/gr) which leads to an increase in its removal efficiency. For this composite, the highest removal efficiency was obtained at the pH = 2 and 30 min contact time. The maximum calculated adsorption capacity was 500 mg/g.

Next-generation ultrafiltration membranes: A review of material design, properties, recent progress, and challenges

Membrane technology utilizing ultrafiltration (UF) processes has emerged as the most widely used and cost-effective simple process in many industrial applications. The industries like textiles and petroleum refining are promptly required membrane based UF processes to alleviate the potential environmental threat caused by the generation of various wastewater. At the same time, major limitations such as material selection as well as fouling behavior challenge the overall performance of UF membranes, particularly in wastewater treatment. Therefore, a complete discussion on material design with structural property relation and separation performance of UF membranes is always exciting. This state-of-the-art review has exclusively focused on the development of UF membranes, the material design, properties, progress in separation processes, and critical challenges. So far, most of the review articles have examined the UF membrane processes through a selected track of paving typical materials and their limited applications. In contrast, in this review, we have exclusively aimed at comprehensive research from material selection and fabrication methods to all the possible applications of UF membranes, giving more attention and theoretical understanding to the complete development of high-performance UF systems. We have discussed the methodical engineering behind the development of UF membranes regardless of their materials and fabrication mechanisms. Identifying the utility of UF membrane systems in various applications, as well as their mode of separation processes, has been well discussed. Overall, the current review conveys the knowledge of the present-day significance of UF membranes together with their future prospective opportunities whilst overcoming known difficulties in many potential applications.

Nanofibrous/biopolymeric membrane a sustainable approach to remove organic micropollutants: A review

Aquifers are severely polluted with organic and inorganic pollutants, posing a serious threat to the global ecological system's balance. While various traditional methods are available, the development of innovative methods for effluent treatment and reuse is critical. Polymers have recently been widely used in a variety of industry sectors due to their unique properties. Biopolymers are a biodegradable material that is also a viable alternative to synthetic polymers. Biopolymers are preferably obtained from cellulose and carrageenan molecules from various biological sources. While compared with conventional non-biodegradable polymeric materials, the biopolymer possesses unique characteristics such as renewability, cost-effectiveness, biodegradability, and biocompatibility. The improvements towards the biopolymeric (natural) membranes have also been thoroughly discussed. The use of nanofillers to stabilise and improve the effectiveness of biopolymeric membranes in the elimination of organic pollutants is one of the most recent developments. This was discovered that the majority of biopolymeric membranes technology consolidated on organic pollutants. More research should be directed toward against emerging organic/persistent organic pollutants (POP) and micropollutants. Furthermore, processes for regenerating and reusing utilized biopolymer-based carbon - based materials are emphasized.

Chromium-Modified Heterogeneous Bipolar Membrane: Structure, Characteristics, and Practical Application in Electrodialysis

The modification of an MB-2 bipolar ion exchange membrane with chromium (III) hydroxide was carried out by a chemical method, namely, by the sequential treatment of the membrane with a solution of chromium (III) salt and alkali. Data on the morphology, phase, and chemical composition of the modified membrane were obtained using scanning electron microscopy and energy-dispersive analysis. In particular, it was shown that the modifier was distributed in a layer 30-50 microns thick at the boundary of the cation- and anion-exchange layers of the bipolar membrane. The electrochemical behavior of the modified membrane in the process of sodium sulfate conversion was studied by measurements of the following characteristics: the current efficiency of the acid and base, the energy consumption of the process, and the degree of contamination of the target products with salt ions. It was shown that the resulting membrane has an alkali and acid yield of 61% and 57%, respectively. This is higher than the same yields for the industrial unmodified MB-2 membrane (38% and 30%). The results of this study demonstrated that the modified samples allowed obtaining a higher yield of acid and base, reducing the content of salt ions in the target products and also reducing the electricity consumption for obtaining a unit of the target product. The concentration dependences of the electrical conductivity of the MK-40 heterogeneous ion-exchange membrane, which is a cation-exchange layer of MB-2, in sodium sulfate solutions before and after its modification with chromium (III) oxide were obtained. A decrease in the specific electrical conductivity of the membrane with the introduction of a modifier was established. A quantitative assessment of the influence of the modifier on the current flow, volume fraction, and spatial orientation of the conductive phases of MK-40 was carried out using an extended three-wire model for the description of the model parameters of ion-exchange materials. When a modifying additive was introduced into MK-40, the fraction of the current passing through the inner solution and the intergel phase decreased. This was due to the substitution of part of the free solution in the pore volume by the modifier. A variant of the practical application of electrodialysis with the chromium-modified bipolar ion-exchange membranes is recommended.

Electrochemical removal and recovery of uranium: Effects of operation conditions, mechanisms, and implications

Removing and recovering uranium (U) from U-mining wastewater would be appealing, which simultaneously reduces the adverse environmental impact of U mining activities and mitigates the depletion of conventional U resources. In this study, we demonstrate the application of a constant-voltage electrochemical (CVE) method for the removal and recovery of U from U-mining wastewater, in an ambient atmosphere. The effects of operation conditions were elucidated in synthetic U-bearing water experiments, and the cell voltage and the ionic strength were found to play important roles in both the U extraction kinetics and the operation cost. The mechanistic studies show that, in synthetic U-bearing water, the CVE U extraction proceeds exclusively via a single-step one-electron reduction mechanism, where pentavalent U is the end product. In real U-mining wastewater, the interference of water matrices led to the disproportionation of the pentavalent U, resulting in the formation of tetravalent and hexavalent U in the extraction products. The U extraction efficacy of the CVE method was evaluated in real U-mining wastewater, and results show that the CVE U extraction method can be efficient with operation costs ranging from $0.55/kgU ~ $64.65/kgU, with varying cell voltages from 1.0 V to 4.0 V, implying its feasibility from the economic perspective.

A multifunctional and low-energy electrochemical membrane system for chemical-free regulation of solution pH

A proper pH environment is essential for a wide variety of industries and applications especially related to water treatment. Current methods for pH adjustment including addition of acid/base and electrochemical processes demonstrate disadvantages associated with environment and energy. Here, we designed a multifunctional electrochemical membrane system (EMS) with one piece of filtration membrane inserted into an electrochemical cell. When electrical field was applied, OH^- and H⁺ ions were produced from reduction and oxidation reactions at cathode and anode, respectively. The membrane posed a resistance for the transport of OH^- and H⁺ ions and prevented their mixing in the cell. The EMS can be also operated in a filtration mode, which could simultaneously regulate permeate and feed pH and accomplish water filtration. In both non-filtration and filtration modes, EMS could achieve effective control of solution pH over a wide range by exerting different voltages without dosing any chemicals. Under the voltage of 1.2 V, the solution pH could reach and be maintained at 10.7 and 3.3 in cathodic and anodic channels, respectively. Furthermore, it was experimentally demonstrated that the EMS only consumed extremely low energy. This, together with membrane filtration in an integrated manner, highlights the huge potential of the EMS for applications in various water industries.

Enhanced adsorptive removal of Cr(III) from the complex solution by NTA-modified magnetic mesoporous microspheres

The Fe₃O₄@nSiO₂@mSiO₂/NTA (FNMs-NTA) was prepared by grafting magnetic mesoporous microspheres with nitrilotriacetic acid (NTA) and applied as an adsorbent for the removal of Cr(III) from complex solutions. Some characterization techniques including Brunauer-Emmett-Teller (BET), Fourier transform infrared spectrometer (FT-IR), X-ray diffraction (XRD), small-angle X-ray diffraction (SAXS), vibrating sample magnetometer (VSM), and thermal gravimetric analysis (TGA) were used to characterize functional groups and pore structure of FNMs-NTA, which proved that NTA was successfully decorated onto the magnetic Fe₃O₄@nSiO₂@mSiO₂ (FNMs) and FNMs-NTA featured a regular mesoporous structure. The batch adsorption of Cr(III) by FNMs-NTA exhibited high adsorption capacity (16.0 mg·g^-1 at pH 3.0, and 25 °C). Adsorption data followed Freundlich isotherm and adsorption process was a spontaneous adsorption process. Moreover, the kinetics of adsorption were well explained by pseudo-second-order kinetic model. FNMs-NTA showed resistance to interfering inorganic cations (Na⁺, Ca²⁺) and complexing agents (EDTA). Furthermore, FNMs-NTA exhibited remarkable regeneration performance and easy separation under external magnetic field. X-ray photoelectron spectroscopy (XPS) analysis showed the FNMs-NTA had excellent adsorption ability for Cr(III) because of the ion exchange and surface complexation.

Birefringence of Thin Uniaxial Polymer Films Estimated Using the Light Polarization Ellipse

A simple method for determining the linear birefringence of the thin layers based on the determination of the orientation of the polarization ellipse of totally polarized light is proposed and it is applied to PVA thin foils. Theoretical notions and the experimental procedure are described. The linear birefringence of polymer thin foils with different degrees of stretching is determined and the applicability of the method is discussed.

Mixed metal oxide anodes used for the electrochemical degradation of a real mixed industrial wastewater

Mixed industrial wastewaters are often highly contaminated with heavy metals and organic pollutants. Treating these mixed wastewaters requires many stagewise unit operations. Our work investigates using an electrochemical oxidation-in-situ coagulation (ECO-IC) process as a pre-treatment step toward the efficient treatment of real mixed industrial wastewater rich with heavy metals and organic contaminants. The process degraded organic contaminants in the wastewater via anodic electrochemical oxidation. Simultaneously, heavy metals were precipitated in the solution by coagulants (iron hydroxides) formed in-situ by cathode-generated hydroxyl ions reacting with the significant amounts of dissolved iron in the wastewater. IrO₂-RuO₂ mixed metal oxide anodes were identified as the best electrodes for organic compound degradation demonstrating 97% degradation of methyl orange (MO) as a model compound within 15 min. These anodes were used to treat real industrial wastewater produced from the industrial cleaning of train tanker cars transporting industrial solvents. The electrochemical treatment experiments resulted in a treated solution with a lower heavy metal content, achieving 96% reduction in Fe and 30% reduction in As content. Only moderate decreases in organic content were observed up to a maximum of 13% reduction in total organic carbon after 1 h of treatment. Electrochemical treatment of the mixed industrial wastewater produced greater effective diameter of the suspended particles and distinct sediment, liquid, and suspended foam phases that could be easily separated for further treatment. ECO-IC shows promise as an efficient and chemical-free method to coagulate heavy metals in real industrial wastewaters and could be an effective pre-treatment in their separation.

Biogenic Synthesis of Iron Oxide Nanoparticles Using Enterococcus faecalis: Adsorption of Hexavalent Chromium from Aqueous Solution and In Vitro Cytotoxicity Analysis

The biological synthesis of nanoparticles is emerging as a potential method for nanoparticle synthesis due to its non-toxicity and simplicity. In the present study, a bacterium resistant to heavy metals was isolated from a metal-contaminated site and we aimed to report the synthesis of Fe3O4 nanoparticles via co-precipitation using bacterial exopolysaccharides (EPS) derived from Enterococcus faecalis_RMSN6 strains. A three-variable Box-Behnken design was used for determining the optimal conditions of the Fe3O4 NPs synthesis process. The synthesized Fe3O4 NPs were thoroughly characterized through multiple analytical techniques such as XRD, UV-Visible spectroscopy, FTIR spectroscopy and finally SEM analysis to understand the surface morphology. Fe3O4 NPs were then probed for the Cr(VI) ion adsorption studies. The important parameters such as optimization of initial concentration of Cr(VI) ions, effects of contact time, pH of the solution and contact time on quantity of Cr(VI) adsorbed were studied in detail. The maximum adsorption capacity of the nanoparticles was found to be 98.03 mg/g. The nanoparticles could retain up to 73% of their efficiency of chromium removal for up to 5 cycles. Additionally, prepared Fe3O4 NPs in the concentration were subjected to cytotoxicity studies using an MTT assay. The investigations using Fe3O4 NPs displayed a substantial dose-dependent effect on the A594 cells. The research elucidates that the Fe3O4 NPs synthesized from EPS of E. faecalis_RMSN6 can be used for the removal of heavy metal contaminants from wastewater.

A Review on Promising Membrane Technology Approaches for Heavy Metal Removal from Water and Wastewater to Solve Water Crisis

Due to the impacts of water scarcity, the world is looking at all possible solutions for decreasing the over-exploitation of finite freshwater resources. Wastewater is one of the most reliable and accessible water supplies. As the population expands, so do industrial, agricultural, and household operations in order to meet man’s enormous demands. These operations generate huge amounts of wastewater, which may be recovered and used for a variety of reasons. Conventional wastewater treatment techniques have had some success in treating effluents for discharge throughout the years. However, advances in wastewater treatment techniques are required to make treated wastewater suitable for industrial, agricultural, and household use. Diverse techniques for removing heavy metal ions from various water and wastewater sources have been described. These treatments can be categorized as adsorption, membrane, chemical, or electric. Membrane technology has been developed as a popular alternative for recovering and reusing water from various water and wastewater sources. This study integrates useful membrane technology techniques for water and wastewater treatment containing heavy metals, with the objective of establishing a low-cost, high-efficiency method as well as ideal production conditions: low-cost, high-efficiency selective membranes, and maximum flexibility and selectivity. Future studies should concentrate on eco-friendly, cost-effective, and long-term materials and procedures.

Removal of As(III) by Electrically Conducting Ultrafiltration Membranes

As(III) species are the predominant form of arsenic found in groundwater. However, nanofiltration (NF) and reverse osmosis (RO) membranes are often unable to effectively reject As(III). In this study, we fabricate highly conducting ultrafiltration (UF) membranes for effective As(III) rejection. These membranes consist of a hydrophilic nickel-carbon nanotubes layer deposited on a UF support, and used as cathodes. Applying cathodic potentials significantly increased As(III) rejection in synthetic/real tap water, a result of locally elevated pH that is brought upon through water electrolysis at the membrane/water interface. The elevated pH conditions convert H₃ASO₃ to H₂AsO₃^-/HAsO₃^2- that are rejected by the negatively charged membranes. In addition, it was found that Mg(OH)₂ that precipitates on the membrane can further trap arsenic. Importantly, almost all As(III) passing through the membranes is oxidized to As(V) by hydrogen peroxide produced on the cathode, which significantly decreased its overall toxicity and mobility. Although the high pH along the membrane surface led to mineral scaling, this scale could be partially removed by backwashing the membrane. To the best of our knowledge, this is the first report of effective As(III) removal using low-pressure membranes, with As(III) rejection higher than that achieved by NF and RO, and high water permeance.

Highly efficient and ultrafast removal of Cr(VI)in aqueous solution to ppb level by poly(allylamine hydrochloride) covalently cross-linked amino-modified graphene oxide

We herein report a facile strategy to prepare poly(allylamine hydrochloride) cross-linked amino-modified graphene oxide (PAH-ASGO) by Schiff-base reactions. The resulting PAH-ASGO exhibited a maximum adsorption capacity of 373.1 mg/g for Cr(VI), which was nearly 9 times higher than that of pure graphene oxide, exceeding that of most GO-based materials previously reported. More significantly, PAH-ASGO can effectively diminish the Cr(VI) concentration from 9.9 mg/L to the extremely low level of 0.004 mg/L within 10 s, far below the maximum allowable level of Cr(VI) (0.05 mg/L) in drinking water. In addition, the adsorbents still displayed excellent removal efficiency of 91.8% after 10 cycles. Considering the broad diversity, we developed also a magnetic PAH-ASGO/Fe₃O₄ adsorbent by a simple cross-linking reaction to achieve rapid separation of PAH-ASGO from their aqueous solution. Finally, the PAH-ASGO was successfully utilized to treat the actual industrial effluent.

Highly Conductive and Permeable Nanocomposite Ultrafiltration Membranes Using Laser-Reduced Graphene Oxide

Electrically conductive membranes are a promising avenue to reduce water treatment costs due to their ability to minimize the detrimental impact of fouling, to degrade contaminants, and to provide other additional benefits during filtration. Here, we demonstrate the facile and low-cost fabrication of electrically conductive membranes using laser-reduced graphene oxide (GO). In this method, GO is filtered onto a poly(ether sulfone) membrane support before being pyrolyzed via laser into a conductive film. Laser-reduced GO composite membranes are shown to be equally as permeable to water as the underlying membrane support and possess sheet resistances as low as 209 Ω/□. Application of the laser-reduced GO membranes is demonstrated through greater than 97% removal of a surrogate water contaminant, 25 μM methyl orange dye, with an 8 V applied potential. Furthermore, we show that laser-reduced GO membranes can be further tuned with the addition of p-phenylenediamine binding molecules to decrease the sheet resistance to 54 Ω/□.

Reactive electrically conducting membranes for phosphorus recovery from livestock wastewater effluents

We present a novel 'proof-of-concept' electrochemically based membrane filtration process for the recovery of nitrogen and phosphorus from livestock wastewater following an anaerobic digestion step. Reactive electrically conducting membranes are shown to precipitate and separate struvite, an eco-friendly fertilizer from synthetic livestock wastewater, resulting in the production of a solid fertilizer and a high-quality water stream, fit for irrigation. The recovery process is based on electrochemical hydrolysis and control of local pH in proximity to the surface of the membrane, and therefore, does not require chemical additives for pH adjustment. The system was assessed at varying concentrations of nitrogen and phosphorus corresponding to diluted and concentrated livestock wastewater (up to 1000 mg/L of N and P). Experimental results show up to 65% removal of phosphorus and nitrogen in the first 30 min of electrochemical filtration, and the precipitates were analytically confirmed to be struvite. In addition, the recovery process was shown efficient as it resulted in limited membrane fouling and flux reduction. Fouling and precipitation results were explained by a mathematical model describing the concentration of N, P, Mg ions in the presence of an external electric field. Accordingly, precipitation takes place in proximity to the membrane's surface but not directly on it, thus, limiting surface fouling. The electrochemical filtration system does not require chemical additives for pH adjustment, and the cost associated with electrochemical membrane-based struvite recovery was calculated to be $158 per ton of dry struvite, which is about 1.4 times lower in comparison to conventional recovery approaches. Overall, the electrochemical filtration system may be a promising alternative for nutrient recovery from livestock wastewater in terms of operational costs, recovery efficiency, and fouling mitigation.

Enhanced removal of Cr(III)-EDTA chelates from high-salinity water by ternary complex formation on DETA functionalized magnetic carbon-based adsorbents

A novel amino-functionalized magnetic adsorbent (Fe₃O₄@C@DETA) was developed for adsorption of Cr(III) and Cr(III)-EDTA from wastewater. Fe₃O₄@C@DETA were successfully prepared by modification of Fe₃O₄@C with diethylenediamine (DETA), which exhibits a core-shell structure and sufficient saturation magnetization. Fe₃O₄@C@DETA exhibits much better adsorption performance for Cr(III) and its chelates than the Fe₃O₄@C because of newly introduced amino active sites. The enhanced adsorption capacity of Fe₃O₄@C@DETA for Cr(III) is 44.74 mg g^-1 (at 25 °C and pH 3.0), which is due to the surface coordination with the newly introduced amino functional sites. The Cr(III)-EDTA anions as a whole was adsorbed through the electrostatic interaction with protonated amino species of the Fe₃O₄@C@DETA and have maximum adsorption capacity of 47.27 mg g^-1 (at 25 °C and pH 3.0). The adsorption data of free and EDTA coordinated Cr(III) were followed the Langmuir equation, while the adsorption dynamics was well explained by pseudo second order model indicating the chemical nature of adsorption process. The higher concentration of Ca²⁺ ions in the wastewater compete for adsorption sites and inhibit the Cr(III) removal, while on other hand Ca²⁺ ions promotes the adsorption of Cr(III)-EDTA, because of electrostatic interaction with adsorbent active sits. Furthermore, the adsorbent can be easily separated by external magnetic field and regenerated in acidic solution. The adsorbent is stable, recyclable and have more than 75% regeneration efficiency and can be repeatedly used in the adsorption process.

Electrochemical Removal of Organic and Inorganic Pollutants Using Robust Laser-Induced Graphene Membranes

The removal of emerging environmental pollutants in water and wastewater is essential for high drinking water quality or for discharge to the environment. Electrochemical treatment is a promising technology shown to degrade undesirable organic compounds or metals via oxidation and reduction, and carbon-based electrodes have been reported. Here, we fabricated a robust, porous laser-induced graphene (LIG) electrode on a commercial water treatment membrane using the multilasing technique and demonstrated the electrochemical removal of iohexol, an iodine contrast compound, and chromium(VI), a highly toxic heavy metal ion. Multiple lasing resulted in a more ordered graphitic lattice, a more physically robust carbon layer, and a 3-4-fold higher electrical conductivity. These properties ultimately led to a more efficient electrochemical process, and the optimized LIG electrodes showed a higher hydrogen peroxide (H₂O₂) generation. At 3 V, 90% of Cr(VI) was removed after 6 h and reached >95% removal after 8 h at pH 2. Cr(VI) was mainly reduced to Cr(III), with small amounts of Cr(I) and Cr(0), which were partially deposited on the electrode membrane surface, confirmed with X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy analysis. Under the same conditions, 50% of iohexol was degraded after 6 h and the transformation products (TPs) were identified using ultra-performance liquid chromatography coupled with mass spectroscopy. A total of seven main intermediates were identified including deiodinated TPs (m/z = 695, 570, and 443), probably occurring via three transformation pathways including oxidative deiodination, amide hydrolysis, and deacetylation. The electrical energy costs calculated for the removal of 2 mg L^-1 Cr(VI) was ∼$0.08/m³ in this system. Taken together, the porous LIG electrodes might be utilized for electrochemical removal of emerging contaminants in multiple applications because they can be rapidly formed on flexible polymer substrates at low cost.

Preserving nanoscale features in polymers during laser induced graphene formation using sequential infiltration synthesis

Direct lasing of polymeric membranes to form laser induced graphene (LIG) offers a scalable and potentially cheaper alternative for the fabrication of electrically conductive membranes. However, the high temperatures induced during lasing can deform the substrate polymer, altering existing micro- and nanosized features that are crucial for a membrane's performance. Here, we demonstrate how sequential infiltration synthesis (SIS) of alumina, a simple solvent-free process, stabilizes polyethersulfone (PES) membranes against deformation above the polymers' glass transition temperature, enabling the formation of LIG without any changes to the membrane's underlying pore structure. These membranes are shown to have comparable sheet resistance to carbon-nanotube-composite membranes. They are electrochemically stable and maintain their permeability after lasing, demonstrating their competitive performance as electrically conductive membranes. These results demonstrate the immense versatility of SIS for modifying materials when combined with laser induced graphitization for a variety of applications.

A WO3/PPy/ACF modified electrode in electrochemical system for simultaneous removal of heavy metal ion Cu2+ and organic acid

Heavy metal ions and organic acids are common pollutants in electroplating wastewater. Effective and economic treatment of such wastewater needs novel technologies. In this study, WO₃/PPy-1/ACF electrode was prepared using a hydrothermal modification method and it has large specific area (788.27 m² g^-1), high areal capacitance (2.58 F cm^-2 under 5 mA cm^-2 charge and discharge) and excellent conductivity. The modified electrode was used in an electrochemical system with activated carbon fiber felt (ACF) as counter electrode. The system simultaneously and successfully removed 97.8 % Cu²⁺ and 80.1 % citric acid (CA) from a simulated electroplating wastewater (typically 100 mg L^-1 Cu²⁺ and 800 mg L^-1 CA) in five- hour optimized operation. The influence of operating parameters (circulating inflow rate, applied voltage and influent pH) on the treatment performance was compared. There is interplay between Cu²⁺ reductive deposition and CA oxidation. The synergetic electrochemical treatment mechanism involves formation of hydrogen peroxide, free radicals, and catalytic effect of Cu species was proposed. This electrochemical system which is low-cost, easy to operate and highly efficient, may be applicable in treating acid-wash or electroplating wastewater, containing heavy-metal ions and organic acids.

Application of enhanced bioreduction for hexavalent chromium-polluted groundwater cleanup: Microcosm and microbial diversity studies

Hexavalent chromium (Cr⁶⁺) is a commonly found heavy metal at polluted groundwater sites. In this study, the effectiveness of Cr⁶⁺ bioreduction by the chromium-reducing bacteria was evaluated to remediate Cr⁶⁺-contaminated groundwater. Microcosms were constructed using indigenous microbial consortia from a Cr⁶⁺-contaminated aquifer as the inocula, and slow-releasing emulsified polycolloid-substrate (ES), cane molasses (CM), and nutrient broth (NB) as the primary substrates. The genes responsible for the bioreduction of Cr⁶⁺ and variations in bacterial diversity were evaluated using metagenomics assay. Complete Cr⁶⁺ reduction via the biological mechanism was observed within 80 days using CM as the carbon source under anaerobic processes with the increased trivalent chromium (Cr³⁺) concentrations. Cr⁶⁺ removal efficiencies were 83% and 59% in microcosms using ES and NB as the substrates, respectively. Increased bacterial communities associated with Cr⁶⁺ bioreduction was observed in microcosms treated with CM and ES. Decreased bacterial communities were observed in NB microcosms. Compared to ES, CM was more applicable by indigenous Cr⁶⁺ reduction bacteria and resulted in effective Cr⁶⁺ bioreduction, which was possibly due to the growth of Cr⁶⁺-reduction related bacteria including Sporolactobacillus, Clostridium, and Ensifer. While NB was applied for specific bacterial selection, it might not be appropriate for electron donor application. These results revealed that substrate addition had significant impact on microbial diversities, which affected Cr⁶⁺ bioreduction processes. Results are useful for designing a green and sustainable bioreduction system for Cr⁶⁺-polluted groundwater remediation.

Single and binary protein electroultrafiltration using poly(vinyl-alcohol)-carbon nanotube (PVA-CNT) composite membranes

Electrically conductive composite ultrafiltration membranes composed of carbon nanotubes have exhibited efficient fouling inhibition in wastewater treatment applications. In the current study, poly(vinyl-alcohol)-carbon nanotube membranes were applied to fed batch crossflow electroultrafiltration of dilute (0.1 g/L of each species) single and binary protein solutions of α-lactalbumin and hen egg-white lysozyme at pH 7.4, 4 mM ionic strength, and 1 psi. Electroultrafiltration using the poly(vinyl-alcohol)-carbon nanotube composite membranes yielded temporary enhancements in sieving for single protein filtration and in selectivity for binary protein separation compared to ultrafiltration using the unmodified PS-35 membranes. Assessment of membrane fouling based on permeate flux, zeta potential measurements, and scanning electron microscopy visualization of the conditioned membranes indicated significant resulting protein adsorption and aggregation which limited the duration of improvement during electroultrafiltration with an applied cathodic potential of -4.6 V (vs. Ag/AgCl). These results imply that appropriate optimization of electroultrafiltration using carbon nanotube-deposited polymeric membranes may provide substantial short-term improvements in binary protein separations.

The interactions and adsorption mechanisms of ternary heavy metals on boron nitride

In this work, the interactions and adsorption mechanisms of Cu²⁺, Cd²⁺, and Ni²⁺ on boron nitride (BN) were tested by the simultaneous removal of metal ions from synthetic wastewater. BN was characterized using XRD, SEM, and FTIR spectroscopy. The adsorption differences between BN and the metal ions were explored through comparative studies in a single and ternary system. In the ternary system, adsorption occurs rapidly in the first 2 min for the metal ions, and the affinity order follows Cu²⁺>Cd²⁺>Ni²⁺. However, adsorption behavior changes due to the interaction between metal ions in the ternary system. Cu²⁺ showed an antagonistic effect on the adsorption of Cd²⁺ and Ni²⁺, while Cd²⁺ and Ni²⁺ produced a synergistic effect on Cu²⁺. In addition, the effect of metal ion concentration on the interaction between ions was studied based on a surface response experiment. An increase in Ni²⁺ or Cd²⁺ concentrations plays a synergistic effect on the adsorption of Cu²⁺, while an antagonistic adsorption for Ni²⁺ occurred with increasing Cu²⁺ or Cd²⁺ concentrations. We also discussed the various adsorption mechanisms as complexation, ion exchange, and electrostatic adsorption based on XPS analysis.

Mineral Scale Prevention on Electrically Conducting Membrane Distillation Membranes Using Induced Electrophoretic Mixing

The growth of mineral crystals on surfaces is a challenge across multiple industrial processes. Membrane-based desalination processes, in particular, are plagued by crystal growth (known as scaling), which restricts the flow of water through the membrane, can cause membrane wetting in membrane distillation, and can lead to the physical destruction of the membrane material. Scaling occurs when supersaturated conditions develop along the membrane surface due to the passage of water through the membrane, a process known as concentration polarization. To reduce scaling, concentration polarization is minimized by encouraging turbulent conditions and by reducing the amount of water recovered from the saline feed. In addition, antiscaling chemicals can be used to reduce the availability of cations. Here, we report on an energy-efficient electrophoretic mixing method capable of nearly eliminating CaSO₄ and silicate scaling on electrically conducting membrane distillation (ECMD) membranes. The ECMD membrane material is composed of a percolating layer of carbon nanotubes deposited on porous polypropylene support and cross-linked by poly(vinyl alcohol). The application of low alternating potentials (2 V_pp,1Hz) had a dramatic impact on scale formation, with the impact highly dependent on the frequency of the applied signal, and in the case of silicate, on the pH of the solution.

Evolution of Environmentally Friendly Strategies for Metal Extraction

The demand for the recovery of valuable metals and the need to understand the impact of heavy metals in the environment on human and aquatic life has led to the development of new methods for the extraction, recovery, and analysis of metal ions. With special emphasis on environmentally friendly approaches, efforts have been made to consider strategies that minimize the use of organic solvents, apply micromethodology, limit waste, reduce costs, are safe, and utilize benign or reusable materials. This review discusses recent developments in liquid- and solid-phase extraction techniques. Liquid-based methods include advances in the application of aqueous two- and three-phase systems, liquid membranes, and cloud point extraction. Recent progress in exploiting new sorbent materials for solid-phase extraction (SPE), solid-phase microextraction (SPME), and bulk extractions will also be discussed.

Boosting Cr(VI) detoxification and sequestration efficiency with carbon nanotube electrochemical filter functionalized with nanoscale polyaniline: Performance and mechanism

Being able to simultaneously reduce and adsorb highly toxic Cr(VI) from bodies of water would provide a significant advance in the treatment of water pollution. Herein, we report on the design of an electroactive filter consisting of nanoscale polyaniline modified carbon nanotubes (PANI-CNT). In an electric field, both Cr(VI) reduction kinetics and Cr(III) sorption capacity were enhanced as the flow rate and voltage increased. At pH 7, Cr(VI) removal efficiency increased from 29.3% at 0 V to 70.2% at 2.5 V. This can be ascribed to superior electrical conductivity, more available active sites and limited pore size of the PANI-CNT filter, further boosted by the flow-through operation. Due to convection-enhanced mass transport, the proposed continuous-flow PANI-CNT filter demonstrated an evidently enhanced kinetic process when compared to conventional batch system. Various advanced assessments and density functional theory calculations verified the essential role of the electric field during Cr(VI) removal. Scanning transmission electron microscopy (STEM) examinations showed that the Cr was mainly sequestered by the PANI. It was also confirmed that an exhausted PANI-CNT filter can be regenerated by chemical washing with an HCl solution. Experiments with actual Cr(VI)-contaminated electroplating wastewater further verified the effectiveness of the system, with complete Cr(VI) transformation and a 50% Cr_total removal efficiency at 2.5 V with a flow rate of 3 mL/min. This study provides new insights into practical continuous-flow solutions to the effective removal of highly toxic Cr(VI).

Enhanced adsorption-coupled reduction of hexavalent chromium by 2D poly(m-phenylenediamine)-functionalized reduction graphene oxide

To improve the mass transfer efficiency of poly(m-phenylenediamine) for the effective removal of hexavalent chromium (Cr (VI)) from aqueous solution, a facile and one-step method to prepare two-dimensional poly(m-phenylenediamine) functionalized reduction graphene oxide (rGO-PmPD) by dilution polymerization is developed. The structure and morphology of rGO-PmPD as well as rGO and PmPD were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer-Emmett-Teller (BET), Fourier-transformed infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD). The preparation mechanism, adsorption performance, and mechanism of rGO-PmPD were then investigated in detail. The obtained rGO-PmPD exhibited thin 2D nanosheet morphology with much improved specific surface area and pore volume (18 and 25 times higher than that of PmPD, respectively). The Cr (VI) adsorption of rGO-PmPD was fitted well with the pseudo-second-order kinetic model and Langmuir isotherm model, and the maximum adsorption capacity of rGO-PmPD reached 588.26 mg g^-1, higher than that of PmPD (400 mg g^-1) and rGO (156.25 mg g^-1). Moreover, the regeneration efficiency of the rGO-PmPD nanosheet is also promising that the adsorption performance after five times of adsorption-desorption cycles still maintains more than 530 mg g^-1. The removal mechanism involved reduction coupled with adsorption and electrostatic interaction between rGO-PmPD and Cr (VI), and ~ 65% of Cr (VI) removal was attributed to reduction and ~ 35% was ascribed to adsorption and electrostatic interaction. This study thus provides a simple and effective route to achieve high accessible surface area of adsorbent materials with enhanced mass transfer efficiency and thereafter improved adsorption performance.

Potential-Driven Electron Transfer Lowers the Dissociation Energy of the C-F Bond and Facilitates Reductive Defluorination of Perfluorooctane Sulfonate (PFOS)

The widespread environmental occurrence of per- and polyfluoroalkyl substances (PFAS) has attracted significant regulatory, research, and media attention because of their toxicity, recalcitrance, and ability to bioaccumulate. Perfluorooctane sulfonate (PFOS) is a particularly troublesome member of the PFAS family due to its immunity to biological remediation and radical-based oxidation. In the present study, we present a heterogeneous reductive degradation process that couples direct electron transfer (ET) from surface-modified carbon nanotube electrodes (under low potential conditions) to sorbed PFOS molecules using UV-generated hydrated electrons without any further chemical addition. We demonstrate that the ET process dramatically increases the PFOS defluorination rate while yielding shorter chain (C₃-C₇) perfluorinated acids and present both experimental and ab initio evidence of the synergistic relationship between electron addition to sorbed molecules and their ability to react with reductive hydrated electrons. Because of the low energy consumption associated with the ET process, the use of standard medium-pressure UV lamps and no further chemical addition, this reductive degradation process is a promising method for the destruction of persistent organic pollutants, including PFAS and other recalcitrant halogenated organic compounds.

A novel superparamagnetic micro-nano-bio-adsorbent PDA/Fe3O4/BC for removal of hexavalent chromium ions from simulated and electroplating wastewater

In order to improve the adsorption efficiency of the adsorbent and solve the problem of separation difficulty, a novel superparamagnetic micro-nano-bio-adsorbent (PDA/Fe₃O₄/BC) was prepared by in situ self-assembly of polydopamine (PDA). The results of scanning electron microscope (SEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FT-IR), X-ray photoelectron spectrometer (XPS), and vibrating sample magnetometer (VSM) characterization showed that the size of bio-adsorbent was about 200 nm. PDA and Fe₃O₄ modifications increased the specific surface area of adsorbent, changed the surface functional group of biochar (BC), and made the adsorbent have super-high magnetization (294.76 emu g^-1). PDA/Fe₃O₄/BC was applied to treat Cr wastewater. The results show that the adsorption of Cr by PDA/Fe₃O₄/BC includes single-layer and multilayer adsorption. The adsorption follows the pseudo-second-order kinetic model. The adsorption is spontaneous and endothermic, and its maximum adsorption capacity and activation energy are 25.25 mg g^-1 at 318 K and 23.108 kJ mol^-1, respectively. After adsorption treatment, PDA/Fe₃O₄/BC still possesses high magnetization (233.04 emu g^-1). PDA/Fe₃O₄/BC can treat actual electroplating wastewater with Cr(VI) concentration from 20 mg L^-1 to less than 0.2 mg L^-1, which met the PRC discharge standard (GB/21900-2008) of electroplating pollutants. Graphical abstract.

Electroactive Membranes for Water Treatment: Enhanced Treatment Functionalities, Energy Considerations, and Future Challenges

To meet the increasing demand for water, potable water providers are turning toward unconventional waters, such as seawater and wastewater. These highly saline and/or heavily contaminated water sources are difficult to treat, demanding the use of advanced technology not typically used to treat conventional water sources such as river water or fresh groundwater. Of these advanced technologies, membrane separation processes are fast becoming the most widely used methods to convert these marginal waters into useful resources. The main factors contributing to the widespread adoption of membrane separation processes for water treatment include their modular nature, small physical footprint, and relative energy efficiency compared to traditional distillation processes. In addition, membranes present a physical barrier to pathogens, which is an attractive feature in terms of disinfection credits. However, traditional membrane materials suffer from several distinct drawbacks, which include membrane fouling (the accumulation of material on the membrane surface that blocks the flow of water), the need for high-pressure membranes (such as reverse osmosis (RO) or nanofiltration (NF)) or membrane/thermal processes (e.g., membrane distillation (MD)) to remove small contaminant compounds (e.g., trace metals, salt, endocrine disrupting compounds), and a pressure-driven membrane's inability to effectively remove small, uncharged molecules (e.g., N-nitrosodimethylamine (NDMA), phenol, acetone, and boron). Electrically driven physical and chemical phenomena, such as electrophoresis, electrostatic repulsion, dielectrophoresis, and electricity-driven redox reactions, have long been coupled to membrane-based separation processes, in a process known as electrofiltration. However, it is only in recent years that appropriate membrane materials (i.e., electrically conducting membranes (EMs)) have been developed that enable the efficient use of these electro-driven processes. Specifically, the development of EM materials (both polymeric and inorganic) have reduced the energy consumption of electrofiltration by using the membrane as an electrode in an electrochemical circuit. In essence, a membrane-electrode allows for the concentrated delivery of electrical energy directly to the membrane/water interface where the actual separation process takes place. In the past, metal electrodes were placed on either side of the membrane, which resulted in large potentials needed to drive electrochemical/electrokinetic phenomena. The use of a membrane-electrode dramatically reduces the required potentials, which reduces energy consumption and can also eliminate electrocorrosion and the formation of undesirable byproducts. In this Account, we review recent developments in the field of electrofiltration, with a focus on two water treatment applications: desalination and water reuse (wastewater or contaminated groundwater recycling). Specifically, we discuss how EMs can be used to minimize multiple forms of fouling (biofouling, mineral scaling, organic fouling); how electrochemical reactions at the membrane/water interface are used to destroy toxic contaminants, clean a membrane surface, and transform the local pH environment, which enhances the rejection of certain contaminants; how electric fields and electrostatic forces can be used to reorient molecules at the membrane/water interface; and how electrical energy can be transformed into thermal energy to drive separation processes. A special emphasis is placed on explicitly defining the additional energy consumption associated with the electrochemical phenomena, as well as the additional cost associated with fabricating EM materials. In addition, we will discuss current limitations of the electrofiltration process, with particular attention given to the current limitations of membrane materials and the future research needs in the area of membrane materials and module development.

Crossflow electrochemical filtration for elimination of ibuprofen and bisphenol a from pure and competing electrolytic solution conditions

For the first time, a crossflow electrochemical filtration system containing multiwalled carbon nanotubes (MWNTs) blended with buckypaper as a flat sheet dual membrane electrode was investigated for the removal of two contaminants of emerging concern, Ibuprofen and Bisphenol A. Breakthrough experiments revealed that a crossflow configuration could be highly efficient in eliminating both contaminants at applied DC potentials of 2 and 3 V over an extended period, from pure salt electrolyte as well as from synthetic secondary wastewater effluent. The shear flow provided consistent surface coverage resulting in excellent sorption performance. The long residence time of the two contaminants within the membrane (18.3 s) was sufficient enough to allow for almost complete degradation of phenolic aromatic products and quinoid rings and the resulting formation of aliphatic carboxylic acids, which was more evident at a higher applied potential (3 V). The formation of the non-toxic aliphatic carboxylic acids is a clear indication of the superior electrochemical performance of the crossflow mode over the dead-end flow-through system. Moreover, this study provides an in-depth understanding of different factors such as filter surface area and residence time that can greatly affect the removal of the contaminants considered.