Removal of As(iii) and As(v) from water using green, silica-based ceramic hollow fibre membranes via direct contact membrane distillation

Solar-based technologies for removing potentially toxic metals from water sources: a review

Technological advances have led to a proportional increase in the deposition of contaminants across various environmental compartments, including water sources. Heavy metals, also known as potentially toxic metals, are of particular concern due to their significant harmful impacts on environmental and human health. Among the available methods for mitigating the threat of these metals in water, solar radiation-based technologies stand out for their cleanliness, cost-effectiveness, and efficiency in removing or reducing the toxicity of heavy metals. The performance and productivity of these methods in removing heavy metals such as arsenic (As), chromium (Cr), mercury (Hg), and uranium (U) from water still need to be comprehensively synthesized. Thus, this work aims to address that gap. The performance, potential, and challenges of real-world applications of conventional solar stills (CSS), membrane-based solar stills, and solar heterogeneous photocatalysis are concisely summarized and critically reviewed. CSS and membrane-based stills are highly effective (efficacy > 98%) in removing and capturing heavy metals from water. However, structural and functional improvements are needed to enhance productivity (especially for CSS) and usability in real-world environmental remediation and drinking water supply scenarios. Solar heterogeneous photocatalysis is highly effective in removing and/or converting As, Cr, Hg, and U into their non-toxic or less toxic forms, which subsequent processes can easily remove. Further research is necessary to evaluate the safety of photocatalytic materials, their integration into scalable solar reactors, and their usability in real-world environmental remediation applications.

In Situ-Grown Al2O3 Nanoflowers and Hydrophobic Modification Enable Superhydrophobic SiC Ceramic Membranes for Membrane Distillation

Membrane distillation (MD) is considered a promising technology for desalination. In the MD process, membrane pores are easily contaminated and wetted, which will degrade the permeate flux and salt rejection of the membrane. In this work, SiC ceramic membranes were used as the supports, and an Al2O3 micro-nano structure was constructed on its surface. The surface energy of Al2O3@SiC micro-nano composite membranes was reduced by organosilane grafting modification. The effective deposition of Al2O3 nanoflowers on the membrane surface increased membrane roughness and enhanced the anti-fouling and anti-wetting properties of the membranes. Simultaneously, the presence of nanoflowers also regulated the pore structures and thus decreased the membrane pore size. In addition, the effects of Al2(SO4)3 concentration and sintering temperature on the surface morphology and performance of the membranes were investigated in detail. It was demonstrated that the water contact angle of the resulting membrane was 152.4°, which was higher than that of the pristine membrane (138.8°). In the treatment of saline water containing 35 g/L of NaCl, the permeate flux was about 11.1 kg⋅m-2⋅h-1 and the salt rejection was above 99.9%. Note that the pristine ceramic membrane cannot be employed for MD due to its larger membrane pore size. This work provides a new method for preparing superhydrophobic ceramic membranes for MD.

Bio-Inspired Functional Materials for Environmental Applications

With the global population expected to reach 9.7 billion by 2050, there is an urgent need for advanced materials that can address existing and developing environmental issues. Many current synthesis processes are environmentally unfriendly and often lack control over size, shape, and phase of resulting materials. Based on knowledge from biological synthesis and assembly processes, as well as their resulting functions (e.g., photosynthesis, self-healing, anti-fouling, etc.), researchers are now beginning to leverage these biological blueprints to advance bio-inspired pathways for functional materials for water treatment, air purification and sensing. The result has been the development of novel materials that demonstrate enhanced performance and address sustainability. Here, an overview of the progress and potential of bio-inspired methods toward functional materials for environmental applications is provided. The challenges and opportunities for this rapidly expanding field and aim to provide a valuable resource for researchers and engineers interested in developing sustainable and efficient processes and technologies is discussed.

Green and Sustainable Membranes: A review

Membranes are ubiquitous tools for modern water treatment technology that critically eliminate hazardous materials such as organic, inorganic, heavy metals, and biomedical pollutants. Nowadays, nano-membranes are of particular interest for myriad applications such as water treatment, desalination, ion exchange, ion concentration control, and several kinds of biomedical applications. However, this state-of-the-art technology suffers from some drawbacks, e.g., toxicity and fouling of contaminants, which makes the synthesis of green and sustainable membranes indeed safety-threatening. Typically, sustainability, non-toxicity, performance optimization, and commercialization are concerns centered on manufacturing green synthesized membranes. Thus, critical issues related to toxicity, biosafety, and mechanistic aspects of green-synthesized nano-membranes have to be systematically and comprehensively reviewed and discussed. Herein we evaluate various aspects of green nano-membranes in terms of their synthesis, characterization, recycling, and commercialization aspects. Nanomaterials intended for nano-membrane development are classified in view of their chemistry/synthesis, advantages, and limitations. Indeed, attaining prominent adsorption capacity and selectivity in green-synthesized nano-membranes requires multi-objective optimization of a number of materials and manufacturing parameters. In addition, the efficacy and removal performance of green nano-membranes are analyzed theoretically and experimentally to provide researchers and manufacturers with a comprehensive image of green nano-membrane efficiency under real environmental conditions.

Surface Modification of UiO-66 on Hollow Fibre Membrane for Membrane Distillation

The hydrophobicity of metal-organic frameworks (MOFs) is critical in enhancing the separation process in membrane distillation. Herein, a new superhydrophobic University of Oslo 66 (UiO-66) MOFs was successfully constructed on the top of alumina hollow fibre (AHF) membrane for desalination purposes. The fabrication methodology of the membrane involved in situ growth of pure crystalline UiO-66 on top of AHF and post-synthetic modification by fluorosilane grafting. The resultant membrane was characterised to study the physicochemical properties of the pristine and modified membrane. A superhydrophobic UiO-66 with a contact angle of 163.6° and high liquid entry pressure was obtained by introducing a highly branched fluorocarbon chain while maintaining its crystallinity. As a result, the modified membrane achieved 14.95 L/m²∙h water flux and 99.9% NaCl rejection with low energy consumption in the direct contact membrane distillation process. Furthermore, the high surface energy contributed by UiO-66 is maximised to produce the maximum number of accessible sites for the grafting process. The synergistic effect of surface hydrophobicity and porous UiO-66 membrane in trapping water vapour shows great potential for desalination application.

Recent developments in porous ceramic membranes for wastewater treatment and desalination: A review

The development of membrane technology has proved vital in providing a sustainable and affordable supply of clean water to address the ever-increasing demand. Though liquid separation applications have been still dominated by polymeric membranes, porous ceramic membranes have gained a commercial foothold in microfiltration (MF) and ultrafiltration (UF) applications due to their hydrophilic nature, lower fouling, ease of cleaning, reliable performance, robust performance with harsh feeds, relative insensitivity to temperature and pH, and stable long-term flux. The enrichment of research and development on porous ceramic membranes extends its focus into advanced membrane separation technologies. The latest emerging nanofiltration (NF) and membrane distillation (MD) applications have witnessed special interests in constructing porous membrane with hydrophilic/functional/hydrophobic properties. However, NF and MD are relatively new, and many shortcomings must be addressed to compete with their polymeric counterparts. For the last three years (2018-2020), state-of-the-art literature on porous ceramic membranes has been collected and critically reviewed. This review highlights the efficiency (permeability, selectivity, and antifouling) of hydrophilic porous ceramic membranes in a wide variety of wastewater treatment applications and hydrophobic porous ceramic membranes in membrane distillation-based desalination applications. A significant focus on pores characteristics, pore sieving phenomenon, nano functionalization, and synergic effect on fouling, the hydrophilic porous ceramic membrane has been discussed. In another part of this review, the role of surface hydrophobicity, water contact angle, liquid entry pressure (LEP), thermal properties, surface micro-roughness, etc., has been discussed for different types of hydrophobic porous ceramic membranes -(a) metal-based, (b) silica-based, (c) other ceramics. Also, this review highlights the potential benefits, drawbacks, and limitations of the porous membrane in applications. Moreover, the prospects are emphasized to overcome the challenges in the field.

Rice husk-SiO2 supported bimetallic Fe-Ni nanoparticles: as a new, powerful magnetic nanocomposite for the aqueous reduction of nitro compounds to amines

This paper reports a novel green procedure for immobilization of bimetallic Fe/Ni on amorphous silica nanoparticles extracted from rice husk (RH-SiO2). The heterogeneous nanocomposite (Fe/Ni@RH-SiO2) was identified using SEM, EDX, TEM, BET, H2-TPR, TGA, XRD, VSM, ICP-OES, and FT-IR analyses. The Fe/Ni@RH-SiO2 nanocomposite was applied as a powerful catalyst for the reduction of structurally diverse nitro compounds with sodium borohydride (NaBH4) in green conditions. This procedure suggests some benefits such as green chemistry-based properties, short reaction times, non-explosive materials, easy to handle, fast separation and simple work-up method. The catalyst was separated by an external magnet from the reaction mixture and was reused for 9 successive cycles with no detectable changes of its catalytic efficiency.

Separation and Recycling of Concentrated Heavy Metal Wastewater by Tube Membrane Distillation Integrated with Crystallization

Tube membrane distillation (MD) integrated with a crystallization method is used in this study for the concurrent productions of pure water and salt crystals from concentrated single and mixed system solutions. The effects of concentrated Zn2+ and Ni2+ on performance in terms of membrane flux, permeate conductivity, crystal recovery rates, and crystal grades are investigated. Preferred crystallization and co-crystallization determinations were performed for mixed solutions. The results revealed that membrane fluxes remained at 2.61 kg·m-2·h-1 and showed a sharp decline until the saturation increased to 1.38. Water yield conductivity was below 10 μs·cm-1. High concentrated zinc and nickel did not have a particular effect on the rejection of the membrane process. For the mixed solutions, membrane flux showed a sharp decrease due to the high saturation, while the conductivity of permeate remained below 10 μs·cm-1 during the whole process. Co-crystallization has been proven to be a better method due to the existence of the SO42- common-ion effect. Membrane fouling studies have suggested that the membrane has excellent resistance to fouling from highly concentrated solutions. The MD integrated with crystallization proves to be a promising technology for treating highly concentrated heavy metal solutions.