Membranes prepared from layers of graphene oxide (GO) offer substantial advantages over conventional materials for water treatment (e.g. greater flux), but the stability of GO membranes in water has not been achieved until now. In this study the behavior of GO membranes prepared with different quantities and species of cations has been investigated to establish the feasibility of their application in water treatment. A range of cation-modified GO membranes were prepared and exposed to aqueous solutions containing specific chemical constituents. In pure water, unmodified and Na-modified GO membranes were highly unstable, while GO membranes modified with multivalent cations were stable provided there were sufficient quantities of cations present; their relative capability to achieve GO stability was as follows: Al 3+ > Ca 2+ > Mg 2+ > Na + . It is believed that the mechanism of cross-linking, and membrane stability, is via metal-carboxylate chelates and cation-graphite surface interactions (cation-π interaction), and that the latter appears to increase with increasing cation valency. The instability of cation (Ca or Al)-modified GO membranes by NaCl solutions during permeation occurred as Na + exchanged with the incorporated multivalent cations, but a high content of Al 3+ in the GO membrane impeded Al 3+ /Na + exchange and thus retained membrane stability. In solutions containing biopolymers representative of surface waters or seawater (protein and polysaccharide solutions), Ca-GO membranes (even with high Ca 2+ content) were not stable, while Al-GO membranes were stable if the Al 3+ content was sufficiently high; Al-formed membranes also had a greater flux than Ca-GO membranes. PAPEROriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

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Graphene oxide (GO), as an emerging nanofiller, is commonly incorporated into the polypiperazine amide (PPA) selective layer to enhance nanofiltration (NF) performance. However, GO’s dispersibility and compatibility present outstanding challenges for fabricating high-performance GO-incorporated NF membranes. In this study, polyvinyl alcohol (PVA) and GO were introduced into the aqueous solution at the same time to enhance the membrane performance by taking advantage of the synergetic effects of GO and PVA. GO’s dispersibility in aqueous solution and its compatibility within the PPA matrix were significantly improved because of the introduction of PVA. Furthermore, both PVA and GO contributed to the diffusion behavior of aqueous monomers for constructing defect-free, rougher, thinner, and more hydrophilic NF membranes. With the introduction of 100 ppm GO and 0.05 wt % PVA into the aqueous solution during the interfacial polymerization process, the optimal thin-film nanocomposite NF membrane exhibited a water flux of 84.2 LMH without reducing the salt rejections (97.6% for Na2SO4), which were 69.1 and 36.7% higher than the pristine thin-film composite (TFC)-blank and TFC-PVA membranes, respectively. Meanwhile, an excellent antifouling performance was achieved with a flux recovery rate of 92.1% because of the lower negative charge and excellent hydrophilicity of the membrane surface. This study develops a simple and effective approach to construct high-performance NF membranes.

To improve CO 2 adsorption performance of nanoparticle absorbents, a novel tertiary amine functionalized nano-SiO 2 (NS-NR 2 ) was synthesized based on the 3-aminopropyltrimethoxysilane (KH540) modified nano-SiO 2 (NS-NH 2 ) via methylation. The chemical structure and performances of the NS-NR 2 were characterized through a series of experiments, which revealed that NS-NR 2 can react with CO 2 in water and nanofluid with low viscosity revealed better CO 2 capture. The CO 2 capture mechanism of NS-NR 2 was studied by kinetic models. From the correlation coefficient, the pseudo second order model was found to fit well with the experiment data. The influencing factors were investigated, including temperature, dispersants, and cycling numbers. Results has shown the additional surfactant to greatly promote the CO 2 adsorption performance of NS-NR 2 because of the better dispersity of nanoparticles. This work proved that NS-NR 2 yields low viscosity, high capacity for CO 2 capture, and good regenerability in water. NS-NR 2 with high CO 2 capture will play a role in storing CO 2 to enhanced oil recovery in CO 2 flooding.

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Adsorptive ultrafiltration mixed matrix membranes (MMMs) are a new strategy, developed in recent years, to remove harmful cations and small-molecule organics from wastewater and drinking water, which achieve ultrafiltration and adsorption functions in one unit and are considered to be among the promising technologies that have exhibited efficiency and competence in water reuse. This mini review concerns the research progress of adsorptive ultrafiltration MMMs for removing heavy metal ions and small-molecule organics. We firstly introduce the types and classifications of adsorptive ultrafiltration MMMs (their classifications can be established based on the type of the adsorbent used). Furthermore, we discuss the removal mechanism of adsorptive ultrafiltration MMMs, as well as summarizing the main fabrication techniques for adsorptive ultrafiltration membranes. In addition, we identified some of the issues and challenges of the practical application for adsorptive ultrafiltration.

Using polyethylenimine (PEI) as the aqueous reactive monomers, a positively charged thin-film nanocomposite (TFN) nanofiltration (NF) membrane with enhanced performance was developed by successfully incorporating graphene oxide (GO) into the active layer. The effects of GO concentrations on the surface roughness, water contact angle, water flux, salt rejection, heavy metal removals, antifouling property, and chlorine resistance of the TFN membranes were evaluated in depth. The addition of 20 ppm GO facilitated the formation of thin, smooth, and hydrophilic nanocomposite active layers. Thus, the TFN-PEI-GO-20 membrane showed the optimal water flux of 70.3 L·m−2·h−1 without a loss of salt rejection, which was 36.8% higher than the thin-film composite (TFC) blank membrane. More importantly, owing to the positively charged surfaces, both the TFC-PEI-blank and TFN-PEI-GO membranes exhibited excellent rejections toward various heavy metal ions including Zn2+, Cd2+, Cu2+, Ni2+, and Pb2+. Additionally, compared with the negatively charged polypiperazine amide NF membrane, both the TFC-PEI-blank and TFN-PEI-GO-20 membranes demonstrated superior antifouling performance toward the cationic surfactants and basic protein due to their hydrophilic, smooth, and positively charged surface. Moreover, the TFN-PEI-GO membranes presented the improved chlorine resistances with the increasing GO concentration.

To construct antifouling polyvinylidene fluoride (PVDF) membranes, L-aspartic acid (L-asp)-modified graphene quantum dots (AGQDs) were covalently anchored on the PVDF membrane surface via a three-step modification method. The pristine PVDF membrane was first dehydrofluorinated to generate internal double bonds under the alkali solution. Then diamine (EDA) and two types of hyperbranched polyethyleneimines (HPEI) were grafted on the alkali-treated surfaces through the Michael addition reaction. Finally, the as-synthesized AGQDs were chemically immobilized on the amine-grafted surfaces via an amidation reaction. The surface morphologies and surface properties of the pristine and modified PVDF membranes were comprehensively characterized by X-ray photoelectron spectroscopy, ATR-FTIR, scanning electron microscope, atomic force microscope, and dynamic antifouling experiments. Meanwhile, the grafting efficiency of AGQDs were found to be strongly dependent on the type of amine used. Compared with EDA and HPEI L , HPEI H with a high molecular weight preferred to be grafted on membrane surfaces rather than membrane pores because of its larger steric hindrance, which facilitate the covalent anchorage of AGQDs on the HPEI H -grafted surface. Thanks to the high grafting efficiency of AGQDs, the resulting PVDF-HPEI H -AGQDs membrane possessed the excellent hydrophilicity (water contact angle as low as 58.7°). Most importantly, this membrane demonstrated superior antifouling performance over the pristine PVDF membrane in the presence of either the positively charged foulant (e.g., lysozyme) or the negatively charged foulant (e.g., bovine serum albumin). This novel membrane fabrication approach developed in this study provides a promising solution to covalently anchor hydrophilic nanoparticles on the PVDF membrane surface for antifouling enhancement.