Polyamide/UiO-66-NH2 nanocomposite membranes by polyphenol interfacial engineering for molybdenum(VI) removal (2023)

Molybdenum (Mo) is a strategic rare metal with high density, strength, hardness, and melting point, which has been widely used in industrial and military fields such as thermocouples, high-strength steel alloys, catalysts, electron tubes and heat-resistant materials [1,2]. Meanwhile, it is an essential but micro intaked nutrient element for the physiological processes of both plants and animals, but its high concentration will induce anemia, hypothyroidism, tissue degeneration and growth delay, finally leading to disease [3]. The fast development of industry not only rapidly increased a considerable and extensive demand for mineral resources, but also consequently released high amounts of wastes into the environment, and Mo is one of the hazardous pollutions [[4], [5], [6]]. China has the reserve and production of molybdenum resources with about 8.3 and 0.13 million tonnes in 2021, respectively. Thus, it is of great importance to develop effective and robust technologies for removing toxic molybdenum ions from wastewaters and also contributing to the enrichment and recovery, especially in China.

Various technologies have been employed to treat heavy metal wastewaters, such as membrane separation, reduction/oxidation, ion exchange, adsorption, and chemical precipitation [7,8]. Compared with other methods, nanofiltration has some features including high selectivity and efficiency, low cost, deep purification and environment-friendly nature, therefore it was broadly exploited and used in wastewater purification [7,9]. Adsorption nanocomposite membrane has been widely used for the treatment of heavy metal ions [10], therefore, a nanofiltration membrane with adsorption ability may provide a potential option for the treatment of Mo(VI) wastewaters. Currently, predominant NF membranes are polyamide thin film composite (TFC) membranes fabricated via interfacial polymerization (IP) of trimesoyl chloride (TMC) and diamine (m-phenylenediamine (MPD) or piperazine (PIP)) upon the porous substrate, exhibiting a great stability and selectivity [11]. Further, various nanomaterials have been incorporated into polyamide layer for optimizing the structure of the polyamide layer and forming thin-film nanocomposite (TFN) membrane, such as metal organic frameworks (MOFs), covalent organic frameworks, zeolite, titanium dioxide, silver nanoparticles, carbon nanotubes, g-C₃N₄ and graphene oxide [12,13].

MOFs are types of versatile function crystalline porous materials, which are consisted of inorganic metal ions or clusters coordinated to organic linkers [14]. To date, MOFs have garnered extensive attention in constructing high performance TFN membranes due to their high porosity, large specific surface area, tunable and well-defined pore size, designable functionality and extraordinary compatibility. Among various MOFs, UiO-66 exhibits exceptionally high water and chemical stability, and has gained lots of applications particularly in eliminating inorganic salt, heavy metal ions, organic dyes and other water pollutants [15,16]. Chung et al. developed UiO-66 incorporated thin-film nanocomposite membranes to concurrently remove Se(IV), Se(VI) and As(V) with a permeate flux of 5.4 L m⁻² h⁻¹ bar⁻¹ and a rejection of higher than 96 % [17]. Song et al. reported a UiO-66 TFN membrane showing a remarkable increase in water flux (about 50 %) and a marginal increase in rejection as compared with the benchmark TFC membrane [18]. Additionally, a series of UiO-66 MOFs with abundant active sites were emerged as superb adsorbents [15]. The amino UiO-66 (UiO-66-NH₂) demonstrated a particularly impressive performance as its additional electron-donated amine groups could significantly increase the positive charge of active sites and adsorb anions from aqueous solutions [15]. However, the agglomeration of MOFs is still a crucial bottleneck for their wide application in complex membranes [19].

To solve the above-mentioned problems, some reports explored the fabrication of interlayer-enhanced TFN membranes, in which organic coating layers or inorganic nanomaterials were uniformly deposited on the substrate membrane before the IP process [20]. The functional interlayer was designed as the “adhesive tape” to effectively eliminate membrane defects and form robust crosslinked structures within TFN membranes [20]. Wu et al. engineered a novel TFC nanofiltration membrane mediated by a polydopamine-covalent organic framework interlayer [21]. The interlayer manipulated the distribution and release of amine monomers and thereby tuned polyamide structure during the IP process, while serving as a barrier against the penetration of polyamide into the porous substrate [22]. Recently, a class of polyphenol structures have served in this regard, such as tannic acid (TA) [23], polydopamine [24,25] and polyphenol/PEI [20]. As a much cheaper natural plant polyphenol compound, tannic acid contains high content of phenolic hydroxyl, and it can be readily extracted from the bark and leaf of plentiful natural plants [23]. TA is capable of interacting with nearly all kinds of polymeric membranes and forming a coating, which can extremely improve the hydrophilicity and adhesive ability of substrate [26]. Additionally, TA is proved to provide a high reactivity with many groups, such as carboxyl, amino and acyl chloride due to the presence of catechol and pyrogallol groups [23]. Therefore, TA has the potential application in reactively adhesive modification. Moreover, TA has shown great affinity for toxic metal ions in wastewaters, such as chromium, manganese, zinc, copper, nickel, mercury, lead, strontium, cesium and gold [[27], [28], [29]]. More importantly, it has been proved that polyphenol structures were effectively competent to the adsorption and removal of Mo(VI) ions [30,31].

In this work, we aim to explore an interlayer-enhanced thin-film nanocomposite membrane for nanofiltration of molybdenum(VI) solution containing salts. Firstly, an amphiphilic poly(vinylidene fluoride)-grafting-poly(acrylic acid) (PVDF-g-PAA) copolymer was synthesized and then prepared as a substrate. The substrate had high hydrophilicity and abundant channels, and its carboxylated surface was conducive for the attachment of the interlayer. Secondly, on the surface of the substrate, the thin-film nanocomposite membrane featuring an interlayer (TFNi) was fabricated by in situ self-assembling of TA/UiO-66-NH₂ and the interface polymerization. The interlayer composed of polyphenol TA and porous amino UiO-66-NH₂ could tune viably the construction of TFNi membrane by Michael addition, Schiff base or other reactions, which was avail to form a non-defect, robust and thin cross-linking structure. In order to investigate the effect of interlayer on the membrane structure and performance, three preparation schemes were designed. On the surface of the interlayer, a polyamide thin layer was prepared by the interface polymerization of PIP and TMC. Thirdly, TFNi membranes were used for nanofiltration of molybdenum(VI) solutions containing salts (sodium chloride and/or sodium sulfate). Moreover, based on adsorption experiments and theoretical computations, the adsorption mechanism of TFNi membrane towards Mo(IV) was elucidated. The separation of Mo(VI) ions by TFNi membrane would not only treat toxic wastewaters, but also facilitate its enrichment and recovery. To our knowledge, there has been no study on Mo(IV) separation by a TFN membrane. The strategy for developing a novel TFNi membrane with high separation performance was also provided in this work.