Water contamination is a pivotal issue due to ultrarapid industrialization [1]. Specifically, increased plastic consumption worldwide and its low recyclability (∼9%) generate microplastics (MP) over a period of time through biological and chemical processes [2]. Every year between 4.8 and 12.7 million tons of MP enters seawater, which accumulates with time’s passage [3]. The smaller size of MP is more of a concern, as they are untraceable, and small invertebrates can ingest MP [4], [5]. Moreover, smaller MP can adsorb pollutants due to their high specific surface area, which can be ingested by aquatic animals and ultimately consumed by humans posing an unpredictable risk to health [6]. Recently MP has been declared as contaminants of emerging concern. Current techniques for removing solids from aquatic systems include traditional filtration, photodegradation, adsorption, and membranes [7], [8], [9]. However, conventional filters such as filter papers can only filter around 20 µm size particles from the water and are not sufficient to filter a few microns or nano size MP [10]. Nano or microfiltration has been trialled; however, the pore-size distribution of membranes must be tailored to each pollutant’s size [10]. Slow filtration rates, difficulty in recovery, and need high pressure to operate increase costs significantly [10]. On the other hand, photodegradation found the somehow effective approach, but a lower removal rate required long time as well, as releasing toxic by-products in aquatic systems are great hurdles. For example, only 65 % photodegradation of 150 µm size polypropylene (PP) occurred in 456 h on the surface of zinc oxide nanorods; however, such long times are not acceptable by industry [8]. The time of photodegradation can be reduced from weeks to days with smaller plastic particles, however, it reduces dangerous carbon emissions like CO, thus, limiting its applications [11].
Similar to MP pollution, methylene blue (MB, dissolved contaminant) is another toxic pollutant due to its adverse impact on aquatic life and the environment [12]. Methylene blue is an industrial waste effluent, and effective removal is desirable. It is pivotal to investigate the MP removal in the presence of other contaminants, such as MB, because the presence of other pollutants might impact the removal abilities of adsorbent or kinetics.
Therefore, quick adsorption of such solids remains the best possible pathway but requires the development of effective adsorbents which not only adsorb these solid pollutants but can also be separated easily. Different materials as adsorbents, including carbon nanotubes, and polyoxometalate‐supported ionic liquid phases, have been applied to remove MP through adsorption, but low adsorption capacities and difficulty in separation are ongoing challenges [13], [14]. Metal-organic frameworks (MOFs) have recently gained attention due to their large surface area, pore-volume, and tuneable properties for MP removal [15]. Further, MOF and MOF-based materials have been successfully used for adsorption [16], [17], [18]. For example, MOF-based foam (membrane) has been developed to remove MPs with good adsorption capability, however, it suffers from the typical membrane issues such as fouling scaling and needs high flow rates that affect membrane permeability [9], [19]. It has been found that transforming three-dimensional (3D) MOF to 2D MOF further provides an opportunity to tune its surface chemistry which can help in overcoming the aforementioned issues [20], [21].
One of the biggest challenges in production of 2D MOFs sheets in solution is blocking the 3D growth of MOF and staking 2D MOFs sheets without compromising the unique characteristics of 2D MOFs, including active surface sites and surface area. Developing hybrid/heterostructures with other materials was found one possible pathway to avoid re-stacking of sheets and harvest unique features of 2D MOF in improving the system performance. For example, 0D MXenes quantum dots were used to develop a hybrid material with 2D Ni MOF to enhance light absorption capability and catalytic efficiency for N 2 reduction [22]. However, simple construction of hybrid still results in restaking of composite sheets, which buried the active surface chemistry of 2D MOF. Therefore, unique engineering is required to build a stable structure that normalizes the highly active surface of 2D MOFs and avoids its transformation to the bulk structure during solution processing. Nanopillared structures with a similar chemistry to 2D MOF can solve the issue of restacking while maintaining MOF properties. Such nanopillared structures with magnetic properties can assist the separation of MOF from solution when is used as an adsorbent rather than employing filtration and centrifugation.
By keeping in mind the above-mentioned challenges, here we developed an engineered structure where 2D MOF sheets are separated using carbon encapsulated iron oxide ([email protected]); carbon provides a strong connection with MOF sheets to stabilize them while magnetic core helps easy separation, schematically shown in Scheme 1. Importantly, the nanopillars ([email protected]) self-assembled between the 2D MOF during the synthesis of MIL-100 (Fe) through hydrothermal treatment. The nanopillared structure possesses a high surface area and unique surface chemistry, which we hypothesize will enable the efficient removal of solid microplastics (MP) and methylene blue (MB) in a mixed pollutants system. Most importantly, removing adsorbent with MP is carried out using external magnetic in contrast to traditionally used filtration or centrifuge, which are not practical. The MP removal is characterized using Dynamic light scattering (DLS), UV–vis, and thermogravimetric analysis (TGA). The ex situ microscopic, Energy-dispersive X-ray spectroscopy and Brunauer, Emmett, and Teller results will be further applied to investigate materials stability.