Aluminum Copper bimetallic metal organic gels/sodium alginate beads for efficient adsorption of ciprofloxacin and methylene Blue: Adsorption isotherm, kinetic and mechanism studies (2023)

Among the most important environmental issues in the world today are organic dye pollution and antibiotic residue, which can harm the health of humans, plant and animal development, and the ecosystem as a whole (Guellati et al., 2022, Kumar et al., 2019, Reichert et al., 2019, Wang et al., 2021a). Ciprofloxacin (CIP) is a common quinolone antibiotic. Nowadays, we can find ciprofloxacin in various concentrations in sewage of all types (Herrera-Herrera et al., 2010), surface water (Yan et al., 2013, Zhang et al., 2013), and aquaculture sites (Castrignano et al., 2020, Zou et al., 2011). Long-term accumulation of ciprofloxacin in water will destroy ecological balance and may even lead to drug resistance of bacteria, posing a great threat to human health. Methylene blue (MB) is another harmful environmentally persistent pollutant and is a toxic cationic dye with a complex aromatic structure that can lead to nervous system, skin, heart, vision and respiratory damage (Chen et al., 2023, de Oliveira Guidolin et al., 2021, Viet et al., 2021). Therefore, it is imperative to find effective ways to remove CIP and MB in water.

In order to efficiently remove dyes and antibiotics from polluted water, a series of technologies such as adsorption (Yu et al., 2019a, Zhao et al., 2022b), biological treatment (Yang et al., 2018, Zhang et al., 2022a), photocatalytic degradation (Liu et al., 2021a, Zhao et al., 2021a), advanced oxidation (Chen et al., 2020, Chen et al., 2022), electrocatalysis (Yang et al., 2018, Zhang et al., 2020) and membrane technology (Alonso et al., 2018, Nasrollahi et al., 2022, Nguyen et al., 2019, Wang et al., 2020) have been explored. Adsorption techniques have drawn the most interest among these due to their adaptability, affordability, potent capacity to remove contaminants, lack of secondary contamination, stability, and high efficiency (Ahmed et al., 2020). Modified activated carbon (Barasarathi et al., 2022, Wang et al., 2019) zeolite (Atugoda et al., 2021, Yuan et al., 2018), bentonite (Antonelli et al., 2020, Wang et al., 2021b), graphene (Othman et al., 2018, Wei et al., 2018), MOFs (Metal Organic Framework) (Bai et al., 2020, Zhao et al., 2021b) and other substances have been employed as adsorbents for ciprofloxacin and methylene blue in the past, however, they have the disadvantages of poor recovery and limited adsorption capacity. Therefore, it is still an urgent need to develop adsorbents with large adsorption capacity, economic durability and good adaptability. It helps to improve the quality of human life while protecting the environment.

MOGs have a layered porous structure, and their micro/mesoporous properties increase the diffusion rate of pollutants, thus speeding up the adsorption process. Moreover, due to its network structure, MOG usually has a large specific surface area and porosity, which provides a good basis for its application in adsorption, separation, catalysis, etc. In addition, the morphology of spherical, nano-linear or two-dimensional films can be formed by adjusting synthesis conditions and ligand types. Compared with traditional synthesis methods, ultrasonic assisted synthesis of MOG can improve the reaction rate. It can be used for rapid detection and identification of pollutants by improving the yield and showing fluorescence characteristics in some specific cases. In addition, their excellent chemical/mechanical stability, low reaction temperature, short reaction time and cheap solvent solutions make MOGs promising adsorbents (Wang et al., 2016a). MOGs have seen significant usage recently in adsorption, catalysis, drug detection, and other domains. (Fang et al., 2022, Gao et al., 2023, Yu et al., 2022). In the past research, it has been found that metal organic gels had a good removal effect on dyes and antibiotics. Liu et al. synthesized JLUE-MOGs with good adsorption effect on CTC by adjusting the ratio of Fe and Eu ions(Liu et al., 2021b);Wang et al. prepared Zn-MOG by reacting xanthine and zinc acetate in water, with a removal efficiency of up to 80.63% for MO(Wang et al., 2021a); Gu et al. synthesized Ga-MOG through solvothermal method and found a competitive adsorption relationship between CIP and CTC, but the adsorption capacity of Ga-MOG for them is still 207.90/126.90mg/g (Gu et al., 2023); Wang et al. reported a kind of iron organic gel (Fe-MOG) with high surface area, which showed excellent adsorption capacity for MO (182.82mg/g) (Wang et al., 2016b). Among them, bimetal-organic gels (Bimetal MOGs) exhibit more active sites and higher porosity than mono-metal MOGs in the adsorption process and can capture more pollutant molecules making them an ideal adsorption material. Research into bimetallic MOGs are limited, adding bimetals to MOGs may enhance their adsorption potential, which is worth exploring (Abednatanzi et al., 2019; Chen et al., 2020b; Liu et al., 2020; Xia et al., 2022).

SA is also non-toxic and odorless, with good biodegradability and biocompatibility, low cost and renewable, making it a promising hydrogel for use in the field of water pollutant adsorption. If the powder adsorption material is fixed on the sodium alginate material, then the formed coated granular material will have good water permeability and adsorption performance. Several previous studies have investigated contaminant removal through the SA coating of powdered materials. For instance, Hu et al. created TA-PVA /SA hydrogel beads by mixing TA with PVA and sodium alginate solution, and found that it had a good removal effect on MB, and the maximum adsorption capacity reached 147.06mg/g at 30°C(Hu et al., 2018). Zhang et al. used SA to immobilize Ti₃C2T_x and form Ti₃C₂T_x/SA beads. Ti3C2Tx/SA-30% was found to have the greatest unit adsorption capacity of MB, reaching 92.17mg/g at 25 °C and pH 7(Zhang et al., 2021a). Malakpour et al. combined SA and CuO, and discovered that SA/CuO composite beads had a good reduction effect on Cr (IV), MB and p-nitrophenol. After 10 cycles, the catalytic activity of the nano-catalyst did not significantly decline(Mallakpour et al., 2022). Given these previous successes with SA composites, the immobilization of MOGs on sodium alginate beads seems an effective strategy to prevent the aggregation of nanomaterials and promote their separation.

The solvothermal approach was used in this study to create a novel bimetallic MOG with TATB serving as the organic ligand and aluminum and copper serving as the metal centers. Powdered MOGs materials and [emailprotected] gel beads were prepared (Fig.1). To examine the possible the structural morphology, specific surface area, component elements, and distinctive functional groups of the adsorbent, a variety of characterisation techniques including XRD, SEM, TEM, EDS, BET, XPS, and Zeta potential were utilized. With ciprofloxacin (CIP) and methylene blue (MB) as the target pollutants, once the adsorption data were fitted using kinetic and isotherm analytical techniques, the adsorption behavior of the materials was thoroughly examined. In addition, the influence of pH level, starting concentration, interference ions and other factors on CIP and MB adsorption and the reuse of materials was also discussed. Lastly, the structure of MOGs was characterized both pre and post-adsorption, and the adsorption mechanism was determined. This work provides some ideas for designing adsorbents with high adsorption capacity and easy recovery for adsorption of various pollutants (quinolone antibiotics and cationic dyes), to expand the application of MOGs materials and sodium alginate in environmental protection.