Dual-function advanced magnetic bacterial cellulose materials: From enhanced adsorption phenomena to an unprecedented circular green catalytic strategy

Copper (Cu) is widely regarded as one of the most important metals due to its exceptional physicochemical properties and impressive catalytic abilities [1], [2], [3]. These characteristics make it indispensable in modern organometallic catalysis and a variety of industrial applications, including batteries, electroplating, and electronics [4], [5], [6], [7]. However, despite its valuable applications, the excessive utilization and accumulation of Cu(II) in environmental systems pose significant health risks, such as hypoglycemia, insomnia, neurological impairments, and Wilson’s disease [8], [9], [10]. To mitigate these dangers, the removal of Cu(II) ions from wastewater is urgently needed and remains a major challenge [11], [12], [13]. In recent decades, various methods including electrochemical treatment, ion exchange, chemical precipitation, membrane separation, coagulation/flocculation, advanced oxidation and adsorption have been employed to remove Cu(II) ions from wastewater [14], [15], [16]. However, many of these methods have limitations, such as low efficiency and the production of sludge [16]. Among these approaches, adsorption remains one of the most accessible and widely used methods for wastewater decontamination due to its low operating costs, reversibility, flexibility, and broad application range [17], [18], [19].

A significant advantage of cost-effective adsorbents lies in their diverse sources, which include minerals, biological materials, and organic compounds [20]. As a result, extensive research has been conducted on various sorbents, such as activated carbon, clays, metal–organic frameworks, porous polymers, and carbon nanomaterials [21], [22], [23]. In recent years, there has been a growing demand for environmentally friendly biomass-based adsorbents, including chitosan, cellulose, and various lignocellulosic wastes, due to their renewable, biodegradable, and biocompatible properties [24], [25], [26]. Among these, bacterial cellulose (BC), as a renewable and sustainable natural resource, has garnered significant research interest [27]. Its low cost, environmental benignity, high specific surface area, and exceptional hydrophilicity make it a promising material for various applications [27], [28], [29]. Bacterial cellulose is a pure and renewable form of cellulose produced by certain bacterial species [30]. Unlike plant-derived cellulose, BC consists of nanofibrils that are free of lignin, pectin, and hemicellulose, eliminating the need for additional processing for commercial applications [31]. BC possesses a well-ordered crystalline structure composed of β-1,4-bonded glucopyranose units, with reactive hydroxyl (–OH) groups located at the C₂, C₃, and C₆ positions along the cellulose chain [32]. These characteristics make BC an excellent candidate for the development of adsorbents capable of removing metal ions from aqueous solutions [33].

Due to the abundant hydroxyl groups on the backbone of BC, chemical modification has become a crucial method for preparing functional materials [34]. This approach enhances the adsorption capacity by increasing the number of reactive sites through various derivatization reactions, such as esterification, etherification, crosslinking, and graft copolymerization [35]. Chemical modifications involving functional groups such as pyridine, amino, carboxyl, hydroxyl, oxime, and phosphonic acid have been incorporated into BC structures to improve their adsorption capacity and efficiency for heavy metal ions [36], [37], [38], [39]. Additionally, BC has been doped with materials like chitosan and graphene oxide sheets, resulting in a diverse range of modified BC-based materials [40], [41], [42]. Amino groups are highly effective grafting groups due to their strong chelation abilities toward heavy metal ions, making them excellent scavengers [43], [44], [45]. For example, Cheng et al. demonstrated that bacterial cellulose membranes (BCMs) modified with ethylenediaminetetraacetic acid (EDTA) via (3-aminopropyl) triethoxysilane (APTES) exhibited a maximum adsorption capacity of 0.512 mmol.g⁻¹ for Sr(II) ions, surpassing the performance of activated carbon [46].

Despite these advancements, challenges remain. BC-based materials often exhibit high dispersibility in water, complicating their recovery after use [47]. To address this issue, magnetic BC-based adsorbents have been developed by incorporating magnetic nanoparticles, enabling their retrieval using an external magnetic field [48]. Magnetite nanoparticles (Fe₃O₄NPs) are particularly well-suited for this application due to their cost-effective synthesis and the ability to manipulate materials at the nanoscale, offering excellent versatility in separation and environmental technologies [49]. Fe₃O₄NPs can be integrated with BC-based materials through various methods, such as co-precipitation and solvothermal synthesis [50]. Among these methods, the co-precipitation method is especially popular due to its environmentally friendly nature [51]. For example, Zhu et al. prepared spherical Fe₃O₄NPs-BC nanocomposites as adsorbents for removing Pb(II), Mn(II), and Cr(III) ions, achieving maximum adsorption capacities of 0.314 mmol.g⁻¹, 0.601 mmol.g⁻¹, and 0.481 mmol.g⁻¹ for Pb(II), Mn(II), and Cr(III), respectively [52]. Additionally, Chen et al. designed a magnetic purifier with a biomass-based structure by blending attapulgite/chitosan (ATP/CS) composite with bacterial cellulose nanofibrils (BCNs), referred to as Fe₃O₄/ATP@(BCNs/CS)₇. They investigated Fe₃O₄/ATP@(BCNs/CS)₇ as an adsorbent for the removal of Pb(II), Cu(II), and Cr(VI) ions from aqueous solutions, reporting Langmuir maximum adsorption capacities of 0.327 mmol.g⁻¹, 1.110 mmol.g⁻¹, and 1.750 mmol.g⁻¹ for Pb(II), Cu(II), and Cr(VI), respectively [53].

While the development of magnetic BC-based adsorbents has advanced wastewater treatment technologies, the disposal of Cu(II)-loaded spent adsorbents remains a critical challenge [54], [55]. Most studies focus solely on adsorption performance, often neglecting the potential to valorize spent materials. Utilizing Cu(II)-loaded adsorbents as precursors for catalysts represents an innovative approach to address this gap. Copper catalysts, particularly those based on Cu(I) and Cu(II), are extensively employed in Huisgen’s 1,3-dipolar cycloaddition (CuAAC) for synthesis of 1,4-disubstituted 1,2,3-triazoles valuable compounds with applications in pharmaceuticals, agrochemicals, and materials science [56], [57], [58], [59], [60], [61]. However, Cu(I)-based catalysts often suffer from thermodynamic instability and susceptibility to oxidation, necessitating the exploration of more stable Cu(II)-based catalytic systems [62], [63], [64], [65], [66]. Heterogeneous Cu(II) catalysts have gained significant attention due to their ability to reduce Cu(II) to Cu(I) in the presence of a reducing agent, such as sodium ascorbate, during the azide-alkyne cycloaddition reaction in Click chemistry. For instance, Zhang et al. reported a heterogeneous catalyst based on an ethylenediamine-functionalized PVC-anchored Cu(II) complex (PVC-EDA-Cu(II)) for the regioselective synthesis of 1,4-disubstituted 1,2,3-triazoles from phenylacetylenes, benzyl halides, and sodium azide in water, with sodium ascorbate as the reducing agent [67]. Despite the merits of these catalytic systems, they often face significant drawbacks, including prolonged reaction times to achieve reasonable yields, the need for high reaction temperatures, laborious product purification steps, challenges in catalyst separation and recovery, and the necessity of using multiple catalysts [68], [69], [70].

To the best of our knowledge, the dual application of magnetic BC-based materials for both environmental remediation and catalysis in the CuAAC reaction has yet to be explored. This dual functionality represents a substantial advancement, combining the efficient removal of pollutants with the facilitation of valuable organic transformations. Furthermore, pyridine groups are well-established as highly effective chelating agents that stabilize Cu(II) species, enhancing catalytic activity through their strong and stable interactions with Cu(II) ions [71], [72]. By integrating pyridine groups into magnetic BC-based materials, it is possible not only to improve pollutant adsorption efficiency but also to enable the reuse of spent adsorbents as active organometallic catalysts. This approach presents a novel and sustainable strategy for materials development, contributing simultaneously to waste valorization and catalytic processes within a single system.

This study aims to address the interconnected challenges of Cu(II) removal and catalytic functionality by developing dual-purpose magnetic BC-based material. The proposed strategy offers a cost-effective and eco-friendly strategy for Cu(II) removal, achieved through the grafting of N,N-bis(2-pyridylmethyl)ethylenediamine (BPEM) onto BC, followed by the in situ synthesis of Fe₃O₄NPs. The impact of BPEM functionalization on the adsorption capacity of BC was systematically evaluated using various adsorbents, including BC, BC-Cl, BC-BPEM, and (BC-BPEM)@Fe₃O₄NPs. The adsorption behavior of these materials toward Cu(II) ions was thoroughly investigated by studying the effects of pH and contact time on adsorption capacity. To better understand the adsorption mechanism and the nature of Cu(II) ion interactions with the BC-based materials, the experimental data were fitted to pseudo-first-order, pseudo-second-order, and Elovich kinetic models. In addition to its adsorption capabilities, the Cu(II)-loaded BC material was used as a heterogeneous catalyst for the regioselective diazide-alkyne 1,3-dipolar cycloaddition reaction, a key process in green chemistry. This reaction enables the synthesis of high-value 1,4-disubstituted bis-1,2,3-triazoles under mild conditions. The catalytic efficiency of the recovered material was assessed in terms of reaction yields and environmental metrics, offering valuable insights into its sustainability and practical applicability.