Carbon dots (CDs) are an emerging class of carbon-based nanomaterials with typical particle sizes below 10 nm [1]. They exhibit tunable photoluminescence, good chemical stability, low cytotoxicity, and favorable biocompatibility, which has led to increasing interest in applications such as biosensing, bioimaging, drug delivery, and environmental remediation [2], [3], [4], [5], [6]. CDs consist of amorphous or partially crystalline carbon cores bearing oxygen-containing groups, such as hydroxyl, carboxyl, and carbonyl moieties [7], [8]. These features provide active sites for pollutant adsorption and allow further chemical modification for environmental and biomedical applications [9], [10], [11], [12].

Two main approaches are commonly employed for CD synthesis. In the top-down method, bulk carbon materials are converted into nanoscale carbon structures through techniques such as arc discharge [13], laser ablation [14], [15], or electrochemical oxidation [16]. The bottom-up method forms CDs from organic molecular precursors through processes such as pyrolysis [17], [18], microwave-assisted synthesis [19], [20], solvothermal treatment [21], [22], and hydrothermal processing [23], [24], [25]. Hydrothermal synthesis is particularly attractive due to its operational simplicity, scalability, and environmental compatibility [26], [27], [28], [29], [30]. Recent studies have shown that natural biomass can serve as an efficient precursor for hydrothermal synthesis. CDs synthesized from Lantana camara flower extract via a one-step hydrothermal method demonstrated a quantum yield of 29% and exhibited stable fluorescence for intracellular imaging and Cr(VI) sensing [31]. Similarly, hydrochar produced from Acacia falcata leaves achieved an adsorption capacity of 30.47 mg g−1 for Cr(VI), attaining over 95% removal in real wastewater, primarily due to its oxygen-rich surface functionalities [32]. However, pristine CDs have notable limitations in water treatment applications, as their ultrasmall size and high aqueous dispersibility hinder efficient separation from treated water and limit their reusability. One approach to overcoming these limitations is to incorporate magnetic nanoparticles, such as Fe₃O₄, into CD-based hybrid structures, which enables magnetic recovery and enhances adsorption capacity and catalytic efficiency [33], [34].

In this context, green synthesis offers a sustainable and environmentally friendly method for fabricating nanomaterials by using plant extracts as natural reducing and stabilizing agents. Plant extracts from Camellia sinensis [35], Moringa oleifera [36], Citrus sinensis [37], banana peels [38], and olive leaves [39] have been employed for the green synthesis of Fe₃O₄ nanoparticles. These natural sources are effective due to their high content of polyphenols, flavonoids, and organic acids, which provide functional groups for nanoparticle nucleation and stabilization. Recent studies have shown that magnetic CD-based nanocomposites synthesized from renewable biomass precursors exhibit high adsorption capacity, visible-light activity, and good reusability. Singh et al. reported N-doped magnetic CDs derived from Allium cepa, achieving visible-light-driven degradation efficiencies of 92.76% for methylene blue and 83.05% for rhodamine B [40]. Ghereghlou et al. synthesized Fe₃O₄@CDs from Elaeagnus angustifolia and reported a high adsorption capacity of 124.39 mg g−1 for methylene blue, with good recyclability and pseudo-second-order kinetics [41]. Similarly, Esmail et al. prepared a CDs@Fe₃O₄ nanocomposite from Crocus cancellatus, achieving 94.4% photodegradation of fluorescein under natural sunlight and retaining 75% efficiency after five reuse cycles [42].

E. japonica (loquat, “Malta eriği”) is a plant species rich in phytochemicals, including polyphenols, flavonoid glycosides, terpenoids, carotenoids, triterpenic acids, vitamins, pectin, and organic acids [43], [44]. These constituents provide a chemical environment that facilitates electron transfer and helps limit nanoparticle aggregation during synthesis. E. japonica naturally grows in Mediterranean-type climates, including regions of Türkiye, where its leaves are an abundant, low-cost, and renewable biomass source for sustainable nanomaterial production. In this study, E. japonica leaf biomass was used directly for the green hydrothermal synthesis of carbon dots, while its aqueous extract served as a bio-reducing and capping medium for Fe₃O₄ nanoparticle formation.

To the best of our knowledge, the use of E. japonica leaves as the carbon source for CDs synthesis has not been previously reported. To enhance the structural and photocatalytic properties of the resulting CDs, TiO₂ nanoparticles were incorporated during the hydrothermal synthesis process [45], [46]. TiO₂ is a well-established photocatalyst due to its strong oxidative ability, chemical stability, and non-toxicity; however, its wide band gap restricts photoactivity mainly to the ultraviolet region [47], [48]. CDs modification of TiO₂ has been widely reported to enhance visible-light absorption and suppress charge recombination by improving interfacial charge transfer. These effects are commonly attributed to heterojunction formation and chemical interactions between CDs and TiO₂, such as Ti–O–C bonding, which contribute to band-gap narrowing and enhanced photocatalytic performance [49]. In this context, Sharif et al. reported that modification of TiO₂ photoelectrodes with carbon quantum dots reduced the band gap from 3.38 to 3.09 eV due to Ti–O–C bond formation and the introduction of mid-gap states [50]. Similarly, Kuldeep et al. demonstrated that green-synthesized CD–TiO₂ nanocomposites exhibit enhanced visible-light absorption and higher photocatalytic efficiency than pristine TiO₂, primarily due to Ti–O–C bonding-induced band-gap narrowing [51].

Chitosan is a biocompatible polymer containing amino and hydroxyl groups, which facilitate metal coordination and interactions with dye molecules in wastewater treatment applications. Owing to these properties, chitosan–TiO₂ hybrid materials have attracted significant attention for environmental applications, as they integrate the adsorption capability of chitosan with the photocatalytic activity of TiO₂ [52], [53], [54], [55]. Midya et al. reported a chitosan-based hybrid nanocomposite consisting of TiO₂ nanoparticles and CDs deposited on polyvinyl-imidazole–crosslinked chitosan [cl-Ch-p(VI)/TiO₂NPs–CDs], which efficiently degraded organic pollutants under sunlight within 180 min [56]. Kashi et al. developed a chitosan–microalgae–carbon-doped TiO₂ composite (CsMaTk/S) for brilliant green adsorption, reporting a specific surface area of 9.1 m2 g−1 and favorable initial removal performance [57]. However, removal efficiency decreased with repeated use, showing a 36% reduction after five cycles and a 48.5% mass loss due to adhesion to the reaction vessel. Despite these advances, practical wastewater treatment still requires hybrid systems that integrate high surface accessibility, rapid pollutant removal, efficient recovery, and stable adsorption–photocatalytic performance over repeated cycles.

CV is a synthetic triphenylmethane dye widely used in textile dyeing, printing, and biological staining, and is considered a major environmental contaminant due to its toxicity, mutagenicity, and persistence in aquatic systems [58], [59]. It is also classified as a carcinogenic compound and poses significant health risks; exposure to below 1 ppm may cause respiratory disorders, renal impairment, and visual disturbances, while higher concentrations can lead to skin irritation and gastrointestinal distress [60]. Due to its environmental and health risks, CV requires efficient wastewater treatment and is therefore commonly used as a model pollutant in remediation studies.

To the best of our knowledge, the adsorption and photocatalytic removal of CV using a magnetically recoverable Fe₃O₄/Chi/TiO₂–CDs nanocomposite has not been previously reported. Previous studies on dye removal have mainly focused on TiO₂-based photocatalysts, CDs-modified materials, or chitosan-based systems, which have generally been investigated as individual components. Despite their promising performance, these approaches still face key limitations regarding material recovery, structural integrity, and reusability over repeated cycles. Furthermore, although interest in green synthesis strategies is increasing, E. japonica has not been explored as a carbon precursor for CDs fabrication. This study therefore reports a green hydrothermal route for fabricating a magnetically recoverable Fe₃O₄/Chi/TiO₂–CDs nanocomposite and evaluates its adsorption–photocatalytic performance, reusability, and operational stability for CV removal. It is hypothesized that the presence of TiO₂, carbon dots, chitosan, and Fe₃O₄ will enhance both adsorption and photocatalytic degradation of CV, while enabling efficient magnetic recovery and reuse.