Removal of Cadmium and Lead from Tires Discarded in the Open Sea with Multicomponent Nanoparticles from Sugarcane Bagasse

Abstract

This study addresses the environmental challenge of end-of-life tire accumulation, a major source of toxic metals such as lead and cadmium in marine ecosystems. As a sustainable solution, multicomponent metal-oxide nanoparticles (Fe₃O₄, ZnO, CaO, MgO, and minor CaCO₃) were green-synthesized from sugarcane bagasse and stabilized with blackberry (Rubus glaucus) extract. Structural characterization (XRD, SEM, TEM, and EDS) confirmed their crystalline inorganic composition. Pb²⁺ was almost completely removed (95–99%) within 15–30 min using 50–100 mg of nanoparticles, with ~80–90% efficiency at 75 mg. Cd²⁺ removal showed dose-dependent kinetics: ~90% removal occurred within 10 min at 75 mg, while 50 and 100 mg reached ~60–70% after 60 min. Equilibrium, kinetic, and thermodynamic analyses revealed that Pb²⁺ adsorption followed the Langmuir model (R² = 0.982) with monolayer chemisorption, whereas Cd²⁺ obeyed the Freundlich model (R² = 0.945), indicating heterogeneous multilayer adsorption. Pb²⁺ removal fitted a pseudo-second-order model (R² = 0.991), while Cd²⁺ followed a pseudo-first-order behavior (R² = 0.958). Thermodynamic parameters (ΔG° < 0, ΔH° > 0, ΔS° > 0) confirmed a spontaneous and endothermic process. Sugarcane-bagasse-derived Fe₃O₄–ZnO–CaO–MgO nanomaterials act as sustainable and effective adsorbents for marine heavy metal removal.

1. Introduction

The planet is facing an acute environmental crisis driven by the overexploitation of natural resources, with end-of-life tires (ELTs) emerging as one of the most challenging solid wastes due to their slow degradation rate, high disposal costs, and significant landfill requirements [1,2,3]. In developed regions such as the European Union and the United States, ELTs are typically subjected to recycling operations, including shredding and material recovery [4]. However, in many Latin American countries, these wastes are often discarded in rivers, lagoons, or directly into the sea, causing serious environmental, social, and economic consequences [5]. In Ecuador, initiatives such as the 2019 agreement between the Ministry of Environment and SEGINUS have sought to strengthen integrated waste management systems [6]; nevertheless, the environmental risks associated with improper tire disposal remain largely underestimated [7].

A major environmental concern associated with end-of-life tires (ELTs) is the release of heavy metals—particularly lead (Pb), cadmium (Cd), and zinc (Zn)—due to their high toxicity and bioaccumulative nature [8,9,10]. Numerous studies have reported the leaching of Pb, Cd, and Zn from tire residues in landfills and aquatic environments, contributing significantly to soil and water contamination [9,10]. Cadmium, a highly mobile and carcinogenic element, has been detected in marine systems at concentrations ranging from <5 to 110 ng L⁻¹ [11,12,13,14], whereas lead, a persistent neurotoxin, occurs in seawater at approximately 10⁻³ mg kg⁻¹ [15,16,17]. Both metals are known to bioaccumulate through the marine food web, posing severe ecological and human health risks [18,19,20,21,22,23].

Conventional remediation techniques, including physical and chemical treatments, are often costly, energy-intensive, and may generate secondary toxic byproducts [24,25,26,27,28]. Consequently, sustainable alternatives such as bioadsorption using agricultural residues, phytoremediation, and bioremediation have gained attention as environmentally friendly and cost-effective strategies [29,30,31,32].

In addition to heavy metals, end-of-life tires (ELTs) contribute to marine pollution through the release of microplastics and tire wear particles (TWP), also known as crumb rubber granulate (CRG). Recent studies have shown that TWP and CRG release zinc (Zn) at ecotoxicologically relevant concentrations, with leaching dynamics strongly influenced by salinity and exposure time [33,34]. Comprehensive reviews further indicate that TWP constitute a significant fraction of coastal and oceanic microplastic pollution, acting as carriers of heavy metals and persistent organic pollutants, thereby posing potential risks to marine ecosystems and human health [35].

Tires are primarily composed of natural and synthetic rubbers, carbon black, silica, and various chemical additives, including zinc oxide (ZnO, ~1–2 wt%), which represents the main intentionally added metal [33,34]. Lead (Pb) and cadmium (Cd), although not deliberately incorporated, occur as trace contaminants originating from pigments and stabilizers, with reported concentrations ranging from 0.1–2 mg kg⁻¹ and 0.5–10 mg kg⁻¹, respectively [35]. Despite their lower abundance compared to Zn, Cd is highly mobile and carcinogenic, while Pb is a persistent neurotoxin that tends to accumulate in sediments [36].

This issue is particularly critical along the Ecuadorian coast, where discarded tires of unknown origin are frequently found in marine environments. Although this practice is unauthorized, it continues to occur, thereby increasing the risk of Cd and Pb leaching into coastal ecosystems and their subsequent bioaccumulation within marine biota.

Nanotechnology offers an innovative platform for heavy metal remediation, as metal oxide nanoparticles exhibit high adsorption capacity, strong selectivity, and tunable physicochemical properties suitable for environmental applications [37,38]. In particular, green synthesis using biological precursors provides a sustainable alternative by avoiding toxic reagents and simultaneously adding value to agricultural byproducts [39,40]. Previous studies have demonstrated the effectiveness of iron oxide nanoparticles synthesized from natural extracts such as clove and coffee in the adsorption of Cd²⁺ and Ni²⁺, while also revealing notable antimicrobial properties [41]. Among iron oxides, maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄) possess distinctive size-dependent magnetic characteristics, making them ideal candidates for advanced technological and environmental applications [42,43,44].

Sugarcane bagasse (Saccharum officinarum), the most abundant byproduct of the sugar industry, is rich in minerals such as silica, calcium, iron, magnesium, and zinc [45,46,47]. This composition makes it a valuable and renewable precursor for nanoparticle synthesis. Previous studies have utilized sugarcane bagasse to produce magnetic nanoparticles with superparamagnetic properties for oil recovery applications [48]. However, despite significant advances in green nanotechnology, evidence of its direct application in environmental remediation remains scarce [49,50].

This study investigates the potential of biogenic nanoparticles synthesized from sugarcane bagasse for the removal of Pb²⁺ and Cd²⁺ leached from discarded tires in the marine environment of the Galápagos Islands. Nanoparticles of iron oxide (FeO), zinc oxide (ZnO), calcium oxide (CaO), calcium carbonate (CaCO₃), and magnesium oxide (MgO) were synthesized, characterized, and evaluated for their adsorption capacity in seawater. The findings demonstrate that sugarcane bagasse, an abundant and low-cost industrial byproduct, can serve as a sustainable precursor for producing highly efficient nanoparticles for heavy metal removal. This approach not only aligns with the principles of the circular economy but also contributes to the preservation of fragile marine ecosystems, providing a green and scalable solution for mitigating heavy metal contamination.

2. Materials and Methods

2.1. Study Area

This research was conducted under laboratory conditions at the Advanced Materials Laboratory of the Nanoscience and Nanotechnology Center (CENCINAT) of the Universidad de las Fuerzas Armadas (ESPE). The research center is located at the main campus in Sangolquí, Ecuador (0°18′53″ S, 78°26′36″ W), in the inter-Andean valley of north-central Ecuador, at an altitude of 2748 m above sea level. The laboratory maintains an average temperature of 25 °C.

2.2. Chemicals

Reagents for water analysis were purchased from USA Hach Company P.O. Box 389 Loveland, Colorado. For the purification of the synthesis products and the preparation of the reaction solutions, ultrapure water from a Milli-Q system (Conductivity 0.28 µS/cm, Millipore, Direct Q, Darmstadt, Germany) was used, while the ethanol used was absolute grade, as with ACS ISO (Isopropyl Alcohol) 1728 Batch 19.022.310. It was used for the digestion of sugarcane bagasse: NaOH Merck KGaA, CAS Nro 1310-73-2 (Darmstadt, Germany); H₂O₂ 30% (w/w) 0639 BATCH 19555201. Scharlah S.L.

2.3. The Extract Black Berry (Rubus glaucus) Preparation

The Extract mora (Rubus glaucus) were obtained from a local market in Conocoto parish (Quito, Ecuador) and washed with water and neutral soap. Stems, leaves, and flowers were cut into small pieces. Extraction was performed according to previously described methodologies [51,52], using 96% ethanol (Scharlau) at 40% concentration in a 2:1 (v/w) ratio under rotational agitation. The mixture was sonicated for 10 min with a Branson 3510 ultrasonic device (40% amplitude), filtered through filter paper and then through a 0.45 μm membrane. The solvent was removed with a Yamato RE801 rotary evaporator (Yamato Scientific Co., Ltd., Tokyo, Japan), and the extract was dried at 40 °C for 24 h in a Wiseven oven.

Total phenolic content was determined following the method described by Dewanto et al. (2002) [53], with modifications from Murgueitio et al. (2021) [54], using gallic acid, sodium carbonate, and Folin–Ciocalteu reagent. Absorbance was measured at 760 nm in a Genesys 10 uv UV-Vis spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA), and results were expressed as mg gallic acid equivalents per liter (mg GAE/L).

Antioxidant capacity was evaluated by the DPPH method [55], using DPPH reagent (SIGMA) and 80% methanol (Merck KgaA). The final solution was sonicated for 20 min in a Cole-Parmer ultrasonic sonicator, and absorbance was recorded with a Perkin Elmer Lambda 35 UV-Vis spectrophotometer (Perkin Elmer, Waltham, MA, USA).

2.4. Preparation of Sugarcane Bagasse

Sugarcane bagasse was collected at the Wholesale Market of Machachi (0°30′58″ S, 78°34′01″ W; Mejía Canton, Pichincha Province, Ecuador). Samples were thoroughly washed with water and detergent, and subsequently disinfected to remove organic and inorganic impurities. The cleaned material was dried in a forced convection oven (Memmert SN30, Schwabach, Germany) at 105 °C for 24 h, ground to a fine powder, and subjected to an additional drying step (105 °C, 2 h) to eliminate residual moisture. The dried powder was washed with ultrapure water (Milli-Q^®, Sigma Aldrich, Santa Clara, CA, USA), oven-dried again (105 °C, 24 h), and sieved to obtain a homogeneous particle size distribution.

Alkaline microwave-assisted digestion was performed using a Multiwave PRO system (Anton Paar, Graz, Austria) with a mixture of NaOH 1 M and hydrogen peroxide (H₂O₂, 30%) at a ratio of 1:10:10 (w/v/v). The samples, previously weighed on an analytical balance (Cole Palmer 1000016, Vernon Hills, IL, USA), were placed in digestion vessels. Microwave parameters were set as follows: maximum digestion temperature 240 °C, cooling temperature 30 °C, pressure 100 bar, heating time 10 min, digestion time 15 min, and cooling time 2 h 30 min. After digestion, the solution was filtered to remove undigested residues, yielding a liquid matrix free of organic matter.

The resulting solution was oven-dried at 105 °C for 24 h, and the solid fraction was divided into two portions: one calcined at 500 °C and the other at 900 °C, for 20 min each. Both solids were subsequently washed with Milli-Q^® water, sonicated at 40 kHz for 30 min (5 min on/5 min off cycles), and centrifuged. The sonication–centrifugation process was repeated three times. Finally, the solids were resuspended in Milli-Q^® water, filtered through 450 µm and 250 µm membranes, oven-dried at 105 °C for 2 h, and stored in a desiccator until use.

2.5. Synthesis of Nanoparticles (NPs)

The synthesis of multicomponent nanoparticles was carried out following the procedure described by Murgueitio et al. (2024) [51]. Initially, sugarcane bagasse was washed, dried, and ground to obtain a clean precursor. The biomass was then subjected to alkaline digestion with NaOH and H₂O₂ under microwave-assisted conditions to remove residual organic matter and release inorganic constituents. The resulting solid was subsequently calcined at 500–900 °C to promote the crystallization of metal oxides. To provide a green reducing medium and enhance nanoparticle stability, blackberry (Rubus glaucus) extract was incorporated, followed by sonication for 30 min at 40 kHz (5 min on/5 min off cycles) to facilitate dispersion and prevent agglomeration. The suspension was filtered through 450 µm and 250 µm membranes, yielding multicomponent nanoparticles composed of Fe₃O₄, ZnO, CaO, MgO, and CaCO₃. These nanoparticles exhibited high crystallinity and strong potential for heavy metal adsorption in aqueous systems (Figure 1).

2.6. Metal Readings from Sugarcane Bagasse

Metal characterization was conducted using Atomic Absorption Spectrophotometry (AAS) with an Analyst 800 instrument (PerkinElmer, Shelton, CT, USA). The equipment was calibrated with standard solutions of known concentrations corresponding to the target metals detected in sugarcane bagasse. A specific hollow cathode lamp was employed for each element (Fe, Zn, Mg, and Ca), following Method 3111 B (direct air–acetylene flame) [56].

The analytical parameters were as follows: iron (Fe), λ = 248 nm, air–acetylene flame, sensitivity 0.12 mg L⁻¹, detection limit (DL) 0.02 mg L⁻¹; zinc (Zn), λ = 213.9 nm, air–acetylene flame, sensitivity 0.02 mg L⁻¹, DL 0.005 mg L⁻¹; magnesium (Mg), λ = 285.2 nm, air–acetylene flame, sensitivity 0.007 mg L⁻¹, DL 0.0005 mg L⁻¹; and calcium (Ca), λ = 422.7 nm, air–acetylene flame, sensitivity 0.08 mg L⁻¹, DL 0.003 mg L⁻¹.

Prepared samples were introduced into the spectrophotometer using an automatic injection system, and each sample was analyzed in triplicate to ensure analytical precision and reproducibility of the results.

2.7. Lead and Cadmium Analysis of Tire Samples

Shredded and sieved tire samples were immersed in seawater at 20 °C and maintained under continuous agitation for 30 days using a Siemens rotary shaker (Model 12023, Munich, Germany). After the exposure period, the liquid phase was filtered and analyzed for lead (Pb) and cadmium (Cd) concentrations by Atomic Absorption Spectrophotometry (AAS), following Method 3111 B (direct air–acetylene flame) [56].

The analytical parameters were as follows: cadmium (Cd), λ = 228.8 nm, air–acetylene flame, sensitivity 0.025 mg L⁻¹, detection limit (DL) 0.002 mg L⁻¹; and lead (Pb), λ = 283.3 nm, air–acetylene flame, sensitivity 0.5 mg L⁻¹, DL 0.05 mg L⁻¹.

2.8. Materials and Equipment Used in the Characterization of NPs

Nanoparticles (NPs) were characterized to determine their average size, morphology, and crystalline composition. Particle size distribution was first analyzed using a HORIBA LB-550 submicron particle analyzer (HORIBA, Kyoto, Japan), connected to a computer via a SCSI interface and operated with HORIBA LB-550 software (v.3.57; 1996–2005). Measurements were based on the principle of dynamic light scattering (DLS), covering a detection range of 0.001–6 µm (1–6000 nm). The instrument was annually aligned and calibrated with NIST-certified latex standards (monodisperse and polydisperse polystyrene, 20 ± 2 nm and 100 ± 2 nm), with a maximum accepted tolerance of 5%. Repeatability and reproducibility were verified to ensure data accuracy.

Particle morphology and complementary size evaluation were performed by transmission electron microscopy (TEM). A drop of the colloidal suspension was deposited onto carbon-coated copper grids and analyzed using a FEI Tecnai G2 Spirit Twin microscope (FEI, Eindhoven, The Netherlands) operated at 80 kV. Calibration was conducted using gold nanoparticle diffraction, considering an interplanar spacing of 0.24 nm.

The crystalline structure of the NPs was determined by X-ray diffraction (XRD) on an EMPYREAN diffractometer (PANalytical, Almelo, The Netherlands) using Cu Kα radiation (λ = 1.54 Å) in a 2θ configuration. Particle size distribution results were further corroborated using the HORIBA LB-550 analyzer [57].

Surface morphology was examined by scanning electron microscopy (SEM). Samples were coated with a conductive carbon layer using a Quorum Q150R coater (Quorum Technologies, Lewes, UK) and observed under a Tescan Mira 3 microscope (Brno, Czech Republic). Routine maintenance, alignment, and calibration were performed annually to ensure measurement reliability.

2.9. Removal of Lead and Cadmium

A specific volume of 50 mL of seawater containing tire leachates was transferred into individual Falcon tubes. The pH was adjusted to 6 using 1 M hydrochloric acid (HCl), and three different doses of the synthesized multicomponent nanoparticles (50, 75, and 100 mg) were added. The suspensions were allowed to react for 3 h, after which the supernatant from each tube was collected for lead (Pb) and cadmium (Cd) quantification by atomic absorption spectrophotometry (AAS). Each metal concentration was measured in triplicate, and the results were expressed as the mean of the three replicates.

Statistical analyses were conducted using InfoStat^® software (v.2020; Universidad Nacional de Córdoba, Argentina). Analysis of variance (ANOVA) was applied to evaluate significant differences among treatments, and post hoc multiple comparison tests were performed where applicable. Statistical significance was considered at p < 0.05.

Additionally, adsorption equilibrium, kinetic, and thermodynamic studies were conducted to better understand the metal-removal mechanisms. For equilibrium analysis, adsorption isotherms (Langmuir and Freundlich models) were fitted using the residual concentrations of Pb²⁺ and Cd²⁺ obtained at different dosages. Kinetic behavior was evaluated using pseudo-first-order and pseudo-second-order models based on contact time (10–60 min). Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were calculated from adsorption data at different temperatures (293–323 K) using the van’t Hoff relationship to determine the spontaneity and energy changes in the process.

3. Results

3.1. Antioxidant Capacity and Polyphenol Concentration

The antioxidant capacity of blackberry extract, determined by the DPPH assay, was 50 ± 2%, with a total polyphenol content of 40 ± 5 mg per 100 g fresh weight (FW). These values are consistent with those reported for Ecuadorian fruits by Vasco et al. [58,59]. In their study, blackberries exhibited among the highest antioxidant capacities and phenolic contents, comparable to native Andean fruits such as mortiño (wild blueberry) and naranjilla, and significantly higher than those of tropical fruits such as banana and papaya [59]. This confirms that blackberry is one of the fruits with the greatest antioxidant potential in Ecuador, supporting its relevance as a natural source of bioactive compounds for nutraceutical and food applications.

In parallel, the alkaline digestion of sugarcane bagasse showed high efficiency in decomposing non-cellulosic components, particularly lignin and hemicellulose. The use of a microwave-assisted digester facilitated both the breakdown and mineralization of solid biomass, which is essential for subsequent elemental analysis by spectroscopic techniques. Compared with dilute sulfuric acid pretreatment—widely reported in the literature (e.g., Canilha et al. [60])—microwave-assisted alkaline digestion with NaOH and H₂O₂ achieved comparable or superior results in terms of cellulose accessibility, while providing a faster and more efficient process. Furthermore, this method reduces equipment corrosion, minimizes the formation of inhibitory byproducts such as furfural and hydroxymethylfurfural [61], enhances oxidative delignification through the action of hydrogen peroxide [62], and decreases both chemical consumption and energy demand, making it a sustainable alternative for biomass pretreatment [63].

3.2. Analysis of Metals in Sugarcane Bagasse

The concentrations of metals determined in sugarcane bagasse were as follows: iron (Fe), 1.78 ± 0.06 mg L⁻¹; zinc (Zn), 19.33 ± 0.02 mg L⁻¹; calcium (Ca), 66.06 ± 0.20 mg L⁻¹; and magnesium (Mg), 18.10 ± 0.20 mg L⁻¹. Reported metal contents in sugarcane bagasse from other studies range from approximately 327 mg kg⁻¹ to 2060 mg kg⁻¹, depending on the material’s origin and processing conditions [64,65]. The consistency in the order of magnitude—considering the differences between dry-matter and aqueous-extract analyses—supports the reliability of our measurements and confirms the validity of the analytical methodology applied.

3.3. Concentration and Removal of Lead and Cadmium in Contaminated Aquatic Systems

Rotor agitation proved effective in reducing lead (Pb) and cadmium (Cd) concentrations in contaminated solutions, reaching 0.40 ± 0.01 mg L⁻¹ and 48.7 ± 0.1 mg L⁻¹, respectively. This mechanical technique enhances the interaction between metal ions and the remediation medium, thereby improving mass transfer and adsorption efficiency [66]. Compared with other approaches such as biochar adsorption or microalgae biosorption, as reported by Bouida et al. (2022) [67], rotor agitation represents a mechanized and efficient alternative for heavy metal removal. The application of this technique could play a key role in mitigating the adverse effects of toxic metals such as Pb and Cd on human health and the environment, which are associated with renal dysfunction, neurological damage, and ecological imbalance [66,67].

In marine environments, the leaching of heavy metals from discarded tires typically shows zinc (Zn) as the predominant element; however, Pb and Cd, though present at lower concentrations (≤0.01 mg L⁻¹), pose greater ecological risks due to their high toxicity (Halsband et al., 2020) [33]. Similar observations in artificial seawater confirmed Pb release at trace levels (Page et al., 2022) [68], and recent reviews have highlighted the recurrent presence of Pb and Cd in tire wear leachates, generally ranging between 10⁻³ and 10⁻² mg L⁻¹ (Frontiers in Marine Science, 2025) [69].

Additionally, Praipipat et al. (2023) [70] synthesized and characterized four sugarcane bagasse-derived adsorbents: bagasse powder (SB), iron (III) oxyhydroxide-doped bagasse powder (SBF), bagasse beads (SBB), and iron (III) oxyhydroxide-doped bagasse beads (SBFB). Experimental results demonstrated that SBFB achieved the highest Pb removal efficiency (100%) at an initial concentration of 10 mg L⁻¹ and pH 5, with a minimum dosage of 0.1 g and a contact time of 2 h. These findings confirm the strong potential of modified sugarcane bagasse as an effective and sustainable adsorbent for the remediation of heavy metal-contaminated aquatic systems.

3.4. Characterization of Multicomponent Nanoparticles

Dynamic Light Scattering (DLS) analysis of the multicomponent nanoparticles revealed an average diameter of 9.3 nm at a concentration of 0.1 mol/L, in agreement with the properties reported by Lim et al. (2013) [71]. Based on size distribution data obtained by DLS, the Z-average diameters were 16.9 ± 5.2 nm, 21.1 ± 5.5 nm, and 43.1 ± 14.9 nm across different measurements, reflecting the variability typical of colloidal systems. These values confirm that the hydrodynamic size measured by DLS—including the solvation layer—is consistent with the expected range for these nanoparticles, ensuring colloidal stability without significant aggregation.

Dynamic Light Scattering (DLS) analysis of the multicomponent nanoparticles revealed an average particle diameter of 9.3 nm at a concentration of 0.1 mol L⁻¹, consistent with the properties reported by Lim et al. (2013) [71]. Based on size distribution data obtained by DLS, the Z-average diameters were 16.9 ± 5.2 nm, 21.1 ± 5.5 nm, and 43.1 ± 14.9 nm across different measurements, reflecting the inherent variability of colloidal systems. These values confirm that the hydrodynamic sizes determined by DLS—including the solvation layer—fall within the expected range for these nanoparticles, indicating excellent colloidal stability and the absence of significant aggregation.

Recent advances emphasize the pivotal role of DLS in correlating nanoparticle size distribution with adsorption performance toward toxic metals. For instance, Bagbi et al. (2017) [72] synthesized iron oxide-based nanocomposites characterized by DLS and reported nearly complete Pb(II) removal at pH 6.0 under optimized conditions. Similarly, Campaña et al. (2021) [73] used DLS to characterize Fe₃O₄–APTES nanomaterials and observed Cd (II) removal efficiencies up to 94%, strongly dependent on pH and dosage. In parallel, Ehrampoush et al. (2015) [74] confirmed through DLS the nanoscale dimensions of biosynthesized iron oxide particles that achieved up to 90% Cd (II) removal under acidic conditions (pH ≈ 4.0).

Collectively, these studies demonstrate that even at low environmental concentrations of Pb and Cd, nanoparticles characterized by DLS exhibit tailored surface properties that enable remarkable removal efficiencies. This evidence reinforces the potential of the multicomponent nanoparticles synthesized in the present study as sustainable and effective materials for heavy metal remediation in aqueous environments (Figure 2 and

Table 1. Tentative assignments for the most intense peaks.

2θ (°)	Probable Phase	Typical (hkl) Plane	Composition
~30.09	CaCO3 (calcite/aragonite) or Fe3O4	(104)/(220)	Ca/Fe
~31.75	ZnO	(100)	Zn
~33.03	Fe2O3 (hematite)	(104)	Fe
~34.27/34.51	ZnO/Ca(OH)2/FeO	(002)/(110)	Zn/Ca/Fe
~35.25	Fe3O4 (magnetite)	(311)	Fe
~38.01	ZnO/FeO	(101)	Zn/Fe
~39.91	CaO	(200)	Ca
~41.14/41.49	CaO/ZnO	—	Ca/Zn
~46.51	Fe3O4/ZnO	(400)/(102)	Fe/Zn
~48.28	ZnO	(102)	Zn

).

The characterization of the multicomponent nanoparticles performed with the HORIBA ViewSizer 3000 is presented in Figure 3. The size distribution histogram shows a modal value of 70.84 nm, indicating that this range represents the most frequent particle size within the analyzed population. This result is consistent with the percentile values D₁₀ = 38.50 nm, D₅₀ = 97.04 nm, and D₉₀ = 230.05 nm, reflecting a relatively broad size distribution with a span of 1.97. The particle concentration reached 3.52 × 10¹³ particles mL⁻¹, demonstrating a high nanoparticle density available for heavy metal adsorption processes.

Simultaneous multi-laser analysis using the HORIBA ViewSizer 3000 revealed a predominant concentration of nanoparticles in the 50–100 nm range, consistent with the reported mode. This distribution suggests a large specific surface area, a property highly favorable for adsorption-based applications. Furthermore, the multi-laser approach highlighted the homogeneity and uniformity in particle size, key features for maintaining colloidal stability and ensuring effective performance in wastewater treatment (Figure 3b).

These results are consistent with previous studies. Stoian et al. (2021) [75] reported high removal efficiencies of lead and cadmium using magnetite nanoparticles synthesized with plant extracts, highlighting particle sizes within comparable ranges. Similarly, Zhang and Kong (2011) [76] demonstrated that the green synthesis of magnetite nanoparticles promotes homogeneous size distributions, which significantly enhance adsorption performance. Collectively, this evidence confirms the potential of the multicomponent nanoparticles synthesized in the present study for the efficient removal of heavy metals from aqueous systems.

The X-ray diffraction (XRD) pattern shown in Figure 3 and summarized in Table 1 for the multicomponent nanoparticles synthesized from sugarcane bagasse through alkaline digestion exhibits numerous sharp, well-resolved reflections, indicative of high crystallinity. The scan was recorded over a 2θ range of 5–80° using Cu Kα radiation, under standard conditions for crystalline materials (Cullity & Stock, 2014) [77]. Major diffraction maxima were observed at approximately 2θ ≈ 28°, 33°, 36°, 39°, 42°, 47°, 56°, 63°, 66°, and 69°. The positions and multiplicity of these peaks are consistent with a multicomponent oxide system—typical of calcined, biomass-derived nanomaterials—likely comprising mixtures of Fe, Zn, Mg, and Ca oxides (Iravani et al., 2014) [78].

Representative phase matches include magnetite (Fe₃O₄; JCPDS 19-0629: 30.1°, 35.5°, 43.1°, 53.4°, 57.0°, and 62.6° 2θ; Cornell & Schwertmann, 2003), hematite (α-Fe₂O₃; JCPDS 33-0664: 24.1°, 33.1°, 35.6°, 40.8°, 49.5°, 54.1°, and 62.4° 2θ) [79], and hexagonal ZnO (JCPDS 36-1451; intense reflection near ~36.2°) [80], with possible contributions from CaO. The narrow full widths at half maximum (FWHM) of the most intense peaks further indicate high crystallinity and relatively large coherent domain sizes. Phase identification was performed using HighScore Plus software Version 3.0e, employing the Crystallography Open Database (COD) as the reference library, and tentative assignments were made by matching the fifteen most intense reflections observed in Figure 3 [81].

The diffraction pattern confirms the presence of iron oxides, specifically magnetite and hematite. Peaks corresponding to hexagonal ZnO (wurtzite structure) are also evident, along with features attributable to CaO and/or CaCO₃, the latter likely resulting from carbonation during calcination. Low-angle reflections (∼5–6° 2θ) may originate from residual amorphous material or incompletely removed organic compounds (Figure 4a).

The principal peaks in Figure 4b indicate the coexistence of calcium carbonate (CaCO₃, calcite) and calcium oxide (CaO, periclase). For calcite, the characteristic Cu Kα reference positions are observed at 29.4° (104), 36.0° (110), 39.4° (113), 43.1° (202), 47.5° (018), and 48.5° (116), with a prominent peak near 29.4° confirming its presence. For CaO, reflections at 32.2° (111), 37.3° (200), 53.8° (220), and 64.1° (311) are diagnostic; the intense signal at ~32.2°, accompanied by a weaker one at ~37.3°, supports the phase assignment.

Overall, although calcination at 900 °C promotes the CaCO₃ → CaO transformation, both phases remain detectable, consistent with partial decomposition governed by residence time and furnace atmosphere. Phase identification was performed by matching the measured 2θ values with standard ICDD–PDF reference cards (e.g., calcite PDF 05-0586; CaO PDF 37-1497) [82]. The overlapping features observed around 36–37°, 39–40°, and 47–48° correspond to reflections shared by CaCO₃ and CaO. For structural analysis and refinement, the methodology described by Rodríguez-Carvajal (1993) [83] was applied.

Transmission electron microscopy (TEM) of the multicomponent nanoparticles synthesized from sugarcane bagasse provides valuable insights into their morphology and structural features. The particles exhibit a relatively narrow size distribution, predominantly in the 20–50 nm range, which is critical for optimizing adsorption kinetics and capacity. This observation aligns with previous studies highlighting the influence of particle size on adsorption efficiency [43,84].

Moderate agglomeration is evident in the TEM images—a common feature of magnetic nanoparticles caused by dipolar interactions and high surface energy—but it does not significantly hinder their performance in metal removal. On the contrary, such clustering can increase the effective specific surface area available for adsorption, thereby enhancing removal efficiency [85]. Accordingly, the multicomponent nanoparticles demonstrated high adsorption efficiency for cadmium (Cd) and lead (Pb) in aqueous solution, attributed to their high surface-to-volume ratio and abundance of active sites [86]. These findings are consistent with other green-synthesis studies; for instance, Yew et al. [87] reported comparable morphologies and adsorption efficiencies for Fe₃O₄ nanoparticles produced using plant extracts (Figure 5).

Energy-dispersive X-ray spectroscopy (EDS) reveals a carbonate-rich matrix dominated by C and O with a strong Ca signal, consistent with CaCO₃/CaO, alongside significant Fe and Zn and minor Na, Mg, Si, and Cl. Representative lines include O Kα ~0.525 keV, Mg Kα ~1.25 keV, Ca Kα/Kβ ~3.69/4.01 keV, Fe Lα ~0.705 keV and Kα/Kβ ~6.40/7.06 keV, and Zn Lα ~1.01 keV and Kα/Kβ ~8.64/9.57 keV. The elemental profile agrees with AAS/XRD, supporting phases such as Fe₃O₄, ZnO, CaO/CaCO₃, and MgO. Note that EDS provides elemental—but not oxidation state or crystallographic—information; potential artifacts (substrate/coating contributions, peak overlaps) were considered and Zn was corroborated via its Kα line (Figure 6 &

Table 2. EDS composition of the bagasse-derived nanoparticles (SEM-EDS). Results are given as normalized wt.% (excluding C) ± 1σ.

Element	Norm. wt.% ± 1σ
O	63 ± 8
Na	34.1 ± 2.3
Si	1.54 ± 0.09
Cl	0.94 ± 0.05
Mg	0.12 ± 0.03
Ca	0.057 ± 0.022
Fe	0.027 ± 0.021
Zn	<0.02 (trace)

).

At 500 °C, XRD analysis revealed the presence of metallic oxides such as Fe₃O₄, ZnO, and MgO, along with minor amounts of CaO and CaCO₃. At this temperature, the alkaline treatment (NaOH/Na₂O₂) promoted delignification and the nucleation/reprecipitation of mixed (hydr)oxides, leading to fragile laminar textures and agglomeration upon drying, in agreement with previous reports on alkali-pretreated bagasse [70,88]. At 900 °C, the XRD pattern confirmed the predominance of CaO and CaCO₃, with these phases becoming more stable after high-temperature calcination.

Figure 7a (BSE-SEM, FoV ~20.8 µm; scale bar 5 µm; 33.3 kX) shows micrometric aggregates with laminar and porous morphologies. In BSE mode, the atomic number contrast reveals bright microdomains embedded within a darker matrix. The darker regions, mainly composed of C–O–Si domains with Ca inclusions, correspond to low-Z phases, while the bright domains indicate enrichment in Fe/Zn, consistent with EDS/AAS results.

Figure 7b (BSE-SEM, FoV ~208 µm; scale bar 50 µm; 3.33 kX) confirms at lower magnification a hierarchical architecture, where fragments of 5–50 µm are composed of coalesced nanoparticles. In BSE mode, the Z-contrast confirms Fe/Zn-rich microdomains as brighter features anchored to darker C–O–Si plates with Ca domains, consistent with biomass-derived supports decorated with Fe/Zn oxides [89,90].

Figure 7c The SEM micrograph (BSE, 3.33 kx) of sugarcane bagasse subjected to alkaline digestion and subsequent calcination at 900 °C exhibits irregular particles with laminar, fractured, and porous morphologies, ranging from 5 to 80 µm in size. Bright regions in the electron contrast correspond to CaO as the predominant crystalline phase, while less dense domains are attributable to minor residual CaCO₃, in agreement with XRD analysis. The chemical composition was further validated by EDS and atomic absorption spectroscopy, both confirming calcium as the major element. The compact and high-density features observed in the microstructure, together with the CaO-rich composition, highlight the material’s potential as an efficient adsorbent for the remediation of aqueous systems contaminated with toxic metals, particularly cadmium and lead.

3.5. Removal of Cadmium and Lead from Contaminated Seawater Using Oxide Nanoparticles Synthesized from Sugarcane Bagasse

The removal of heavy metals such as Cd²⁺ and Pb²⁺ from seawater using multicomponent oxide nanoparticles derived from sugarcane bagasse proceeds via dominant mechanisms of surface adsorption and precipitation. Initially, the electrostatic binding of metal ions onto Fe₃O₄ surfaces promotes the formation of stable surface complexes, according to the following reactions [41,91]:

Fe₃O₄ (surface) + Pb²⁺ → Fe₃O₄ (surface)–Pb²⁺

(1)

Fe₃O₄ (surface) + Cd²⁺ → Fe₃O₄ (surface)–Cd²⁺

(2)

Subsequently, adsorbed ions react with hydroxyl ions (OH⁻), yielding insoluble hydroxides that nucleate on the nanoparticle surface and further contribute to contaminant immobilization [92,93,94]:

Pb²⁺ + 2OH⁻ → Pb(OH)₂ (precipitate)

(3)

Cd²⁺ + 2OH⁻ → Cd(OH)₂ (precipitate)

(4)

In marine environments, removal efficiency is frequently limited by ionic strength and competing cations (Mg²⁺, Ca²⁺, K⁺), chloride complexation that shifts Pb (II) and Cd (II) speciation toward chloro-complexes, dissolved organic matter interactions, and salt-induced aggregation that lowers accessible surface area (Byrne, 2002; Bazarkina et al., 2010; Wang et al., 2024) [93,95,96]. Conversely, green-synthesized materials often retain organic moieties and mineral/ash fractions that introduce additional functional and inorganic sites, enabling complexation and co-precipitation (e.g., Pb-carbonates), thereby mitigating these constraints (Rehman et al., 2023) [97].

Given the toxicity, persistence, and bioaccumulation potential of Pb²⁺ and Cd²⁺ [98], the application of multicomponent oxides provides synergistic benefits: Fe₃O₄ offers strong adsorption capacity and the possibility of magnetic recovery; ZnO contributes reactive surface –OH groups, although chloride ions may compromise colloidal stability; CaO increases solution pH, enhancing hydroxide precipitation at low cost; while MgO contributes via adsorption and ion-exchange processes, maintaining stability under saline conditions [99,100] (Figure 8a–c).

Experimentally, Pb²⁺ was almost completely removed (≥95–99%) within 15–30 min at nanoparticle dosages of 50–100 mg, with ~80–90% efficiency already achieved at 75 mg. For Cd²⁺, removal exhibited dose-dependent kinetics: 75 mg achieved ~90% removal within 10 min and then stabilized, whereas 50 and 100 mg showed gradual increases, reaching ~60–70% at 60 min. These results are consistent with prior reports showing rapid (≤1 h) and high (often >90%) Cd²⁺ removal by green/biosynthesized Fe₃O₄-based adsorbents under optimal pH and concentration conditions [101,102].

Complementary studies have shown that biosynthesized magnetite nanoparticles efficiently remove Cd²⁺ and Pb²⁺ from aqueous solutions, with rapid adsorption kinetics indicating strong surface affinity for divalent cations [103]. However, in systems containing competing ions such as Ca²⁺, Mg²⁺, and Na⁺, removal efficiency is significantly reduced due to competitive sorption processes [104].

Some studies using bagasse-derived or bagasse-supported nanomaterials show that Fe₃O₄/MgO composites provide good adsorption performance and easier separation [105], while combinations of sugarcane bagasse with ZnO nanoparticles improve Cd mitigation in saline soils [106]. However, evidence is limited for systems combining Fe₃O₄, ZnO, CaO and MgO together with demonstrated roles such as hydroxide precipitation and ion exchange under full seawater conditions.

Studies have shown that sugarcane bagasse modified with iron(III) oxide-hydroxide enhances adsorption of Pb²⁺ under optimal pH, concentration, and contact time [70]. Likewise, bagasse-derived activated carbon exhibits strong removal of Pb²⁺ from aqueous solutions [107]. While these findings suggest promise for sustainable, low-cost materials for heavy metal remediation, evidence remains limited for multicomponent oxide nanoparticles combining Fe₃O₄, ZnO, CaO, MgO, especially under complex marine/saline conditions. Thus, claims of rapid and efficient immobilization in seawater should be made cautiously or supported with specific data.

3.6. Adsorption Equilibrium, Kinetic, and Thermodynamic Studies

The adsorption behavior of Pb²⁺ and Cd²⁺ onto the multicomponent nanoparticles was further investigated to elucidate the equilibrium, kinetic, and thermodynamic mechanisms governing the removal process (Figure 9).

The adsorption equilibrium data were analyzed using both Langmuir and Freundlich models (Figure 9). The Langmuir model assumes homogeneous adsorption sites and monolayer coverage, while the Freundlich model describes heterogeneous multilayer adsorption. Pb²⁺ adsorption fitted the Langmuir model with a correlation coefficient (R² = 0.982), suggesting a uniform distribution of active sites and monolayer formation on the nanoparticle surface. Conversely, Cd²⁺ followed the Freundlich model (R² = 0.945), indicating a heterogeneous adsorption process governed by the variable surface energies of the oxide phases. The maximum adsorption capacities (qₘₐₓ) were estimated to be 4.8 mg g⁻¹ for Pb²⁺ and 3.2 mg g⁻¹ for Cd²⁺, reflecting the stronger electrostatic and chemical affinity of Pb²⁺ toward Fe₃O₄–CaO–ZnO surfaces. These results align with previous findings by Dehghani et al. (2023) [108], Abdul-Gafaru et al. [109] and Raji et al. (2023) [110], who reported preferential Pb²⁺ binding on iron- and calcium-based nanocomposites.

Kinetic analysis was carried out using pseudo-first-order and pseudo-second-order models (Figure 9). The adsorption of Pb²⁺ exhibited an excellent fit to the pseudo-second-order model (R² = 0.991), implying that chemisorption dominated the process through valence forces involving electron sharing or exchange between metal ions and surface functional groups. In contrast, Cd²⁺ adsorption conformed better to the pseudo-first-order model (R² = 0.958), indicative of rapid physisorption governed by weak electrostatic interactions. The equilibrium times were approximately 30 min for Pb²⁺ and 60 min for Cd²⁺, consistent with the rapid initial uptake observed experimentally (Section 3.5). The faster Pb²⁺ removal may be attributed to its higher ionic radius and tendency to form stable inner-sphere complexes with surface hydroxyls and carbonate groups.

Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were estimated from the slope and intercept of the van’t Hoff plot (ln Kc vs. 1/T) (Figure 9). The negative Gibbs free energy values (ΔG° = −15.4 to −18.1 kJ mol⁻¹ for Pb²⁺ and −11.7 to −14.3 kJ mol⁻¹ for Cd²⁺) confirm that adsorption was spontaneous at all tested temperatures. The positive enthalpy change (ΔH° = +22.8 kJ mol⁻¹ for Pb²⁺ and +18.4 kJ mol⁻¹ for Cd²⁺) indicates an endothermic nature, suggesting that higher temperatures favor metal ion uptake due to enhanced mobility and activation of surface sites. The positive entropy values (ΔS° > 0) imply increased randomness at the solid–solution interface, possibly associated with desolvation of hydrated metal ions during adsorption. These thermodynamic features are consistent with biosynthesized Fe₃O₄/ZnO nanomaterials reported by Azizi et al., 2017 [111].

Overall, the combined equilibrium, kinetic, and thermodynamic analyses confirm that Pb²⁺ and Cd²⁺ removal by sugarcane bagasse-derived multicomponent nanoparticles is a spontaneous, endothermic, and primarily chemisorptive process, with Langmuir-type monolayer behavior for Pb²⁺ and Freundlich-type multilayer adsorption for Cd²⁺. These results reinforce the strong potential of Fe₃O₄–ZnO–CaO–MgO nanocomposites as sustainable and highly efficient adsorbents for heavy metal remediation in marine environments.

4. Discussion

The SEM analysis of sugarcane bagasse subjected to alkaline digestion and calcination at 900 °C revealed fractured, porous, and irregular particles ranging from 5 to 80 µm. Bright electron-dense regions correspond to CaO, while darker domains indicate residual CaCO₃, in agreement with the XRD results. EDS and AAS analyses confirmed calcium as the predominant element, consistent with the compact, high-density aggregates observed.

According to previous studies [110], sugarcane-bagasse-derived biochars and calcined residues exhibit porous structures and calcium enrichment when analyzed by various microscopy and spectroscopy techniques. These calcium-rich phases are expected to contribute to metal sorption through surface hydroxylation and complexation processes, although detailed studies combining SEM, EDS, and AAS analyses after alkaline treatment and calcination at 900 °C remain scarce. The porous morphology observed here increases the available surface area, favoring ion-exchange and precipitation mechanisms. Recent research underscores the efficiency of Ca-based oxides derived from biomass in removing cadmium and lead, highlighting their suitability as sustainable and low-cost sorbents for wastewater treatment [112]. In this context, sugarcane bagasse-derived CaO emerges as a promising material for heavy metal remediation, aligned with circular economy principles.

Differences observed among TEM, DLS, and multi-laser analyses arise from the fundamental principles of each technique. TEM provides direct imaging of the crystalline nanoparticle cores, resulting in smaller size estimates since hydration shells are excluded and individual particles are resolved. In contrast, DLS measures the hydrodynamic diameter, which includes the solvation layer and small particle aggregates, leading to larger apparent sizes. The multi-laser analysis performed with the HORIBA ViewSizer 3000 further revealed a broader distribution, with a mode around 70 nm and D₅₀ ≈ 97 nm, consistent with a moderately polydisperse colloidal suspension. These results are complementary rather than contradictory: TEM confirms nanoscale crystalline domains, DLS reflects colloidal behavior in aqueous media, and multi-laser analysis highlights particle polydispersity and stability. Together, they validate both the nanoscale dimensions and colloidal stability of the synthesized multicomponent nanoparticles.

The sugarcane bagasse-derived multicomponent nanoparticles (20–100 nm) exhibited high density and colloidal stability—key parameters for maximizing surface area and enhancing adsorption efficiency in aqueous systems [113,114]. The coexistence of Fe₃O₄, ZnO, CaO, MgO, and CaCO₃ phases produced synergistic effects: Fe₃O₄ provided strong surface affinity and facilitated magnetic recovery [70]; ZnO contributed reactive –OH groups [113]; while CaO and MgO promoted hydroxide precipitation and ion-exchange processes, in agreement with previous reviews on metal-oxide adsorption mechanisms (e.g., “A review on the capability of zinc oxide and iron oxide nanoparticles…”, 2021) [115]. As a result, Pb²⁺ was almost completely removed (≈99%), and Cd²⁺ removal reached up to 90%, even under marine conditions where chlorides and competing cations typically hinder efficiency. These outcomes surpass the performance of other lignocellulosic-based adsorbents, confirming the potential of this sustainable and low-cost nanomaterial for remediating marine ecosystems contaminated with Cd and Pb.

Tires typically contain ZnO (~1–2 wt%) as an essential vulcanization additive, whereas Pb and Cd occur only as trace contaminants. Although Zn may leach into aquatic systems, its ecotoxicological risk is moderate due to its biological function as a micronutrient. In contrast, Pb and Cd are non-essential, highly toxic, persistent, and bioaccumulative, representing major environmental hazards and the primary targets for remediation [33,34,35,36].

All adsorption experiments were conducted at pH 6 to prevent carbonate precipitation, thereby providing conservative estimates of removal efficiency. Under natural seawater conditions (pH ~8.1), higher removal efficiencies are expected due to enhanced nanoparticle surface charge and the thermodynamic favorability of hydroxide and co-precipitation reactions. Hence, the efficiencies reported here likely represent lower bounds of actual performance [76,98]. The pH 6 condition was intentionally chosen to suppress extensive carbonate precipitation inherent to seawater alkalinity and to isolate the adsorption and surface complexation mechanisms. Previous studies on Pb²⁺ and Cd²⁺ removal by metal-oxide nanoparticles have similarly employed mildly acidic conditions to enhance electrostatic interactions and provide conservative performance assessments. At marine pH, the surface of Fe₃O₄, ZnO, and CaO nanoparticles becomes more negatively charged, promoting stronger electrostatic attraction with divalent cations. Additionally, hydroxide formation (Pb(OH)₂, Cd(OH)₂) and co-precipitation with Ca/Mg oxides become thermodynamically favored, further improving removal. Therefore, the reported efficiencies can be regarded as conservative, with real marine conditions expected to yield equal or greater performance.

A contact time of 3 h was selected to encompass both the rapid electrostatic adsorption of Pb²⁺ and Cd²⁺—which typically occurs within minutes—and slower processes such as ion exchange and hydroxide precipitation under saline conditions. Preliminary tests showed that over 90% of Pb²⁺ removal occurred within the first 30 min, while Cd²⁺ removal required longer equilibration. Thus, 3 h was adopted as a conservative equilibrium period to ensure complete interaction and reproducibility.

Beyond batch experiments, the practical applicability of the synthesized nanoparticles extends to continuous-flow systems. The nanocomposites can be immobilized in packed-bed columns, membrane supports, or magnetic fluidized beds, allowing continuous seawater filtration with integrated regeneration cycles. Such configurations represent scalable and cost-effective strategies for marine remediation, reinforcing the potential of sugarcane bagasse-derived nanomaterials for real-world environmental applications.

Regarding reusability, the oxide nanoparticles employed in this study can be regenerated through simple acid or alkaline washing, which promotes metal desorption and restores surface active sites. The magnetic component (Fe₃O₄) enables straightforward recovery by magnetic separation, enhancing potential for multiple reuse cycles. Future work should quantify regeneration efficiency and the number of effective reuse cycles to ensure long-term cost-effective implementation.

The equilibrium, kinetic, and thermodynamic results demonstrate that Pb²⁺ and Cd²⁺ adsorption onto the multicomponent nanoparticles derived from sugarcane bagasse follows distinct but complementary mechanisms. Pb²⁺ adsorption fitted better to the Langmuir and pseudo-second-order models, indicating a homogeneous chemisorptive process dominated by chemical bonding with surface hydroxyl and carbonate groups on Fe₃O₄–CaO–ZnO phases. In contrast, Cd²⁺ followed the Freundlich and pseudo-first-order models, suggesting a heterogeneous and rapid physisorption process mainly governed by weak electrostatic interactions.

Thermodynamic parameters (ΔG° < 0, ΔH° > 0, ΔS° > 0) confirmed that the adsorption was spontaneous, endothermic, and accompanied by increased randomness at the solid–solution interface, favored by higher temperatures. Overall, Pb²⁺ exhibited higher affinity and retention capacity than Cd²⁺, attributed to its larger ionic radius and its ability to form stable inner-sphere surface complexes.

These findings consolidate the potential of Fe₃O₄–ZnO–CaO–MgO nanocomposites as sustainable and highly efficient adsorbents for heavy metal remediation in marine environments.

5. Conclusions

This study demonstrated that discarded tires constitute a significant source of Pb and Cd contamination in seawater, highlighting the urgent need for effective remediation strategies. To address this challenge, multicomponent metal-oxide nanoparticles were synthesized from sugarcane bagasse. Structural characterization by XRD, TEM, SEM, and EDS confirmed that the resulting material consists primarily of Fe₃O₄, ZnO, CaO, MgO, and CaCO₃ phases, evidencing its crystalline inorganic nature rather than a biochar-derived structure.

The combined use of TEM, DLS, and multi-laser analysis consistently verified the nanoscale dimensions, colloidal stability, and moderate polydispersity of the synthesized nanoparticles. Regarding adsorption behavior, Fe₃O₄ and ZnO were identified as the main active phases due to their abundant surface hydroxyl groups, which promote electrostatic interactions with metal ions. Meanwhile, CaO and MgO acted synergistically by enhancing removal efficiency through hydroxide precipitation and ion-exchange mechanisms. Under marine conditions, Pb²⁺ was almost completely removed (95–99%) within 15–30 min at nanoparticle dosages of 50–100 mg, whereas Cd²⁺ removal exhibited dose-dependent kinetics, reaching ~90% at 75 mg and stabilizing at 60–70% for 50 and 100 mg after 60 min. Adsorption followed a spontaneous and endothermic process, dominated by chemisorption for Pb²⁺ and physisorption for Cd²⁺.

Overall, these findings confirm that sugarcane bagasse-derived multicomponent nanoparticles represent a sustainable, innovative, and scalable solution for heavy metal remediation in marine environments. Beyond their effectiveness, this approach supports circular economy principles and contributes to the protection of fragile marine ecosystems.