Synthesis and evaluation of novel nanocomposites for removal of zinc ions from aqueous solutions

Abstract

Zn(II) contamination in aquatic systems poses significant risks to environmental safety and public health. In this study, BaCO₃/BaAl₂O₄/C nanocomposites, denoted BA600 and BA800, were synthesized by a facile Pechini sol-gel route at 600 and 800 °C and evaluated for Zn(II) removal from aqueous solutions. Besides, XRD data evidenced the presence of crystalline BaCO₃ and BaAl₂O₄ phases, and the average crystal size increased from 54 nm for BA600 to 66 nm for BA800. Morphological analyses showed that BA600 had a denser aggregated structure, whereas BA800 exhibited coarser and more uniform particles. Under the optimum conditions of pH 6, adsorbent dose 0.1 g, and 298 K, equilibrium was reached within 60 min for BA600 and 80 min for BA800. The maximum adsorption capacities were 124.38 mg/g for BA600 and 90.58 mg/g for BA800. Zn(II) adsorption followed the pseudo-second-order kinetic model and the Langmuir isotherm and was found to be spontaneous, exothermic, and predominantly chemical in nature. After five adsorption/desorption cycles, the removal efficiencies decreased from 79.62 to 69.76% for BA600 and from 56.31 to 44.57% for BA800, corresponding to retention of 87.62% and 79.15% of their initial performance, respectively. In real wastewater, the adsorption capacities reached 110.40 mg/g for BA600 and 77.60 mg/g for BA800. These findings demonstrate that BA600, in particular, is an efficient adsorbent for Zn(II) removal from aqueous media.

Introduction

Aquatic contamination caused by heavy metals is considered a serious environmental problem due to industrial, agricultural, and urban activities and the release of untreated effluents^1–4. Heavy metals pose serious threats to environmental and human health by disrupting aquatic ecosystems and causing toxic effects on vital organs after prolonged exposure^5–7. Zn(II) contamination in aquatic systems mainly originates from anthropogenic sources such as mining, galvanization and electroplating, battery manufacturing, pigment and textile industries, fertilizer application, and untreated industrial and municipal effluents. Excessive exposure to Zn(II) may harm aquatic life and cause adverse health effects in humans, including gastrointestinal disorders and disruption of elemental balance. Therefore, the efficient elimination of Zn(II) ions from contaminated water is of considerable environmental and public health importance^8–10. A variety of treatment approaches have been adopted for the elimination of heavy metals from aqueous media, comprising adsorption¹¹, membrane filtration¹², coagulation/flocculation¹³, electrodialysis¹⁴, ion exchange¹⁵, chemical precipitation¹⁶, electrochemical treatment^17,18, solvent extraction¹⁹, and bioremediation²⁰. Although membrane-based technologies, coagulation/flocculation, chemical precipitation, electrodialysis, and bioremediation have been used for metal removal, each method has important limitations. These include membrane fouling, sludge generation, high cost and energy demand, low efficiency at low metal concentrations, and sensitivity to operating conditions. Other methods such as flotation²¹, photocatalytic treatment²², and advanced oxidation²³ have also been investigated, although many remain limited by process complexity, high cost, or the generation of secondary pollution. Recent reports have emphasized that adsorption remains one of the most reliable and versatile approaches for the elimination of heavy metals from polluted water because of its high efficiency, economic feasibility, operational simplicity, and regeneration potential. In addition, the fabrication of composite adsorbents has emerged as an effective strategy to enhance adsorption behavior by improving the availability of active sites, physicochemical stability, and resistance during practical applications. For example, Asadullah et al. demonstrated that a biochar/bentonite composite monolith could effectively remove Cr(III), Ni(II), and Zn(II) ions from aqueous solutions in fixed-bed operation, confirming the importance of composite design and adsorbent durability in adsorption-based water treatment²⁴. For these reasons, adsorption has become one of the most promising strategies for the purification of metal-contaminated water^25,26. Metal oxide nanoparticles and their composites have attracted considerable interest in adsorption applications because of their high surface area, abundant active sites, and tunable physicochemical properties. Their performance may be further enhanced through combination with organic components or with other metal oxides^27,28. The incorporation of more than one metal oxide may generate synergistic effects that enhance adsorption capacity, selectivity, and structural stability. Recently developed nanocomposite adsorbents such as CBCS@LDH, SiO₂/GO/Al₂O₃, and graphene oxide/halloysite nanotube@polyaniline have demonstrated promising adsorption behavior toward aqueous pharmaceutical pollutants, which further emphasizes the importance of rational composite design in enhancing pollutant uptake and regeneration characteristics^29–31. The Pechini sol-gel approach is particularly advantageous for preparing multicomponent oxide systems because it enables homogeneous incorporation of metal ions and provides excellent compositional control, purity, and microstructure^32–36. These characteristics render the Pechini sol-gel technique specifically desirable for the fabrication of efficient nanocomposite adsorbents^37–39. A number of materials have been utilized for the sequestration of Zn(II) ions. Kaolinite recorded an adsorption capacity of 15.52 mg/g, which reflects its low cost and natural abundance but also indicates relatively limited adsorption performance⁴⁰. Amino-functionalized magnetic nanoparticles showed a capacity of 24.21 mg/g and offered the advantage of magnetic separation, although their adsorption efficiency remained moderate, and their preparation required additional surface modification⁴¹. Graphene/Fe₃O₄ composite achieved 121.5 mg/g⁴², while Fe₃O₄ nanoparticles reached 107.27 mg/g⁴³, which demonstrates the potential of magnetic nanomaterials for Zn(II) adsorption. Despite the progress achieved in the construction of adsorptive systems for Zn(II) elimination, many previously reported substances still exhibit one or more shortcomings, such as low adsorption capacity, insufficient structural stability, particle aggregation, difficult regeneration, or complex synthesis procedures. These limitations indicate that there remains a clear need for novel adsorbents that can be prepared by a comparatively straightforward and controllable method while offering high efficiency and favorable physicochemical characteristics for Zn(II) uptake. Hence, the present study was designed with the following objectives: (1) to synthesize, for the first time, BaCO₃/BaAl₂O₄/C nanocomposites by the Pechini sol-gel process at 600 and 800 °C, denoted BA600 and BA800, respectively; (2) to characterize the prepared nanocomposites in terms of their structural and morphological features; (3) to evaluate their adsorption performance toward Zn(II) ions in aqueous media; (4) to elucidate the adsorption behavior using kinetic, isotherm, and thermodynamic models; and (5) to investigate their regeneration ability, reusability, resistance to interfering species, and applicability for Zn(II) removal from real wastewater. The Pechini sol-gel route was selected because it is a relatively low-cost and controllable method for preparing multicomponent materials. It enables homogeneous mixing and good compositional control without the need for high-pressure conditions, complex templates, or highly specialized equipment. Zn(II) ions were chosen as the target pollutant because they frequently occur in aqueous effluents and become harmful at elevated concentrations. They also provide a suitable model for evaluating the adsorption efficiency of the fabricated nanocomposites. This work should therefore be viewed as a focused environmental case study. It does not imply that other toxic heavy metals are less important. The applicability of the prepared nanocomposites toward other hazardous metal ions may be investigated in future studies. Consequently, the main innovation of the present study is the first-time development of BaCO₃/BaAl₂O₄/C as a multicomponent adsorbent for removal of Zn(II) ions. It also includes a systematic comparison of the corresponding products obtained at two calcination temperatures in order to clarify the influence of thermal treatment on phase composition, crystal growth, morphology, and adsorption efficiency. From a compositional perspective, BaCO₃ is expected to provide carbonate-associated and oxygen-containing active sites capable of interacting with Zn(II) ions. Besides, BaAl₂O₄ offers a stable oxide framework with accessible Ba–O and Al–O surface centers that can participate in binding of Zn(II) ions. In addition, the carbon phase improves the textural characteristics of the composite by increasing surface exposure, reducing agglomeration, and enhancing the accessibility of adsorption sites, while also contributing residual oxygen-containing functionalities. The integration of these components within a single nanocomposite is therefore expected to generate a synergistic adsorption system with enhanced efficiency toward removal of Zn(II) ions.

Materials and methods

Materials

Tartaric acid (C₄H₆O₆), barium nitrate (Ba(NO₃)₂), sodium hydroxide (NaOH), polyethylene glycol 400 (H(OCH₂CH₂)_nOH), hydrochloric acid (HCl), aluminum nitrate nonahydrate (Al(NO₃)₃.9H₂O), potassium chloride (KCl), and zinc nitrate hexahydrate (Zn(NO₃)₂.6H₂O) were employed in this study. All chemicals were purchased from Sigma-Aldrich, were of analytical grade, and were used as received without further purification.

Synthesis of BaCO₃/BaAl₂O₄/C nanocomposites

The BaCO₃/BaAl₂O₄/C nanocomposites were synthesized by the Pechini sol-gel approach, as depicted in Fig. 1. Primarily, 21 g of tartaric acid (0.140 mol), as a chelating agent, was carefully solubilized in 50 mL of distilled water. In a second beaker, 12 g of Al(NO₃)₃.9H₂O (0.0320 mol) and 12 g of Ba(NO₃)₂ (0.0459 mol), as metal precursors, were carefully solubilized in 100 mL of distilled water. After that, the chelating agent was then added to the metal precursors, and the resulting blend was agitated continuously for 30 min to ensure thorough complexation. Thereafter, 10 mL of polyethylene glycol 400 (0.0280 mol) was gradually added to the reaction system for crosslinking. No external pH adjustment was applied during the Pechini sol-gel synthesis, and the reaction mixture was used at its naturally developed acidic pH, which was approximately 2.5 owing to the presence of tartaric acid and metal nitrate precursors. The molar ratio between tartaric acid: Al(NO₃)₃.9H₂O: Ba(NO₃)₂: polyethylene glycol 400 was approximately 4.38:1.00:1.43:0.88. This ratio was selected to provide sufficient complexation of the dissolved metal ions by tartaric acid, maintain the required Ba and Al precursor balance for formation of the targeted BaCO₃/BaAl₂O₄/C system, and ensure adequate polymeric crosslinking by polyethylene glycol 400 during gel formation in the Pechini sol-gel process. The obtained sol was heated under continuous stirring at 200 °C until entire evaporation of the solvents took place and a dry precursor was formed. A homogeneous sol was first formed after mixing the precursor solutions, and this sol gradually evolved into a viscous gel during heating and stirring as solvent evaporation and polymeric network formation progressed. Continued heating at 200 °C then yielded the dry precursor before calcination. The use of 200 °C was intended to ensure complete solvent removal and to favor polyesterification/polymeric network development between the organic constituents and the metal-containing species, thereby yielding a stable dry precursor before calcination. The resulting powder was subsequently calcined at 600 and 800 °C for 3 h in order to obtain the BaCO₃/BaAl₂O₄/C nanocomposites denoted as BA600 and BA800, respectively. The calcination step was carried out in air under conventional muffle-furnace conditions.

Fig. 1.

Synthesis of BaCO₃/BaAl₂O₄/C nanocomposites.

Instrumentation

The crystalline phases of all produced samples were identified utilizing an X-ray diffraction diffractometer (D8 Discover, Bruker, USA). Also, the surface morphology as well as elemental composition of all synthesized samples were investigated through field emission scanning electron microscopy (FE-SEM) integrated with energy dispersive X-ray spectroscopy (EDX) using a Quanta 250 FEG instrument (Thermo Fisher Scientific, USA). Besides, further insight into the internal structure and particle morphology of all synthesized samples was obtained utilizing a high-resolution transmission electron microscope (HR-TEM) on a JEM-2100Plus instrument (JEOL Ltd., Japan). Besides, the concentration of Zn(II) ions was determined by inductively coupled plasma analysis using an inductively coupled plasma instrument (6500 Duo Thermo Scientific, USA).

Removal of Zn(II) ions from aqueous solutions using fabricated nanocomposites

The removal of Zn(II) ions from aqueous solutions by BA600 and BA800 nanocomposites was investigated under the experimental conditions summarized in Table 1. For the pH study, 100 mg of adsorptive material was added to 150 mL of Zn(II) solution with 100 mg/L as the initial concentration at 298 K, and the blend was agitated for 300 min while the pH was varied from 2 to 6. The pH of the Zn(II) solutions was adjusted to the desired value by the dropwise addition of dilute HCl or NaOH solutions before the adsorption experiments. For the contact time study, 100 mg of adsorptive material was mixed with 150 mL of Zn(II) solution with 100 mg/L as the initial concentration at 298 K and pH 6, and the suspension was stirred for periods ranging from 10 to 100 min. To evaluate the effect of solution temperature, 100 mg of adsorptive material was brought into contact with 150 mL of Zn(II) solution with 100 mg/L as the initial concentration at pH 6 while the temperature was varied from 298 to 328 K and the contact time was fixed at 60 min for BA600 and 80 min for BA800. The effect of adsorbent dosage was examined by varying the mass of the adsorbent from 0.025 to 0.4 g while maintaining the solution volume at 150 mL, the initial Zn(II) concentration at 100 mg/L, and the temperature at 298 K. The influence of the initial Zn(II) concentration was studied by varying the concentration from 25 to 200 mg/L in 150 mL of solution using 100 mg of adsorptive material at 298 K and pH 6 with stirring for 60 min in the case of BA600 and 80 min in the case of BA800. All batch adsorption experiments were performed under continuous agitation using a laboratory magnetic stirrer operated at 500 rpm.

Table 1.

Investigated conditions for the evaluation of Zn(II) adsorption performance.

Impact	Investigated conditions
	V (mL)	Co (mg/L)	T (K)	W (g)	t (min)	pH
pH	150	100	298	0.1	300	2–6
Contact time	150	100	298	0.1	10–100	6
Solution temperature	150	100	298–328	0.1	60 (BA600) 80 (BA800)	6
Adsorbent dosage	150	100	298	0.025–0.4
Concentration of Zn(II) ions	150	25–200	298	0.1	60 (BA600) 80 (BA800)	6

After the adsorption experiments, the adsorbent was separated by centrifugation, and the concentration of Zn(II) ions remaining in the filtrate was determined by inductively coupled plasma analysis. The removal percentage of Zn(II) ions (% R) and the adsorption capacity (Q, mg/g) of the adsorbent were calculated using Eqs. 1 and 2, respectively^44–46.

1

2

In these equations, C_o and C_e denote the initial and equilibrium concentrations of Zn(II) ions, respectively, while V represents the solution volume and W denotes the mass of the adsorbent.

To ensure the reliability and reproducibility of the obtained results, all adsorption experiments were performed three times under identical conditions. The reported values are expressed as mean ± standard deviation (SD), and the error bars in the corresponding figures denote the standard deviation of triplicate measurements.

The regeneration of the Zn-laden adsorbent was examined by treating the Zn-loaded material with 100 mL of HCl solution at three different concentrations (1 M, 1.5 M, and 2 M) for 60 min. The desorption process was conducted in order to evaluate the efficiency of hydrochloric acid as an eluting agent for the release of retained Zn(II) ions from the surface of the adsorbent. The extent of Zn(II) ion desorption was expressed as %D and calculated employing Eq. 3⁴⁷.

3

In this equation, C_d represents the concentration of desorbed Zn(II) ions in the eluting solution, where V_d represents the volume of the eluent.

The reuse potential of BA600 and BA800 was assessed over five successive adsorption/desorption cycles under identical operating conditions. In each cycle, 0.1 g of adsorbent was contacted with 150 mL of Zn(II) solution at an initial concentration of 100 mg/L at 298 K and pH 6. A contact time of 60 min was used for BA600, whereas 80 min was applied for BA800. After completion of each adsorption run, the exhausted nanocomposite was regenerated employing 100 mL of 2 M HCl. After each desorption step, the regenerated adsorbent was thoroughly washed several times with distilled water to remove any residual acid and desorbed species. The washed adsorbent was then dried before being reused in the subsequent adsorption cycle.

A binary adsorption study was carried out in order to assess the influence of accompanying ions on the disposal of Zn(II) ions by the BA600 and BA800 nanocomposites. Also, the experiments were conducted at 298 K and pH 6, employing a preliminary Zn(II) concentration of 100 mg/L, whilst preserving a 1:1 ratio between Zn(II) ions and every accompanying species. The accompanying ions comprised K⁺, Na⁺, Mg²⁺, Ca²⁺, Cu²⁺, Ni²⁺, Cl⁻, NO₃⁻, SO₄²⁻, and methylene blue dye, which were selected to represent common cationic and anionic species encountered in aqueous media. The batch adsorption procedure was implemented using 0.1 g of each nanocomposite with 150 mL of Zn(II) solution incorporating the pertinent competing ion. The suspensions were stirred for 60 min in the case of BA600 and for 80 min in the case of BA800, until equilibrium was attained. The remaining Zn(II) concentration was subsequently determined by inductively coupled plasma analysis. Methylene blue dye was introduced as a representative organic co-contaminant to simulate the presence of dissolved organic pollutants in the wastewater matrix and to evaluate the Zn(II) removal performance under more realistic competitive conditions.

Results and discussion

Characterization results

XRD

Figure 2 demonstrates that the XRD patterns of the BA600 (Fig. 2A) and BA800 (Fig. 2B) samples consist of well-defined crystalline phases belonging to BaCO₃ and BaAl₂O₄. The reflections attributed to BaCO₃ are located at 2θ values of 23.99^o, 27.87^o, 29.66^o, 32.56^o, 33.93^o, 39.60^o, 42.07^o, 43.05^o, 44.29^o, 44.99^o, 46.90^o, 48.98^o, 49.80^o, 52.84^o, 55.88^o, 59.18^o, 59.88^o, 61.12^o, 68.16^o, 69.67^o, 71.05^o, 72.99^o, 76.44^o, and 78.90^o and correspond to the Miller indices of (111), (002), (012), (102), (112), (220), (221), (041), (202), (132), (113), (222), (042), (310), (241), (223), (043), (330), (332), (062), (400), (260), (234), and (171), respectively. These diffraction peaks agree well with JCPDS card No. 00-005-0378 and confirm the presence of orthorhombic barium carbonate. In the same figure the peaks assigned to BaAl₂O₄ appear at 2θ values of 19.58^o, 22.19^o, 28.42^o, 34.50^o, 36.01^o, 40.15^o, 41.11^o, 45.95^o, 53.80^o, 54.64^o, 56.43^o, 57.94^o, 67.20^o, 74.24^o, and 76.99^o and are indexed to the planes of (100), (101), (102), (110), (111), (112), (004), (104), (210), (114), (105), (212), (106), (116), and (311), respectively. These reflections are consistent with JCPDS card No. 01-072-1331 and verify the formation of hexagonal barium aluminate. The average crystal size grew from 54 nm for BA600 to 66 nm for BA800. This increase is most reasonably attributed to the higher treatment temperature of BA800, which enhances atomic mobility and promotes crystallite growth, thereby yielding a coarser and more highly developed crystalline structure. No distinct carbon-related diffraction peaks were observed in the XRD patterns, which indicates that any non-carbonate carbon present in the samples was not in a well-crystallized form. This is consistent with the presence of only a minor amount of poorly ordered or amorphous carbonaceous residue, which would not be expected to generate sharp XRD reflections.

Fig. 2.

XRD analysis of (A) BA600 and (B) BA800 products.

EDX and Raman

Figure 3; Table 2 collectively confirm the elemental composition of the BA600 (Fig. 3A) and BA800 (Fig. 3B) nanocomposites. The EDS spectra indicate that the two samples are composed predominantly of oxygen and barium together with aluminum, while carbon is also clearly detected, which is consistent with the successful production of the targeted barium-based nanocomposite system. Noticeable variation in percentages may also reflect temperature-induced changes in crystallization behavior, particle growth, and the distribution of phases across the analyzed surface.

Fig. 3.

EDS analysis of (A) BA600 and (B) BA800 products.

Table 2.

Elemental composition of BA600 and BA800 samples.

Nanocomposite	Weight percentages (wt%)
	% C	% O	% Al	% Ba
BA600	4.9	20.1	7.1	67.9
BA800	4.4	14.6	4.6	76.4

The carbon signal observed in the EDS analysis arises mainly from the carbonate-containing phase, particularly BaCO₃, which is an intrinsic component of the synthesized nanocomposite. In addition, a fraction of carbon may also originate from residual carbonaceous species left after partial decomposition of the organic precursors, namely tartaric acid and polyethylene glycol 400, during the Pechini sol-gel process. Because calcination was carried out in air, most organic species are expected to decompose extensively; therefore, the detected carbon is attributed mainly to BaCO₃, while any non-carbonate carbon contribution is considered minor and poorly ordered rather than graphitic. A relevant previous study on BaAl₂O₄ synthesis also showed that phase evolution in Ba-containing aluminates is strongly dependent on precursor composition and calcination temperature, and that BaCO₃ may still appear under certain preparation conditions⁴⁸. This supports the view that the final composition of the present system is highly sensitive to the thermal treatment conditions. The compositional differences between BA600 and BA800 can be attributed principally to the effect of calcination temperature on phase evolution and surface chemistry. The sample treated at the higher temperature is expected to undergo more extensive decomposition of organic remnants together with a greater degree of structural reorganization and oxidation, which would reduce the relative carbon contribution and modify the proportions of the remaining elements. In order to support the EDX results, the carbon content of the synthesized nanocomposites was additionally measured using a CHN analyzer. The analysis showed carbon contents of 4.7 wt% and 4.1 wt% for BA600 and BA800, respectively, which further confirms the presence of carbon in both samples. Raman analysis was carried out for the BA600 sample as an illustrative example to identify the type of carbon present and the other vibrational groups in the synthesized nanocomposite. As shown in Fig. 4, the Raman spectrum displays bands at 1420, 1061, 694, 626, 543, 219, 137, and 75 cm⁻¹. The prominent bands at 1420 and 1061 cm⁻¹ are characteristic of the carbonate group and confirm the presence of BaCO₃ in the BA600 structure. In addition, the bands observed at 694, 626, and 543 cm⁻¹ can be assigned to metal–oxygen stretching vibrations and lattice modes related to the barium aluminate phase, while the low-frequency bands centered at 219, 137, and 75 cm⁻¹ are attributed to external lattice vibrations of the Ba–O and Al–O framework. Importantly, Fig. 4 does not show the characteristic Raman features of graphitic carbon, particularly the D band near 1350 cm⁻¹, the G band near 1580 cm⁻¹, and the 2D band around 2700 cm⁻¹. Accordingly, the carbon detected in BA600 is more reasonably assigned to residual carbon originating from the incomplete combustion or carbonization of the organic starting materials employed during the Pechini sol-gel procedure rather than to graphitic carbon. This interpretation is further supported by the weak carbon contribution and the dominance of carbonate and inorganic lattice vibrations in the Raman spectrum, indicating that the carbonaceous species are limited in amount and poorly ordered instead of being graphitized.

Fig. 4.

Raman spectrum of the BA600 sample.

FE-SEM

Figure 5 depicts the FE-SEM micrographs of the BA600 (Fig. 5A) and BA800 (Fig. 5B) nanocomposites and demonstrates that both materials possess aggregated particulate architectures rather than isolated individual grains. In Fig. 5A, the BA600 sample is characterized by a densely compact surface composed predominantly of cauliflower-like agglomerates together with quasi-spherical particles and irregular clustered domains. This morphology suggests the presence of relatively fine particles that are strongly packed and partially fused, giving rise to a rough and compact texture. By contrast, Fig. 5B shows that the BA800 sample exhibits a more evolved morphology marked by the appearance of well-developed faceted particles together with polyhedral grains, block-like domains, and aggregated fine crystallites. Several particles display a polygonal or near-hexagonal outline, which indicates a greater degree of structural ordering and crystal development. The distinction between the two morphologies is primarily attributable to the influence of calcination temperature on nucleation growth and sintering behavior. At the lower treatment temperature, the particles remain finer and more heavily agglomerated, whereas the higher temperature enhances atomic mobility and promotes grain growth, crystallite coarsening, and the formation of more clearly defined crystalline faces. As a consequence, BA800 exhibits a coarser and more morphologically distinct surface than BA600, which reflects the stronger thermal driving force for particle growth and structural reorganization.

Fig. 5.

FE-SEM analysis of (A) BA600 and (B) BA800 products.

HR-TEM

Figure 6 shows the HR-TEM images of BA600 (Fig. 6A) and BA800 (Fig. 6B) and demonstrates a distinct change in morphology after increasing the calcination temperature. The BA600 nanocomposite in Fig. 6A exhibits compact irregular aggregates together with obvious rod-like features surrounded by fine quasi-spherical and irregular nanoparticles. This image reflects a less uniform structure with strong particle adhesion that led to the formation of dense clustered domains. In contrast, the BA800 nanocomposite in Fig. 6B is mainly composed of near-spherical and irregular polygonal nanoparticles assembled into clustered and chain-like agglomerates with clearer particle boundaries and better structural uniformity, while the rod-like features observed in BA600 are no longer evident. This difference can be ascribed to the elevated calcination temperature, which enhanced atomic diffusion recrystallization and crystal growth and therefore promoted the transformation of the heterogeneous compact structure into a more stable and homogeneous granular morphology. These HR-TEM observations confirm that BA600 possesses an irregular aggregated structure containing rod-like units, whereas BA800 is characterized by a more uniform arrangement of agglomerated near-spherical nanoparticles.

Fig. 6.

HR-TEM analysis of (A) BA600 and (B) BA800 products.

Surface textures

The surface textural properties of the synthesized BA600 and BA800 nanocomposites were further elucidated from the nitrogen adsorption–desorption analysis presented in Fig. 7 and the related parameters are presented in Table 3. As shown in Fig. 7A, both samples exhibit type IV adsorption–desorption isotherms, which are characteristic of mesoporous materials, while Fig. 7B displays broad BJH pore size distribution curves, confirming the existence of pores within the mesopore region. In agreement with these observations, Table 3 shows that BA600 possesses a higher BET surface area (13.93 m²/g) and a larger total pore volume (0.1180 cm³/g) than BA800 (7.22 m²/g and 0.0959 cm³/g, respectively), whereas BA800 exhibits a greater average pore diameter (53.19 nm) compared with BA600 (33.90 nm). These data demonstrate that increasing the calcination temperature led to pore enlargement accompanied by a reduction in surface area and pore volume, most likely as a result of particle growth and partial pore coalescence or collapse. Accordingly, the more favorable textural characteristics of BA600, particularly its higher surface area and pore volume, can provide a greater number of accessible active sites and thus contribute to its greater adsorption performance toward Zn(II) ions. In addition, the slightly higher carbon content retained in BA600 may contribute to the presence of more residual oxygen-containing surface functionalities, thereby further enhancing its interaction with Zn(II) ions.

Fig. 7.

N₂ adsorption–desorption isotherms (A) and BJH pore size distribution curves (B) of BA600 and BA800 nanocomposites.

Table 3.

Surface features of BA600 and BA800 nanocomposites.

Nanocomposite	BET surface area (m2/g)	Total pore volume (cm3/g)	Average pore diameter (nm)
BA600	13.93	0.1180	33.90
BA800	7.22	0.0959	53.19

Optimization of Zn(II) adsorption

pH

Figure 8 exhibits that the elimination of Zn(II) ions by BA600 or BA800 increased substantially as the solution pH rose from 2 to 6. At pH 2, both adsorbents exhibited very limited removal efficiency as the high concentration of H⁺ ions strongly competed with Zn(II) ions for the available active sites and thereby suppressed zinc uptake. Under these conditions, BA600 achieved a removal efficiency of 5.84%, whereas BA800 showed a slightly lower value of 5.15%. By contrast, at pH 6, the removal performance of both samples improved markedly, with BA600 reaching 80.39% and BA800 attaining 56.89%. This pronounced enhancement indicates that the reduction in proton competition at higher pH facilitated the interaction of Zn(II) ions with the active surface sites of both adsorbents. The comparison further reveals that BA600 displayed a considerably greater adsorption performance than BA800 at pH 6, which suggests that BA600 possessed a higher affinity for Zn(II) ions under the investigated conditions. This pH-dependent adsorption trend is consistent with that reported for related thermally engineered nanocomposites, where pollutant uptake increased at higher pH values because the reduced proton interference and the more favorable surface charge promoted stronger interaction between the adsorbate species and the available active sites^47,49,50.

Fig. 8.

Impact of solution pH on the elimination efficacy of Zn(II) ions employing BA600 and BA800 samples.

pH values higher than 6 were not investigated because Zn(II) may undergo hydrolysis and precipitation under more alkaline conditions, which could lead to an overestimation of the actual adsorption performance.

Adsorption mechanism

As an illustrative example for elucidating the adsorption mechanism, the BA600 nanocomposite was selected for FTIR analysis before and after Zn(II) adsorption, as shown in Fig. 9. Figure 9 provides strong evidence for the adsorption mechanism of Zn(II) ions onto BA600 through a direct comparison between the FTIR spectrum before adsorption (Fig. 9A) and that recorded after adsorption (Fig. 9B). Before adsorption, the main band at 1420 cm⁻¹ can be assigned to the carbonate stretching vibration, while the band at 1061 cm⁻¹ is attributed to C–O vibration of carbonate-containing species. In addition, the bands located at 854, 694, 633, 557, and 426 cm⁻¹ are related to oxygen–metal vibrations, mainly Ba–O and Al–O bonds in the BaCO₃/BaAl₂O₄/C structure. After Zn(II) adsorption, significant spectral changes were observed. A broad band appeared at 3434 cm⁻¹, which is attributed to surface hydroxyl groups and/or adsorbed water molecules, whereas a band at 1585 cm⁻¹ appeared and may be associated with bending vibration of adsorbed water or surface hydroxyl-related species. Moreover, the carbonate band at 1420 cm⁻¹ changed markedly after adsorption, giving bands at 1488, 1427, and 1365 cm⁻¹, indicating that carbonate-containing active sites were involved in Zn(II) binding. The band at 1061 cm⁻¹ slightly shifted to 1053 cm⁻¹ and a new band appeared at 964 cm⁻¹, further confirming the participation of oxygen-containing surface groups in the adsorption process. Likewise, the metal–oxygen bands also shifted from 854 to 633 cm⁻¹ to 833 and 619 cm⁻¹, respectively, while bands at 694, 557, and 426 cm⁻¹ remained with slight variation, suggesting that Zn(II) ions interacted with the BA600 surface through oxygen donor atoms associated with carbonate groups, hydroxyl groups, and metal–oxygen linkages^47,50,51. These spectral modifications suggest that the disposal of Zn(II) ions by BA600 mainly proceeded through surface complexation/chemisorption, accompanied by electrostatic attraction under the favorable pH conditions.

Fig. 9.

FTIR spectra of BA600 (A) before and (B) after adsorption of Zn(II) ions.

Figure 10 illustrates the FE-SEM image of the BA600 nanocomposite after adsorption of Zn(II) ions and provides further support for the proposed adsorption mechanism when compared with the pristine BA600 surface shown in Fig. 5A. Before adsorption, Fig. 5A revealed that BA600 possessed a densely compact and rough surface composed mainly of cauliflower-like agglomerates, quasi-spherical particles, and irregular clustered domains. In contrast, after adsorption, Fig. 9 indicates that the surface morphology of BA600 became noticeably altered owing to the attachment of Zn(II) ions onto the available active sites. The post-adsorption surface appears more covered and less distinct than the original one, which suggests that Zn(II) ions were successfully deposited on the nanocomposite surface and occupied a considerable portion of the accessible adsorption centers. This morphological variation confirms that the adsorption process was not merely a superficial physical event, but rather involved effective interaction between Zn(II) ions and the BA600 surface. Therefore, the comparison between Figs. 5A and 10 provides clear morphological evidence for the successful uptake of Zn(II) ions by BA600 and is compatible with the FTIR results, which indicated the participation of surface functional groups in the adsorption process.

Fig. 10.

FE-SEM image of BA600 nanocomposite after adsorption of Zn(II) ions.

Figure 11 presents the EDS spectrum of the BA600 nanocomposite after adsorption of Zn(II) ions and offers decisive evidence for the successful uptake of zinc when compared with the spectrum of the pristine sample shown in Fig. 3A. Before adsorption, Fig. 3A confirmed that BA600 was composed mainly of C, O, Al, and Ba, with weight percentages of 4.9, 20.1, 7.1, and 67.9%, respectively, and no Zn signal was detected. In contrast, after adsorption, Fig. 11 clearly reveals the appearance of a distinct Zn peak, and the elemental composition changed markedly to 9.6% C, 34.0% O, 16.5% Al, 11.6% Ba, and 28.3% Zn. The emergence of Zn in the post-adsorption spectrum provides direct confirmation that Zn(II) ions were successfully immobilized on the BA600 surface. Moreover, the substantial changes in the relative elemental percentages after adsorption further indicate that the interaction of Zn(II) ions with the adsorbent was significant and not merely incidental. In particular, the pronounced increase in oxygen content supports the involvement of oxygen-bearing surface groups in Zn(II) binding, while the reduction in the relative Ba percentage can be attributed to coverage of the original surface by adsorbed zinc species. Therefore, the comparison between Figs. 3A and 11 strongly confirms the successful disposal of Zn(II) ions by BA600 and supports the proposed mechanism that adsorption proceeded mainly through interaction of Zn(II) ions with oxygen-containing active sites on the nanocomposite surface.

Fig. 11.

EDS pattern of BA600 nanocomposite after adsorption of Zn(II) ions.

The elemental mapping images presented in Figs. 12 and 13 provide additional visual confirmation for the successful disposal of Zn(II) ions by BA600 nanocomposite. Before adsorption (Fig. 12), the elemental distribution is mainly associated with the original constituent elements of the nanocomposite, namely Ba, Al, O, and C, with no detectable presence of Zn, which confirms the initial composition of the pristine material. In contrast, after adsorption (Fig. 13), the elemental maps clearly reveal the appearance and uniform distribution of Zn over the BA600 surface, demonstrating that Zn(II) ions were effectively immobilized on the adsorbent. This distinct difference between the two figures provides strong evidence for the successful uptake of zinc ions and further supports the EDS results, confirming that the adsorption process occurred through effective interaction between Zn(II) ions and the active sites distributed across the nanocomposite surface.

Fig. 12.

Elemental mapping images of the BA600 nanocomposite before adsorption of Zn(II) ions.

Fig. 13.

Elemental mapping images of the BA600 nanocomposite after adsorption of Zn(II) ions.

Time

Figure 14 illustrates that the elimination of Zn(II) ions by BA600 or BA800 increased progressively with contact time until equilibrium was attained for each adsorbent. This behavior indicates that the gradual occupation of the available active sites enhanced zinc uptake as the contact period was extended. At the beginning of the process, both materials exhibited comparatively modest removal efficiencies because the interaction between Zn(II) ions and the adsorbent surfaces was still at an early stage. After 10 min, BA600 achieved a removal efficiency of 44.44%, whereas BA800 exhibited a distinctly lower value of 27.15%, which demonstrates that BA600 possessed a greater affinity for Zn(II) ions from the outset. As the contact time increased, the removal efficiency of both adsorbents continued to rise, although BA600 consistently outperformed BA800 throughout the investigated period. Equilibrium was reached after 60 min for BA600 and after 80 min for BA800 with corresponding removal efficiencies of 79.62% and 56.31%, respectively. The negligible change in removal beyond these equilibrium times may be attributed to saturation of the available adsorption sites on both materials. Taken together these findings show that BA600 not only removed a greater proportion of Zn(II) ions than BA800 but also attained equilibrium more rapidly, which reflects its superior adsorption performance under the examined conditions. This time-dependent adsorption behavior is in good agreement with that reported for related thermally engineered nanocomposites, where the removal efficiency increased progressively with contact time until equilibrium was attained, followed by a plateau attributed to gradual occupation and eventual saturation of the available active sites; notably, the lower-temperature products consistently reached equilibrium faster than their higher-temperature counterparts^47,49,50.

Fig. 14.

Impact of time on the elimination efficacy of Zn(II) ions employing BA600 and BA800 products.

The kinetics of Zn(II) ions onto BA600 and BA800 were evaluated using the pseudo-first-order model given in Eq. 4 as well as the pseudo-second-order model given in Eq. 5^52–54.

4

5

In these equations, Q_e​ denotes the adsorptive capacity at equilibrium, and Q_t​ represents the adsorption capacity at time t. The term K₁​ is the rate constant of the pseudo-first-order model, and K₂​ denoting the rate constant of the pseudo-second-order model.

As shown in Fig. 15 and summarized in Table 4, both kinetic models considered to interpret the elimination behavior of Zn(II) ions on the two nanocomposites. Nevertheless, the pseudo second-order model provided a markedly superior description of the experimental results for both BA600 and BA800. For BA600, the calculated equilibrium adsorption capacity obtained from the pseudo-second-order model was 121.51 mg/g, which corresponds closely to the experimental value of 119.51 mg/g, whereas the pseudo-first-order kinetic model gave a much lower value of 65.67 mg/g. A similar trend was observed for BA800, where the pseudo second-order model yielded a Q_e​ value of 87.79 mg/g, which closely matched the experimental capacity of 84.51 mg/g, while the pseudo first-order kinetic model produced a substantially lower value of 48.84 mg/g. Furthermore, the correlation coefficient (R²) values for the pseudo-second-order model reached 0.9999 for both adsorbents, whereas the corresponding values for the pseudo-first-order model were only 0.9675 for BA600 and 0.9692 for BA800. These data demonstrate that the elimination of Zn(II) ions onto BA600 and BA800 is consistent with the pseudo-second-order kinetic model and indicate that this model describes the adsorption process with much greater accuracy than the pseudo-first-order approach.

Fig. 15.

(A) Pseudo-first-order and (B) pseudo-second-order kinetic models.

Table 4.

Kinetic constants of the pseudo-first-order and pseudo-second-order models for the elimination of Zn(II) ions by BA600 and BA800 products.

Nanocomposite	QExp (mg/g)	Pseudo-first-order			Pseudo-second-order
		K1 (1/min)	R 2	Qe (mg/g)	K2 (g/mg.min)	R 2	Qe (mg/g)
BA600	119.51	0.03077 ± 0.00281	0.9675	65.67 ± 6.12	0.0010003 ± 0.0000004	0.9999	121.51 ± 0.01
BA800	84.51	0.02549 ± 0.00185	0.9692	48.84 ± 4.04	0.0010013 ± 0.0000160	0.9999	87.79 ± 0.32

Temperature

Figure 16 displays that the elimination of Zn(II) ions by BA600 or BA800 declined as the temperature increased, which indicates that the adsorption process became less significant at elevated temperatures. At 298 K, BA600 exhibited an elimination efficiency of 79.62%, whereas BA800 showed a lower value of 56.31%, which confirms that BA600 possessed a markedly greater affinity for Zn(II) ions under these conditions. As the temperature rose to 328 K, the elimination efficiency of both adsorbents decreased substantially and reached 44.07% for BA600 and 19.68% for BA800. This pronounced reduction suggests that the elimination of Zn(II) ions on both nanocomposites was thermodynamically less stable at higher temperatures and was therefore favored at lower temperatures. The comparison between the two materials further shows that BA600 consistently outperformed BA800 across the examined temperature range and retained a higher removal capacity even at 328 K. These findings indicate that although increasing temperature adversely affected Zn(II) uptake for both adsorbents, BA600 remained the more efficient material and exhibited a stronger overall adsorption performance than BA800. This temperature-dependent adsorption behavior is in close agreement with that reported for analogous thermally engineered nanocomposites, where adsorption performance decreased with increasing temperature, confirming that the uptake process was more favorable at lower temperatures and predominantly exothermic in nature^47,49,50.

Fig. 16.

Impact of solution temperature on the elimination percentage of Zn(II) ions by BA600 and BA800 samples.

The thermodynamic parameters governing the adsorption of Zn(II) ions onto BA600 and BA800 were evaluated by means of Eqs. 6, 7, and 8^55–57.

6

7

8

In the above equations, K_d​ denotes the distribution coefficient, whilst the symbols ΔH^o, ΔS^o, and ΔG^o correspond to the standard enthalpy, entropy, and Gibbs free energy changes, respectively, whereas R is the universal gas constant and T is the absolute temperature.

As presented in Fig. 17 and summarized in Table 5, the adsorption of Zn(II) ions onto both nanocomposites was associated with negative ΔH^o values of − 45.33 kJ/mol for BA600 and − 43.12 kJ/mol for BA800. These values demonstrate that the adsorption process was exothermic and indicate that the uptake mechanism was predominantly chemical in nature since the magnitude of ΔH^o exceeded 40 kJ/mol for both adsorbents. The magnitude of ΔH° provides important insight into the nature of the adsorption mechanism. Generally, enthalpy changes lower than 40 kJ/mol are associated with physisorption, whereas values exceeding this threshold are indicative of chemisorption involving stronger interactions such as surface complexation or chemical bonding. In the present study, the ΔH° values for both BA600 and BA800 exceed 40 kJ/mol, which strongly supports the predominance of a chemisorption mechanism^50,51. It should be noted that the Van’t Hoff analysis assumes that ΔH° remains constant over the investigated temperature range^50,51. This assumption is considered reasonable in the present study because the temperature interval (298–318 K) is relatively narrow, and significant variations in enthalpy are not expected within such a limited range. Therefore, the linear relationship between ln K_d and 1/T can be reliably applied. The negative ΔG^o values obtained over the entire temperature range further confirm that the adsorption proceeded spontaneously under the investigated conditions. Moreover, the positive ΔS^o values of 0.1373 and 0.1392 kJ/mol.K for BA600 and BA800, respectively, suggest that the process was feasible and accompanied by an elevated degree of disorder at the solution-solid interface. A comparison between the two materials further reveals that BA600 exhibited a slightly more negative ΔH^o value than BA800, which points to somewhat stronger interaction with Zn(II) ions. Taken together, these thermodynamic findings establish that the elimination of Zn(II) ions by BA600 and BA800 was spontaneous, feasible, exothermic, and predominantly chemisorptive.

Fig. 17.

Van’t Hoff plots of ln K_d against 1/T for the adsorption of Zn(II) ions onto BA600 and BA800 products.

Table 5.

Thermodynamic constants for the elimination of Zn(II) ions by BA600 and BA800 products.

Nanocomposite	△So (kJ/mol.K)	△Ho (kJ/mol)	△Go (kJ/mol)
			298 K	308 K	318 K	328 K
BA600	0.1373 ± 0.0124	− 45.33 ± 3.88	− 86.26 ± 5.36	− 87.26 ± 5.45	− 89.00 ± 5.54	− 90.38 ± 5.62
BA800	0.1392 ± 0.0151	− 43.12 ± 4.73	− 84.59 ± 6.59	− 85.99 ± 6.70	− 87.38 ± 6.81	− 88.77 ± 6.92

Amount of adsorbent

Figure 18 shows that the elimination of Zn(II) ions by BA600 or BA800 was markedly dependent on the adsorbent dose. The removal efficiency increased substantially as the dose rose from a very low value to an intermediate level and then declined when the adsorbent amount was increased further. At 0.025 g, both materials exhibited very poor removal performance, as the number of available active sites was limited, and BA600 achieved 5.55%, whilst BA800 showed a lower value of 3.78%. A pronounced improvement was observed at 0.1 g, where the removal efficiency reached 79.62% for BA600 and 56.31% for BA800, which indicates that this dose provided a far greater number of accessible adsorption sites for Zn(II) uptake. When the dose was increased to 0.4 g, the removal efficiency of both adsorbents decreased to 69.35% for BA600 and 43.22% for BA800, although these values remained considerably higher than those recorded at 0.025 g. This behavior suggests that the initial rise in removal resulted from the increase in available surface-active sites, whereas the subsequent decline at higher dosage was most likely caused by particle aggregation and overlap of adsorption sites, which reduced the effective surface area. The comparison between the two nanocomposites further demonstrates that BA600 consistently outperformed BA800 at all examined doses and displayed a more favorable adsorption behavior towards Zn(II) ions under the investigated conditions.

Fig. 18.

Impact of adsorbent dosage on the elimination percentage of Zn(II) ions by BA600 and BA800 samples.

Concentration

Figure 19 shows that the elimination of Zn(II) ions by BA600 or BA800 decreased progressively as the initial metal ion concentration increased. This behavior indicates that the available adsorption sites became increasingly insufficient relative to the growing number of Zn(II) ions in solution, and consequently, the removal efficiency declined at higher concentrations. At 25 mg/L, both adsorbents exhibited very high removal performance, with BA600 achieving 96.16% and BA800 reaching 91.04%, which demonstrates that both materials were highly effective when the zinc concentration was low. By contrast, at 200 mg/L, the elimination efficacy decreased markedly to 40.89% for BA600 and 29.72% for BA800, which reflects the reduced ability of the adsorbents to accommodate the larger quantity of Zn(II) ions present in solution. The comparison between the two nanocomposites further reveals that BA600 consistently outperformed BA800 at both the lowest and highest concentrations and therefore possessed a stronger overall affinity for Zn(II) ions under the investigated conditions. These findings confirm that increasing the initial Zn(II) concentration adversely affected the percentage removal for both adsorbents, although BA600 remained the more efficient material throughout the examined concentration range. This concentration-dependent adsorption trend is in excellent agreement with that reported for analogous thermally engineered nanocomposites, where the removal efficiency was highest at lower initial concentrations and declined progressively as the adsorbate concentration increased, owing to the gradual saturation of the available active sites and the limited number of adsorption centers relative to the increasing pollutant load^47,49,50.

Fig. 19.

Impact of initial Zn(II) concentration on the elimination percentage of Zn(II) ions by BA600 and BA800 nanocomposites.

The equilibrium behavior of Zn(II) adsorption onto BA600 and BA800 was analyzed using the Langmuir isotherm given in Eq. 9 and the Freundlich isotherm given in Eq. 10^58–60.

9

10

In these equations, the terms Q_max and K₃​ correspond to the greatest monolayer adsorption capacity and the Langmuir constant, whereas K₄​ and 1/n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively.

As shown in Fig. 20A–B and summarized in Table 6, both isotherm models were applied in order to interpret the adsorption characteristics of the two nanocomposites. Nevertheless, the Langmuir model provided a markedly better description of the experimental data than the Freundlich model. For both BA600 and BA800, the correlation coefficient (R²) derived from the Langmuir equilibrium model reached 0.9998, whereas the corresponding Freundlich values were only 0.7418 and 0.7271, respectively. These results demonstrate that the elimination of Zn(II) ions by BA600 and BA800 products obeys the Langmuir isotherm and therefore suggests predominantly monolayer adsorption on a finite number of energetically defined surface sites. The Langmuir model further yielded maximum uptake capacities of 124.38 mg/g regarding BA600 and 90.58 mg/g concerning BA800, which proves the superior elimination performance of BA600 under the investigated conditions. Taken together these findings establish that the Langmuir isotherm is more appropriate than the Freundlich model for describing the equilibrium uptake of Zn(II) ions by both adsorbents.

Fig. 20.

(A) Langmuir and (B) Freundlich isotherms.

Table 6.

Equilibrium parameters of the Langmuir and Freundlich isotherms for the elimination of Zn(II) ions by BA600 and BA800 products.

Nanocomposite	Langmuir			Freundlich
	Qmax (mg/g)	R 2	K3 (L/mg)	K4 (mg/g)(L/mg)1/n	1/n	R 2
BA600	124.38 ± 0.61	0.9998	0.5755 ± 0.0993	50.45 ± 8.03	0.2186 ± 0.0476	0.7418
BA800	90.58 ± 0.48	0.9998	0.3380 ± 0.0475	38.39 ± 6.16	0.1899 ± 0.0428	0.7271

Table 7 compares the maximum uptake capacities of several adsorbents reported for Zn(II) removal and clearly displays that the samples established in the present study perform very favorably relative to a lot of previously published materials. By comparison, BA600 exhibited the highest adsorption capacity of 124.38 mg/g, whereas BA800 also showed a high value of 90.58 mg/g, which confirms the strong adsorption ability of both nanocomposites towards Zn(II) ions.

Table 7.

Survey and comparative analysis of Q_max for disposal of Zn(II) ions.

Adsorbent	Qmax (mg/g)	pH	Time (min)	T (K)	References
Kaolinite	15.52	5	120	298	40
Amino-functionalized magnetic nanoparticles	24.21	6	90	333	41
Graphene/Fe3O4 composite	121.5	7	60	303	42
Fe3O4 nanoparticles	107.27	5	60	298	43
Hydroxyapatite	37.53	6	30	298	62
Magnetic chlorapatite	76.95	10	120	298	63
Magnetic multi-walled carbon nanotubes	3.76	5	350	298	64
Cross-linked magnetic modified chitosan	32.16	5	30	298	65
MnFe2O4/chitosan/Schiff-base nanocomposite	289.86	7.5	100	328	61
BA600	124.38	6	60	298	This study
BA800	90.58	6	80	298	This study

To further evaluate the role of the individual components, attempts were made to isolate the relevant phases from the present synthetic system by varying selected preparation parameters. However, suitable control materials could not be obtained under conditions directly comparable to those used for the BaCO₃/BaAl₂O₄/C nanocomposites. Therefore, for comparison purposes, BaCO₃ and BaAl₂O₄ were prepared separately as reported by Salehabadi et al.⁴⁸, and their adsorption performances toward Zn(II) ions were examined under the same standard conditions of pH 6, contact time 80 min, and 298 K. The adsorption capacities of BaCO₃ and BaAl₂O₄ were found to be 55.76 and 30.48 mg/g, respectively, both of which are markedly lower than those of BA600 (124.38 mg/g) and BA800 (90.58 mg/g). These findings indicate that the adsorption performance of the BaCO₃/BaAl₂O₄/C nanocomposites arises from a synergistic interaction among their components rather than from the simple individual contribution of BaCO₃ or BaAl₂O₄ alone.

The expanded comparative data presented in Table 6 demonstrate that, in addition to their high maximum adsorption capacities, BA600 and BA800 also perform favorably in terms of the main operating parameters, confirming the strong potential of the prepared nanocomposites for efficient Zn(II) removal from aqueous media. This performance may be attributed to the favorable surface characteristics of BA600 and BA800 together with the existence of plentiful binding sites for zinc ions. The better performance of BA600 relative to BA800 may further be associated with the lower calcination temperature, which likely preserved a greater number of accessible adsorption sites. However, it should also be acknowledged that some recently developed systems have achieved higher maximum adsorption capacities under their respective experimental conditions. For instance, MnFe₂O₄/chitosan/Schiff base nanocomposite was reported to exhibit a Zn(II) adsorption capacity of 289.86 mg/g⁶¹, which is substantially higher than that of BA600. Nevertheless, the importance of the present work lies not in claiming the highest reported adsorption capacity, but in the first-time development of a BaCO₃/BaAl₂O₄/C multicomponent adsorbent, its comparatively simple preparation by the Pechini sol-gel process, and the systematic demonstration of the influence of calcination temperature on its structure and adsorption behavior. In addition, Schiff-base-based systems generally involve extra functionalization steps and organic reagents, which increase the synthetic complexity and likely the preparation cost. Therefore, although the present material shows lower adsorption capacity than some advanced Schiff-base-containing adsorbents, it remains attractive as a simpler and potentially more economical alternative with good practical applicability.

Regeneration

Figure 21 shows that the desorption of Zn(II) ions from BA600 or BA800 increased markedly as the concentration of HCl was raised from 1.0 to 2.0 M. Using 1.0 M HCl, the desorption efficiency was 86.80% for BA600 and 83.09% for BA800, which indicates that both adsorbents released a substantial proportion of the retained zinc ions even at the lowest acid concentration examined. A further increase in HCl concentration enhanced the desorption process significantly, and the highest efficiencies were achieved at 2.0 M, where BA600 and BA800 exhibited values of 99.43% and 99.37%, respectively. This behavior demonstrates that hydrochloric acid was highly effective for the regeneration of both nanocomposites and that the desorption process became increasingly favorable with increasing proton concentration. The excellent desorption performance of HCl may be attributed to the strong competition of H+ ions with adsorbed Zn(II) ions for the active surface sites together with the ability of the acidic medium to disrupt the interactions established between zinc ions and the adsorbent surface.

Fig. 21.

Impact of HCl concentration on the desorption percentage of Zn(II) ions from BA600 and BA800 products.

To further verify the stability of the adsorbent after regeneration, BA600 was selected as a representative example. After the regeneration process using 2 M HCl, the resulting filtrate was investigated by inductively coupled plasma analysis, and none of the constituent elements of BA600, namely Ba and Al, were detected. This result confirms that no measurable leaching of the adsorbent components occurred during regeneration, highlighting the excellent chemical stability of BA600. Moreover, as shown in Fig. 22 and in comparison, with Fig. 2B, the regenerated sample retained the main diffraction peaks characteristic of the original material, although a slight change in the intensity of some peaks was observed. This minor variation in peak intensity may be attributed to slight changes in crystallinity during the adsorption–desorption process, while the preservation of the principal diffraction features confirms that the overall crystal structure of BA600 remained essentially stable after regeneration.

Fig. 22.

XRD analysis of the regenerated BA600 product.

Reusability

Figure 23 shows that the elimination of Zn(II) ions by BA600 or BA800 gradually declined with increasing cycle number. This behavior indicates that repeated reuse of the adsorbents led to a progressive reduction in the number or effectiveness of the available active sites. BA600 exhibited a removal efficiency of 79.62% in the initial cycle, and this value decreased steadily to 69.76% after the fifth cycle. A similar trend was observed for BA800, where the removal efficiency declined from 56.31% at the beginning to 44.57% after five reuse cycles. The gradual loss in performance may be attributed to incomplete desorption of Zn(II) ions together with partial blocking or deterioration of adsorption sites during successive adsorption and regeneration steps. Despite this decrease, BA600 consistently retained higher removal efficiency than BA800 throughout all cycles, which confirms its superior stability and stronger adsorption performance under repeated use. Overall, Fig. 23 demonstrates that both nanocomposites preserved a considerable proportion of their adsorption capacity after several cycles, although BA600 remained the more effective and more reusable adsorbent for Zn(II) removal.

Fig. 23.

Cyclic adsorption/desorption effect over five consecutive runs.

Interference

The binary adsorption experiments were designed as a simplified competitive model to examine the selectivity of the prepared nanocomposites toward Zn(II) ions in the presence of representative coexisting species. The 1:1 ratio between Zn(II) and each competing ion was selected to provide a controlled basis for comparing the competitive influence of different inorganic ions and the model organic co-contaminant under equivalent conditions. Table 8 demonstrates that the adsorption of Zn(II) ions by both BA600 and BA800 was influenced by the presence of coexisting species, although the degree of interference depended strongly on the nature of the competing ion or molecule. The most pronounced suppression was observed in the presence of Cu²⁺ and Ni²⁺, which caused the largest decreases in adsorption capacity, indicating strong competition with Zn(II) ions for the available active sites. Monovalent and alkaline earth cations such as Na⁺, K⁺, Ca²⁺, and Mg²⁺ also reduced Zn(II) uptake, but to a lesser extent. In contrast, the common anions Cl⁻, NO₃⁻, and SO₄²⁻ exerted only minor effects, suggesting that these species did not significantly hinder Zn(II) adsorption under the investigated conditions. Methylene blue dye, however, caused a noticeable decline in adsorption capacity, with reductions of 22.26 mg/g for BA600 and 16.11 mg/g for BA800, which confirms that organic co-contaminants may also compete for surface-active sites. Despite the presence of all examined interfering species, BA600 consistently retained higher Zn(II) adsorption capacities than BA800, reflecting its superior resistance to competitive interference and its greater selectivity toward Zn(II) ions.

Table 8.

Effect of accompanying ions on adsorption capacity of BA600 and BA800 products towards Zn(II) ions.

Accompanying ion	Q (mg/g) BA600	Q (mg/g) BA800	Decrease in Q (mg/g) BA600	Decrease in Q (mg/g) BA800
None (Control)	119.51	84.51	–	–
Na+	111.84	78.96	7.67	5.55
K+	110.92	78.14	8.59	6.37
Mg2+	101.63	71.24	17.88	13.27
Ca2+	99.84	69.85	19.67	14.66
Cu2+	83.42	57.33	36.09	27.18
Ni2+	88.76	61.42	30.75	23.09
Cl−	117.26	82.94	2.25	1.57
NO3−	117.88	83.36	1.63	1.15
SO42−	115.74	81.48	3.77	3.03
Methylene blue dye	97.25	68.40	22.26	16.11

Assessment of Zn(II) removal from wastewater

To assess the practical applicability of the synthesized samples, separation investigations were accomplished employing real wastewater gathered from chemistry teaching laboratory. Before the adsorption experiments, the effluent was spiked with Zn(II) ions and methylene blue dye to adjust the initial concentration of each pollutant to 100 mg/L, in line with the single-component synthetic tests whilst preserving the native ionic configuration of the practical matrix. Analysis of the fortified effluent revealed the presence of common inorganic species, namely Na⁺ (58.4 mg/L), K⁺ (12.7 mg/L), Mg²⁺ (7.9 mg/L), Ca²⁺ (24.3 mg/L), NO₃⁻ (21.6 mg/L), Cl⁻ (71.5 mg/L), SO₄²⁻ (46.2 mg/L), HCO₃⁻ (118.0 mg/L), and PO₄³⁻ (4.6 mg/L), together with trace amounts of Cu (0.06 mg/L), Fe (0.23 mg/L), and Zn (0.11 mg/L). In addition to the ionic composition, the wastewater matrix exhibited an approximate chemical oxygen demand (COD) of 192 mg/L, a total organic carbon (TOC) content of 39 mg/L, and an electrical conductivity of 910 µS/cm, and a turbidity of 16 NTU. Under the previously optimized conditions, BA600 and BA800 achieved adsorption capacities of 110.40 and 77.60 mg/g, respectively, in the real wastewater. These values were slightly lower than those obtained in distilled water, which may be attributed to competition from coexisting ions and partial blockage of active sites by dissolved constituents, although both adsorbents still maintained strong removal performance.

Conclusions

This investigation successfully established that BaCO₃/BaAl₂O₄/C nanocomposites can be produced through a facile Pechini sol-gel process and can serve as effective adsorbents for the removal of Zn(II) ions from aqueous media. The results confirmed that calcination temperature exerted a decisive influence on crystal growth, structural development, and adsorption behavior of the prepared materials, since the average crystallite size increased from 54 nm for BA600 to 66 nm for BA800. BA600 exhibited the more favorable physicochemical characteristics and therefore showed high adsorption performance compared with BA800. The adsorption of Zn(II) ions onto both samples followed the pseudo-second-order kinetic model and the Langmuir isotherm, while thermodynamic evaluation confirmed that the process was spontaneous, feasible, and exothermic. The maximum adsorption capacities reached 124.38 mg/g for BA600 and 90.58 mg/g for BA800, indicating competitive adsorption performance, although higher Zn(II) uptake values have been reported for some other advanced adsorbent systems. Therefore, the significance of the present work lies not in establishing the highest adsorption capacity reported in the literature, but in the first-time development of a BaCO₃/BaAl₂O₄/C multicomponent adsorbent, its comparatively simple preparation, and its demonstrated applicability in regeneration and real-wastewater treatment. In real wastewater, the adsorption capacities reached 110.40 mg/g for BA600 and 77.60 mg/g for BA800, further confirming the practical applicability of the prepared nanocomposites under realistic matrix conditions.

Author contributions

Nada S. Al-Kadhi (Writing—Review & Editing, Funding acquisition), Saad A. Aljlil (Writing—Review & Editing, Supervision), Hesham H. El-Feky (Methodology, Investigation, Resources); Ehab A. Abdelrahman (Conceptualization, Methodology, Writing—Review & Editing).

Funding

Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2026R85), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

Data availability

All data generated or analyzed in this study are included in the article.

Declarations

Competing interests

The authors declare no competing interests.

Consent to participate

Not applicable.