Introduction

Water is essential for each organism on the planet. It is a known fact that there is a limited source of water. Industrial activities require a vast amount of water, where the water used in the course of pursuing such activities usually ends up contaminated. Heavy metal ions are among the most dangerous contaminants which can be found in such waters (Khan et al. 2019; Morrow 2010). Lead, zinc, chromium, copper, mercury, arsenic, and cadmium are commonly found in wastewater (Alp Kavlo et al. 2023; Dağci et al. 2021; Goudarzi 2007; Kaya et al. 2010; Khan et al. 2022; Kurnaz Yetim et al. 2022). Heavy metal ions can be released through natural processes and human activities such as various industrial activities, mining, agriculture. (Jagirani et al. 2021; Kaya et al. 2010; Ozcan et al. 2018). Once heavy metal ions are released, they can have both short- and long-term impacts on soil and water ecosystems which can have a range of both direct and indirect consequences is better (Elliott et al. 1986; Ozcan et al. 2018; Raskin et al. 1994; Saleh et al. 2022).

Heavy metal ions can be bioaccumulated in tissues, which can have a deleterious effect on the food chain (Özdemir et al. 2019). Similarly, the release of heavy metal-containing waters can also contaminate the soil, where heavy metal ions can build up in vegetation (Cheng 2003; Elliott et al. 1986; Ozcan et al. 2018; Rascio et al. 2011; Raskin et al. 1994; Saleh et al. 2022). Heavy metal ions can damage the genetic materials of such organisms, disrupt their metabolic processes, and can lead to immune-related dysfunctions (Guo et al. 2009; Sultana et al. 2019; Zhou et al. 2008). Moreover, consumption of heavy metal exposed foods can lead to indirect intake of heavy metals (Khan et al. 2022; Sultana et al. 2019), which can have long-term effects on humans (Gacía et al. 2018; Gupta et al. 2015; Ijomone et al. 2020; Lin et al. 2018; Mishra 2009; Zheng et al. 2023, 2012).

Prevention and mitigation of heavy metal ions from entering freshwater are essential to minimizing any potential direct effects. Traditional wastewater management technologies may have certain limitations in terms of removing heavy metal ions. To this end, innovative technologies are required to deal with any such issues. At this point, one such technology that has great potential is nanoparticles, which may be used in the removal of heavy metal ions (Salem et al. 2017; Xiong et al. 2015).

Nanoparticles are very small particles the diameter of which is between a couple of nanometres to a few hundred nanometres (Yu et al. 2006). They exhibit interesting chemical, physical, and electronic properties compared to the bulk material. Nanoparticles have large surface area/volume ratios, as noted by Xu et al. (2012), which makes them suitable candidates for heavy metal absorption. A high surface area/volume ratio also leads to high electron concentration on the surface, which also leads to high reactivity (Xu et al. 2012). Hence, high catalytic activity can be obtained from such. Various types of nanostructures have previously been reported regarding their applications in heavy metal recovery. Among those, metallic nanoparticles are of some particular importance due to their unique properties. The heavy metal removal characteristics of Bi nanoparticles, Ti nanoparticles, Au nanoparticles, Mg nanoparticles, and Ag nanoparticles have previously been reported (Guevara‐bernal et al. 2022; Kurnaz Yetim et al. 2022; Lo et al. 2012; Mahdavi et al. 2012; Qian et al. 2013; Razzaz et al. 2016; Talukder et al. 2022). In heavy metal removal applications, microextraction, liquid–liquid extraction, cloud point extraction, solid-phase extraction, dispersive liquid–liquid microextraction, and chemical precipitation methods have commonly been used (Alp Kavlo et al. 2023; Altunay et al. 2022; Goudarzi 2009; Habila et al. 2017; Kinaree et al. 2014; Kurnaz Yetim et al. 2022; Rihana-Abdallah et al. 2022; Saleh et al. 2022; Soylak et al. 2007; Taghani et al. 2018). In addition, the potential for heavy metal removal via magnetic nanoparticles has also been evidenced in the literature (Almomani et al. 2020; Dave et al. 2014). Depending on their magnetic properties, magnetic nanoparticles can display superparamagnetic, ferromagnetic, and ferrimagnetic properties. Iron oxide, cobalt oxide, and nickel oxide nanoparticles, for instance, are commonly used in magnetic nanoparticle applications (Karaçam et al. 2020; Kurnaz Yetim et al. 2020a; Yetim 2021). Producing composite nanostructures using such magnetic cores has been favoured in terms of obtaining nanocomposites with magnetic characteristics. Using magnetic nanoparticles in heavy metal ion removal provides certain advantages. For example, magnetic nanoparticles have quite a high potential for heavy metal removal (Wang et al. 2015; Zarei et al. 2018). In addition, magnetic nanoparticles can be collected by a simple magnet, which makes them suitable candidates for magnetic filtration applications (Kurnaz Yetim et al. 2020a).

The heavy metal adsorption characteristics of magnetic nanocomposites have been previously assessed in the literature. For example, Safinejad et al. assessed the removal properties of magnetic zeolites for Pb and Cd (Safinejad et al. 2017). Shishehbore investigated the heavy metal removal properties of silica-coated ferrite nanoparticles (Shishehbore et al. 2011). Warner et al. assessed the heavy metal adsorption potential of manganese-doped iron oxide nanoparticles (Warner et al. 2012). Fe3O4@SiO2 nanocomposites were assessed for Ni, Cu, and Fe removal by Huang et al. (Huang et al. 2014). It may be noted, however, that the heavy metal removal properties of Co3O4@Fe3O4 nanoflowers have not previously been addressed in the literature.

In this report, Co3O4@Fe3O4 nanoflowers were produced via hydrothermal synthesis (Hasanoğlu Özkan et al. 2022). Cd2+ and Pb2+ heavy metal ion removal by Co3O4@Fe3O4 nanoflowers from seawater, mineral water, tap water, and bottled water was studied, and a new extraction enrichment method was developed. It can be seen in the literature that nanoflowers have been used in the removal of heavy metal ions (Aybike et al. 2023; Kurnaz Yetim et al. 2022; Ozalp et al. 2023); however, the report assess the potential of Co3O4@Fe3O4 composite nanoflowers for heavy metal removal.

Materials and methods

Spectral data measurements

A Rikagu Miniflex600 X-ray diffractometer that had a Ni-filtered Cu Kα source was used for the X-ray diffraction (XRD) analysis. The scanning range was 10° < 2θ < 90° and signals obtained in the investigations were recorded. The peaks were identified using the software of the device and compared with the similar results in the literature. Additionally, scanning electron microscopy (SEM) was used to analyse the surface morphology of the nanoflowers. Analysis was performed in different electron energies 10 kV, 20 kV and 30 kV in back scattering mode and secondary electron mode. Images with acceptable visual quality were used in the assessment. The energy dispersive X-ray spectroscopy (EDX) analysis was performed during SEM investigation no additional investigation was performed. For EDX analysis, a Fei Quanta 400F equipped with apparatus was used. Magnetic measurements were performed at room temperature by a vibrating sample magnetometer, Cryogenic Limited PPMS with the maximum magnetic field of ± 1 T. An Isolab ultrasound wave-supported ultrasonic bath was used in liquid extraction. A Quantachrome Corporation Autosorb-6 was used for the pore size characterization. After the recovery of analytes with MNP, measurements were made in the decanted and eluted solutions. An Agilent 240 AA Duo model atomic absorption spectrometer was used to perform the analytical work where the analytes. Pb and Cd, were identified using hollow cathode lamps (Agilent). The parameters for metal analysis can be shown in Table 1. The experimental process conducted in the work is shown in Fig. 1.

Table 1 Parameters pertaining to FAAS for metals
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Fig. 1
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Schematic summarizing the experimental workflow of the study

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Merck quality chemicals and regents were used, which are of analytical grade. Ultrapure water with an 18.2 MΩ cm specific resistance was used in the experiments.

For standard solutions of metals, 1000 mg/L NIST standard stock solutions were used. The Pb and Cd standards to be analysed were prepared for the appropriate working range of 0.05–3 mg/L. Standard solutions were prepared by dissolving with 0.5 M HNO3. Solutions of 0.05, 0.1, 0.25, 0.5, 1, 2, and 3 mg/L prepared from standards of Pb and Cd were read in the FAAS.

Synthesis of Co3O4@Fe3O4 nanoflowers

For the synthesis of Co3O4@Fe3O4 magnetic nanoflowers, Co3O4 was first synthesised via the hydrothermal method. For this purpose, 9.7 g of cobalt(II) nitrate hexahydrate (Co(NO3)2.6H2O) and 0.5 g urea were stirred together using a magnetic stirrer for 1 h and dissolved in 35 mL deionized water. Then, the mixed solution was transferred into a 50 mL Teflon-lined autoclave. The mixture was kept there for 12 h at 160 °C. The suspension was cooled to room temperature and centrifuged. The sediment obtained from the suspension was washed several times with deionized water and ethanol. The resultant product was calcinated at ash oven at 300 °C and kept there for 1 h (Kurnaz Yetim et al. 2020b). The hydrothermal nanofabrication method was used to produce Co3O4@Fe3O4 nanoflowers. 0.656 g iron (II) sulphate heptahydrate (FeSO4.7H2O) and 3.2 g NaOH were dissolved in 40 mL pure water and stirred for 15 min. In a second, separate beaker, 0.4 g Co3O4 was dissolved in 25 mL pure water and stirred for 10 min. Co3O4 mixture was added dropwise to the first mixture and stirred for 5 min. The final product was transferred to an autoclave and kept there at 150 °C for 6 h. The autoclave was then cooled at room temperature and the adsorbent was magnetically separated in solution using a magnet. The solid-phase was washed repeatedly with water and ethanol and then dried in an oven at 80 °C for 24 h (Hasanoğlu Özkan et al. 2022).

Solid-phase extraction procedure

The solid-phase extraction method used in the recovery of metal ions and the preparation of freshwater samples, seawater and standard reference materials (SRM) in Supplementary were shown. The same protocol is also shown in the Fig. 1.

Results and discussion

Characterization of Co3O4@Fe3O4 composite nanoflowers

XRD characterization of composite nanoflowers

X-ray diffraction patterns of Co3O4@Fe3O4 composite nanoflowers and Fe3O4 magnetic nanoparticles are shown in Fig. 2. It can be seen that the patterns are coherent with each other, where peaks related to Fe3O4 nanoparticles can be identified in the pattern of Co3O4@Fe3O4 composites. It can be seen that the sharpness of the peaks decreases subsequent to doping with Co3O4. However, the intensities of the peaks appear similar. No contamination-related peaks are shown in the pattern, indicating that the nanoparticles are of high purity.

Fig. 2
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X-ray diffraction patterns of Co3O4@Fe3O4 nanocomposites, and Co3O4 and Fe3O4 magnetic nanoparticles

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The diffraction peaks related to Fe3O4 are shown with the orange lines in Fig. 2, where peaks at 18.3°, 30.4°, 35.6°, 37.08°, 43.3°, 53.36°, 57.3°, 62.8°, 70.98°, 74.12°, 74.98°, and 78.94° are defined, which correspond to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0), (5 3 3), (6 2 2), and (4 4 4) face-centred cubic (fcc) crystal formations, respectively (Karaçam, Yetim, and Koç 2020; Kurnaz Yetim et al. 2020a). Co3O4 nanoflowers show two distinctive peaks at 36.8° and 38.5°, which illustrate fcc crystal structures with (3 1 1) and (2 2 2) orientations. The XRD pattern of Co3O4 nanoflowers exhibit peaks at 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.6°, 59.4°, 65.2°, and 77.3°, which are indicative of the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) fcc orientations, respectively (Kurnaz Yetim et al. 2020b). The XRD pattern of Co3O4@Fe3O4 nanoflowers is shown via a purple line in Fig. 2, which can be seen to contain peaks associated with Fe3O4 and Co3O4 (Hasanoğlu Özkan et al. 2022; Yetim 2021). The sharp and intense peaks seen in the XRD patterns indicate good crystallinity.

SEM and EDX analysis of Co3O4@Fe3O4 composite nanoflowers

The co-deposition of Co3O4 with Fe3O4 nanoparticles slightly alters the structure of the Co3O4. It can be seen that Fe3O4 nanoparticles fill the pores and gaps in the Co3O4 nanoflowers. The size of the Fe3O4 nanoparticles was found to be smaller than 100 nm; and therefore can easily fill the pores between spikes of Co3O4 nanoflowers. Figure 3 illustrates the SEM image, EDX spectra, and EDX mapping results for Co3O4@Fe3O4 nanoflowers. The SEM image confirms the clumpy structure of the Co3O4@Fe3O4 nanoflowers when the Fe3O4 nanoparticles fill the gaps in the former. The EDX spectrum contains Co-, O- and Fe-related peaks, which also confirms the chemical form of the Co3O4@Fe3O4 nanoflowers. The EDX maps also confirm that Co3O4@Fe3O4 nanoflowers consist of Co, Fe and O. It can be seen that Co accumulated on the inner sections of the nanoflowers, whereas Fe was found on the outer shell. O, by contrast, spreads all over the nanocomposites.

Fig. 3
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Scanning electron microscopy image, energy dispersive X-ray spectra, and energy dispersive X-ray spectra mapping results for Co3O4@Fe3O4 nanoflowers (Hasanoğlu Özkan et al. 2022)

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VSM analysis of magnetic composites

Vibrating sample magnetometry (VSM) results for Co3O4@Fe3O4 nanoflowers were assessed and the associated hysteresis plot is shown in Fig. 4. These hysteresis plots illustrate a large coercivity; therefore, it was deduced that Co3O4@Fe3O4 nanoflowers exhibit ferromagnetic characteristics. The magnetic saturation of Fe3O4 was found to be around 90 emu/gr, while Co3O4@Fe3O4 nanoflowers were defined as being around 38.3 emu/g. It can be seen that the addition of Co3O4 to the Fe3O4 structure slightly decreases the overall magnetic saturation. In all, the magnetic saturation of Co3O4@Fe3O4 nanoflowers was found to be quite high compared to the nanoflower-based magnetic composites. Different magnetic saturations have been reported in the literature for Fe3O4 nanostructures; for instance, the magnetic saturation of Bi2S3@Fe3O4 was found to be between 2.24 emu/g and 28.6 emu/g (Karaçam et al. 2020), and as 22.1 emu/g for Fe3O4@SiO2 magnetic nanoparticles (Tumturk et al. 2014). The magnetic saturation of Fe3O4@G2 and Fe3O4@G2/Au/Bi magnetic nanocomposites were found to be 34.7 emu/g and 34.4 emu/g, respectively; that of Fe3O4–PbS nanostructures was found to be 8.2 emu/g (Hedayati et al. 2016), and those of Fe3O4/gelatine/metoprolol and Fe3O4/gelatine/aspirin were determined to be 3.5 emu/g and 8 emu/g, respectively (Kavousi et al. 2019). Different reports indicate that Co-based materials could exhibit ferromagnetic characteristics. The hysteresis plot of the Co3O4@Fe3O4 nanoflowers indicates that they were exposed to cation diffusion, and thus that cobalt ions spread in magnetite. Such a case might alter the superparamagnetic structure of Fe3O4-based nanoparticles and force Co3O4@Fe3O4 nanoflowers to exhibit ferromagnetic characteristics (Hasanoğlu Özkan et al. 2022).

Fig. 4
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Vibrating sample magnetometry characteristics of Co3O4@Fe3O4 nanoflowers

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Adsorption results for nanoflowers

To optimize the Co3O4@Fe3O4 nanoflowers for the recovery of Pb2+ and Cd2+ heavy metal ions, eluent type, pH, adsorbent amount, starting sample volume, extraction duration, and repeatability were studied, the results of which are shown in the following sections. While, evaluating the results of the analyses in the study, the data were evaluated by performing five repeated analyses. Calculations were made at 95% confidence level (t = 2.78 probability with n − 1 degree of freedom).

pH scan

pH optimization was studied using Co3O4@Fe3O4 nanoflowers for the recovery of Pb2+ and Cd2+ heavy metal ions. To investigate this, recovery experiments were conducted at various pH ranges from 4.0 to 8.0, as shown in Fig. 5. Increased recoveries were achieved for both Pb2+ and Cd2 heavy metal ions. Figure 5 illustrates pH-related Pb2+ and Cd2+ heavy metal ion recovery rates in the presence of Co3O4@Fe3O4 nanoflowers. The highest recovery was achieved at pH 7.5 at 88.95% and 85.8% for Pb2+ and Cd2+, respectively.

Fig. 5
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Effect of pH on Pb2+ and Cd2+ recovery (amount of Co3O4@Fe3O4: 50 mg; eluent volume: 25 mL; sonication period: 10 min, for both Pb2+ and Cd2+)

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Eluent type scan

Eluent type was investigated to determine the optimum conditions for Pb2+ and Cd2+ recovery in the presence of Co3O4@Fe3O4 nanoflowers. Here, 0.1, 1, 2, 3 M HNO3 and 1 M HCl were used as eluents. The recovery values obtained by analysing the eluent type and concentration with five repetitions are shown in Fig. 6. The highest recovery was achieved using 1 M HNO3 at 88.1% for both Pb2+ and Cd2+ heavy metal ions.

Fig. 6
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Effect of eluent type for Pb2+ and Cd2+ recovery (solution pH: 7.5; amount of Co3O4@Fe3O4: 50 mg; eluent volume: 25 mL; sonication period: 10 min, for both Pb2+ and Cd2+)

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Adsorbent amount scan

Optimization of the amount of adsorbent was studied for 50, 100, 150, and 200 mg Co3O4@Fe3O4 nanoflowers for Pb2+ and Cd2+ heavy metal recovery, the results of which are shown in Fig. 7. The highest recovery rate achieved for Pb2+ was 92.2% for 100 mg adsorbent. The highest recovery achieved for Cd2+ was 88.6% for 200 mg adsorbent. An increasing recovery rate was achieved with increasing amount of adsorbent for Cd2+ heavy metal ions. The recovery rate for Pb2+ was found to be Gaussian in nature, where the highest recovery was achieved for 100 mg adsorbent, after which an increasing amount of adsorbent resulted in a decreased recovery rate.

Fig. 7
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Effect of amount of adsorbent on the recovery of Pb2+ and Cd2+ (solution pH: 7.5; eluent volume: 25 mL; sonication period: 10 min)

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Eluent volume scan

Optimization of the volume of solvent was studied for 5 mL, 10 mL, 20 mL, and 30 mL for Pb2+, and 2.5 mL, 5 mL, 10 mL, 20 mL, and 30 mL for Cd2+ heavy metal ion recovery in the presence of Co3O4@Fe3O4 nanoflowers. The eluent type-related recovery characteristics of the Co3O4@Fe3O4 nanoflowers are shown in Fig. 8. The highest Cd2+ recovery was achieved with a 5 mL solvent volume at 96.9%. Decreased Cd2+ recovery was observed for increasing solvent volumes. The highest recovery for Pb2+ was achieved at a solvent volume of 20 mL at 97.3%. Different eluent volumes give different recovery rates for different heavy metals; therefore, 20 mL and 5 mL eluent volumes showed the maximum recoveries of Pb2+ and Cd2+ ions, respectively.

Fig. 8
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Effect of sample volume on the recovery of Pb2+ and Cd2+ (solution pH: 7.5; sonication period: 10 min; amount of Co3O4@Fe3O4: 100 mg and 200 mg for Pb2+ and Cd2+, respectively)

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Extraction duration

The effect of extraction duration on Pb2+ and Cd2+ ion recovery was studied in the presence of Co3O4@Fe3O4 nanoflowers. Extraction durations were studied for 5, 10, 20, 30, and 60 min, and the recovery rates shown in Fig. 9. The highest recovery was achieved for 30 min extraction for both Pb2+ and Cd2+ heavy metal ions. Recovery rates for Pb2+ and Cd2+ heavy metal ions were determined to be 99.6% and 99.9%, respectively. Optimum recovery conditions were determined and are shown in Table 2.

Fig. 9
figure 9

Effect of extraction duration on the recoveries of Pb2+ and Cd2+ (solution pH: 7.5; amount of Co3O4@Fe3O4: 100 mg and 200 mg; eluent volume: 20 mL and 5 mL; for Pb2+ and Cd2+, respectively)

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Table 2 Optimum conditions for the Co3O4@Fe3O4 enrichment process (95% confidence level)
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Validation parameters for linearity, limit of detection (LOD), limit of quantification (LOQ), and relative standard deviation (RSD%) for the metals are shown in Table 3.

Table 3 Parameters relating to the validation of the metal ions (n = 12, 95% confidence level)
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Matrix interference is a physical interference, and can either suppress or enhance absorbance signal of analyte. For this effect of foreign ions on heavy metal removal was investigated in a solid-phase extraction study which was conducted in the presence of Co3O4@Fe3O4 nanoflowers. It could be seen that matrix ions in real samples do not have any apparent effect on the recovery rates of Pb2+ and Cd2+ heavy metal ions (see in Table 4). It was thus proposed that foreign ions do not interfere with the enrichment process.

Table 4 Effect of foreign ions on Pb2+ and Cd2+ heavy metal ion recovery (Sample volume: 25 mL, amount of Pb2+ and Cd2+: 25 µL from 50 mg/L, n = 5. 95% confidence level)
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The reusability of the Co3O4@Fe3O4 nanoflowers was also studied. For the reusability study, the solid-phase used in the extraction duration study was used. The adsorbent used in the extraction duration study was then washed several times using deionized water and reused for the optimization. For reusability tests, the nanomaterials obtained after the adsorption process were washed once with 1 M nitric acid and then washed three times with pure water. The presence of Pb2+ and Cd2+ in the nitric acid and pure water used in the washing process was analysed using FAAS. However, as a result of the analysis, it was seen that it was below the detection limit for metal ions. With regard to the repeatability of the nanomaterial, while we observed 99.6% and 99.6% recovery in the first use for Pb2+ and Cd2+, respectively, these decreased to 98.2% and 98.9% after the fifth use, respectively. The average recoveries for Pb2+ and Cd2+ over the first five uses were found to be 99.0% and 99.5%, respectively. In addition, the results for the repeatability were determined over five repetitions. It should be noted that there was a negligible decrease in the recovery performance of the magnetic nanoparticles used for Pb2+ and Cd2+.

To test the reliability of the process, SRM (standard reference material) seawater (NASS-6) was used. The results pertaining to the reliability process are shown in Table 5.

Table 5 Analysis results for SRM seawater (NASS-6) (n = 3)
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The percent preconcentration factors (PF%) for Pb2+ and Cd2+ heavy metal recoveries were 200 and 260, respectively, which were evaluated via the following equation:

$${\text{PF}}\left( \% \right) = C_{{{\text{ap}}}} /C_{{{\text{sp}}}} \times R \times 100,$$

where Cap is the final concentration, and Csp is the initial concentration of the analytes in the elution phase and initial phase, respectively. In order to calculate the PFs for the above for Pb2+ and Cd2+, at least three repetitions were conducted for water samples.

In the enrichment process of Co3O4@Fe3O4 nanoflowers, the optimum conditions obtained for Pb2+ and Cd2+ heavy metal ion recovery were applied to the real samples (seawater, tap water, mineral water, and bottled water). Before the preconcentration method, 10 µL each of Pb2+ and Cd2+ standards were added to 25 mL water samples, and readings were taken via FAAS. After enrichment, the optimum conditions (solution pH: 7.5; sonication period: 30 min; amount of Co3O4@Fe3O4: 100 mg and 200 mg; eluent volume: 20 mL and 5 mL; for Pb2+ and Cd2+, respectively) were applied, the results of which are shown in Table 6.

Table 6 Pb2+ and Cd2+ levels before and after enrichment in various samples (solution pH: 7.5; sonication period: 30 min; amount of Co3O4@Fe3O4: 100 mg and 200 mg; eluent volume: 20 mL and 5 mL; for Pb2+ and Cd2+, respectively) (n = 3)
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Different factors altering the recovery rate were investigated, where eluent type, pH, adsorbent amount, eluent volume and sonication time were considered with regard to Pb2+ and Cd2+ recovery. 99.9% Cd2+ and 99.6% Pb2+ recovery rates were achieved in the investigation. Nanoflowers were used in different studies for heavy metal ion recovery. We compare our results with those of previous reports in Table 7, from which it can be seen that our results exhibit reasonable to good recovery characteristics.

Table 7 Comparison of recovery characteristics of nanoparticles
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Conclusion

In this work, Co3O4@Fe3O4 nanoflowers were produced via hydrothermal synthesis and their Pb2+ and Cd2+ heavy metal ion recovery potential investigated. An adsorption procedure was applied to seawater, tap water, and mineral water for Pb2+ and Cd2+ ion enrichment.

Hydrothermally synthesized Co3O4@Fe3O4 nanoflowers were characterized via XRD, SEM, EDX, and VSM. The results indicated that the Co3O4@Fe3O4 nanocomposites have flower-like structures that incorporate Co, Fe, and O atoms. Contamination-related signal was not observed. Magnetic characterization illustrated that the nanoflowers had ferromagnetic characteristics, which makes them a suitable candidates for magnetic filtration and magnetic solid-phase extraction applications.

Recovery rates for Pb2+ and Cd2+ heavy metal ions were calculated after solid-phase extraction using the concentration difference between the samples prior to and after the extraction process. In our investigations, optimum eluent type, pH, adsorbent type, amount of adsorbent, and solution volume were assessed to achieve maximum Pb2+ and Cd2+ heavy metal ion recovery. The highest recovery rates were found to be 99.6% and 99.9% for Pb2+ and Cd2+ heavy metal ions, respectively. A repeatability test was also performed up to 5 repetitions a slight recovery rate decrease was observed.

It was illustrated that Co3O4@Fe3O4 nanoflowers are reusable, and their recovery rates do not alter drastically with repeated tests for Pb2+ and Cd2+ heavy metal ion recovery. It was also seen that the extraction time of the Co3O4@Fe3O4 nanoflowers is quite short, and that adsorption rates are extremely high. It was also found that Co3O4@Fe3O4 nanoflowers provide good enrichment in real world samples such as seawater, tap water. Obtaining a very high heavy metal ion recovery rate illustrates that Co3O4@Fe3O4 nanoflowers have the potential to be used in real world scenarios and industrial applications.

In conclusion, our work illustrates that Co3O4@Fe3O4 nanoflowers have very high adsorption potential and can be used as an adsorbent material for heavy metal recovery applications. Having magnetic characteristics also enables nanoflowers to be used in magnetic filtration applications. It was found that Co3O4@Fe3O4 nanoflowers exhibit outstanding performance; therefore, have the potential to be used in applications involving the removal of heavy metals.