Facile synthesis of a novel nanocomposite for determination of mercury and copper ions in food and water samples

2.1 Chemicals

Sodium hydroxide (NaOH), (3-aminopropyl)trimethoxysilane (C₆H₁₇NO₃Si), hydrochloric acid (HCl), 3-bromo-5-chlorosalicylaldehyde (C₇H5BrClO₂), toluene (C₇H₈), ethanol (C₂H₆O), mercury(II) chloride (HgCl₂), copper(II) chloride dihydrate (CuCl₂·2H₂O), nitric acid (HNO₃), potassium nitrate (KNO₃), thiourea (CH₄N₂S), and ethylenediaminetetraacetic acid disodium salt dihydrate (C₁₀H₁₈N₂Na₂O₁₀) were obtained from Sigma Aldrich Company and utilized without additional refining. The rice husk was obtained from a rice mill located near the Egyptian city of Zagazig. Besides, aluminum cans were obtained from a local market in Zagazig city, Egypt.

2.2 Synthesis of composite

Firstly, sodium aluminum silicate hydrate sample was synthesized as described by Ehab et al (Abdelrahman et al., 2021). The reaction between sodium aluminum silicate hydrate and (3-aminopropyl)trimethoxysilane was carried out as described by Khalifa et al but with slight modifications (Khalifa et al., 2020). 2 g of sodium aluminum silicate hydrate and 2.20 mL of (3-aminopropyl)trimethoxysilane was refluxed for 24 hrs using 50 mL of toluene. The obtained product was separated utilizing a centrifuge, washed with ethanol, and dried at 65 °C for 24 hrs. Besides, 1 g of sodium aluminum silicate hydrate/(3-aminopropyl)trimethoxysilane sample, 1 g of 3-bromo-5-chlorosalicylaldehyde, and 50 mL of ethanol were mixed then transferred to a 100 mL Teflon-lined stainless-steel autoclave for hydrothermal treatment at 120 °C for 3 hrs utilizing a microwave oven. The product was separated with a centrifuge, washed with ethanol, and then dried at 55 °C for 24 hrs. Scheme 1 represents the steps required for the modification of sodium aluminum silicate hydrate by 3-bromo-5-chlorosalicylaldehyde.

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Scheme 1

Modification of sodium aluminum silicate hydrate by 3-bromo-5-chlorosalicylaldehyde.

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2.3 Instrumentation

X-ray diffraction pattern of the synthesized composite was obtained using a D8 Advance X-ray diffractometer. Infrared spectrum of the synthesized composite was obtained using a Bruker FT-IR spectrometer. The surface morphology of the synthesized composite was obtained using a JEOL 2000 scanning electron microscopy. The morphology of the synthesized composite was obtained using an EM-2100 high-resolution transmission electron microscopy. The percentages of CHN of the synthesized composite was obtained using a 2400 PerkinElmer CHN Elemental Analyzer. Utilizing a 210 Buck Scientific atomic absorption spectrometer with an acetylene–air flame, the concentration of copper and mercury ions was determined. The wavelengths for mercury and copper were 184.89 and 324.80 nm, respectively.

2.4 Removal of mercury and copper ions from aqueous media

0.05 g of the synthesized composite was mixed with 50 mL of 150 mg/L of the copper or mercury solution at different pH (2–8) to investigate the effect of pH. After stirring the mixture for 3 hrs, the composite was separated using a centrifuge. Using an atomic absorption spectrometer, the final concentration of the copper or mercury ions in the filtrate was estimated. 0.1 M NaOH or HCl was used for modifying the pH. The effect of time was examined by mixing 0.05 g of the synthesized composite with 50 mL of 150 mg/L of the copper or mercury solution at pH = 6 for different times (5–120 min). After certain intervals of stirring, the composite was separated. Using an atomic absorption spectrometer, the final concentration of the copper or mercury ions in the filtrate was estimated. The effect of temperature was examined by mixing 0.05 g of the synthesized composite with 50 mL of 150 mg/L of the copper or mercury solution at pH = 6 and different temperatures (298–322 K). After 80 min of stirring, the composite was separated. Using an atomic absorption spectrometer, the final concentration of the copper or mercury ions in the filtrate was estimated. The effect of concentration was examined by mixing 0.05 g of the synthesized composite with 50 mL of different concentrations (5–200 mg/L) of the copper or mercury solution at a pH = 6. After 80 min of stirring, the composite was separated. Using an atomic absorption spectrometer, the final concentration of the copper or mercury ions in the filtrate was estimated. The amount of removed mercury or copper ions per gram of the composite (Q, mg/g) was determined using equation (1).

The percent removal (% R) of mercury or copper ions was determined using equation (2).

C_i (mg/L) is the initial concentration of mercury or copper ions. C_e (mg/L) is the equilibrium concentration of mercury or copper ions in the filtrate. V (L) is the volume of mercury or copper solution. M (g) is the quantity of the composite. To assess the effect of desorption, 0.05 g of the synthesized composite was mixed for 80 min with 50 mL of 5 mg/L of the mercury or copper solution at a pH = 6. The composite was then separated using a centrifuge. Besides, the ions adsorbed on the synthesized composite were desorbed by shaking the loaded composite for 15 min with 5 mL of 0.5 M desorbing agents, such as thiourea, EDTA disodium salt, and nitric acid. The percentage of desorption (% D) was determined using equation (3).

The concentration of mercury or copper ions in the extractant is denoted by C_d (mg/L). Besides, V_d (L) is the volume of the extractant. To assess the reusability of the synthesized composite, five adsorption/desorption cycles utilizing nitric acid as a desorbing agent were performed in succession. After each run, the removal percentage (% R) was determined using equation (1). The method described by Khalifa et al (Altowayti et al., 2021) was used to determine the point of zero charge (pH_PZC) of the synthesized composite: Using 0.1 M hydrochloric acid or sodium hydroxide, the initial pH value of 50 mL of 0.025 M potassium nitrate solutions was adjusted to different values (pH = 2–12). Besides, 0.10 g of the composite was added to each potassium nitrate solution, followed by 6 hrs of stirring. After separating the composite from the liquid phase, the final pH value (pH_final) of the filtrate was measured. The pH_final values were plotted versus the pH_initial values. pH_pzc is the pH_final value at which a typical plateau is obtained (Altowayti et al., 2021).

2.5 Actual real sample collection and pretreatment

Several water samples were collected in polypropylene flasks previously cleaned with 10 % nitric acid. Before putting the samples in the refrigerator, 0.45 m membranes were used to filter them. In Benha City, Egypt, food samples (fish, spinach, and chicken) were acquired from a local market. Before each sample was sliced into little pieces, the edible component was extracted and cleaned with distilled water. After 12 hrs of drying at 60 °C, the samples were ground until uniform and then placed in polyethylene grabs. To expedite the procedure, microwave-assisted acid digestion was used to digest the materials. In separate Teflon digesting tubes, 0.5 g of dry sample was inserted, followed by the addition of 2 mL of 30 % hydrogen peroxide and 4 mL of nitric acid. After 15 min at room temperature, the tubes were hermetically sealed and heated using the one-step approach described below (power: 1650 W; temperature: 200 °C; ramp time: 16 min; cooling time: 16 min; hold time: 16 min). After bringing the test tubes to room temperature, nearly all of the solutions were evaporated and diluted with distilled water to 50 mL.

2.6 Preconcentration process

In a conical flask, 30 mL of digested solution or water sample was combined with 0.05 g of the synthesized composite. The pH of the mercury or copper solution was adjusted to 6.0 using sodium hydroxide and/or hydrochloric acid solutions, and the volume was increased to 50 mL using distilled water. In addition, the contents were stirred for 80 min before the adsorbent was separated and transferred to a 25 mL beaker. After adding 5 mL of 0.5 M nitric acid, the loaded composite was agitated for 10 min to allow the mercury or copper ions to desorb. Using an atomic absorption spectrometer, the concentration of mercury or copper ions in the filtrate was determined.

3.1 Characterization of the synthesized composite

Fig. 1. A–B shows the X-ray diffraction patterns of sodium aluminum silicate hydrate and sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite, respectively (Abdelrahman et al., 2021). The sodium aluminum silicate hydrate peaks at 2Θ = 13.9°; 24.3°, 31.6°, 34.7°, 37.4°, 42.8°, 52.0°, 58.2°, 62.0°, and 63.9° correspond to lattice plans (1 1 0), (2 1 1), (3 1 0), (2 2 2), (3 2 1), (4 1 1), (5 1 0), (4 4 0), and (6 1 1), respectively as obtained from JCPDS No. 42–0216 (Abdelrahman et al., 2021). The XRD pattern of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite shows a halo at 2Θ = 25°, confirming the destruction of a crystalline structure of sodium aluminum silicate hydrate owing to the association of it with an amorphous background (Khalifa et al., 2020).

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Fig. 1

The X-ray diffraction patterns of the sodium aluminum silicate hydrate (A) and sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite (B).

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Fig. 2 shows the FT-IR spectrum of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite. The bands, which were observed at 3450 and 1650 cm⁻¹, are due to the stretching and bending vibration of OH and/or C = N, respectively. The bands, which were observed at 3085 and 3003 cm⁻¹, are due to the stretching vibration of aromatic and aliphatic CH, respectively. The bands, which were observed at 760 and 806 cm⁻¹, are due to the out of plane bending vibration of aromatic CH. The band, which was observed at 1348 cm⁻¹, is due to the bending vibration of CH. The bands, which were observed at 1496, 1552, and 1597 cm⁻¹, are due to the stretching vibration of aromatic C = C (Abdelrahman et al., 2021; Khalifa et al., 2020). Moreover, the bands, which were observed at 1450 and 1095 cm⁻¹, are due to the external and internal asymmetric stretching of Y-O-Y (Y = Si and/or Al), respectively. Also, the bands, which were observed at 695 and 645 cm⁻¹, are due to the external and internal symmetric stretching of Y-O-Y ((Y = Si and/or Al), respectively. Besides, the bands, which were observed at 560 and 466 cm⁻¹, are due to the double ring vibration and bending vibration of Y-O-Y (Y = Si and/or Al), respectively (Abdelrahman et al., 2021; Khalifa et al., 2021).

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Fig. 2

The FT-IR spectrum of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

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The elemental analysis of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite shows carbon, hydrogen, and nitrogen content of 22.35 %, 2.94 %, and 2.07 %, respectively. The presence of carbon, nitrogen, and hydrogen in the composite is further proof that organic substance (composed of these components) was effectively loaded onto sodium aluminum silicate hydrate (Khalifa et al., 2020). The SEM and TEM investigations suggest that the composite's structure resembles that of cotton. As seen in Figs. 3 and 4, this indicates that the crystalline phase is associated with an amorphous background (Khalifa et al., 2020).

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Fig. 3

The FE-SEM image of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

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Fig. 4

The HR-M image of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

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3.2 Removal of mercury and copper ions from aqueous media

3.2.1

3.2.1 Effect of pH

The influence of pH on the % R of mercury or copper ions and the adsorption capacity of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite are illustrated in Fig. 5A-B, respectively. If the pH is raised from 2 to 6, both the removal rate and adsorption capacity improve substantially. In addition, it has been demonstrated that a slight increase occurs when the pH changes from 6 to 8. Therefore, pH = 6 was chosen as the optimal pH for further tests. At pH = 6, the removal percentages of mercury and copper ions are 69.67 and 85.29 %, respectively. At pH = 6, the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite has an adsorption capacity of 104.50 mg/g for mercury ions and 127.94 mg/g for copper ions. The pH_final versus pH_initial curve for several potassium nitrate solutions is clarified in Fig. 5C to get the point of zero charge (pH_PZC) (Abdelrahman et al., 2021; Khalifa et al., 2020). Also, the results confirmed that the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite point of zero charge is 3.18. It was found that when the pH of the mercury or copper solution is less than 3.18, the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite has a positive charge owing to the presence of positive hydrogen ions which repelled the mercury or copper ions, resulting in a decrease in the removal percentage and the adsorption capacity. Besides, when the pH of the mercury or copper solution is greater than 3.18, the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite has a negative charge owing to the presence of negative hydroxide ions which are attracted to the mercury or copper ions and hence increase the removal percentage and the adsorption capacity (Khalifa et al., 2020).

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Fig. 5

The influence of pH of the mercury or copper solution on % removal (A) and adsorption capacity (B). The plot of pH_final versus pH_initial (C).

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3.2.2

3.2.2 Effect of time

The influence of time on the percent removal of mercury or copper ions and the adsorption capacity of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite are illustrated in Fig. 6-B, respectively. If the time is raised from 5 to 80 min, both the removal rate and adsorption capacity improve immensely. In addition, it has been demonstrated that there is a slight increase when the time is raised from 80 to 120 min due to the saturation of active sites. As a result, time = 80 min was chosen as the optimal duration for future tests. At time = 80 min, the percent removal of mercury and copper ions is 70.67 and 86.00 %, respectively. At time = 80 min, the composite's adsorption capacities for mercury and copper ions are 106.0 and 129.0 mg/g, respectively.

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Fig. 6

The influence of contact time of the mercury or copper solution on % removal (A) and adsorption capacity (B).

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Furthermore, two kinetic models were utilized to investigate the influence of contact time: pseudo-second-order (Equation (4)) and pseudo-first-order (Equation (5)) (Khalifa et al., 2020).

(4)

$$\frac{t}{Q_t}=\frac{1}{K_2{Q}_e^2}+\frac{1}{Q_e}t$$

(5)

$$\log \left(Q_e-Q_t\right)=log{Q}_e-\frac{K_1}{2.303}t$$

Q_e (mg/g) is the equilibrium adsorption capacity of the synthesized composite. Q_t (mg/g) is the adsorption capacity of the synthesized composite towards the mercury or copper ions at contact time t. The rate constant of the pseudo-first-order model is denoted by K₁ (1/min), while the rate constant of the pseudo-second-order model is denoted by K₂ (g/mg.min). The pseudo-first-order and pseudo-second-order models are clarified in Fig. 7A-B, respectively. According to Table 1, the correlation coefficients (R²) of the pseudo-first-order model are less than those of the pseudo-second-order model. Moreover, the pseudo-second-order adsorption capacity is more in line with experimental adsorption capacity than the pseudo-first-order adsorption capacity. Consequently, the pseudo-second-order model provided a more accurate explanation for the kinetic data than the pseudo-first-order model. Thus, it appears that the chemical reaction is involved in the rate-controlling process (Khalifa et al., 2020). Nonlinear kinetic models were studied as described by Altowayti et al. (Figures omitted for brevity). The R² value derived from the nonlinear plot is much smaller than that derived from the linear model. The results of this study showed that the linear method is a good way to show the kinetic data (Altowayti et al., 2021).

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Fig. 7

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

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Table 1 Kinetic constants for the removal of mercury and copper ions using the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

Metal ion	Pseudo first order				Pseudo second order
	Qe (mg/g)		K1 (1/min)	R2	Qe (mg/g)		K2 (g/mg.min)	R2
	Calculated	Experimental			Calculated	Experimental
Hg(II)	53.52	106.00	0.0278	0.9749	102.78	106.00	0.00175	0.9968
Cu(II)	61.22	129.00	0.0266	0.9363	123.92	129.00	0.0016	0.9995

3.2.3

3.2.3 Effect of temperature

The influence of temperature on the percent removal of mercury or copper ions and the adsorption capacity of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite are illustrated in Fig. 8A-B, respectively. If the temperature is raised from 298 to 328 K, both the removal percentage and adsorption capacity drop. Therefore, temperature = 298 K was chosen as the optimal temperature for the subsequent studies. The removal percentages of mercury and copper ions at 328 K are 29.83 and 52.97 %, respectively. At 328 K, the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite had an adsorption capacity of 44.75 mg/g for mercury ions and 79.45 mg/g for copper ions. Utilizing equations (6) and (7), the thermodynamic parameters such as change in entropy (ΔS^o), change in enthalpy (ΔH^o), and change in free energy (ΔG^o) were calculated (Khalifa et al., 2020).

(6)

$$\mathrm{l}\mathrm{n}{K}_d=\frac{\mathrm{\Delta }{S}^o}{R}-\frac{\mathrm{\Delta }{H}^o}{\mathit{RT}}$$

(7)

$${\mathrm{\Delta }G}^o=\mathrm{\Delta }{H}^o-T{\mathrm{\Delta }S}^o$$

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Fig. 8

The influence of temperature of the mercury or copper solution on % removal (A) and adsorption capacity (B). The plot of lnK_d versus temperature.

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T is the adsorption temperature (K) whereas K_d is the distribution constant (L/g). R is a gas constant (kJ/mol K). Utilizing equation (8), the distribution constant was determined (Khalifa et al., 2020).

(8)

$$K_d=\frac{Q_e}{C_e}$$

The plot of lnK_d versus temperature is illustrated in Fig. 8C. Table 2 is a listing of thermodynamic parameters. The fact that the enthalpy value exceeds 40 kJ/mol indicates that the adsorption of mercury or copper ions by the synthesized composite is chemical in nature as shown in Scheme 2 (Khalifa et al., 2020). Owing to the negative sign of enthalpy, the adsorption of mercury or copper ions is exothermic (Khalifa et al., 2020). Besides, owing to the negative sign of Gibbs free energy, the spontaneous adsorption of mercury or copper ions occurs when the synthesized composite is used. At the solution boundary/composite, the adsorption of mercury or copper ions occurs disorderly owing to the positive sign of entropy (Khalifa et al., 2020).

Table 2 Thermodynamic parameters for the removal of mercury and copper ions using the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

Metal ion	ΔGo (KJ/mol)				ΔSo (KJ/molK)	ΔHo (KJ/mol)
	Temperature (kelvin)
	298	308	318	328
Hg(II)	−91.36	−92.86	−94.35	−95.85	0.1498	−46.71
Cu(II)	−84.94	−86.29	−87.65	−89.00	0.1355	−44.56

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Scheme 2

Adsorption mechanism of mercury and copper ions onto the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

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3.2.4

3.2.4 Effect of concentration

The influence of concentration on the percent removal of mercury or copper ions and the adsorption capacity of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite are illustrated in Fig. 9A-B, respectively. The percentage of mercury or copper ion removal reduces as the concentration rises from 5 to 200 mg/L, although the adsorption capacity rises (Khalifa et al., 2020).

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Fig. 9

The influence of concentration of the mercury or copper solution on % removal (A) and adsorption capacity (B).

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Using Freundlich (Equation (9)) and Langmuir (Equation (10)) equilibrium isotherms, the concentration data were examined (Khalifa et al., 2020).

(9)

$$\mathit{ln}{Q}_e=ln{K}_F+\frac{1}{n}ln{C}_e$$

(10)

$$\frac{C_e}{Q_e}=\frac{1}{K_L{Q}_m}+\frac{C_e}{Q_m}$$

The maximum adsorption capacity of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite is denoted by Q_m (mg/g). The Langmuir constant is K_L (L/mg), while the Freundlich constant is K_F ((mg/g)(L/mg)^1/n). The heterogeneity constant is 1/n. Using equation (11), the Freundlich isotherm can be used to determine Q_m (Khalifa et al., 2020).

(11)

$$Q_m=K_F\left(C_i^{1/n}\right)$$

The Langmuir and Freundlich isotherms are clarified in Fig. 10A-B, respectively. According to Table 3, the Freundlich isotherm correlation coefficients (R²) are smaller than Langmuir's. Consequently, the Langmuir isotherm represented equilibrium data more accurately than the Freundlich isotherm. The composite's maximal absorption capacity for mercury and copper ions is 107.53 and 130.89 mg/g, respectively. The maximum adsorption capacity of sodium aluminum silicate hydrate for mercury and copper ions is 60.34 and 85 mg/g, respectively. This confirms the effectiveness of the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite owing to the ability of 3-bromo-5-chlorosalicylaldehyde to remove mercury or copper ions through the formation of chelates.

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Fig. 10

The Langmuir (A) and Freundlich (B) isotherms.

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Table 3 Equilibrium constants for the removal of mercury and copper ions using the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite.

Metal ion	Langmuir			Freundlich
	Qm (mg/g)	KL (L/mg)	R2	Qm (mg/g)	KF (mg/g)(L/mg)1/n	R2
Hg(II)	107.53	0.4997	0.9997	186.93	23.88	0.8736
Cu(II)	130.89	0.5872	0.9967	235.97	35.23	0.9554

3.2.5

3.2.5 Effect of desorption and reusability

Fig. 11 illustrates the relationship between the amount of desorption and a few desorbing agents. Desorbing agents included EDTA disodium salt, thiourea, and nitric acid at a concentration of 0.5 M. Thiourea is bidentate ligand while EDTA is a hexadentate ligand forming highly stable complexes with metal ions in aqueous solution. So, they have the affinity to uptake studied metal ions from the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite. Also, the use of nitric acid increases remarkably the concentration of hydrogen ion leading to desorption of studied metal ions from the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite due to the instability of the formed complexes at highly acidic solutions. The results reveal that 0.5 M nitric acid is the optimum desorbing agent for extracting the greatest quantity of investigated ions from the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite. Furthermore, Fig. 12 illustrates the relationship between percent removal and cycle number. The small decrease in removal percentage shows that the sodium aluminum silicate hydrate/3-bromo-5-chlorosalicylaldehyde composite can be effectively regenerated and reused for the removal of mercury and copper ions from aqueous media.

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Fig. 11

The plot of % desorption versus several desorbing solutions.

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Fig. 12

The plot of % removal versus the cycle number.

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3.3 Application

Prior to atomic absorption spectrometer examination, the proposed separation method was utilized to preconcentrate mercury and copper ions in real samples (sea water, river water, fish muscles, spinach leaves, and chicken muscles). The findings of the preconcentration of mercury and copper ions, as well as the recoveries for the spiked samples, are summarized in tables 5 and 6, respectively. The recovery results indicate the process's precision, adaptability, and quantitative separation (greater than95%). In addition, the % RSD was less than 3.5%, showing excellent reproducibility. Standard solutions of each metal ion were processed at the optimum conditions to construct the calibration curves. The dynamic analytical ranges are 0.8–380 μg/L and 1.00–550 μg/L for copper and mercury ions, respectively. The preconcentration factor is 10.