Preconcentration of trace nickel lons from aqueous solutions by using a new and low cost chelating polystyrene adsorbent

2.1 Preparation of adsorbent

We applied PSAHSB adsorbent to adsorb trace amount of nickel ions from the model aqueous solutions. The chemical structure of adsorbent is illustrated in Fig. 1. The adsorbent was purchased from IGEM Laboratories, Russia. The adsorbent was synthesized according to procedures in (Basargin, 2005; Shyaa, 2012; Philippides, 1993). The synthesis included four successive stages: (i) nitration of polystyrene to polynitrostyrene; (ii) reduction of this product to polyaminopolystyrene; (iii) diazotization of the produced amino group; (iv) azo-coupling of the diazotized amino with a monomeric 4-hydroxybenzenesulfonic acid ligand. The adsorbent was insoluble in water, acids, alkalis, and organic solvents and do not swell. The absorbent was ground in an agate mortar and bolted through a a 200-mesh sieve (74 µm) (Shaaban, 2014).

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Fig. 1

Chemical structure of the PSAHB-SO₃H adsorbent.

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2.2 Structure characterization

The PSAHSB adsorbent is characterized by; (i) Fourier transform infrared (FTIR) (Perkin Elmer Spectrum BX FT-IR, Shimadzu, Kyoto, Japan) in pressed KBr pellets (spectral resolution: 1 cm − 1; (ii) Atomic force microscopy (AFM) (Multimode NS3a, No. 3327, Milwaukee, USA); (iii) Potentiometric determination of the content of chelating groups and the dissociation constants of the adsorbent.

2.3 Adsorption experiments

The adsorption of Ni (II) ions on PSAHSB adsorbent was performed by static methods under ambient conditions; 25 mg of adsorbent was added into 25 mL of Ni (II) solutions (added as Ni (NO₃)₂·6H₂O, Aldrich – Spain) in different initial concentrations with stirred at 300 rpm, then an adsorbate onto adsorbent was separated from the mixture by filtration using 0.45 μm filter paper. The pH was adjusted up to the desired value with 0.1 M HCl or 0.1MNaOH solutions by using a digital pH meter (781 pH/Ion, Metrohm, India). The adsorption isotherms, which indicate Ni (II) ions adsorption behavior, were investigated at temperatures 293.15, 313.15 and 333.15 K respectively. The residual concentration of ions in the filtrate C_e was determined by using a UV–Vis spectrophotometer (LI-295, Lasany, India) with H₂Dm Dimethylglyoxime reagent (Sigma-Aldrich, Mumbai, India) as director at λ = 445 (Shpigun et al., 1986), and by helping the earlier constructed standard calibration curves. The equilibrium adsorption capacity q_e (mg g⁻¹) and degree of adsorption (R,%) were calculated according to the Eqs. (1) and (2) (Ajmal, 2000).

2.4 Adsorption kinetics

Two kinds of kinetic models were applied to investigate the kinetic mechanism of the adsorption process. The pseudo-first-order kinetics model is one of the most broadly used to describe the adsorption of metal ions from aqueous solutions (Lagergren, 1898; Khan, 2012; Din and Mirza, 2013). The linear form of pseudo-first-order equation was expressed by Eq. (3).

The other one is the pseudo-second-order model. The linearity of this model is given by Eq. (4) (Ho, 2006).

2.5 Desorption and recycle test

The study of desorption for the regeneration of the adsorbent is necessary to make the adsorption process more economical. Therefore, desorption of adsorbate from the adsorbent was carried out by washing it to a beaker with 10 mL of 1 to 4 M concentrations of HCl and HNO₃ desorbing agents (Merck – Germany). Then, the system was mixed using a magnetic stirrer for 1 h. The degree of desorption of ions (D, %) from the adsorbent was determined with Eq. (5).

3.1 Characterization of adsorbent

3.1.1

3.1.1 FTIR analysis before and after Ni complexation

The FTIR spectrum of PSAHSB adsorbent was shown in Fig. 2(a). FTIR spectrum of this adsorbent confirmed the presence of functional groups characteristic of the adsorbent. Hydroxyl group was identified by the broadband attributed to stretching vibrations of O—H bonds in the range 3600–3100 cm⁻¹, the signals at 2925, 1600, 1450, 1110, 854, 700, cm⁻¹ associated with 2-phenol-4-sulfonic acid moiety. The peak at 1519 cm⁻¹ is assigned to the stretching vibrations of (—N⚌N—) bonds and the peak at 3060 cm⁻¹ attributed to Ar—H, signal at 1350 cm⁻¹ assigned to the stretching vibrations of C—OH in the structure of adsorbent (Mondal et al., 2002). The aliphatic structure is attributed to the vibrations of C—H bonds in the range 2940–2850 cm⁻¹. The signal 710– 680 cm⁻¹ is assigned to deformation vibrations of C—C groups in the benzene ring. Thus, the IR spectrum confirms the structure of the adsorbent presented in Fig. 1.

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Fig. 2

FT-IR spectra before (a) and after (b) adsorption of Ni (II) ions onto PSAHSB adsorbent.

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The IR spectrum of the PSAHSB adsorbent and its complex with Ni (II) is shown in Fig. 2(b). In this study, a process is proposed based on the exchange of ions by the following active chelating groups: negatively charged of a phenolic oxygen atom with positively charged of Ni(II) ions. The groups are capable of bonding Ni(II) ions during the dissociation of H + cations (Wu and Li, 2013; Kolcu, 2019). It is clear from obtained FT-IR spectra after the adsorption, that the intensity of the relevant bands is changed (hypsochromic shift). The obtained signals in IR spectra show visibly that the maximal wavenumber of the signal —OH groups, is around 3435 cm⁻¹, after adsorption of nickel ions, it is shifted to the 3385 cm⁻¹. Similarly, the maximal wavenumber of signal N⚌N groups is around 1519 cm⁻¹ before adsorption. After adsorption, it is shifted to the 1508 cm⁻¹ (Wu and Li, 2013; Kolcu, 2019).

3.1.2

3.1.2 Surface morphology

Fig. 3(a) shows the scanning force microscopy SFM image of the initial surface of the adsorbent under study (Wang et al., 1998). There is an occasional horizontal interference. This interference indicates the presence of a small number of particles with low adhesion on the surface which are displaced by the probe at scanning. Fig. 3(b) clearly shows the globular structure of the surface of the adsorbent. The adsorbent was grains with a grain size in the range of 40–170 nm, a pore size in the range of 10–100 nm, pore volume 2.032 mL/g, and surface area 970 m²/g⁻¹. This essentially led to the formation of mixed meso/macro-pores (Dutta et al., 2014).

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Fig. 3

SFM image of a adsorbent surface in 2D (a) 725 × 725 pixels (b) 1200 × 1200 pixels.

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3.1.3

3.1.3 Acidic properties of the adsorbent

The determination of acidic dissociation constants $({\mathrm{p}\mathrm{K}}_{\mathrm{i}on})$ of adsorbents plays a fundamental role in many analytical procedures such as: the examination of the physic-chemical properties of chelating adsorbents, stability and complex formation and the determination of the pH range in which active groups are ionized and capable of participating in reactions of ion exchange (Wang, 2008). The ionization constants of the adsorbent were determined by potentiometric titration following the standard procedure (Doshi, 2017; Orth, 2016; Basargin, 2006). Experiments were performed with the H⁺ form of the PSAHSH adsorbent and with total static capacity (SC) = 1.64 mmol g⁻¹ (Experiment S1 and Table S1). The method is as follows:0.1 g portions of the adsorbent were placed in 40-mL bottles (10bottels), 20 mL of 1 M NaCl (to create the ionic strength μ = 1) was added to each bottle and bottles were left for 90 min. Then, different volumes of 0.02 M NaOH solutions were added to the bottles, so that the degree of neutralization of active acidic groups (α) of the adsorbent varied from 0 to SC value. At last, the mixtures were kept at 293.15 K for 24 h in a desiccator full of nitrogen to attain the ionic equilibrium of the replacement of the chelating group protons in the adsorbent by sodium cationsn in NaOH solution.

The potentiometric titration results (Table S2) were plotted as a differential titration curve in the coordinates ΔpH/Δ α–α (see Fig. 4). It can be noticed that the differential curve exhibits three jumping positions (inflection points), which can be attributed to the ionization of the betainic protons of the ionogenic groups: N⚌N—, —SO₃H and —OH groups during the titration of adsorbent. These results unambiguously confirm the chemical structure of adsorbent in Fig. 1. The α values for each acidic group and their corresponding pH values (Table S3) were used for constructing pH vs. log α/(1 – α) dependences Fig. 5. The pK_ion can be determined either graphically by the plot in Fig. 5 or mathematically by using the Henderson–Hasselbalch equation (Eq. (6)) (Po and Senozan, 2001).

(6)

$$p{K}_{\mathit{ion}}=pH-n.log\left(\frac{\alpha }{1-\alpha }\right)$$ where n is the slope of the straight line in Fig. 5.

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Fig. 4

(a) Integral and (b) differential curves of potentiometric titration for adsorbent.

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Fig. 5

Determination of dissociation constants for PSAHSB by the graphical method.

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The pK_ion values were tabulated in Table 1. It is clear that the pK_ion values are relatively strong. They lead to a higher affinity of Ni (II) ions towards the adsorbent, and that made the adsorption faster and form more stable complex.

Table 1 Ionization constants (pK_ion) of chelating groups of PSAHSB adsorbent.

Ionization stages of chelating groups	pKion (graphical)	pKion (calculated), n = 3¶
		$\overline{x}\pm$ st/ $\surd$ n(±RSD %)
pKi1 of 1st IS of —N⚌N—	4.33	4.31 ± 0.12 (±2.83%)
pKi2 of 2nd IS of —SO3H	6.33	6.35 ± 0.04 (±0.61%)
pKi3 of 3th IS —OH	7.95	7.68 ± 0.03 (±0.32%)

¶ The calculated results were expressed as $\overline{\mathrm{x}}$  ± st/n^1/2 where $\overline{\mathrm{x}}$ is the mean of n observations, s is the standard deviation, t is distribution value chosen for 95% confidence level and S_r is the relative standard deviation.

3.2 Effect of pH and initial Ni (II) concentration

The pH of the nickel solution plays a major role in all adsorption process, especially on the adsorption capacity. The degree of adsorption is significantly affected by the surface charge on the adsorbent, which is highly dependent on the pH of the solution (Leyva-Ramos, 1989). The optimal pH for the ideal adsorption was determined experimentally from the plot of the adsorption degree (R %) vs pH in the range of 1 to 10. The Fig. 6 shows the effects of pH on the adsorption of Ni (II) ions onto PSAHSB adsorbent. The adsorption degree (R, %) was calculated to be ≥98% in the pH range of 3 to 7. The adsorption of trace amount of nickel ions increased sharply within the pH range of 1–3, then remained constant within the 3–7 pH range, but decreased sharply at pH large than 8, which demonstrated that the adsorption was strongly conditioned by the pH. At low pH, the R% was low also, due to the increase in the positive charges on active chelating sites of N⚌NH⁺, C—OH₂⁺ in acidic form, leading up to electrostatic repulsion between the Ni ²⁺ ions and these active sites on the adsorbent surface. The increase in pH leads to decrease in electrostatic repulsion because of the reduction in the positively charged surface, thus resulting in higher adsorption (Özcan et al., 2004). In the basic solution at pH large than 7, the adsorbent surface becomes negatively charged, In addition, the degree of adsorption decreased rapidly where Ni ²⁺ ions precipitated as nickel hydroxide. As shown in Fig. 6, it is clear that the adsorption of Ni ions increased with an increase in initial Ni concentration. Increasing concentration gradient acts as increasing the driving force, and in turn, leads to an increase in equilibrium adsorption until adsorbent saturation is reached.

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Fig. 6

Effect of pH on Ni(II) adsorption on 20 mg PSAHSB, 300 rpm, 90 min & 293.15 K.

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3.3 Effect of contact time

The adsorption of Ni (II) on the adsorbent was achieved in less than 20 min at 293.15 K as shown in Fig. 7. The removal amounts of Ni ions by PSAHSB adsorbent were maintained in the same rate even extending the contact time. Noteworthy, the adsorption degree was calculated to be ≥98% at a concentration of 0.98 μg ^_ mL⁻¹ of nickel ions. Based on these results, PSAHSB adsorbent was proved to be an efficient adsorbent possessing relatively large adsorption capacity. An increase in the temperature to 333 K shortened the time of adsorption but it led to the partial degradation of formed complex and a decrease in the value of R as shown in Fig. 7.

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Fig. 7

Effect of contact time on the adsorption equilibrium at different temperatures, PH = 5.5.

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3.4 Adsorption kinetic

The liner plot of $log(q_e-q_t)$ vs. t and t/q_m vs. t for the pseudo-first-order model and the pseudo second-order model respectively were shown in Fig. 8(a and b). The parameters of the two models were shown in Table. 2. The obtained results suggest that the kinetic data are fit with the pseudo second-order kinetic model. Comparison of experimental and calculated q_e values, the q_e of this model is approximate to the experimental data and the R² is 0.999, which is very close to1. It is clear from Table. 2, that the rate constant of the pseudo second-order model (k₂) is very small (0. 0.006 g/min.mg), indicating that the adsorption equilibrium can occur in a short time.

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Fig. 8

Kinetic model for Ni(II) adsorption. (a) Pseudo-firstorder, (b) pseudo-second order models [initial Ni(II) conc. = 25 mg/L; contact time = 5–90 min; T = 293; V = 25 mL; pH = 6].

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3.5 Adsorption isotherm

The fit of adsorption data to an isotherm is important to both theoretical and practical applications. In order to optimize the adsorption system, it is essential to establish the most appropriate correlations for the equilibrium data of each system (Altınışık et al., 2010). Equilibrium isotherm equations are used to describe the experimental adsorption data, and to understand the adsorption mechanism (Bulut et al., 2008). In this work, three different temperatures at initial pH = 6 ± 0.02 were used to describe the adsorption isotherms which was illustrated in Fig. 9(a).

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Fig. 9

(a) Adsorption isotherm at three different temperatures, (b) Langmuir and (c) Freundlich isotherms for Ni(II) ions adsorption on PSAHSB adsorbent [initial Ni(II) conc. = 0.01–1.5 mg/L; pH = 6; Adsorbent dose = 25 mg; t = 20 min].

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The Langmuir and Freundlich models are adopted to correlate the experimental data (Chen and Wang, 2006; Wang, 2012). The Langmuir model is based on the assumption that the maximum adsorption occurs when a saturated monolayer of solute molecules is present on the adsorbent surface, the energy of adsorption is constant and there is no migration of adsorbate molecules in the surface plane. The equation of the Langmuir adsorption model was expressed in linear form as (Eq. (7))

The Freundlich model assumes that different sites with several adsorption energies are involved. The equation of Freundlich adsorption model was expressed in linear form as Eq. (8) (Bulut et al., 2008).

The experimental data of Ni (II) ions adsorption (Fig. 9) (a) were regressively analyzed with the Langmuir and Freundlich adsorption model (Fig. 9b & c) with the relative values calculated from the two models in Table 3. It can be concluded that Langmuir adsorption model was coincides with the experimental data better than Freundlich model,which indicates that the Langmuir model with complete monolayer coverage of the particles can fit the experimental data very well (Tan, 2007).

3.6 Adsorption Thermodynamics**

The thermodynamic parameters including free energy $\left(\mathrm{\Delta }{G}^0\right)$ , enthalpy $\left(\mathrm{\Delta }{H}^0\right)$ and entropy $\left(\mathrm{\Delta }{S}^0\right)$ can be calculated from the slope and intercept of the plot of lnK_d vs. 1/T (Fig. 10) by applying the following Eq. (9) & Eq. (10) equations:

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Fig. 10

Effect of temperature on the distribution coefficients of Ni onto PSAHSB adsorbent at different initial Ni solution concentrations.

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The thermodynamic parameters were collected as shown in Table 4.The positive values of ΔH⁰ show an endothermic process (Hosseini-Bandegharaei, 2011), where the Ni (II) ions were solvated in water and when these ions get adsorbed, they require energy to complete their hydration process (Chen and Wang, 2006). The ΔG⁰ was negative as expected for a spontaneous process under the conditions applied. The decreases in free energy with the increase of T indicated more efficient adsorption at high temperature. The positive value of ΔS⁰ reflects the affinity of PSAHSB toward Ni (II) ions in aqueous solutions (Bulut et al., 2008; Genç-Fuhrman et al., 2004; Soner Altundoğan, 2000). The result of Ni (II) ions adsorption on PSAHSB was a spontaneous and endothermic process.

3.7 Desorption and reusability

As shown in Fig. 11(a), the HNO₃ is a weak desorbing agent. This is evidence of the strong bond between Ni ions and the adsorbent. Only when10ml 2 M HCl acid was used; a significant increase in the degree of desorption was realized. It was found that the desorption degree increased with increasing the HCl concentration. Desorption mechanism is based on the exchange of hydrogen ions with the Ni (II) ions.

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Fig. 11

(a) Desorption test and (b) Adsorption–desorption cycles of adsorption of Ni on PSAH adsorbent.

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On the other hand, results presented in Fig. 11(b) show the cyclic adsorption–desorption performance to estimate the ability of regeneration. The adsorption degree showed consistently at a level of ˃90% for each cycle after desorption with 12 cycles. The results proved the high regeneration ability of the adsorbent.

3.8 Effect of coexisting ions

The effects of coexisting ions such as sodium, potassium, calcium, barium, iron (II) and (III), magnesium, chromium and cadmium, usually found in natural samples were examined using model solutions containing 10 μg of Ni(II)) and excess matrix ions in 50 mL of model solutions. The tolerance limits (error ≤ 4.3%) are given in Table 5. The results show that the presence of high concentrations of major cations has no obvious effect on the studied Ni(II)) ion recoveries.

3.9 Analytical applications

The analytical applicability of modified PSAHSB adsorbent was tested for the collection of metal ions from real samples (Tap water, Potatoes and Carrot) obtained from Hodeida city, Yemen. The real samples tested by direct and standard addition through batch mode under optimized conditions. The Ni (II) ions were pre-concentrated and determined. The results (Table 6), showing that recoveries of Ni (II) ions (≥95%) could be made with a good precision of RSD < 5%, indicate the reliability of the present method for metal analyses in the real samples without significant interference.