Synthesis and surface protection of nano zerovalent iron (NZVI) with 3-aminopropyltrimethoxysilane for water remediation of cobalt and zinc and their radioactive isotopes

Abstract

A method is described to synthesize a novel magnetic nano-sorbent via surface modification and protection of nano-zerovalent iron (NZVI) with 3-aminopropyl trimethoxysilane for the formation of NZVI-NH₂. The produced magnetic nanomaterial was characterized by FT-IR, SEM, TEM, TGA, surface area analysis (BET) and zeta potential. Based on the TEM analysis, the particle size of the NZVI-NH₂ sorbent was identified in the range 45–46 nm. The NZVI-NH₂ sorbent was examined to evaluate its adsorption characteristics towards cobalt(II), zinc(II) and their radioactive isotopes in aqueous solutions and water samples using the batch equilibrium and microcolumn techniques. The metal capacity values of Co(II) and Zn(II) were optimized in the presence of different physico-chemical parameters such as pH, contact time, sorbent dose, metal ion concentration and interfering ions on the adsorption process. We also aimed to explore, investigate and evaluate using the microcolumn technique the potential applications of the NZVI-NH₂ adsorbent for remediation of cobalt(II), zinc(II) and their radioactive isotopes from radioactive wastewater samples.

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1. Introduction

Nano zerovalent iron (NZVI) is known as a highly efficient magnetic adsorbent for removal of organic and inorganic pollutants.^1,2 NZVI has a high reducing capability and large specific surface area.^3,4 It has been frequently used for reduction of a wide variety of pollutants, such as trichloroethylene,⁵ chlorinated compounds,⁶ phenolic,⁷ some aromatic compounds, nitrate and heavy metallic ions.^8–12

NZVI has some advantages of mild reaction conditions, rapid reaction rates, low cost, and suitable for green process technology that does not need additional chemical additives.^13–16 Although NZVI is well known and applied in remediation of contaminants, there are still some challenges associated with its application. The aggregation of nano-particles may minimize their reactivity and mobility.^17,18 Due to its high reactivity, NZVI can be easily oxidized forming an oxide layer which blocks the active surface sites.¹⁸ Therefore, controlling the aggregation and oxidation of NZVI are the main concerns. To overcome these obstacles, immobilizing of NZVI over supports provides another solution. Different ways to protect NZVI surface were done like using amorphous activated carbon,¹⁹ nano-silica SBA-15,²⁰ poly(vinyl alcohol) microspheres,²¹ alginate bead,²² PEG/nylon membrane.²³ Also, supporting materials such as bentonite, chitosan and kaolinite have been used to immobilize NZVI and keep the reactivity of the nano-material to adsorb metal ions and organic pollutants from water.^23–26 Such surface modification does not affect the magnetic properties of NZVI which facilitate its magnetic separation.^27–31 The chemical agents which could be used for surface modification should fulfill some criteria. These criteria include the ability to form a bond with NZVI surface, and it should have a suitable functional group which can bond to the target metal ion. Silylating agents meet these requirements, and hence can be considered as a potential surface modification agents. Silylating agents are compounds which have two parts, one of them is organic chain and the other part is hydrolyzable substituents.³² The general structure of silane coupling agent is R–(CH₂)_n–Si–X₃ where R is an organofunctional group, (CH₂)_n is a linker attached to silicon atom and X is a hydrolyzable group like ethoxy or methoxy groups. Different silylating agent are commonly used to modify the surface of nano-materials to enhance the adsorption of metal ions and inorganic pollutant in aqueous solutions.³² Silylating agents are known to have the ability to form complexes with metal ions, act as a corrosion inhibitors and can be used for modification of clays such as montmorillonite and bentonite to increase their capacities.^34,35

In the present study, 3-aminopropyltrimethoxysilane (silylating agent) was chosen and used as a surface modifying agent for NZVI, where the methoxy groups are hydrolyzed to silanol containing species which can bonded to the surface of NZVI, and the other side (amino groups) can chelate the target metal ion. This paper deals with designing a new surface-modified-nano zerovalent iron to remove zinc(II) and cobalt(II) as well as their radioactive isotopes from liquid solutions by immobilization of 3-aminopropyltrimethoxysilane on the surface of nano zerovalent iron as a core–shell structure (NZVI-NH₂) to prevent the oxidation process and increase the reactivity of the surface.

2. Experimental

2.1 Materials

Unless otherwise stated, the chemicals used were all of AR purity grade. Ferric chloride hexahydrate (FeCl₃·6H₂O, FW = 270.3 and 97%) and sodium borohydride (NaBH₄, FW = 37.3 and 98%) were purchased from Oxford, India. Sodium acetate anhydrous (CH₃–COONa, FW = 82.03 and 99%), 3-aminopropyltrimethoxysilane (H₂N(CH₂)₃Si(OCH₃)₃ FW = 221.37 and 99%) were purchased from sigma Aldrich, USA. Cobalt chloride (CoCl₂, FW = 129.84 and >97.0%) and zinc chloride (ZnCl₂, FW = 136.30 and >98.0%) were purchased from BDH, UK. Disodium ethylenediaminetetraacetate dihydrate (C₁₀H₁₄N₂Na₂O₈·2H₂O, FW = 372.24 and 99.0–100.5%), hydrochloric acid (HCl, FW = 36.46 and 37%) and toluene (HPLC grade) were purchased from Sigma Aldrich, USA.

2.2 Synthesis

2.2.1 Synthesis of nano zero valent iron (NZVI). The nano zerovalent iron (NZVI) employed in the present work was synthesized according to the procedure reported by Kržišnik et al.²⁷

2.2.2 Synthesis of NZVI immobilized-3-aminopropyltrimethoxysilane (NZVI-NH₂). The synthesized NZVI was refluxed in presence of a mixture containing 50.0 mL of toluene and 3.0 mL of 3-aminopropyltrimethoxysilane for 3 hours at 120 °C. The produced black solid material was collected, washed with methanol and then dried at 50 °C.

2.3 Characterization of the prepared NZVI-NH₂

Thermal gravimetric analysis (TGA) of NZVI-NH₂ sorbent was obtained using a Perkin-Elmer TGA7 Thermobalance. The selected operating conditions were a temperature range of 20–600 °C, a heating rate of 10 °C min⁻¹, a flow rate of 20 mL min⁻¹ pure nitrogen atmosphere and the sample mass was taken in the range of 5.0–6.0 mg. The Fourier transform infrared (FT-IR) spectra of NZVI and NZVI-NH₂ sorbents were recorded from KBr pellets using a BRUKER Tensor 37 Fourier transform infrared spectrophotometer in the range of 400–4500 cm⁻¹. Scanning electron microscope (JSM-6360LA, JEOL Ltd.), (JSM-5300, JEOL Ltd.) and an ion sputtering coating device (JEOL-JFC-1100E) were used to examine the image and particle size of NZVI-NH₂. High resolution-transmission electron microscopy (HR-TEM) model JEM-2100 was used to image the various modified magnetic nano-sorbents. The HR-TEM technique includes scanning image observation device to give bright and dark-field STEM images at 200 kV. Also, the unit comprises energy dispersive X-ray analyzer model JED-2300T to examine the images and particle size of NZVI and NZVI-NH₂ sorbents. Surface area analysis was performed by using (Nova 3200 Nitrogen physisorbtion Apparatus, USA) in order to determine the surface area of nano-sorbents. The zeta potential η (mV) and electrophoretic mobility were determined using zetasizer (marvel instrument). The electrophoretic mobility of the suspensions of NZVI-NH₂ was measured in water. All measurements were run in duplicate under 25 °C and count rate 503.6 kcps. Also, the study of zeta potential against different value of pH over range (3–9) has been done.

2.4 Batch experiments

Batch technique was used to study the sorption processes of cobalt(II) and zinc(II) using the proposed NZVI-NH₂ nanosorbent under room temperature. This study was performed to investigate the effect of different experimental factors such pH of the medium, contact time, amount of sorbent, initial metal ion concentration and competing ions on the metal ion removal from aqueous solutions.

2.4.1 Effect of pH. 10.0 ± 1.0 mg of NZVI-NH₂ sorbent was placed in a 50 mL measuring flask to which 9.0 mL of acidic and buffer solutions of pH (1.0–7.0) was added. 1.0 mL of 0.1 mol L⁻¹ of metal ion solution (Co(II) or Zn(II)) was then added to the reaction mixture. The flasks were shaken using horizontal shaker at 100 rpm for 30 min at room temperature. The solution was filtered and the filtrate was titrated against 0.01 mol L⁻¹ EDTA solution using the suitable buffer and indicator.

2.4.2 Effect of contact time. 10.0 ± 1.0 mg of NZVI-NH₂ sorbent was placed in a 50 mL measuring flask. 1.0 mL of 0.1 mol L⁻¹ of metal ion solution and 9.0 mL of the optimum buffer solution were then added. The mixture was shaken for a varied period of time (1–60 min). The mixture was filtered and the filtrate was titrated against 0.01 mol L⁻¹ EDTA solution using the suitable buffer and indicator.

2.4.3 Effect of sorbent dosage. The effect of nanosorbent dosage on the metal uptake of Co(II) and Zn(II) was studied by the batch equilibrium technique in presence of different masses (5.0–100.0 mg of sorbent). The optimum conditions were used as described above.

2.4.4 Effect of metal ion concentration. The sorption process was studied using 10.0 ± 1.0 mg of nano-sorbent with different metal ion concentration (0.01–0.20) mol L⁻¹ of Co(II) and Zn(II) using the optimum conditions as described above.

2.4.5 Effect of competing ions. The effect of other interfering ions on the metal uptake was studied as following. 10.0 ± 1.0 mg of NZVI-NH₂ and 100 ± 1.0 mg of the competing ions (ammonium chloride, sodium chloride, calcium sulfate and magnesium sulfate) was added into the flask. 9.0 mL of optimum buffer solution and 1.0 mL of 0.1 mol L⁻¹ of Co(II) or Zn(II) solution was also added to the reaction mixture. Shaking was taken place for 30 min and the procedure was completed as described above.

2.5 Adsorptive removal of metal ions by a preconcentration microcolumn

Removal of Co(II) and Zn(II) ions from various real water samples were performed according to the following procedure.^33,34 Real water samples were collected from different sources (tap water, sea water and wastewater) and spiked with 1.0–2.0 mg L⁻¹ of the selected metal ions. A 0.5 L of water sample which contains a mixture of the selected metal ions was passed over a micro-column system packed with 50.0 ± 1.0 mg of nano-sorbent at a constant flow rate of 5 mL min⁻¹ under air pressure. Effluent solution was collected and subjected to metal determination using a Perkin Elmer flame atomic absorption spectrophotometer, model 2380.

2.6 Adsorptive removal of radioactive isotopes by a preconcentration microcolumn

Removal of Co-60 and Zn-65 radioisotopes from simulated low-level radioactive liquid waste was performed according to the following procedure. Simulated samples with radioactive cobalt and zinc were prepared by irradiation of Co(II) and Zn(II) compounds in Egypt Second Research Reactor. A 500 mL of radioactive sample was passed over a micro-column system packed with 50.0 ± 1.0 mg of NZVI-NH₂ sorbent at a constant flow rate of 5 mL min⁻¹ under air pressure. Effluent solution was collected and the residual radioactivity was determined using NaI scintillator.

3. Result & discussion

3.1 Synthesis and characterization

Synthesis of magnetic NZVI nano-sorbent employed in this work was based on the reduction of ferric ions using a strong oxidizing agent, sodium borohydride. After addition of the sodium borohydride solution, the mixture was stirred for additional 30 min to complete the reduction reaction of Fe³⁺ into zerovalent iron. NZVI particles appeared immediately after addition of the oxidizing agent. Finally, NZVI nano-particles were isolated, collected and stored under alcohol to prevent oxidation process.³⁸

Fe³⁺ + 2BH₄⁻ + 6H₂O → Fe⁰ + 2BH(OH)₃ + 7H₂↑

Wet NZVI was refluxed in toluene with 3-aminopropyltriethoxysilane for protection and functionalized the surface of nano zerovalent iron with amine group as represented in Scheme 1.

Scheme 1 Route for synthesis of NZVI and NZVI-NH₂ sorbents.

3.1.1 FT-IR spectra of magnetic nano-sorbents. The FT-IR spectra obtained for unmodified and modified NZVI are shown in Fig. 1a and b, respectively. Comparing the spectra of NZVI and NZVI-NH₂ shows that there are some common bands in both spectra. These bands include: (1) broad band at 3521 cm⁻¹ corresponding to the surface adsorbed water molecules, (2) iron bands appear at 400–700 cm⁻¹.^39,40 In addition, there are several characteristic bands in the FT-IR spectrum of NZVI-NH₂ sorbent (Fig. 1b) which are used as strong evidences for the successful modification of NZVI with 3-aminopropyltrimethoxysilane. These include: (1) a band at 2939 cm⁻¹ which is corresponding to the aliphatic C–H bond, (2) two peaks at 2326 and 2349 cm⁻¹ related to the bending of free –NH₂ group, (3) a peak at 1022 cm⁻¹ which assigned to the organic siloxane bond Si–O–Si and a peak at around 1100 cm⁻¹ due to the Si–O–C bond, (4) medium peaks in range 1661–1645 cm⁻¹ are corresponding to N–H bending, (5) a peak at 924 cm⁻¹ due to waging of 1° amine and (6) the successful modification of iron surface was confirmed by observation of iron-carbon stretching band at 453 cm⁻¹ as previously reported by Al-Mustafaa et al.⁴¹

Fig. 1 FTIR of nanosorbents (a) NZVI, (b) NZVI-NH₂.

3.1.2 SEM of magnetic nano-sorbents. The images of NZVI and NZVI-NH₂ using scanning electron microscopy are represented in Fig. 2a and b, respectively. Spherical shape and homogeneous distribution of the particles of NZVI and NZVI-NH₂ are evident in these two images. The average particles size of NZVI and NZVI-NH₂ were characterized in the range of 17–52 and 34–45 nm, respectively.

Fig. 2 SEM images of nanosorbents (a) NZVI, (b) NZVI-NH₂.

3.1.3 HR-TEM of magnetic NZVI-NH₂. The high resolution-transmission electron microscope show 2D HR images of NZVI and NZVI-NH₂ particles. Fig. 3a shows NZVI which has spherical particles in the range 23–36 nm, while Fig. 3b shows the HR-TEM image of NZVI-NH₂ sorbent as spherical and semi-spherical shape and the particle size was identified in the range 43–46 nm.

Fig. 3 TEM images of nanosorbents (a) NZVI, (b) NZVI-NH₂.

3.1.4 Determination of surface area. The sorption and desorption curve for determination of the surface area of NZVI-NH₂ sorbent is represented in Fig. 4. The surface area of NZVI-NH₂ was determined and characterized using the multi-point BET method as 2.420 × 10² m² g⁻¹, and using the t-method external surface area as 2.420 × 10² m² g⁻¹. All this data confirm that the NZVI-NH₂ sorbent has a large surface area which enhances the adsorption process of metal ions on the surface of this material.

Fig. 4 Sorption and desorption curve for determination of the surface area.

3.1.5 Thermal gravimetric analysis (TGA). The thermal gravimetric analysis (TGA) of NZVI-NH₂ sorbent confirms four degradation steps as shown in the Fig. 5. The first step was identified at 25–125 °C (percent loss = 4.9%) and due to possible loss of adsorbed water from the surface of NZVI-NH₂ sorbent. The other three degradation steps at 125–230 °C (percent loss = 9.0%), 230–450 °C (percent loss = 3.1%) and 450–600 °C (percent loss = 3.2%) is mainly due to decomposition of loaded 3-aminopropyltri-methoxysilane from the NZVI-NH₂ sorbent. However, Maity et al. have previously discussed and reported the TGA of some similar silane compounds.⁴²

Fig. 5 TGA thermogram of NZVI-NH₂ sorbent.

3.1.6 Determination of zeta potential. The zeta potential curve is shown in Fig. 6a and the results of zeta potential measurements of NZVI-NH₂ in water were identified as 15.5 mV, conductivity (mS cm⁻¹) was found as 0.0179, mobility was characterized as 1.217 μm cm V⁻¹ s⁻¹ under effective voltage equal 148. The zeta potential curve as a function of pH was studied over pH range (3–9) as shown in Fig. 6b.

Fig. 6 (a) Zeta potential curve of NZVI-NH₂ adsorbent, (b) zeta potential as function of pH.

3.2 Metal sorption uptake characteristics of NZVI-NH₂ sorbent

3.2.1 Effect of pH on metal sorption. The results of the uptake process of Co(II) and Zn(II) using NZVI-NH₂ sorbent in presence of different solutions with pH in the range 1.0–7.0 are shown in the Fig. 7. The NZVI-NH₂ sorbent was found to exhibit good sorption characteristics for the evaluated metal ions in neutral pH medium.⁴³ The nitrogen atoms in the form of amino groups already present on the surface of NZVI-NH₂ sorbent have a four lone pairs of electrons which increase the ability of this sorbent to bind and form complexes with Co(II) and Zn(II) at neutral pH.⁴⁴ The maximum metal capacity values of Co(II) and Zn(II) were identified at pH 7.0 as 3400 and 3000 μmol g⁻¹, respectively in presence of pH 7.0. However, the high change in metal capacity value of Co(II) from 850 to 3400 μmol g⁻¹ upon increasing the pH of contact solution from 6.0 to 7.0 may be due the strong ability and affinity of Co(II) to combine with the unprotonated neutral nitrogen atoms in order to form adsorbed metal ion complex on the surface of NZVI-NH₂ adsorbent.⁴⁴ At low pH, the metal capacity values of Co(II) and Zn(II) were low due to the expected high competition between solution free hydrogen ion and metal ion for the active centers on the surface of modified nano-sorbent. Therefore, the metal capacity decrease at low pH conditions.⁴⁵

Fig. 7 Effect of pH on metal capacity values by NZVI-NH₂ sorbent.

3.2.2 Effect of shaking time on metal sorption. The effects of contact time on sorption process of NZVI-NH₂ sorbent for cobalt(II) and zinc(II) are shown in the Fig. 8. In case of cobalt, it was observed that the metal capacity increases gradually with the increase in contact time and the maximum metal capacity was 3900 μmol g⁻¹. The metal capacity of zinc(II) was found also to increase with time until the contact time became 40 minute. After that the graph formed a plateau at 50 and 60 minute with the maximum metal sorption value 3200 μmol g⁻¹. This difference in metal-sorbent binding is mainly dependent on the binding process of each metal with the active site on the surface. Some metals such as cobalt(II) require more time to bind with the surface of NZVI-NH₂ sorbent and therefore, it was observed that metal capacity increases as the contact time increases along all studied shaking time values. Other trend may be due to a fast binding process between metal ion and nitrogen donor atoms until the surface is saturated and equilibrium condition takes place such as the sorption process of zinc(II) on the surface of modified nano-sorbent.^36,37

Fig. 8 Effect of shaking time (min) on metal capacity values by NZVI-NH₂ sorbent.

The adsorption kinetics models are generally correlated with the solution uptake rate. Therefore, these models are generally important to evaluate the design of water treatment process. The kinetics of adsorbate binding onto the surface of adsorbent materials is considered of significant importance for selection of the best operating conditions if the full-scale batch process is aimed. In addition, the adsorption kinetics study provides some idea about the residence time of the adsorbate at the solid-solution interface as well as the solute uptake rate. The rate of reaction is very importance in designing the adsorption system and can be calculated from kinetic study. In order to elucidate the adsorption mechanism and potential rate controlling step, models including the pseudo-first-order and pseudo-second-order equations are investigated to fit the obtained experimental data from batch metal ions abstraction experiments. The conformity between experimental data and the model predicted values was evaluated on the basis of correlation coefficient (R² values close or equal to 1). A relatively high R² value indicates that the model successfully describes the adsorption kinetics.⁴⁶

The pseudo-first order from the integrated linear expression is given by eqn (1).

ln(q_e − q_t) = ln q_e − k₁t (1)

where k₁ is the pseudo-first order rate constant. The plot of ln(q_e − q_t) versus time (t) for adsorptive removal of Zn(II) and Co(II) by NZVI-NH₂ sorbent yielded two graphs with R² were identified in the range of 0.970–0.979.

The pseudo-second-order kinetic model is expressed by eqn (2).

t/q_t = 1/k₂q_e² + t/q_e (2)

where k₂ is the pseudo-second-order-rate constant (g mg⁻¹ min⁻¹) which can be determined for different concentrations according to the linear plots of t/q_t versus t (Fig. 9). It is evident from these graphs that higher correlation coefficients, R², were identified in the range of 0.9990–0.9998 and 0.9993–0.9998 for Zn(II) and Co(II), respectively as shown in Fig. 9. The calculated equilibrium adsorption capacity values, q_e(calc), were found to correspond to 10.571–14.619 and 10.799–12.563 mg g⁻¹ for removal of Zn(II) and Co(II), respectively by NZVI-NH₂ sorbent. This confirms that pseudo-second-order kinetics is the correct model to describe the mechanism of adsorptive removal of Zn(II) and Co(II) by NZVI-NH₂ sorbent.

Fig. 9 Adsorption of (a) Zn(II) and (b) Co(II) by NZVI-NH₂ using pseudo-second-order model.

3.2.3 Effect of sorbent dosage on metal sorption. Batch technique was also implemented to evaluate the factor of sorbent dosage on the metal capacity of Zn(II) and Co(II) by NZVI-NH₂ sorbent by using the optimum buffering and contact time conditions in presence of different NZVI-NH₂ sorbent (5.0–50.0 mg) sorbent. Fig. 10 shows the effect of sorbent dose on the determined metal capacity values of Zn(II) and Co(II) by NZVI-NH₂ sorbent. A gradual decrease in the metal capacity of Zn(II) and Co(II) ions upon increasing the sorbent dosage. The maximum metal capacity values were identified at 5.0 mg sorbent and the lowest metal capacity at 50.0 mg sorbent. The characterized metal capacity values were found in the range of 1600–5800 μmol g⁻¹ (Co(II)) and 650–5400 μmol g⁻¹ (Zn(II)). The reason for this behavior is mainly due to the greater availability of surface active sites in the case of high sorbent dosage which permit good binding possibility of the metal ion to the sorbent surface. In addition, the high metal sorption capacity values at lower sorbent dosage may be also explained on the basis of an increased metal to sorbent ratio.⁴⁶

Fig. 10 Effect of sorbent dosage on metal capacity values by NZVI-NH₂ sorbent.

3.2.4 Effect of initial metal ion concentration on metal sorption properties. The effect of initial metal ion concentration on the removal processes of Co(II) and Zn(II) by NZVI-NH₂ sorbent was carried out by the batch adsorption experiments using different initial metal ion concentrations ranging from 0.02 to 0.20 mol L⁻¹ at the optimum contact time, buffering and sorbent dose conditions. The results of this study are represented in Fig. 11. In case of Zn(II), a gradual increase in metal capacity is observed as the metal ion concentration increases up to 0.20 mol L⁻¹ with the identified highest value was 4100 μmol g⁻¹. Similarly, the metal capacity of Co(II) exhibited a gradual increase in metal capacity value as the initial metal ion concentration increases up to 0.18 and 0.20 mol L⁻¹ to produce 4050 μmol g⁻¹. The reason for this behavior and trend are mainly related to the driving force that created from the initial metal ion concentration and affected the mass-transfer barrier between the NZVI-NH₂ sorbent and metal ion in the contact medium. Therefore, a higher initial concentration of metal ion is expected to enhance and increase the metal capacity values of Co(II) and Zn(II) using NZVI-NH₂ sorbent.⁴⁵

Fig. 11 Effect of metal ion concentration on metal capacity values by NZVI-NH₂ sorbent.

The Langmuir theory is based on the assumption that the adsorption process is considered as a type of chemical combination is unimolecular form and the Langmuir adsorption model is represented by eqn (3). The Freundlich adsorption isotherm model considers a heterogeneous adsorption surface that has unequal available sites with different energies of adsorption. The Freundlich adsorption isotherm model is represented by eqn (4).⁴⁶

q_e = q_maxbC_e/(1 + bC_e) (3)

q_e = K_FC_e^1/n (4)

where C_e is the equilibrium concentration (mg L⁻¹), q_e is the amount of adsorbed ion at equilibrium (mg g⁻¹), q_max (mg g⁻¹) and b (L mg⁻¹) are the Langmuir constants which are related to the adsorption capacity and adsorption energy respectively. K_F is a constant for the system and defined as the adsorption or distribution related to the bonding energy. The regression coefficients (R²) for Zn(II) was detected by the Langmuir adsorption isotherm model which as 0.995 (Fig. 12a). The adsorption model for removal of Co(II) by NZVI-NH₂ sorbent are represented in Fig. 12b. The of regression coefficient (R²) for Co(II) by Freundlich adsorption isotherm model was found as 0.998. The results of this study confirm that removal processes of Zn(II) using NZVI-NH₂ sorbent are well described and more fitted to the Langmuir isotherm rather than Freundlich isotherm via chemical formation of monolayer adsorption processes of Zn(II) on the surface of NZVI-NH₂ sorbent.⁴⁴ While adsorption of Co(II) using NZVI-NH₂ sorbent are well described and more fitted by the Freundlich adsorption isotherm model.

Fig. 12 Langmuir adsorption isotherm model of (a) Zn(II) and Freundlich adsorption isotherm model of (b) Co(II) by NZVI-NH₂ sorbents.

3.2.5 Effect of interfering ions on metal sorption capacity. Batch adsorption technique was also used to study the influence of interfering ions on the removal processes of studied metal ions by using 0.01 g NZVI-NH₂ sorbent, 9.0 mL of optimum buffer, 1.0 mL metal ion and 0.100 g of competing ions (ammonium chloride, sodium chloride, calcium sulphate, magnesium sulfate). The obtained results of this study are shown in the Fig. 13. The interfering ions were found to show a decrease in all metal capacities of cobalt and zinc ions. However, the order of decreasing metal capacity was identified as sodium chloride, ammonium chloride, calcium sulphate and finally the lowest metal capacity for both zinc and cobalt were observed with magnesium sulphate. The surface of NZVI-NH₂ is characterized by the presence of amino (–NH₂) as the surface functional group which acts as an electron pair donor via nitrogen atom. This allow the electron deficient ions such as Ca(II) or Mg(II) to strongly bound to the surface of NZVI-NH₂ sorbent. On the other hand, a repulsion interaction is expected to take place between the negatively charged sulphate ion and the nitrogen donor atom.

Fig. 13 Effect of interfering ion on metal capacity values by NZVI-NH₂ sorbent.

3.3 Adsorptive removal of metal ions by a preconcentration microcolumn

Microcolumn technique was used to evaluate the potential removal of Co(II) and Zn(II) ions from water. Real water samples were collected from tap water, sea water and wastewater and spiked with 1.0–2.0 mg L⁻¹ of the selected metal ions. The column system was packed with 50.0 ± 1.0 mg of magnetic NZVI-NH₂ sorbent by using a flow rate of 5 mL min⁻¹ under air pressure. The extraction of Co(II) and Zn(II) ions from different water samples. The percentage extraction values of Zn(II) and Co(II) from tap water were found to correspond to 91.81 and 88.98%, respectively. In sea water sample, the percentage extraction values of Zn(II) and Co(II) were identified as 96.27% and 93.90%, respectively. Finally the percentage extraction values of Zn(II) and Co(II) from wastewater were characterized as 97.33 and 92.12%, respectively.

3.4 Adsorptive removal of radioactive cobalt and zinc

Treatment and removal of radioactive of cobalt-60 and zinc-65 ions from simulated low-level radioactive liquid waste was performed according to the following procedure. Real wastewater samples were collected and spiked with radioactive Co(II) and Zn(II). A 0.5 L of water sample which contains radioactive elements with the mentioned concentration was passed over a micro-column system packed with 50.0 ± 1.0 mg of magnetic NZVI-NH₂ sorbent at flow rate of 5 mL min⁻¹ under air pressure. The percentage extraction values of radioactive zinc and cobalt from water were identified as 96.2 and 93.3%, respectively.

4. Conclusion

The characterization was performed to provide a full understanding of the newly designed and synthesized NZVI-NH₂ sorbent. The FT-IR proved a successful preparation of magnetic NZVI-NH₂ sorbent. The SEM and TEM provided accurate particle size 43–46 nm. NZVI-NH₂ sorbent was characterized to exhibit a high thermal stability and a high surface area. Batch technique experiments showed that sorption processes of Zn(II) and Co(II) from aqueous solutions using NZVI-NH₂ were established at neutral pH. NZVI-NH₂ sorbent was also found selective for both Co(II) and Zn(II) and the metal capacity were very close under most of the studied experimental factors. By using microcolumn technique, excellent percent removal values of inactive zinc and cobalt ions from different real water samples were established. Similarly, excellent removal recovery values were obtained in case of radioactive zinc (96.2%) and radioactive cobalt (93.3%) using NZVI-NH₂ sorbent as the packing materials in the microcolumn technique.

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