Superparamagnetic magnetite-alginate nanoparticles (M-AlgNPs) were synthesized utilizing a coprecipitation method. Then, the prepared M-AlgNPs were characterized via Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction spectroscopy (XRD) to affirm the formation and the crystallinity of this composite. In addition, the surface morphology of the prepared nanoparticles was inspected by transmission electron microscopy (TEM) which revealed well-dispersed nanoparticles of Fe3O4 into alginate. The adsorption characteristics of the synthesized nanoparticles for removing Ni+2 and Co+2 from wastewater were evaluated via atomic absorption measurements (AAS). The elimination efficiency of the M-AlgNPs was detected at in 100 ppm (initial concentration) of Ni+2 and Co+2, separately. The M-AlgNPs provided the maximum equilibrium uptake percentage for Ni+2 and Co+2 of 97.88 and 95.01%, respectively. The adsorption of Ni+2 and Co+2 onto the M-AlgNP surface was found to fit the Langmuir model with values higher than that obtained from the Freundlich model for both metal ions. Moreover, “separating factor” for the adsorption process was assessed and found to be less than unity; this expresses the higher ability of the investigated metal ions to be adsorbed onto the M-AlgNP surface. The adsorption method was discovered to be pH-dependent and well-suited to the isothermal equations of Langmuir and Freundlich. For regeneration studies, M-AlgNPs have been investigated and results confirmed that it could be reused with effective sorption capacity over three cycles.

1. Introduction

Water pollution with metal ions is an enormous ecological problem, as they damage even at small levels. Metal ions, for instance, nickel, cobalt, lead, and iron, are toxic and have nonbiodegradable characteristics that cause difficulty for both the earth and life. Removing these metal ions from the sewage stream is an important demand for a healthy setting. Several techniques have been used to eliminate metal ions from wastewater such as exchange of ions, extraction of solvents, flotation, oxidation, and adsorption [13]. Adsorption has gradually gained a lot of attention since late in perspective of the fact that this technique is essential and it generally has minimal effort and energy in expelling important metal particles from water.

Numerous sorts of adsorbents for treatment have been produced, for example, activated carbon, natural bentonite, tobacco stem ash, and calcium alginate beads [47]. As of late, with the quick advancement of nanotechnology, nanoparticles are at present concentrated. Generally, the research found that nanosorbents have great adsorption limits concerning metal ion elimination from water due to the large surface area and high reusability of these nanomaterials.

The adsorption technique combined with magnetic features for metal ion separation is utilized widely in wastewater remediation and green treatment, because of their little size and superparamagnetic properties. Also, these particles can be efficiently regained with an outer attractive field “magnet” and have a low threat effect.

Latterly, magnetic polymer nanoparticles got developing consideration in various fields because of their higher surface area and active sites to adsorb various pollutants. The magnetic polymer composite can be prepared via different methods; the most basic one is the independent preparation for the magnetic nanoparticles and polymer materials and after that blending them physically or chemically. Another technique to get the magnetic polymer composite depends on in situ precipitation of magnetic material within the polymer, where the magnetic particles are encompassed by a hydrophilic polymer shell [8, 9].

Alginate is an unbranched polymer formed by linear block copolymerization of d-mannuronic acid and l-guluronic acid, characterized by high hydrophilicity and is considered as an ecofriendly material, biocompatible, and biodegradable [10].

In this study, magnetite-alginate nanoparticles (M-AlgNPs) were synthesized via the coprecipitation method. The structure of the prepared M-AlgNPs was expressed using FTIR and XRD. The morphology and particle size of the synthesized nanomaterial were detected using TEM. In addition, M-AlgNPs were applied as a green adsorbent for removing Ni+2 and Co+2 from wastewater. The effect of pH values and adsorbent doses was studied. Moreover, the adsorption isotherm and equilibria of Ni+2 and Co+2 adsorption onto M-AlgNPs were studied using both Langmuir and Freundlich models.

2. Materials and Experiments

2.1. Materials

FeCl3·6H2O (iron(III) chloride hexahydrate), FeCl2·4H2O (iron(II) chloride tetrahydrate), sodium hydroxide, nickel sulfate (NiSO4·5H2O), and cobalt nitrate (Co(NO3)2) are of analytical grade and are obtained from Merck, Germany. Sodium alginate was purchased, and Pluronic PE 6400 was from Sigma.

The fresh concentration of Ni+2 and Co+2 was prepared by means of adequate dilutions for the purchased solutions.

2.2. Synthesis of M-AlgNPs

M-AlgNPs were synthesized via the coprecipitation technique, where FeCl3·6H2O and FeCl2·4H2O have been dissolved separately in the ratio of 2 : 1. Then, the two previous solutions were mixed and heated (65°C) under mild stirring using a mechanical stirrer. An aqueous solution of NaOH (3 M) is then added drop by drop until the formation of black suspended particles. The reaction continued for 30 min under the same conditions [11, 12]. Then, magnetite alginate was prepared via a ionotropic gelation method. Firstly, the previously synthesized magnetite nanoparticles were redispersed in a solution 2.5% of sodium alginate using an ultrasonic bath for 5 min. The resulting M-AlgNPs were washed several times with deionized water; then, the particles were magnetically separated.

2.3. Characterization of M-AlgNPs

FTIR was used to distinguish the functional groups of M-AlgNPs using FTIR-1615 (Perkin Elmer (USA)) using the KBr disc, in wavenumbers that ranged from 4000 to 400 cm-1. The crystal structure of the synthesized M-AlgNPs was estimated via an X-ray diffraction technique (Panalytical X’Pert PRO MPD (Netherlands)). The instrument is outfitted with a copper anode (Cu-Kα) producing wavelength radiation of 1.54 Å. The diffraction pattern was registered at ambient temperature in the angular width of 4–80 (2θ) applying a scan time of 0.4 (s) and a step size of 0.02 (2θ). Zeta potential values were measured (Malvern Zetasizer ZS-HT, United Kingdom) to detect the optimum pH for the adsorption process. The morphology of the prepared M-AlgNPs was viewed utilizing the transmission electron microscope, JEM 2100 (JEOL, Japan).

2.4. Adsorption Studies

The concentration of Ni+2 and Co+2 in the aqueous solution was determined using a ZEEnit 700P atomic absorption spectrometer (Analytik Jena, Germany) equipped with an air-acetylene and a slot burner of 100 mm. The instrumental conditions were balanced by the producer’s recommendations. The accuracy of measurement is accomplished through triple repetition for each concentration.

The batch experiments are used to detect the adsorption manner of the prepared M-AlgNPs for Ni+2 and Co+2 (at room temperature). In this step, M-AlgNPs were added (known weight) to the metal ion aqueous solution separately and checked well (200 rpm) for two hours (selected time). Then, the M-AlgNPs were separated by a magnet, and the remaining metal ion concentration is determined using FAAS.

The adsorption of Ni+2 and Co+2 onto the M-AlgNP surface was estimated in the pH range from 2 to 7.5 (using 0.1 M NaOH or 0.1 M HNO3). Moreover, the effect of the different weights of the synthesized M-AlgNPs was investigated at ambient temperature.

The capacity of adsorption at equilibrium () is calculated in mg/g using the following equation: where is the weight of the synthesized M-AlgNPs (g), is the volume of the sample (L), is the initial metal ion concentration (mg/L), and is the concentration of the metal ions at equilibrium (mg/L).

3. Results and Discussion

The procedure of the coprecipitation reaction of formation of Fe3O4 can be summed up by the formation of oxygenated bridges between Fe(III) and Fe(II). Before the synthesis steps, the initial ratio of Fe(III) and Fe(II) is obtained as an acid solution to avoid the formation of hydroxide form before mixing. Then, the two ratios are mixed and dispersed well, and NaOH solution is added causing the precipitation of iron ions as hydroxide. The previously formed hydroxide precipitate is converted to the oxides through the formation of the oxygenated bridge [11, 13].

The reaction can be described by the following equation:

3.1. Characterization of M-AlgNPs

Figure 1 shows the FTIR spectra of M-AlgNPs, which exhibit three peaks at 635, 589, and 441 cm-1 caused by the vibration of FeO bond in both octahedral and tetrahedral sites of the inverse spinel cubic of Fe3O4, respectively [14, 15]. The peaks at 1623 and 1352 cm-1 were attributed to the stretching of the C=O group in the alginate. In addition, the peak that appeared at 951 cm-1 was caused by the C-O group. A broad band that ranged from 3200 to 3500 cm-1 is due to the OH group [16].

Figure 2 shows the spectrum of M-AlgNPs which is greatly matched to JCPDS file No. 19-0629. This confirms the formation of magnetite nanoparticles, as evidenced by the peaks at 2θ values of 30.33, 35.79, 43.65, 57.35, 62.99, and 74.49° that attributed to the crystal plan of magnetite at (220), (311), (400), (422), (511), and (440) (Figure 2). The presence of alginate is elucidated from the appearance of broad diffraction peaks at 2θ less than 20° [17].

The average particle size of M-AlgNPs can be assessed utilizing the Debye-Scherrer relation [18]: where describes the mean diameter of M-AlgNPs, is the wavelength of the radiation source, is FWHM (the full width at half-maximum) in radians, is constant, and is the half diffraction angle—Bragg angle.

According to the Debye-Scherrer equation, particle sizes of M-AlgNPs were estimated to be 9.22 nm with respect to the most intense peak (311).

Figure 3 shows that the synthesized M-AlgNPs are present as irregular spherical shape and have uniform size distribution with average diameters of around 7-9 nm.

3.2. Adsorption Studies

It is outstanding that the pH values are a standout amongst the most essential elements that influence the manner of the adsorption process, since the pH of the metal ion solution controls the adsorption of these metals on the solid surfaces. Zeta potential measurement is determined for M-AlgNPs at different pH to detect the charge of the surface. Concerning Figure 4, the point of zero charge (isoelectric point) is found around 4.2. Above this point, the surface becomes negatively charged.

Experiments are performed to find the optimum pH on the adsorption of Ni+2 and Co+2 onto M-AlgNPs using various pH values of 2 up to 7.5; above this pH, the investigated metal ions begin to precipitate. Figure 5 shows the variation of the pH value on the adsorption behavior of Ni+2 and Co+2 onto M-AlgNPs using sorbent weight of 0.2 g/100 L at 25°C and initial metal ion concentration of 100 mg/L. The efficiency of adsorption was affected linearly by increasing the value of pH with the maximum value at pH 6.5. From zeta potential and experimental results, the removal efficiency increases above the isoelectric point reaching maximum before the neutral (pH 6.5) and higher negative value of zeta potential found around this pH. Thus, the carboxylate groups at this pH () have more negative charges leading to more attraction to the positively charged metal ions, while at the pH value of 2, the hydroxyl groups of alginate become protonated causing a decrease in the efficiency of the adsorption of Ni+2 and Co+2 onto M-AlgNPs [19, 20].

Different sorbent doses were used that ranged from 0.1 to 0.5 g/mL for 100 mg/L metal ion concentration keeping other variables constant as shaking time, rpm, and temperature. Figure 6 reveals that the elimination of Ni+2 and Co+2 increases with increasing the weight of M-AlgNPs. This is due to more available sites present with increasing the weight of adsorbent (M-AlgNPs) until 0.4 g/L. The decrease in the removal percentage with increasing the dose of the adsorbent surface is due to the repulsion between the large numbers of dispersed nanoparticles. So, the chance of metal ions to be attracted onto the M-AlgNP surface decreases.

3.3. Adsorption Isotherms

The Langmuir isotherm describes the adsorption as the formation of one layer that covers the sorbent surface and no further adsorption can occur [21], and it is represented by the following equation: where (mg/g) is the maximum capacity of adsorption and (L/mg) is the Langmuir constant. Figure 7 shows the relation between and ; the intercept and the slope facilitate the calculation of and , respectively. Table 1 represents the calculated adsorption parameters which confirm the obligation of the adsorption of Co+2 and Ni+2 onto the M-AlgNP surface to the Langmuir relation.

Also, the following relation is applied for the Langmuir model to announce whether Co+2 and Ni+2 have a great affinity to be adsorbed on the M-AlgNP surface or not: where is the unit less constant called “separation factor,” is the Langmuir constant, and is the initial concentration of polluted metal ions.

The values of have expressed the tendency of the pollutants to be adsorbed on the adsorbent surface. When the value exceeds unity, it indicates that there is no tendency for adsorption “undesirable.” Meanwhile, the higher tendency of adsorption is achieved when the value of becomes more than zero and less than one. The calculated values for Co+2 and Ni+2 are 0.030 and 0.053, respectively, indicating their higher tendency for adsorption onto M-AlgNPs.

Also, the adsorption manner can be described by the Freundlich isotherm model using the following equation: where is the adsorption capacity at equilibrium (mg/g), is the concentration of each metal ions in the solution at equilibrium (mg/L), is the capacity of adsorption () (mg/g), and describes the intensity of the adsorption and is defined as the extent of reliance of the adsorption process with the concentration at equilibrium [22].

The Freundlich model states that the sorbent surfaces are heterogeneous and possess various binding energies. Figure 8 shows Freundlich adsorption isotherm, the values of obtained from the slope and are observed to be more than unity, as shown in Table 1, which reveals the cooperative adsorption [8, 23]. Langmuir isotherm gives higher values for adsorption of both metals than that obtained from Freundlich isotherm as listed in Table 1 indicating that metal ions are adsorbed as monolayer on the surface of M-AlgNPs.

The previous results suggest that Co+2 and Ni+2 are adsorbed on the prepared magnetite-alginate nanoparticle surface forming monolayer through the attraction between positive metal ion and outer oxygen atom as presented in Scheme 1. The prepared M-AlgNPs and the adsorbed metals are then separated using a strong magnet. Reusability of the M-AlgNPs as adsorbents for the metal ions was examined (under the same condition) due to financial and environmental concerns [24]. After the first adsorption process, the M-AlgNPs were tested again by stirring the adsorbent for 2 hours in 5 M HNO3 [25]. M-AlgNPs were carefully cleaned, separated with powerful magnets after adsorption-desorption with deionized water, and then dried and reconditioned to the second cycle for adsorption. The findings indicate, for Ni+2 and Co+2, respectively, that the effectiveness of M-AlgNPs was 94.1 and 92.6% after being used three times.

4. Conclusion

Magnetite-alginate nanoparticles (M-AlgNPs) are synthesized, and the structure is characterized using FTIR and XRD measurements. Also, the morphology and the particle size were determined using TEM analysis. The prepared nanoparticles were applied as a green adsorbent for removing hazard metal ions “cobalt and nickel ions” as selected pollutants from an aqueous water sample separately. The efficiency of adsorption of Co+2 and Ni+2 onto the prepared M-AlgNP surfaces were 95.01 and 97.88%, respectively. Moreover, the maximum capacity of adsorption () was 33.557 and 37.313 mg/g for Co+2 and Ni+2. Both Langmuir and Freundlich isotherm models were applied, and values were obtained and showed more fitting to Langmuir models for Co+2 and Ni+2. Additionally, the “separation factor” values are calculated depending on Langmuir models and confirming the higher ability of adsorption .

Data Availability

All measurements are undergone in the Egyptian Petroleum Research Institute, and the condition is inserted inside the article. The authors are ready for any question.

Conflicts of Interest

The authors declare that there is no conflict of interests.