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Polyethylenimine Grafted on Nano-NiFe2O4@SiO2 for the Removal of CrO42‒, Ni2+, and Pb2+ Ions from Aqueous Solutions

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04 December 2023

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04 December 2023

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Abstract
Polyethyleneimine (PEI) has been reported to have good potential for the adsorption of metal ions. In this work, PEI was covalently bound to NiFe2O4@SiO2 nanoparticles to form the new adsorbent NiFe2O4@SiO2–PEI. The material allowed magnetic separation and was characterized via powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform IR spectroscopy (FT-IR), and thermogravimetry-differential thermal analysis (TGA-DTA). The adsorption of CrO42‒, Ni2+, and Pb2+ ions from aqueous solutions was studied at different pH, temperatures, metal ion concentration, and adsorbent dosage. Maximum adsorption capacities of 149.23, 156.68, and 161.25 mg/g were obtained for CrO42‒, Ni2+, and Pb2+, respectively under optimum conditions using 0.075 g of the adsorbent material 250 mg/L ion concentration, at pH = 6.5 and room temperature.
Keywords: 
Subject: Chemistry and Materials Science  -   Inorganic and Nuclear Chemistry
Keywords polyethyleneimine; core-shell NiFe2O4@SiO2; magnetic separation; toxic metals; adsorption

1. Introduction

Toxic metals such as lead have been used by human mankind for thousands of years, but only with the industrial revolution and the rapid growth of the human population and industrial activities in the last 70 years, the penetration of toxic metals into the natural environment and water resources has enormously increased and represents a thread to human health [1]. Many of the metals under concern are heavy metals such as Hg, Cd, and Pb. This is why the term “heavy metals” is frequently used for all toxic metals. However, toxic metals such as Be, Cr, and Ni should not be termed heavy metals as their density does not exceed 5 g/cm2 and their chemistry is also dissimilar to heavy metals that have a high binding affinity to sulfur-based (bio)ligands in common [1,2].
The detection of toxic metals and their removal from water resources has been continuously an important scientific topic and various methods have been developed to remove toxic metals such as chemical precipitation (including coagulation and flocculation), adsorption, electrochemical reduction, removal through membrane processes, reverse osmosis, and ion exchange methods [3]. Important goals are reducing the costs, simplifying the methods, avoid double contamination, and increase sensitivity [4,5,6,7]. Amongst these methods, adsorption (chemi- and physisorption) is the most interesting in terms of sensitivity and selectivity and functionalized nanoparticular materials seem to be favorable for their large surface to mass ratio [4,6,8,9,10,11,12,13,14].
From the chemical viewpoint main challenges in the removal of toxic metal ions from wastewater lies in the efficiency and in the recyclability of the adsorbents [3]. Polyethyleneimine (PEI) has been reported to be an efficient material showing fast uptake and fast release under different pH conditions [15,16,17,18,19]. For recovery, most adsorbents were separated and then recycled through centrifugation or filtration but in recent years the use of magnetically separable adsorbent materials was introduced and seems very promising to achieve high recycling rates [5,17,20,21,22,23]. Frequently, the magnetic separation is based on hematite and magnetite embedded in core-shell nanoparticles [17,20,24,25,26,27,28,29] and in recent years hematite and magnetite structures have been successfully replaced with ferrite structures such as nickel ferrite (NiFe2O4) [30,31], CoFe2O4 [25,32], or MnFe2O4 [21,33].
Herein we report a study on the use of polyethylene imine (PEI) grafted on core-shell NiFe2O4@SiO2 nanoparticles as adsorbent for the removal of CrO42‒, Ni2+, and Pb2+ ions from water. We studied the influence of parameters such as pH, temperature, metal ion concentration, and amount of NiFe2O4@SiO2–PEI adsorbent and also investigated the kinetics of the adsorption system.
In a recent very similar approach Fe3O4@SiO2–PEI–NTDA nanoparticles (NP) in which Fe3O4@SiO2 nanoparticles were functionalized with PEI and 1,4,5,8-naphthalenetetracarboxylic-dianhydride (NTDA) were used to adsorb Pb2+ ions in the presence of Cd2+, Ni2+, Cu2+, and Zn2+ [17]. Further similar materials were Fe3O4@MIL-88A(Fe)‒APTMS NP based on the Fe-containing MOF MIL-88A(Fe) and (3-aminopropyl)trimethoxysilan (APTMS) applied for the removal of CrO42‒, Cd2+, and Pb2+ [34], CoFe2O4@MWCNT‒CTS NP based on multi-walled carbon nanotubes and chitosan (CTS) for the adsorption of Pb2+ [32], MnFe2O4@GO‒TPA based on graphene oxide (GO) and tetraethylenepentamine (TPA) for the adsorption of Pb2+ [33], and very recently Fe3O4@SiO2‒CTS–DTPA with diethylenetriaminepentaacetate (DTPA) binding at the NH functions of CTS for the removal of Pb2+ [35]. A similar comparative study for CrO42‒, Ni2+, and Pb2+ (along with Cd2+ and Hg2+) was previously led using amino-functionalized Fe3O4@GS nanomaterials based on non-further defined graphene (GS) [36]. CrO42– was efficiently removed using sodium lignosulfonate/PEI/sodium alginate beads very recently [37]. Very recently polyaniline-grafted pine sawdust was used to efficiently adsorb Cu2+, Co2+, Cd2+, Ni2+, Pb2+, Zn2+ and Fe2+ in a comparative study [38]. In a very recent approach, 8-chloroacetyl–aminoquinoline (CAAQ) was attached through PEI as ligand to Fe3O4@SiO2 nanoparticles for the capture of Fe3+, Cu2+, and Cr3+ [23].

2. Materials and Methods

2.1. Instrumentation

Powder X-ray diffraction (PXRD) measurements were carried out on a Shimadzu 6100 using Cu-Kα irradiation on solid powder samples of freshly prepared and recovered materials of NiFe2O4@SiO2-PEI. PXRD of NiFe2O4 were recorded on an STOE-STADI MP diffractometer equipped with a Cu-Kα1 radiation (λ = 0.15406 Å) source and operating in transmission mode. Atomic absorption spectroscopy (AAS) was measured using a Shimadzu AA-6880 spectrophotometer. TGA-DTA was recorded on a Shimadzu TGA-DTG-60H instrument. FT-IR spectra were recorded on a Bruker Alpha I spectrophotometer on KBr disks. FE-SEM and EDX analysis were carried out on a JEOL JSM-IT 100 instrument.

2.2. Reagents

All chemicals including NiCl2.6H2O, FeCl3.6H2O, FeCl2.6H2O, Si(OEt)4, trimethoxy(3-(oxiran-2-ylmethoxy)propyl)silane, and toluene were purchased from Merck and Sigma-Aldrich and used without further purification.

2.3. Synthesis of the NiFe2O4@SiO2–PEI adsorbent

NiFe2O4@SiO2 was prepared according to our previously reported work [30]. In brief, 10 g of PEI (Sigma-Aldrich, 5000 average molecular weight) were dissolved in 50 mL hot toluene and then cooled. This was mixed with 472 mg (2 mmol) trimethoxy(3-(oxiran-2-yl-methoxy)propyl)silane (MW = 248.35 g/mol, from Sigma-Aldrich) and the mixture heated under reflux for 8 h. Then, 2.5 g of the NiFe2O4@SiO2 nanoparticles were added at room temperature and the mixture was heated under reflux for 5 h. The resulting colorless solid was filtered, washed with toluene, and dried at 50 °C. For the analytics, see the results and discussion section.

2.4. Adsorption experiments

For the metal standard solutions 5, 10, 15, 20, 25, and 30 mg/L of Na2Cr2O7.2H2O (for CrO42‒, MW = 297.99 g/mol), Ni(NO3)2.6H2O (for Ni2+, MW = 229.54 g/mol), and Pb(NO3)2.6H2O (for Pb2+, MW = 370.84 g/mol) were dissolved in 100 mL deionized water. This translates to 0.3356‒2.0135 mmol/L for CrO42‒, Ni2+, and Pb2+, 0.2178‒1.3070 mmol/L for Ni2+, and 0.1348‒0.8090 mmol/L for Pb2+. The pH was adjusted to 3 to 8 using diluted NaOH (1M) or HCl (1M) solutions. Adsorption procedure: A 250 mL Erlenmeyer flask was equipped with 50 mL of each metal ion solution (50‒300 mg/L), the adsorbent (0.01‒0.1 g) was added and the mixture was stirred at room temperature for 45 min at pH values ranging from 3 to 8.
The adsorption capacity at equilibrium (qe) and the adsorption capacity at time t (qt) are defined as:
Preprints 92196 i001.
with C0: initial concentration (mg/L), Ce: equilibrium concentration(mg/L), m: amount of adsorbent (g), V: volume of the solution (L) [39].

2.5. Adsorbent recovery

After adsorption of the CrO42‒, Ni2+, and Pb2+ ions at pH = 6.5; 0.075 g adsorbent; 250 mg/L metal ions; 298 K for 45 min, the magnetic adsorbent was separated from the reaction batch with the help of an external magnet and was recycled by washing with HCl solution (5%) followed by NaOH solution (5%) to remove the metals.

3. Results and discussion

3.1. Synthesis and characterization of the adsorbent

The NiFe2O4@SiO2–PEI adsorbent was prepared as shown in Scheme 1. First, trimethoxy(3-(oxiran-2-ylmethoxy)propyl)silane was reacted with the PEI polymer followed by reaction with core-shell NiFe2O4@SiO2 nanoparticles (details in the Materials and Methods Section).
The powder XRD pattern of the NiFe2O4@SiO2–PEI adsorbent showed signals at 2Ɵ = 37.3, 43.4, 46.8, 53.7, 57.7, 62.9, 71.4, and 74.6 ° characteristic for the cubic phase of magnetite and nickel ferrite (NiFe2O4, reference code: 00-003-0875) while reflections corresponding to silica were absent (Figure 1).
The FT-IR analysis of NiFe2O4@SiO2–PEI (Figure 2) showed bands at 3678, 3609, and 3573 cm–1 assignable to N–H stretching vibrations, the broad peak between 3100 and 3700 cm–1 is due to O–H stretching modes, while C(sp3)–H stretches appear sharp at 2945 and 2879 cm–1. The C–H bending modes are found at 1348 cm–1. The bands located at 1634, 1210, 1153, 1110, 1038, 924, and 879, cm–1 can be assigned to the Si–O–Si, Si–O, C–C, C–N, and C–O functionalities. Finally, the Fe–O lattice vibrations appear as a broad band centered at 530 cm–1 [23,30,37].
Field emission scanning electron microscopy (FE-SEM) images of NiFe2O4@SiO2–PEI show irregular shaped and partially agglomerated particles with diameters ranging from 50 to 100 nm (Figure 3) similar to what we recently reported for NiFe2O4@SiO2–PSA particles (PSA = propylsulfonic acid) that were prepared in a similar manner [30]. Energy-dispersive X-ray spectroscopy (EDX) analysis showed C (36.31%), N (38.96%), Ni (2.12%), O (6.78%), Fe (4.38%), and Si (4.12%) (Figure 3).
Thermogravimetric (TGA) and differential thermal analysis (DTA) showed a small weight loss of about 5% around 100 °C which is due to loss of adsorbed water. The major weight of about 77% of the original mass occurred in the range 200 to 500 °C (Figure 4). The residual 18% represent the NiFe2O4@SiO2 nanoparticles without the “organic” functionalization in excellent agreement with the EDX analysis showing a total of 75% for C and N.

3.2. Adsorption studies for CrO42‒, Ni2+, and Pb2+ ions

3.2.1. Effect of the pH

In order to evaluate the effect of pH on the adsorption of CrO42‒, Ni2+, and Pb2+, the pH war varied from 3 to 8 was while other parameters are fixed at 250 mg/L metal ion initial concentration, 0.075g adsorbent, 50 mL volume, and 298 K. The adsorption capacity showed a maximum at a pH of 6.5 and decreased with increasing pH (Figure 5). In acidic media at pH ˂ 6 we assume a competition between protons (H+) and the metal ions Ni2+ and Pb2+ in their coordination to the NH2 groups of the adsorbent. The maximum is reached at pH = 6.5 with adsorption capacities of 149.25, 156.68, and 161.25 mg/g for CrO42‒, Ni2+, and Pb2+, respectively.
Very similar pH-dependent behavior as our materials with the maximum adsorption at pH = 6 was previously reported for the Pb2+-adsorbing materials Fe3O4@SiO2@PEI–NTDA [17], PEI-bacterial cellulose [19], and sodium alginate (ALG)/PEI composite hydrogels [16], in line with PEI acting as coordinating agent in these materials. However, also for Fe3O4@SiO2@PEI–CAAQ in which CAAQ acts as additional ligand [23] the same behavior was found. The very recently reported adsorbent Fe3O4@SiO2‒CTS–DTPA (DTPA = diethylenetriaminepentaacetate) [35] shows a maximum Pb2+ adsorption already in acidic solutions at pH = 3 and no loss of binding capacity between pH = 3 and pH = 6.
On the other hand, the very similar behavior of our adsorbent towards the cations Ni2+ and Pb2+ on one side and the anionic CrO42‒ is peculiar as for other amine-containing materials such as the recently reported sodium lignosulfonate/PEI/sodium alginate beads [37], the ethylenediamine-functionalized Fe3O4 (EDA@Fe3O4) particles [40], amino-functionalized Fe3O4@GS nanomaterials [36], polydopamine modified chitosan aerogels [41], or the MOF APTMS@MIL-88A(Fe) (APTMS = (3-aminopropyl)trimethoxysilan) [34], better CrO42– adsorption was found at low pH (2 to 3) while cation adsorption is superior at higher pH. This is reasonable in view of the protonated amine functions at low pH allowing to strongly adsorb the CrO42– anion while neutral amine function coordinate cations. The only explanation we have so far is that the CrO42‒ has been largely reduced to Cr3+ ions which then would absorb in a similar way as Ni2+ and Pb2+. This idea is supported by several reports that could show that Cr3+ can be formed from CrO42– through electron transfer from various materials [15,18,26,37,41,42,43,44]. Such CrO42– to Cr3+ reduction upon adsorption can be very efficient if a distinct electron-donating material is present as in the CTAB-intercalated MoS2 nanosheets (CTAB = cetyl trimethyl ammonium bromide) that can be used for the simultaneous removal of Cr(IV) and Ni(II) [45] or in the chitosan-modified multi-walled carbon nanotube composites (MWCNT-CTS) that adsorb CrO42– exclusively as Cr3+ [46].
In future studies we will use X-ray photoelectron spectroscopy (XPS) to study the oxidations states of the adsorbed Cr as was done in the last two mentioned studies.

3.2.2. Effect of the contact time

Examining the effect of contact time in the adsorption process of the metals allowed us calculating the reaction rate and the time to reach equilibrium. For this purpose, the reaction parameters were kept constant with an initial concentration of metal salts of 250 mg/L, 0.075 g adsorbent, and pH = 6.5. The adsorption capacity increased rapidly within the first 5 min, at high rate within the first 20 min and then continues slowly. An almost equilibrium was reached within 45 min (Figure 6).
For the previously reported similar materials Fe3O4@SiO2@PEI–NTDA [17] and Fe3O4@SiO2@PEI–CAAQ [23], the adsorption capacities reached plateau values only after more than 200 min [17] or 90 min [23], respectively which indicates that our system is markedly more active and lies in the same time-range as the previously reported PEI-bacterial cellulose [19]. In contrast to this, very fast adsorption of Cu2+, Co2+, Cd2+, Ni2+, Pb2+, Zn2+ and Fe3+ within 10 to 20 min was achieved with polyaniline grafted on pine saw dust [38], underlining the suitability of polyamines and anilines in efficiently coordinating the metals.

3.3. Adsorption kinetic and mechanism

The mechanism of the adsorption of the metal ions were studied via different kinetic models e.g. pseudo-first order, pseudo-second order, and Elovich models [47]. The correlation coefficient (R2) values for the different kinetic models were calculated by drawing log(qe‒qt) vs. t (pseudo-first order), t/qt vs. t (pseudo-second order), and qt vs ln t (Elovich) diagrams (Table 1). The agreement with pseudo-first order kinetics is slightly better than the pseudo-second order fit and much better than with the Elovich equation. This stands in contrast to the behavior of the reported adsorption of Pb2+ by an activated carbon [47] and we ascribe this to the more unspecific surface of the carbon in contrast to the well-defined coordination sites of PEI. This is supported by the very similar behavior of the Fe3O4@SiO2@PEI–NTDA [17] that showed also pseudo-first order kinetics for the Pb2+ adsorption. The better agreement of experimental data with pseudo-second order kinetics reported for Fe3O4@SiO2@PEI–CAAQ [23] is in line with the additional CAAQ ligand showing superior binding than PEI.

3.3.1. Effect of the amount of adsorbent

The effect of the amount of adsorbent was studied in the range from 0.01 g to 0.1 g while other parameters are held constant (conc. of adsorbates: 250 mg/L, pH = 6.5). With increasing amounts of adsorbent, the adsorption capacity increased up to about 0.08 g (Figure 7). Further increase did not give higher adsorption. The slight decrease in adsorption capacity at high adsorbent loads might be due to aggregation and accumulation of particles and overall reduction of their surface.
A marked maximum adsorption maximum for Pb2+ was found in the dosage-behavior for the recently reported adsorbent material Fe3O4@SiO2@PEI-NTDA [17]. For this material also, aggregation was assumed to be responsible for this phenomenon. When comparing the two curves, our maximum is less pronounced, meaning that our system is more tolerant to larger amounts of adsorbent.

3.3.2. Effect of the metal ion concentration

In order to determine the maximum adsorption capacity of the adsorbent, the effect of different concentrations of metal ions was evaluated. Figure 8 shows that the adsorption capacity of the adsorbent increases with the increase of the initial concentration of CrO42‒, Ni2+, and Pb2+ ions. The highest adsorption capacity was observed at a concentration of 250 mg/L. At higher concentrations, the adsorption capacity of the adsorbent remains constant pointing to the saturation of the adsorbent sites.

3.4. Adsorption isotherms

The equilibrium isotherm studies could provide the information about the nature of the interaction between the adsorbed material and the adsorbent and can be used to determine the adsorption capacity of the adsorbent. In order to conduct a view of the way of CrO42‒, Ni2+, and Pb2+ ions adsorption, the mechanism was investigated by applying the linear forms of Langmuir, Freundlich, and Temkin isotherm models [48]. The calculated parameters for different isotherms are depicted in Table 2. By looking at the parameters of isotherms we are able to get an insight into the adsorption mechanism. The Langmuir isotherm model is based on the hypothesis that a single-layer of adsorbent material on the surface structure of the adsorbent is saturated during adsorption, the adsorption sites are identical, the energy of the adsorption is not dependent on the surface coverage, and there is no interaction between the adsorbates (here, the adsorbed metal ions) [47].
The correlation coefficient (R2) was calculated for all isotherms and fitted to the experimental data. Values of 0.972 (CrO42‒), 0.976 (Ni2+), and 0.997 (Pb2+) R2 show that the Langmuir isotherm fitting agrees very well with the experimental results. Accordingly, the mechanism of the adsorption is monolayer adsorption on the surface of the adsorbent [48,49]. The same behavior was also found for similar adsorbent materials such as Fe3O4@SiO2@PEI–CAAQ [23], Fe3O4@SiO2@PEI–NTDA [17], and Fe3O4@SiO2‒CTS/DTPA [35], while for the CrO42– adsorption on sodium lignosulfonate/PEI/sodium alginate beads [37] the Langmuir and Freundlich models gave very similar R2 values. The Freundlich isotherm applies to non-ideal adsorption on heterogeneous surfaces [50,51] and the Freundlich-type of behavior is in line with the very heterogeneous surface of the lignosulfonate/PEI/sodium alginate material [37] in contrast to our adsorbent.

3.5. Adsorption thermodynamics

The effect of temperature on the adsorption capacity was investigated to determine the thermodynamic parameters and investigate the spontaneity of the adsorption process. The adsorption capacity decreased with increasing temperature from room temperature to 75°C (Figure 9). This is in line with an exothermic nature of the adsorption process.
From this data, we calculated also the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) of the system. The change in ΔG of the CrO42‒, Ni2+, and Pb2+ ions at six different temperatures is obtained through the relationships between ΔG, ΔH, ΔS and ln Kc in the equations shown in Table 3. ΔH and ΔS are obtained by plotting ln Kc against 1/T (Figure 10).
Thermodynamic parameters ΔG, ΔH and ΔS are shown in Table 3. The negative value of ΔG is in line with spontaneous adsorption processes on the surface of the adsorbent. The negative values of ΔH and ΔS show the exothermic nature of the adsorption reaction, which is very probably binding to the amine functions of the PEI. The overall exothermic binding is in line with the rapid binding (compare Figure 6) and is an important pre-requisite for the use of this material for efficient metal-recovery from solution.
As in the pH-dependent experiments, the anionic CrO42‒ behaves remarkably similar to the Ni2+ and Pb2+ cations with negative ΔH0, ΔS0, and ΔG0. This stands in contrast to the related sodium lignosulfonate/PEI/sodium alginate beads for which the CrO42‒ adsorption showed positive ΔH0 (7.5 kJ/mol) and ΔS0 (70 J/K.mol) values, but a negative ΔG0 of –13.36 kJmol–1 at 298 K [37]. This difference supports our assumption that the CrO42‒ ions in our test solutions are adsorbed as Cr3+ ions on the adsorbent.

3.6. Scanning electron microscope (SEM) / energy-dispersive X-ray (EDX) analysis

Figure 11A shows the SEM image of the as-prepared NiFe2O4@SiO2-PEI adsorbent having a cauliflower-like morphology. The morphology does not change upon loading with Cr(VI), Pb(II) and Ni(II) (Figure 11B to 11D). EDX spectra show the characteristic peaks of Cr(VI), Pb(II) and Ni(II) ions (Figure 11F to 11G). For comparison we recorded the EDX of a re-cycled NiFe2O4@SiO2#-PEI sample and we found traces of Na+ and Cl– (Figure 11E) stemming from the washing procedure (first HCl, then NaOH, see Materials and Methods).

3.7. Adsorbent recovery

The recyclability was tested in 11 consecutive runs and the adsorbent showed good recovery (Figure 12) for all three ions.
For the recently reported Fe3O4@SiO2@PEI–CAAQ (CAAQ = 8-chloroacetyl–aminoquinoline) [23], efficient recycling was only achieved when using Na2EDTA2+ solutions for the stripping of the metal cations, while desorption using HCl or HNO3 steadily decreased the adsorption capacity. This underlines that the additional CAAQ ligand helps to stronger bind metal cations, but at the same time is detrimental to a rapid and efficient desorption.

4. Conclusions

In this study, a new adsorbent material NiFe2O4@SiO2–PEI which is polyethylene imine (PEI) grafted on core-shell NiFe2O4@SiO2 nanoparticles was synthesized through a simple and easy procedure and characterized using PXRD, FE-SEM, EDX, FT-IR, and TGA-DTA analyses. The potential of NiFe2O4@SiO2–PEI in the adsorption of CrO42‒, Ni2+, and Pb2+ ions from aqueous solutions were investigated under variation of pH, adsorbent amount, metal ion concentration, and temperature. The maximum adsorption was achieved at pH = 6.5 and 250 mg/L CrO42‒, Ni2+, and Pb2+ ions concentration and 0.075 g adsorbent at room temperature. The adsorption mechanism was investigated using pseudo-first order, pseudo-second order, and Elovich models with the best match of the pseudo-first order model with the experimental results. The best fit to the adsorption isotherms was the Langmuir model and both findings are in line with a smooth homogeneous mono-layer adsorption. The adsorption of both the anionic CrO42‒, and the cations Ni2+, and Pb2+ ions increased with increasing time and decreased with increasing temperature and also the pH-dependency was very similar. Deconvolution of the T-dependent adsorption gave negative values for ΔG, ΔH and ΔS. For the metal cations Pb2+ and Ni2+ this is in line with binding of these cations to the amine-functions of the PEI. The very similar behavior of the anionic CrO42‒ is probably due to reduction of CrO42‒ to Cr3+ which shows comparable binding properties to Pb2+ and Ni2+. In future studies we will elaborate on this using XPS for the determination of the oxidation states of the Cr species bound to the adsorbent.
For the moment we can state that the new adsorbent material NiFe2O4@SiO2–PEI is an interesting candidate for removal of toxic metals from wastewater, in view of its simple preparation, simple adsorbing kinetics, exothermic thermodynamics (chemical binding) and good recovery and recyclability. In the future we will further explore its potential by studying the adsorption of further metal cations such as Cu2+, Cr3+, and Gd3+ as well as the co-dependence of the adsorption of toxic metals with other cationic and anionic components in wastewater.

Author Contributions

The experimental work has been carried out by M.Kj., S.M.K., M.Z., and E.T.A. The original draft was written by M.Kj., A.K. and both authors have edited and revised the original draft. All authors agree with the submitted version of the manuscript.

Funding

E.T.A and A.K. acknowledge funding by the German Academic Exchange Service (DAAD; Eric Tobechukwu Anthony 91732061).

Availability of Data and Materials

Data will be made available on request.

Declarations Ethical Approval

Not applicable

Acknowledgments

Laboratory support by the Islamic Azad University, Buinzahra Branch is highly acknowledged.

Competing Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

TOC text:

Polyethyleneimine (PEI) was covalently bound to NiFe2O4@SiO2 nanoparticles to form the new magnetically-separable material NiFe2O4@SiO2–PEI for the adsorption of CrO42‒, Ni2+, and Pb2+ ions from aqueous solutions.

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Preprints 92196 sch001
Scheme 1. Preparation of the polyethylene imine (PEI) grafted on core-shell NiFe2O4@SiO2 nanoparticles (NiFe2O4@SiO2–PEI) though trimethoxy(3-(oxiran-2-yl-methoxy)propyl)silane.
Scheme 1. Preparation of the polyethylene imine (PEI) grafted on core-shell NiFe2O4@SiO2 nanoparticles (NiFe2O4@SiO2–PEI) though trimethoxy(3-(oxiran-2-yl-methoxy)propyl)silane.
Preprints 92196 sch001
Preprints 92196 g001
Figure 1. XRD pattern of NiFe2O4@SiO2–PEI (left) and of pristine NiFe2O4 (right).
Figure 1. XRD pattern of NiFe2O4@SiO2–PEI (left) and of pristine NiFe2O4 (right).
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Preprints 92196 g002
Figure 2. FT-IR spectrum of NiFe2O4@SiO2–PEI.
Figure 2. FT-IR spectrum of NiFe2O4@SiO2–PEI.
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Preprints 92196 g003
Figure 3. FE-SEM photograph (left) and EDX analysis (right) of NiFe2O4@SiO2–PEI.
Figure 3. FE-SEM photograph (left) and EDX analysis (right) of NiFe2O4@SiO2–PEI.
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Preprints 92196 g004
Figure 4. TGA and DTA analysis of NiFe2O4@SiO2–PEI.
Figure 4. TGA and DTA analysis of NiFe2O4@SiO2–PEI.
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Preprints 92196 g005
Figure 5. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of the pH (45 min; 0.075 g adsorbent; 250 mg/L adsorbate, 298 K).
Figure 5. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of the pH (45 min; 0.075 g adsorbent; 250 mg/L adsorbate, 298 K).
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Preprints 92196 g006
Figure 6. Adsorption of CrO42‒, Ni2+, and Pb2+ ions over time (pH = 6.5; 0.075 g adsorbent; 250 mg/L metal ions; 298 K).
Figure 6. Adsorption of CrO42‒, Ni2+, and Pb2+ ions over time (pH = 6.5; 0.075 g adsorbent; 250 mg/L metal ions; 298 K).
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Preprints 92196 g007
Figure 7. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of adsorbent amount (45 min; pH = 6.5; 250 mg/L adsorbates; 298 K).
Figure 7. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of adsorbent amount (45 min; pH = 6.5; 250 mg/L adsorbates; 298 K).
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Preprints 92196 g008
Figure 8. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of initial metal ion concentration (45 min; 0.075 g adsorbent; pH = 6.5; 298 K).
Figure 8. Adsorption of CrO42‒, Ni2+, and Pb2+ ions: effect of initial metal ion concentration (45 min; 0.075 g adsorbent; pH = 6.5; 298 K).
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Preprints 92196 g009
Figure 9. Adsorption of CrO42‒, Ni2+, and Pb2+ ions over T (45 min; 0.075 g adsorbent; 250 mg/L metal ion conc. concentration; pH = 6.5).
Figure 9. Adsorption of CrO42‒, Ni2+, and Pb2+ ions over T (45 min; 0.075 g adsorbent; 250 mg/L metal ion conc. concentration; pH = 6.5).
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Preprints 92196 g010
Figure 10. Linear plot of ln Kc vs 1/T for the adsorption of CrO42‒, Ni2+, and Pb2+ ions.
Figure 10. Linear plot of ln Kc vs 1/T for the adsorption of CrO42‒, Ni2+, and Pb2+ ions.
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Preprints 92196 g011
Figure 11. SEM images of NiFe2O4@SiO2–PEI before (A) and after Cr(VI) (B), Ni(II) (C), and Pb(II) (D) adsorption. EDX spectra of recovered NiFe2O4@SiO2–PEI before metal ion adsorption (E) and after Cr(VI) (F), Ni(II) (G), and Pb(II) (H) adsorption. (Metal concentrations 10 mg L–1). Scale bars in A to D: S3400 15.0 kV 5.2 mm x 20.0 k SE (2.00 μm).
Figure 11. SEM images of NiFe2O4@SiO2–PEI before (A) and after Cr(VI) (B), Ni(II) (C), and Pb(II) (D) adsorption. EDX spectra of recovered NiFe2O4@SiO2–PEI before metal ion adsorption (E) and after Cr(VI) (F), Ni(II) (G), and Pb(II) (H) adsorption. (Metal concentrations 10 mg L–1). Scale bars in A to D: S3400 15.0 kV 5.2 mm x 20.0 k SE (2.00 μm).
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Preprints 92196 g012
Figure 12. Adsorption of CrO42‒, Ni2+, and Pb2+ ions at pH = 6.5; 0.075 g adsorbent; 250 mg/L metal ions; 298 K for 45 min in 11 consecutive runs.
Figure 12. Adsorption of CrO42‒, Ni2+, and Pb2+ ions at pH = 6.5; 0.075 g adsorbent; 250 mg/L metal ions; 298 K for 45 min in 11 consecutive runs.
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Table 1. Parameters and correlation coefficient (R2) of kinetic models.
Table 1. Parameters and correlation coefficient (R2) of kinetic models.
Linear equations a Parameters
Pseudo-first order CrO42‒ Ni2+ Pb2+
k R2 qe (mg/g) k R2 qe (mg/g) k R2 qe (mg/g)
0.118 0.999 151.07 0.097 0.992 169.78 0.102 0.996 159.66
Pseudo-second order 0.0014 0.981 163.93 0.0008 0.968 175.43 0.0011 0.980 178.57
Elovich equation α R2 β α R2 β α R2 β
135.57 0.928 0.026 10.85 0.983 0.022 79.89 0.963 0.023
a Pseudo-first order: log(qe–qt) = logqe(k t /2.303) with k is the rate constant (min‒1), t is the contact time (min),
Pseudo-second order: t/qt = (1/kqe2)+(t/qe), Elovich equation: qt = (ln(αβ)/β)+(lnt/β)with β, α are the Elovich constants.
Table 2. Parameters and correlation coefficient (R2) of isotherm models.
Table 2. Parameters and correlation coefficient (R2) of isotherm models.
CrO42‒ Ni2+ Pb2+
Model Parameters
Langmuir b R2 qmax (mg/g) b R2 qmax (mg/g) b R2 qmax (mg/g)
0.075 0.972 181.81 0.105 0.976 178.57 0.441 0.997 166.67
Freundlich kf R2 n kf R2 n kf R2 n
0.015 0.910 0.646 0.016 0.893 0.675 0.0014 0.814 0.553
Temkin B R2 A B R2 A B R2 A
0.0247 0.931 367.1 0.0226 0.885 298.3 0.0272 0.838 1.584
a qmax is the maximum monolayer adsorption capacity (mg/g); qe is the sorption capacity at equilibrium (mg/g); Ce is the concentration of CrO42‒ at equilibrium (mg/L). R2 is the correlation coefficient.: Langmuir linear equation: Ce/qe = 1/bqmax + Ce/qmax with b (L/mg) is the Langmuir constant; Freundlich linear equation: lnqe = lnkf + lnCe/n with kf and n are the Freundlich constants; Temkin linear equation: qe = BlnA + BlnCe with A and B are the Temkin constants.
Table 3. Thermodynamic parameters for the adsorption of CrO42‒, Ni2+, and Pb2+ ions.a.
Table 3. Thermodynamic parameters for the adsorption of CrO42‒, Ni2+, and Pb2+ ions.a.
ΔG° (kJ/mol) (T = 298 K) ΔS° (J/K.mol) ΔH° (kJ/mol)
CrO42‒ Ni2+ Pb2+ CrO42‒ Ni2+ Pb2+ CrO42‒ Ni2+ Pb2+
‒4.18 ‒5.58 ‒7.37 ‒67.96 ‒98.92 ‒106.64 ‒24.43 ‒35.06 ‒38.96
a Kc (L/mg) is the equilibrium constant, R = 8.314 J/mol·K, T is the absolute temperature (K), Gibbs free energy ∆G°(kJ/mol), Enthalpy ∆H°(kJ/mol), Entropy ∆S° (J/K·mol). ΔG0 = –RTlnKC with KC = qe/Ce and RTlnKC = TΔS0.ΔH0.
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