Polyethylenimine Grafted onto Nano-NiFe 2 O 4 @SiO 2 for the Removal of CrO 4 2− , Ni 2+ , and Pb 2+ Ions from Aqueous Solutions
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 for magnetic separation and was characterized via powder X-ray diffraction (PXRD), showing the pattern of the NiFe2O4 core and an amorphous shell. Field emission scanning electron microscopy (FE-SEM) showed irregular shaped particles with sizes ranging from 50 to 100 nm, and energy-dispersive X-ray spectroscopy (EDX) showed high C and N contents of 36 and 39%, respectively. This large amount of PEI in the materials was confirmed by thermogravimetry–differential thermal analysis (TGA-DTA), showing a mass loss of about 80%. Fourier-transform IR spectroscopy (FT-IR) showed characteristic resonances of PEI dominating the spectrum. The adsorption of CrO42−, Ni2+, and Pb2+ ions from aqueous solutions was studied at different pH, temperatures, metal ion concentrations, and adsorbent dosages. The maximum adsorption capacities of 149.3, 156.7, and 161.3 mg/g were obtained for CrO42−, Ni2+, and Pb2+, respectively, under optimum conditions using 0.075 g of the adsorbent material at a 250 mg/L ion concentration, pH = 6.5, and room temperature.
Keywords: polyethyleneimine; core-shell NiFe2O4@SiO2; magnetic separation; toxic metals; adsorption
1. Introduction
Toxic metals such as lead have been used by humans 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 has the penetration of toxic metals into the natural environment and water resources enormously increased, and this represents a threat to human health [[1]]. Many of the metals of concern are heavy metals such as Hg, Cd, and Pb. This is probably why the term "heavy metals" is frequently but wrongly 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]].
The detection of toxic metals and their removal from water resources has continuously been 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 costs, simplifying methods, avoiding double contamination, and increasing sensitivity [[4], [6]]. 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 due to their large surface-to-mass ratio [[4], [6], [8], [10], [12], [14]].
From a chemical viewpoint, the main challenges in the removal of toxic metal ions from wastewater lie in the efficiency and recyclability of adsorbents [[3]]. Polyethyleneimine (PEI) has been reported to be an efficient material that shows fast uptake and fast release under different pH conditions [[15], [17], [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 has been introduced and seems very promising in order to achieve high recycling rates [[5], [17], [20], [22]]. Frequently, the magnetic separation process is based on hematite and magnetite embedded in core-shell nanoparticles [[17], [20], [24], [26]], and in recent years, hematite and magnetite structures have been successfully replaced with ferrite structures such as nickel ferrite (NiFe2O4) [[28]], CoFe2O4 [[25], [30]], or MnFe2O4 [[21], [31]].
Magnetically separable core-shell Fe3O4@SiO2 nanoparticles, functionalized with PEI and 1,4,5,8-naphthalenetetracarboxylic-dianhydride (NTDA), were recently 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 metal-organic framework MOF MIL-88A(Fe) and (3-aminopropyl)trimethoxysilan (APTMS) applied for the removal of CrO42−, Cd2+, and Pb2+ [[32]], CoFe2O4@MWCNT–CTS NP based on multi-walled carbon nanotubes and chitosan (CTS) for the adsorption of Pb2+ [[30]], MnFe2O4@GO–TPA based on graphene oxide (GO) and tetraethylenepentamine (TPA) for the adsorption of Pb2+ [[31]], and very recently Fe3O4@SiO2–CTS–DTPA with diethylenetriaminepentaacetate (DTPA) binding at the NH functions of CTS for the removal of Pb2+ [[33]]. A similar comparative study for CrO42−, Ni2+, and Pb2+ (along with Cd2+ and Hg2+) was previously conducted using amino-functionalized Fe3O4@GS nanomaterials based on non-further defined graphene (GS) [[34]]. CrO42– was efficiently removed using sodium lignosulfonate/PEI/sodium alginate beads very recently [[35]]. Very recently, polyaniline-grafted pine sawdust was used to efficiently adsorb Cu2+, Co2+, Cd2+, Ni2+, Pb2+, Zn2+, and Fe2+ in a comparative study [[36]]. In a very recent approach, 8-chloroacetyl–aminoquinoline (CAAQ) was attached through PEI as a ligand to Fe3O4@SiO2 nanoparticles for the capture of Fe3+, Cu2+, and Cr3+ [[23]].
Herein, we report a study on the use of polyethylene imine (PEI) grafted onto core-shell NiFe2O4@SiO2 nanoparticles (Scheme 1) as an 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 the amount of a NiFe2O4@SiO2–PEI adsorbent and also investigated the kinetics of the adsorption system.
2. Results and Discussion
2.1. Characterization of the Adsorbent
The powder XRD pattern of the NiFe2O4@SiO2–PEI adsorbent showed signals at 2Ɵ (assigned hkl values) = 30.0 (220), 37 (311 + 222), 43.4 (400), 53.7 (422), 57.7 (511), 62.9 (440), 71.4 (620), and 74.6° (533) characteristic of the cubic phase of nickel ferrite (NiFe2O4, reference code: 00-003-0875) (Figure 1). A further signal at 46.8° could not be assigned. Reflections corresponding to crystalline silica were absent, but we assigned the broad features from 12 to 42° and from 50 to 78° to the amorphous SiO2–PEI shell, in keeping with a relatively large total of about 1 g SiO2 + PEI on 2 g of NiFe2O4 (see synthesis). In the recovered NiFe2O4@SiO2–PEI adsorbent, all PXRD features were retained.
In contrast with PXRD, the FT-IR analysis of NiFe2O4@SiO2–PEI (Figure 2) also revealed the amorphous part, with bands at 3678, 3609, and 3573 cm−1 assigned to N–H stretching vibrations and the broad peak between 3100 and 3700 cm−1 assigned 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 [[28], [35]]. Finally, the NiFe2O4 core of the material causes the Ni/Fe–O lattice vibrations to appear as a broad band centered at 530 cm−1 [[23], [28], [35]].
The field emission scanning electron microscopy (FE-SEM) images of NiFe2O4@SiO2–PEI showed irregularly 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 [[28]]. The contrast of all particles is identical which is in line with the complete surface of the initial NiFe2O4 particles covered with large amounts of SiO2 and PEI, in keeping with the PXRD results. 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) and thus confirms the relatively large amounts of SiO2 compared to Fe and Ni, which were found in the correct 2:1 ratio. Based on PEI (~CH2CH2NH), the weight ratio of C:N should be 24:14. However, the EDX analysis shows a C:N = 36:39. We suspect that the EDX method overestimates the surface of the material, which is dominated by the end-NH2 groups, thus producing the high values for N compared with C. The overestimation of the surface by EDX is supported by the overall large amounts of C and N originating from surface-bound PEI compared with the core-shell elements Ni, Fe, and Si.
Thermogravimetric (TGA) and differential thermal analysis (DTA) showed a small weight loss of about 5% around 100 °C. As we used a sample dried at 50 °C in vacuo, we assigned the corresponding endothermic peak to a loss of residual water bound in the PEI. The major weight of about 77% of the original mass occurred in the range of 200 to 500 °C (Figure 4). The residual 18% represents the NiFe2O4@SiO2 nanoparticles without the "organic" functionalization, which is in excellent agreement with the EDX analysis showing a total of 75% for C and N and thus a large coverage of the particles with PEI.
2.2. Adsorption Studies for CrO 4 2− , Ni 2+ , and Pb 2+ Ions
2.2.1. Effect of pH
In order to evaluate the effect of pH on the adsorption of CrO42−, Ni2+, and Pb2+, the pH was varied from 3 to 8, while the other parameters were fixed at a 250 mg/L metal ion initial concentration, 0.075 g of 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 competition between protons (H+) and the metal ions Ni2+ and Pb2+ in their coordination with the NH2 groups of the adsorbent. The maximum was reached at pH = 6.5 with adsorption capacities of 149.3, 156.7, and 161.3 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 a coordinating agent in these materials. However, also for Fe3O4@SiO2@PEI–CAAQ (CAAQ = 8-chloroacetyl–aminoquinoline) in which CAAQ acts as an additional ligand [[23]], the same behavior was found. The previously reported adsorbent Fe3O4@SiO2–CTS–DTPA (DTPA = diethylenetriaminepentaacetate) [[33]] already shows maximum Pb2+ adsorption in acidic solutions at pH = 3, no loss of binding capacity between pH = 3 and pH = 6, and an adsorption capacity of around 105 mg/g at pH = 6, which is markedly lower compared with our adsorbent material.
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 compared to other amine-containing materials such as the previously reported sodium lignosulfonate/PEI/sodium alginate beads [[35]], the ethylenediamine-functionalized Fe3O4 (EDA@Fe3O4) particles [[37]], amino-functionalized Fe3O4@GS nanomaterials [[34]], polydopamine modified chitosan aerogels [[38]], or the MOF APTMS@MIL-88A(Fe) (APTMS = (3-aminopropyl)trimethoxysilan) [[32]]; 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 the neutral amine function coordinates cations. The only explanation we have so far is that the CrO42− was largely reduced to Cr3+ ions which would then absorb in a similar way to Ni2+ and Pb2+. This idea is supported by several reports that show that Cr3+ can be formed from CrO42– through electron transfer from various materials [[15], [18], [26], [35], [38], [40]]. 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) [[42]] or in the chitosan-modified multi-walled carbon nanotube composites (MWCNT-CTS) that adsorb CrO42– exclusively as Cr3+ [[43]]. In future studies, we will use X-ray photoelectron spectroscopy (XPS) to study the oxidation states of the adsorbed Cr as was carried out in the last two mentioned studies.
The approximately 145 mg/g total adsorption capacity of our adsorbent for Cr compares to about 290 mg/g for the Fe3O4@SiO2@PEI–CAAQ adsorbent [[32]] which is only outnumbered by the 340 mg/g reported for an EDTA-inspired polydentate hydrogel [[44]]. In view of the additional mass of the metal oxide cores of the Fe3O4@SiO2@PEI–CAAQ and our adsorbent and the easy magnetic separation, the core-shell systems are superior even in capacity.
The capacity for Ni2+ absorption of about 150 mg/g found for our adsorbent material compares well with the amino-functionalized Fe3O4@GS nanomaterials [[34]].
2.2.2. Effect of Contact Time
Examining the effect of contact time in the adsorption process of the metals allowed us to calculate 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 of adsorbent, and pH = 6.5. The adsorption capacity increased rapidly within the first 5 min, at a high rate within the first 20 min, and then continued slowly. 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 onto pine sawdust [[36]], underlining the suitability of polyamines and anilines in efficiently coordinating the metals.
2.3. Adsorption Kinetics and Mechanism
The mechanism of the adsorption of the metal ions was studied via different kinetic models, e.g., pseudo-first order, pseudo-second order, and Elovich models [[45]]. 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 [[45]] 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]] which also showed 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 to PEI.
2.3.1. 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 the other parameters were held constant (conc. of adsorbates: 250 mg/L and pH = 6.5). With increasing amounts of adsorbent, the adsorption capacity increased up to about 0.08 g (Figure 7). The further increase did not give higher adsorption. The slight decrease in adsorption capacity at high adsorbent loads might be due to the aggregation and accumulation of particles and the overall reduction in 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 as well, 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 of larger amounts of adsorbent.
2.3.2. 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 in 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.
2.4. Adsorption Isotherms
Equilibrium isotherm studies can provide 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 produce a view of the path of CrO42−, Ni2+, and Pb2+ ions' adsorption, the mechanism was investigated by applying the linear forms of Langmuir, Freundlich, and Temkin isotherm models [[46]]. The calculated parameters for different isotherms are depicted in Table 2. By looking at the parameters of the isotherms we are able to gain 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) [[45]].
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 adsorption is monolayer adsorption on the surface of the adsorbent [[46]]. 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 [[33]], while for the CrO42– adsorption on sodium lignosulfonate/PEI/sodium alginate beads [[35]], the Langmuir and Freundlich models gave very similar R2 values. The Freundlich isotherm applies to non-ideal adsorption on heterogeneous surfaces [[48]] and the Freundlich-type behavior is in line with the very heterogeneous surface of the lignosulfonate/PEI/sodium alginate material [[35]] in contrast with our adsorbent.
2.5. 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 the exothermic nature of the adsorption process.
From these data, we also calculated 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 was determined through the relationships between ΔG, ΔH, ΔS, and ln Kc in the equations shown in Table 3. ΔH and ΔS were obtained by plotting ln Kc against 1/T (Figure 10).
The thermodynamic parameters ΔG, ΔH, and ΔS are shown in Table 3. The negative ΔG values are in line with spontaneous adsorption processes on the surface of the adsorbent. The negative values for ΔH show the exothermic nature of the adsorption reaction, which is very probably binding to the amine functions of the PEI. The negative ΔS values point to a non-spontaneous reaction. However, the overall exothermic binding is in line with rapid binding under these conditions (compared in 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 [[35]]. This difference supports our assumption that the CrO42− ions in our test solutions are adsorbed as Cr3+ ions on the adsorbent.
2.6. Scanning Electron Microscope (SEM)/Energy-Dispersive X-Ray (EDX) Analysis
Figure 11A shows the SEM image of the as-prepared NiFe2O4@SiO2–PEI adsorbent as nanoparticles of approximately 50 to 100 nm but more agglomerated than in Figure 3. The morphology does not change upon loading with Cr(VI), Pb(II), and Ni(II) (Figure 11B–D). The EDX spectra show the characteristic peaks of Cr(VI), Pb(II), and Ni(II) ions (Figure 11F–G). For comparison, we recorded the EDX of a recovered NiFe2O4@SiO2–PEI sample, and we found traces of Na+ and Cl– (Figure 11E) stemming from the washing procedure (first HCl and then NaOH; see Materials and Methods, Section 3).
2.7. Adsorbent Recovery
The recyclability was tested in 11 consecutive runs, and the adsorbent showed good recovery (Figure 12) for all three ions.
For the previously 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 more strongly bind metal cations but at the same time is detrimental to rapid and efficient desorption.
3. Materials and Methods
3.1. Instrumentation
Powder X-ray diffraction (PXRD) measurements were carried out on a Shimadzu 6100 using Cu-Kα (λ = 0.15406 Å) radiation at 298 K on solid powder samples of freshly prepared and recovered materials of NiFe2O4@SiO2–PEI. The PXRD of NiFe2O4 was recorded on an STOE-STADI MP diffractometer equipped with a Cu-Kα1 radiation (λ = 0.15406 Å) source and operating in transmission mode. Thermogravimetry–differential thermal analysis (TGA-DTA) was recorded on a Shimadzu TGA-DTG-60H instrument on a powder sample. Fourier-transformed (FT)-IR spectra were recorded on a Bruker Alpha I spectrophotometer on KBr disks. Field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX) were carried out on a JEOL JSM-IT 100 instrument.
3.2. Reagents
All chemicals including NiCl2·6H2O (Merck, Darmstadt, Germany, puriss.p.a. >98%), FeCl3·6H2O (Merck, reagent grade > 98%), FeCl2·6H2O (Merck, puriss.p.a. > 99%), tetraethylorthosilicate Si(OEt)4 (TEOS) (Merck, reagent grade 98%), trimethoxy(3-(oxiran-2-ylmethoxy)propyl)silane (Sigma-Aldrich, St. Louis, MO, USA, MW = 248.35 g/mol, 99%), PEI (Sigma-Aldrich, 5000 average molecular weight, 99%), and toluene (Merck, anhydrous 99.8%) were used without further purification.
3.3. Synthesis of the NiFe 2 O 4 @SiO 2 –PEI Adsorbent
First, NiFe2O4@SiO2 NPs were prepared according to our previously reported work [[28]]. In brief, NiFe2O4 NPs were produced by adding a mixture of 160 mL 1 M aqueous FeCl3·6H2O and 40 mL 1 M of NiCl2·6H2O quickly to 1 L of a boiling aqueous solution of 1 M NaOH under vigorous stirring. Then, the solution was cooled to room temperature and stirred continuously for 90 min. The resulting precipitate was then purified during four repeated washing–centrifugation–decantation cycles, each using 50 mL of water. 2 g of the NiFe2O4 NPs were dispersed in 25 mL EtOH by ultrasonic treatment for 2 h at 60 °C, and then, 10 mL 25% aqueous ammonia was added to the mixture and stirred at 60 °C for 40 min. Then, 1 mL of TEOS was added, and stirring was continued at the same T for another 24 h. The suspended silica-coated particles were separated from the solution by placing an external magnet in the flask and decanting the supernatant solution. The NPs were washed 3x with 15 mL MeOH and dried in vacuum for 48 h. Finally, the NiFe2O4@SiO2 NPs were calcinated at 800 °C for 2 h.
For the preparation of the NiFe2O4@SiO2–PEI adsorbent, 10 g of PEI was dissolved in 50 mL of hot toluene and then cooled. This was mixed with 472 mg (2 mmol) trimethoxy(3-(oxiran-2-yl-methoxy)propyl)silane, and the mixture was heated under reflux for 8 h. Then, 2.5 g of the NiFe2O4@SiO2 nanoparticles was 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 in vacuo affording 2.9 g of NiFe2O4@SiO2–PEI adsorbent.
3.4. 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−, 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 (1 M) or HCl (1 M) solutions. Adsorption procedure: a 250 mL Erlenmeyer flask was supplied 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:
qe = ((C0 − Ce)V)/m qt = ((C0 − Ct)V)/m
where C0: initial concentration (mg/L), Ce: equilibrium concentration (mg/L), Ct: concentration at the time t, m: amount of adsorbent (g), and V: volume of the solution (L) [[50]].
3.5. Adsorbent Recovery
After the adsorption of the CrO42−, Ni2+, and Pb2+ ions at pH = 6.5, using 0.075 g adsorbent and 250 mg/L metal ions at 298 K for 45 min, the magnetic adsorbent was separated from the reaction batch with the help of an external magnet. For the recovery of the adsorbent material, the adsorbed metals were removed through means of washing with HCl solution (5%) followed by NaOH solution (5%).
4. Conclusions
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 was investigated under variation of pH, adsorbent amount, metal ion concentration, and temperature. The maximum adsorption was achieved at pH = 6.5 and a 250 mg/L CrO42−, Ni2+, and Pb2+ ion concentration and 0.075 g of 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 for the adsorption isotherms was the Langmuir model, and both findings are in line with smooth homogeneous mono-layer adsorption. The adsorption of both the anionic CrO42− and the cation Ni2+ and Pb2+ ions increased with increasing time and decreased with increasing temperature. 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 the binding of these cations to the amine functions of the PEI. The very similar behavior of the anionic CrO42− is probably due to the 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 the 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.
Figures, Scheme and Tables
Graph: Scheme 1 Preparation of the polyethylene imine (PEI) grafted onto core-shell NiFe2O4@SiO2 nanoparticles (NiFe2O4@SiO2–PEI) though trimethoxy(3-(oxiran-2-yl-methoxy)propyl)silane.
Graph: Figure 1 Powder XRD (PXRD) pattern of the as-synthesized (top left) and recovered NiFe2O4@SiO2–PEI after washing with HCl solution (5%) followed by NaOH solution (5%) (bottom left). PXRD pattern of pristine NiFe2O4 (right). Note that the two 2Ɵ scales are not identical.
Graph: Figure 2 FT-IR spectrum of NiFe2O4@SiO2–PEI.
Graph: Figure 3 FE-SEM photograph (left) and EDX analysis (right) of NiFe2O4@SiO2–PEI.
Graph: Figure 4 TGA and DTA analysis of NiFe2O4@SiO2–PEI.
Graph: Figure 5 Adsorption of CrO42−, Ni2+, and Pb2+ ions: effect of pH (45 min, 0.075 g of adsorbent, 250 mg/L of adsorbate, and 298 K).
Graph: Figure 6 Adsorption of CrO42−, Ni2+, and Pb2+ ions over time (pH = 6.5, 0.075 g of adsorbent, 250 mg/L of metal ions, and 298 K).
Graph: Figure 7 Adsorption of CrO42−, Ni2+, and Pb2+ ions: effect of adsorbent amount (45 min, pH = 6.5, 250 mg/L of adsorbates, and 298 K).
Graph: Figure 8 Adsorption of CrO42−, Ni2+, and Pb2+ ions: effect of initial metal ion concentration (45 min, 0.075 g of adsorbent, pH = 6.5, and 298 K).
Graph: Figure 9 Adsorption of CrO42−, Ni2+, and Pb2+ ions over T (45 min, 0.075 g of adsorbent, 250 mg/L of metal ion conc. concentration, and pH = 6.5).
Graph: Figure 10 Linear plot of ln Kc vs. 1/T for the adsorption of CrO42−, Ni2+, and Pb2+ ions.
Graph: 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 concentration: 10 mg L−1). Scale bars in (A–D): 50 nm.
Graph: Figure 12 Adsorption of CrO42−, Ni2+, and Pb2+ ions at pH = 6.5, 0.075 g of adsorbent, 250 mg/L of metal ions, 298 K, and for 45 min in 11 consecutive runs.
Table 1 Parameters and the correlation coefficient (R2) of the kinetic models.
| Linear Equations a | Parameters |
|---|
| CrO42− | Ni2+ | Pb2+ |
|---|
| k | R2 | qe (mg/g) | k | R2 | qe (mg/g) | k | R2 | qe (mg/g) |
|---|
| Pseudo-first order | 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 as the rate constant (min−1), t is the contact time (min), pseudo-second order: t/qt = (1/kqe2) + (t/qe), and the Elovich equation: qt = (ln(αβ)/β) + (lnt/β) with β, α as the Elovich constants.
Table 2 Parameters and the correlation coefficient (R2) of the isotherm models.a.
| Models | | Parameters | |
|---|
| CrO42− | Ni2+ | Pb2+ |
|---|
| 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) as the Langmuir constant; Freundlich linear equation: lnqe = lnkf + lnCe/n with kf and n as the Freundlich constants; Temkin linear equation: qe = BlnA + BlnCe with A and B as the Temkin constants.
Table 3 Thermodynamic parameters for the adsorption of CrO42−, Ni2+, and Pb2+ ions.a.
| ΔG0 (kJ/mol) (T = 298 K) | ΔS0 (J/K·mol) | ΔH0 (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 is ΔG0 (kJ/mol), enthalpy is ΔH0 (kJ/mol), and entropy is ΔS0 (J/K·mol). ΔG0 = −RTlnKC with KC = qe/Ce and RTlnKC = TΔS0·ΔH0.
Author Contributions
The experimental work was carried out by M.K. (Mehdi Khalaj), S.-M.K., M.K. (Mehdi Kalhor), M.Z., and E.T.A. The original draft was written by M.K. (Mehdi Khalaj) and A.K., and both authors edited and revised the original draft. All authors have read and agreed to the published version of the manuscript.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data presented in this study are available on request from the corresponding author.
Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to have influenced the work reported in this paper.
Acknowledgments
Laboratory support provided by the Islamic Azad University, Buinzahra Branch is highly acknowledged.
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Footnotes
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By Mehdi Khalaj; Seyed-Mola Khatami; Mehdi Kalhor; Maryam Zarandi; Eric Tobechukwu Anthony and Axel Klein
Reported by Author; Author; Author; Author; Author; Author
