Partially Reduced Graphene Oxide Modified with Polyacrylonitrile for the Removal of Sm³⁺ from Water

Abstract

An in situ emulsion polymerization method was used for the synthesis of polyacrylonitrile nanoparticles amino-functionalized partially reduced graphene oxide (PAN-PRGO). After that, hydrolyzed polyacrylonitrile nanoparticles amino-functionalized partially reduced graphene oxide (HPAN-PRGO) nanocomposite was achieved by the modification of nitrile groups of the composite polymer chains to carboxylic groups, aminoethylene diamine, and amidoxime functional groups through partial hydrolysis using a basic solution of sodium hydroxide for 20 min. Different synthesized materials were characterized and compared using well-known techniques including transmission electron microscope (TEM), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectra, and X-ray diffraction (XRD). The nanocomposite was structured through the interaction between acrylonitrile’s (AN) nitrile groups and amino-functionalized graphene oxide nanosheets’ amino groups to successfully graft polyacrylonitrile over the surface of functionalized nanosheets as approved by characterization techniques. The synthesized composite was examined for the removal of samarium ions (Sm³⁺) from water. Different experimental conditions including pH, contact time, initial concentration, and adsorbent dose were investigated to determine the optimum conditions for the metal capture from water. The optimum conditions were found to be a contact time of 15 min, pH 6, and 0.01 g of adsorbent dosage. The experimental results found, in a good agreement with the Langmuir isotherm model, the maximum adsorption capacity of Sm³⁺ uptake was equal to 357 mg/g. A regeneration and reusability study of synthesized composite up to six cycles indicated the ability to use HPAN-PRGO nanocomposite several times for Sm³⁺ uptake. The obtained results prove that this polymer-based composite is a promising adsorbent for water treatment that must be studied for additional pollutants removal in the future.

1. Introduction

Recently, one of the most worldwide problems is water pollution with different contaminants [1,2,3,4]. Lanthanide elements are considered one of these harmful contaminants that have increased in water due to their universal demand as significant innovation basic elements in various applications [5]. For instance, optical glasses, catalysts, steel, and ceramics industrial applications utilize lanthanides 17 elements due to their exceptional chemical and physical properties [6]. Unfortunately, due to their bioaccumulation, propensity, and toxicity, unsustainable quantities of these elements are getting into water sources and food chains, causing harmful effects on human health, especially in the vicinity of mines and refineries [7]. For example, the exposition, swallowing, or inhalation of toxic samarium soluble salts can irritate the respiratory tract as well as the eyes and skin [8]. As a result, the removal of Sm³⁺ from water has been a top priority. Several published studies on the removal of Sm (III) by various adsorbents and biosorbents were identified based on the literature [9,10,11,12]. In terms of economy, high-quality products, reliability, performance, versatility, and fabrication, the adsorption-based technique has been widely regarded as the most durable and promising of all existing water treatment techniques [13,14,15,16]. Until today, polystyrene, clays, zeolites, and carbon nanomaterials have also been used as adsorbents in water treatment. However, due to some drawbacks, such as nonbiodegradability and poor adsorption capability, some of these adsorbents are not widely used [17]. It is necessary to go through a chemical modification process to increase the ability to recover from treated water, dissolve in acidic media, avoid swelling, and improve adsorption efficiency [18]. The coordination of metal ions with these adsorbents efficiently depends on their surfaces functional groups, such as amidoxime, amino, hydrazine, and imidazole groups that have chelation properties toward target toxic metals [19,20,21]. Water purification has been achieved using aliphatic polymers with nitrogen functional groups, but their surface area is limited. Though, the polymeric composites of carbon and carbon-based materials have shown very effective adsorption of toxic ions from water/wastewater. Amongst all carbon-based materials, graphene oxide (GO) is a 2-dimensional single thick graphitic carbon atom that can covalently bond to other groups due to the existence of functional groups (reactive oxygen) on its top [22]. Graphene has excellent tailorable functionalities, a large active area, and electrical, mechanical, and thermal properties [23]. Graphene functionalization with polymers containing chelating groups has received a lot of interest in recent years as a way to improve graphene’s adsorption capacity. Polypyrrole functional groups have been utilized for the modification of reduced graphene oxide to capture Hg⁺² ions from water with high selectivity and capacity [24]. Exceptional physiochemical stability can be achieved via covalently linked hybrid structure. In the current study, polymer-grafted reduced GO (rGO) hybrids were synthesized. Polyacrylonitrile (PAN)-grafted ethylene diamine partially reduced GO (GO-NH₂) was synthesized as a novel promising hybrid nanomaterial. The excellent physical properties of polyacrylonitrile, such as high chemical resistance and harness, beside its low cost, simple synthesis, good environment stability, and non-toxicity, encouraged us to use this polymer in the fabrication of this nanocomposite. Additionally, the polyacrylonitrile affinity toward toxic species can be improved via the simple chemical modification of its suspended nitrile (CN) functional groups to amidoxime (AO) groups [25]. Polyacrylonitrile’s non-toxicity and high stability, besides the partially reduced graphene oxide’s large surface area, had driven the synthesis of PAN-partially reduced graphene oxide (PAN-PRGO) nanocomposite for the uptake of samarium ions (Sm³⁺) from water. Herein, the surface of functionalized GO nanosheets were grafted covalently with PAN nanoparticles. PAN-PRGO highly covalently attached nanocomposite was synthesized by the emulsion polymerization of acrylonitrile (AN) with ethylene diamine functionalized graphene oxide (GO), in which surface free amino groups of functionalized GO reacted with the nitrile group of AN for the covalent attachment. This step was followed by formation of carboxylate, amidoxime, and amide groups over the nanocomposite surface by partial hydrolysis of free nitrile groups using NaOH to enhance the chelation and adsorption properties of the nanocomposite toward Sm³⁺ ions.

2. Materials and Methods

2.1. Chemicals

Samarium (III) nitrate hexahydrate (Sm(NO₃).₆H₂O, 99.9%), ethylene diamine tetraacetic acid (EDTA, 98%), azo-bis-isobutyronitrile (98%), sodium dodecyl sulfate (SDS) (99%), ethylene diamine (99%), acrylonitrile (99%), and graphite flakes (99.9%) were supplied from Sigma-Aldrich and used as received without any purification. In all experiments, the used water was deionized water.

2.2. PAN-PRGO Nanocomposite Synthesis

Improved Hummer’s method [26] was used for the synthesis of functionalized graphene oxide (NH₂-GO). A three-necked round-bottomed flask was used to mix 121 mL distilled H₂O with 0.42 g SDS and 0.52 g of NH₂-GO. The micelles were produced after a half-hour of stirring and sonication. After that, 70 °C was adjusted to heat the suspension solution and kept at the same temperature. Then, 2, 2′-azobisisobutyronitrile (0.151 g dissolved in 1.5 mL N,N-Dimethylformamide) was added to the suspension. Acrylonitrile as a monomer (11 mL) was injected after 16 min. The reaction was then permitted to proceed with continuous stirring and nitrogen atmosphere for 5 h. Lastly, the synthesized PAN-PRGO nanocomposite was collected by centrifugation and washed several times with water and ethyl alcohol to eliminate excess unreacted acrylonitrile, then heated at 70 °C overnight for dryness. For the synthesis of hydrolyzed polyacrylonitrile/partially reduced graphene oxide nanocomposite (HPAN-PRGO) and hydrolyzed polyacrylonitrile nanoparticles (HPAN), synthesized according to the literature [27], NaOH (4%, 4 mL) was added to 0.22 g of PAN-PRGO or PAN nanoparticles then refluxed for 25 min at 70 °C. Lastly, the HCl and H₂O were used to wash the filtered mixture to achieve pH 7.5, after which it was dried at 70 °C under vacuum.

2.3. Materials Characterization

The synthesized materials, including HPAN-PRGO, HPAN, GO-NH₂, and GO, were characterized using different techniques. Fourier-transform infrared spectroscopy (FT-IR-6100 Jasco, Tokyo, Japan) was used to perform FT-IR spectra in the range of 400–4000 cm⁻¹ at room temperature and collected at a resolution of 4 cm⁻¹. X-ray diffractometer (XRD, X’Pert Pro, PANanalytical, Almelo, The Netherlands) was used to measure XRD using Cu Ka radiation (λ = 1.5406 Å) operated at 45 kV. The diffraction intensities were recorded over the 2θ ranging from 5° to 90° with the constant scanning rate of 1° min⁻¹. The morphology and size of different materials were determined using a scanning electron microscope (SEM, Quanta FEG 250, Eindhoven, The Netherlands) and a transmission electron microscope (TEM, JEOL JEM-2100 Plus, Hillsboro, USA), respectively, operated at 200 KV. The Raman spectra were investigated with a 532 nm laser beam using a Raman spectrometer (Horiba Inc., Michigan, USA).

2.4. Sm³⁺ Adsorption Experiments

Batch adsorption experiments were used for the study of the adsorption capacities of synthesized materials toward Sm³⁺ removal from water, with the investigation of the effect of different factors on the adsorption process including contact time, initial concentration of Sm³⁺, adsorbent dose, and pH value. During the study of initial concentration effect, 30 mL of Sm³⁺ ions in acetate buffer with a concentration range from 10 to 300 mg/L was mixed with 0.01 g of adsorbent and shaken for 30 min, then filtered and analyzed for the presence of Sm³⁺ ions using atomic absorption spectrometry. The pH effect on Sm³⁺ adsorption was also investigated in the range of 2 to 8 adjusted using HCl and NaOH by mixing 0.01 g of adsorbent with 30 mL of 100 mg/L Sm³⁺ solution. The solution was shaken for one hour, then filtered and analyzed. To determine the dosage effect on Sm³⁺ adsorption, 30 mL of 100 mg/L of Sm³⁺ solution was mixed with different masses of adsorbent in the range of 0.001 to 0.05 g with continuous shaking for 30 min, followed by filtration and analyzing. To study the adsorption kinetics, 30 mL of 100 mg/L of Sm³⁺ solution was mixed with 0.01 g of adsorbent at different times of adsorption in the range of 0.5 to 60 min. Additionally, the adsorption isotherm of Sm³⁺ ions was studied by mixing 0.01 g of adsorbent with 30 mL of Sm³⁺ ions in the concentration range of 100 to 1500 mg/L with shaking for 30 min, followed by filtration and analyzing. Furthermore, the reusability of HPAN-PRGO for the removal of Sm³⁺ ions was studied up to six cycles. After each adsorption cycle the adsorbent was filtered and the adsorbed ions were desorbed by washing with 2 M HCl, buffer solution, and distilled water, followed by drying at 45 °C to be used in the next cycle. All previous experiments were performed in triplicate.

3. Results and Discussions

3.1. Materials Characterization

The synthesized materials were characterized using different techniques, as discussed in this section. For the morphology characterization of HPAN-PRGO, PAN-PRGO, HPAN, PAN, GO, and GO-NH₂, SEM and TEM images are shown in Figure 1 and Figure 2. A smooth surface with several folds of GO was confirmed with TEM images, besides its layered structure with lateral dimensions of some micrometers. GO-NH₂ reserved this multilayered composition of GO due to the scrolling of nano-sheets as shown in its TEM image that retained its nanostructure and high surface area. This large surface area of GO was also confirmed from the SEM image that showed the agglomeration of GO in a 2-D multilayered composition with lateral sizes of some nanometers and a smooth surface. Shaded and bright areas can be formed on the surface of GO via the overlap of dissimilar layers. There was a similarity between the GO and GO-NH₂ SEM images with much folded and roughed GO-NH₂ that retained the high surface area with a functionalization feature. The PAN nanoparticle growing over graphene sheets homogenously was confirmed from SEM and TEM images of HPAN-PRGO and PAN-PRGO. Hence, the amine-functionalized graphene oxide nanosheets grafted with PAN through polymerization. Figure 1g shows the particle size distribution of HPAN-PRGO nanocomposite. From the TEM image and the histogram, the average particle size of HPAN-PRGO nanocomposite was 4.7 nm. The average particle size was calculated by 174 measuring the diametersnanoparticles in TEM image by ImageJ software (ver 2, LOCI, Wisconsin, USA, 2014).

According to SEM, the HPAN and PAN nanoparticles had a narrow size distribution, mostly below 60 nm. In addition, the SEM image’s high magnification revealed spherical particles with sharp edges of PAN latex. The HPAN particles showed an increase in diameter when compared to PAN particles, due to the formation of amide (CONH₂) and carboxyl (COOH) groups on the polymer chains by hydrolysis of cyano (CN) groups, but no major change in overall morphology [28]. Although the bulk of the latexes are still made up of spherical crystals, evidence of neighboring latex conglutinating to one another at lateral points due to hydrolysis or crosslinking was discovered.

The Raman spectra with two characteristic D and G bands of HPAN-PRGO, GO-NH₂, and GO are shown in Figure 3a. D and G bands are at 1357 and 1571 cm⁻¹, respectively. The quality of graphitic constructions was determined using the D band to G band intensity ratio (I_D/I_G), where this ratio for strongly ordered pyrolytic graphite reaches zero as an indicator of the defects and degree of disorder in the graphitic system. This ratio was calculated from three peaks and the value was the average of three values. This ratio in GO was found to be 0.94 which was a little more than the ratio in aminated GO that was equal to 1.13, which was attributed to the increased defects in GO structure due to the covalent bonding between GO sheets and ethylene diamine. The GO-NH₂ I_D/I_G ratio confirms the better quality of the prepared reduced GO, since this value should be between 1.12 and 1.42, as reported in the literature [29]. This ratio was also found to equal to 0.86 in HPAN-PRGO, which was lower than the ratio in GO, indicating the disordered structure due to the combination between free amino groups of GO sheets and nitrile groups of HPAN [24]. Synthesized materials including HPAN-PRGO, HPAN, GO-NH₂, and GO were characterized using XRD as shown in Figure 3b. Due to the intermolecular repulsion of the nitrile dipoles, the HPAN spectrum shows a diffraction peak at 2θ = 17.40, which corresponds to the (100) crystallographic planes of PAN (due to PAN intermolecular spacing) [30]. The spectrum for HPAN-PRGO also indicates a poor reflection peak at 2θ = 17.0, which can be attributed to polyacrylonitrile’s (100) crystal planes, implying that GO aids in the forming of PAN nanoparticles. The occurrence of a peak at 2θ = 10.97 in GO sheets produced by chemical oxidation of graphite is attributed to the functional oxygen-containing groups on the sheets [31]. The sharp peak of GO diminished after chemical functionalization with ethylene diamine, and a wide peak at 2θ = 25.0 existed, suggesting that the GO was partly reduced, possibly due to the restacking of the aminated GO nanosheets [32]. Based on Bragg’s Law, the interlayer spacing for GO and GO-NH₂ was 8.753 and 8.185 Å, respectively [33]. The decreased intensity of PAN peaks after implanting HPAN over aminated GO sheets indicated the reaction between the nitrile group of PAN and free amino groups of GO that caused the damage of bonds in the inner crystal area, and a reduction in the polar interaction between the molecular chains.

FT-IR spectra of synthesized materials including HPAN-PRGO, HPAN, PAN-PRGO, PAN, GO-NH₂, and GO are shown in Figure 4a,b to approve their formation. The FT-IR of GO showed several peaks that corresponded to various functional oxygen groups, such as epoxy and alkoxy stretching, that appeared at 1158 and 1082 cm⁻¹, respectively [34,35]; hydroxyl groups stretching vibrations appeared as a wide band at 3337 cm⁻¹, and the carbonyl of the carboxylic group appeared at 1732 cm⁻¹ [35]. The FT-IR spectra of amino-functionalized GO showed two new peaks, for N–H bending that appeared at 1652 cm⁻¹, and N–H stretching that was represented at 3273 cm⁻¹ [36]. These bands show that the amine group and the amide linkage effectively functionalized graphene oxide with ethylene diamine. The characteristic peaks of PAN that appeared at 1460 and 2921 cm⁻¹ represented CH₂ bending and stretching vibrations, while the nitrile stretching vibration was represented by the peak at 2238 cm⁻¹ [37]. Additionally, the nitrile group was confirmed by the peak at 1644 cm⁻¹. The FT-IR spectra of PAN-PRGO showed a peak at 2921 cm⁻¹, as in PAN, that corresponded to CH₂ stretching vibration, and a peak corresponding to a nitrile group at 2239 cm⁻¹. Additionally, the formation of N–C=N or NH–CO due to the interaction between AN nitrile groups and amino-functionalized GO was confirmed by the appearance of a new peak corresponding to an amide group at 1574 cm⁻¹, and the disappearance of or reduction in the amino-functionalized GO peaks at 1652 and 3273 cm⁻¹. Additionally, NH–C=N–CH₂ formation was represented by the peaks at 1219 and 1460 cm⁻¹. The FT-IR of PAN partially hydrolyzed using sodium hydroxide showed peaks at 1667 and 1574 cm⁻¹, corresponding to amino and carbonyl stretching bands, and a reduced band at 2239 cm⁻¹ after hydrolysis. The HPAN-PRGO FTIR spectra showed two new peaks at 1667 and 1574 cm⁻¹ confirming the occurrence of hydrolysis. Additionally, the nitrile group band at 2239 cm⁻¹ completely disappeared, while two overlapped peaks corresponding to COONa and N–H were observed at 1574 and 1667 cm⁻¹. The GO was confirmed to be partially reduced by the reduction in the epoxy group peak at 1082 cm⁻¹. Thus, these results demonstrate that aminated GO was successfully produced, and that AN was simultaneously implanted and polymerized at the amino group of functionalized GO during the polymerization process, yielding covalently hybrid-prepared (HPAN-PRGO) nanocomposite. To determine the enhanced surface area of synthesized nanocomposite, the N₂ adsorption–desorption isotherm of synthesized materials was performed and is shown in Figure 4c. The surface areas of HPAN, GO-NH₂, and HPAN-PRGO were 27.0, 36.3, and 58.0 m²/g, respectively. The effect of modification on the surface area is clear from the isotherm curve that further affects the adsorption capacities toward the uptake of Sm³⁺ ions.

3.2. Initial Concentration Effect

The effect of initial concentration on the adsorption of Sm³⁺ over synthesized materials was studied in the concentration range of 10–300 mg/L, as shown in Figure 5a. According to Figure 5a, the adsorption capacity was increased as the initial concentration increased from 10 to 100 mg/L. The maximum adsorption capacity for the removal of Sm³⁺ was reached at initial concentrations of 100, 100, and 50 mg/L in the case of HPAN-PRGO, GO-NH₂, and HPAN, respectively. After the equilibrium was reached there was no increase in the adsorption capacity with the increase in the initial concentration, meaning that there were no available sites for adsorption after reaching the equilibrium. The maximum adsorption capacities were 92.5, 74.0, and 22.7 mg/g for Sm³⁺ removal over HPAN-PRGO, GO-NH₂, and HPAN, respectively.

3.3. PH Effect

The removal of Sm³⁺ ions over the synthesized HPAN-PRGO, GO-NH₂, and HPAN at different pH values was studied, as shown in Figure 5b. The studied pH range was 2.0 to 8.0, adjusted using 0.1 N of HCl and NaOH. For all studied adsorbents, the maximum adsorption capacities were reached at pH equal to 6.0, and found to be 92.5, 74.0, and 22.7 mg/g for HPAN-PRGO, GO-NH₂, and HPAN, respectively. According to Figure 5b, the relationship between sorption and solution pH has an extreme principle. The adsorption of Sm³⁺ ions increased with pH increasing, peaked around pH 6.0, and then started to drop again. The low adsorption capacities of different adsorbents at pH < 6 was attributed to the protonated state of adsorbent functional groups (O=C–NH⁺, NH₃⁺, and COOH⁺₂) causing an increase in the electrostatic repulsion with the positive samarium ions, while at pH > 6 the adsorption capacities decreased due to the precipitation of Sm³⁺ ions as hydroxides, which made their capture by the adsorbent decrease and may have been due to competitive adsorption from hydroxyl ions caused by hydrogen bonding formation, resulting in a reduction in the amount of possible adsorption sites. Further experiments were performed at pH equal to 6.0 as the results in this section indicated that the optimum pH value was 6.0.

3.4. Contact Time Effect

The contact time effect on Sm³⁺ adsorption over HPAN, GO-NH₂, and HPAN-PRGO was studied over a time range of 10 to 60 min and the results are shown in Figure 6a. According to Figure 6a, most Sm³⁺ ions interacted rapidly with the synthesized materials HPAN, GO-NH₂, and HPAN-PRGO during the first 10 min of the removal process. This rapid stage occurred due to the existence of empty adsorption sites with a large number on the surface of the adsorbent that caused the rapid capture of ions within a short period. After that, the equilibrium was achieved within 15 min, which caused this time to be chosen as the optimum contact time. The repulsive forces between the adsorbed ions were augmented when the quantity of Sm³⁺ adsorbed onto the adsorbent increased, and adsorption tolerance for free metal ions was exacerbated accordingly. Hence, after reaching the equilibrium, there was no further increase in the adsorption capacities due to repulsion forces, as well as the occupancy of adsorption sites with ions, and there were no free sites to capture additional ions, so time became without effect on the adsorption process.

3.5. Dose Effect

The dose effect on the adsorption of Sm³⁺ ions over HPAN, GO-NH₂, and HPAN-PRGO was studied over a range of 0.1 to 2.5 g/L and the results are shown in Figure 6b. This study aimed to investigate the effect of adsorbent weight on Sm³⁺ removal using synthesized nanocomposites. According to Figure 6b, the adsorption capacities reached their maximum at 0.3 g/L, which was equal to 23, 74, and 93 mg/g for HPAN, GO-NH₂, and HPAN-PRGO, respectively. However, at higher doses, the adsorption capacities decreased. The range from 0.1 to 0.3 g/L showed an increase in the adsorption capacities toward Sm³⁺ ions, with approximately 15% of that attributed to the fixed volume of solution in this case that lead to an increase in the chance of adsorbent interaction with pollutant ions. At definite dosage, a redistribution of the metal ions in a higher volume of adsorbent occurs with any further increase in the adsorbent dosage, and the contact efficacy stops limiting the adsorption. Accordingly, there was a drop in the adsorption activity per unit mass of adsorbent at this dosage.

3.6. Adsorption Isotherm

The maximum adsorption capacity of captured ions can be possibly investigated by plotting the adsorption isotherm. Different isotherms can describe the interaction between pollutant ions and active sites of adsorbent and describe also the isotherm behavior, such as the length of the linear section, the existence of a bending section, and degree of curvature, etc. The studied isotherms hit plateaus in a given concentration range, as shown in Figure 7. Langmuir and Freundlich isotherm models were used to fit the equilibrium data, as shown in Figure 7a,b, respectively, and the obtained parameters for both isotherms are introduced in

Table 1. Freundlich and Langmuir adsorption isotherms’ different parameters for Sm³⁺ adsorption over synthesized nanomaterials.

Adsorbent	Freundlich			Langmuir
	KF	N	R2	qm	RL	R2
HPAN-PRGO	84.523	4.486	0.846	357	0.015	0.998
GO-NH2	38.553	3.7	0.937	218	0.035	0.994
HPAN	21.76	3.954	0.947	82	0.032	0.996

.

For HPAN-PRGO, GO-NH₂, and HPAN materials as adsorbents for the capture of Sm³⁺ ions, the experimental data fit lower to the Freundlich model isotherm than to the Langmuir model isotherm, based on the R_L (dimensionless constant) values that fell in the favorable range (0–1) and based on R² (correlation coefficient) values. These values indicated that Sm³⁺ ion capture using HPAN-PRGO, GO-NH₂, and HPAN nanomaterials were a better fit to the Langmuir isotherm model than the Freundlich model. The monolayer of adsorbed ions is designed on the adsorbent surface according to Langmuir’s theory and the enthalpy of adsorption sites, and the energy is identical while a heterogeneous adsorption is described according to Freundlich’s theory [38].

According to this, the Sm³⁺ ions are adsorbed over HPAN-PRGO, GO-NH₂, and HPAN nanomaterials in a monolayer by active adsorption sites that are energetically identical. Subsequently, it is fair to say that the synthesized substances have homogeneous composition, with the same adsorption core geometry, resulting in identical adsorption forces for ion extraction from aqueous solutions. According to the Langmuir isotherm, the maximum adsorption capacities are 357, 218, and 82 mg/g for the Sm³⁺ ions’ adsorption over HPAN-PRGO, GO-NH₂, and HPAN, respectively. These results of the isotherm indicate the capture of a high quantity of Sm³⁺ ions that is attributed to the existence of carboxylate and amine groups with a large number over the nanocomposite surface.

3.7. Reusability Study

The ability to reuse any adsorbent is a key factor determining the overall cost of water treatment processes [39,40]. The reusability study of the adsorbent has been carried out by repeating the pollutant ion adsorption–desorption cycle many times by the same adsorbent [41,42,43]. The desorption here of Sm³⁺ ions was achieved using 2 M HCl and buffer solution. Then, the desorbed HPAN-PRGO nanocomposite was washed with sodium hydroxide to retain the –ve charge of the adsorbent carboxylic groups. After that, the adsorbent was washed several times with deionized H₂O and dried to be used in the next cycle. The adsorption–desorption process for Sm³⁺ ion removal using HPAN-PRGO nanocomposite was performed up to six successive cycles. The reversibility of the adsorption process and excellent regeneration properties of the HPAN-PRGO nanocomposite for Sm³⁺ removal was indicated from the loss in the removal efficiency after six cycles, which was found to be only 5% compared to the first cycle, as shown in Figure 8. After the last cycle, the removal efficiency was maintained above 90%. Based on these findings, it can be suggested that the HPAN-PRGO adsorbent can be used continuously to remove Sm³⁺ ions from water with low cost and high efficiency.

3.8. Comparative Study of Samarium Ions Adsorption

To highlight the effectiveness of the synthesized material as an Sm³⁺ adsorbent, the maximum adsorption capacity obtained here for Sm³⁺ ions was compared with previously reported results. The previous studies for Sm³⁺ adsorption are summarized in

Table 2. Comparison between the removal of Sm³⁺ ions over HPAN-PRGO nanocomposite and previous studies.

Adsorbent	Adsorption Capacity (mg/g)	Ref.
HPAN-PRGO	357.0	This study
Oxidized MWCNTs	89.3	[49]
Fe2O3/SiO2/R1R2PO3Na	180.0	[50]
Mesoporous Fe3O4 mSiO2–DODGA nanoparticles	28.60	[51]
ZIF-8 NPs	281.10	[52]
SiO2/PVI/H2PO4− NPs	160.0	[53]
Sargassum Sp.	51.10	[54]
PAN SDS	97.70	[55]
samarium (III) ion-imprinted polymer (IIP) particles	12.25	[56]
Heulandite	8.53	[57]

. According to previously reported results, HPAN-PRGO nanocomposite revealed excellent adsorption capacity for Sm³⁺ ion removal compared to reported adsorbents. Therefore, synthesized HPAN-PRGO nanocomposite may be effectively used for the capture of Sm³⁺ ions from aqueous solution. An important study that must be considered in the future is to determine the thermodynamic parameters (ΔG, ∆H, and ∆S) [44,45,46,47,48] of the adsorption process to get a complete picture regarding the applicability of the HPAN-PRGO nanocomposite in large-scale water treatment.

However, is very important to point out that the material cost is an important factor prohibiting the widespread use of composites at a large scale. In some cases, recent advances in composites are driving composites to be more competitive and cost effective. Concerning the material tested in the present study, more investigations are needed to evaluate the process efficiency using real wastewater at a large scale. Additionally, the competitive applicability of this innovative adsorbent should be evaluated, taking into consideration various parameters related to the material proprieties (degradation life cycle, regeneration, etc.) and to the waste generated by the process, including the loaded pollutant’s disposal and chemicals used for the adsorption/desorption process. Moreover, cost comparison between this innovative adsorbent and other materials should be conducted.

4. Conclusions

In this study, HPAN-PRGO nanocomposite as a novel adsorbent was synthesized successfully and applied for the removal of samarium ions from water. The synthesized HPAN-PRGO nanocomposite was characterized using various techniques (SEM, TEM, FT-IR, and XRD). The experimental results for Sm³⁺ ion removal over HPAN-PRGO nanocomposite were found to fit well with the Langmuir model, with a maximum adsorption capacity equal to 357 mg/g. This indicated that the ions were adsorbed over the surface of the nanocomposite as a monolayer via energetically identical adsorption sites. Interestingly, the nanocomposite reuse was investigated up to six successive cycles with no significant loss in the Sm³⁺ removal efficiency. Finally, we can conclude that HPAN-PRGO nanocomposite can be considered as a promising material for water treatment that could be studied for the removal of additional metals and dyes.