Unleashing the Power of Graphene-Based Nanomaterials for Chromium(VI) Ion Elimination from Water

Abstract

Graphene-oxide-based nanomaterials have gained attention in recent years in the field of heavy metal removal. In this work, graphene oxide (GO) and graphene-oxide-coated silica nanoparticles (GO/SiO₂) were synthesized for the efficient removal of Cr(VI) ions from water. Synthesized nanosorbents were characterized by FTIR, Raman spectroscopy, and Transmission Electron Microscopy (TEM). The effects of the pH and the concentration of Cr(VI) ions in adsorption, using GO and GO/SiO₂, was studied using the batch process. The results of the study indicated that the maximum removal percentage was shown at pH 3 for both adsorbents. Comparatively, GO/SiO₂ showed a higher removal percentage (92.28%) than GO (86.15%) for Cr(VI) at a concentration of 50 ppm. The results validate that the removal of Cr(VI) ions is highly concentration-dependent and pH-dependent. This study shows that GO and GO/SiO₂ are efficient adsorbents and that GO/SiO₂ has great potential over GO for the removal of Cr(VI) ions from water.

1. Introduction

The availability of clean drinking water is critical for maintaining a healthy life. Freshwater resources have been steadily declining in recent decades due to increased human consumption, pollution, and climate change. Among the toxic pollutants found in water bodies, heavy metals deserve special attention. Heavy metals are generally referred to as those metals which possess a specific density of more than 5 g/cm³ and adversely affect the environment and living organisms [1]. They are very toxic even at low doses, very persistent in the environment and living tissues, and easily migrate upstream of the food chain. The toxic effects of these metals are harmful to the human body and its proper functioning. Moreover, monitoring and removing them are costly procedures. The most commonly found heavy metals in wastewater include arsenic, cadmium, chromium, copper, lead, nickel, mercury, and zinc, all of which badly affect human health and the environment [2]. Heavy metals enter into the surrounding environment by natural means and as a result of human activities. Various sources of heavy metals include soil erosion, natural weathering of the earth’s crust, mining, industrial discharge, urban runoff, sewage discharge, pest- or disease-control agents applied to crops, and many others [3].

Chromium (Cr) is the seventh most abundant element on earth [4]. It occurs in several oxidation states in the environment, ranging from Cr²⁺ to Cr⁶⁺.The most commonly occurring forms of Cr are trivalent- Cr³⁺ and hexavalent- Cr⁶⁺, and both states are toxic to animals, humans, and plants [5]. Hexavalent chromium is thought to have 100-fold extra toxicity than trivalent chromium for each acute and persistent exposure, due to its excessive water solubility and mobility and easy reduction. The respiratory tract is the important goal organ for hexavalent chromium after inhalation exposure in humans. Chronic human exposure to high concentrations of hexavalent chromium through inhalation or oral exposure may produce effects on the liver, kidney, gastrointestinal, and immune systems, and possibly the blood. Dermal exposure to hexavalent chromium can result in contact dermatitis, sensitivity, and ulceration of the skin [6].

Heavy metal pollution has become a serious problem in both developed and developing countries [7,8,9,10]. In addition to the detection of heavy metal ions, their elimination from water is quite a crucial concern. Many strategies can be employed for the removal process, and common methods for removing heavy metals from water sources include precipitation, flocculation, membrane separation, ion exchange, and evaporation [11,12,13]. Among the many methods, adsorption is often the most effective technique of choice, due to its low cost, simple design, strong operability, and especially its high elimination performance from dilute solutions [14,15]. Some widely used adsorbents for the adsorption of metal ions include activated carbon, clay minerals, biomaterials, industrial solid wastes, and zeolites [16]. The development of nanotechnology has provided various nanomaterials as efficient adsorbents for removing metal ions from water [17,18,19,20]. Heavy metal removal using graphene and graphene-based nanomaterials has grown significantly during these years. Development of various functionalized nanomaterials, such as graphene oxide (GO), enhances the removal efficiency toward metal ion uptake. The presence of oxygen-containing functional groups (hydroxyl, carboxyl, epoxy, etc.) for metal ion complexation, large specific surface area, fast adsorption speed, affinity for a large number of metal ions, and low preparation cost make GO a promising adsorbent for heavy metal removal [21,22]. Rajesh et al. reported a novel and straightforward approach for effectively removing toxic chromium by using the intriguing interaction between exfoliated graphene oxide (EGO), trioctylamine (TOA), and Cr(VI) [23]. Recently, numerous research works have studied the development of graphene-oxide-based nanomaterials and their environmental applications [24]. For instance, Deng et al. synthesized magnetic graphene oxide and used it as an adsorbent for the removal of Cd²⁺ from wastewater [25]. Likewise, a synthesized magnetic graphene oxide (mGO) grafted with amine groups (mGO-PAMAM) was found to be effective in the removal of Cd²⁺, Pb²⁺, and Cu²⁺ from wastewater and to have excellent regeneration capability for future use [26]. Prepared magnetic chitosan and graphene nanocomposites functionalized with EDTA (EDTA-MCS/GO) were used for the removal of Cr³⁺ from water [27]. In this study, we synthesized graphene oxide (GO),amine-modified silica using APTES and graphene-oxide-coated silica nanoparticles (GO/SiO₂), for the removal of Cr(VI) ions from water. Generally, the interaction between metal ions and the surface oxygen-containing groups plays an important role in the adsorption capacity of heavy metals on CNTs and graphene-based nanomaterials [28]. Since silica nanoparticles and their derivatives have been found to possess excellent qualities, such as a definite pore size, high surface area, strong selectivity, and adsorption capability, a lot of studies have been conducted employing them as adsorbents [29]. Further modification of nano silica and synthesized GO is possible and thereby overcomes the self-aggregation and also increases the removal efficiency. When APTES-modified silica is coated with GO, the adsorption rate is found to be increased due to its enhanced properties.

In the present work, we successfully prepared an amine-modified silica-GO self-assembled novel high-efficiency adsorbent for the successful removal of Cr(VI) from the solution and compared the result with GO. The combination of amine-modified silica and GO creates a synergistic effect that improves Cr(VI) adsorption. Graphene oxide possesses a large surface area and abundant oxygen-containing functional groups, such as hydroxyl and carboxyl groups, which can form coordination complexes with metal ions. The amine-modified silica, with its amino groups, can interact with the functional groups on GO, creating a favorable environment for Cr(VI) adsorption. The synergistic effect between the amine-modified silica and GO enhances the overall adsorption capacity for Cr(VI). The properties of the prepared adsorbent were studied using various spectroscopic and microscopic methods. We achieved a high-percentage removal of Cr(VI) in the silica-GO hybrid system.

2. Materials and Methods

2.1. Materials

Potassium dichromate (K₂Cr₂O₇), graphite (20-micron, synthetic CAT#282863), potassium permanganate (KMnO), Conc.H₂SO₄, sodium nitrate (NaNO₃), hydrochloric acid (HCl), sodium carbonate (Na₂CO₃), Nano silica (10 nm diameter, spec. surface area 175–225 m²/g (BET), 99.8% trace metals basis. CAT#718483), 3-Aminopropyltriethoxysilane (APTES), HCl, ammonia, ethanol, and 30% H₂O₂ were purchased from Sigma Aldrich (St. Louis, MI, USA). All the chemicals were used as received, unless otherwise specified.

2.2. Methods

2.2.1. Synthesis of Graphene Oxide

Graphene oxide was synthesized by modified Hummer’s method [30]. Then, 5 g of graphite, 115 mL of sulfuric acid, and 5 g of NaNO₃ were taken in a beaker and stirred for 30 min in an ice bath. After that, 15 g of KMnO₄ was added very slowly and stirred for 40 min. We then slowly raised the temperature to 35 °C for 12 h, and to this, 230 mL of distilled water was added slowly. The temperature was further raised to 90 °C for 1 h, and to this, 400 mL of distilled water and 50 mL of 30% H₂O₂ were added. Then, we removed the supernatant and washed it with 5% HCl several times to remove sulphate salt. The solution was then made alkaline by treating it with 5% Na₂CO₃. Afterward, it was centrifuged and dried it in an oven.

2.2.2. Synthesis of Amine Modified Silica

The APTES and silica nanoparticles were used to create amine modified silica nanoparticles. This was accomplished by dissolving 1 mL of APTES in 200 mL of ethanol in a beaker. This solution was then mixed with 10 g of SiO₂. The resulting mixture was sonicated and stirred vigorously for one hour. The solution was then given 150 mL of water, followed by 30 min of stirring and 15 min of sonication. The mixture was centrifuged for 20 min at 14,000 rpm, the solid residue recovered and repeatedly rinsed with ethanol. The amine modified silica nanoparticles collected by gently heating the gel-like substance for three hours at 50 °C and 0.2 mbar of reduced pressure.

2.2.3. Synthesis of Self-Assembled Graphene-Oxide-Coated Silica Nanoparticle

Synthesized graphene oxide was dispersed in distilled water. Modification of amine on nano silica was accomplished by using 3-aminopropyl triethoxysilane in ethanol/water mixture (9:1). Nano silica purchase from sigma Aldrich is modified by using 3-Aminopropyl triethoxysilane (APTES) in ethanol medium (Figure 1) and the amine modified silica was also dispersed in distilled water and prepared the self-assembled structures of graphene oxide coated silica nanoparticles by mixing along with stirring and the core shell precipitates out and washed and dried. Then,10 g of amine-modified silica was dispersed in 500 mL distilled water via sonication and stirring. In another beaker, 0.1 g of graphene oxide was dispersed in 500 mL of distilled water. The two 500 mL solutions were mixed thoroughly and stirred for another 6 h. Centrifuge the precipitate and dried in vacuum oven at 80 °C.

2.2.4. Preparation of Stock Solution

Stock solution of chromium was prepared by dissolving 100 mg of potassium dichromate (K₂Cr₂O₇) in a 100 mL standard flask. From this stock solution of 1000 ppm, different concentrations of standard solutions (1, 10, 30, 50, 100, and 200) were prepared.

2.2.5. Optimization of pH

A series of standard solutions, 1 ppm, 10 ppm, 30 ppm, 50 ppm, 100 ppm, and 200 ppm, were prepared in a 100 mL standard flask. The pH was adjusted to 3, 5, 7, and 9 for a 30 ppm solution, and 20 mg of GO was added to each, sonicated for 20 min, and allowed to settle down for 24 h. The solutions were decanted using Whatman No. 41 filter paper, and the samples to be analyzed were filtered using a syringe filter. Then ICP-MS analysis was performed, and the results were used for determining the pH at which the maximum adsorption takes place. The same procedure was carried out using silica-coated GO for a 30 ppm standard solution, and the optimum pH was determined.

2.2.6. Batch Adsorption Test on GO and GO-Coated Silica Nanoparticles

To investigate the adsorption of heavy metal ions (Cr (VI)) on GO and GO-coated silica nanocomposites, a batch adsorption test was performed. First, 20 mg of GO was added to 20 mL of different concentrations of standard solutions (1 ppm, 10 ppm, 30 ppm, 50 ppm, 100 ppm, and 200 ppm). Then, 20 mg of GO-coated silica nanoparticles was added to the next set of 20 mL standard solutions. The pH was optimized, then the sample was sonicated for 20 min, allowed to cool at room temperature, and it was left at room temperature for 24 h for equilibration. It was then decanted using Whatman No.1 filter paper, and the sample to be analyzed was filtered using a syringe filter. Then ICP-MS analysis was carried out for the filtrate that was obtained after sonication, filtration, and syringe filtration. The quantity of adsorbed heavy metal ions was calculated as the percentage of the difference between concentrations of metal ions before and after adsorption as follows [31,32]:

$$\mathit{P}\mathit{e}\mathit{r}\mathit{c}\mathit{e}\mathit{n}\mathit{t}\mathit{a}\mathit{g}\mathit{e} \mathit{A}\mathit{d}\mathit{s}\mathit{o}\mathit{r}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n} \left(\mathit{R}\mathit{e}\mathit{m}\mathit{o}\mathit{v}\mathit{a}\mathit{l}\right)=\frac{{\mathit{C}}_{\mathit{i}}-{\mathit{C}}_{\mathit{e}}}{{\mathit{C}}_{\mathit{e}}}\times \mathbf{100}$$

where C_i is the initial heavy metal concentration (ppm), and C_e is the heavy metal concentration at final equilibrium (ppm).

2.3. Characterizations

Characterization of graphite and graphene oxide was performed using FTIR spectrometer (FTIR spectrometer Perkin Elmer Spectrum 2). The Raman spectra were obtained using WITEC Alpha300 RA—Confocal Raman Microscope with AFM. Morphological studies of GO and GO-coated silica nanoparticle were performed with a transmission electron microscope (TEM—JEOL 2100, high resolution, Tokyo, Japan). Amounts of heavy metal removal from standard solutions were determined by using ICP-MS (iCAP Q-model-Thermo).

3. Results and Discussion

3.1. Characterization of GO and GO/SiO₂

FTIR measurement was employed to investigate the bonding interactions in graphite before and after the oxidation process. Figure 2 shows the FTIR spectra of graphite, graphene oxide, and GO/SiO₂.

Evidence of graphite oxidation could be obtained from the analysis of spectral data. The different bands present in the spectra indicate different functional groups present on GO. Figure 2 shows that the synthesized GO has a peak between 1000 and 1100 cm⁻¹ that is attributed to the epoxy (-O-) group [33]. The peak between 1700 cm⁻¹ and 1720 cm⁻¹ is associated with the stretching of C=O bonds of the carboxyl group (-COOH) [34] or carbonyl group [26,35]. The peak located between 3500 and 3400 cm⁻¹ is ascribed to the -OH stretching mode hydroxyl group [26]. The FTIR spectrum shows oxygen-containing functional groups such as carbonyl (C=O), carboxyl—(COOH), epoxy (C-O-C), and hydroxyl (O-H) groups present on GO. The FTIR spectra of graphite do not show such properties, thereby confirming the successful incorporation of oxygen-containing groups into the graphite. From the FTIR spectra of GO-SiO₂, we can confirm the successful incorporation of SiO₂ into the GO sheets. The characteristic peaks at 1199 cm⁻¹, 1060 cm⁻¹, 970 cm⁻¹, and 800 cm⁻¹ are attributed to the Si-O-C asymmetric stretching vibration Si-O-Si asymmetric stretching vibration, Si-OH stretching vibration, and Si-O-Si symmetrical stretching vibration, respectively. Moreover, the peaks were due to the Si-O-Si groups having a higher intensity than the Si-O-C and Si-OH groups. Furthermore, the peak at 1721 cm⁻¹ on GO (C=O) completely disappeared, suggesting the conversion of carbonyl groups into Si-O-C bonds.

The FTIR analysis shown in Figure 3 proves the existence of modified silica nanoparticles. The bands at 810 and 475 cm⁻¹ found in the FTIR spectrum of silica nanoparticles illustrated in Figure 3 can be attributed, respectively, to the stretching and bending of Si-O-Si bonds. Furthermore, siloxane vibrations from (SiO)n groups are implicated according to the absorption bands at 1100 cm⁻¹. The O-H stretching band of the surface silanol groups and the remaining adsorbed water molecules are responsible for the peaks at 3450 and 1630 cm⁻¹, respectively, which developed after the APTES modification of the silica nanoparticles (Figure 3). The bands around 2800 cm⁻¹ can be connected to the asymmetrical and symmetrical stretching vibrations of CH₂ groups after silica nanoparticles were modified with APTES (Figure 3). The bending vibration of the N-H group causes the appearance of a new band at 1560 cm⁻¹ in response to the addition of an amine group. Furthermore, the stretching vibration of the N-H group can be linked to the peak around 3500 cm⁻¹. These combined results show that amine-functionalized silica nanoparticles were successfully prepared.

Raman spectroscopy is a standard non-destructive technique employed for the characterization of carbon-based materials [36]. The oxidation of graphite to GO can be further confirmed by Raman spectroscopy. Characteristic peaks of graphitic carbon-based materials are the G and D bands and their overtones [37]. As shown in Figure 4, characteristic bands for graphite and GO appear at around 1300–1350 cm⁻¹ (D band) and between 1550 and 1600 cm⁻¹ (G band) [26,38]. Their intensity ratio (I_D/I_G) indicates the degree of the disorder and defects [38]. A universal observation is that the higher the disorder of graphite, the broader the G and D bands, which have a higher relative intensity compared to the G band [39]. As shown in Figure 3, it is observed that there is a small shift for GO, and, also, the bands are broader than they are in graphite. The G peak and D peak of GO are shifted to higher frequencies with respect to that of graphite. The increase in intensity and shifting of the D peak to a higher wavelength corresponds to the presence of defects and the distortion of the sp2 crystal structure of graphite [37]. The 2D band appears at 2660 cm⁻¹ [40]. The intensity of the 2D band is smaller after oxidation because of the breaking of the stacking order and incorporation of functional groups due to the oxidation reaction [41]. Thus, the intense D band in GO confirms the oxygen functionalization of graphite. The ratio of I_D and I_G portrays the defect window regime in GO and GO/SiO₂. In general, the oxidation of graphite is accomplished by the increase of I_D/I_G. Hence, the higher the value of this I_D/I_G ratio, the more there will be structural defects or disorders in the system.

Table 1. I_D/I_G values of graphite, GO, and GO/SiO₂.

Sample Code	ID/IG
Graphite	0.05
GO	0.60
GO/SiO2	0.94

shows the I_D/I_G values of graphite, GO, and GO/SiO₂.

The I_D/I_G ratios of GO and GO/SiO₂ were 0.05 and 0.60, respectively. Thus, the increased value of the I_D/I_G ratio of GO/SiO₂ indicates an increased degree of disorder and improved lamellar spacing, which would assist in the better dispersion of GO-SiO₂ nanoparticles, and this result is in line with the TEM images.

Transmission electron microscopy (TEM) is considered as an effective method for the morphological studies of GO. With increases in the oxidation level, GO becomes highly transparent, since it possesses a high amount of the oxygenated functional group, which makes them suitable for exfoliation into monolayers or just a few layers of GO after ultra-sonication. As shown in Figure 5, the TEM images of GO sheets are coupled with numerable wrinkles and ripples on them. The transparency level of the image indicates the presence of more layers and functionalization with oxygen-containing groups [42]. Moreover, the GO-coated silica shows a rough external surface, with visible irregular cavities (Figure 5b). This must be due to the nanoparticles embedded in the silica backbone. The TEM images containing the inner core are outer shell shows that the GO was successfully coated on the silica particles.

3.2. Effect of pH on Adsorption of Cr(VI) Ions

The pH is the key factor that affects the removal efficiency of metal ions. It affects the surface charge of the adsorbent, the degree of ionization, and the speciation of the adsorbate [43]. To study the effect of the pH on adsorption, experiments were conducted under the same experimental conditions, at a concentration of 30 ppm and a pH in the range of 3.0–9.0. The experimental results are shown in

Table 2. Percentage of adsorption of Cr(VI) ions using GO and GO-coated-silica nanoparticles at 30 ppm.

pH of the Sample	Initial Concentration (Ci) (ppm)	Final Concentration (Ce) (ppm)	Adsorbance Percentage (%)
		GO	Si/GO	GO	Si/GO
3	30	8.08331	5.77240	73.06 ± 0.6	80.76 ± 0.125
5	30	10.09871	10.97567	66.34 ± 0.3	63.41 ± 0.312
7	30	10.00400	10.68623	66.65 ± 0.025	64.38 ± 0.421
9	30	10.11465	11.00769	66.28 ± 0.532	60.97 ± 0.25

.

From Table 2, it is clear that the optimum pH for the efficient removal of chromium(VI) ions is pH 3. Basic medium, i.e., pH 9, leads to the least removal of Cr(VI) for both of the adsorbents (Figure 6). As reported by Zhao et al. [44] and Shirkhanloo et al. [45], at low pH values, the surface charge of GO is positive due to the protonation reaction. At lower pH values, the predominant Cr(VI) species mainly exists in the monovalent HCrO₄⁻ form, which is then gradually changed to the divalent CrO₄²⁻ and Cr₂O₇²⁻ as the pH increases [46,47]. Hence, at a lower pH, the surface of GO and GO-SiO₂ becomes protonated. Then, the positively charged surface of the adsorbents strongly attracts oppositely charged Cr(VI) species through electrostatic interaction, and the adsorption rate increases [48]. Moreover, the adsorption free energy of HCrO₄⁻ ion is lower than that of CrO₄²⁻, and, hence, HCrO₄⁻ is more favorably adsorbed than CrO₄²⁻ at the same concentration [49,50]. As the pH increases, the availability of the OH⁻ ions’ concentration increases, thus causing them to compete with the Cr(VI) ions, leading to the reduction in percentage of adsorption [32]. Thus, this study reveals that the adsorption quantities of Cr(VI) ions at a lower pH are larger than those of at a higher pH.

3.3. Evaluation of the Adsorption Efficiency of GO and Si/GO at Different Concentrations of Solutions

Adsorption experiments were conducted with different concentrations of heavy metal solutions (C_i, 1–200 ppm) at pH 3.

Table 3. Percentage of adsorption of Cr(VI) ions from different concentrations of samples, using GO and GO-coated silica nanoparticles.

Initial Concentration of the Sample (Ci) (ppm)	Concentration after Adsorption (Ce) (ppm)		Percentage of Adsorption (%)
	GO	Si/GO	GO	Si/GO
1	0.61504	0.29760	38.49 ± 0.25	70.24 ± 0.65
10	5.94674	2.46630	40.53 ± 0126	75.34 ± 35
30	8.08331	5.77240	73.06 ± 0.6	80.76 ± 0.69
50	6.92333	3.85825	86.15 ± 0.8	92.28 ± 0.45
100	65.34009	52.31453	34.66 ± 0.58	47.69 ± 0.52
200	132.2878	118.8226	33.86 ± 0.87	40.59 ± 0.85

shows the effect of initial concentration of the solution under the optimum pH.

At a concentration of 50 ppm, it was noticed that the percentages of adsorption of Cr(VI) by GO and GO-SiO₂ were at the maximum, i.e., at optimum concentration; the removal percentage was 86.15% and 92.28% for GO and GO-SiO₂, respectively. As the initial concentration increases, the adsorption percentage of Cr(VI) increases and then decreases for both the adsorbents. At lower concentrations, the number of Cr(VI) ions present in the solution is comparatively lower compared to the number of available sites on the GO and GO-SiO₂ nanoparticles [51]. The highest heavy metal concentration (200 ppm) leads to the lowest removal percentage [52]. Imtithal et al. [31] reported that, at a lower concentration of metal ions, vacant sites are available for the adsorption of metal ions, and, hence, there is a higher adsorption rate. At higher concentrations, the capacity of the adsorbent becomes exhausted due to the non-availability of the surface sites; i.e., as the concentration increases, the surface sites of the adsorbents become saturated, leading to less or no more adsorption (Figure 7). Moreover, it is clear that the adsorption efficiency of GO-SiO₂ is higher than GO at all concentrations. Although the functional groups, such as epoxy (C-O-C), hydroxyl (-OH), carboxyl (-COOH), and carbonyl (C=O), on the surface of GO promote the adsorption of metal ions, the aggregation of its layers due to strong interplanar interactions leads to a reduced metal ion adsorption capacity [53]. GO and amine-modified silica are good adsorbents, and their combination leads to the formation of an excellent adsorbent, i.e., GO-SiO₂. The increases in the specific surface area and the number of binding sites of the GO-SiO₂ composite contribute to its excellent removal efficiency [54]. Both amine-modified silica and graphene oxide (GO) are good adsorbents for removing heavy metals from wastewater. Due to the abundance of functional groups such as -OH, -COOH, and -C=O, GO has a wide surface area and high adsorption capacity. These groups have high interactions with heavy metal ions through coordination and electrostatic interactions, making GO an efficient adsorbent. On the other hand, amino functional groups in amine-modified silica show potent metal-ion-chelating capabilities. It is a great adsorbent for metal ions because the amino groups can form complexes with metal ions through coordination interactions. The GO-SiO₂ composite that results from the combination of these two substances has improved the adsorption abilities for the removal of Cr(VI) from wastewater. The amine-modified silica provides chelating sites for Cr(VI) ions, whereas GO provides a high surface area for the adsorption of metal ions. A better adsorption effectiveness can also result from the presence of amine functional groups on the silica surface, which can encourage the dispersion of GO and prevent its aggregation. The interaction between protonated amine and hydro chromate anion exhibits a strong affinity for graphene oxide, primarily through cation-pi and electrostatic interactions [23,55,56]. Chromium(VI) is easily adsorbed onto the surface of GO due to hydrogen bonding and the electrostatic interaction of CrO₄²⁻ with the oxygen functionalities in GO. Figure 8 illustrates the interaction between the amine and silica, graphene oxide with amine-modified silica, and Cr(VI).

3.4. Comparison of Removal Efficiency of Cr(VI) Ions Using Different Adsorbents

In order to compare the efficiency of synthesized nanoparticles for the removal of Cr(VI) ions from water with the ones reported in the literature, it is appropriate to compare the removal percentage or adsorbance capacity of graphene oxide (GO) and graphene-oxide-coated silica nanoparticles (GO/SiO₂). In this paper, a comparison of the removal percentage of different adsorbents with synthesized adsorbents is presented

Table 4. Summary of removal percentage of Cr(VI) using various adsorbents.

No.	Adsorbent	Optimum pH	Removal % (a)/Adsorption Capacity (mg/g) (b)	Reference
1	Graphene oxide/silica nanosheets with multistage pores	2	90 (a)	[57]
2	Surface-modified Jacobsite (MnFe2O4) nanoparticles	2	31.55 (b)	[58]
3	GO prepared from Graphite	4	92.8 (a)	[32]
4	Maghemite nanoparticles	2.5	97.3 (a)	[59]
5	Magnetite nanoparticle	2.5	99.8 (a)	[49]
6	Nanostructured graphite oxide	3	63 (a)	[31]
7	Silica/graphite oxide composite	3	60.24 (b)	[31]
8	Chitosan-grafted graphene oxide (CS-GO) nanocomposite	2	96 (a)	[51]
9	Chitosan GO	3	310.4 (b)	[60]
10	Graphene oxide chitosan microspheres modified with α–FeO(OH)	3	97.69 (a)	[61]
11	Trioctylamine-exfoliated graphene oxide (TOA–EGO)	2.5–3	96.3	[23]
12	Graphene oxide (GO)	3	86.15 ± 0.8 (a)	Present study
13	GO-SiO2	3	92.28 ± 0.45 (a)	Present study

.

It can be observed that an acidic pH in the range from 2.0 to 3.0 was found to be optimum in most of the studies. An acidic pH is often preferred for the removal of Cr(VI) from wastewater because it helps to convert the highly toxic and mobile Cr(VI) into its less toxic and less mobile Cr(III) form. This is because, at lower pH levels, the reduction of Cr(VI) to Cr(III) is more favorable and occurs more quickly. The reduction of Cr(VI) to Cr(III) occurs through various mechanisms, such as chemical reduction, microbial reduction, or electrochemical reduction. However, the efficiency of these mechanisms is affected by the pH of the solution. At a low pH, the chemical reduction of Cr(VI) is favored, while at higher pH values, microbial reduction is more favorable. Moreover, at lower pH values, the solubility of Cr(III) is significantly reduced, which leads to its precipitation and removal from the solution. This precipitation can occur in the form of various Cr(III) compounds, such as hydroxides, oxides, or salts. These precipitates are less soluble and less mobile than Cr(VI), reducing the risk of groundwater contamination and other adverse environmental impacts.

Compared with other adsorbents, a high absorbance percentage was recorded for synthesized GO and GO/SiO₂ at pH 3. Other adsorbents that are reported in the literature with a high removal percentage showed maximum adsorption at a pH < 3. A high surface area and the presence of oxygen-containing functional groups on the surface of GO facilitate the removal efficiency by complexation with Cr(VI) ions. The higher removal efficiency of GO/SiO₂ can be justified by its enhanced properties.

4. Conclusions

In this study, graphene oxide (GO) and graphene-oxide-coated silica nanoparticles were successfully synthesized and characterized by FTIR and Raman spectroscopy and TEM. The synthesized nanoparticles were used for the removal of chromium (VI) ions from water. The pH was optimized to 3, and the maximum adsorption percentage of Cr(VI) was observed at a concentration of 50 ppm for both of the adsorbents. A difference of 6.13% adsorption was observed between GO and GO-coated silica nanoparticles at optimum conditions. The results of the present study showed that both GO and GO-coated silica nanoparticles are effective adsorbents for the Cr(VI) removal from water. The adsorption of Cr(VI) on these synthesized adsorbents is strongly dependent on the pH values and initial concentration of the solution. GO-coated silica nanoparticles are excellent adsorbents compared to GO alone, and we highly recommend that they can be used in water treatment for removing Cr(VI). Further optimization studies are essential for the various applications of these nanoparticles.