Sustainable Remedy Waste to Generate SiO₂ Functionalized on Graphene Oxide for Removal of U(VI) Ions

Abstract

The Hummer process is applied to generate graphene oxide from carbon stocks’ discharged Zn-C batteries waste. SiO₂ is produced from rice husks through the wet process. Subsequently, SiO₂ reacted with graphene oxide to form silica/graphene oxide (SiO₂/GO) as a sorbent material. XRD, BET, SEM, EDX, and FTIR were employed to characterize SiO₂/GO. Factors affecting U(VI) sorption on SiO₂/GO, including pH, sorption time, a dosage of SiO₂/GO, U(VI) ions’ concentration, and temperature, were considered. The experimental data consequences indicated that the uptake capacity of SiO₂/GO towards U(VI) is 145.0 mg/g at a pH value of 4.0. The kinetic calculations match the pseudo second-order model quite well. Moreover, the sorption isotherm is consistent with the Langmuir model. The sorption procedures occur spontaneously and randomly, as well as exothermically. Moreover, SiO₂/GO has essentially regenerated with a 0.8 M H₂SO₄ and 1:50 S:L phase ratio after 60 min of agitation time. Lastly, the sorption and elution were employed in seven cycles to check the persistent usage of SiO₂/GO.

1. Introduction

Uranium ions may develop into the environment through applications, industry, and mining, posing risks to health of humans and biological situations because of radiation and toxicity [1,2]. Nevertheless, by the widespread use of atomistic energy and further applications, U(VI) is exceptionally loose in running solutions; it threatens environmental safety and biological balance [3,4]. Thus, U(VI) elimination and deposition from liquid origins are important for nucleate energy engendering and peripheral protection [5,6]. To acquire U(VI) outputting from radioactive wastewater that is financially workable, the extracting materials must maintain tremendous extracting ability, selectivity, departure rate, and reusability applications. Until recently, most studies have been directed towards the application of extracting materials for U(VI) adsorption due to its high performance, capacity, low cost, and broad versatility [7,8].

Recently, numerous investigations were conducted for the removal and extraction of U(VI), comprising ion exchange [9,10,11,12], precipitation [13], liquid extraction [14], film separation [14,15], and sorption [16,17,18,19,20]. Adsorption methods were commonly examined because they remain faithful and are low risk and low cost; furthermore, the separation power could be enormously promoted by originating relevant sorption materials [21,22,23,24,25,26,27]. Graphene-based sorbents are commonly criticized as subjective sorbents due to their distinctive qualities, such as low mass, chemical force, and fundamental variability.

Recently, advances in current nano-technology, science, and standardized hybrid permeable substances have gained considerable attention for sorption and separation purposes, owing to their orderly, uniform, and interpenetrating mesoporous, tunable hole sizes and extended surface areas. Numerous organic-inorganic porous hybrids evolved for excluding heavy metals (Ba(II), Hg(II), Cd(II), Cu(II), and Pb(II)) from wastewater [28,29,30]. Graphene-based mesoporous silica was utilized for Pb(II) ions’ elimination [31]. Reduced graphene oxide was combined with heavy metal oxides to enhance the efficiency of sonocatalytic degradation and photocatalytic hydrogen production. Multifunctional WO₃ and tungsten trioxide/reduced graphene oxide (WO₃/rGO) composite photocatalysts were used to investigate the sonocatalytic degradation of toxic Congo red azo dye as well as photocatalytic hydrogen production. Furthermore, Platinum has been used as a co-catalyst with WO₃/rGO to boost hydrogen production [32]. Reduced graphene oxide/SnO₂ (rGO/SnO₂) nanocomposite used ultrasound-generated cavitational events to improve the uniform deposition of SnO₂ on rGO nanotubes. These rGO/SnO₂ nanocomposites were explored for their potential gas sensing and electrochemical implementations [33]. The spongy ball-like CuO and CuO/rGO photocatalysts achieved optical results, which proved that the bandgap is within the visible range of the solar spectrum. In addition, the CuO/rGO composite produce more photocatalytic hydrogen compared to CuO [34].

Furthermore, graphene oxide-related materials have been fabricated widely and exploited in several applications. Nevertheless, several convergent surveys have been performed, particularly inorganic/GO for uranium removal. It was assumed that the recent advancement of inorganic/GO for adsorption of U(VI) promotes the refinement of inorganic/GO materials for uranium disposal and recovery purposes. GO/MnO₂ and magnetite nanoparticle/GO composites have been utilized for U(VI) removal [35,36]. Fe-Ni/GO composite was manufactured to monitor the exclusion of U(VI) from liquid solution in environmental conditions [37]. Magnetic Fe₃O₄/SiO₂ composite particles were applied for swift removal of U(VI) from liquid solutions. The maximum uranium adsorption capacity onto magnetic Fe₃O₄/SiO₂ composite particles was estimated to be about 52 mg/g at 25 °C [38]. A functional surface layer based on quaternary ammonium is produced in situ on the surface of silica-coated magnetic nanoparticles for uranium removal, with an uptake efficiency of 87 mg/g at 120 min of equilibrium time [39]. The graphene oxide/bentonite composite was synthesized to adsorb uranium ions at room temperature with 234.19 mg/g sorption capacity [40].

Here, the SiO₂/GO sorbent was produced from the obtained graphene oxide from spent carbon rods batteries, and silica was established from the rice husk to remove U(VI) ions from the aqueous solution. The experimental parameters are pH, initial U(VI) concentration, contact time, SiO₂/GO dose, and temperature, with which we examined U(VI) adsorption by batch experiments. In addition, adsorption isotherms and thermodynamic considerations were evaluated.

2. Materials and Methods

2.1. Chemicals and Techniques

A UV/Vis 6405 spectrophotometer was utilized for analyzing U(VI) at 650.0 nm with Arsenazo III (as a chromophore) [41]. Furthermore, SEM, EDX, BET, and XRD were exploited to designate the morphologies of GO, SiO₂, SiO₂/GO, and U/SiO₂/GO. FTIR (IRPrestige-21, Shimadzu, Kyoto, Japan) analysis with IR steadfastness software was utilized to expose the function groups. A FT-Raman spectrometer RAMII (Bruker, Billerica, MA, USA) equipped with a 500 mW Nd: YAG laser was used to collect Raman spectra at 1064 nm. The experimental conditions lead to a spectral resolution of 5 cm⁻¹.

The chemicals and reagents can be used without purification. Arsenazo III, UO₂(CH₃COO)₂·2H₂O, phosphoric acid, FeSO₄.7H₂O, sodium hydroxide, and NaNO₃ were obtained from Merch. HCl, hydrogen peroxide, methanol, ethyl acetate, and sulfuric acid were obtained from Sigma Aldrich. The chosen source for rice husk adopted here was a local rice harvester at Mina El-Qamh, Qalyubia Governorate, Egypt. The spent carbon rods were taken from Egyptian Zn-C battery solid wastes.

2.2. Fabrication of Silica

Silica is fabricated from low-cost, agricultural sources such as rice husk (RH) [42,43]. The collected RH (200 g) was washed by tap water to eliminate dust and further contaminations and then dried at 100 °C. Subsequently, it was ignited under suction; the black RH was refluxed with 1.0 M HCl at 80 °C and stirring constantly for 5 h. After cooling, the treated RH was filtered and washed with Milli-Q water until it became acid-free (pH 7.0) and then dried at 100 °C overnight; the attained product was calcinated at 750 °C for 8 h. A white silica product was acquired after calcination.

A 20 g white product was dissolved completely in 200 mL of 2.5 M NaOH at 70 °C under reflux. The produced sticky solution was filtered before being washed with 200 mL of boiled Mili-Q water, and then permitted to cool. The mixture was stirred for 2 h, and subsequently, 50.0% H₂SO₄ was added dropwise to the mixture, stirring constantly, till white silica formed at pH ~8.0. The white silica was rendered alkali-free after washing with hot Milli-Q H₂O and then desiccation for 12 h at 70 °C. The desiccated silica was dried at 112 °C for 11 h (Figure 1a).

2.3. Graphene Oxide Preparation

The spent Zn-C batteries (dry cell battery wastes) were broken to attain waste carbon rods, which were milled to obtain a fine powder. Hot water was used to wash the carbon powder three times, and this was followed by washing with 30.0% sulfuric acid to eliminate inorganic impurities such as MnO₂ and NH₄Cl. The Hummer method was employed using treated carbon powder (graphite) to obtain graphene oxide [44,45]. Briefly, treated graphite powder (30 g) and NaNO₃ (20 g) were added into a 3.0 L container. An acid mixture of 500 mL (98.0%) H₂SO₄ and 130 mL (85.0%) H₃PO₄ was combined inside a container with stirring. The compounds were frozen to 0–4 °C in an ice bath.

Thereafter, KMnO₄ (36 g) was regularly added after 125 min with severe stirring and refrigeration to keep the mixture’s temperature beneath 15 °C for 33–125 min. Subsequently, the mixture warmed to 32 °C within a water bath for 11 h, where it converted to a sickly brownish-gray color. Then, 600 mL purified water was gradually added; moreover, the mixtures were constantly stirred for 24 min below 95 °C. Vast volumes of H₂O (1.8 L) and 95 mL H₂O₂ (29%) were added into the earlier mix over 32 h, mixing to complete the reaction, which resulted in a bright yellow color. Subsequently, the mixture was treated with 85 mL (17%) HCl to eliminate any residual ions, centrifuged, and washed multiple times to eliminate HCl. Subsequently, a 32 min centrifugation (3100 rpm) was undertaken to eliminate any impurities. The sample was filtering under a vacuum, rinsed with deionized H₂O many times, and placed to dry at 85 °C. Eventually, the gained GO was desiccated at 55 °C for 24 h (Figure 1b).

2.4. Preparation of Silica/GO

The GO was functionalized with a silanol unit of silica to gain silica/graphene oxide (GO/SiO₂) by in situ hydrolysis [46]. For this purpose, 12 g of GO was dispersed in 85 mL water and 66 mL ethanol with 60 min sonication. Then, a 3.5 g SiO₂ dissolving in 90 mL ammonia was added to the solution with constant stirring for 11 h. Afterwards, acetic acid and ammonia was used to adjust the pH of the mixture to 8.5; the mixture was kept for 3 h at 65 °C for complete condensation of the silica on GO. The SiO₂/GO precipitate was washed many times by Milli-Q H₂O. The SiO₂/GO was heated to 82 °C in a furnace for 12 h (Figure 1c).

2.5. Adsorption Measures

A series of trials were performed to obtain the optimal parameters of the related conditions governing a U(VI) sorption method, such as pH of solution, equilibrium time, SiO₂/GO quantity, U(VI) ions concentration, and reaction temperature. Group tests were completed in replicate. The sorption experiments were developed by incorporating 50 mL of various U(VI) concentrations, (10–100 mg) SiO₂/GO dose at 220 rpm mechanical shaker, agitation time changing from 5–120 min, and (25–55 °C) temperature. The pH was varied from 1–6 and was adjusted by 1.0 M NaOH or 1.0 M H₂SO₄. The sorption experiments studied the sorption kinetics, equilibrium, and thermodynamic isotherms. In each sorption trial, U(VI) were put in SiO₂/GO, and the uptake q_e (mg/g) and distribution constant (K_d) were estimated using the subsequent equations:

$$q_e=\left(U_o-U_e\right)\times \frac{V_L}{m_g}$$

(1)

$$K_d=\left(\frac{U_o-U_e}{U_e}\right)\times \frac{v_{mL}}{m_g}$$

(2)

where U_o and U_e = initial and equilibrium U(VI) ions concentration (mg/L), respectively, V_L= volume per L, v_mL = volume per mL, and m_g= SiO₂/GO weight (g).

3. Results and Discussion

3.1. Descriptions

3.1.1. XRD Scrutiny

XRD analyses of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO are displayed in Figure 2. The SiO₂ exhibited major broad peaks at 2θ = 19°, 22°, 24°, 28°, 29°, 32°, 34°, 36°, 39°, and 49°, which correspond to the database of Bruker software COD 9014256 (Figure 2a). The data of the SiO₂ pattern revealed that the SiO₂ was composed mainly of cristobalite [47]. As shown in Figure 2b, a broad peak at 2θ = 14° and two peaks with low intensity at 2θ = 20° and 42° were characterized of graphene oxide (GO), which correspond to the database of Bruker software COD 2000183 and 2002929.

XRD spectra of SiO₂/GO and U/SiO₂/GO are displayed in Figure 2c,d. In the XRD of SiO₂ and GO (Figure 2a,b), a new peak appears at 2θ = 9°; the peak position and peak shape of GO with high intensity at 2θ = 14° were not altered, whereas the broad peaks of SiO₂ overlap with the GO pattern. A new composite (SiO₂/GO) was formed from these data due to the surface chemical and electrostatic interaction between SiO₂ and GO. It has a broad peak at 2θ = 20°, as well as small two peaks, which appear at 2θ= 27° and 28°, according to the corresponding database of Bruker software COD 4025951 and 4124041 (Figure 2). In the XRD pattern of U/SiO₂/GO (Figure 2d), some new peaks were observed after adsorption according to the database of Bruker software COD 8103695, 9000080, and 9009686. In contrast, the intensity of peaks was slightly changed, signifying that the SiO₂/GO crystallinity did not imply alteration after U(VI) adsorption.

3.1.2. SEM Analysis

This analysis was established to check the reconstruction of exterior and natural configurations of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO (Figure 3). SEM of SiO₂ and GO demonstrate definite shapes and include distinct irregular compositions. The particle sizes of SiO₂ and GO may be 18 nm and 55 nm. However, the particle size of SiO₂/GO may be 40 nm due to SiO₂ being fixed on GO. The SiO₂ and GO skeletons were established of small irregular bits with minute diameters, and they were not related to each other.

In contrast, the photograph of SiO₂/GO shows smooth surfaces with several cavities due to the collection and affixation of SiO₂ on the surface of GO. Additionally, the photograph of SiO₂/GO was constructed of aggregate particles with bigger interstitial holes. After U(VI) sorption on SiO₂/GO, the photograph shows that the holes were packed by U(VI). The SiO₂/GO surface was irregular and agglomerated by U(VI), and the particle size of U/SiO₂/GO may be 88 nm, which is larger than the other sizes. Additionally, the SEM results reveal that the SiO₂/GO structure gave a more widespread surface area for U(VI) adsorption, which was better than that of GO or SiO₂.

3.1.3. EDX Analysis

EDX spectra were used to conduct semi-quantitative analyses of SiO₂, GO, and SiO₂/GO, besides U/SiO₂/GO (Figure 4). From the outcomes, Si and O bands were conferred in the SiO₂ scale, and no extra bands were disclosed (Figure 4a), but the GO scale has C and O peaks (Figure 4b). The EDX analysis of SiO₂/GO contained C, O, and Si bands (Figure 4c). These results confirmed that the Si, C, and O bands happened due to SiO₂/GO formation. Ultimate U(VI) sorption above SiO₂/GO only discerned plain U(VI) bands that admitted and validated U(VI) sorption above SiO₂/GO (Figure 4d).

3.1.4. BET Analysis

The BET is operated to assess solid materials’ surface area and pore size. Figure 5a–d demonstrated the N₂ sorption–desorption isotherms of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO. The obtained data in

Table 1. Surface area, pore size, and pore volume of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO.

Materials	SBET, m2/g	Pore-Volume, cc/g	Pore-Size, nm
SiO2	25.89	0.037	2.87
GO	37.37	0.052	3.19
SiO2/GO	35.45	0.045	2.99
U/SiO2/GO	32.15	0.039	2.85

showed that the surface areas of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO were 25.89, 37.37, 35.45, and 32.15 m²/g, respectively. The surface area (S_BET) and pore volume, besides the pore size of SiO₂/GO, were measured between the consistent SiO₂ and GO values, and it might be considered that SiO₂ decorated GO pores. The SiO₂ surface area and porosity improved the GO surface to pick up uranium ions. From the data, the surface area, pore size, along with pore volume of SiO₂/GO were reduced after U(VI) sorption due to pore-blocking with ions. The results showed that U(VI) ions were powerfully captured on SiO₂/GO due to more active sites.

3.1.5. FTIR Analysis

The FTIR spectra of SiO₂, GO, SiO₂/GO, and U/SiO₂/GO were determined at 4000–400 cm^–1, as characterized in Figure 6. Regarding the SiO₂ in Figure 6a, the extensive band at 3450 cm⁻¹ offered O–H of Si–OH, but a distinctive band at 1646 cm⁻¹ of reacted H₂O on SiO₂ was distinguished, which was not wholly displaced via drying [48]. The influential bands at 1216 and 1088 cm⁻¹ were determined through a siloxane group (Si–O–Si) [47,49,50]. The attendance peak at 956 cm⁻¹ was also accepted in the silanol group (Si–OH). The absorption peak at 795 cm⁻¹ was documented in the Si–O–Si group, and the detected peak was 466 cm⁻¹ in the Si–O–Si group [51,52].

Moreover, Figure 6b of the prepared GO demonstrates a deep band at 3432 cm⁻¹, consistent with the O–H group. The vibrational band at 1646 cm⁻¹ expressed C=C or OH of H₂O sorbed upon GO. An assignment at 1704 cm⁻¹ was declared to C=O of COOH groups. The slight vibrations at 1183 and 957 cm⁻¹ correspond to epoxy groups (C–O), and the 1102 cm⁻¹ is in agreement with C–O of COOH groups [53].

The spectrum of SiO₂/GO (Figure 6c) showed an abroad peak at 3398 cm⁻¹ that belonged to –OH groups of SiO₂ and GO. The assignments at 3112 cm⁻¹ match –CH aromatic, and features at 1676 and 1593 cm⁻¹ are consistent with C=O and C=C of GO. Additionally, the peaks at 1246, 1145, and 1093 cm⁻¹ were concerned with the siloxane groups. Moreover, a Si–OH peak at 943 cm⁻¹ was documented, and the vibrational band at 788 cm⁻¹ corresponds to the Si–O–Si group. From these data, it was confirmed that SiO₂/GO was formed.

After U(VI) sorption in the U/SiO₂/GO spectrum (Figure 6d), the –OH, epoxy (–O–), and C=O stretching vibration bands were condensed and relocated to redshift with 4–11 cm⁻¹, probably because of U(VI) adsorbed on SiO₂/GO. Moreover, fresh peaks consistent with of O=U=O are found near 974 and 894 cm⁻¹ [54]. Moreover, two weak peaks appeared near ≈465 and ≈425 cm⁻¹ due to the U–O group [55]. Accordingly, it realized that the SiO₂/GO acted more friendly to U(VI) sorption.

3.1.6. Raman Analysis

Raman spectra of SiO₂, GO, and SiO₂/GO are illustrated in Figure 7(a–c). The spectrum of the prepared SiO₂ from rice husk waste is given in Figure 2a, the distinctive bands of the SiO₂ network, D1 and D2, can be observed, which are associated with the silica material, besides the regular silica band observed at 985 cm⁻¹, which is assigned to the SiOH groups [56]. The Raman spectrum of the prepared graphene oxide from rod batteries wastes is depicted in Figure 2b. The G band is noticed at 1585 cm⁻¹, whereas the D peak is also observed at 1355 cm⁻¹. The G band is standard for all sp² carbon forms, and it appears from the C-C bond stretch. This band is assembled from first-order Raman scattering [57,58]. The G band in GO is due to the presence of oxygen atoms in the GO structure, forming from sp² ordered crystalline graphite-like configurations. The D band in GO is broadened due to a disordered sp³ carbon structure.

The Raman spectrum of the prepared SiO₂/GO is shown in Figure 7c. It detected the characteristic bands of SiO₂ and GO. It was observed that the bands of the SiO₂ network wrre D1, D2, and SiOH groups for SiO₂, whereas the D and G bands were attributed to GO. All bands of SiO₂/GO are minor when compared with the bands of its constituents (SiO₂ and GO). The relative intensity of the D and G peaks offers crucial information concerning the chemical environment. The intensity of both D and G bands display a significant alteration without changing the position or shape when GO reacts with silica particles that acquire covalent attached to the GO [59,60,61]. However, in the SiO₂/GO spectrum (Figure 7c), the D and G bands’ intensity ratio was small, demonstrating a noncovalent interaction between silica and GO [62,63]. Hence, it is quite evident that silica and GO accumulated via the random packing procedure according to the van der Waal potential.

3.2. Sorption Data

U(VI) sorption was intended on SiO₂/GO from the uranium sulfate complex solution. Some testing experiments were directed on U(VI) sorption to study the impact of pH, SiO₂/GO dose, contact time, initial U(VI) concentration, and sorption temperature.

3.2.1. pH Influence

pH of solution is an important factor affecting U(VI) extraction on the SiO₂/GO sorbent. The pH result at SiO₂/GO was intended with several pH values from 1.0–6.0 at permanent settings of 50 mg SiO₂/GO and the 50 mL assaying 150 mg/L U(VI), 40 min sorption time, and 25 °C (Figure 8a). The U(VI) uptake increased to 115 mg/g through an increasing pH of 4.0. Successively, as pH increased to 6.0, U(VI) uptake dropped to 40 mg/g. As a consequence, the maximum uptake was attained at pH 4.0.

The graphene oxide active sites are –OH, –COOH, and –O–; however, SiO₂/GO active sites are increased to improve uptake. The active sites increased in the SiO₂/GO due to existing –OH, –COOH, –O–, and silanol (Si–OH) groups on the SiO₂/GO surface. At high acidity (pH < 4.0), the sorption sites of SiO₂/GO take positive charges. However, the electrostatic attraction between the positive active sites of SiO₂/GO and anionic species [UO₂(SO₄)₃]⁴⁻ and [UO₂(SO₄)₂]²⁻ were carried out due to a great volume of HSO₄⁻ that easily reacted with positive active sites. Some neutral complexes of uranium ions were formed by reacting some H⁺ ions with uranium anionic species. Additionally, HSO₄⁻ ions competed with uranium anionic species during sorption. Hence, U(VI) sorption was decreased. At pH 4.0, U(VI) was observed in a hydrolyzed appearance, and the resulting cationic varieties recognized UO₂²⁺, [(UO₂)₂(OH)₂]²⁺ dimmer, along with trimmer [(UO₂)₃(OH)₅]⁺ [64]. Oxygen atoms of SiO₂/GO deprotonated. Multiple uranium hydrolysis outputs formed were exceedingly adsorbed by electrostatic magnetism and complexation operations. Hence, the uranium uptake realized maximum sorption at pH 4.0. Alternatively, at pH ˃ 4.0, uptake progressively decreased, owing to the precipitation of uranyl hydroxide species [65]. For that reason, it was not necessary to examine the effect of pH 6.0.

3.2.2. Dose Influence

The effect of SiO₂/GO dose on uranium sorption was measured using a variety of doses from 10 to 100 mg, at 4.0 pH, 150 mg/L of U(VI), and 40 min sorption time (Figure 8b). The results informed that U(VI) sorption improved by increasing the SiO₂/GO dose. Due to increasing the active sites, more active sites were available for U(VI) sorption at higher dosages. Henceforth, U(VI) sorption improved to 96.66% by increasing the SiO₂/GO doses to 50 mg. Nevertheless, the sorption percent prevailed constantly until the 100 mg dose. Hence, the 50 mg SiO₂/GO dose was chosen for sorption tests.

3.2.3. Sorption Time and Kinetics

The role of contact time on U(VI) uptake was applied on SiO₂/GO at an altered contact time (5–120 min) at pH 4.0, 150 mg/L of U(VI), 50 mg of SiO₂/GO dose (Figure 9a). The data showed that the uranium uptake was increased from 62–145 mg/g by increasing contact time from 5–50 min. Instantaneously varying sorption time to 120 min should have no noticeable impact on U(VI) uptake. Henceforth, a contact time of 50 min was an appropriate supplementary condition of uptake.

Pseudo-first-order as well as pseudo-second-order model was maintained for the good data to estimate U(VI) sorption kinetics on SiO₂/GO sorbent. The nonlinear equation of the two forms is as follows [66]:

$$q_t=q_e\left(1-e^{-k_{1}t}\right)$$

(3)

$$q_t=\frac{k_2{q}_{\mathrm{e}}^{2}t}{1+k_2{q}_{\mathrm{e}}t}$$

(4)

where q_e and q_t (mg/g) are U(VI) amounts sorbed at equilibrium and at sorption time t (min), and k₁ (1/min) refers to the first-order constant, while k₂ (g/mg.min) refers to the second-order constant. q₁, k_1, and R² were 124 mg/g, 0.115 1/min, and 0.89, respectively, based on the nonlinear regression of first-order rate in Figure 9b, as well as the reduced Chi-square was 67.72. Nonetheless, for the nonlinear regression of second-order, q₂, k₂, and R² were 150 mg/g, 9.67 g/mg.min, and 0.96, respectively, with a reduced Chi-square of 18.88. From the data in Figure 9b, the R² for the second-order model was close to unity, and q₂ = 150 mg/g was closer to experimental uptake, q_e (145 mg/g). Therefore, it suggested that the second-order model described U(VI) sorption kinetics upon SiO₂/GO. Hence, U(VI) sorption by SiO₂/GO signified that it was most likely controlled by the chemisorption process [67].

3.2.4. Initial U(VI) Concentration and Equilibrium Isotherms

Many experiments were conducted to determine the effect of U(VI) concentration and its uptake on SiO₂/GO. These tests were performed by stirring 50 mL of various U(VI) concentrations vary between 25–600 mg/L at pH 4.0, 50 mg of SiO₂/GO dose, and 50 min sorption time. Figure 10a shows that as initial U(VI) concentration increased, the U(VI) uptake augmented to reach a maximum uptake of 145 mg/g at 150 mg/L; after that, the uptake remained constant.

Adsorption isotherms are critical for understanding the specifics of adsorption conduct, SiO₂/GO sorbent surface properties, and further factors that affect sorption procedures. The sorption experiments were carried out at the optimum conditions to determine the respectable well-matched isotherm. Nonlinear models of Langmuir and Freundlich engaged in regulating the convenient model and competently related the practical information. Nonlinear most minor square optimization performance was employed, and isotherm factors were estimated with the assistance of nonlinear fit software. The reduced Chi-square was operated as a condition for the fitting advantage. The squared alteration between the experimental and calculated data was divided into the calculated data gained from the Langmuir alongside Freundlich models [68]. The reduced Chi-square was signified by:

$$x^2={\displaystyle \sum }\frac{{\left(q_{e\left(\exp \right)}-q_{e\left(mod\right)}\right)}^2}{q_{e\left(mod\right)}}$$

(5)

where q_e(exp) and q_e(mod) (mg/g) are the uptakes of experimental and model calculation. x² has reduced Chi-square. The assessment of x² was utilized to assess the fit of the best isotherm to experimental data, where the smaller the reduced Chi-square, the better the isotherm fit. Permitting to Langmuir model sorption employed at homogeneous sites inside the SiO₂/GO adsorbent and formerly U(VI) occupied at sites. The nonlinear Langmuir isotherm was determined using the following equation:

$$q_e=\frac{q_{max}{k}_L{C}_e}{1+k_L{C}_e}$$

(6)

where q_e (mg/g) is the sorbed U(VI) amount on SiO₂/GO adsorbent, C_e (mg/L) is the equilibrium U(VI) concentration, q_max (mg/g) is the maximum sorbed U(VI) amount on SiO₂/GO adsorbent, and k_L (L/mg) is defined as the Langmuir sorption constant.

The empirical nonlinear Freundlich model according to sorption via heterogeneous surface was set as:

$$q_e=k_f{C}_e^{1/n}$$

(7)

where k_f and n symbolize for the uptake and sorption intensity, respectively. From 10b shows that the Langmuir model’s sorption uptake (148.5 mg/g) is relevant to the experimental uptake (145.0 mg/g), and its R² (0.962) is close to unity when compared to the Freundlich model’s k_f (79.71 mg/g) and R² (0.691). Additionally, the Langmuir’s Chi-square is smaller than Freundlich’s Chi-square. As a result, Langmuir’s model is the most accurate for the experimental data.

3.2.5. Temperature and Thermodynamic Investigations

The temperature impact on U(VI) uptake on SiO₂/GO sorbent was measured at 25–55 °C at pH 4.0, 50 g of SiO₂/GO dose, 50 min contact time, and 150 mg/L of U(VI). The data in Figure 11a show that as the sorption temperature rose to 55 °C, the uptake decreased to 142.20 mg/g. As a result, it was probably the increase in temperature that led to the decomposition of the SiO₂/GO sorbent. Additionally, it may be the decrease in SiO₂/GO active sites that started the diminishing uptake.

The sorption mechanism was assessed through thermodynamic calculations for U(VI) on SiO₂/GO sorbent, and they were assessed from the Van’t Hoff calculations [69,70]:

$$log{K}_d=\frac{\Delta S^{°}}{2.303R}-\frac{\Delta H^{°}}{2.303RT}$$

(8)

$$\Delta G^{°}=\Delta H^{°}-T\Delta S^{°}$$

(9)

where K_d (L/g) refers to the distribution constant, ΔG° (kJ/mol) symbolizes for the sorption free energy, ΔH° (kJ/mol) stands for the enthalpy sorption changes, ΔS° represents the entropy sorption changes (J/mol.K), R is 8.314 J/mol.K, and T expresses the Kelvin temperature (K). Thermodynamic considerations of SiO₂/GO sorbent are shown in Figure 11b and

Table 2. Thermodynamic parameters of U(VI) sorption upon SiO₂/GO sorbent.

Temp, K	∆G°, kJ/mol	∆H°, kJ/mol	∆S°, kJ/(mol.K)
298	−8.2980	−12.33	−1.35 × 10−2
303	−8.230
308	−8.163
313	−8.095
318	−8.027
323	−7.959
328	−7.892

. The negative ΔG° values stated that the adsorption of (VI) on SiO₂/GO was achievable spontaneously. The sorption progression also formed U(VI) electrostatic interaction with SiO₂/GO. The negative ΔH° value suggested that the sorption method was exothermic. Correspondingly, the negative ΔS° value evidenced the feasibility and randomness of the sorption.

3.3. Desorption Investigation

3.3.1. Eluting Type

Eluting HNO₃, HCl, H₂SO₄, NaCl, NaNO₃, and Na₂SO₄ agents were performed to acquire the U(VI) desorption from U/SiO₂/GO. Still, the other desorption factors were kept constant at a 1.0 M eluting concentration, 60 min eluting time, and 1:30 (S:L) phase ratio. As presented in Figure 12a, U(VI) elution from U/SiO₂/GO using 1.0 M H₂SO₄ was extended to the maximum elution (88.0%). Consequently, H₂SO₄ is the best eluting agent.

3.3.2. Acid Concentration

U(VI) desorption from U/SiO₂/GO was studied. Many concentrations of H₂SO₄ ranging from 0.2–1.2 M were employed to detect the best acid concentration for desorption, and other parameters were constant at 1:30 S:L ratio, 60 min desorption time, and 25 °C. The acquired data in Figure 12b appeared to improve U(VI) desorption by increasing the eluting concentration until realizing the highest value (85.0%) at 0.8 M H₂SO₄. Subsequently, 0.8 M of H₂SO₄ was suitable for eluting of U(VI).

3.3.3. Solid: Liquid Ratio

The S:L ratio’s outcome was inspected by a difference of the S:L phase range from 1:10–1:70, while other parameters were settled at 0.8 M acid for 60 min contact time (Figure 12c). The comprehensive data indicate that U(VI) desorption was continuously improved through increasing the S:L phase ratio until 89.0% at 1:50 S:L and constantly persisted from 1:50 to 1:70 S:L phase ratio. Hence, the S:L phase ratio of 1:50 optimized U(VI) desorption.

3.3.4. Eluting Time

Eluting time affects U(VI) desorption or elution from U/SiO₂/GO, and it was examined many times from 10 to 90 min. The other parameters constantly persist at 0.8 M H₂SO₄, 1:50 S:L phase ratio. In the data in Figure 12d, desorption was improved from 75.0 to 99.0% by increasing the agitation time from 10 to 60 min, but the desorption constantly persisted after 60–90 min. Therefore, 60 min was the best eluting time.

3.4. Regeneration

Regeneration remains the most significant advantage in the re-reusability of SiO₂/GO adsorbent. The SiO₂/GO adsorbent was refreshed using 0.8 M H₂SO₄, 1:50 ratio of S:L, and 60 min for recycling (Figure 13). The sorption–desorption behaviors were recurrent until desorption dropped to 80.0% after the seventh cycle for SiO₂/GO. Hence, this validated the use of SiO₂/GO for U(VI) sorption.

4. Conclusions

Silica/GO was prepared from the environmental solid wastes and employed for U(VI) sorption. SiO₂/GO adsorbent was produced from graphene oxide obtained from spent carbon rod batteries and silica was obtained from rice husk to eliminate U(VI) ions from their solutions. The best sorption parameters were pH of 4.0, 150 mg/L U(VI), 50 mg SiO₂/GO dosage, and 50 min of equilibrium time. The recognized superior uptake was at 145.0 mg/g. In addition, kinetic results were stated to suit the second-order kinetic model.

Furthermore, the Langmuir isotherm model was well-matched for labeling adsorption developments. The negative ∆S° and negative ∆H° values suggested that the thermodynamic calculations recognized U(VI) sorption randomness and exothermic nature. Moreover, the negative ∆G° values pointed out to spontaneous sorption. The U(VI) ions were also stripped from U/SiO₂/GO by 0.8 M H₂SO₄, a 1:50 ratio of S:L, and 60 min of stripping time. The sorption–desorption developments were frequently iterated till desorption was reduced to 80.0% seven recurring times. Finally, SiO₂/GO adsorbent permitted a good U(VI) adsorption from aqueous sources.