Green Hydrogel-Biochar Composite for Enhanced Adsorption of Uranium.

Uranium is the backbone of the nuclear fuel used for energy production but is still a hazardous environmental contaminant; thus, its removal and recovery are important for energy security and environmental protection. So far, the development of biocompatible, efficient, economical, and reusable adsorbents for uranium is still a challenge. In this work, a new orange peel biochar-based hydrogel composite was prepared by graft polymerization using guar gum and acrylamide. The composite's structural, morphological, and thermal characteristics were investigated via Fourier transform infrared (FTIR), scanning electron microscope (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA) methods. The composite's water absorption properties were investigated in different media. The performance of the prepared composite in adsorbing uranium (VI) ions from aqueous media was systematically investigated under varying conditions including solution's acidity and temperature, composite dose, contact time, and starting amount of uranium. The adsorption efficiency increased with solution pH from 2 to 5.5 and composite dose from 15 to 50 mg. The adsorption kinetics, isotherms, and thermodynamics parameters were analyzed to get insights into the process's feasibility and viability. The equilibrium data were better described through a pseudo-second-order mechanism and a Langmuir isotherm model, indicating a homogeneous composite surface with the maximum uranium (VI) adsorption capacity of 263.2 mg/g. The calculated thermodynamic parameters suggested that a spontaneous and endothermic process prevailed. Interference studies showed high selectivity toward uranium (VI) against other competing cations. Desorption and recyclability studies indicated the good recycling performance of the prepared composite. The adsorption mechanism was discussed in view of the kinetics and thermodynamics data. Based on the results, the prepared hydrogel composite can be applied as a promising, cost-effective, eco-friendly, and efficient material for uranium (VI) decontamination.

Introduction

The quick development of the nuclear industry can be attributed to the introduction of considerable amounts of uranium (U) to the surrounding environment, especially nearby water bodies as well as groundwater systems.[1] Uranium poses major threats to the environmental ecosystems and human health owing to its long-term high toxicity and radioactivity;[2] hence, the safe treatment and disposal of uranium-contaminated water has become a public concern. Uranium can be removed by physical, chemical, and biological methods.[3] Among the numerous treatment methods applied for uranium removal from water bodies, the adsorption technique is a preferred, facile, efficient, and cost-effective choice.[4] Biocompatible adsorbents have been employed as a more environmentally friendly option for uranium decontamination from aqueous solutions.[5]

Currently, biochar (B) is attracting great interest as a renewable, efficient, environmentally friendly, economic, and green biomaterial for environmental remediation practices.[6] It is a carbon-rich pyrolysis product that has numerous biomass sources, usually either vegetal or animal waste, where biochar’s physical and chemical characteristics are determined by the source type and pyrolysis conditions.[7] The abundance of surface functional groups (e.g., hydroxyl, carboxyl, amino),[8] high porosity, large surface area, and excellent ion-exchange capacity result in a significantly high adsorption efficiency of biochar-based adsorbents.[9] Besides its good adsorption performance, biochar shows the advantages of sustainability, simple preparation, facile functionalization, stable structure, enhanced physicochemical properties, and recyclability.[10]

The performance of biochar-based adsorbents is influenced by their surface area, pores size, and surface functionality in addition to pollutant type and size.[11,12] To increase biochar’s adsorption capacity and environmental applicability, modification of raw biochar, either by chemical or physical route, is a current research focus.[13−20] Polymeric materials are among the applied biochar modifying substrates that result in better physicochemical properties and enhanced removal efficiency for various pollutants.[18] Recent studies have demonstrated that polymeric biochar adsorbents offer better mechanical strength, enhanced durability, high adsorption capacity, and improved recyclability.[21] Furthermore, polymeric biochar hydrogel adsorbents are expected to be a good addition to the biocompatible adsorbents for uranium removal due to the combination of the porous network of biochar with the high chemical affinity of polymeric hydrogels.

Polymeric hydrogels have attracted increasing attention as potential effective candidates for environmental remediation[22,23] thanks to their features such as hydrophilicity, high water retention, biocompatibility, low cost, and ability to sense and respond to the changes in environmental conditions (e.g., medium acidity, temperature, ionic strength, etc.). Polymeric hydrogels are three-dimensional (3D) cross-linked networks with hydrophilic properties that can readily swell in water, aqueous solutions, or biological fluids and retain a large amount of water.[24,25] The ionic functional groups of polymeric hydrogels render them the ability to remove various metal ions from aqueous media due to their high complexing ability.[26] Over the past decade, polymeric hydrogels, based on synthetic and biopolymers, have been widely applied for the removal of various contaminants from aqueous media due to their synthesis simplicity, facile application, viscoelasticity, porous structure, and availability of numerous raw materials.[27,28]

While hydrogels can be prepared using synthetic polymers, natural polymers are preferred for hydrogel fabrication as they fit the requirements for “green” and sustainable development.[29] Guar gum (GG)-based hydrogels have been introduced as a benign, low-price, and biodegradable material in the field of water purification.[29] Additionally, GG possesses many reactive functional groups, which allows the graft copolymerization of synthetic monomers. The combination of advantages of natural and synthetic polymers via graft copolymerization allows obtaining functional biomaterials with tailored properties for a wide usage range.[30,31]

To enhance the adsorption capacity of GG-based hydrogels, chemical modification is often applied. Therefore, in this study, a hydrogel composite was prepared by grafting acrylamide (Am) onto GG, followed by grafting orange peel biochar to Am-GG via radical polymerization. The potential applicability of the prepared composite as an efficient material for uranium removal from aqueous solutions was explored. The main parameters that impact uranium adsorption from aqueous media were systematically evaluated. To our knowledge, this is the first report investigating uranium removal from aqueous solutions by the guar gum-acrylamide-orange peel biochar (GGAmB) hydrogel composite.

Results and Discussion

Synthesis and Characterization of the GGAmB Hydrogel Composite

Graft polymerization of Am with GG in the presence of a chemical cross-linking agent (N,N′-methylene bisacrylamide, MBA) and B was carried out to form the GGAmB hydrogel composite. Scheme represents the predicted mechanism for the graft polymerization and cross-linking process of Am onto GG chains using potassium persulfate (APS) as a radical initiator. APS decomposed by heating at 70 °C under N2 gas to give sulfate radicals that could abstract hydrogen of −OH groups of the GG matrix to create macroradicals. Am monomer could be grafted onto these active radicals while MBA cross-linked the monomer with the radicals to form a 3D network structure. Through the formation of a 3D network, biochar was dispersed and bounded to the hydrogel network.

Scheme 1

Synthesis of GGAmB via Graft Polymerization

Grafting polymerization and biochar incorporation into the network were confirmed by comparing the Fourier transform infrared (FTIR) spectra of GG, raw biochar, and GGAmB composite as presented in Figure . The GG spectrum showed peaks at 3444 and 2925 cm–1 corresponding to O–H and C–H (of −CH2 groups) stretching vibrations,[32] while O–H bending and −CH2 twisting vibrations were observed at 1082 and 1023 cm–1, respectively.[32] The peaks at 1652, 1457, and 1157 cm–1 were due to mannose ring stretching, −CH2 (of the C–OH group) symmetrical deformation, and C–O–C (of glycosidic linkage) asymmetric stretching vibrations, respectively.[33] The peak noticed at 868 cm–1 was due to the skeletal stretching vibrations of galactose and mannose units.[32] The raw biochar spectrum revealed a peak at 3428 cm–1 that represents the O–H stretching of alcoholic, carboxylic, and phenolic groups,[34] while the C=O stretching peak appeared at 1634 cm–1. The peaks at 1045 and 874 cm–1 were related to the aromatic C=C and C–O stretching vibrations, respectively,[35] while the peak at 535 cm–1 was due to Si–O bending vibration. The GGAmB hydrogel spectrum revealed new characteristic peaks at 1665 and 1636 cm–1 that were attributed to C=O stretching and NH2 angular deformity stretching vibrations of the amide group[36] that confirms the grafting of Am onto GG. The peaks at 3664, 3438, and 2946 cm–1 represented the O–H, N–H, and C–H stretching vibrations, respectively.[35] The peaks at 1410 and 1445 cm–1 corresponded to C–N and C–H vibrations, whereas the peaks at 1123 and 670 cm–1 were due to C–O–C stretching and N–H wagging vibrations, respectively. The characteristic band of Si–O disappeared, which confirms the formation of the GGAmB hydrogel composite.

Figure 1

FTIR spectra of GG, biochar, and GGAmB hydrogel composite.

Figure shows the thermal decomposition behavior (a function of weight loss with temperature increment) of biochar and the GGAmB hydrogel composite. Biochar mass loss was noticed over a broad temperature range up to 600 °C. A slight weight loss (10.21 wt %) was first observed up to 200 °C that corresponded to the entrapped moisture release. Following, a relatively slow weight loss was noticed with temperature increment, where a considerable mass was lost (40.1 wt %) between 200 and 600 °C. It was noticed that approximately 49.79 wt % biochar mass exists at 600 °C, which indicates its thermal stability.[37] The weight loss for the GGAmB composite could be divided into four phases, an initial phase (100–200 °C) in which a small amount of weight loss was observed (11 wt %) that was attributed to the dehydration and volatile compounds elimination. The second phase occurred at 200–330 °C with 17 wt % loss and was attributed to side-chain decomposition. The third phase was noted at 330–450 °C with a 16 wt % loss that could be due to the degradation of amide groups. Finally, the fourth phase was noticed at 460–600 °C with a 40 wt % loss that corresponded to the cross-linked structure collapse.

Figure 2

Thermogravimetric analysis (TGA) curves of (a) biochar and (b) GGAmB hydrogel composite.

The crystallinity of biochar, GG, and GGAmB hydrogel composite was analyzed via X-ray diffraction (XRD), and the recorded patterns are illustrated in Figure . The biochar XRD pattern showed poor crystallinity with one broad peak at 2θ° = 30.32 related to crystalline CaCO3, which is ascribed to the (104) plane of calcite trigonal crystal structure, which is consistent with previous research.[38,39] In the case of GG, an amorphous structure with low overall crystallinity can be noticed with an observed peak at 2θ° = 20.2.[33] The disappearance of the diffraction peak related to pure biochar in the GGAmB XRD pattern refers to the high crystallinity of the hydrogel composite and indicates the dispersion of biochar into the polymeric network.

Figure 3

XRD patterns of (a) biochar, (b) GG, and (c) GGAmB hydrogel composite.

The surface morphology of biochar and GGAmB was analyzed by scanning electron microscope (SEM). Biochar micrograph revealed a smooth, nonporous, and less uneven surface (Figure a), whereas the GGAmB composite (Figure b) showed a rougher, porous, and irregular surface morphology, which is desirable for uranium (VI) adsorption. The internal pores favor the intraparticle diffusion of uranium (VI) ions to GGAmB and enhance the adsorption process. These data reveal the successful grafting polymerization process.

Figure 4

SEM images of (a) biochar and (b) GGAmB hydrogel composite.

Swelling Behavior

Equilibrium swelling is one of the significant considerations for assessing the hydrogel’s effectiveness. In practical applications, high swelling rate and capacity are required. Figure shows the swelling kinetics of the GGAmB hydrogel in pure water and saline solution. Water diffused into the GGAmB network chains when the hydrogel was brought in contact with the aqueous solution, leading to an extensive segmental movement that expands the space between the network chains.[40] It can be noticed that GGAmB swelled relatively fast, attaining 50% absorbency in 90 min, due to capillary action through the pores. After that, the swelling consistently increased at a slower rate and attained its maximum equilibrium swelling (1600 g/g) in 290 min. The high water absorbency of GGAmB hydrogel is caused by the hydrophilic groups that exist in the 3D network structure in addition to the porous property and high surface area, which enhanced and increased the water absorbance capacity.

Figure 5

Swelling behavior of GGAmB hydrogel composite in different media.

The water absorbance of GGAmB hydrogel in 0.9 wt % NaCl solution was relatively lower than that obtained in d-water (1400 g/g). This behavior was attributed to the charge screening effect of ionic hydrogels[41] producing imperfect anion–anion repulsion, which led to a reduced ionic pressure between the hydrogel network and water molecules, thereby decreasing the water absorption capacity. D-water displayed a better absorption since it had a less ionic concentration that aided in the enhanced water intake by osmotic pressure.

Application of the GGAmB Hydrogel Composite for Uranium (VI) Adsorption

Impact of Initial Solution pH

Medium acidity is a significant factor in uranium adsorption, as it may affect both the distribution of uranium ions and the protonation–deprotonation reactions of adsorbent functional groups and adsorbent surface charge.[42] Since uranium (VI) adsorption is governed by its interaction mechanism with the composite active groups, solution pH could promote or suppress uranium adsorption.[43] Therefore, the pH dependence of uranium adsorption by the prepared hydrogel composite was determined at different values from 2 to 8, and the results are shown in Figure . Initially, uranium adsorption increased with the pH increment from 2 to 5.5, where the maximal adsorption was detected; then, it decreased with further pH increase. Under acidic conditions, uranium (VI) adsorption onto the hydrogel composite was low due to the existence of more hydrogen ions, which compete with uranium (VI) for the available functional groups.[44] In addition, the functional groups’ protonation under strong acidic pH conditions reduced the number of active sites for uranium (VI) adsorption.[45] pH increase led to a decrease in the amount of hydrogen ions and the deprotonation of active sites that resulted in a higher adsorption capacity. Additionally, the pH increase caused the hydrogel dilation that eventually led to an increased influx of uranium (VI) to the hydrogel to react with the unreachable adsorption sites at lower pHs owing to the hydrogel shrinking.[46] However, under alkaline conditions, various uranium–hydroxyl complexes were formed, such as UO2(OH)+ and (UO2)2(OH)22+, that reduced the available quantity of uranium (VI) cations engaged in the adsorption process, and consequently, a lower adsorption capacity was noticed.

Figure 6

Impact of pH on U(VI) adsorption by the GGAmB hydrogel composite.

The effect of solution pH on the adsorption efficiency was further investigated via point of zero charge (PZC) analysis. PZC, i.e., the pH at which the GGAmB surface is globally neutral, was determined by the ζ potential measurements method, and the plot of ζ potential vs pH is represented in Figure . The PZC value of GGAmB was 2.2; thus, the hydrogel will remain neutral at this pH. Below this value, the GGAmB surface is positively charged and it is difficult to adsorb the positively charged uranium ions. However, at pH values higher than 2.2, the GGAmB surface is negatively charged and can adsorb the positively charged uranium (VI) species via the electrostatic interaction mechanism.

Figure 7

Variation of ζ potential of GGAmB with solution pH.

Impact of Hydrogel Composite Dose

Adsorbent dose is a considerable parameter that influences the adsorption performance. Generally, using a small adsorbent quantity that can attain a considerable adsorption percentage is desirable for economic adsorption.[40] The impact of GGAmB hydrogel dose on uranium (VI) adsorption efficiency was investigated to attain the most appropriate amount of hydrogel. Figure shows the effect of GGAmB hydrogel dose (15–125 mg) on uranium (IV) adsorption at 30 °C and constant initial uranium (VI) concentration. It can be noticed that uranium (VI) removal % increased from 31.6 to 96.1% with increasing adsorbent dose from 15 to 50 mg, while the higher GGAmB hydrogel dose did not produce a dramatic change. The increase in removal efficiency (R%) with dose increment can be attributed to the increased surface area and the available number of adsorption sites at higher hydrogel doses.[47] In contrast, a contradicting behavior was noticed for the adsorption capacity. The lower uranium (VI) adsorption capacity observed for a higher hydrogel dose was because more active sites of hydrogel remained unsaturated during the adsorption process due to uranium deficiency with respect to these sites.[48] Since no appreciable variation was noticed in uranium (VI) removal at the hydrogel dose higher than 50 mg, it was selected for further adsorption experiments.

Figure 8

Impact of GGAmB hydrogel composite dosage on U(VI) adsorption.

Impact of Initial Solution Concentration

To figure out the adsorption behavior, it is necessary to understand the relationship between the adsorbed quantity and the equilibrium concentration of the adsorbate. The adsorption rate is dependent on the adsorbate initial concentration; thus, it should be considered when designing the adsorption system. The impact of initial uranium (VI) ions concentration on the GGAmB adsorption capacity and removal efficiency was studied at various concentrations (50–500 mg/L) at ambient temperature while fixing the reaction time, pH, and shaking rate. Data represented in Figure showed that the adsorption capacity increased while the removal efficiency decreased with an increase in the initial uranium (VI) concentration. The uranium (VI) ions were easily adsorbed at low initial concentrations due to the abundance of hydrogel’s free binding sites. By increasing the uranium concentration, most of these binding sites were occupied as the adsorption process proceeded and thus the total available active sites became limited.[49] The observed increased adsorption capacity resulted from the greater driving force that allowed for overcoming the mass transfer resistance between uranium (VI) ions and the hydrogel, and consequently, the adsorption rate was increased.[50] With further increase of uranium (VI) concentration, the removal percentage decreased while the adsorption capacity reached a plateau, signifying the saturation of the hydrogel active sites.

Figure 9

Impact of initial concentration on U(VI) adsorption by the GGAmB hydrogel composite.

Adsorption Isotherm Studies

The adsorption isotherms are critical to perceive the adsorbate allocation on the adsorbent surface at equilibrium.[51] They can be used to delineate the relationship between the solute adsorbed quantity and its concentration at equilibrium under fixed temperature. Hence, for optimizing the adsorbent utilization, the best-correlated adsorption isotherm to the equilibrium data should be determined. To investigate the uranium (VI) adsorption mechanism on the prepared composite, the equilibrium data were examined by Langmuir and Freundlich isotherms, and the results are displayed in Figure .

Figure 10

(a) Langmuir (b) and Freundlich adsorption isotherms for U(VI) adsorption by the GGAmB hydrogel composite.

Forming a uniform adsorbate monolayer that coats the whole adsorbent surface is defined from the Langmuir equation[52]where qmax is the maximum adsorption capacity (mg/g), qe is the quantity of uranium (VI) adsorbed at equilibrium (mg/g), Ce is the uranium (VI) concentration at equilibrium (mg/L), and b is the Langmuir constant (L/mg). Plotting Ce/qe versus Ce gives a straight line, where qe and b can be calculated from the slope and intercept, respectively. Uranium (VI) adsorption data were plotted according to the linear Langmuir formula and are displayed in Figure a.

Freundlich isotherm is another common adsorption model that is applied in describing the adsorption on nonhomogeneous surfaces and has the form[53]where KF (L/g) and n are the Freundlich parameters associated with the adsorption capacity and intensity, respectively, and can be computed from the log qe vs log Ce graph. Figure b represents the linear plot of log qe versus log Ce as experimentally obtained, and Table lists the specific fitting parameters of both Langmuir and Freundlich isotherm models.

Table 1

Isotherm Parameters of U(VI) Adsorption by the GGAmB Hydrogel Composite

adsorption isotherm	fitting parameters
Langmuir	R2 = 0.999
	qmax = 263.2 mg/g
	b = 0.073 L/mg
Freundlich	R2 = 0.901
	KF = 45.24 mg/g
	n = 0.336

The superior correlation to the experimental data besides the good agreement of the calculated monolayer adsorption capacity exhibited by the Langmuir isotherm (263.2 mg/g) to the experimentally obtained value (260.1 mg/g) indicated that the Langmuir model is more suitable to describe uranium (VI) adsorption onto GGAmB hydrogel compared to the Freundlich adsorption model. This demonstrates that the hydrogel active binding sites have uniform distribution, resulting in a single-layer adsorption process.

The adsorption feasibility was investigated via the RL values that were calculated according to the following formula

The calculated RL values ranged from 0.2 to 0.02 within the applied initial concentrations in this study, indicating favorable adsorption of uranium (VI) ions on the GGAmB hydrogel.[54] Additionally, data driven from Freundlich isotherm showed n value below 1, confirming the favorable uranium (VI) adsorption by the prepared hydrogel composite.[54]

Kinetics Modeling

Adsorption rate is a critical factor in understanding the adsorption nature and evaluating the adsorbent’s overall performance in practical applications. The results of uranium (VI) adsorption progress with time (Figure a) indicate that the adsorption process initially proceeded relatively fast and then slowed down till the equilibrium was reached after 150 min; then, the uranium (VI) adsorption capacity remained constant with a further increase in time. The incipient rapid adsorption phase was ascribed to the elevated uranium (VI) affinity of the hydrogel that limited the intraparticle diffusion resistance and increased the adsorbent exchange surface. The subsequent slower adsorption phase could be ascribed to the significant reduction of available binding sites as they were being occupied by uranium (VI) ions.[55]

Figure 11

(a) Impact of adsorption time on U(VI) adsorption by the GGAmB hydrogel composite, (b) pseudo-first-order, and (c) pseudo-second-order model fitting.

Generally, the adsorption kinetics of metal ions is important in designing effective adsorption systems; thus, uranium (VI) adsorption kinetics on the GGAmB hydrogel composite was analyzed using pseudo-first-order and pseudo-second-order kinetic models. The pseudo-first-order model assumes a single adsorbate for each adsorption site at the adsorbent surface, and its linear formula is presented as[56]where qe and qt are uranium (VI) quantities adsorbed per unit composite weight (mg/g) at equilibrium and time t, respectively, and k1 is the rate constant of pseudo-first-order adsorption (1/min). The k1 and qe values were computed from the slope and intercept of log (qe – qt) versus t linear plot as represented in Figure b and Table .

Table 2

Kinetics Parameters of U(VI) Adsorption by the GGAmB Hydrogel Composite

	pseudo-first-order			pseudo-second-order
qe, exp (mg/g)	qe, cal (mg/g)	k1(min–1)	R2	qe cal (mg/g)	k2 × 10–3 (g/mg min)	R2
122.851	95.600	0.218	0.975	128.110	0.040	0.997

The pseudo-second-order linear equation is given as[57]where k2 is the pseudo-second-order rate constant (g/(mg min)). Plotting t/qt against time yields a straight line (Figure c), where k2 and qe values could be computed from the intercept and slope, as listed in Table .

The results revealed better fitting of the pseudo-second order to the experimental data. Moreover, the adsorption capacity value calculated by pseudo-second order was closer to the experimentally determined one. These results emphasize that the pseudo-second-order equation is more appropriately describing uranium (VI) adsorption on the GGAmB hydrogel, indicating a chemical adsorption mechanism.

Thermodynamic Studies

To describe the significance of temperature on the amount of uranium (VI) ions adsorbed onto the GGAmB hydrogel at optimum contact time, the adsorption process was studied at different reaction temperatures (30–70 °C) and the data are graphed in Figure . The results showed that the maximum monolayer adsorption capacity of uranium (VI) onto the GGAmB hydrogel increased with the adsorption medium temperature, suggesting a spontaneous and endothermic process.[58] The increase in uranium (VI) adsorption with temperature could be due to the enlargement of pores and/or the activation of the adsorbent surface.[59] Additionally, the higher temperature leads to an increase in the mobility of uranium (VI) ions due to the acquired energy in the system; thus, the uranium (VI) ions became more energetic to react with the adsorbent surface sites.[60] This behavior suggests that the uranium (VI) interaction with the active sites at the GGAmB hydrogel could be a chemical one.[60]

Figure 12

Impact of temperature on U(VI) adsorption by GGAmB hydrogel composite.

The thermodynamic parameters afford insights to understand the adsorption type and mechanism. Therefore, the thermodynamic parameters related to the adsorption process of uranium (VI) on the GGAmB hydrogel at equilibrium were evaluated. The plot of ln Kd against 1/T (Figure ) was used to determine the change in standard free energy (ΔG°), change in standard enthalpy (ΔH°), and change in standard entropy (ΔS°) applying the subsequent formula[58]where T(K) is the temperature, R (8.314 J/mol.K) is the gas constant, and Kd is the distribution coefficient (mL/g).

Figure 13

Plot of ln K versus 1/T for the calculation of adsorption thermodynamic parameters.

The recorded values of ΔG°, ΔH°, and ΔS° are presented in Table . Generally, G° indicates the degree of spontaneity of the adsorption process and the higher negative value reflects a more energetically favorable adsorption.[61] The negative values of Gibbs free energy as shown in Table denote the feasibility and spontaneity of uranium (VI) adsorption on the GGAmB hydrogel. Additionally, ΔG° had more negative values at higher temperatures, indicating an energetically favorable adsorption process at elevated temperatures,[62] which affirms the endothermic adsorption pathway. A decrease in ΔG° with an increase in temperature reflects better sorption at elevated temperatures, resulting from the higher spontaneity extent of the adsorption at elevated temperatures.[63]

Table 3

Thermodynamic Parameters of U(VI) Adsorption by the GGAmB Hydrogel Composite

temp (K)	ΔH° (kJ/mol)	ΔG° (kJ/mol)	ΔS° (J/mol K)
303	20.785	–2.369	75.212
313		–3.072
323		–3.758
333		–4.750
343		–5.392

The positive value of ΔH° for uranium (VI) adsorption on GGAmB refers to an endothermic adsorption process[64] and the increase of the system randomness resulting from the solid–liquid interaction during adsorption.

The positive entropy change value denotes the GGAmB hydrogel affinity toward uranium (VI) ions in the aqueous solution[65] and reflects higher randomness at the adsorbate–solution interface, indicating the adsorption process stability. Additionally, the positive ΔS value indicates that the degree of free active sites increased at the solid–liquid interface during the adsorption of uranium (VI) onto the GGAmB hydrogel[63] and the adsorption is mediated by entropy-driven spontaneous process.[66] Similar thermodynamic behavior was reported for uranium adsorption on different adsorbents.

Effect of Interfering Ions

The selectivity of GGAmB hydrogel toward uranium (VI) ions was examined in binary batch systems at equivalent concentrations. The selectivity was evaluated as a function of uranium (VI) adsorption efficiency obtained in the presence and absence of the interfering ions (nitrates) under the optimum conditions. As per Figure , the efficiency of the adsorption process was slightly lowered by the presence of interfering ions in the solution matrix, and the extent of this effect depended on the metal ion type. This lowering of the adsorption efficiency could be due to the competition of the interfering ions with uranium (VI) ions for the limited active sites of the GGAmB hydrogel.

Figure 14

Effect of interfering ions on U(VI) adsorption efficiency by the GGAmB hydrogel composite.

Desorption and Recyclability Behavior

Desorption and recyclability are important parameters when assessing adsorbents for practical application. To study the effect of the eluent type, the uranium (VI) adsorbed onto GGAmB hydrogel was eluted with different desorbing agents (HCl, HNO3, H2SO4, 0.5 mol/L) and then the solution was filtered and uranium (VI) concentration was analyzed. The desorption efficiency was calculated as the ratio of desorbed uranium ions to the adsorbed uranium ions by GGAmB hydrogel. The results showed that HNO3 was the most efficient eluent for uranium desorption from the GGAmB hydrogel and achieved a desorption efficiency of 92%. Slightly lower desorption efficiency values were noticed for H2SO4 (88%) and HCl (90%). The good desorption efficiency under highly acidic conditions could be due to the sufficiently high hydrogen-ion concentration resulting in strong competitive adsorption on the GGAmB surface and the surface protonation of GGAmB, which allows desorption of positively charged uranium (VI) ions.

Recyclability was studied through several successive adsorption–desorption processes using 0.5 mol/L HNO3. Figure reveals that the decrease of GGAmB hydrogel adsorption efficiency was about 15% after it was recycled four times. This good desorption and recyclability property of the GGAmB hydrogel indicates its potential application for uranium recovery from aqueous solutions.

Figure 15

Recyclability of the GGAmB hydrogel composite.

Adsorption Mechanism

The results showed that the adsorption kinetics is better described by the pseudo-second-order kinetic model, indicating that chemisorption is the rate-limiting step of uranium (VI) adsorption on GGAmB.[67] Additionally, the adsorption equilibrium data fitted best to the Langmuir isotherm, reflecting that the adsorption mechanism is primarily chemical.[68] Besides, the Freundlich constant (n) had a value less than unity, which further confirms that uranium adsorption is a chemical process.[69] These recommend that the interaction between uranium (VI) ions and GGAmB is chemisorption involving covalent or ionic valence forces of bonds through electrons sharing or exchange.[70] Based on the results of ZPC, adsorption isotherms, kinetic models, and thermodynamics, uranium adsorption mechanism on GGAmB could be as follows: first, uranium (VI) ions migrate from the solution and reach the GGAmB surface. Since at pH 5.5 the overall charge of the GGAmB surface is negative, uranium ions have a positive charge, the adsorption could initially occur via the electrostatic interaction mechanism. However, apart from the electrostatic forces, other chemical interactions, e.g., ion-exchange and coordination, of uranium (VI) ions with the GGAmB active sites distributed along the hydrogel structure contribute to the overall uranium uptake. Negative ΔH means a chemically exothermic process; hence, the chelating mechanism may generally dominate over the ion-exchange mechanism.[69] The amide and hydroxyl groups bounded on GGAmB can share electrons and form chemical coordinate bonds between the donor atoms of the binding sites and uranium (VI) ions to form chelate compounds.

FTIR spectra were applied to further predict the adsorption mechanism via various peaks that correspond to the functional groups and surface properties. Comparing the FTIR spectra before (Figure ) and after (Figure ) uranium adsorption, a new peak at a wavelength of 935 cm–1 is noticed, which corresponds to the asymmetric stretching vibration mode of the O=U=O moiety.[71] Additionally, the change in the intensity and transmittance of some peaks was clearly observed after uranium adsorption. The shift in the frequency of the C=O band from 1665 to 1671 cm–1 and NH2 band from 1636 to 1609 cm–1 could be caused by the coordination of uranyl ions.[72] These shifts suggest major participation of the amide groups in uranium adsorption. Changes seen in the FTIR spectral analysis explicitly endorse the contribution of functional groups of the GGAmB in binding uranium, with amide groups being more responsible for uranium binding than the hydroxyl groups.

Figure 16

FTIR spectrum of the GGAmB hydrogel composite after uranium (VI) adsorption.

Conclusions

In this study, a low-cost, environmentally sound, and industrially applicable hydrogel was developed and applied to remove uranium (VI) from contaminated water. SEM analysis showed the porous and irregular surface of the GGAmB hydrogel, which supports the adsorption of uranium ions onto the hydrogel surface. A range of experimental batch adsorption factors were investigated to optimize the adsorption process. The adsorption kinetic data correlated to the pseudo-second-order equation, indicating a chemisorption process. The Langmuir isotherm model was more compatible with the uranium (VI) adsorption data on the prepared hydrogel. The maximum adsorption capacity of the GGAmB hydrogel composite based on the Langmuir isotherm model was 263.2 mg/g, and the adsorption equilibrium was attained at 150 min. This study indicates that GGAmB is a promising adsorbent for uranium cleanup in nuclear waste management and pollution control applications.

Materials and Methods

Reagent-grade chemicals and distilled water (d-water) were utilized in all experiments. Guar gum (GG), acrylamide (Am), arsenazo III, N,N′-methylene bisacrylamide (MBA), ethanol, and methanol were purchased from Sigma-Aldrich. Potassium persulfate, 37% HCl, and NaOH (Merck, Germany) were used as obtained. Stock uranium (VI) solution obtained from the corresponding nitrate salt was used for adsorption experiments with proper dilutions.

Biochar Production

Orange peels (the feed material for biochar production) were obtained locally from Shibin El-Kom, Egypt. Orange peels were initially dehydrated at 70 °C in an oven (MOV-212S, Sanyo, Japan); then, the pyrolysis process was executed at 350 °C under N2 atmosphere for 3 h.[2] Afterward, the product (biochar, B) was ground, rinsed by 0.1 M HCl followed by d-distilled water for the removal of impurities, dried overnight at 70 °C, and stored in a desiccator for later use.[37] The C, H, N, O, and S elemental percentage of the produced biochar was analyzed by a Vario MACRO cube elemental analyzer (Germany), and the results are presented in Table .

Table 4

Elemental Analysis (%) of Orange Peel Biochar

sample	C	H	N	O	S
B	67.43	4.91	1.12	25.21	0.33

Preparation of the GGAmB Hydrogel Composite

The composite was prepared via the free radical graft polymerization technique. Guar gum (5 g) was dissolved in a 50 mL flask with a specific quantity of d-water and placed in a thermostat water bath. At that point, the addition of biochar (0.6 g) followed by Am (3 g) and consistent blending at 40 °C was conducted. The temperature was increased to 70 °C, and then 0.7 g of potassium persulfate in d-water was added with continuous mixing. After 15 min, MBA (0.05 g) was added to the mixture with the purge of N2. The water bath was kept at 70 °C to complete the polymerization reaction and obtain the GG-g-poly(Am)/biochar (GGAmB) hydrogel composite. The formed hydrogel was cut into little pieces and washed by a suitable ratio of d-water and ethanol to get rid of unreacted chemicals, dried overnight at 60 °C, and then stored in a desiccator.

Characterization of the GGAmB Hydrogel Composite

The functional groups of biochar and GGAmB hydrogel composite were detected using a Fourier transform infrared (FTIR, ATI Matson Genesis Series FTIR) spectrometer, while their thermal properties were investigated by thermogravimetric analysis (TGA 55, Meslo). For thermal analysis, specimens were put in a platinum container and the temperature was increased in the range of 30–600 °C under N2 atmosphere with a 10 °C/min heating rate. The morphological features of the orange peel biochar and the GGAmB composite were investigated through a scanning electron microscope (SEM, Quanta FEG 250, FEI Company). To investigate the crystalline structures, an X-ray diffraction (XRD) instrument (Shimadzu XRD-6000 lab x, Japan) with Cu Kα radiation (40 kV, /30 mA) was used. The point of zero charge was measured by a zetasizer (ZS, Malvern, U.K.) over the pH range 2–9.

Swelling Properties of the GGAmB Hydrogel Composite

D-water and saline water (0.9 wt % NaCl solution) were used to evaluate the swelling performance of the GGAmB hydrogel composite. Briefly, a dried disk of GGAmB hydrogel (0.5 g) was immersed in d-water or saline water (200 mL) at room temperature for a variable time till the swelling equilibrium state was obtained. The swollen gel was removed from the solution after a precisely defined time and then weighed after eliminating the excess solution. The swelling capacity was computed as[73]where Qeq is the equilibrium swelling capacity and Ws and Wd are the swollen and dry hydrogel weight (g), respectively.

Uranium (VI) Adsorption Studies

Uranium (VI) adsorption was conducted by the batch adsorption technique using a thermostatic water bath (SWB 15, Thermo Scientific) at 25 ± 1 °C except for thermodynamics experiments. Typically, a fixed amount of the GGAmB composite (50 mg) was added to certain uranium (VI) aliquot and stirred for 150 min. Then, the mixture was filtrated and the remaining uranium in the liquid phase was spectrophotometrically determined with a UV–visible spectrophotometer (Thermo, evolution 300, U.K.) via the arsenazo-III procedure.

Aiming to assess the GGAmB adsorptive performance, the uranium (VI) removal efficiency (R%) and adsorption capacity (qe, mg/g) were computed from the subsequent formulas[74]where Co and Ce are the uranium concentrations in solution before adsorption and at equilibrium (mg/L), respectively, V is the solution volume (L), and W is the GGAmB composite weight (g).