Synthesis, Characterization and Investigation of Cross-Linked Chitosan/(MnFe₂O₄) Nanocomposite Adsorption Potential to Extract U(VI) and Th(IV)

Abstract

A cross-linked chitosan/(MnFe₂O₄) CCsMFO nanocomposite was prepared using co-precipitation methods and used as a nanomaterial to extract U(VI) and Th(IV) from an aqueous solution based on adsorption phenomena. The contact time of experiments shows a rapid extraction process within 30 min by the CCsMFO nanocomposite. The solution pH acts a critical role in determining q_m value, where pH 3.0 was the best pH value to extract both ions. The pseudo-second-order equilibrium model illustrated the kinetics equilibrium modal extraction process. Adsorption isotherm of U(VI) at pH 3.0 by CCsMFO nanocomposite is an endothermic process. In contrast, the adsorption isotherm of Th(IV) at pH 3.0 by CCsMFO nanocomposite is an exothermic process. The reusability of CCsMFO nanocomposite was tested using basic eluents as suitable conditions for the chemical stability of CCsMFO nanocomposite; the reusability results show promising results for the removal of U(VI) adsorbed onto CCsMFO nanocomposite with 77.27%, after 12 h by Na₂CO₃ as eluent. At the same time, the reusability results show good reusability for removal of U(VI) adsorbed onto CCsMFO nanocomposite with 21.82%, after 8 h by EDTA as eluent.

1. Introduction

A natural heteropolymer produced from chitin by chemical, enzymatic methods through full or partially chitin’s alkaline deacetylation to produce a poly(β-1-4)-2-amino-2-deoxy-D-glucopyranose (Figure 1) is known as Chitosan (CS), where the deacetylation degree (%DD) plays a significant role in the physical, chemical, and biological properties of CS [1,2]. The measured pKa of chitosan gel-aqueous solution ranges 6.5 to 6.7 because it has three different chemical functional groups: primary and secondary hydroxyl groups at the C-6 and C-3 positions, and an amino group at the C-2 place (Figure 1) [3,4,5]. At a relatively low pH < 6.5, CS has a high positive charge because of amino group protonation [6,7]. Chitosan shows a high sensitivity to high temperatures which causes thermal analysis at high temperatures. In addition, the presence of oxidizable and hydrolyzable bonds in the backbone of chitosan makes it chemically sensitive to some materials. In addition, enhanced CS accessibility for microorganisms leads to biodegradation with time [2]. Despite these obstacles, and due to Cs’s chemical and physical features, it has several biological and medicinal applications as hemodialysis membrane materials, artificial skin, drug delivery, and substituting or regenerating blood/tissue interfaces [8,9,10]. In addition, it is used as an effective material in water treatment applications in coagulant [7] and membrane manufacturing [2,11]. Moreover, the advantageous characteristics of low price, abundance, nontoxicity, biocompatibility, biodegradability, hydrophilicity, ionic active sites (NH₂ and OH group), and high adsorption capability that DD determined CS is used as an adsorbent for organic and dye pollutants removal [5,12]. In addition, it is used as an adsorbent for Fe(II), Cu(II), Zn(II), Cr(VI), Ni(II), Hg(II), Pb(II), Cr(III) and Cd(II) remediation due to its chelation capability [12,13,14,15,16,17]. Moreover, it is used to remove stable nuclide ions such as U(VI), Th(IV), and radionuclide Sr⁹⁰, Co⁶⁰, and Cs¹³⁷ [18,19,20,21,22,23]. Th(IV) and U(VI) ions cause diverse health problems such as kidney toxicity, neurotoxin, immune toxin, mutagen, and carcinogen, which increase the probability of lung, pancreatic, and colorectal cancers, in addition to chronic respiratory diseases, liver damage, anemia, RBC hemolysis, fatigue syndrome, and low blood pressure [24].

Chitosan is widely used as a surface-modified material for several nanomaterials [25,26,27]. Most surface-modified materials are magnetic materials such as Fe₂O₄ and spinal ferrite MFe₂O₄ (M = Fe, Mn, Zn, Ni, Mg, and Co) [28]. Mainly, magnetic materials are fabricated using cross-linker molecules such as glyoxal, glutaraldehyde, isatin, epichlorohydrin, and formaldehyde called modified cross-linked nanocomposites [5,19,29,30,31,32,33,34,35,36,37]. The cross-linked chitosan was used for coating a magnetic nanoparticle to enhance the adsorption capacity of magnetic nanoparticles due to NH₂ and OH groups which make ionic active sites. At the same time, the magnetic properties of cross-linked chitosan metal-oxide nanocomposites facilitate separating it from aqueous media by a magnet, an important feature that shortens many steps in the water purification process [29,30,31,32,33,34,35,36,37].

The present research aims to fabricate and characterize cross-linked chitosan Manganese spinel ferrite (CCsMFO) nanocomposites by FTIR, TGA, DSC, SEM, and TEM. The extraction of Th(IV) and U(VI) ions in this research is based on adsorption phenomena. The adsorption conditions were examined using batch experiments to optimize the adsorption conditions at different contact times, pH, temperatures, and initial metal-ion concentrations at constant ionic strength. To find the adsorption capacity of Th(IV) and U(VI) by (CCsMFO), the kinetic model, adsorption isotherm, and thermodynamic adsorption results were obtained based on optimization results.

2. Results and Discussion

2.1. Characterization of (CCsMFO) Nanocomposite

The FT-IR spectrum of MnFe₂O₄, chitosan, and (CCsMFO) are shown in (Figure 2A–C), respectively. A stretching band at 568.4 cm⁻¹ for (Fe–O) and 649.6 cm⁻¹ for (Mn–O) of the spinel structure of iron is shown in (Figure 2A). While a broad band from 3272.70 cm-1 (N–H stretching), 3524.75 cm⁻¹ (O–H stretching), 2933 and 2879.92 cm⁻¹ (C–H stretching), 1664.05 cm⁻¹ (amide II band, C–O stretching of the acetyl group), a 1592.09 cm⁻¹ strong band corresponding to (amide II band, N–H stretching), 1380–1320 cm⁻¹ (asymmetrical C–H bending of the CH₂ group) and 1079.35 cm⁻¹ (O bridge stretching) of the glucosamine residue corresponding to chitosan (Figure 2B). In Figure 2C, the FT-IR spectrum shows the stretching band 568.95 cm⁻¹ (Fe–O) and 655.95 cm⁻¹ (Mn–O) for MnFe₂O₄ and the characteristic band at 3298.73 cm-1 (O–H stretching) and 2932.29 cm⁻¹ (C–H stretching), and a 1635.72 cm⁻¹ medium band corresponding to (C=N (imine) group), 1559.00 cm⁻¹ (amide II band, N–H stretching) 1421.59 cm⁻¹ (asymmetrical C–H bending of the CH₂ group) and 1068.00 cm⁻¹ (O–bridge stretching) of the glucosamine residue for CS. The IR results for (CCsMFO) showed the presence of characteristic bands corresponding to (Fe–O), (Mn–O), and (C=N), which proves that the method of preparation is proper and produced the target nanoparticles.

The diffractogram of MnFe₂O₄ NPs, chitosan, and (CCsMFO) is given in (Figure 3A–C, respectively). Figure 3A shows the characteristic diffraction peaks of MnFe₂O₄ NPs with an inverse cubic spinel structure (cubic, space group: Fd3m; JCPDS No. 73-1964) at 18.66° (110), 30.32° (220), 35.38° (311), 43.12° (400), 52.12° (422), and 57.12° (511), which is confirmed by the previous literature [38,39]. In Figure 3B, two characteristic peaks of chitosan were suggested, the first at 10.7° (020) and 20.0° (220), which were semi-crystalline due to the hydroxyl and amine groups of chitosan [6]. At the same time, Figure 3C shows the diffraction peaks of the (CCsMFO) nanocomposite which contains peaks of MnFe₂O₄ and chitosan at 14.84° (110), 31.36° (220), 28.24° (311), 37.12° (400), 51.82° (422), and 56.52° (511). That shows that the width of CS peaks (220) became narrower in size, indicating that the size became smaller and CS peaks (020) vanished due to imine bond forming (C=N). In addition, the peaks of chitosan and MnFe₂O₄ positions were changed. At the same time, this indicates MnFe₂O₄ nanoparticle binding to glutaraldehyde-cross linked chitosan.

The particle size of the (CCsMFO) nanocomposite was quantitatively calculated using XRD peaks from Debye–Scherrer (Equation (1)), which finds a relationship between particle size and peak broadening. The peak sizes of the (220), (311), (400), and (511) for MnFe₂O₄-CS obtained from this equation were found to be about 20, 42, 30, 24, and 51 nm, respectively.

$$D=\frac{ k\lambda }{\beta cos\left(\theta \right)}$$

(1)

where k, λ, β, and θ are Sherrer constant (0.89), X-ray wavelength (nm), the peak width at half maximum, and Bragg diffraction angle, respectively.

The thermal stability of MnFe₂O₄ NPs, chitosan, and CCsMFO nanocomposite was studied by thermal gravity analysis as shown in Figure 4A–C for MnFe₂O₄ NPs, chitosan, and CCsMFO nanocomposite, respectively. The TGA curve for MnFe₂O₄ NPs (Figure 4A) shows no mass changes during the overheating process from rt to 1000 °C; the residual mass is 98.32% indicating that MnFe₂O₄ NPs are thermally stable. For the TGA curve of chitosan (Figure 4B), a 14% weight loss occurred between (100–150 °C) due to water evaporation. However, the weight stayed constant up to 248 °C. After that, a significant weight loss happened up to 750 °C (68.53% total weight loss) because of the thermal degradation of CS organic matter. The TGA curve of the CCsMFO nanocomposite (Figure 4C) shows three main weight-loss stages. The first stage illustrates a solvent loss (water evaporation) at 120 °C in CCsMFO nanocomposite. In comparison, the thermal degradation of chitosan is responsible for the second and third weight-loss phases at approximately 230 and 650 °C, respectively. The residual mass is 39.41% at 800 °C of the heating process, where, due to MnFe₂O₄, it remained unchanged_, indicating CCsMFO nanocomposite consists of 39.41% metal oxide and 60.59% chitosan.

The morphology of CCsMFO nanocomposite was explored using SEM, TEM, and EDS. SEM micrographs (Figure 5) show the irregular particle size and shape of CCsMFO nanocomposite particles. At the same time, TEM images (Figure 6) show miscellaneous flake forms with different size particles, and the particle size was measured using the “Image-J” program and expressed in

Table 1. CCsMFO particle size.

Samples	Area (nm)2	Length (nm)
1	10.24	15.10
2	13.85	17.65
3	12.11	17.88
4	20.96	49.65
5	28.16	63.29
Average	17.06	32.71

. In SEM micrographs, energy dispersive X-ray spectroscopy (EDS) determines each element’s kind and weight percent. The proportion of each element after normalization is displayed in

Table 2. EDS of CCsMFO nanocomposite and adsorbed metal ions percent.

Weight%	C Kα	O Kα	Mn Kα	Fe Kα	U Kα	Th Kα
Materials
MnFe2O4-CS	35.02	18.24	12.78	34.44	-	-
MnFe2O4-CS /U(VI)	30.74	14.41	14.23	32.32	8.3	-
MnFe2O4-CS /Th(IV)	33.23	16.07	11.32	30.56	-	8.82

and Figure 7. The source of Si Kα peaks detected in EDS charts breaks in using conventional Si(Li) detectors that produced a silicon internal fluorescence peak from a Si dead layer of the Si-Li detector [40].

The electrochemical potential between a charged surface and a liquid flowing near the charged surface’s plane is known as the zeta-potential [40]. According to Figure 8, CCsMFO has a negative value (−20.20 mV), making it a good material for positive species adsorption, such as U(VI) and Th(IV).

2.2. Optimization Adsorption Protocols

2.2.1. Effect of Adsorbent Amount

A total of 5.0 mL of 50.0 ppm of U(VI) and Th(IV) solution at pH 3.0 were contacted with various masses of adsorbent (1.0–5.0 mg) for 12 h at 25 °C. Figure 9A shows that the uptake of U(VI) and Th(IV) increases as CCsMFO nanocomposite mass increases to 87% and 91%, respectively, at mass = 5.0 mg for both metal ions. The increased amount of the adsorbent substance increases with its active adsorption sites, and, thus, the rate of uptake increases [41]. Where the uptake increases as U(VI) and Th(IV) concentration increases, which may be explained by the improved ratio of total active sites to the metal ions in solution; these ions interact fully with the active site [42,43,44,45].

2.2.2. Solution pH Effect

A total of 5.0 mg of CCsMFO nanocomposite was conducted on 5.0 mL of 50.0 ppm of different pH (1.0–7.0) metal ions solution at 25 °C for 12 h. Figure 9B,C show the maximum% uptake of U(VI) at 48% and Th(IV) at 87% at pH 3.0.

As noted in Figure 9B, the maximum uptake of U(VI) is at pH = 7 ˃ 6 ˃ 3 ≥ 5 = 4 ˃ 2 ˃ 1. In the aqueous solution, uranium forms oligomeric hydrolysis species depending on the pH of the solution. Dissociation of UO₂(NO₃)₂ uranyl nitrate in water involves several ions at pH > 3.0, but (UO₂)₂(OH)₂²⁺ uranyl hydroxide is the main ion as pH increases from pH = 3 to pH = 7, in addition to oligomeric and monomeric hydrolysis species (UO₂)₃(OH)⁵⁺, (UO₂)₃(OH)₄²⁺, (UO₂)₂OH₃⁺, (UO₂)₃(OH)₇^-, (UO₂)₄(OH)₇⁺, UO₂OH⁺, UO₂(OH)₂, UO₂(OH)₃^- and UO₂(OH)₄^2- [42,43,44,45]. In addition, as pH increases from pH = 3 to pH = 7, uranyl cation UO₂²⁺ reacts with carbonate anion CO₃²⁻ to form monomeric carbonate compounds: UO₂CO₃, UO₂(CO₃)₂²⁻ and UO₂(CO₃)₃⁴⁻, and oligomeric carbonate compounds: (UO₂)₃(CO₃)₆⁶⁻ [43]. In the chemistry of uranium ions in an aqueous solution, the most soluble species of total U(VI) is UO₂²⁺. In contrast, the other species UO₂(OH)⁺, (UO₂)₂(OH)₂²⁺, (UO₂)₃(OH)₅³⁺, (UO₂)₂(OH)₂, UO₂(CO₃)₂²⁻, UO₂(CO₃)₃⁴⁻, and (UO₂)₃(CO₃)₆⁶⁻ enhance the precipitation as a yellow precipitate on the surface of the adsorbent and compete for sorption efficiency of UO₂²⁺ on the adsorption active sites, so the suitable pH for the adsorption of uranyl ion on CCsMFO nanocomposite is 3.0. At the same time, Figure 9B notes that the maximum% uptake of Th(IV) is at pH 3.0, which corresponds to the hydrolysis of Thorium ion in an aqueous solution which depends on the pH of the solution. As pH increases from pH 3 to pH 7, Th(IV) salts hydrolyze to produce monomeric hydrolysis species [ThOH]³⁺ and [Th(OH)₂]²⁺. Monomeric species have a strong tendency to polymerize [ThOH(H₂O)n]³⁺, where n = 7 or 8, and dimerize to [Th₂(OH)₂(H₂O)₁₂]⁶⁺, which makes the slightly soluble white precipitate compete with a soluble Thorium species to adsorb on the adsorbent surface [42,43,44,45]. As noted in (Figure 9C) at pH 3.0, Th⁴⁺ ions are a predominant soluble species over other thorium ions species that can easily adsorb on the surface of CCsMFO nanocomposite.

2.2.3. Contact Time Effect

Using a solution of 5.0 mL of 50.0 ppm U(VI) and Th(IV) at pH 3.0 and 25.0 °C, the contact time of adsorption by CCsMFO nanocomposite was performed at regular periods (0.5, 1, 3, 6, 8, and 12 h). Figure 9D expresses that the maximum adsorption capacity (q_m) of U(VI) and Th(IV) onto CCsMFO nanocomposite increases as contact time increases. The value of q_m for U(VI) required 3 h at pH values of 3.0, whereas the value of q_m for Th(IV) needed 1 h.

2.2.4. Metal Ion Concentration Effect

A total of 5.0 mL of solutions of different U(VI) and Th(IV) concentrations in the range (10.0–100.0) ppm at pH 3.0 contacted 5.0 mg of CCsMFO nanocomposite for 12 h. Figure 9E shows that the uptake reaches the maximum at 50.0 ppm. This may be illustrated by referring to the linear driving-force law accumulation rate of adsorbent (U(VI) and Th(IV)) on adsorbate (CCsMFO nanocomposite) as a concentration gradient function [45].

2.2.5. Adsorption Kinetics Modeling Studies

Most adsorption reactions are controlled by several successive steps, including (a) resistance to film diffusion, (b) resistance to intraparticle diffusion, and (c) the proper sorption reaction rate [46,47]. Since the first step is excluded by sufficiently shaking the solution, the rate-determining step is one of the other two steps. Two kinetic models are used to postulate time-dependent adsorption models: pseudo-first-order and pseudo-second-order. The proposed model should explain the sorption kinetics modal of U(VI) and Th(IV) on CCsMFO nanocomposite, which is comprised of two phases: a rapid adsorption-driven early phase which greatly influences equilibrium sorbent uptake (first step) and a second, slower phase which adds to the minimal amount of metal-ion sorption [46,47]. The slower step, known as the rate-determining step of metal-ion sorption on sorbent surfaces, involves interactions between bulk solution and sorbent surface [46,47].

Equations (2) and (3) describe liner pseudo-first-order and pseudo-second-order adsorption, respectively.

ln (qₑ − q_t) = ln (qₑ) − k₁t

(2)

$$\frac{t}{q_e}=\frac{1}{k_2{q_e}^2}+\frac{1}{q_e} t$$

(3)

Figure 10A,B expresses pseudo-first-order and pseudo-second-order kinetic models results. At the same time, the values of (q_e) calculated, (q_e) experimental, and R² correlation coefficients are tabulated in

Table 3. Parameters of kinetic models.

Metal Ion, pH	Pseudo 1st Order			Pseudo 2nd Order			qe (mg·g−1) Experimental
	qe (mg·g−1)	k1 (h−1)	R2	qe (mg·g−1)	k2 (g·mg−1·h−1)	R2
U(VI), pH 3	24.72	0.06	0.98	19.49	0.08	0.99	19.06
Th(IV), pH 3	19.45	0.10	0.91	43.29	1.07	1.00	43.33

, where the values of R², q_e calculated, and q_e experimental fit the pseudo-second-order kinetic model more than the pseudo-first-order model. This indicates that in the rate-determining step of the adsorption of U(VI) and Th(IV) on CCsMFO, the nanocomposite mechanism occurs by chemisorption [48,49,50]. The adsorption behavior involves the valency forces through sharing electrons between the U(VI) and Th(IV) ions and the adsorbent active sites of chitosan on CCsMFO as amine, carboxymethyl, and hydroxyl groups [48,49,50]. The value of k₂ (Table 2) shows the uptake of U(VI) by CCsMFO nanocomposite at pH 3.0 (0.08 g·mg⁻¹·h⁻¹) and k₂ for uptake of Th(IV) by CCsMFO nanocomposite at pH 3.0, (1.07 g·mg⁻¹·h⁻¹) achieves equilibrium faster than the uptake of U(VI) because the hydrolysis of Th(IV) and U(VI), as mentioned in Section 2.2, drives the hydration radius of thorium ions less than uranyl ions species; thus, the adsorption of thorium ions is easier than uranyl ions [51].

2.2.6. Temperature’s Effect and Adsorption Isotherms Modeling

The temperature effect in this work was investigated using pH, contact time, and metal-ions-concentration optimized conditions by variation in temperature at 25, 35, and 45 °C. The results of temperature variations (Figure 11A) show the adsorption isotherm of U(VI) by CCsMFO nanocomposite belonging to the S-type adsorption isotherm, which explains the adsorption process experienced by cooperative adsorption mechanisms, where adsorbate interaction at the surface of the adsorbent is more vital than adsorbate interaction in bulk by creating a cluster of the multilayers of the adsorbate at the surface of the sorbent [49,50]. Meanwhile, Figure 11A shows the adsorption isotherm of Th(IV) by CCsMFO nanocomposite belonging to the H-type adsorption isotherm, which suggests extremely high affinity of (adsorbate) Th(IV) for (adsorbent) CCsMFO nanocomposite [51,52,53].

The adsorption isotherms model is often described using linear equations of the Langmuir, Freundlich, and Dubinin–Radushkevich (D-R) isotherm models (Equations (4), (6) and (7), respectively). Equation (4) is the linear equation form of the Langmuir (II) model, where the intercept is used to find q_m (mg/g), and the obtained value is used with slop to find K_L (L/mg). The calculated value of q_m and K_L are used to find the value of R_L (Equation (5)), which expresses the adsorption process to be either unfavorable if R_L > 1), favorable if 0 < R_L < 1, linear if R_L = 1, and irreversible if R_L = 0 [51,52,53].

$$\frac{1}{q_e}=\left(\frac{1}{q_m{K}_L}\right)\frac{1}{C_e}+\frac{1}{q_m}$$

(4)

$$R_L=\frac{1}{1+K_L{C}_o}$$

(5)

Equation (6) is the linear equation form of the Freundlich modal, where n (heterogeneity factor) was obtained from slop, and the intercept was used to find the adsorption capacity value (K_f). A value of (n) above one reflects normal sorption, but if it is less than one, this reflects that the sorption process undergoes a cooperative adsorptions process [54,55].

$$log q_e=log K_f +\frac{1}{n}log C_e$$

(6)

Equation (7) is the linear (D-R) model that explains the adsorption process’s physical and chemical characteristics. It helps to calculate potential binding energy E (kJ/mol), which describes the transferring of one mol of adsorbate from the solution to the sorbent surface [56]. Polanyi potential (ε) is from Equation (11), where R is the gas constant (kJ K⁻¹ mol⁻¹), and T is the temperature (K).

$$ln\left(q_e\right)=ln\left(q_m\right)-\beta {\epsilon }^2$$

(7)

$$\epsilon =R Tln(1+\frac{1}{C_e})$$

(8)

The constant (mol²/kJ²) (β) value is obtained from Equation (7) and Equation (9) was used to obtain E.

$$E=\frac{1}{\sqrt{2\beta }}$$

(9)

The results of adsorption isotherms for U(VI) and Th(IV) uptake by CCsMFO nanocomposite are expressed in Figure 12A–C and Figure 13A–C and the parameters q_m, n, linear regression (R²), and E listed in

Table 4. Parameters Langmuir(II), Freundlich and D-R adsorption isotherm models at 25.0, 35.0, and 45.0 °C for U(VI) and Th(IV) ions onto CCsMFO nanocomposite, at pH 3.0.

Mn+, pH	U(VI), pH 3.0			Th(IV), pH 3.0
T (°C)	25	35	45	25	35	45
Langmuir
qm (mg/g)	500.00	250.00	111.11	19.31	28.99	25.64
KL (L/mg)	1.90 × 10−3	4.10 × 10−3	0.01	0.06	0.04	0.02
RL (mg/g)	0.91	0.83	0.62	0.26	0.33	0.50
R2	1.00	1.00	1.00	0.97	0.96	0.94
Freundlich
Kf (L/mg)	0.97	0.96	0.97	0.47	0.48	0.38
n	1.00	1.02	1.01	0.56	0.59	0.56
R2	1.00	1.00	1.00	0.99	0.99	0.98
D-R
qm (mg/g)				77.19	87.96	82.35
β (mol2/kJ2)				1.00 × 10−5	2.00 × 10−5	2.00 × 10−5
E (kJ/mol)				223.71	158.13	158.13
R2	0.75	0.73	0.75	0.97	0.96	0.97

were used to explain the adsorption process.

As shown in Table 4 and Figure 12A–C, Langmuir (II) and Freundlich isotherms, the model’s R² values are > 0.89, signifying that the U(VI) adsorption by CCsMFO nanocomposite can be described using the Langmuir (II) and Freundlich models. R_L values were calculated and found to be 0 < R_L < 1, indicating a favorable adsorption of U(VI) onto CCsMFO nanocomposite by the formation of a monolayer at the surface of the sorbent [56,57]. The values of q_m are 500.00, 250.00, and 111.11 (mg/g) at T = 25, 35, 45 °C, respectively. The Langmuir (q_m) values of U(VI) uptake onto CCsMFO nanocomposite decrease as the temperature increases, which reflects exothermic adsorption. At the same time, n obtained from the liner Freundlich model (Table 4) suggested that the adsorption process underwent some difficulties on the CCsMFO nanocomposite surface as a heterogeneous surface [58,59].

For the adsorption isotherm modals parameters of Th(IV) onto CCsMFO nanocomposite listed in Table 3 and Figure 13A–C, where Langmuir (II), Freundlich, and D-R models R² values are >0.89, but the R² value obtained from Freundlich modal was the best value, implying that the adsorption of Th(IV) onto CCsMFO nanocomposite can be described using the Freundlich model, where (n) was obtained from the liner Freundlich model (Table 3), suggesting that the adsorption process experiences some difficulty on the CCsMFO nanocomposite surface as a heterogeneous surface. At the same time, R² values of the D-R model were better than the R² of Langmuir value, where binding energy (E) values obtained from the D-R model are ≥ 8.00 kJ/mol at different temperatures, indicating that the adsorption mechanism occurred by the chemical adsorption process [52,59,60].

2.2.7. Adsorption Thermodynamics

Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) as thermodynamic parameters were obtained at 25 °C and listed in

Table 5. ΔG°, ΔH°, and ΔS° of U(VI) and Th(IV) adsorption onto CCsMFO nanocomposite at 25 °C.

Thermodynamic Parameters	U(VI)	Th(IV)
	pH = 3.0	pH = 3.0
ΔG° kJ/mol	0.15	−10.53
ΔH° kJ/mol	12.15	−44.65
ΔS° J/K·mol	40.03	−114.49

using Van’t Hoff (Equation (11)). Kd was calculated using (Equation (12)), then ΔH°/R and ΔS°/R were calculated from the slope and intercept values of lnK_d Vs. 1/T plot (Figure 14). Where ΔH° and entropy ΔS° were easily calculated and facilitated the calculation of ΔG° using Equation (12) [61,62,63,64].

$$K_d=\frac{q_e}{C_e}$$

(10)

$$Ln{K}_d=\frac{\mathsf{\Delta }S°}{R}-\frac{\mathsf{\Delta }H°}{RT}$$

(11)

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

(12)

The Van’t Hoff plot shows that U(VI) adsorption processes onto CCsMFO nanocomposite at pH 3.0 are endothermic. In contrast, Th(IV) adsorption processes onto CCsMFO nanocomposite at pH 3.0 are classified as exothermic [61,62,63,64].

2.3. Reusability of CCsMFO Nanocomposite

The reusability of CCsMFO nanocomposite was examined by batch desorption test, started by loading U(VI) and Th(IV) onto CCsMFO nanocomposite surfaces, then leached using five cycles with different intervals of time. Figure 15A,B shows the % desorption of U(VI) and Th(IV) from CCsMFO nanocomposite using 0.10 M NaCl, EDTA, and Na₂CO₃ eluents. The used eluents have a basic pH to keep the CCsMFO nanocomposite stable. The results in Figure 15A show that the highest desorption percentage of U(VI) was 77.27% using Na₂CO₃ eluent for 12 h, and the highest desorption percentage of Th(IV) was 21.82% using EDTA eluent for 8 h (Figure 15B) [38].

2.4. Comparative Study

Table 6. Maximum adsorption capacity q_m (mg/g) of U(VI) and Th(IV) on different nanosorbents with present work MnFe₂O₄ NPs at 25 °C.

Metal Ions	Nanosorbent	qm	pH	Reference
U(VI)	Fe3O4 magnetic carboxymethyl chitosan nano- particles functionalizedwith ethylenediamine	175.40	4.5	[50]
U(VI)	phosphonate grafted mesoporous carbon	150.00	4.0	[65]
Th(IV)	Fe3O4 magnetic ion-imprinted chitosan	147.10	4.0	[66]
Th(IV)	amino- Fe3O4 magnetic glycidyl methacrylate nanoparticles	50.89	3.7	[67]
Th(IV)	amino- Fe3O4 magnetic glycidyl divinylbenzene nanoparticles	68.98	3.7	[67]
Th(IV)	Magnetic Fe3O4 /SiO2/ PVA/aminopropyltriethoxysilane (APTES) nanoparticles	62.50	5.0	[68]
U(VI)	Polyethyleneimine Modified Magnetic Chitosan	181.80	6.0	[69]
U(VI)	MnFe2O4	80.96	3	[38]
		104.04	4
		76.50	5
Th(IV)	MnFe2O4	179.81	3.0	[38]
U(VI)	CCsMFO nanocomposite	500.00	3.0	Current study
Th(IV)	CCsMFO nanocomposite	77.19	3.0	Current study

shows qm for adsorption U(VI) and Th(IV) by different nanosorbents at different pH and 25 °C. CCsMFO nanocomposite has a unique maximum adsorption capacity qm with 500.00 mg/g, describing the adsorption of U(VI) compared to other nanomaterials that contain NH₂ and OH groups and MnFe₂O₄ [38], as shown in Table 6. In contrast, the q_m of adsorption Th(IV) by CCsMFO with 77.19 mg/g compared with MnFe₂O₄ [38] and Fe₃O₄ magnetic ion-imprinted chitosan in [65].

3. Material, Instruments, and Methods

3.1. Material

All reagents used were analytical-grade reagents with no further purification. Chitosan (CS) shrimp shells, ≥75%, and glutaraldehyde (25% v/v) in aqueous form were from SIGMA-ALDRICH. Sodium, hydrochloric acid 37% (HCl), sodium carbonate (Na₂CO₃), EDTA, and potassium hydrogen phthalate (KHP) were from BDH PROLABO. Thorium(IV) nitrate tetrahydrate (Th(NO₃)₄∙4H₂O) and Uranyl(VI) nitrate hexahydrate (UO₂(NO₃)₂.6H₂O) from BDH Chemicals Ltd. Poole England. Sodium chloride (NaCl) and hydroxide (NaOH) were from GAINLAND CHEMICAL COMPANY (GCC). Acetone and Ethanol were from SELVO CHEM. 99.5% glacial acetic acid was from TEDIA. Arsenazo(III) indicator was from JANSSEN CHIMICA.

3.2. Instruments

The metal ions’ concentration was determined using a Vis Spectrophotometer from the METASH model V-5100 and a 1.0 cm quartz cell. Fourier-transform Infrared Spectroscopy (FT-IR) spectra were measured using Thermo Nicolet NEXUS 670 FT–IR Spectrophotometer. Thermal gravimetric analysis (TGA) was examined using NETZCH STA 409 PG/PC, a thermal analyzer in the temperature range of 25 °C–1000 °C at a heating rate of 20 °C/min. X-ray diffraction (XRD) was measured using Philips X pert PW 3060, operated at 45 kV and 40 mA. The shape with 3-dimensional (3D) surface morphology was examined with NCFL’s FEI QUANTA 600 FEG scanning electron microscope (SEM). The surface’s nature and average size were carried out using a magneton MORGAGNI FEI 500 tunneling electron microscopy. Solution pH was measured using a METTLER TOLEDO pH–meter. Zeta potential was measured using Zetasizer Nano ZS90 (Malvern Instruments). The weighing was performed by RADWAG ^® AS 220. R2 Electronic Balance. Samples were shaken using a GFL-1083 thermostatic shaker.

3.3. Methods

3.3.1. Cross-Linked Chitosan/MnFe₂O₄ (CCsMFO) Nanocomposite Synthesis

A 2% w/v chitosan solution was prepared by dissolving 1.00 g chitosan into 50 mL of 2% (v/v) acetic acid with vigorous stirring (1300 rpm) for 1 h to obtain a clear, homogenous gel. Then, 0.25 g of previously fabricated MnFe₂O₄ NPs [38] was added under vigorous stirring (1300 rpm) for 1 h to obtain a homogenous black gel. After that, 0.5 mL of 25 wt.% glutaraldehyde was added under vigorous stirring (1300 rpm) for 3 h. Then, the pH was slowly raised to 12.0 by adding 5.0 M NaOH solution dropwise under vigorous stirring (1300 rpm), where the mixture separated into a transparent aqueous layer and a black magnetic precipitate as shown in Scheme 1. A neodymium magnet was used to separate the magnetic product, and then washed with distilled water, ethanol, and acetone, then dried at 100 °C for 24 h.

3.3.2. Sorption and Recovery Experiments

Th(IV) and U(VI) standard solutions were prepared by dissolving UO₂(NO₃)₂.6H₂O) and (Th(NO₃)₄.4H₂O) salt with different concentrations (10–100 ppm), with adjusted pH using buffer solution (HCl)/(KHP) and NaOH/KHP to adjust the solution pH of 1.0, 2.0, and 5.0–7.0, but pH 3.0 and 4.0 were adjusted using NaCl/HCl buffer solution.

Batch methods were performed to optimize adsorption-experiment conditions by executing them at 25 °C using CCsMFO nanocomposite. A total of 5.0 mL of 50 ppm of Th(IV) and U(VI) was used to optimize the mass of sorbent, solution pH, initial metal-ions concentration (Ci), and contact time. Certain amounts of CCsMFO nanocomposite ranging from (1.0–5.0) mg were used to find the optimum mass of sorbent condition. The optimized mass of sorbent was used with 5.0 mL of 50 ppm U(VI) and Th(IV) solutions to optimize solution pH. Then, at optimized pH conditions, 5.0 mL of 50 ppm of U(VI) and Th(IV) samples were shaken for 24 h. A regular sampling was taken every 30 min to investigate the adsorption-process equilibrium conditions. The data produced from optimization experiments were manipulated to investigate Langmuir, Freundlich, and D-R isotherm models by finding adsorption capacity qe (mg/g) (equilibrium amount of adsorbate Th(IV) and U(VI) adsorbed per unit mass of sorbent (CCsMFO nanocomposite)); and removal yield obtained from Equations (13) and (14), respectively.

$$q_e=\left(C_o-C_e\right)\frac{V}{m}$$

(13)

$$\% R=\frac{\left(C_o-C_e\right)}{C_o} \times 100$$

(14)

where C_e is the equilibrium concentration of U(VI) and Th(IV) (mg/L), C_o is the initial concentration of U(VI) and Th(IV) (mg/L), V is the solution volume (L), and m is (CCsMFO nanocomposite) mass (mg).

The recovery of CCsMFO nanocomposite was explored using batch techniques starting by loading U(VI) and Th(IV) onto CCsMFO nanocomposite. Then, U(VI) and Th(IV) were leached through 5 cycles with different time intervals, using 0.10 M of NaCl, Na₂CO₃, and EDTA eluents, where % recovery was obtained by Equation (15):

$$\% R=\frac{\left(C_{ads} – C_{des}\right)}{C_{ads}}\times 100\%$$

(15)

3.3.3. Determination of U(VI) and Th(IV) Concentration

The concentration of U(VI) and Th(IV) was obtained for each metal ion separately by Vis absorption spectroscopy, where Arsenazo(III) indicator was used as a specific colorimetric agent by mixing 0.50 mL of 0.1% Arsenazo(III) indicator with 1.0 mL of the U(VI) sample with 10.0 mL of 0.01 M hydrochloric acid solution and 1.0 mL of Th(IV) solution with 10.0 mL of 9.0 M hydrochloric acid solution [38,70]. Absorbance measurements were carried out after sample preparation at 660 nm to detect Th(IV) ions and 650 nm to detect U(VI) ions using a 1.0 cm quartz cell within one hour [38,70].

4. Conclusions

In this work, a green synthesis method was used to prepare a cross-linked chitosan/(MnFe₂O₄) CCsMFO nanocomposite, and it was used to extract U(VI) and Th(IV) from an aqueous solution based on adsorption phenomena. The Langmuir maximum adsorption capacities (q_m) of U(VI) are 500.00, 250.00, and 111.11 (mg/g) at T = 25, 35, and 45 °C, respectively. The (q_m) values of U(VI) uptake onto CCsMFO nanocomposite decrease as the temperature increases, reflecting exothermic adsorption. The Langmuir and D-R maximum adsorption capacities (q_m) of Th(IV) are unaffected by temperature. Furthermore, the adsorption equilibrium model fitted the pseudo-second-order equilibrium model. Based on adsorption, kinetic and isotherm results prove that the extraction of U(VI) and Th(IV) using CCsMFO nanocomposite is explained by a physical-chemical adsorption process.