Photocatalytic Performance of Functionalized Biopolymer for Neodymium (III) Sorption and the Recovery from Leachate Solution

Abstract

Successive grafting of new sorbent bearing amino phosphonic groups based on chitosan nano magnetite particles was performed through successive coupling with formaldehyde. The produced composite was characterized by the high sorption capacity toward rare earth elements (REEs) and consists of different types of functional groups (phosphonic, hydroxyls and amine groups) that are used for enhancing the sorption properties. The chemical modification and the sorption mechanism were investigated through different analytical tools; i.e., FTIR, SEM, SEM-EDX, TGA, BET (surface area) and pH_pzc. The sorption was investigated toward Nd(III) as one of the REE(III) members under ultraviolet (UV) and visible light (VL) conditions. The optimum sorption was found at pH₀ 4 and the sorption capacity was recorded at 0.871 and 0.779 mmol Nd g⁻¹ under UV and VL respectively. Sorption isotherms and uptake kinetics were fitted by Langmuir and Sips and by pseudo-first order rate equation (PFORE) for the functionalized sorbent, respectively. The sorbent showed a relatively high-speed sorption kinetic (20 min). The bounded metal ions were progressively eluted using 0.2 M HCl solution with a desorption rate 10–15 min, while the loss in the total capacity after a series of sorption recycling (sorption/desorption) (five cycles) was limited (around 3%) with 100% of the desorption efficiency, indicating the high stability of the sorbent toward an acidic medium. The sorbent was used for the recovery of REEs from leach liquor residue after pretreatment for the extraction of particular elements. From these results (high loading capacity, high selectivity and high stability against acid treatments), we can see that the sorbent is a promising tool for the selective recovery of rare earth elements in the field of metal valorization.

1. Introduction

Rare-earth elements (REEs) consist of seventeen members that, with the exception of yttrium and scandium, range in atomic number from 57 (lanthanum) to 71 (lutetium). They are all with many similar properties [1,2] and are thus referred to as “lanthanides”. Because of their unusual physical and chemical properties, they are used in different varieties of industrial and technological applications [3,4]. The need for rare earth elements (REEs) has increased, because of their widespread use in high-tech applications [5] and advanced technologies; as a kind of valuable additive (composite) for automotive catalytic converters, the military, and green technologies; and in mobile phones, televisions, light-emitting diodes, and computer hard disks [6,7]. Some REEs(III) (i.e., Dy, Er) have been used for applications in the nuclear industry, for example in the control rods that regulate nuclear reactors, in shielding materials, and for the quantifying ionizing of radiation [8]. Therefore, the development of more performant separation procedures for REEs has become necessary, especially given that it is limited to a small number of countries [9]. Many policies have been established to promote the development of REE production from geological tailing materials and industrial byproducts [10,11,12,13,14,15]. Other successful method have been performed that use unconventional techniques such as production from red mud tailing material [16,17] or phosphogypsum and phosphoric acid production units [18,19,20,21,22,23,24], fly ash, coal fines, mining tailings, phosphorus-based products and permanent magnets [25,26,27]. Relatively purified REEs have been obtained using a physical separation, followed by a leaching and then selective extraction, with this method being used effectively for the recovery of REEs. Other methods have been used to decrease the environmental impacts (reduce pollution) [28].

The extraction of REEs from leaching solution is generally accomplished by a variety of conventional methods, including chemical precipitation [29,30] and solvent extraction [31], which are mainly used for the extraction of high concentrated metal ions in solutions. Ion exchange methods [32,33,34,35], impregnated resins [36,37,38] and chelating composites [39,40,41,42,43,44] are used as tools for the efficient extraction of these elements in less concentrated solutions. Solid phase extraction (SPE) is based on the association between organic ligands and REEs. Other tools pay specific attention to the extraction of metal ions and heavy elements such as carbon [45,46], graphene-based sorbents [47], silica functionalized composite [48,49,50], clays [51,52], NbCo-MOF [53] and functionalized biopolymers [54,55].

Biosorbents and biopolymers are easily functionalized (due to their hydrophilic properties) by substitute/grafting reactive groups in order to increase the uptake capacity of the sorbent for metal ions. This improves sorption kinetics, facilitates more efficient recovery, and improves other physico-chemical properties. The presence of amine, carboxylic and hydroxyl groups in the structure of these biopolymers (for example chitosan, gums and alginate) facilitates such chemical modification. Among the groups for which grafting improves sorption capacity are sulfonic acid [56], amidoxime [57], amino acids [54,58], heterocyclic groups [59,60,61], phosphorus groups [62,63] quaternary ammonium groups [64,65,66,67], amino-sulfonic acid [68], amine and thion [69], and green synthesized materials [70].

Chitosan is considered one of the most abundant biopolymers and is known as an excellent biosorbent of heavy metal ions, dyes and proteins from various media due to their low cost and their high amino and hydroxyl group contents. Chitosan can be modified easily by chemical or physical processes (acquired by grafting new functional groups) or to condition polymers as membranes, fibers, hollow fibers, and gel beads [71]. It has a singular property, through the known polysaccharides and because of its cationic behavior, that allows it to be dissolved, modified and shaped by protonated amines. This means it can improving sorption kinetics and solid liquid separation by reducing its size, through its coating on magnetite and other nanoparticles, in order to produce nano/micro structured composites. The presence of amines and hydroxyls on their structure allow for chemical modification [72] and the recovery of metal ions and complexes. Several grafting groups have been created, including carboxylic group derivatives [73,74], poly(amines) [75,76], and amino-phosphonic [77,78] and phosphonic [79] moieties. These improve the selectivity of chitosan derivatives towards phosphate ions by immobilizing nano-sized La(OH)₃ [80].

A new process of phosphorylating on chitosan particles has been achieved, one which produces a very efficient sorbent for the recovery of rare earth elements and for which the highest sorption capacities were performed in a mild acid solution. In this work the neodymium element (as an REE with essential uses in many industries, including the automobile industry and in permanent magnets, lasers, cryo-coolers, and high-tech glasses) was tested to evaluate the synthesized sorbents (under UV and VL conditions) before application in REE recovery in pregnant raffinate solution (produced from the acidic leaching of the residual by-product).

A deep characterization of the synthesized material in terms of its structural and textural properties and the binding mechanisms toward metal ions has been discussed in the above, first part of the manuscript. This was performed through a wide diversity of analytical tools, including SEM-EDX, TEM, FTIR, elemental analysis (EA), TGA, pH_pzc and surface area (nitrogen sorption desorption properties). The second part of this work, concerning study of the sorption properties and the sorption capacity toward neodymium metal ion through the effects of pH, uptake kinetics, isotherm experiments, selectivity tests (from equimolar solution of different metal ions, expected to be associated with neodymium in the leaching liquor), metal desorption, and sorbent recycling. This part was performed three times for reproducibility under visible light (VL) and under UV emission and is represented in the figures as curves with a standard deviation obtained through the application of error bars. The final part of this work studies the evaluation of the sorbent’s ability to recover rare earth elements from the pregnant leach liquor (polymetallic solution) that is derived from the acidic leaching of ore residue. From loading experiments, we observed a specific selectivity of REEs in terms of their loading properties, which recovered rare earth oxalate from the eluate using oxalic acid precipitation in acidic medium.

2. Results and Discussion

2.1. Characterization of Prepared Composite

2.1.1. FTIR Analysis

FTIR spectra were used not only to verify the structure of the sorbent through the grafting of phosphonic groups into chitosan particles, nor just to obtain information as to the type of binding and the expected mechanisms through changes in the environments surrounding these groups, but also to verify the stability of the sorbent against the acidic solution used in desorption studies (in the sorption–desorption cycles).

Figure 1 shows the FTIR spectra of the magnetite chitosan nanoparticles after being functionalized by amino phosphonate moieties, after loading with metal ions, and after five cycles of sorption–desorption. Figure 1 shows several peaks related to phosphonate groups, which are not found in the non-functionalized particles. Peaks can be seen at 1421 cm⁻¹ and 1348 cm⁻¹, 1053 cm⁻¹, and 561 cm⁻¹ and 484 cm⁻¹ and represent P=O (asym.), P(O) (phosphate(str.)) and P-O-C (str.) sharing with Fe-O [81,82,83] respectively. Most of these peaks are decreased in their intensity by the effect of metal adsorption and have some shifts, while they are restored to their original state after desorption, emphasizing the high stability of the sorbent against degradation processes and its good resistance against acid in the elution procedures. Peaks at 3412, 3267, 3274 and 3263 cm⁻¹ for MCH, CH-POH, CH-POH+Nd and after five cycles, respectively, are related to OH and NH groups in the chitosan and amino phosphonate groups [84,85,86].

From Figure 1, we can see, through the shifts and changes towards a lower transmittance, that OH, NH, and P-O, are the groups most noticeably affected by metal ion sorption. This indicates that these groups are used for binding with metal ions. The spectra of the sorbent after desorption are closed to the original functionalized material, which emphasizes the remarkable stability of this composite toward acidic environments and also highlights the reversal sorption. Table S1 shows the most significant peaks in the magnetite chitosan nanoparticles (MCH), phosphonate sorbent (CH-POH), after Nd(III) sorption and after five cycles of sorption–desorption, while Figure S1 shows the full range of the FTIR spectra.

2.1.2. TGA Analysis

Thermal degradation of the functionalized sorbent shows four loss stages against the three stages of the MCH shown in Figure 2. The stabilization plateaus of the loss for both the sorbents were noticed at temperatures of around 450 and 670 °C for MCH and CH-POH, respectively. The final weight losses for both sorbents reached 74.726 and 46.96% respectively. This indicates that the phosphonate groups support the resistance of the thermal degradation (the stabilization was performed at 650 °C) and the successive grafting, through an increase in the residual ratio of the hydrocarbons compared with the original moiety that was supported by amino phosphonic groups (46.96 for magnetite chitosan against 74.726 for functionalized polymer).

The loss profiles are discussed in detail as follows. (a) The first loss stage is related to external and internal water release and has a loss percentage of 7.506% at 144.37 °C for CH-POH, while it was recorded at 237.8 °C with a loss percentage of 10.26% for MCH composite. (b) The second loss stage concerns the depolymerization of polysaccharide chains, this was performed at a temperature of 248.17 °C with a loss percentage of 10.94% for CH-POH and 352.1 °C with a loss percentage of 27.01% for MCH. (c) The third stage of degradation concerns the loss of amines and hydroxyl groups and was performed at 483.17 °C with a loss percentage of 38.559% for CH-POH. This stage was the last for MCH with a loss percentage of 9.69%. The last stage of the CH-POH sorbent was related to the degradation of the phosphonate groups (thermal resistance compounds), the organic skeleton (formation of char) and the magnetite particles and showed a loss percentage of 17.72%. Figure S2 shows the DrTG of both sorbents, indicating three bands with different positions at 75.97 °C, 254.8 °C, and 370.3 °C, against 397.3 °C, 476.5 °C and 652.5 °C for MCH and CH-POH, respectively.

2.1.3. BET and pH_PZC Properties

The functionalized sorbent has an S_BET (specific surface area), according to BET measurements (depending on N sorption desorption isotherms), of around 23.85 m² g⁻¹, while V_p (the porous volume) is around 8.32 cm³ STP g⁻¹. The average pore size is around 126–135 Å.

The acid–base properties of the synthesized sorbents were identified using the surface charge characterization (pH_PZC). The sorbents were tested in two different concentrated solutions (0.1 and 1 M NaCl). The data are reported in Figure 3, which shows two identical curves for the same sorbent. The pH_pzc was recorded at 6.58 and 6.82 for 0.1 and 1 M NaCl, respectively, for CH-POH, while it was noticed at 6.23 and 6.28 with the same respective concentrations for MCH. The basic properties of these sorbents are related to the pka of hydroxyls and amines (for the MCH)—which are increased by grafting and by supporting the phosphonate groups (in case of CH-POH)—and it is this which is primarily responsible for the shifts of the pH to more basic levels (compared with those of chitosan magnetite 6.23 [87]).

2.1.4. Morphology and Textural Properties

Figure 4a,c shows a TEM analysis photograph of the pristine and phosphorylated composites, which appear as magnetite nanoparticles of irregular embedment (appearing as dark points with diameters ranging from 2 to 15 nm for both sorbents). This spreads as a heterogeneous shape inside the polymer composite. The size of the magnetite is around 7 (±2) nm. Figure 4b,d show an SEM photograph of the same respective sorbents. The overall size of the particles is less than one micron, which appear as irregular shapes.

2.1.5. Elemental Analysis (EA)

Table S2 shows the EA of chitosan nanoparticles before and after the grafting of phosphorylated groups. There is a noticeable increase in percentage of N and O from 4.12% (2.94 mmol) to 5.58% (3.98 mmol) and 30.22% (18.89 mmol) to 36.92% (23.08 mmol) for MCH and CH-POH, respectively. The main source of this increase was determined to be the amino phosphonate grafted moieties, which contain N:O:P and C with ratios 1:3:1 and 2% respectively. Additionally, the percentage of P for the final sorbent was shown to be 2.95% (0.95 mmol). This emphasizes the successful grafting of phosphonate groups in the chitosan particles.

Figure 5 shows the EDX analysis (semi-quantitative) of both magnetite chitosan before and after the grafting of phosphonate moieties. It can be seen that the results are in line with the elemental analysis results. Increases in N, O and P contents confirm the successful grafting of the amino phosphonate groups to the surface of the chitosan. The N, O, and P increased from 4.11%, 34.55% and 0% for the MCH to 5.42%, 36.48% and 2.11% for MCH-NPOH.

2.2. Loading from Synthetic Solutions

2.2.1. Effect of pH

Figure 6 shows the Nd sorption properties using CH-POH at different pH₀ (1–6) under visible light and UV emission. The sorption experiments were performed three times and the average, with error bars, are exhibited in the figures. It can be seen that both sorbents have the same sorption profiles (the capacity begins low and increases with pH). It is noteworthy that the sorbent has low sorption capacity in acidic conditions, compared with the slightly acidic medium. This is due to repulsion of the positively charged metal ions (mainly Nd³⁺ or NdSO₄⁺) and protonated functional groups (mainly; OH, P(OH) and NH₂). Additionally, with increasing pH the positive charge on the functional groups gradually decreased (partial deprotonation of the sorbent) and the repulsion consequently decreased, allowing for easier binding with the functional groups. The sorption was performed below the pH_pzc, in which the sorbent still partially protonated. The sorption stabilized at pH_eq 4 for both experiment conditions, wherein the average q_max is around 0.79 mmol Nd g⁻¹ and 0.88 mmol Nd g⁻¹ for visible light and UV, respectively. Figure S3 exhibits the speciation diagram of Nd under experimental conditions. The anionic species (Nd(SO₄)²⁻) co-existed only at pH < 3, without exceeding 15%, revealing the sorption with the protonated groups. This also demonstrates that the species were mainly present at pH 4 are Nd³⁺ and NdSO₄⁺, while the precipitation was noticed at pH₀ 7.25.

Figure 6b, reports the average and the error bar of the changes to pH during neodymium sorption under both experimental procedures (UV and VL). Both processes exhibit the same profiles, wherein the UV emission produces the highest changes to pH. By comparing these data with those obtained from the pH_pzc experiment, it was observed that the variation of the pH in the presence of metal ions is less marked than in their absence. From these data it was concluded that the sorption was performed by releasing protons from the phosphonic groups during the sorption of REEs.

Figure 6c includes the plotting of log₁₀ D (D is the distribution ratio equivalent to q_eq/C_eq) vs. pH_eq. The slope from this plot is close to 0.51 and 0.57 for both sorbents. This means that the sorption mechanism was performed through an ionic exchange mechanism using two protons from the sorbent per metal ion (probably mainly with sulfate species, as appeared in the EDX analysis in Figure 7 (the presence of S element in the spectra)).

Figure S4 shows the EDX analysis of the loaded sorbent at pH_eq 4. This represents a high percentage of Nd (3.46%) and reflects the high affinity of this composite toward REEs. The spectra show the presence of S element (which is absent from the original functionalized sorbent; see Figure 5), which confirms the sorption of Nd in a sulfate species beside a trivalent ion. Additionally, the oxygen percentage was increased from 36.48% before loading to 37.21% after Nd sorption, the source of O in this case being provided from the sulfate species.

2.2.2. Uptake Kinetics

The kinetics profiles may be controlled by different parameters, such as resistances to diffusion (film, intraparticle diffusion and bulk modes). The resistance to bulk diffusion and the effects of the film diffusion are limited by proper agitation speed, which causes a homogenous sorbent and solute distribution. The experimental profiles were fitted using different reaction rates associated with pseudo-first-order rate equations (PFORE) as shown in Figure 7, pseudo-second-order rate equations (PSORE) (Figure S5a), and the resistance to intraparticle diffusion (RIDE), (Figure S5b). The PFORE is considered to be a well fitted equation for the experimental profiles for UV and VL conditions, as shown in Figure 7, while the other equations are poorly fitted for the experimental profiles, as shown in Figure S5.

It is noteworthy that the PFORE have closer determinations through a comparison of the equilibrium capacities (i.e., of the sorption capacities in comparison with those obtained from the experimental studies): the q_eq,1 is overestimated with the q_eq,exp in both experimental conditions. From the kinetic profiles, we see that the sorbent shows a total sorption within 20 min, in which around 85% of the sorption was performed in the first 10 min. From these data we can confirm that the sorption was mainly performed on the surface (external layer) and then, to a small extent, in the pores (which is limited).

Table 1. Uptake kinetics of the Nd(III)sorption using CH-POH sorbent under UV and VL parameters.

Model	Parameter	Unit	CH-POH#UV	CH-POH#VL
	qeq,exp	mmol Ndg−1	0.871	0.779
PFORE	qeq,1	mmol Ndg−1	0.891	0.785
	k1 × 10	min−1	0.395	0.582
	R2	-	0.996	0.991
	AIC	-	−102	−95
PSORE	qeq,2	mmol Ndg−1	1.08	0.886
	k2 × 10	gmmol−1 min−1	1.95	2.36
	R2	-	0.893	0.902
	AIC	-	−52	−44
RIDE	De × 108	m2 min−1	2.16	1.86
	R2		0.928	0.953
	AIC		−87	−89

reports the comparison of the models’ parameters. By comparing the q_exp (the calculated sorption capacities value), AIC and the rate coefficients (R²), we see a remarkable preference of the PFORE (physical sorption) compared with the chem-sorption of the PSORE.

2.2.3. Sorption Isotherms

The importance of the sorption isotherms is in their ability to detect the maximum sorption capacities (from sorbent saturation) and the sorbent affinity coefficient toward the selected metal ions. The sorption isotherms performed at an initial pH 4 for both sorption conditions (UV and VL). The saturation plateaus began around 1.5 and 1.2 mmol L⁻¹ for UV and VL, respectively. The maximum loading capacities were recorded as 1.45 mmol Nd g⁻¹ and 1.33 mmol Nd g⁻¹ for the respective experiments. On the other hand, a significant increase of initial slopes under UV conditions emphasizes the improvement of the sorption performances. The different profiles applied for sorption isotherms were Langmuir, Freundlich and the Sips equations, as shown in Figure 8.

The Freundlich (power-type equation) was used for multi-layer sorption. A heterogenous distribution was expected between the molecules. This is usually recognized by a non-asymptotic shape. The Langmuir equation (homogeneous sorption) is assumed to occur through a monolayer and is performed without the interactions of the sorbed molecules. Consequently, it is more fitting for the experimental profiles. The Sips equation (a combination of the Langmuir and Freundlich equations) is performed by the addition of an adjustable parameter (n) to make the experimental profiles more fitting.

Table 2. Sorption isotherms of Nd(III) using CH-POH sorbent under UV and VL.

Model	Parameter	Unit	CH-POH_UV	CH-POH_VL
	qm,exp	mmol Nd g−1	1.45	1.33
Langmuir	qm,L	mmol Nd g−1	1.49	1.36
	bL	L mmol−1	3.68	2.25
	R2	-	0.991	0.989
	AIC	-	−130	−124
Freundlich	kF	L1/nF mmol1−1/nF g−1	1.37	1.95
	nF	-	2.86	2.53
	R2	-	0.855	0.872
	AIC	-	−39	−43
Sips	qm,S	mmol Nd g−1	1.51	1.42
	bS	(L mmol−1)1/nS	1.66	1.59
	nS	-	0.962	0.901
	R2	-	0.995	0.990
	AIC	-	−153	−133

represents the parameters of the three models. By comparison of the R² and AIC, it was shown that the Langmuir and Sips are the most well fitted equations for the experimental profiles, displaying improved performance over the Freundlich equation. The high affinity of the sorbent toward REEs can be explained by the nature of the reactive group activities and the metal ion softness character. Additionally, according to Pearson’s rules, the hard and the soft acid–base theory (HSAB) [88,89] assigns a high affinity and reactivity of hard acids to the hard bases. The phosphonate groups are classified as hard bases [90] and have a high affinity to REEs (classified as hard acids).

Table 3. Nd(III) sorption properties with a comparison of performances (equilibrium time, pH, q_mL and b_L).

Sorbent	pH	Equilibrium Time (min)	qm,L(mmol g−1)	bL(Lmmol−1)	Reference
Ion-imprinted composites	7.7	10	0.24	175	[94]
Sargassum-sp.	5	180	0.70	27.77	[95]
Kluyveromyces marxianus.	1.5	1440	0.083	5.63	[96]
Phosphorus sol-gel	6	180	1.13	-	[97]
Impregnated magnetic microcapsules	4	600–720	1.04	4904	[98]
Calixarene-functionalized with graphene oxide	7	240	2.16	3.38	[91]
Cysteine-magnetite-NPs	7	30	0.59	261.4	[99]
Silica impregnated with IL	3.5	200	0.145	267	[100]
Fumarated- polystyrene	5	50	0.30	5.87	[101]
Chlorella-vulgaris	5	30	0.87	4.18	[102]
Poly γ-glutamic acid	3	-	1.64	8.47	[92]
Graphitic C3N4-nanosheets	8	360	0.91	140	[103]
Carboxylic functionalized corn stalk gel	3	360	2.44	591	[93]
Diatomaceous-earth	5	150	1.17	26.1	[104]
Lanthanide-MOF	6	120	0.99	5.19	[105]
Mesoporous functionalized sorbent	5	40	1.06	1.24	[106]
Phosphorylated chitosan composite under UV	4	20	1.49	3.68	This work
Phosphorylated chitosan composite under VL	4	20	1.36	2.25	This work

reports the sorption capacities and the sorption properties of alternative sorbents in the literature for comparison with the CH-POH sorbent. Different conditions sometimes make such comparison difficult; however, it still demonstrates meaningful trends. The CH-POH sorbent shows relatively high sorption capacities in comparison with most in the literature. Some sorbents have been reported to have high sorption capacities, such as calixarene-functionalized graphene oxide composite [91], poly-γ glutamic acid sorbent [92], and carboxylic acid modified corn stalk gel [93], but CH-POH is preferential in terms of kinetic characteristics and affinity coefficients.

2.2.4. Binding Mechanism

The data collected from the pHpzc (for the surface charge of the sorbent), FTIR (the used functional groups in the binding mechanisms, through changes in their intensities and displacement), speciation diagram of neodymium ions and the studies of the pH effect, provided a prediction of the sorption mechanism. From the experimental conditions, it was found that the maximum adsorption of Nd(III) ions is achieved in slightly acidic pH (around 4) with partial deprotonation of functional groups (from the pHpzc). This collection of functional groups (OH, NH, P-OH and P=O) exhibit an electrostatic attraction with positively charged metal ions and the availability of a lone pair of electrons on these groups makes chelation properties possible. The sharing of these functions was emphasized through the FTIR (change in intensities) of NH and OH bands and the displacement of some functional groups, such as P=O, indicating a change in the environment surrounding these groups that is used for binding. The slope of the plot of log₁₀ D vs. pH_eq gives data close to 0.51 and 0.57 for both sorbents. This indicates that the ionic exchange mechanism was performed with the expectation of two protons from the sorbent per one of the metal ions as shown in Scheme 1.

2.2.5. Selectivity from Multi-Component Aqueous Solutions

The sorbent was transferred to test the selectivity (as a first stage, prior to application) toward metal ions in a multi component equimolar system for the possible extraction of REE from polymetallic solution. These complementary studies were performed with most of the elements associated with REEs in the leachate solutions (i.e., Ca, Mg, Fe, and Al). These elements are found mostly in sedimentary rocks such as shale and gibbsite ore materials. The sorption performances were evaluated at different pH values (ranging from 1 to 5). From

Table 4. Selectivity studies of CH-POH in polymetallic equimolar solution under VL(a) and (UV) conditions.

pHeq	VL				UV
	Nd/Fe	Nd/Ca	Nd/Mg	Nd/Al	Nd/Fe	Nd/Ca	Nd/Mg	Nd/Al
1.1	0.154	1.173	0.428	0.574	0.201	0.630	0.8278	0.531
2.12	4.118	5.578	3.781	6.344	3.665	7.193	4.934	9.321
3.18	6.2098	15.971	9.602	14.599	4.867	17.647	12.475	18.958
3.76	15.3938	34.587	22.628	32.78	16.388	44.328	51.68	44.212
4.75	13.385	34.481	22.213	34.789	19.561	53.803	58.013	50.147

, it can be seen that the sorbent has a high affinity toward REEs at slightly alkaline pH as opposed to representative and transition metals. The data from selectivity coefficients SC_Me1/Me2 = D_Me1/D_Me2, shows a preference for Nd over other metals.

At pH_eq 4.86, the selectively of the CH-POH has the following sequence.

Al(III) (SC: ~34.8) ≈ Ca(II) (SC: ~34.4) >> Mg(II) (SC: ~22.2) >> Fe(III) (SC: ~13.4) in visible light, this selectivity was changed and improved by using UV emission, which obtained a sequence of Mg(II) (SC: ~58) > Ca(II) (SC: ~53.8) > Al(III) (SC: ~50.1) >> Fe(III) (SC: ~19.6).

Figure S6 shows the total capacity of the sorbent toward these metal ions at different pH conditions. This figure emphasizes the improved capacity and selectivity effected by the pH and UV conditions.

2.2.6. Metal Desorption and Sorbent Recycling

The desorption process of adsorbed Nd(III) was achieved using an acidic condition of 0.2 M HCl solution as eluent. The desorption procedures seem faster than the sorption process (the loading processes were performed during 20 min of contacts compared with less than 15 min for complete elution of adsorbed metal ions). Around 10 min is considered to be sufficient for elution of more than 80% and 75% of adsorbed metal ions at UV and VL, respectively. From Figure 9, it can be seen that the elution procedures were improved by using UV emission. The data in the figure are the average of the three elution experiments with error bar, in which the loaded samples were collected from the uptake kinetic experiments.

Table 5. Recycling data of CH-POH sorbent (SE: the sorption efficiency (%); DE: the desorption efficiency (%); StD: the standard deviation (%)).

Cycle #	SE	StD	DE	StD
1	85.23	0.86	100.0	0.21
2	84.67	1.05	100.0	0.14
3	84.01	0.51	99.78	0.12
4	83.66	0.33	100	0.22
5	83.14	0.21	99.83	0.34

shows the sorption–desorption cycles for five successive runs for possible reuse of the sorbent and for its reproducing properties. The sorption shows a limited decrease in the efficiency for both experiments (less than 3% decrease in the efficiency under UV and VL), while the elution remains around 100% after the five cycles. This reflects the stability of the sorbent toward the sorption–desorption process. This also was emphasized by the FTIR as shown in Figure 1.

2.3. Application on Ore Leachate

Table 6. Chemical composition of raffinate with extraction % after treatment for U removal.

Constituents	Conc. (mg/L)	Extraction %	Constituents	Conc. (mg/L)	Extraction %
Fe	1814	30.52	Mg	308	12
Al	1989	21.04	REE	361	17.71
Ca	397	11.4

shows the concentration of metal ions after leaching from ore materials and applying uranium extraction processes using quaternary ammonium resin. The high concentration of REE (361 mg L⁻¹) in the raffinate supports the authors’ proposal to apply it toward extraction and attempt to upgrade the concentration in order to make it suitable for selective precipitation.

The loading experiments were performed under agitation procedures in the presence of UV and VL, and the results are reported in

Table 7. Effect of the pH values on sorption efficiencies of polymetallic ions using CH-POH after treatment with amino-sulfonic chitosan composite.

Conditions	pHeq	Nd/Zr	NdPb	Nd/Mg	Nd/Fe	Nd/Al	Nd/Ca
VL	1.16	12.345	5.4483	5.138	9.138	3.375	6.810
	2.19	7.306	7.1364	3.623	7.883	4.513	8.864
	3.27	11.565	12.461	10.383	19.911	9.197	15.956
	4.11	13.691	23.836	20.360	2.056	8.957	21.552
	4.89	12.567	23.874	23.558	1.424	3.559	24.567
UV	1.11	7.807	6.93	3.697	7.04	3.988	6.912
	2.15	10.029	14.255	3.613	11.313	7.861	9.65
	3.21	12.853	11.369	7.61	20.588	12.378	10.273
	4.1	14.243	26.82	17.862	2.305	12.42	13.745
	4.79	14.427	30.282	27.54	1.98	3.918	18.531

.

2.4. Extraction Results

The leachate solution from acidic attack of the pristine ore materials produced a polymetallic solution with high concentration of metal ions, including RREs. Pretreatment of the leaching solution for removal of particular metal ions was performed using quaternary ammonium resin and the produced raffinate solution had a high concentration of REEs that could be valorized. The residual concentration and removal percentage of the most important metal ions are reported in Table 6. The sorbent shows different extraction percentage toward metal ions with different extraction tendencies, in which the loss percentage of REEs is around 17.8% from the original leaching solution.

Different conditions were applied to test the recovery of REEs in the residual solution using the CH-POH sorbent (different values of pH (ranging from 1 to 5) under UV and VL conditions). Table 7 shows the selectivity of the sorption after treatment for the most important metal ions, indicating (a) that the selectivity and sorption capacity are gradually increased with the pH, (b) the high removal of REE compared with major and heavy elements (this is parallel to the results obtained from the selectivity test) and (c) that the selectivity was improved when under UV conditions. The composite remains a useful tool for REEs recovery. The selectivity was recorded under VL with the order Ca > Pb ≈ Mg >> Zr > Al ≈ Fe and under UV with the order pb > Mg > Ca > Zr >> Al ≈ Fe.

3. Materials and Methods

3.1. Materials

Chitosan (Medium M. Wt, with acetylation degree 75–85%), aminomethyl phosphonic acid (99%), anhydrous sodium hydroxide pellets (98%), ferrous sulfate (99%), ammonium ferric sulfate (>99.9%) and formaldehyde (37%, w%) were supplied by Sigma-Aldrich (Taufkirchen, Germany). Epichlorohydrin (EPI; 99%) was supplied by Shanghai-Makclin, Biochemical Co., Ltd. (Shanghai, China). Neodymium (III) sulfate was purchased by the National Engineering Research Centre (NERC) of Rare Earth Metallurgy and Functional Materials—China. The anhydrous calcium chloride (97%), hydrated salts of aluminum chloride (AlCl₃·6H₂O; 98%), magnesium chloride (MgCl₂·6H₂O; 95%), and ferric chloride (FeCl₃·6H₂O; 97%) used in the selectivity tests were purchased through the Guangdong Guanghua-Sci, Tech Co., (Shantou, China). Acetone (99%), ethanol (95%) and absolute ethanol were supplied from Xilong-Scientific Co., Ltd., (Shantou, China). Other reagents were produced from Prolabo-products (VWR-Radnor; PA, USA).

3.2. Characterizations

The FT-IR spectra (range 4000–400 cm⁻¹) were achieved for the dried sorbents (60 °C) after grinding with KBr; 1% (w/w), and designed as KBr disk using Shimadzu, IR-Tracer, 100-FTIR spectrometer (Shimadzu-Tokyo, Japan). The C, N, P and H contents were analyzed through elemental analysis using 2400 Series II CHNS/O elemental analyzer; PerkinElmer–Waltham (MA, USA). The thermal decomposition of the sorbent was performed under a nitrogen environment with a temperature ramp (10 °C min⁻¹) using TG-DTA; Netzsch-STA449-F3 Jupiter; NETZSCH, Gerätebau, HGmbh, (Selb, Germany). The surface morphology was analyzed via scanning electron microscope; SEM with model Phenom-ProX; Thermo-Fisher Scientific (Eindhoven, The Netherlands), the semi-quantitative analyses were achieved by energy dispersive X-ray; Phenom-ProX, SEM. The concentration of metal ions was detected using ICP-AES; ICPS,7510; Shimadzu (Tokyo, Japan). The pH_PZC analysis was measured using pH-drift [107], about 0.1 g of dried sorbent was agitated in 50 mL solution (0.1 M NaCl) for 24 h and the equilibrium pH (pH_eq) was measured, the pH of the solution ranged from 1 to 14. The pH_PZC was known as pH₀ = pH_eq. The BET surface area was measured through nitrogen sorption–desorption using Micromeritics TrisStar II; Norcross (Gwinnett, GA, USA), with the samples firstly degassed at 120 °C/12 h. The pH of the solution was calibrated by pH iono-meter, S220 Seven; Mettler-Toledo (Shanghai, China). The concentration of metal ions in the solution was measured using the ICP tools (inductively coupled plasma atomic emission spectrometer) by ICPS; 7510; Shimadzu (Tokyo, Japan). The particle size of the synthesized sorbent was investigated by TEM analysis using JEOL-1010, JEOL-Ltd. (Tokyo-Japan).

3.3. Synthesis of Functionalized Sorbent

3.3.1. Synthesis of Magnetite Nanoparticles

Preparation of magnetite nanoparticles was undertaken by thermal precipitation technique through reaction of ferrous sulfate (5 g) and ammonium ferric sulfate (17.35 g) in 50 mL aqueous solution. The mixture undergoes vigorous stirring till dissolved completely. The precipitation was obtained by pH adjustment to 11 using 7 M NaOH solution and the temperature was maintained at around 50 °C for 1 h. The precipitated magnetic nanoparticles were collected from the solution, washed several times by water and acetone then dried at 60 °C overnight.

3.3.2. Functionalization of the Nanoparticles

Chitosan particles were dissolved (1 g) in 50 mL of 7% w/w acetic acid solution. One gram of (aminomethyl)phosphonic acid (99%) was added to the solution with continuous stirring at 45 °C till complete dissolution, followed by addition of 1 g of formaldehyde dropwise and 2 g of the prepared magnetite nanoparticles. The flask was equipped with a condenser and refluxed to 90 °C for 5 h, after cooling, a further 2 mL epichlorohydrin was added to the mixture and refluxed for a further 7 h. The content of the flask was poured into an aqueous solution of 2% NaOH and was left stirring overnight. The precipitated product was filtered, washed with acetone and water then air dried at 65 °C for 10 h to yield amino phosphonic chitosan nano particles (Scheme 2).

3.4. Metal Sorption from Synthetic Solutions

The sorption experiments were achieved through batch techniques, in which an amount of synthesized composite (m, g) was mixed with a volume of solution (V, L) which containing an initial metal concentration (C₀, mmol L⁻¹) at an initial pH (pH₀), both of which are variable depending on the experimental conditions and maximum loading (see below). The agitation velocity was fixed at 165 rpm with a temperature 21 ± 1 °C for most experiments. After sorption, the samples were collected and filtered before ICP analysis to detect the final concentration (C_eq, mmol L⁻¹). The loading capacity (q_eq, mmol g⁻¹) was measured using the mass balance equation q_eq = (C₀ − C_eq) × V/m. The desorption experiment was performed using the batch method with a solution of 0.2 M HCl. The desorption kinetics (as a function of time) were measured on the samples collected from kinetic experiments and treated with eluent. The sorbent recycling was investigated for five cycles, with a rinsing with water performed between each sorption and desorption step. The sorption isotherms and kinetics were studied by conventional equations that are summarized in Table S3a (Freundlich, Sips and Langmuir equations) and Table S3b (pseudo-first-order rate equation (PFORE), pseudo-second-order rate equation (PSORE), and resistance to intraparticle diffusion (RIDE)). The fitting quality of the curves was determined through the Akaike information criterion (AIC) [108] and R² parameters.

3.5. Treatment of Ore Material

The samples enriched with rare earth elements were collected after ore processing and extraction of particular elements. The reprocessed leaching liquor still had a high concentration of rare earth element (over 350 mg L⁻¹). The chemical composition of ore materials is identified in Table S4. The produced raffinate after extraction still had a high concentration of rare earth elements that could be valorized. The leaching solution was produced by the effect of acidic attack with the condition of the mixture being 15% H₂SO₄ and 0.25 M NaCl. The suitable ratio of solid/liquid was assigned to 1/2 with an agitation time of 2 h at 90 °C. The concentration of the most important elements is shown in Table S5, while Table 6 shows the leaching composition after treatment of the leaching liquor by quaternary ammonium resin for the extraction process.

4. Conclusions

A new functionalized chitosan sorbent bearing phosphonic groups was used to support the sorption process of REEs in mild acidic conditions. The sorbent was designed as a nano structure using magnetite nano particles. The structure of the sorbent was investigated using FTIR, SEM-EDX, TGA, BET, pH_pzc, and EA. The sorption procedures were investigated toward Nd(III) ions under visible light and UV emission. The functionalized sorbent shows a high sorption capacity with 20 min required for complete saturation. Kinetic profile was fitted with the PFORE fitted equation. The sorption experiments were performed at slightly acidic conditions (pH 4) with partially protonated properties of the sorbent surface. Using the UV tools caused an improvement of the sorption properties and kinetic efficiency. The sorbent shows highly sorption properties when compared with those found in the literature and appears to be stable against acidic eluents, making it a novel tool for the recovery of REEs from solution. Desorption was achieved by 0.2 M HCl solution, which seems to be fast and sufficient to remove all adsorbed metal ions from the surface, and complete desorption was achieved within 15 minutes. Langmuir and Sips are the most fitted profiles for the sorption isotherms. The sorbent shows stability in terms of sorption–desorption cycles after five runs of loading and elution procedures. The sorbent shows selectivity in a poly-metallic equimolar solution and UV conditions are more efficient than visible light. The sorbent was used for the recovery of REEs from a raffinate solution after treatment with quaternary ammonium resin. This sorbent appears to be a remarkable tool for REEs recovery.