Ionic Speciation and Coordination Mechanisms of Vanadium, Iron, and Aluminum in the Oxalic Acid Leachate of Shale

Abstract

The oxalic acid leachate of vanadium-bearing shale (OALS) is a complex system in which the ion states and coordination mechanisms of the primary metallic elements—vanadium, iron, and aluminum—are not fully understood. This study investigated the ionic speciation and coordination mechanisms of vanadium, iron, and aluminum in OALS. The results indicate that vanadium predominantly existed as VO(C₂O₄)₂²⁻ anions, iron as Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ anions, and aluminum as Al(C₂O₄)₂⁻ and Al(C₂O₄)₃³⁻ anions. The coordination reaction processes and equations of various oxalate complexes were examined. Regardless of whether the molar ratio was 1:1 or 1:2, the iron–oxalate complex exhibited the lowest reaction Gibbs free energy (ΔG), with values of −5343.69 and −1470.72 kJ/mol, respectively. The aluminum–oxalate complex followed, with ΔG values of −5169.23 and −1318.87 kJ/mol, respectively. The vanadium–oxalate complex displayed the highest reaction ΔG, at −2760.65 and −714.12 kJ/mol, respectively. Therefore, the coordination mechanism of vanadium, iron, and aluminum with oxalate ions in OALS is such that iron coordinated with oxalate first, followed by aluminum, and finally vanadium. The research results have important guiding significance for the purification, enrichment, and coordination mechanisms of complex solutions.

1. Introduction

Vanadium-bearing shale is a significant source of vanadium in China [1]. With the growing market demand for vanadium in recent years, extracting vanadium from vanadium-bearing shale has emerged as an important research direction. Traditional vanadium extraction methods from vanadium-bearing shale primarily involved roasting, sulfuric acid leaching, and ammonium salt precipitation [2,3]. However, these methods were associated with issues such as strong sulfuric acid corrosion and the generation of ammonia nitrogen wastewater [4]. To address these challenges, Hu et al. [5,6] proposed using oxalic acid, a clean and environmentally friendly organic acid, to extract vanadium from vanadium-bearing shale. Compared to traditional inorganic acids (e.g., sulfuric acid and hydrochloric acid), oxalic acid leaching offers advantages such as high selectivity, ease of operation, safety, and environmental protection [7]. However, vanadium in vanadium-bearing shale typically exists as aluminum isomorphous substitution in octahedral aluminum silicate minerals [8]. Consequently, regardless of whether inorganic or organic acids were used, aluminum was co-leached with vanadium, leading to a high concentration of impurity aluminum in the oxalic acid leachate of vanadium-bearing shale (OALS), often 10 to 20 times that of vanadium [9,10]. Additionally, vanadium-bearing shale contained minor iron-containing minerals, which are also leached during the process [11]. Therefore, the main ions presented in the OALS include vanadium, aluminum, iron, and oxalate ions. However, the OALS is a highly complex system, and the ionic speciation as well as the coordination reaction processes and mechanisms between metal ions and oxalate ions remain poorly understood. Therefore, conducting in-depth research on the coordination mechanisms within the OALS is of significant importance.

Oxalic acid has good reducing and complexing properties and can form anionic complexes with various high-valent metal cations, such as Fe³⁺, Al³⁺, VO²⁺, Cr³⁺, Cu²⁺, and Ti²⁺ [12,13]. The oxalate leaching-solvent extraction system demonstrates significant potential for practical industrial application in the green recovery of vanadium from resources like shale, red mud, and spent catalysts. Its core advantages lie in high selective leaching, efficient separation, oxalate regeneration, and environmental sustainability. Based on these characteristics, the use of oxalic acid as a leaching agent to extract valuable metals from minerals and solid secondary resources or to remove impurity iron from non-metallic minerals has been reported in recent years [14,15,16]. Red mud contains abundant iron resources. Due to the reducing and complexing properties of oxalic acid, it is often used to extract iron from red mud. In the oxalic acid leaching solution for red mud, iron mainly exists in the form of Fe(C₂O₄)₃³⁻, and iron can be recovered in the form of FeC₂O₄ by using the iron powder reduction precipitation method [16,17]. Yakabe and Minami [18] used tri-n-octylamine (TOA) solvent extraction to extract hafnium from oxalic acid solutions containing hafnium. With 0.08 mol/L TOA as the extractant, hafnium and oxalate ions were co-extracted. The extraction mechanism was that the complex Hf(C₂O₄)₄⁴⁻ of hafnium and oxalic acid formed an ionic association with TOA, and the structure of the hafnium complex was [(R₃NH)₄·Hf(C₂O₄)₄]. Djordjević et al. [19] used di-n-octylamino ethanol (DOAE) and di-n-octylamino propanol (DOAP) to study the extraction of tantalum (Ta) and niobium (Nb) from oxalic acid–metal solutions. Firstly, the complex of M (M = Ta, Nb) and oxalate ions was speculated to be MO(C₂O₄)₃³⁻ based on solution chemistry. The mechanism was that the complex MO(C₂O₄)₃³⁻ of M and oxalate ions formed an ionic association with the extractant DOAP, and the structure of the complex was speculated to be [(R₂R’NH)₃·MO(C₂O₄)₃] [20]. Overall, in recent years, oxalic acid has increasingly been utilized in the field of hydrometallurgy. However, the existing literature predominantly focuses on the extraction of valuable metals or the removal of impurity ions, with limited attention given to the ionic speciation and coordination processes involving metal and oxalate ions. In particular, detailed studies on the coordination mechanisms are scarce. Although our previous work has reported on the ionic speciation of some oxalate complexes [21,22], a systematic investigation of the coordination reaction processes and mechanisms for vanadium, iron, and aluminum with oxalate ions has yet to be conducted.

In this study, the ionic speciation and coordination mechanisms of vanadium, iron, and aluminum in the OALS were investigated. Thermodynamic software was employed to analyze the ionic speciation. Additionally, the coordination reaction processes of various coordinating ions were examined. Finally, quantum chemical calculations were performed on the complexes to elucidate the coordination mechanisms of vanadium, iron, and aluminum with oxalate ions. These results provide valuable insights for the coordination and separation of metal ions.

2. Materials and Methods

2.1. Experimental Materials

The OALS was derived from vanadium-bearing shale collected in Tongshan County, Hubei Province, using a roasting-followed-by-leaching process. Figure 1 presents the phase composition of the vanadium shale, while

Table 1. Primary chemical constituents of vanadium-bearing shale (wt.%).

V2O5	Al2O3	Fe2O3	MgO	CaO	SiO2	K2O	Na2O	P2O5	S	C
0.72	9.15	4.31	1.45	4.59	57.21	2.18	0.56	0.63	3.56	11.80

lists its primary chemical constituents.

The OALS was prepared according to previous studies [23,24]. The vanadium-bearing shale, with a particle size finer than −0.074 mm (100% passing), was subjected to roasting at 850 °C for 1 h. Subsequently, the roasted material was leached under the optimal conditions: a solution containing 60% oxalic acid (H₂C₂O₄) and 5% calcium fluoride (CaF₂), with a liquid-to-solid ratio of 1:1, at a temperature of 95 °C for 4 h [23]. Upon completion of the reactions, the OALS was obtained through filtration. The primary chemical composition of the OALS is presented in

Table 2. The main chemical composition of the OALS (mol/L).

V	TFe	Fe2+	Al	K	Na	P	F	C2O42−	pH *
0.03	0.07	0.027	0.54	0.08	0.02	0.018	0.32	1.40	0.65

* Note: pH has no unit. TFe represents the total concentration of iron.

. It is indicated that the OALS is a very complex solution with low vanadium concentration and high aluminum and iron concentrations; in addition, the OALS contains both organic ligands oxalate and inorganic ligands fluoride and phosphate ions.

2.2. Experimental Operations

The free thermodynamic software Medusa [25] was primarily employed to study ionic speciation. Given the highly acidic nature of the OALS, the pH range for the ionic speciation analysis in this study was set from −1.0 to 6.0. In the simulation of the actual OALS, all constituent ions were included, and their concentrations were set to match those measured in the original OALS. In contrast, for the simulation of the pure oxalic acid system, only oxalate ions and a single metal ion (V, Fe, or Al) were introduced, with their concentrations also matching those present in the actual OALS.

Quantum chemical calculations were performed using Gaussian 16W software [26]. Structural models were constructed with GaussView 6.0 software. Subsequently, density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level were employed to optimize the structures of vanadium, iron, and aluminum complexes with oxalate. Frequency calculations were then conducted on the optimized structures using the same basis set to determine their thermodynamic parameters. The Gibbs free energy (ΔG) of the oxalate complexes was calculated to elucidate the preferential coordination mechanism of vanadium, iron, and aluminum with oxalate.

2.3. Determination Method

The concentration of vanadium was determined using the ammonium ferrous sulfate titration method [27]. The iron concentration was measured by 1,10-phenanthroline spectrophotometry with a UV-5500 spectrophotometer (Metash, Shanghai, China) [28]. The oxalate concentration was determined via potassium permanganate titration [29]. Concentrations of other elements were analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES, Optima 4300DV, PerkinElmer, Boston, MA, USA). The phase compositions were identified by X-ray diffraction (XRD, D8 Advance, Bruker, Berlin, Germany).

3. Results and Discussion

3.1. Ionic Speciation of Vanadium, Iron, and Aluminum in the OLAS

3.1.1. Ionic Speciation of Vanadium

Generally, both tetravalent (V(IV)) and pentavalent vanadium(V(V)) exist in the vanadium-bearing shale. In the sulfuric acid leachate, vanadium usually coexists in the form of VO²⁺ or VO₂⁺ [30], while in the oxalic acid system, vanadium can form various complexes with oxalate ions. Therefore, the ionic speciation of V(IV) and V(V) in the pure oxalic acid solution were studied, and the results are shown in Figure 2.

As shown in Figure 2a, when the pH is in the range of −1.0 to 0, V(IV) mainly exists in the form of free VOC₂O₄ (aq) molecules; when the pH rises to 0–1.0, V(IV) mainly exists in the form of VO(C₂O₄)₂²⁻; and when the pH is greater than 1, over 99% of V(IV) exists in the form of VO(C₂O₄)₂²⁻. In contrast, as shown in Figure 2b, when the pH is in the range of −1.0 to −0.5, V(V) mainly exists in the form of VO₂⁺; when the pH rises to −0.5 to 1.2, V(V) mainly exists in the form of VO₂C₂O₄⁻; and when the pH is greater than 1.2, V(V) mainly exists in the form of VO₂(C₂O₄)₃³⁻ anions. Due to the strong reducing property of oxalate ions, the coordinated anions VO₂C₂O₄⁻ and VO₂(C₂O₄)₃³⁻ are further reduced to VO(C₂O₄)₂²⁻ in high-concentration oxalate solutions. Therefore, in the pure oxalic acid solutions, vanadium mainly exists in the form of coordinated anions of tetravalent vanadium.

Based on the above results, the ionic speciation of vanadium in the actual OALS was studied (Figure 3). The results show that the ionic speciation of vanadium in the actual OALS was basically consistent with that of V(IV) in the pure oxalic acid system. When pH is less than 0, it exists in the form of VOC₂O₄(aq) molecules, and when pH is greater than 0, it exists in the form of VO(C₂O₄)₂²⁻ complex anions. When pH is 0.65, the proportion of VO(C₂O₄)₂²⁻ complex anions is 92.65%, and the proportion of VOC₂O₄(aq) molecules is 7.35%. This indicates that in the high-concentration oxalic acid system, V in the vanadium shale acid leaching solution mainly coordinates with the organic ligand C₂O₄²⁻ to form VO(C₂O₄)₂²⁻ anions and does not coordinate with the inorganic ligands P and F.

3.1.2. Ionic Speciation of Iron

In sulfuric acid system, iron usually exists in two valence states: divalent (Fe²⁺) and trivalent (Fe³⁺) [31]. Due to the reducing property of oxalic acid, Fe³⁺ is usually reduced to Fe²⁺ by oxalate ions. In the OALS, Fe³⁺ accounts for 60% of the total iron, while Fe²⁺ makes up 40%. Therefore, the ionic speciation of Fe(III) and Fe(II) in pure oxalic acid systems and actual OALS was studied, and the results are shown in Figure 4 and Figure 5.

As can be seen from Figure 4a, when the pH is between −1.0 and −0.5, Fe(III) mainly exists as FeC₂O₄⁺ cation; when the pH rises to −0.5 to 1.0, Fe(III) is mainly in the form of Fe(C₂O₄)₂⁻ anion; and when the pH is greater than 1, Fe(III) mainly exists as Fe(C₂O₄)₃³⁻ anions. Compared with Figure 4b, in the actual OALS, the ionic speciation of Fe(III) is relatively similar to that in the pure oxalic acid system. However, when the pH is between −1.0 and 0, the FeH₂PO₄²⁺ species is formed, indicating that Fe(III) can form complexes with P. When the pH is between −0.5 and 1.0 and greater than 1.0, the species of Fe(III) are mainly Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ coordinated anions, respectively. When the solution pH is 0.65, the proportion of Fe(C₂O₄)₂⁻ ions is 73.66%, that of Fe(C₂O₄)₃³⁻ ions is 25.64%, and that of FeC₂O₄⁺ is 0.70%. Therefore, in the oxalic acid leaching solution of vanadium shale, Fe(III) mainly exists in the form of Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ coordinated anions.

As can be seen from Figure 5a, when the pH is between −1.0 and 0.5, Fe(II) mainly exists in the form of Fe²⁺; when the pH rises to 0.5 to 2.5, Fe(II) is mainly in the form of free FeC₂O₄ molecules; and when the pH is greater than 2.5, Fe(II) mainly exists as the Fe(C₂O₄)₂²⁻ anions. Compared with Figure 5b, in the actual OALS, similar to Fe(III), the existence state of Fe(II) is also quite similar to that in the pure oxalic acid system. In the solution with pH ranging from −1.0 to 3.0, the FeH₂PO₄⁺ ion exists; while when the pH is 0.5 to 2.5 and greater than 2.5, Fe(II) mainly exists as FeC₂O₄ and Fe(C₂O₄)₂²⁻ species, respectively. When the solution pH is 0.65, the proportions of Fe²⁺, FeC₂O₄(aq), FeH₂PO₄⁺, and Fe(C₂O₄)₂²⁻ are 41.28%, 41.37%, 17.03%, and 0.27%. The results show that in the OALS, Fe(II) mainly exists in the form of FeC₂O₄ and Fe(C₂O₄)₂²⁻ complex anions.

It is well-established that the stability of a complex increases with the number of coordinated ligands. As evidenced by the coordination stability constants (

Table 3. Stability constants of Fe-C₂O₄ and Fe-F complexes.

	lgβ1	lgβ2	lgβ3	lgβ5
Fe(II)-C2O4	2.9	4.52	5.22	-
Fe(III)-C2O4	9.4	16.2	20.2	-
Fe(II)-F	0.8	-	-	-
Fe(III)-F	5.28	9.30	12.06	15.77

) [32], the formation constants of Fe(III)-C₂O₄ complexes consistently exceed those of Fe(II)-C₂O₄ complexes. This indicates that when Fe³⁺ and Fe²⁺ coexist in an oxalate solution, Fe³⁺ preferentially coordinates with C₂O₄²⁻. Consequently, Fe(III)-C₂O₄ complexes exhibit greater stability in the OLAS. Furthermore, the coordination constant of Fe(II)-C₂O₄ surpasses that of Fe(II)-F complexes, rendering Fe²⁺ more likely to coordinate with C₂O₄²⁻ and precluding the formation of Fe(II)-F complexes in the OLAS. Similarly, regardless of coordination number, the formation constants of Fe(III)-C₂O₄ complexes consistently dominate over those of Fe(III)-F complexes, thereby eliminating Fe(III)-F complexes in the OLAS. These findings were in full agreement with the simulation results presented in Figure 3 and Figure 4.

Based on the aforementioned findings, since Fe also exists in anionic forms within the OALS, this may significantly impede vanadium extraction from the system. Whether employing solvent extraction or ion exchange methodologies, the selection of extractants or ion-exchange resins with high vanadium selectivity was of paramount importance.

3.1.3. Ionic Speciation of Aluminum

Aluminum mainly exists in the form of Al³⁺ in sulfuric acid solutions [31], while in oxalic acid solutions, aluminum readily coordinates with oxalate ions to form various complexes. Thus, the ionic speciation of aluminum in the pure oxalic acid systems and actual OALS was investigated, and the results are shown in Figure 6.

The results in Figure 6a indicate that aluminum can form four complexes, namely, AlHC₂O₄²⁺, AlC₂O₄⁺, Al(C₂O₄)₂⁻, and Al(C₂O₄)₃³⁻, in the pure oxalic acid solutions of different pH values. When the pH is between 0 and 1.5, the Al(C₂O₄)₂⁻ anion is dominant, and when the pH is greater than 1.5, the Al(C₂O₄)₃³⁻ complex anion is stably present.

Compared with the ionic speciation of aluminum in the pure oxalic acid solution, the ionic speciation of aluminum in the actual OALS is more complex. The results in Figure 6b show that, in addition to the above four aluminum–oxalate complexes, aluminum forms various complexes with fluorine, including AlF²⁺, AlF₂⁺, AlF₃, and AlF₄⁻. However, when the pH is between 0 and 1.5, the Al(C₂O₄)₂⁻ anion is still dominant, and when the pH is greater than 1.5, the Al(C₂O₄)₃³⁻ complex anion is still stably present. Moreover, it can be seen from the results in Figure 5b that as the pH increases, the acidity of the solution continuously decreases, and the proportions of C₂O₄²⁻ and F⁻ increase, which leads to the increasing coordination numbers of Al³⁺ with C₂O₄²⁻ and F⁻, thereby forming various aluminum–oxalate and aluminum-fluoride complexes. In the actual OALS, when the pH is 0.65, the proportions of Al(C₂O₄)₂⁻, Al(C₂O₄)₃³⁻, AlF₂⁺, AlF₃, and AlC₂O₄⁺ ions are 48.95%, 8.06%, 20.76%, 10.85%, and 5.05%.

Based on the stability constants of Al-F and Al-C₂O₄ complexes (

Table 4. Stability constants of Al-F and Al-C₂O₄ complexes [32].

	lgβ1	lgβ2	lgβ3	lgβ4	lgβ5	lgβ6
Al-F	6.11	11.12	15.00	18.00	19.40	19.80
Al-C2O4	7.26	13.00	16.30	-	-	-

) [32], when the coordination number ranges from 1 to 3, the stability constants of Al-C₂O₄ complexes consistently exceed those of Al-F complexes. However, when the coordination number increases to 4–6, the stability constants of Al-F complexes surpass 18, indicating that Al-F complexes with coordination numbers exceeding 4 exhibit exceptional stability. It should be noted that higher coordination numbers require elevated ligand ion concentrations. Given that the C₂O₄²⁻ concentration (1.40 mol/L) in OALS solution significantly exceeds the F⁻ concentration (0.32 mol/L), under pH 0.65 conditions, aluminum primarily forms complexes with C₂O₄²⁻ ions, with minimal complexation occurring with fluoride ions.

Similar to the research results on iron, since Al also exists in anionic forms within the OALS, this also may significantly impede vanadium extraction from the system. Whether employing solvent extraction or ion exchange methodologies, the selection of extractants or ion-exchange resins with high vanadium selectivity was of paramount importance.

3.2. Coordination Reaction Process of Vanadium, Iron, and Aluminum with Oxalate Ions

Section 3.1. investigated the ionic forms of vanadium, iron, and aluminum in the OALS. The results showed that vanadium, iron, and aluminum mainly formed complexes with oxalate ions, but the coordination process was not clear. Therefore, a systematic study was further conducted on the coordination process of vanadium, iron, and aluminum with oxalate ions.

3.2.1. Coordination Process of Vanadium and Oxalate Ions

In the inorganic acid sulfuric acid system, vanadium in the leachate of vanadium-bearing shale usually exists in the form of VO₂⁺ [30]. However, according to the research results of Section 3.1., vanadium in the OALS mainly exists in the form of VOC₂O₄ molecules and VO(C₂O₄)₂²⁻ complex anions, and their coordination reactions are shown in Equations (1) and (2) [33].

$${\mathrm{VO}}^{2+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}{=\mathrm{VOC}}_2{\mathrm{O}}_4$$

(1)

$${\mathrm{VOC}}_2{\mathrm{O}}_4+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={{\mathrm{VO}(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{2-}$$

(2)

According to reference [34], pentavalent vanadium can form coordination anions VO₂C₂O₄⁻ and VO₂(C₂O₄)₂³⁻ with oxalate ions. The reaction equations are as follows:

$${\mathrm{VO}}_2^{+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={\mathrm{VO}}_2{{\mathrm{C}}_2{\mathrm{O}}_4}^{-}$$

(3)

$${\mathrm{VO}}_2{{\mathrm{C}}_2{\mathrm{O}}_4}^{-}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}{=\mathrm{VO}}_2{{{(\mathrm{C}}_2{\mathrm{O}}_4)}_2}^{3-}$$

(4)

However, in the OALS, there is a large number of oxalate ions. Under the conditions of heating and an excess of oxalate ions, the V(V) complex with oxalate is further reduced to a V(IV) complex. Therefore, in the OALS, vanadium mainly exists in the form of VOC₂O₄ molecules and VO(C₂O₄)₂²⁻ complex anions. The reaction equation was as follows [35]:

$${\mathrm{H}}^{+}{+\mathrm{VO}}_2{{(\mathrm{C}}_2{\mathrm{O}}_4)}^{-}\rightleftharpoons {\mathrm{HVO}}_2{\mathrm{C}}_2{\mathrm{O}}_4$$

(5)

$${\mathrm{HVO}}_2{(\mathrm{C}}_2{\mathrm{O}}_4)+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}\rightleftharpoons {\mathrm{HVO}}_2{{(\mathrm{C}}_2{\mathrm{O}}_4)}^{-}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{*}$$

(6)

$${\mathrm{HVO}}_2{{(\mathrm{C}}_2{\mathrm{O}}_4)}^{-}+{\mathrm{H}}^{+}\rightleftharpoons {\mathrm{VO}(\mathrm{C}}_2{\mathrm{O}}_4){+\mathrm{H}}_2\mathrm{O}\mathrm{fast}$$

(7)

$${\mathrm{H}}^{+}+{\mathrm{VO}}_2{{(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{3-}\rightleftharpoons {\mathrm{HVO}}_2{{{(\mathrm{C}}_2{\mathrm{O}}_4)}_2}^{2-}$$

(8)

$${\mathrm{HVO}}_2{{{(\mathrm{C}}_2{\mathrm{O}}_4)}_2}^{2-}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}\rightleftharpoons {\mathrm{HVO}}_2{{{(\mathrm{C}}_2{\mathrm{O}}_4)}_2}^{3-}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{*}$$

(9)

$${\mathrm{HVO}}_2{{{(\mathrm{C}}_2{\mathrm{O}}_4)}_2}^{3-}+{\mathrm{H}}^{+}\rightleftharpoons {{\mathrm{VO}(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{2-}+{\mathrm{H}}_2\mathrm{O}\mathrm{fast}$$

(10)

Therefore, when vanadium-bearing shale is leached with oxalic acid, the coordination process of vanadium is as follows: V(IV) directly coordinates with C₂O₄²⁻ to form VOC₂O₄. Under conditions of excess C₂O₄²⁻, VOC₂O₄ is further coordinated to form the VO(C₂O₄)₂²⁻ complex; V(V) is first coordinated by C₂O₄²⁻ to form the VO₂C₂O₄⁻ complex. Under conditions of excess C₂O₄²⁻, VO₂C₂O₄⁻ is further coordinated to form the VO₂(C₂O₄)₂³⁻ complex. However, under heating conditions, the VO₂C₂O₄⁻ and VO₂(C₂O₄)₂³⁻ complexes are further reduced by C₂O₄²⁻, forming the VOC₂O₄ and VO₂(C₂O₄)₂³⁻ complexes.

3.2.2. Coordination Process of Iron and Oxalate Ions

In the sulfate leachate of vanadium-bearing shale, iron usually exists simultaneously as Fe²⁺ and Fe³⁺ [31]. However, in the oxalic acid system, iron exists in multiple complex forms. According to the research results in Section 3.1, the main complexes of Fe²⁺ with C₂O₄²⁻ are FeC₂O₄ and Fe(C₂O₄)₂²⁻, and the reaction equations are shown in Equations (11) and (12) [36,37]; the main complexes of Fe³⁺ with C₂O₄²⁻ are FeC₂O₄⁺, Fe(C₂O₄)₂⁻, and Fe(C₂O₄)₃³⁻, and the reaction equations are shown in Equations (13)–(15) [38].

$${\mathrm{Fe}}^{2+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={\mathrm{FeC}}_2{\mathrm{O}}_4$$

(11)

$${\mathrm{FeC}}_2{\mathrm{O}}_4+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={{\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{2-}$$

(12)

$${\mathrm{Fe}}^{3+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}^{+}$$

(13)

$${\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}^{+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={{\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{-}$$

(14)

$${{\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}_2}^{-}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={{\mathrm{Fe}(\mathrm{C}}_2{\mathrm{O}}_4{)}_3}^{3-}$$

(15)

The initial pH of the OALS is 0.65. According to Figure 3 and Figure 4, Fe²⁺ exists simultaneously as FeC₂O₄ and Fe(C₂O₄)₂²⁻, and Fe³⁺ exists simultaneously as Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻. Therefore, the coordination process of Fe²⁺ with C₂O₄²⁻ is that Fe²⁺ first coordinates with C₂O₄²⁻ to form C₂O₄²⁻ and then is further coordinated by C₂O₄²⁻ to form the Fe(C₂O₄)₂²⁻ complex. The coordination process of Fe³⁺ with oxalate ions is as follows: Fe³⁺ first coordinates with C₂O₄²⁻ to form FeC₂O₄⁺, and then under the condition of excess oxalate ions, FeC₂O₄⁺ is further coordinated to form the Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ complexes.

3.2.3. Coordination Process of Aluminum and Oxalate Ions

In the sulfate leachate, aluminum usually exists in the form of Al³⁺, Al(OH)²⁺, and Al(OH)₂⁺ [39]. However, in the oxalic acid system, aluminum exists in various complex forms, such as AlC₂O₄⁺, Al(C₂O₄)₂⁻, and Al(C₂O₄)₃³⁻. The coordination reaction of aluminum with oxalate ions are shown in Equations (16)–(19) [40].

$${\mathrm{Al}}^{3+}+{{\mathrm{HC}}_2{\mathrm{O}}_4}^{-}={{\mathrm{AlHC}}_2{\mathrm{O}}_4}^{2+}$$

(16)

$${\mathrm{Al}}^{3+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}={{\mathrm{AlC}}_2{\mathrm{O}}_4}^{+}$$

(17)

$${{\mathrm{AlC}}_2{\mathrm{O}}_4}^{+}+{{\mathrm{C}}_2{\mathrm{O}}_4}^{2-}=\mathrm{Al}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}$$

(18)

$$\mathrm{Al}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2}^{-}+{\mathrm{C}}_2{\mathrm{O}}_4^{2-}=\mathrm{Al}{{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_3}^{3-}$$

(19)

According to the results in Figure 5, Al exists in the form of both Al(C₂O₄)₂⁻ and Al(C₂O₄)₃³⁻ in the OALS. Therefore, during the oxalic acid leaching of vanadium-bearing shale, the coordination process of Al with C₂O₄²⁻ is as follows: Al³⁺ first coordinates with HC₂O₄⁻ and C₂O₄²⁻ to form AlHC₂O₄²⁺ and AlC₂O₄⁺, respectively. Under the condition of excess oxalate ions, AlC₂O₄⁺ is further coordinated to form Al(C₂O₄)₂⁻ and Al(C₂O₄)₃³⁻ complexes.

3.3. Coordination Mechanism of Vanadium, Iron, and Aluminum with Oxalate Ions

The previous section determined the ionic speciation and coordination process of vanadium, iron, and aluminum in OALS. The results indicated that vanadium, iron, and aluminum mainly exist in the form of oxalate complexes in OALS, and the complexes with inorganic ligands P and F were relatively few. However, the coordination mechanism between metal ions and oxalate ions was not clear. Therefore, the quantum chemistry software Gaussian 16W was used to conduct an in-depth study on the coordination mechanism of vanadium, iron, and aluminum with oxalate ions.

3.3.1. Structural Optimization of Vanadium–Oxalate Complexes

According to Equations (1) and (2), the vanadium–oxalate complexes were formed by the coordination reaction between VO²⁺ and C₂O₄²⁻, resulting in VOC₂O₄ and VO(C₂O₄)₂²⁻. Therefore, the molecular structures of VO²⁺ and C₂O₄²⁻ were first optimized, and the results are shown in Figure 7.

As shown in Figure 7a, the electrostatic potential of VO²⁺ was deep blue at the V atom, indicating that the V atom had a higher electrostatic potential and exhibited electron acceptor characteristics. As shown in Figure 6b, the C₂O₄²⁻ molecule has good structural symmetry, with the electrostatic potential around the O atoms being red and evenly distributed among the four O atoms, demonstrating electron donor characteristics.

Based on the above analysis, the structure of the complex VOC₂O₄ was optimized, and the results are shown in Figure 8a. On this basis, the structure of the complex VO(C₂O₄)₂²⁻ was further optimized, and the results are shown in Figure 8b and

Table 5. Bond length and angles of VO(C₂O₄)₂²⁻.

Bond Length/Å				Bond Angles/°
V=O	V-O	C=O	C-O	O-V=O	O-V-O	O-V-O	O-C=O
1.60	1.99	1.22	1.30	109.03	88.09	79.68	125.75

. The results indicate that the molecular structure of VO(C₂O₄)₂²⁻ is a symmetrical structure centered on the V=O double bond, with two oxalate ions arranged at certain angles on both sides.

3.3.2. Structural Optimization of Iron–Oxalate Complexes

In the OALS, Fe³⁺ had a larger proportion and was more likely to coordinate with oxalate ions. Therefore, this paper only examines the coordination process between Fe³⁺ and oxalate ions. According to Equations (13) and (14), Fe³⁺ first coordinated with C₂O₄²⁻ to form FeC₂O₄⁺, and its optimized result is shown in Figure 9a. Then, under the condition of excess oxalate ions, it further coordinated to form Fe(C₂O₄)₂⁻, and its optimized result is shown in Figure 9b. Then Bond length and angles of Fe(C₂O₄)₂⁻ is listed in

Table 6. Bond length and angles of Fe(C₂O₄)₂⁻.

Bond Length/Å			Bond Angles/°
Fe-O	C=O	C-O	O-Fe-O	O-Fe-O	O-C=O
1.86	1.21	1.31	95.86	84.14	127.70

. The results show that Fe(C₂O₄)₂⁻ had a planar symmetrical structure with iron atoms at the center and two oxalate ions on both sides.

3.3.3. Structural Optimization of Aluminum–Oxalate Complexes

According to Equations (16)–(18), Al³⁺ coordinates with HC₂O₄⁻ to form AlHC₂O₄²⁺, and then further forms AlC₂O₄⁺ and Al(C₂O₄)₂⁻. To compare with vanadium–oxalate and iron–oxalate complexes in the same system, the aluminum–oxalate complexes with molar ratios of 1:1 and 1:2 were optimized, that is, the structures of AlC₂O₄⁺ and Al(C₂O₄)₂⁻ were optimized. The results are shown in Figure 10. The results indicate that the molecular structure of Al(C₂O₄)₂⁻ is a structure centered on the aluminum atom, with two oxalate ions arranged in a cross-symmetric manner. Bond length and angles of Al(C₂O₄)₂⁻ are listed in

Table 7. Bond length and angles of Al(C₂O₄)₂⁻.

Bond Length/Å			Bond Angles/°
Al-O	C=O	C-O	O-Al-O	O-Al-O	O-C=O
1.77	1.20	1.33	118.67	92.31	125.34

.

3.3.4. Coordination Mechanism of Vanadium, Iron, and Aluminum with Oxalate Anions

The frequencies of the optimized VO²⁺, Fe³⁺, Al³⁺, C₂O₄²⁻, VOC₂O₄, VO(C₂O₄)₂²⁻, FeC₂O₄⁺, Fe(C₂O₄)₂⁻, AlC₂O₄⁺, and Al(C₂O₄)₂⁻ were calculated to determine the ΔG of these structures. At this stage, the ΔG of these compounds was precisely determined. The ΔG of the coordination reactions of vanadium, iron, and aluminum with oxalate ions (Equations (1), (2), (13), (14), (17) and (18)) were calculated. The results are shown in

Table 8. ΔG of coordination reactions between V, Fe, Al, and C₂O₄²⁻ ions.

Reaction Equation	ΔG (kJ/mol)
VO2+ + C2O42− = VOC2O4	−2760.65
VOC2O4 + C2O42− = VO(C2O4)22−	−714.12
Fe3+ + C2O42− = FeC2O4+	−5343.69
FeC2O4+ + C2O42− = Fe(C2O4)2−	−1470.72
Al3+ + C2O42− = AlC2O4+	−5169.23
AlC2O4+ + C2O42− = Al(C2O4)2−	−1318.87

.

As shown in Table 8, regardless of whether the molar ratio was 1:1 or 1:2, the reaction ΔG of the iron–oxalate complex is the smallest, being −5343.69 and −1470.72 kJ/mol, respectively. Next is the aluminum–oxalate complex, with reaction ΔG values of −5169.23 and −1318.87 kJ/mol, respectively. The vanadium–oxalate complex has the largest reaction ΔG, being −2760.65 and −714.12 kJ/mol, respectively. Therefore, the order of coordination ability of vanadium, iron, and aluminum with oxalate is iron > aluminum > vanadium. From this, it can be known that the coordination mechanism of vanadium, iron, and aluminum with oxalate ions is as follows: iron coordinates with oxalate ions first, followed by aluminum, and finally vanadium.

4. Conclusions

(1)

Vanadium, iron, and aluminum can form various complexes with oxalate ions. In the OALS (pH range of 0.5 to 1.0), vanadium predominantly exists as VO(C₂O₄)₂²⁻ anions, iron primarily exists as the Fe(C₂O₄)₂⁻ and Fe(C₂O₄)₃³⁻ anions, and aluminum mainly exists as the Al(C₂O₄)₂⁻ and Al(C₂O₄)₃³⁻ anions.

(2)

In the OALS, vanadium, iron, and aluminum initially coordinate with oxalate ions to form 1:1 molar ratio complexes. Subsequently, these complexes further coordinate to generate anionic complexes with molar ratios of 1:2 and 1:3 between the metal ions and oxalate ions.

(3)

The coordination sequence of vanadium, iron, and aluminum with oxalate ions is as follows: iron > aluminum > vanadium. Specifically, the coordination mechanism proceeds such that iron coordinates with oxalate ions first, followed by aluminum, and finally vanadium in the OALS.