Extraction of Gold Based on Ionic Liquid Immobilized in UiO-66: An Efficient and Reusable Way to Avoid IL Loss Caused by Ion Exchange in Solvent Extraction

Ionic liquids (ILs) have received considerable attention as a promising green solvent for extracting metal ions from aqueous solutions. However, the recycling of ILs remains difficult and challenging because of the leaching of ILs, which is caused by the ion exchange extraction mechanism and hydrolysis of ILs in acidic aqueous conditions. In this study, a series of imidazolium-based ILs were confined in a metal–organic framework (MOF) material (UiO-66) to overcome the limitations when used in solvent extraction. The effect of the various anions and cations of the ILs on the adsorption ability of AuCl4− was studied, and 1-hexyl-3-methylimidazole tetrafluoroborate ([HMIm]+[BF4]−@UiO-66) was used for the construction of a stable composite. The adsorption properties and mechanism of [HMIm]+[BF4]−@UiO-66 for Au(III) adsorption were also studied. The concentrations of tetrafluoroborate ([BF4]−) in the aqueous phase after Au(III) adsorption by [HMIm]+[BF4]−@UiO-66 and liquid–liquid extraction by [HMIm]+[BF4]− IL were 0.122 mg/L and 18040 mg/L, respectively. The results reveal that Au(III) coordinated with the N-containing functional groups, while [BF4]− was effectively confined in UiO-66, instead of undergoing anion exchange in liquid–liquid extraction. Electrostatic interactions and the reduction of Au(III) to Au(0) were also important factors determining the adsorption ability of Au(III). [HMIm]+[BF4]−@UiO-66 could be easily regenerated and reused for three cycles without any significant drop in the adsorption capacity.


Introduction
Ionic liquids (ILs) represent a class of environmentally friendly "green" solvents with unusual physical and chemical properties, such as a low vapor pressure and the absence of volatilization, and they can be used for the extraction of metal ions from aqueous solutions [1][2][3][4][5][6][7][8][9]. The most commonly proposed mechanism for the extraction of metal ions into the hydrophobic ionic liquid phase is ion exchange [2][3][4][5][6][7][8][9]. The simultaneous release of IL ions to the aqueous phase (resulting in IL losses and aqueous phase pollution) when metal ions are extracted into the IL phase has been reported in many cases. Moreover, the chemical stability of ILs has already aroused attention. The hydrolysis of fluorine-based anions of ILs has been reported. Fernandes and co-workers found that the hydrolysis of [BF 4 ] − even occurred at room temperature [10]. The hydrolysis of fluorine-based anions generates an abundance of toxic hydrofluoric acid (HF), which results in environmental pollution.
Supported ionic liquids (SILs) are a new type of solid material prepared through physical adsorption, chemical bonding, or loading of ILs onto porous supports, and they have the characteristics of both ILs and carriers. The process can significantly improve the utilization of ILs, solve the problems of the high viscosity, separation, and mass transfer of ILs, and expand the application field of ILs. Various porous materials have been used as supports, such as covalent organic frameworks [11], porous celluloses [12,13], molecular sieves [14], magnetic materials [15], resins [16], porous silica [17], and biopolymers [18]. SILs have been widely used in various
The surface morphology of UiO−66 before and after IL loading was observed via SEM. Figure S1a, b show the SEM images of UiO−66 and [HMIm] + [BF4] − @UiO−66, indicating that no detectable IL phase formed on the surface of UiO−66 [42]. A large number of irregularly rounded particles can be found closely attached to the characterized surfaces of UiO−66 and [HMIm] + [BF4] − @UiO−66, and this rough surface can enhance the adsorption capacity. This is the same form as previously reported for UiO−66 [25]. At the same time, many pores and voids can be observed in Figure S1, and this porous tertiary structure is beneficial for the following adsorption performance. The dimensions of both samples are the same, indicating that the morphological characteristics of the samples are the same before and after synthesis.
To  [26,27], which indicate the successful synthesis of UiO−66. The strong band at 1105 cm −1 can be attributed to C−N stretching, and the band at 1296 cm −1 can be attributed to the imidazole ring stretching [42], which indicates the successful sequestration of IL onto UiO−66.
The  [20,26,29,42]. In addition, [  According to the reported studies, the composition of ILs is an important factor that affects the extractability of AuCl 4 − when ILs are used in solvent extraction or immobilized on solid supports. The extractability of AuCl 4 − was found to be greater for ILs composed of more hydrophilic anions and more hydrophobic cations [13,41] 4 ] − in UiO-66 was characterized by various microscopic and spectroscopic techniques, such as scanning electron microscopy (SEM), Fourier transform infrared spectra (FT-IR), and X-ray diffraction spectrometry (XRD). The specific surface area and porous structures of UiO-66 and [HMIm] + [BF 4 ] − @UiO-66 were investigated via nitrogen adsorption-desorption isotherms and pore size distribution curves.
The surface morphology of UiO-66 before and after IL loading was observed via SEM. Figure S1a, 4 ] − @UiO-66, and this rough surface can enhance the adsorption capacity. This is the same form as previously reported for UiO-66 [25]. At the same time, many pores and voids can be observed in Figure S1, and this porous tertiary structure is beneficial for the following adsorption performance. The dimensions of both samples are the same, indicating that the morphological characteristics of the samples are the same before and after synthesis.
To  [26,27], which indicate the successful synthesis of UiO-66. The strong band at 1105 cm −1 can be attributed to C-N stretching, and the band at 1296 cm −1 can be attributed to the imidazole ring stretching [42], which indicates the successful sequestration of IL onto UiO-66.   [27], characteristic of microporous materials, proving the existence of microporosity in the materials. IL curing can still maintain the microporous properties of UiO−66, and the stable microporous structure benefits the diffusion of Au(III). The pore size distribution parameters were obtained based on the Brunauer-Emmett-Teller (BET) pore size distribution curves. The BET surface area and pore volume of UiO−66 and [HMIm] + [BF4] − @UiO−66 were 1211.178 m 2 /g and 0.455 cm 3 /g, and 1207.26 m 2 /g and 0.442 cm 3 /g, respectively. Before and after IL curing, the high specific surface area possessed by  [20,26,29,42] The N 2 adsorption-desorption isotherms of the adsorbent materials are shown in Figure 3a. The characteristics of UiO-66 and [HMIm] + [BF 4 ] − @UiO-66 both follow an I-shaped curve [27], characteristic of microporous materials, proving the existence of microporosity in the materials. IL curing can still maintain the microporous properties of UiO-66, and the stable microporous structure benefits the diffusion of Au(III). The pore size distribution parameters were obtained based on the Brunauer-Emmett-Teller (BET) pore size distribution curves. The BET surface area and pore volume of UiO-66 and [HMIm] + [BF 4 ] − @UiO-66 were 1211.178 m 2 /g and 0.455 cm 3 /g, and 1207.26 m 2 /g and 0.442 cm 3 /g, respectively. Before and after IL curing, the high specific surface area possessed by UiO-66 can provide more adsorption sites for Au(III). In addition, the pore diameters of [HMIm] + [BF 4 ] − @UiO-66 are mainly distributed around 0.94 nm and 1.51 nm. These diameters are much larger than those of gold species. Thus, this facilitates the diffusion of gold ions from the surface of the material into the pores, so that there are enough adsorption sites on the surface to adsorb gold ions. The reduction in the specific surface area and pore volume of UiO-66 before and after loading also confirms that the IL was successfully transported into UiO-66.  [27], characteristic of microporous materials, proving the existence of microporosity in the materials. IL curing can still maintain the microporous properties of UiO−66, and the stable microporous structure benefits the diffusion of Au(III). The pore size distribution parameters were obtained based on the Brunauer-Emmett-Teller (BET) pore size distribution curves. The BET surface area and pore volume of UiO−66 and [HMIm] + [BF4] − @UiO−66 were 1211.178 m 2 /g and 0.455 cm 3 /g, and 1207.26 m 2 /g and 0.442 cm 3 /g, respectively. Before and after IL curing, the high specific surface area possessed by UiO−66 can provide more adsorption sites for Au(III). In addition, the pore diameters of [HMIm] + [BF4] − @UiO−66 are mainly distributed around 0.94 nm and 1.51 nm. These diameters are much larger than those of gold species. Thus, this facilitates the diffusion of gold ions from the surface of the material into the pores, so that there are enough adsorption sites on the surface to adsorb gold ions. The reduction in the specific surface area and pore volume of UiO−66 before and after loading also confirms that the IL was successfully transported into UiO−66.
By introducing an IL into the porous framework of an MOF, it has been reported that the interionic interactions become stronger due to the interaction of the anions of the IL with the metal sites of the MOF, and the direct interaction between the imidazolium ring of the IL with either the MOF or the anion of the IL [21].   Since the results confirm that [HMIm] + [BF 4 ] − @UiO-66 adsorbed Au(III) certainly without ion exchange, to further explore the adsorption mechanism of Au(III), the effect of pH on adsorption was investigated, and the X-ray photoelectron spectroscopy (XPS) analyses of [HMIm] + [BF 4 ] − @UiO-66 before and after adsorption were examined.

Effect of pH
The pH effect on the surface charge of the adsorbent and metal ions is one of the fundamental factors affecting the adsorption rate. Here, the effect of pH varying from 1 to 9 on Au(III) adsorption was evaluated. The adsorption ratio of Au(III) was in the range of 88.28% to 94.37% when the pH was 1 to 2. Meanwhile, it decreased from 94.37% to 73.48% with increasing pH from 2 to 9. As far as we know, the species of Au(III) in solutions at pH 1-9 have negative charges, such as [ [38][39][40]. As shown in Figure  fundamental factors affecting the adsorption rate. Here, the eff 9 on Au(III) adsorption was evaluated. The adsorption ratio of 88.28% to 94.37% when the pH was 1 to 2. Meanwhile, it decrea with increasing pH from 2 to 9. As far as we know, the specie  To investigate the adsorption mechanism of Au(III), [HM   4 ] − @UiO-66/Au. This indicates that Au(III) was successfully adsorbed by this material. The peaks in the high-resolution Au4f spectrum of the gold adsorbent can be divided into Au4f 7/2 and Au4f 5/2, as shown in Figure 6b. The two peaks at 87.54 eV (Au4f 5/2) and 83.87 eV (Au4f 7/2) correspond to Au(0), while the two peaks at 88.12 eV (Au4f 5/2) and 84.47 eV (Au4f 7/2) correspond to Au(I) [25,45,46,48]. The results suggest that Au(III) was reduced to Au(0) and Au(I) by [HMIm] + [BF 4 ] − @UiO-66, and that a redox mechanism exists during the adsorption process. The Au(0) area ratio is 64.68%, and the size of the peak area ratio indicates that gold mainly exists on the adsorbent in the form of Au(0) [46].
To understand the interactions between the gold and N atoms, the XPS N1s spectra of [HMIm] + [BF4] − @UiO−66 and [HMIm] + [BF4] − @UiO−66/Au were studied. In Figure 6c, the N1s spectra can be divided into C=N and −NH. The peak of the C=N binding energy changes from 398.57 ev to 398.47 ev, while the peak of the −NH binding energy changes from 399.88 ev to 400.2 ev, after adsorption. The peak area of C=N decreases while the peak area of NH increases relatively after adsorption. The results indicate that the electrons were transferred from N to Au(III). The N−containing functional groups were bound to Au(III) through complexation, and Au(III) was reduced to Au(I) and Au(0) [47,48]. The results prove the mechanism of Au(III) adsorption after IL immobilization, which not only avoids ion exchange in solvent extraction, but also provides a new adsorption site for Au(III).  from 399.88 eV to 400.2 eV, after adsorption. The peak area of C=N decreases while the peak area of NH increases relatively after adsorption. The results indicate that the electrons were transferred from N to Au(III). The N-containing functional groups were bound to Au(III) through complexation, and Au(III) was reduced to Au(I) and Au(0) [47,48]. The results prove the mechanism of Au(III) adsorption after IL immobilization, which not only avoids ion exchange in solvent extraction, but also provides a new adsorption site for Au(III).
In summary, the mechanism of the adsorption of Au(III) by [HMIm] + [BF 4 ] − @UiO-66 has three parts: electrostatic interaction, coordination between Au(III) and N-containing functional groups, and a reduction of Au(III) to Au(I) and Au(0). The loss of [HMIm] + [BF 4 ] − IL in the aqueous phase caused by the miscibility of the water and anion exchange was effectively restrained because of the strong interaction between the IL and UiO-66.   The data obtained were further fitted using pseudo−first−order (PFO, Equation (1)), pseudo−second−order (PSO, Equation (2)), and intraparticle diffusion (Id, Equation (3)) models to describe the adsorption behavior. The pseudo−first−order kinetic model assumes [49] that the adsorption process is physical adsorption, and its rate−limiting step is related to pore diffusion. The pseudo−second−order kinetic model assumes [39] that the adsorption process is chemical adsorption, and its rate−limiting step is a chemical reaction. The intraparticle diffusion model assumes [50] that the external mass transfer process leads to either rapid intraparticle diffusion or rate control steps. The data obtained were further fitted using pseudo-first-order (PFO, Equation (1)), pseudo-second-order (PSO, Equation (2)), and intraparticle diffusion (Id, Equation (3)) models to describe the adsorption behavior. The pseudo-first-order kinetic model assumes [49] that the adsorption process is physical adsorption, and its rate-limiting step is related to pore diffusion. The pseudo-second-order kinetic model assumes [39] that the adsorption process is chemical adsorption, and its rate-limiting step is a chemical reaction. The intraparticle diffusion model assumes [50] that the external mass transfer process leads to either rapid intraparticle diffusion or rate control steps.

Adsorption Kinetics
where q e and q t (mg/g) are the metal amounts when adsorption equilibrium is reached and at time t, respectively, K 1 (1/min) is the pseudo-first-order rate constant, K 2 (mg/g·min) is the pseudo-second-order rate constant, K 3 (mg/g·min 0.5 ) is the particle diffusion rate constant, and C is the intercept, representing the thickness of the boundary layer. The experimental data were analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, and the parameters obtained are shown in Figure 8 and diffusion rate constant, and C is the intercept, representing the thickness of the boundary layer.
The experimental data were analyzed using pseudo−first−order, pseudo−second−order, and intraparticle diffusion models, and the parameters obtained are shown in Figure  8 and Table 1

Isotherm Study
The adsorption isotherm is an important indicator to evaluate the maximum adsorption capacity of the adsorbent. The variation in the adsorption capacity with the initial Au(III) concentration at three temperatures (25, 30, and 35 °C) is shown in Figure 7b. Furthermore, the adsorption capacity increases with increasing initial Au(III) concentration. It can be adsorbed entirely at low concentrations because a sufficient number of adsorp-

Isotherm Study
The adsorption isotherm is an important indicator to evaluate the maximum adsorption capacity of the adsorbent. The variation in the adsorption capacity with the initial Au(III) concentration at three temperatures (25,30, and 35 • C) is shown in Figure 7b. Furthermore, the adsorption capacity increases with increasing initial Au(III) concentration. It can be adsorbed entirely at low concentrations because a sufficient number of adsorption sites are provided by the adsorbent. Due to the limited number of adsorption sites, the adsorption capacity tends to increase gradually and slowly with increasing Au(III) concentration, which leads to adsorption saturation. The obtained data were fitted and analyzed with the Langmuir model (Equation (4)), Freundlich model (Equation (5)), and Dubinin-Radushkevich (D-R) model (Equation (6)). The Langmuir model assumes [54] that adsorption is monolayer adsorption on a uniform surface, and that the adsorption and desorption are in dynamic equilibrium. The Freundlich isotherm model assumes [55] that adsorption is multilayer adsorption and occurs on heterogeneous surfaces. The D-R isotherm model assumes [56] that the adsorption process is not layer-by-layer adsorption on the adsorbent surface but related to the micropore volume.
ln q e = ln K F + 1 n c e , ln q e = ln q m − βε 2 , ε = RT ln 1 where K L (L/mg) is the Langmuir constant related to the affinity of the binding site, K F ((mg/g)/(L/mg) 1/n ) shows the Freundlich constant associated with the adsorption strength, q m (mg/g) and n express the highest adsorption capacity and the coefficient of the Freundlich model, respectively, β represents the D-R isotherm constant, R represents the universal gas constant (8.314 J/mol·K), and T represents the temperature (K). The separation factor (R L ) describes the basic characteristics and feasibility of the Langmuir isotherm: The experimental results were analyzed using the Langmuir, Freundlich, and D-R models, and the parameters obtained are shown in Figure 9a-i and Table 2. The results show that the adsorption isotherms of Au(III) were more consistent with the Langmuir model (R 2 = 0.995, 0.91634, 0.92302), and the theoretical maximum adsorption amounts of Au(III) at the three temperatures were 109.89, 142.05, and 279.33 mg/g, which are very close to the actual maximum adsorption amounts of 111.57, 160.77, and 284.64 mg/g, indicating that the adsorption of Au(III) is monolayer adsorption at a specific homogeneous location on the adsorbent surface [47,48,57]. Moreover, the R L values of Au(III) in the Langmuir model were all below 1, which indicates that the adsorption is appropriate [53]. In addition, the maximum adsorption of [HMIm] + [BF 4 ] − @UiO-66 (284.64 mg/g) was higher than that of UiO-66 (203.42 mg/g) (see SI for detailed results).

Thermodynamic Study
The influence of the adsorption temperature on the adsorption process is significant and explains the adsorption thermodynamics with relevant thermodynamic parameters. The data obtained were evaluated with the following equations (Equations (8)-(10)) containing classical thermodynamic parameters [25]: where , T (K), and R (8.314 J/mol·K) are the thermodynamic equilibrium constant, the adsorption temperature, and the gas constant, respectively, ΔS (J/mol/K), ΔH (KJ/mol), and ΔG (KJ/mol), respectively, are the changes in the entropy, enthalpy, and Gibb's free energy. As shown in Figure 10, the Au(III) adsorption of [HMIm] + [BF4] − @UiO−66 was enhanced with increasing temperature. The thermodynamic parameters at different

Thermodynamic Study
The influence of the adsorption temperature on the adsorption process is significant and explains the adsorption thermodynamics with relevant thermodynamic parameters. The data obtained were evaluated with the following equations (Equations (8)-(10)) containing classical thermodynamic parameters [25]: where K c , T (K), and R (8.314 J/mol·K) are the thermodynamic equilibrium constant, the adsorption temperature, and the gas constant, respectively, ∆S (J/mol/K), ∆H (KJ/mol), and ∆G (KJ/mol), respectively, are the changes in the entropy, enthalpy, and Gibb's free energy. As shown in Figure 10, the Au(III) adsorption of [HMIm] + [BF 4 ] − @UiO-66 was enhanced with increasing temperature. The thermodynamic parameters at different temperatures are summarized in Table 3. The increase in temperature favors the increase in the number of active molecules. Thus, the adsorption of Au(III) by the adsorbent is promoted, indicating that the adsorption process is heat absorption. It was found that ∆G was negative at different temperature conditions, indicating that the reaction process is spontaneous and feasible [20,58]. Additionally, the negative value of ∆G was increased with increasing temperature, indicating that the higher the temperature, the more spontaneous and favorable the adsorption of Au(III). The positive value of ∆H indicates that the adsorption is a heat-absorbing reaction. On the contrary, a positive value of ∆S indicates that the system's degrees of freedom and disorder are increased, which favors an increase in the frequency of collisions between Au(III) and the adsorbent [42,45,48,59].
temperatures are summarized in Table 3. The increase in temperature favors the increase in the number of active molecules. Thus, the adsorption of Au(III) by the adsorbent is promoted, indicating that the adsorption process is heat absorption. It was found that ΔG was negative at different temperature conditions, indicating that the reaction process is spontaneous and feasible [20,58]. Additionally, the negative value of ΔG was increased with increasing temperature, indicating that the higher the temperature, the more spontaneous and favorable the adsorption of Au(III). The positive value of ΔH indicates that the adsorption is a heat−absorbing reaction. On the contrary, a positive value of ΔS indicates that the system's degrees of freedom and disorder are increased, which favors an increase in the frequency of collisions between Au(III) and the adsorbent [42,45,48,59].

Selectivity and Practical Application
E−waste containing Au(III) coexists with other metal ions; therefore, the adsorption selectivity of Au(III) was studied to better evaluate the adsorbent's performance. Mg(II), Cu(II), Zn(II), Pb(II), Fe(II), and Ni(II) were chosen as background ions to study the selectivity of [HMIm] + [BF4] − @UiO−66. As shown in Figure 11a, when the concentration ratio of Au(III) to other coexisting ions was 1:1, Au(III) adsorption on the adsorbent reached 98.5%. In contrast, almost no other metal ions were adsorbed. Considering that the concentration of coexisting ions in e−waste leachate is several times higher than that of Au(III), the adsorption of Au(III) with other coexisting ions at a concentration ratio of 1:150 was investigated. The results show that the adsorption of Au(III) remained unaffected by the high concentration of coexisting ions, and that Au(III) could be 100% adsorbed by the adsorbent. The excellent Au(III) adsorption performance of the adsorbent can be attributed to the physicochemical properties of the metal atoms, such as the ionic radius (R), electronegativity (Xm), and covalent index (Xm 2 r). The Xm 2 r (5.48) and Xm (2.54) of Au(III) are higher, which allows Au(III) to be preferentially adsorbed [46]. Additionally, according to hard-soft acid-base (HSAB) theory, Au(III) can form strong bonds with N−containing functional groups, which may also contribute to its high adsorption [46,60]. In addition, at a lower pH, other coexisting ions may exist as cations or neutrals.

Selectivity and Practical Application
E-waste containing Au(III) coexists with other metal ions; therefore, the adsorption selectivity of Au(III) was studied to better evaluate the adsorbent's performance. Mg(II), Cu(II), Zn(II), Pb(II), Fe(II), and Ni(II) were chosen as background ions to study the selectivity of [HMIm] + [BF 4 ] − @UiO-66. As shown in Figure 11a, when the concentration ratio of Au(III) to other coexisting ions was 1:1, Au(III) adsorption on the adsorbent reached 98.5%. In contrast, almost no other metal ions were adsorbed. Considering that the concentration of coexisting ions in e-waste leachate is several times higher than that of Au(III), the adsorption of Au(III) with other coexisting ions at a concentration ratio of 1:150 was investigated. The results show that the adsorption of Au(III) remained unaffected by the high concentration of coexisting ions, and that Au(III) could be 100% adsorbed by the adsorbent. The excellent Au(III) adsorption performance of the adsorbent can be attributed to the physicochemical properties of the metal atoms, such as the ionic radius (R), electronegativity (Xm), and covalent index (Xm 2 r). The Xm 2 r (5.48) and Xm (2.54) of Au(III) are higher, which allows Au(III) to be preferentially adsorbed [46]. Additionally, according to hard-soft acid-base (HSAB) theory, Au(III) can form strong bonds with N-containing functional groups, which may also contribute to its high adsorption [46,60]. In addition, at a lower pH, other coexisting ions may exist as cations or neutrals. Therefore, the coexisting ions are not adsorbed by [HMIm] + [BF 4 ] − @UiO-66 due to electrostatic repulsion with positively charged [HMIm] + [BF 4 ] − @UiO-66 on the surface [61]. In addition, some anions will be inevitably introduced into the system during the leaching of Au(III) from e-waste. Therefore, the effect of several representative anions (Cl − , SO 4 2− , PO 4 3− , and NO 3 − ) on Au(III) adsorption at different concentrations (0, 0.001, 0.01, and 0.1 mol/L) was investigated. As can be seen in Figure 11b, the adsorption of Au(III) was inhibited to a greater extent by PO 4 3− as the anion concentration increased. When PO 4 3− was 0.1 mol/L, the adsorption of Au(III) was only 36%. On the contrary, the adsorption of Au(III) was slightly inhibited by SO 4 2− . When SO 4 2− was 0.1 mol/L, the adsorption of Au(III) was still above 85%, while Cl − and NO 3 − hardly affected the adsorption of Au(III). In the presence of different Cl − and NO 3 − concentrations, Au(III) adsorption remained at around 95%. Therefore, leaching agents containing Cl − and NO 3 − media are preferred in the Au(III) leaching process. When the commonly used aqua regia ablates e-waste to extract Au(III), the adsorption of Au(III) on the composites is not affected. In addition, the aqua regia−based leaching of e-waste is a flexible and low-cost method. At present, it is also a widespread process in the industry [62,63]. Therefore, the coexisting ions are not adsorbed by [HMIm] + [BF4] − @UiO−66 due to electrostatic repulsion with positively charged [HMIm] + [BF4] − @UiO−66 on the surface [61]. In addition, some anions will be inevitably introduced into the system during the leaching of Au(III) from e−waste. Therefore, the effect of several representative anions (Cl − , SO4 2− , PO4 3− , and NO3 − ) on Au(III) adsorption at different concentrations (0, 0.001, 0.01, and 0.1 mol/L) was investigated. As can be seen in Figure 11b, the adsorption of Au(III) was inhibited to a greater extent by PO4 3− as the anion concentration increased. When PO4 3− was 0.1 mol/L, the adsorption of Au(III) was only 36%. On the contrary, the adsorption of Au(III) was slightly inhibited by SO4 2− . When SO4 2− was 0.1 mol/L, the adsorption of Au(III) was still above 85%, while Cl − and NO3 − hardly affected the adsorption of Au(III). In the presence of different Cl − and NO3 − concentrations, Au(III) adsorption remained at around 95%. Therefore, leaching agents containing Cl − and NO3 − media are preferred in the Au(III) leaching process. When the commonly used aqua regia ablates e−waste to extract Au(III), the adsorption of Au(III) on the composites is not affected. In addition, the aqua regia−based leaching of e−waste is a flexible and low−cost method. At present, it is also a widespread process in the industry [62,63].

Reusability
In order to understand the recyclability of the adsorbent and to judge whether it has practical value, the reusability of the composite was investigated. The Au(III) adsorption + − Figure 12. (a) Percentage of major metal elements; (b) adsorption rate of each metal ion.

Reusability
In order to understand the recyclability of the adsorbent and to judge whether it has practical value, the reusability of the composite was investigated. The Au(III) adsorption rate of [HMIm] + [BF 4 ] − @UiO-66 in the reusability experiments is shown in Figure 13. After three consecutive cycles, the removal of Au(III) was greater than 95%, and there was no obvious decrease. However, no subsequent experiments were performed due to the large material loss during adsorption resolution. The results show that [HMIm] + [BF 4 ] − @UiO-66 has relatively stable adsorption properties and can be used as an excellent adsorbent for separating Au(III) from aqueous media.

Reusability
In order to understand the recyclability of the adsorbent and to judge whether it has practical value, the reusability of the composite was investigated. The Au(III) adsorption rate of [HMIm] + [BF4] − @UiO−66 in the reusability experiments is shown in Figure 13. After three consecutive cycles, the removal of Au(III) was greater than 95%, and there was no obvious decrease. However, no subsequent experiments were performed due to the large material loss during adsorption resolution. The results show that [HMIm] + [BF4] − @UiO−66 has relatively stable adsorption properties and can be used as an excellent adsorbent for separating Au(III) from aqueous media.

Discussion
[HMIm] + [BF4] − @UiO−66 was prepared as an adsorbent material for Au(III) recovery from acidic solutions, and it effectively avoided a series of contamination problems associated with the dissolution of ILs in water due to ion exchange. A series of characterizations, including SEM, FTIR, XRD, and N2 adsorption and desorption experiments, confirmed the successful sequestration of ILs, and the maximum adsorption of Au(III) by [HMIm] + [BF4] − @UiO−66 at pH 2.0 and 35 °C was 284.64 mg/g. In the kinetic and thermodynamic studies, the adsorption process was found to be an endothermic, feasible, and spontaneous reaction, while the rate−limiting step of adsorption was found to be a chemical reaction. In the isotherm studies, the adsorption processes at different temperatures

Discussion
[HMIm] + [BF 4 ] − @UiO-66 was prepared as an adsorbent material for Au(III) recovery from acidic solutions, and it effectively avoided a series of contamination problems associated with the dissolution of ILs in water due to ion exchange. A series of characterizations, including SEM, FTIR, XRD, and N 2 adsorption and desorption experiments, confirmed the successful sequestration of ILs, and the maximum adsorption of Au(III) by [HMIm] + [BF 4 ] − @UiO-66 at pH 2.0 and 35 • C was 284.64 mg/g. In the kinetic and thermodynamic studies, the adsorption process was found to be an endothermic, feasible, and spontaneous reaction, while the rate-limiting step of adsorption was found to be a chemical reaction. In the isotherm studies, the adsorption processes at different temperatures were consistent with the Langmuir model. [HMIm] + [BF 4 ] − @UiO-66 showed good selectivity for Au(III) adsorption and successfully recovered Au(III) from e-waste. The effect of the pH on Au(III) adsorption and the XPS results indicated that the Au(III) adsorption mechanism was either an electrostatic, redox, coordination, or complexation mechanism. The results indicate that [HMIm] + [BF 4 ] − @UiO-66 successfully enclosed ILs. As an advanced adsorptive material for gold recovery from e-waste, [HMIm] + [BF 4 ] − @UiO-66 can be reused for three cycles without any significant decrease in the adsorption rate. It has a simple synthesis, appropriate adsorption kinetics and adsorption capacity, and excellent selectivity and regeneration, and it can successfully recover gold in practical applications. The results of the present work show that the material has application value and practicability in industry.

Materials and Chemicals
The standard stock solution of Au(III) ( The synthesis process of UiO-66 was based on a previously reported method [64]. The details are reported in the Supplementary Materials.

Synthesis of IL/UiO-66
Amounts of 1.5 g UiO-66, 1 g ILs, and 3 mL C 2 H 5 OH were mixed in a glass bottle and stirred at room temperature for 15 h. Then, the composite was filtered, washed with C 2 H 5 OH, and dried in a vacuum oven overnight at 80 • C.

Batch Adsorption Experiment
The batch adsorption experiments were performed to study the Au(III) adsorption performance of [HMIm] + [BF 4 ] − @UiO-66. Generally, 10 mg [HMIm] + [BF 4 ] − @UiO-66 was added into a 50 mL plastic centrifugal tube containing 10 mL Au(III) solution with different concentrations at pH 2. The solution was shaken in a constant oscillator at 170 rpm for the desired duration, and then the mixture was filtered through a 0.45 µm filter membrane to separate the adsorbent from the aqueous phase.
Adsorption experiments were performed under optimized parameters (oscillation frequency: 170 rpm; t: 6 h; pH: 2; T: 35 • C; V Au(III) : 10 mL) unless otherwise indicated. The effect of pH was studied in the pH range of 1-9 (adjusted by adding diluted NaOH or HCl solutions). The experiment of the adsorption kinetics was carried out at different adsorption times ranging from 0 to 360 min. In contrast, the experiment of the adsorption isotherms was evaluated at different initial Au(III) concentrations ranging from 0 to 600 mg/L. The thermodynamic experiment was studied by controlling the temperature (from 298 to 308 K). The selectivity of the adsorbent for Au(III) was studied with different concentrations of Au(III) in the hybrid solution prepared by dissolving HAuCl 4 , MgCl 2 , CuCl 2 , ZnCl 2 , PbCl 2 , FeCl 3 , and NiCl 2 in DI water. With initial Au(III) concentrations of 10 and 100 mg/L, the mass ratios of Au(III) to coexisting ions were 1:150 and 1:1, respectively. The effect of anions was examined in the presence of Cl − , SO 4 2− , PO 4 3− , and NO 3 − , and the concentrations were set to 0, 0.001, 0.01, and 0.1 mol/L.
Liquid-liquid extraction experiments were performed by adding 1 mL [HMIm] + [BF 4 ] − to 10 mL of 150 mg/L Au(III) solution for 6 h with shaking under optimized parameters. After centrifugation (3000 r/min) of the mixed solution for 5 min, the aqueous was taken for measurement.
The regeneration experiment of [HMIm] + [BF 4 ] − @UiO-66 was implemented as follows: 20 mg [HMIm] + [BF 4 ] − @UiO-66 was mixed to 20 mL Au(III) aqueous solution (60 mg/L) at pH 2. Then, the mixed solution was shaken for 6 h at 35 • C and was separated by highspeed centrifugation (8000 r/min). The supernatant was tested to obtain the remaining Au(III) concentration. The residual solid was immersed for about 12 h with 20 mL 1 mol/L HCl and 5% thiourea solution (the gold was eluted into an acid thiourea solution), rinsed three times with DI water, and executed to the second time of adsorption-desorption cycle. The adsorption capacity and ratio of Au(III) were calculated using the following equations: Adsorption ratio = c 0 − c e c 0 × 100% , where q (mg/g) is the Au(III) adsorption capacity, c 0 and c e (mg/L) are the initial and equilibrium concentrations of Au(III) in solution, respectively, m (mg) is the mass of the adsorbent used, and V (mL) is the volume of the Au(III) solution.

Recovery of Au(III) from Waste CPUs
The waste CPUs we used had an array of pins. The pins were detached from the CPU and immersed in aqua regia solution for 2 h (magnetically stirred for 1 h at room temperature and then 1 h at 75 • C) until the pins were wholly dissolved without residue. The obtained solution was diluted 10 times with DI water. Then, 10 mL of the diluted solution and 10 mg of adsorbent were added into a centrifuge tube, shaken for 6 h, and then filtered through a filter membrane.