Construction of Magnetic S‐Doped CoWO4 Composite for Efficient and Selective Recovery of Gold from Wastewater via Adsorption–Reduction Pathway

The design and synthesis of efficient adsorbents for the recovery of precious metals from secondary resources are of great environmental and economic significance. Herein, a magnetic sulfur‐doped composite CoFe2O4@S–CoWO4 (CF@S–CoWO4) is developed through a hydrothermal synthesis method, which is used to selectively recover gold in aqueous media. Significantly, CF@S–CoWO4 exhibits the best overall performance with gold ions adsorption capacity (Qmax) and distribution coefficient (Kd) are 1049 mg g−1 and 4.4 × 106 mL g−1, respectively, which are much higher than those of other gold adsorption materials. The selectivity coefficients (K) toward other metal ions (Pd2+, Ca2+, Mg2+, Cd2+, Al3+, Li+, Ni+) are also higher, which suggests that CF@S–CoWO4 had a preferential selectivity for Au3+ in coexisting ion solutions. Moreover, the antianion interference of the composite follows the order: SO42− > PO43− > NO3− > CO32−, and it also shows very good reusability with adsorption efficiency at 81.78% after four repeated cycles. Based on characterizations and calculation, it is found that Au(III) mainly undergoes chelation and reduction reactions in the S sites in CF@S–CoWO4, which indicates the important role of S sites. Hence the CF@S–CoWO4 composite demonstrates a promising application for the recycling of gold ions from electronic wastewater.

The design and synthesis of efficient adsorbents for the recovery of precious metals from secondary resources are of great environmental and economic significance.Herein, a magnetic sulfur-doped composite CoFe 2 O 4 @ S-CoWO 4 (CF@S-CoWO 4 ) is developed through a hydrothermal synthesis method, which is used to selectively recover gold in aqueous media.Significantly, CF@S-CoWO 4 exhibits the best overall performance with gold ions adsorption capacity (Q max ) and distribution coefficient (K d ) are 1049 mg g À1 and 4.4 Â 10 6 mL g À1 , respectively, which are much higher than those of other gold adsorption materials.The selectivity coefficients (K ) toward other metal ions (Pd 2þ , Ca 2þ , Mg 2þ , Cd 2þ , Al 3þ , Li þ , Ni þ ) are also higher, which suggests that CF@S-CoWO 4 had a preferential selectivity for Au 3þ in coexisting ion solutions.Moreover, the antianion interference of the composite follows the order: SO 4 2À > PO 4 3À > NO 3 À > CO 3 2À , and it also shows very good reusability with adsorption efficiency at 81.78% after four repeated cycles.Based on characterizations and calculation, it is found that Au(III) mainly undergoes chelation and reduction reactions in the S sites in CF@S-CoWO 4 , which indicates the important role of S sites.Hence the CF@S-CoWO 4 composite demonstrates a promising application for the recycling of gold ions from electronic wastewater.
adsorbent for copper ions. [6]Li et al. reported that the generation of •CO 2 À radicals on the (001) crystalline surface of FeWO 4 imparts a strong reduction capacity, thus exhibiting an effective removal of Cr(VI). [7]Zhao et al. synthesized nanosized ZnWO 4 materials to simultaneously reduce vanadium(V) and chromium(VI), and the removal efficiency reached 68.8% and 97.3%, respectively. [8]otably, the strong reduction ability of MWO 4 and the highoxidation potential of Au(III) could achieve excellent adsorption capacity by redox reactions between M(II) ion and Au(III). [9]We thus select CoWO 4 as a matrix material for the recovery of gold ions.Based on the hard-soft acid-base (HSAB) theory, sulfur atoms were doped into the CoWO 4 matrix to improve the selectivity and adsorption ability, because gold ions (soft acid) can be preferentially bounded by sulfur/nitrogen-containing groups (soft Lewis). [3]Note that, to facilitate the magnetic separation of the S-CoWO 4 matrix material from the solution, we mounted CoFe 2 O 4 (CF) nanoparticles onto S-CoWO 4 considering its good chemical stability, moderate saturation magnetic field, and insensitivity to redox reaction. [10]However, to the best of our knowledge, there is no relevant report on the use of magnetic S-CoWO 4 composite to capture gold ions.Therefore, the aim of this study is to synthesize CF@S-CoWO 4 composite and to explore its feasibility for the recovery of gold from aqueous solution.Batch adsorption experiments against Au(III) ions were carried out to evaluate the adsorption properties of the composite.The effect of the sulfur adsorption sites on the adsorption capacity is explained by characterization and density functional theory (DFT) calculations.The dynamic experiment of CF@S-CoWO 4 composite for selective recovery of Au(III) ions from simulation wastewater is also conducted to verify its potential application in industry.

Structural Characterization
CF@S-CoWO 4 was synthesized by a one-step hydrothermal method as shown in Figure 1A (Experimental details are shown in the "Experimental Section" and "Supporting Information").Scanning electron microscopy (SEM) image of CF@S-CoWO 4 is displayed in Figure 1B, it could be observed that the surface morphology of the synthesized composites is a flower-like structure, this is due to the sheet-like F127 polymer added in the synthesis to restrict the growth of metal oxides in the thickness direction. [11]5b] Then, the elemental composition of the sample was also tested by the mapping, the signals of Co, W, Fe, O, and S elements were detected as shown in Figure 1F-K and Figure S1 (Supporting Information), and their corresponding atomic percentages are 16.39%, 20.94%, 18.5%, 38.41%, and 5.75%, respectively.From the color distribution of each element, it could be found that these small particles attached to the petals are CoFe 2 O 4 .In addition, elemental sulfur was successfully doped into CoWO 4 , as shown in the S mapping in Figure 1K.Since inductively coupled plasma optical emission spectroscopy is precise and have very good sensitivity in atomic element identification, we tested the sulfur content, and the doping percentage of S was 3.79%.To confirm the crystal structure of these composites, CF@S-CoWO 4 and CF@CoWO 4 were characterized by X-ray diffraction (XRD) as shown in Figure 2A.The strong and sharp diffraction peaks indicate the excellent crystallization of the obtained CoWO 4 and the diffraction peaks of the composite material shifted after sulfur doping.All diffraction peaks can be assigned pure tetragonal phase of CoWO 4 with cell parameters of a = 4.948 Å, b = 5.683 Å, c = 4.669 Å, β = 90°and a space group P2/a(13), which is in good agreement with the JCPDS card file no.15-0867. [12]In addition, we also carried out XRD test on CoFe 2 O 4 , as shown in Figure S2, Supporting Information, it is consistent with JCPDS card file no.22-1086, which indicates that CoFe 2 O 4 was successfully synthesized.However, compared with the XRD pattern of CF@S-CoWO 4 , no characteristic peak of CoFe 2 O 4 was found on CF@S-CoWO 4 , which might due to particle size too small to be detected by XRD.
Fourier transform infrared spectroscopy (FT-IR) characterization of CF@S-CoWO 4 , CF@CoWO 4 , and CoFe 2 O 4 (CF) was also carried out over the frequency range of 400-2000 cm À1 .In Figure 2B, the strong spectral band at 1636 cm À1 is associated with the O─H bending vibration, indicating the presence of surface hydroxyl groups, whereas the in-plane bending vibration at 1383 cm À1 is assigned to hydroxyl groups. [13]And the band observed at 1116 cm À1 is the characteristic of cobalt ferrite system, which may be due to the residual FeOOH.And the strong characteristic peak at 591 cm À1 is attributed to the vibration of the chemical O─M─O bonds at the octahedral and tetrahedral positions of CoFe 2 O 4 . [14]5c] Interestingly, by comparing with CF@CoWO 4 , it can be found that a new peak belonging to S-metal bond appears at 506 cm À1 on the spectra of CF@S-CoWO 4 . [15]The above results demonstrate that CF@S-CoWO 4 was successfully synthesized.
The magnetic properties of the magnetic adsorbent were considered to directly affect the separation efficiency, so the magnetic properties of the synthesized material were measured by vibrating sample magnetometer (VSM) at 300 K under the maximum magnetic field.10b] And the saturation magnetization intensity of CF@S-CoWO 4 is about 22.59 emu g À1 , which is lower than that of CF (100.75 emu g À1 ) but still exhibits good super paramagnetism.To verify that the saturation magnetization strength of the composite is sufficient to separate from the suspension by magnets, a magnetic separation experiment was performed, and the composite could be separated quickly within 5 min as shown in Figure S3(Supporting Information).All in all, under the premise of ensuring fast magnetic separation, the unit mass ratio of CF in CF@S-CoWO 4 composite should be as small as possible due to the poor adsorption capacity of magnetic nanoparticles, so that the adsorption capacity of the composite will be higher.
Specific surface area is one of the most important factors determining the adsorption capacity of a material.So N 2 gas adsorption-desorption experiments were conducted on CF@S-CoWO 4 .As seen in Figure 2D and S4 (Supporting Information), the isotherm of CF@S-CoWO 4 conforms to the type II with an H3-type hysteresis loop, [1] this also proves that the composite is a porous material and multilayer reversible adsorption occurs.For porous materials, a larger specific surface area could provide more adsorption sites and promote fast mass transfer for Au(III) adsorption.By calculation, the surface area, pore volume, and pore diameter of CF@S-CoWO 4 are 158.95m 2 g À1 , 0.196 cm 3 g À1 , and 4.923 nm, respectively.

Au(III) Recovery Performance of CF@S-CoWO 4
Solution pH has a vital effect on Au(III) aqueous species in the solution and the activity of the functional groups on the surface of the adsorbent, thereby affecting the adsorption performance of the adsorbent. [16]The effect of solution pH on the adsorption of gold ions by CF@S-CoWO 4 is shown in Figure 3A.Since the adsorption capacity of the composite material was unknown at the beginning, a trial experiment of 10 mg of adsorbent and 20 mL of gold ion solution (200 mg L À1 ) was performed on it.The experimental results found that the adsorption ratio was close to 100% from pH 3.0 to 5.0, and the optimal pH was not obtained.Therefore, the experiment was adjusted to 5 mg of adsorbent and 30 mL of gold ion solution (200 mg L À1 ), and it was found that CF@S-CoWO 4 had the best adsorption capacity of gold ion up to 692.3 mg g À1 at pH 3, so the gold ion solution of pH 3 was used in the next experiments.According to the zeta potentials of CF@S-CoWO 4 and gold ions solution presented in Figure 3B, the zero-point charge (pH zpc ) of CF@S-CoWO 4 is 5.5, so the higher adsorption capacity of Au(III) ions at lower pH values (pH < pH zpc ) was attributed to the protonation of sulfur adsorption sites on the CF@S-CoWO 4 , because the more positive CF@S-CoWO 4 could preferentially capture more negative gold ions (AuCl 4 À ) with the enhanced binding of the sulfur sites to Au(III) via Lewis soft-soft interactions. [17]However, the high concentration of Cl À will compete with AuCl 4 À for the adsorption sites at pH 1-2,and when the pH value exceeds pH zpc , the increased hydroxide ions were inclined to compete with the adsorption sites and the increased rejection of the negative adsorbents (deprotonation) to gold ions, leading to lower adsorption capacity.
Subsequently, the effect of adding different concentration of thiourea in preparing CF@S-CoWO 4 on the adsorption performances of gold ions was investigated.It can be seen from Figure 3C that when the thiourea concentration are 10 and 30 mM, the adsorption performances are lower than that in 20 mM thiourea, which indicates that 20 mM thiourea concentration can lead to the optimal S-doping amount and thus best gold adsorption capacity.And the maximum adsorption capacity of gold ions on CF@S-CoWO 4 (prepared with 20 mM thiourea) and CF@CoWO 4 reached up to 1049 and 380 mg g À1 , respectively (see Figure S5, Supporting Information for details).The above results show that moderate S-doping on CF@CoWO 4 has a significant effect on Au(III) adsorption.Moreover, we also evaluated the adsorption capacity of CoFe 2 O 4 , S-CoFe 2 O 4 , CoFe 2 O 4 @CoWO 4 , and CoFe 2 O 4 @S-CoWO 4 .It can be found from Figure S6, Supporting Information, that the adsorption capacity of CoFe 2 O 4 has not changed greatly after posttreatment with thiourea (S-CoFe 2 O 4 ), but the performance of the adsorbent has been significantly improved after adding thiourea during the synthesis of magnetic CoWO 4 , which indicates that sulfur is mainly doped in CoWO 4 .
Additionally, the actual industrial wastewater or leachate not only has coexisting metal ions, but also different anions will exist to affect the recovery of gold ions.It is known that chloride ions and gold ions have a strong affinity, and our pH experiments have showed that the presence of chloride ions have a greater impact on the adsorption of gold ions, so the effect of chloride ions on adsorption will not discuss in this section.Here, the adsorption performance of CF@S-CoWO 4 for gold ions in the presence of different anions (NO 3 À , PO 4 3À , SO 4 2À , and CO 3 2À ) was investigated by adding 5 mg adsorbent to 40 mL gold ion solution (100 mg L À1 , pH = 3).From Figure 3D, we can see that the order of influence of each anion on gold ion adsorption is: SO 4 2À < PO 4 3À < NO 3 À < CO 3 2À , which follows the HSAB theory, and this theory suggests that S,P, and N elements containing anions can promote the adsorption of noble metals. [18]t could also be found that when these four anions exist alone, the adsorption capacity of CF@S-CoWO 4 for gold ions all exceeded 600 mg g À1 , which indicates that the adsorbent (CF@S-CoWO 4 ) has excellent anti-interference ability for most anions.
Since gold ions often coexist with other metal ions in complex environments, the adsorption selectivity is also a vital factor for Au(III) adsorption because gold ions often coexists with other metal ions in a complex environment.The selective experiment was carried out by adding 5 mg of CF@S-CoWO 4 to 30 mL of the simulated wastewater containing Au(III), Pd(II), Ca(II), Mg(II), Cd(II), Al(III), Li(I), and Ni(I) (the initial concentration and residual concentration of metal ions are listed in Table S1, Supporting Information).The pH of the simulated wastewater was adjusted to 3.0 and the sediment was filtered.By comparing the concentration of each metal ion before and after adsorption, as shown in Figure 3E, the composite could completely capture gold ions and the recovery efficiency exceeds 99%, without being significantly affected by other seven kinds of metal ions.This is due to the better affinity between gold ions and oxygen/sulfur-containing groups, thus exhibiting excellent selectivity for Au(III) ions.
For further explanation of the selectivity of CF@S-CoWO 4 for Au(III) ions, the partition coefficient (K d ) along with the selectivity coefficient (K ) were calculated separately for each metal ions.Higher K d suggested that the binding sites were more readily adsorbed with gold ions while having a weak affinity for their coexisting ions. [19]K d and K could be calculated by next Equation (1 and 2) where C i (mg L À1 ) is the initial concentration of metal ions in solution.C e (mg L À1 ) is the residual concentration of metal ions in the solution.V is the solution volume (mL) and m is the adsorbent mass (g).The values of two parameters (K d and K ) for the adsorption of gold ions by this composite are listed in Table S1 (Supporting Information).The distribution coefficient (K d ) of the composite for Au(III) reaches 4.4 Â 10 6 mL g À1 , which is much higher than that of the other coexisting metal ions (6.2-770.7 mL g À1 ).Thus, these results indicate that CF@S-CoWO 4 has an excellent selectivity to distinguish Au(III) from other coexisting metal ions.Moreover, The K d value of Au(III) is 5.7 Â 10 3 -7.1 Â 10 5 higher than that of other metal ions, and the high K value (5.7 Â 10 3 -7.1 Â 10 5 ) indicated that CF@S-CoWO 4 had a preferential selectivity for Au(III) in a mixed solution of multimetal ions.
Desorption and regeneration of adsorbents are considered one of the most important factors for practical applications.In the adsorption-desorption experiment, 40 mg of CF@S-CoWO 4 was placed in 80 mL of 100 mg L À1 Au(III) ions solution for 10 h.After that, the gold ions were desorbed using a mixture solution of 1.0 mol L À1 thiourea and 1.0 mol L À1 HCl as the eluent. [20]The adsorption-desorption cycle was repeated four times as shown in Figure 3F.As the number of cycles increases, the adsorption ratio of gold ions decreases slightly, and the eluent could release gold ions from the composite well.The above results show that CF@S-CoWO 4 has excellent recyclability, which is very promising for the recovery of gold ions in the actual environment.

Adsorption Kinetics Study
The kinetics study of the adsorption process could not only explore the kinetics of gold ions on the adsorbent, but also reveal the adsorption mechanism and the rate-controlling steps of the adsorption process.The two commonly accepted and used adsorption kinetic models in the current research field are pseudo-first-order and pseudo-second-order kinetic models (Equation (S3) and (S4), Supporting Information). [21]The above two kinetic equations were used to perform a nonlinear fitting on the experimental data of different gold ion concentrations (50, 100, and 150 mg L À1 ).The adsorption ratio, fitting curves, and related parameters are presented in Figure 4 and Table S2 (Supporting Information).The adsorption ratio of gold ions by CF@S-CoWO 4 is shown in Figure 4A.It could be found that the adsorption of Au(III) ions occurs rapidly within the first 60 min, and about 75-93% of Au(III) ions is captured in these three concentration ranges.This is because there are enough adsorption sites on the composite surface to bind to gold ions during this period.After 60 min, since most of the adsorption sites were occupied, the adsorption speeds slow down, and the adsorption equilibrium were reached after 240 min and the adsorption ratio approaching to 100%.This indicates that CF@S-CoWO 4 has fast and excellent adsorption kinetics.
In comparison, the correlation coefficient (R 2 ) according to pseudo-second-order model are 0.936, 0.984, and 0.989, respectively, which are higher than those according to pseudo-firstorder model (0.973, 0.939, and 0.932), as shown in Figure 4B.In addition, the actual adsorption amounts (296.52,596.16, and 750.84 mg g À1 ) are in good agreement with the theoretical adsorption amounts calculated from the pseudo-second-order model (361.63,642.57, and 853.82 mg g À1 ).The adsorption of gold ions on CF@S-CoWO 4 at different concentrations (50, 100, and 150 mg L À1 ) all satisfies the pseudo-second-order kinetic equation, indicating that the adsorption process is chemical adsorption, and the rate-controlling step is the adsorption process of the adsorbate on adsorption sites. [22]Note that, the increase in the concentration of gold ions will lead to a decrease in the pseudo-second-order kinetic adsorption rate constant, which may be the increase in the content of gold ions leads to increased competition among gold ions for adsorption sites, thereby reduce the adsorption rate of gold ions.

Adsorption Isotherms Study
The adsorption isotherm is of great importance for exploring the adsorption behavior of the adsorbate on the adsorbent during the adsorption process, revealing the affinity between the adsorbate and the adsorbent. [23]Therefore, the relationship between the adsorption isotherm data of CF@S-CoWO 4 on Au(III) and the temperature was fitted using Langmuir and Freundlich isotherm models (Equation (S5) and (S6), Supporting Information).
The effect of initial concentration on the adsorption capacity of gold ions by CF@S-CoWO 4 at different temperatures (303, 313, and 323 K) is shown in Figure 4C.It could be concluded that the adsorption capacity of the composite for Au(III) ions are 1049 mg g À1 at 303 K, 1274 mg g À1 at 313 K, and 1386 mg g À1 at 323 K, respectively.As the reaction temperature increased, the adsorption capacity of CF@S-CoWO 4 for gold ions also increased, which indicated that the adsorption process was an endothermic reaction.
The fitting curves of Langmuir and Freundlich isotherm models, and the calculated isotherm parameters are presented in Figure 4D and Table S3 (Supporting Information).Comparing with the correlation coefficient of the Langmuir (R 2 = 0.877, 0.867, and 0.774) isotherm model, the Freundlich isotherm model (R 2 = 0.949, 0.942, and 0.907) have higher correlation coefficients at different temperatures, indicating that the adsorption process of the composite material for gold ions is more consistent with Freundlich isotherm model, and the adsorption reaction is multilayer adsorption on a homogeneous surface, which may be the reason for the excellent adsorption performance, because in the case of multilayer adsorption, one adsorption site can adsorb multiple metal ions.According to the literature, the adsorption capacity of the adsorbent for metal ions increases with the increase of the K F (constants of Freundlich models) value, and the adsorption is feasible and favorable when the index n is in the range of 1-10. [24]It could be concluded from Table S3 (Supporting Information) that the K F parameters increased from 167.32 to 242.28 with temperature increased from 303 to 323 K, which is also consistent with the increase of adsorption capacity with the increase in temperature.The Freundlich exponent n values for the adsorption of gold ions by CF@S-CoWO 4 were 3.13 at 303 K, 3.57 at 313 K, and 3.85 at 323 K, respectively.The above results showed that it is feasible to use the Freundlich model to describe this adsorption process.

Adsorption Thermodynamics Study
Adsorption is not only affected by the surface properties of the adsorbent, but also related to the physical and chemical properties of the gold ion solution, such as temperature, pH, and coexisting ions.In particular, exploring the influence of temperature on the performance of the adsorbent involves the choice of environmental conditions for the application of the adsorbent, which has an important reference value for exerting the best adsorption performance of the adsorbent. [25]To investigate the changes in thermodynamic parameters during the Au(III) adsorption on CF@S-CoWO 4 at a different temperature, the thermodynamic properties of the interactions involved could be evaluated (Equation (S7)-(S9), Supporting Information).The adsorption capacity at different temperatures (293, 303, 313, and 323 K) were shown in Figure 5A and the calculated thermodynamic parameters were shown in Table S4 (Supporting Information), and Van't Hoff plot of the adsorption of Au(III) on CF@S-CoWO 4 is shown in Figure 5B.It could be concluded that the adsorption capacity of the composite for gold ions increased from 546.42 to 839.82 mg g À1 when the temperature increased from 293 to 323 K (Figure 5A), which may be due to the higher adsorption capacity by making the adsorption sites more active with increasing temperature.As shown in Table S4, Supporting Information, the positive values of ΔH (25.625 kJ mol À1 ) and ΔS (0.101 kJ mol À1 ) indicate that CF@S-CoWO 4 for adsorption of Au(III) is a randomly increasing endothermic process.Also, the negative ΔG at each temperature indicates that Au(III) adsorption is spontaneous process.The absolute value of ΔG increases from 3.929 to 7.084 kJ mol À1 with the increase of temperature (from 293 to 323 K), indicating that higher temperature is beneficial to the adsorption reaction. [26]The results of thermodynamic experiments show that the adsorption process of gold ions on CF@S-CoWO 4 is spontaneous and irreversible.

Comparison of Adsorption Performance
Although the adsorption performance is one of the most concerned indicators in the field of adsorption, the selectivity should also be considered as an important parameter at the same time.Therefore, we first compared only the adsorption performance of CF@S-CoWO 4 with that of the adsorbents already reported in the literature.By comparing the maximum adsorption capacity of gold ions with other adsorbents (Figure 5C), the adsorption capacity of CF@S-CoWO 4 (only 5 mg dosage) is better than other adsorbents including IM-IUA, [4a] DACS-TA, [4b] TUCS, UiO-66-NH 2 , PCN-222-MBA, CuS NPs, GO, Ni 0.6 Fe 2.4 O 4 -MTD, DAVFs-CS, [27] except F-WS 2 , [12] MoS 2 [20] and metal-organic polymer. [28]However, these literatures did not report the corresponding distribution coefficient (K d ).Hence, we also compared the literatures that reported both adsorption capacity (Q max ) and distribution coefficient (K d ), as shown in Figure 5D.Our CF@S-CoWO 4 composite demonstrated a high Au(III) adsorption capacity of 1049.04 mg g À1 and high affinity K d (4.5 Â 10 6 mL g À1 ), which are better than previously reported materials including GCC51, [22] UiO-66-BTU, [9] CuS NPs, [27d] UiO-66-MTD,BCS, DONA-MOF, 2,5-TP, MTpPa-1, and CysR. [29]

Investigation of Adsorption Mechanism
From the SEM image (Figure 6A), it could be found that the morphology of CF@S-CoWO 4 hardly changed after adsorption,  which suggests its structural stability, and energy-dispersive Xray spectroscopy (EDS) spectrum in Figure 6B shows that, the atomic and weight percentages of Au elements after adsorption are as high as 13.88% and 33.26%, respectively, this result confirmed that gold ions were adsorbed in large quantities on CF@S-CoWO 4 .Moreover, TEM image shows that after adsorption, there is no structure changes, as shown in Figure 6C.In order to clarify which elements in CF@S-CoWO 4 contribute to the adsorption of gold, EDS element mapping was performed, as shown in Figure 6D-I.It can be found that the tungsten and sulfur elements in the composite play an important role in the adsorption of gold.In addition, the sample after adsorption of gold ions was further analyzed by XRD, it is observed from Figure 6J that five characteristic diffraction peaks appear at 38.3°(111), 44.5°(200), 64.7°(220), 77.6°(311), and 81.8°( 222), which correspond to metallic gold peaks according to JCPDS card file no.10a] More importantly, the X-ray photoelectron spectroscopy (XPS) spectra provide further evidence for the mechanism.In Figure 7A, a comparison before and after adsorption reveals the appearance of a new peak after adsorption, which corresponds to the characteristic peak of gold.To elucidate the oxidation state of the gold adsorbed on the composite, the peaks were further deconvoluted into two doublets of Au 4f 7/2 and Au 4f 2/5 by analyzing the Au 4f peak of the XPS spectrum (Figure 7B).The deconvoluted peaks located at 83.90 and 84.22 eV in the Au 4f 7/2 peak are attributed to Au(0) and Au(I), respectively.While these two oxidation states of Au are located at 87.59 and 87.91 eV in Au 4f 5/2 peak, respectively. [30]Meanwhile, according to the fitted peak ratio, the area percentage of Au(0) (61.41%) is higher than that of Au(I) (38.59%), this indicates that the reduction reaction is the main mechanism for this adsorption process.the Au(III) adsorption onto CF@S-CoWO 4 reached saturation, the Au(III) and Au(I) gradually reduced to Au(0).The reduction process of Au species is shown in the following Equation ( 3) and (4) For better understanding of the mechanism of the adsorptionreduction of gold ions on CF@S-CoWO 4 ,the XPS spectra of Co, W, O, and S elements in this composite were analyzed.By comparing XPS spectra of Co 2p (Figure 7C) before and after adsorption, it can be found that the two characteristic peaks at 782.64 and 788.33 eV before adsorption are attributed to Co 3þ and Co 2þ in the Co 2p 3/2 peak, respectively.After adsorption, the two characteristic peaks were shifted to 781.22 and 786.95 eV, respectively.The two characteristic peaks attributed to Co 3þ (798.42-798.51eV) and Co 2þ (798.11-798.24eV) in the Co 2p 1/2 peak were only slightly changed before and after adsorption.The above results suggest that the oxidation of Co species in this adsorption reaction induces electron transfer, allowing the gold ions to be reduced. [14]In the XPS spectrum of W 4f (Figure 7D), two peaks located at 35.45 eV (W 4f 3/2 ) and 37.59 eV (W 4f 5/2 ) before adsorption have both shifted to lower binding energy position with 0.15 eV and the total area has not changed after adsorption, suggesting that electron transfer also occurs on the W species. [31] As the binding energy of W 4f is slightly positively shifted after adsorption, indicating a decrease in the electron density of the W atom.At the same time, a significant negative shift in the binding energy of Co 2p 3/2 was observed, indicating an increase in the electron density of the Co atom.Therefore, we suggest that the redox loop between Co and W atoms promotes the adsorption performance.Figure 7E shows the O 1s high-resolution spectrum of CF@S-CoWO 4 , the spectra can be deconvoluted into three peaks at 530.53, 532.11, and 533.58 eV, corresponding to the lattice oxygen (O α ), the surface oxygen species (O β ), and the oxygen within the hydroxyl group(O γ ), respectively. [32]After Au(III) adsorption, the binding energy of the O α did not change, indicating that the O α is not involved in the adsorption of gold ions.While the binding energy of the O β and O γ moved to lower binding energy of 531.89 and 533.07 eV, respectively.The results indicate that these two adsorption sites undergo a chelation reaction with gold ions. [33]Note that the XPS spectra of Fe 2p (Figure S7, Supporting Information) also proved that the binding energy of Fe─O bond did not change.In the S 2p XPS spectrum (Figure 7F), two peaks before adsorption at 162.21 and 164.06 eV could be assigned to S 2p 3/2 and S 2p 1/2 of S 2À , while the two weak peaks at 168.09 and 169.32 eV corresponds to S 2 2À , indicating the partial oxidation of S 2À . [34]This indicates that sulfur atoms have replaced some of the oxygen atoms of CoWO 4 .
After adsorption of gold ions, these four peaks moved to 162.25, 164.84, 168.64, and 169.77eV, respectively.And the relative signal contribution of S 2À in the S 2p 3/2 peak decreased significantly from 40.7% to 20.5%, whereas that of S 2À in the S 2p 1/2 peak is accordingly increased from 34.2% to 17.1%, the phenomena confirm the strong Au 3þ -S 2À bonding interactions according to previous literature. [23]And two characteristic peaks area belonging to S 2 2À from 38.8% increased to 48.7%, which indicates that the electrons transferred from the Co─O─W oxo-bridge to the S sites enable the reduction of the adsorbed gold ions, so the signal contribution of S 2 2À from S 2À oxidation increases.
To explore in depth the effect of S atom doping on the Au adsorption capacity, the first-principle calculation was carried out using Vienna ab initio simulation package code, which was based on density functional theory (DFT).The specific calculation detail is described in the Supporting Information.The adsorption models and energies of Au on a) CoWO 4 (100), b) S-CoWO 4 (100) are shown in Figure 8. Theoretical results show that the adsorption energy of an Au metal on S-CoWO 4 (E ad = À1.46 eV) is more negative than CoWO 4 (E ad = À0.87 eV), which indicates the introduction of S atoms has more excellent stability and chemisorption efficiency than O atoms. [35]This is in good agreement with the experimental data.The adsorption energy of two reactions were all negative, it indicated that the adsorption processes were indeed influenced by coordination.
Based on the above analysis, the main adsorption-reduction mechanism of CF@S-CoWO 4 on Au(III) is proposed, as shown in Figure 9.That is, the gold ions are first adsorbed on the S atoms of CF@S-CoWO 4 , and then the electrons are transferred to the trivalent gold on the S site through the Co─O─W oxo-bridge, so that it gets electrons to be gradually reduced to monomeric gold.

Conclusion
In this work, a novel magnetic S-doped CoWO 4 composite (CF@S-CoWO 4 ) was successfully synthesized by hydrothermal method for the efficient recycling of gold.In the recovery process, only 5 mg of the composite has an adsorption capacity of 1049 mg g À1 at pH 3 and 298 K, and it still has high-distribution coefficient (K d = 4.4 Â 10 6 mL g À1 ) and efficiency (99.86%) for Au(III) adsorption in the presence of various metal ions (Pd 2þ Ca 2þ , Mg 2þ , Cd 2þ , Al 3þ , Li þ , and Ni þ ).And the antianion interference of CF@S-CoWO 4 follows the order: SO 4 2À > PO 4 3À > NO 3 À > CO 3 2À .Moreover, it also exhibits excellent reusability (81.78%) after four desorption-adsorption cycles.The kinetics and isotherm studies show that pseudosecond-order and Freundlich models could better explain the adsorption process, indicating that the reaction is homogeneous multilayer chemical adsorption.And thermodynamic studies show that adsorption is a spontaneous and irreversible endothermic reaction.In the study of this mechanism, it was found by characterization and DFT calculation that the reactions of Au(III) on the composite are chelation and reduction, and Au(III) has a better adsorption efficiency in the S sites than in the O sites.Consequently, CF@S-CoWO 4 is a promising candidate and has good prospects in industrial practice for recovering gold.

Experimental Section
Chemicals: The sources and grades of the chemicals and the characterization methods used in this study are provided in Supporting Information.
Material Synthesis: CoFe 2 O 4 (CF) was synthesized according to the previously reported work, and the details are described in the Supporting Information.The detailed procedure of as-prepared CF@S-CoWO 4 was as follows: (NH 4 ) 6 W 12 O 40 •xH 2 O (0.33 mM), thiourea (10, 20, and 30 mM), CoCl 2 (4.0 mM), F127 (0.02 mM), and CoFe 2 O 4 (2.5 mM) were weighed by analytical balance and mixed in a 150 mL of deionized water.After stirring with a glass rod, the mixture was completely dispersed by sonication for 10 min.Next, the solution was transferred to an autoclave and hydrothermally treated at 220 °C for 24 h.The black product was washed three times with deionized water and anhydrous ethanol alternately.Finally, the product was dried under a vacuum at 60 °C for 12 h.CF@CoWO 4 was prepared by the same procedure as above, except that thiourea was not added.Among them, the preparation method of CoFe 2 O 4 is provided in supporting information.
Metal Ion Adsorption Tests: Batch adsorption experiments are described in detail in Supporting Information.

Figure 3 .À
Figure 3. A) Affect of solution pH for Au(III) adsorption; B) zeta potential of CF@S-CoWO 4 and gold ions solution; C) effect of the thiourea concentration on the maximum adsorption capacity of Au(III) in the preparation of CF@S-CoWO 4 ; D) the effect of anions (NO 3 À , PO 4 3À , SO 4 2À , and CO 3 2À ) on the Au(III) adsorption by CF@S-CoWO 4 ; E) selectivity of CF@S-CoWO 4 for gold ions in simulated wastewater; F) adsorption ratio of CF@S-CoWO 4 for Au(III) as a function of adsorption-desorption cycle numbers.

Figure 4 .
Figure4.A) Impact of reaction time on adsorption capacity of CF@S-CoWO 4 ; B) the fitting curves of pseudo-first-order and pseudo-second-order kinetic models; C) impact of initial concentration on the adsorption capacity of CF@S-CoWO 4 ; D) fitting curves of Langmuir and Freundlich isotherm models.

Figure 5 .
Figure 5. A) Influence of temperature on adsorption capacity of CF@S-CoWO 4 ; B) Van't Hoff plots for Au(III) adsorption by CF@S-CoWO 4 ; C) comparison of maximum adsorption capacity among CF@S-CoWO 4 and other adsorbents; D) comparison of the adsorption capacity (Q max ) and distribution coefficient(K d ) of CF@S-CoWO 4 with those of other adsorbents .