Highly efficient ZnO/WO3 nanocomposites towards photocatalytic gold recovery from industrial cyanide-based gold plating wastewater

Discharging the gold-contained wastewater is an economic loss. In this work, a set of ZnO/WO3 was facile synthesized by hydrothermal method in order to recover gold from the industrial cyanide-based gold plating wastewater by photocatalytic process. Effect of ZnO contents coupled with WO3 was first explored. Then, effects of operating condition including initial pH of wastewater, type of hole scavenger, concentration of the best hole scavenger and photocatalyst dose were explored. A series of experimental results demonstrated that the ZnO/WO3 nanocomposite with 5 wt% ZnO (Z5.0/WO3) depicted the highest photocatalytic activity for gold recovery due to the synergetic effect of oxygen vacancies, a well-constructed ZnO/WO3 heterostructure and an appropriate band position alignment with respect to the redox potentials of [Au(CN)2]− and hole scavengers. Via this ZnO/WO3 nanocomposite, approximately 99.5% of gold ions was recovered within 5 h using light intensity of 3.57 mW/cm2, catalyst dose of 2.0 g/L, ethanol concentration of 20 vol% and initial pH of wastewater of 11.2. In addition, high stability and reusability were observed with the best nanocomposite even at the 5th reuse. This work provides the guidance and pave the way for designing the ZnO/WO3 nanocomposite for precious metal recovery from a real industrial wastewater.

their actual application is strictly restricted by their specific drawbacks.For example, the electrolysis is the energy intensive process with low metal selectivity, while the ion exchange process is costly and requires the precise control 14 .Both adsorption and chemical oxidation usually produce the waste product or sludge, thus requiring the downstream process to complete the waste treatment 14 .
Besides all mentioned processes, the photocatalytic process is another promising process that can simultaneously remove free cyanide and recover gold ions in metallic form, which can be reused in subsequent processes.As brief, the photocatalytic process is a chemical-based process that can be proceeded via the joint operation between appropriate intensive light and semiconductor (photocatalyst) 15 .When the photocatalyst (S) is irradiated by the suitable light, the electrons will transfer from valance band (VB) to conduction band (CB), leaving the photogenerated holes (reaction (R1)) 16,17 .The contained [Au(CN) 2 ] − species can readily react with the photogenerated electrons at CB, yielding the metallic gold deposited on the surface of photocatalyst and free cyanide (CN − ) (reaction (R2)) 18 .Meanwhile, the contained hydroxide ions (OH − ) can react with the photogenerated holes to produce the hydroxyl radicals (OH • ) (Reaction (R3)).Subsequently, the formed OH • can consecutively react with the released free cyanide ions (CN − ), yielding the cyanate species (OCN − ) as the main product (reaction (R4)) Based on the proposed mechanism, it can be noticed that the photocatalytic reduction of [Au(CN) 2 ] − can well proceed in the basic environment.Nevertheless, the reduction reaction of [Au(CN) 2 ] − is still sluggish due to its high negative reduction potential (− 0.57 V/NHE 19 ), thus limiting the use of variety photocatalysts.That is, only the photocatalysts that have a more positive VB position than the oxidation potential of OH − and also have a more negative CB position than the reduction potential of [Au(CN) 2 ] − can complete the photocatalytic gold recovery from the metal-cyanocomplexes contained solutions such as ZnO 20,21 , ZnS 22 , TiO 2 and TiO 2 -based materials 23,24 .Among all explored photocatalysts, it is noteworthy that TiO 2 or TiO 2 -based materials exhibited the best photocatalytic activity, probably due to its good surface property and high thermal-chemical resistance 25,26 .Nevertheless, the popular used materials have a considerably high cost, which is not practical in actual operation.Therefore, the development of cheap and efficient photocatalyst for gold recovery is a key issue in this circumstance.
Tungsten trioxide (WO 3 ) is an attractive semiconductor for the photocatalytic applications, owing to its non-toxic, biocompatibility, inexpensive, bandgap tunability, high electron storage capability and high stability in violent conditions 27,28 .Besides, it possesses a narrow bandgap (2.4-2.8 eV), which can theoretically absorb almost 12% of solar light 29 .Nevertheless, it still has a poor photocatalytic activity due to its low kinetics, sluggish charge transfer, high recombination rate of photogenerated charge carriers 29,30 .To overcome these drawbacks, many strategies have been carried out to improve the photocatalytic activity of WO 3 by structural modification, co-catalyst hybridization, metal doping and heterojunction [29][30][31][32] .Among all developed strategies, the hybrid heterojunction is recognized as the most effective strategy to improve the photocatalytic performance of WO 3 because of the formation of a more negative CB, which can suppress the rate of charge recombination 29 .Several materials have been selected to hybrid with WO 3 such as Ag 3 PO 4 , AgI, TiO 2 , RGO, g-C 3 N 4 , Bi 2 WO 6 30,32 .Hybridizing WO 3 with ZnO is currently gaining attention due to the improvement of surface acidity of resultant nanocomposites and visible light absorption as well as the improved photocatalytic activity of the single counterpart 28 .Nevertheless, the ZnO/WO 3 nanocomposites were often applied for the photocatalytic degradation of pollutants.For examples, the ZnO/WO 3 nanocomposite synthesized by hydrothermal method displayed a high charge separation and a redshift of light absorption compared with the counterpart semiconductors due to W-O-Zn linkage formation, which can promote a high photocatalytic degradation of methyl orange (MO) 33 .The ZnO-WO 3 synthesized by ultrasound-assisted synthesis exhibited a great visible light response as well as the degradation of NO x , approximately twice as high as that of conventional composites 34 .The ZnO/WO 3 photocatalyst synthesized by a calcination method induced the formation of the immanent electric field between both semiconductors, which can drive the Z-scheme charge transfer mechanism for photocatalytic degradation of MO, rhodamine B (RhB) and bisphenol A (BPA) 35 .The addition of ZnO on WO 3 induced by the Pechini sol-gel method induced the creation of a W-O-Zn bond, which can enhance the UV-Vis light absorption, reduce the rate of charge recombination, increase the specific surface area and promote the photocatalytic desulfurization of thiophene 36 .The ZnO/WO 3 photocatalyst synthesized by hydrothermal and calcination methods exhibited the outstanding photocatalytic H 2 O 2 production due to the generation of interfacial internal electric field (IEF) in the S-scheme heterojunction, which can further suppress the charge recombination and empower the electrons to participate the photocatalytic reaction 37 .A ZnO@WO 3 nanocomposite synthesized by the hydrothermal method exhibited a short rod-like structure with a long lifespan of charge carriers and a low charge transfer resistance 38 .Besides, hybridizing WO 3 with ZnO importantly changed the forbidden band width of the nanocomposites, thus improving the visible light response and the photocatalytic degradation of RhB.The WO 3 /ZnO composite synthesized by the co-precipitation and ultra-sonication routes exhibited the synergetic effect of counterpart semiconductors to increase the charge separation and reduce the charge recombination, which can remarkably improve the photocatalytic degradation of methylene blue (MB) and RhB 39 .
Based on our literature survey, the photocatalytic applications of ZnO/WO 3 nanocomposites for the metal removal/recovery from wastewater are still limited.Here, a set of ZnO/WO 3 nanocomposites was hydrothermally synthesized and used as the photocatalyst for gold recovery from the industrial cyanide-based gold plating wastewater.Effects of various parameters were explored in order to find the best ZnO/WO 3 photocatalyst and the best operating condition.A series of experimental results realized that the as-synthesized Z 5.0 /WO 3 nanocomposite exhibited the highest photocatalytic activity to recover gold than the pristine semiconductors due to the synergetic effect of generated defects, a well-developed heterojunction and an appropriate alignment of band position with respect to the redox potential of active species.Besides, it possessed the excellent stability and reusability for the photocatalytic gold recovery.

Experimental
Synthesis of ZnO/WO 3 nanocomposites A series of ZnO/WO 3 nanocomposites was synthesized by a facile hydrothermal method adopted from Zaw et al. 40 .Briefly, a certain quantity of zinc nitrate 6-hydrate (Zn(NO 3 ) 2 •6H 2 O, KemAus) was dissolved in 11.4 mL of i-propanol (i-C 3 H 7 OH, QRëC) and 50 mL of distilled (DI) water.Approximately 2 g of tungsten (VI) oxide (WO 3 , Sigma Aldrich) was subsequently added under the thorough stirring at 400 rpm at 25 °C for 4 h.Next, 1 mL of 3 M of hydrochloric acid (HCl, Merck) was gradually added.The slurry was stirred consecutively at the same stirring rate and temperature for 1 h.Subsequently, the obtained slurry was meticulously transferred into Teflon-lined stainless-steel autoclave and hydrothermally treated at 200 °C for 2 h.After a natural cooling, the solid portion was separated from the aqueous solution by centrifugation at 11,000 rpm (5804R, Eppendorf) for 10 min, washed several times by ethanol and DI water and finally dried overnight at 100 °C.The ready-to-use ZnO/WO 3 nanocomposites at different weight contents of ZnO, denoted as Z x /WO 3 (x is the weight percent of ZnO) were gained after calcination in air at 400 °C for 2 h.
Characterization of ZnO/WO 3 nanocomposites The morphologies and optical properties of synthesized ZnO/ WO 3 nanocomposites were analyzed as follows.The external microstructure and elemental distribution were observed by a field emission scanning electron microscopy (FE-SEM, JSM7610FPlus, JEOL) attached by an energy dispersive X-ray (EDS, ULTIM MAX 65) spectrometer.The interplanar spacing was monitored by a highresolution transmission electron microscopy (TEM, JEM-3100F, JEOL) with an accelerating voltage of 300 kV.The crystallographic structure and local symmetry were evaluated by an X-Ray diffractometer (XRD, Bruker D2 Phaser) using Cu Kα X-ray and Raman spectroscopy (Perkin Elmer Spectrum GX).The Brunauer-Emmett-Teller (BET) surface area was examined by N 2 adsorption/desorption according to the Barrett-Joyner-Halenda (BJH) technique at 77 K using a gas adsorption analyzer (Quantachrome® ASiQwin™).The surface element and electronic state was examined by an X-Ray photoelectron spectrometer (XPS, Axis Ultra, Kratos) with a delay line detector (DLD) and a monochromatic Al Kα (hν = 1486.6eV) source.The optical absorption property was recorded by an UV visible near infrared spectrometer (Perkin Elmer Lambda 95).The behavior of photoinduced charge carriers was elucidated by a photoluminescence (PL) spectrometer (Perkin-Elmer LS-55) in air at 298 K with an excitation wavelength of 286 nm.The qualitative unpaired electron and defects were determined by an electron paramagnetic resonance spectroscopy (EPR, model, EMXmicro, Bruker) at 298 K.
Photocatalytic activity of ZnO/WO 3 nanocomposites The photocatalytic activity of all synthesized ZnO/WO 3 nanocomposites was assessed for the gold recovery from the industrial cyanide-based gold plating wastewater collected from the circuit board manufacturing industry in Thailand.The concentrations of all metal ions were first measured by an inductive coupled plasma mass spectrometry (ICP-MS, PerkinElmer, NexION 2000).In each batch, the required quantity of explored photocatalyst was dispersed in 300 mL of the cyanide-based gold plating wastewater in cylindrical glass photoreactor having an inside diameter of 9 cm and height of 7 cm.The solid-wastewater mixture was thoroughly stirred at 400 rpm for 30 min in the absence of irradiated light to allow the uniform dispersion and adsorption of gold species on the photocatalyst surface.Subsequently, the reactor was irradiated by a high-pressure mercury lamp (400 W, 200-600 nm, RUV 533 BC).The distance between the reactor and light source was fixed at 28.5 cm, getting the light intensity of 3.57 mW/cm 2 .At particular time, approximate 5 mL of mixture was sampled and centrifuged at 11,000 rpm to separate the solid catalyst from the processed wastewater.The concentration of gold ions in the processed wastewater was analyzed by the flame atomic absorption spectrometry (Flame-AAS, Analyst 200 + flas 400; Perkin-Elmer).The percentages of gold recovery at particular time were estimated according to Eq. (1).
Where R is the gold recovery percentage, m o is the initial mass of gold ions in wastewater and m t is the mass of gold ions at time t.

Results and discussion
The photocatalytic activity of all synthesized ZnO/WO 3 nanocomposites was tested for gold recovery from the industrial cyanide-plating bath wastewater.The original wastewater depicted a weak basic property with the initial pH of 9.02-9.11.It contained gold ions at the concentration of 8-10 mg/L together with a trace quantity of copper ions (Cu 2+ ), nickel ions (Ni 2+ ), potassium ions (K + ) and zinc ions (Zn 2+ ) of less than 0.00006, 0.00047, 0.20285 and 2.27 mg/L, respectively.
To further confirm the crystal structures of ZnO/WO 3 nanocomposites, the Raman spectroscopy was carried out.As shown in Fig. 2(b), the pure ZnO sample exhibited a sharp peak centered at 435.5 cm −1 , arising from the oxygen-sub-lattice of hexagonal wurtzite ZnO sample 42 .The pure WO 3 displayed the Raman spectrum at 135.5 cm −1 due to the lattice vibration, at 272.2 and 327.5 cm −1 due to the W-O-W bending mode of bridging oxide ions and at 715.5 and 805.7 cm −1 due to the W-O-W stretching mode [42][43][44] .All synthesized ZnO/WO 3 nanocomposites illustrated a predominate Raman spectra of WO 3 with different intensities.Typically, an intensive www.nature.com/scientificreports/sharp Raman peak represents a high crystallinity of material 45 .Among all synthesized ZnO/WO 3 nanocomposites, the Z 5.0 /WO 3 nanocomposite exhibited the most intense and sharp Raman spectra, indicating its highest crystallinity.A blue shift of Raman spectra of all synthesized ZnO/WO 3 was observed compared to those of pure WO 3 , indicating the existence of lattice defects 46 .
As mentioned elsewhere, the surface properties of photocatalyst may affect the adoption of active molecule and subsequent charge transfer 16 , the surface area of all synthesized ZnO/WO nanocomposites was evaluated by the N 2 adsorption/desorption isotherms.As demonstrated in Fig. 3, all samples exhibited the Type IV isotherm, which was the characteristic of mesoporous materials based on the recent IUPAC classification.A greatly increase of N 2 adsorption volume at a relative pressure at 0.99 was attributed to the capillary condensation, which was a well index of a high homogeneity of synthesized samples.Quantitatively, the BET surfaces of pristine ZnO and WO 3 were 17.8 and 8.8 m 2 /g, respectively, while those of Z 2.5 /WO 3 , Z 5.0 /WO 3 , Z 10 /WO 3 , Z 15 /WO 3 and Z 25 / WO 3 were 5.2, 7.9, 14.7, 17.3 and 22.0 m 2 /g, respectively.An increased BET surface area as the increased ZnO contents in nanocomposites might be caused by the increase of well-dispersed ZnO NPs on the WO 3 surface.
To explore the chemical surface composition and electronic state, the XPS analysis was carried out using C 1s peak of 284.6 eV as the reference position.As depicted in Fig. 4a, the survey XPS spectra of ZnO, WO 3 and ZnO/WO 3 nanocomposite showed the clear spectra of Zn 2p, O 1s and W 4f peaks and also the reference C www.nature.com/scientificreports/1s peak.For the O 1s XPS spectra (Fig. 4b), the ZnO possessed asymmetric broad peak, centered at ~ 531 eV (Fig. 4b).After deconvolution, two symmetric peaks appeared at 529.81 and 531.57eV, corresponding to the Zn-O-Zn and surface -OH species 35,48 , respectively.The O 1s XPS spectra of WO 3 were found at 529.89, 531.94 and 533.84 eV, belonging to the lattice oxygen, oxygen-absorbing H 2 O/adsorb oxides and surface -OH groups, respectively 49,50 .The O 1s XPS spectra of both nanocomposites displayed three principal peaks of lattice oxygens and adsorbed oxygens, which slight negative shift with respect to WO 3 .For the W 4f XPS spectra (Fig. 4c), two symmetric peaks at 35.26 and 37.37 eV were observed for WO 3 , respectively assigning to the W 4f 7/2 and W 4f 5/2 components, corresponding to the W 6+ oxidation state 51 .Both Z 5.0 /WO 3 and Z 10 /WO 3 composites displayed the spectra of both W 4f 7/2 and W 4f 5/2 components with slight negative shifts by 0.01 and 0.12 eV and 0.06 and 0.10 eV compared with WO 3 , respectively.For the Zn 2p XPS spectra (Fig. 4d), the ZnO exhibited two XPS peaks at 1021.40 and 1044.50 eV, respectively assigning to the Zn 2p 3/2 and Zn 2p 1/2 components 52 .The positive shifts of either Zn 2p 1/2 or Zn 2p 3/2 elements were observed by 0.35 and 0.35 eV for Z 5.0 /WO 3 and 0.07 and 0.10 for Z 10 /WO 3 .The positive shift of Zn elements and negative shift of W and O elements well indicated the interfacial electron transfer from ZnO to WO 3 after hybridization 35,37 .Theoretically, the electron movements generally induce the formation of IEF at the ZnO-WO 3 interface, which allows the transfer of electrons from WO 3 to ZnO in the presence of light illumination according to the S-scheme mechanism 37,53,54 .
To investigate intrinsic defects in all synthesized samples, the EPR was conducted.Typically, the EPR signal ascribes the presence of unpaired electrons of the oxygen vacancies 50 .A high intensity of EPR signal indicates a high concentration of bulk oxygen vacancies 55,56 .As plotted in Fig. 5, the pristine ZnO displayed a very strong EPR signal at g-value of ~ 1.96, assigning to the singly ionized oxygen vacancy 57 .On the other hands, the pristine WO 3 depicted a very weak EPR signal at g-value of ~ 2.01 (inset figure), suggesting a low defect quantity in its structure.Interestingly, all ZnO/WO 3 nanocomposites exhibited strong EPR signals at g-value of ~ 2.01 indicating the formation of bulk oxygen vacancies in WO 3 counterpart after hydrothermal and calcination, reasonably agreement with the Raman results.The highest EPR intensity was observed for the Z 2.5 /WO 3 and Z 5.0 /WO 3 nanocomposites, suggesting their comparable abundance of oxygen vacancies.
To elucidate the energy level and light absorption performance, the UV-Vis near infrared spectroscopy was carried out.As presented in Fig. 6a, pure ZnO NPs possessed a strong absorbance at the wavelength below 400 nm, indexing an excellent UV light absorption performance, while pure WO 3 NPs exhibited the absorption band edge at 480 nm, indicating an excellent absorption performance during the UV light and short-wave visible light.All synthesized ZnO/WO 3 nanocomposites demonstrated the blueshift of absorption band edge with respect to the WO 3 sample.The absorption intensity during the short-wave visible light decreased as the increase of ZnO contents.Via the use of Tauc equation (Eq.2), plot of (αhv) 1/n and hv allowed to determine the value of bandgap (E g ) by a linear extrapolation to the x-axis 58 .
where h is Plank's constant, v is the photon frequency, α is the absorption coefficient, E g is the band gap, A is the proportional constant, and n is a factor related to the nature of the electron transition (½ for direct and 2 for indirect transition).From Tauc plots, the pure ZnO and WO 3 NPs possessed the bandgap energy of 3.24 and 2.66 eV, respectively and all synthesized ZnO/WO 3 nanocomposites exhibited the bandgap between that of ZnO and WO 3 counterparts.That is, the Z 2.5 /WO 3 , Z 5.0 /WO 3 and Z 10 /WO 3 nanocomposites possessed the similar bandgap of 2.69 eV, while the Z 15 /WO 3 and Z 25 /WO 3 nanocomposites depicted the bandgap of 2.71 and 2.73 eV, respectively.The  www.nature.com/scientificreports/VB edge and CB edge of pure ZnO and WO 3 samples can be acquired from the following empirical equations (Eqs.3-4) 29,35 , providing the E VB of 2.92 and 3.43 eV and E CB of − 0.32 and 0.77 eV for ZnO and WO 3 , respectively.
where E CB and E VB are the potential of CB and VB edges, respectively, E e is the energy of free electron (4.5 eV) and X e is the electronegativity of ZnO and WO 3 (5.80 and 6.60, respectively 35 ).
The qualitative recombination rate of photogenerated charge carriers of all synthesized ZnO/WO 3 nanocomposites was explored via the PL spectrometer.Typically, the stronger the PL intensity, the higher the rate of charge recombination and vice versa 35 .For pure samples, as shown in Fig. 6b, the ZnO sample possessed the PL spectra during the UV and visible regions, and were higher than that of WO 3 , indicating a faster recombination rate of charge carriers.Among all ZnO/WO 3 nanocomposites, the Z 5.0 /WO 3 possessed the lowest PL intensity compared with other nanocomposites, suggesting its lowest recombination rate of charge carriers.This might be attributed to the presence of oxygen vacancies which can act as the electron trapping sites and extend the lifetime of electron-hole pairs as well as suppress the rate of charge recombination 35,59 .

Photocatalytic activity of ZnO/WO 3 nanocomposites
Effect of coupled ZnO contents The photocatalytic activity of all synthesized ZnO/WO 3 nanocomposites was explored for gold recovery from the cyanide-based gold plating wastewater.As illustrated in Fig. 7a and Fig. S1, both pure ZnO and pure WO 3 NPs exhibited a comparable adsorption capacity of gold ions (2.41 and 1.97% for ZnO and WO 3 , respectively).In the presence of irradiated light, the ZnO sample possessed a total photocatalytic activity for gold recovery up to 31.8%, approximately 1.6-time higher than that of WO 3 .A significant improved photocatalytic gold recovery was observed via the use of ZnO/WO 3 nanocomposites.That is, total quantity of gold recovery was increased from 51.4 to 75.2% as the increase of ZnO content from 2.5 to 5.0 wt%.Further increasing the weight content of coupled ZnO to 75 wt% lessened the total quantity of gold recovery to 34.5%.The highest photocatalytic performance accounted from both adsorption (~ 10.7%) and photocatalytic reaction (~ 64.5%) was achieved at 75.2% via the use of Z 5.0 /WO 3 nanocomposite.The rate of photocatalytic gold recovery was then fitted by the Langmuir-Hinshelwood model as expressed by Eq. ( 5) 60 .A plot of ln(C t /C 0 ) versus reaction time t provides a negative slope, which allows to estimate the pseudo-first order reaction rate constant.Based on www.nature.com/scientificreports/ the model fitting (Fig. 7b), the Z 5.0 /WO 3 nanocomposite possessed the highest value of pseudo first-order rate constant of 0.1876 h −1 , approximately 3.4 and 7.1 times higher than that of ZnO and WO 3 , respectively.
where C 0 is the concentration of gold ions at initial time, C t is the concentration of gold ions at particular time t and k is the pseudo first-order rate constant.Due to fact that the photocatalytic process is consisted of two sequential steps; adsorption and photocatalytic reaction, the adsorption experiment was also conducted for ZnO, WO 3 and Z 5.0 /WO 3 photocatalysts under the dark environment.As illustrated in Fig. S2, approximately 14.7, 11.2 and 15.31% of gold ions were removed by adsorption in the presence of ZnO, WO 3 and Z 5.0 /WO 3 photocatalysts, respectively.This suggested the main contribution of photocatalytic reaction for gold recovery compared with the adsorption.
As well known that the catalytic performance of photocatalyst is tightly related to the textural property, morphology, optical property and also the charge separation efficiency.Based on the obtained results, it seemed to be that the photocatalytic gold recovery of ZnO/WO 3 photocatalysts did not depending on the crystal morphology and textural property.However, it can be presumed that the generated oxygen vacancies played a profound effect on the photocatalytic gold recovery of Z 5.0 /WO 3 nanocomposite.This is because the generated oxygen vacancies can serve as the electron trapping sites, which can extend the lifetime of e − -h + pairs and alleviate their recombination rate, which are beneficial for the photocatalytic performance.Besides, the heterojunction between WO 3 and ZnO can act as the electron transfer bridge, which thereby extends the lifetime of charge separation as well as suppress the rate of charge recombination 35,37,61 .
Effect of initial pH of wastewater The effect of initial pH of wastewater on the photocatalytic gold recovery was explored in the range of 7.02-11.2via the Z 5.0 /WO 3 nanocomposite.As displayed in Fig. 9a and Fig. S3, the quantity of adsorbed gold ions slightly increased as the increase of initial pH of wastewater from 7.02 to 9.11 and dropped afterward.The variation of ionic gold adsorption might be attributed to the interaction between the surface charges of photocatalyst and forms of gold cyanide complexes.That is, the semiconductors generally demonstrate positive surface charges the pH is less than the zero-point charge pH (pH ZPC ) and exhibit negative surface charges when the solution pH is more than the pH zpc value 65 and the gold cyanide complexes usually exhibit the stable form as [Au(CN) 2 ] − over a whole pH range at 25 °C and 1 atm 66 .In this case, the pH zpc value of Z 5.0 /WO 3 is ~ 8.0 (Fig. S4).A poor adsorption capacity at pH < pH zpc was probably attributed to the positive charge repulsion between H + to approach the W-OH surface 67 .A high adsorption capacity at pH of 9.11 may be due to the formation of a weak repulsive force between the surface charges and the gold cyanide species, which still allowed an effective adsorption of active species on the surface of Z 5.0 /WO 3 nanocomposite.However, a repulsive force became higher in a strong basic solution which can diminish the adsorption of active species.In the presence of irradiated light, the photocatalytic gold recovery increased as the increase of initial pH of wastewater, providing the pseudo first-order rate constants of 0.0087, 0.0168, 0.1876, 0.2535 and 0.4464 h −1 for pH 7.02, 8.07, 9.11, 10.1 and 11.2 respectively (Fig. 9b).A high photocatalytic gold recovery in a strong basic environment might be attributed to the fact that the contained OH − species can effectively react with the photogenerated h + to form the OH • at VB, thus effectively suppressing the charge recombination as well as promoting the photocatalytic performance for gold recovery.
Effect of hole scavenger types and concentrations As mentioned elsewhere, the addition of hole scavengers can improve the photocatalytic metal removal by the hole interception, thus allowing the photogenerated electrons to bound the adsorbed gold ions to proceed the photocatalytic reaction 69 .In this part, the effect of different hole scavengers including methanol (CH 3 OH), ethanol (C 2 H 5 OH) and isopropanol (i-C 3 H 7 OH) on the photocatalytic gold recovery was explored via the Z 5.0 /WO 3 nanocomposite.As shown in Fig. 10a and Fig. S5, approximately 49% of gold ions was recovered within 5 h in the absence of hole scavenger.An improved photocatalytic activity for gold recovery was obviously observed in the presence of hole scavengers.The gold recovery percentages increased as the order of ethanol > methanol > i-propanol > no hole scavenger, providing the pseudo first-order rate constants of 0.4464, 0.2687, 0.1835 and 0.1240 h −1 , respectively (Fig. 10b), probably attributed to the effect of different oxidation potentials of each hole scavenger.That is, the lower the oxidation potential, the better the hole scavenger performance 70,71 .On the basis of different oxidation potentials; methanol (0.016 V/NHE 71 ), www.nature.com/scientificreports/ethanol (0.084 V/NHE 71 , i-propanol (0.105 V/NHE 71 ), the methanol-contained system should illustrate the highest photocatalytic gold recovery.Bases on the obtained results, the exception from the trend indicated that there are some other factors that may play a crucial role on the photocatalytic gold recovery.A low photocatalytic activity of methanol might be attributed to a strong surface adsorption of formed intermediate species.Based on the find out of Santasalo-Aarnio et al. 72 , both formate and bonded CO were the main intermediate species generated in the methanol oxidation, which can adsorb and block the active sites of photocatalyst to complete the photocatalytic reaction 71 .In the presence of ethanol, it can react with the photogenerated h + , yielding C 2 H 4 O as the reaction product 73 .Figure 11a and Fig. S6 illustrated the effect of ethanol concentrations in the range of 10-30 vol% on the photocatalytic gold recovery.It is noteworthy that the increase of ethanol concentrations from 10 to 20 vol% led to the increase of gold recovery from 70.4 to 99.5%.However, further increasing the ethanol concentration to 30 vol% diminished the gold recovery.In terms of pseudo first-order rate constants, they were 0.1741, 0.1874, 0.4464, 0.0751 and 0.0367 h −1 at ethanol concentrations of 10, 15, 20, 25 and 30 vol%, respectively (Fig. 11b).A poor performance of gold recovery at low ethanol concentration was probably attributed to a fast recombination rate of charge carriers due to the limitation of hole scavenger quantity.In presence of high ethanol concentration, excess ethanol molecules may compete to each other or other active molecules to complete the oxidation reaction 74 and probably hinder the adsorption of other reactive species on active sites, which in turn reduced the photocatalytic performance.A poor photocatalytic activity in the presence of inappropriate content of hole scavengers was also reported for other photocatalytic systems [74][75][76] .
Effect of photocatalyst doses The effect of photocatalyst doses on the photocatalytic gold recovery was also explored as depicted in Fig. 12a and Fig. S7.It is worth noting that the gold recovery increased from 77.6 to 99.5% as the increase of photocatalyst doses from 1.0 to 2.0 g/L.However, further rising the photocatalyst dose to 2.5 g/L decreased the photocatalytic performance.The system with the photocatalyst dose of 2.0 g/L exhibited the pseudo first-order rate constant of 0.4464 h −1 , approximately 2.1, 1.4 and 3.5 times higher than that of 1.0, 1.5 and 2.5 g/L, respectively (Fig. 12b).Typically, a high photocatalyst dose usually exhibits a high active surface area and electrons, which can facilitate an effective photocatalytic reaction 76 .However, too high photocatalyst dose induced a high suspension turbidity, consequently causing a low light penetration and light scattering 77 .In addition, a high photocatalyst dose usually conducted a non-homogeneous suspension because of the photocatalyst agglomeration 78 .
The comparative efficacy of gold recovery from gold-cyanide complexes was finally conducted between the as-synthesized Z 5.0 /WO 3 nanocomposite and other photocatalysts.As shown in Table 1, different works depicted different percentages of photocatalytic gold recovery, attributing to their different utilized matrix/wastewater as well as operating conditions to test the photocatalytic performance.The photocatalytic performance of Z 5.0 /WO 3 is seemed to be lower than ZnO, diagnosing from the use of longer operating time with low initial concentration 10.Effect of hole scavenger types on (a) photocatalytic gold recovery of Z 5.0 /WO 3 nanocomposite at irradiation time of 5 h and (b) linear variation of ln (C t /C 0 ) versus time (t) using light intensity of 3.57 mW/cm 2 , catalyst dose of 2.0 g/L, hole scavenger concentration of 20 vol% and initial pH of wastewater of 11.2. of gold ions.Via rGO/TiO 2 photocatalyst, it is hard to compare the between them because they used the gold ions with high concentration (300 mg/L).Under almost similar working conditions, the photocatalytic performance of Z 5.0 /WO 3 nanocomposite was comparable to TiO 2 /WO 3 , but lower than that of Au/TiO 2 .
Mechanism of gold recovery by ZnO/WO 3 nanocomposites The reaction mechanism of photocatalytic gold recovery from the cyanide-based gold plating wastewater via ZnO/WO 3 nanocomposites was roughly proposed on the basis of obtained results and literature.According to the density functional (DFT) calculation of He et al. 35 , the heterojunction of ZnO and WO 3 induced the positive electric surface on ZnO and negative electric surface on WO 3 , thus forming the p-n junction where ZnO and WO 3 were the n-type and p-type, respectively.Via this junction, electrons can transfer from ZnO to WO 3 , while the holes can migrate from WO 3 to ZnO after hybridization 28,33,35,79 .In this work, the transfer of electrons from ZnO to WO 3 was clearly evidenced by a positive shift of Zn 2p XPS spectra and a negative shift of W 4f XPS and O 1s XPS spectra (Fig. 4b-d).Based on the calculated E VB and E CB of ZnO and WO 3 according to Eqs. ( 3)-( 4), the ZnO had a more negative value of E CB than that of WO 3 (− 0.32 and 0.77 eV for ZnO and WO 3 , respectively).According to a low Fermi energy levels of WO 3 (− 4.52 eV 37 ) compared with ZnO (− 4.29 eV 37 ), the electrons can certainly transfer from ZnO to WO 3 after heterojunction to equilibrate the Fermi levels of both counterparts (Scheme 1a).Such electron movements induced the build-up of IEF at the ZnO-WO 3 interface directing from ZnO to WO 3 35,37 .In the presence of irradiation, the generated IEF can accelerate the recombination of weak redox power of electrons at CB of WO 3 and holes at VB of ZnO according to the S-scheme mechanism 37,53,54 , thus allowing a spatial separation of strong redox power of electrons at CB of ZnO and holes at VB of WO 3 to proceed the photocatalytic reaction.Therefore, the photoreduction of [Au(CN) 2 ] − to metallic gold (reaction (R2)) occurred at the CB of ZnO, while photooxidation of OH − (reactions (R3)-(R4)) and C 2 H 5 OH emerged at the VB of WO 3 as proposed in Scheme 1b.This statement was clearly confirmed by a high photocatalytic gold recovery of ZnO compared with that of WO 3 (Fig. 7).
Reusability of ZnO/WO 3 nanocomposites The reusability of Z 5.0 /WO 3 nanocomposite was also examined for the photocatalytic gold recovery for several consecutive runs.As demonstrated in Fig. 13, no noticeable change of the photocatalytic gold recovery was observed even at the 5th reuse (99.4%).This indicated a high stability and reusability of the hydrothermally synthesized ZnO/WO 3 nanocomposites.The resultant Au decorated ZnO/ WO 3 (Au/ZnO/WO 3 ) obtained from this work may be used for other applications such as desulfurization of thiophene 36 , H 2 O 2 production 37 , or gas sensing 41 .However, the application of resultant Au/ZnO/WO 3 was not conducted in the work due to it beyonds the scope.www.nature.com/scientificreports/

Conclusions
A set of ZnO/WO 3 nanocomposites was facile synthesized by hydrothermal method for photocatalytic gold recovery from the industrial cyanide-based gold plating wastewater.An appropriate content of ZnO NPs in the ZnO/WO 3 nanocomposites was first explored and subsequently followed by the investigation of optimum operating condition.The experimental results demonstrated that the ZnO/WO 3 nanocomposites exhibited a higher photocatalytic gold recovery than both pristine counterparts.The Z 5.0 /WO 3 nanocomposite possessed the highest photocatalytic activity for gold recovery due to the synergetic effect of oxygen vacancies and the formed S-scheme heterojunction, which can serve as electron trapping sites to extend the lifetime of electron-hole pairs and suppress the combination rate of charge carriers.Besides, appropriate band position alignment of nanocomposite with respect to the redox potential of gold-cyanide species encouraged the reduction of [Au(CN) 2 ] − at CB of ZnO and the oxidation of hole scavengers at VB of WO 3 .Via the Z 5.0 /WO 3 nanocomposite, approximately 99.5% of gold ions was recovered within 5 h using the light intensity of 3.57 mW/cm 2 , catalyst dose of 2.0 g/L, ethanol concentration of 20 vol% and initial pH of wastewater of 11.2.In addition, it possessed a high stability https://doi.org/10.1038/s41598-023-49982-6

( 3 )Figure 7 .
Figure 7. Effect of photocatalyst types on (a) photocatalytic gold recovery at irradiation time of 5 h and (b) linear variation of ln (C t /C 0 ) versus time (t) using light intensity of 3.57 mW/cm 2 , catalyst dose of 2.0 g/L, 20 vol% C 2 H 5 OH and initial pH of wastewater of 9.11.

Figure 9 .
Figure 9.Effect of initial pH of wastewater on (a) photocatalytic gold recovery of Z 5.0 /WO 3 nanocomposite at irradiation time of 5 h and (b) linear variation of ln (C t /C 0 ) versus time (t) using light intensity of 3.57 mW/cm 2 , catalyst dose of 2.0 g/L and 20 vol% C 2 H 5 OH.

Figure 11 .
Figure 11.Effect of ethanol concentrations on (a) photocatalytic gold recovery of Z 5.0 /WO 3 nanocomposite at 5 h and (b) linear variation of ln (C t /C 0 ) versus time (t) using light intensity of 3.57 mW/cm 2 , catalyst dose of 2.0 g/L and initial pH of wastewater of 11.2.

Figure 12 .
Figure 12.Effect of Z 5.0 /WO 3 nanocomposite doses on (a) photocatalytic gold recovery at 5 h and (b) linear variation of ln (C t /C 0 ) versus time (t) using light intensity of 3.57 mW/cm 2 , ethanol concentration of 20 vol% and initial pH of wastewater of 11.2.

Scheme 1 .
Scheme 1. Proposed mechanism of photocatalytic gold recovery from the cyanide-based gold plating wastewater via ZnO/WO 3 nanocomposites in the (a) absence and (b) presence of UV-Vis irradiation.

Figure 13 .
Figure 13.Reusability of Z 5.0 /WO 3 nanocomposite for gold recovery at 5 h using light intensity of 3.27 mW/ cm 2 , catalyst dose of 2.0 g/L, ethanol concentration of 20 vol% and initial pH of wastewater of 11.2.

Table 1 .
Comparative efficacy of gold recovery from gold-cyanide complexes by photocatalytic process.*Not detected.