Toward Eco‐Friendly E‐Waste Recycling: New Perspectives on Ozone‐Assisted Gold Leaching

Global demand for more effective methods to reclaim gold from electronic waste (E‐waste) has never been greater. Alternatives to hydrometallurgical methods, such as cyanide, are still limited. This work examines utilizing ozone and chlorides to recycle Au from E‐waste. It is started with a fundamental investigation of Au dissolution processes on the extended surface of Au polycrystalline and Au nanoparticulated electrodes. An online electrochemical scanning flow cell coupled with inductively coupled plasma mass spectrometry quantifies the rates and amounts of Au leaching. Identical‐location scanning electron microscopy (IL‐SEM) further correlates dissolution events with electrode morphological changes. It is demonstrated that ozone in the electrolyte imposes an anodic potential on the electrode, leading to anodic Au dissolution. Passivation disappears when small amounts of chlorides are added to the electrolyte, significantly enhancing the leaching yield. IL‐SEM images of gold nanoparticles (NPs) before and after exposure to ozone reveal heterogeneity in NP size‐dependent dissolution, showing higher dissolution for smaller particles. An effective Au leaching procedure is further demonstrated in a lab‐scale reactor using real E‐waste where almost complete recovery of Au is achieved. This research suggests that with engineering optimization in reactor applications based on ozone‐stimulated gold, dissolution can pave the way for environmentally friendly gold recycling.


Introduction
Electric and electronic equipment (EEE), like printed circuit boards, is part of our daily life.Gold is one of the main components of EEE due to its good electric conductivity and high chemical inertness.The electronic industry produces over 300 tons of gold each year through EEE. [1,2]Since this equipment has a relatively short lifespan, and due to the growing demand for gold and the consumption of natural ores, it is crucial to develop efficient methods for recovering gold from electronic wastes (E-wastes).
[5][6] Both of these processes are considered mature.However, they suffer from high energy-consuming processes and downstream waste generation.While dangerous global warming gases are released in the pyrometallurgical methods, the hydrometallurgical methods produce high amounts of toxic waste.In addition, most E-waste is made up of plastic housing, which makes it necessary to mill it and burn it to remove as many impurities as possible and expose the precious metals.Due to the intermixed metals, an additional hydrometallurgical washing step must be implemented to separate these elements.Hydrometallurgy is based on relatively cheap chemical leaching treatments, for instance, highly acidic solutions of strong oxidizing nature such as hot fuming aqua regia or pressurized alkaline cyanide solutions. [7,8]The benefit of these processes is a high dissolution rate during leaching achieved by controlling the redox potential.At the same time, the production of dangerous gases and residual toxic waste is considered the main drawback.
Global demand for more effective methods to reclaim gold from electronic waste (E-waste) has never been greater.Alternatives to hydrometallurgical methods, such as cyanide, are still limited.This work examines utilizing ozone and chlorides to recycle Au from E-waste.It is started with a fundamental investigation of Au dissolution processes on the extended surface of Au polycrystalline and Au nanoparticulated electrodes.An online electrochemical scanning flow cell coupled with inductively coupled plasma mass spectrometry quantifies the rates and amounts of Au leaching.Identical-location scanning electron microscopy (IL-SEM) further correlates dissolution events with electrode morphological changes.It is demonstrated that ozone in the electrolyte imposes an anodic potential on the electrode, leading to anodic Au dissolution.Passivation disappears when small amounts of chlorides are added to the electrolyte, significantly enhancing the leaching yield.IL-SEM images of gold nanoparticles (NPs) before and after exposure to ozone reveal heterogeneity in NP size-dependent dissolution, showing higher dissolution for smaller particles.An effective Au leaching procedure is further demonstrated in a lab-scale reactor using real E-waste where almost complete recovery of Au is achieved.This research suggests that with engineering optimization in reactor applications based on ozone-stimulated gold, dissolution can pave the way for environmentally friendly gold recycling.
Exploration into more benign systems has been carried out in recent years.Bioleaching is an up-and-coming procedure but often suffers from slow kinetics and long leaching times. [9][12][13] However, the current technologies do not make this approach economically viable.The thiosulfate-based gold leaching processes are still far from large-scale application because the overall gold recovery is generally lower than in other commonly used processes and the reagent consumption is high.Also, the use of thiosulfate favors the formation of a resistant passivation layer.The process is frequently improved by using copper and ammonia, but ammonia is difficult to dispose of, transported and stored. [14]n contrast, bioleaching frequently encounters issues related to toxicity; metals present in E-wastes can obstruct microorganism growth and activity.This necessitates the prior adaptation of microorganisms to the E-waste, typically through incremental subculturing, before bioleaching and can increase the cost. [15]herefore, developing new methods for efficient and clean recovery of gold from E-waste is still necessary.
In addition to its presence in EEE, gold has widespread use in industrial and scientific applications.One of the latter is its use as electrode material in electrochemical energy storage, conversion devices, and electrochemical sensors due to its wide stability window in aqueous electrolytes. [16,17][33][34] One of these fundamental studies also investigated its dissolution behavior. [35]Although gold often improves the stability and duration of electrocatalysts, gold is by no means genuinely inert. [36]Recently, with the help of online electrochemical inductively coupled plasma mass spectrometry (ICP-MS), new insights into Au dissolution have been obtained, significantly improving our understanding of the processes governing its stability. [37]On the one hand, this improved understanding of gold dissolution is essential to design strategies to avoid its degradation in applied electrode materials.On the other hand, this knowledge can be used to develop new methods for dissolving and recovering gold from E-waste, as demonstrated in the current work.
Online ICP-MS studies using a scanning flow cell (SFC) show that, in the absence of complexing agents in the electrolyte, Au starts to dissolve when subjected to potentials above 1.2-1.3V reversible hydrogen electrode (RHE). [35][39] The amount of gold lost in the electrolyte directly correlates to the amount of oxide that is getting oxidized/reduced, which is dependent on potential limits and the scan rate of potential scans. [37]hen presenting online dissolution results, it is usual to use the terms "anodic" and "cathodic" dissolution."Anodic dissolution" corresponds to dissolution processes that take place at the same time of reactions that involve anodic (positive) currents, like for example a surface oxidation reaction.On the other side, "cathodic dissolution" refers to dissolution processes that occur at the same time as reactions that involve cathodic (negative) reactions, like for example an oxide reduction.However, it is important to note that the cathodic dissolution process itself is not necessarily a reduction reaction, but it has to take place at the same time of a reduction reaction.For example, during the reduction of the gold oxide, some of the oxidized Au atoms can dissolve into the electrolyte solution as Au 3þ soluble species instead of being reduced to metallic Au 0 , and this is what usually the term "cathodic dissolution" refers to.Although moderate at lower potentials, complexing agents can promote oxidative dissolution, moving the dissolution onset potential to less positive values.[47] The dissolution is also affected by the size of gold nanoparticles (NPs). [42,48]However, to this point, there have been limited studies about the chemically (without externally applied potential) induced gold dissolution, which could provide valuable insights for different applications like recycling.That extensive knowledge about gold electrochemical dissolution can be used inversely for its recovery through chemically induced potential alterations.
Analogously to what was previously reported for platinum and palladium [49] and recently also iridium, [50] the use of ozone as an oxidizing agent and chloride as a complexing element is being explored as a new, safer approach for gold leaching.This research pivots toward environmental stewardship by advocating for ozone in treating E-waste, thus exploiting its robust oxidative properties for the recuperation of gold.Some previous works explored the possibility of using ozone as leaching agent for the recovery of gold.For example, Van Antwerp and Lincoln patented a methodology that prescribed ozone for extracting precious metals from ores, proposing that UV-generated ozone also contains other "activated oxygen" species that help breaking down the chemical bonds between the precious metal and the surrounding ore's molecular structure, but they do not highlight the possible effect of chlorides. [51]The work of Torres and Lapidus entails the extraction of Pt, Pd, and Au from magnetite ore utilizing high-concentration chloride-rich solutions and ozone. [52]Furthermore, Viñals et al. [53] studied the effect of different parameters on the leaching of polycrystalline gold by ozone in presence of high concentrations of chlorides.Distinct from preceding studies that concentrated on goldbearing ores or polycrystalline gold, in the present wok, we focus on the mechanisms of how ozone helps dissolve gold itself and how the presence of chloride anions promotes this process, using electrochemical characterization and analytical techniques such as identical-location scanning electron microscopy (IL-SEM) and online ICP-MS dissolution measurements, which allows a better understanding in a much smaller scale.The current research goes beyond the aforementioned previous works, analyzing dissolution mechanisms at reduced chloride concentrations, accentuating the role of NP and extrapolating these insights for E-waste processing applications.Amid the expansive existing literature, the present study distinguishes itself by orienting these advanced leaching methodologies toward E-waste, concentrating on NP interactions, and deciphering the variances in dissolution processes between NPs and polycrystalline substrates in ozone-chloride systems.
In the present study, the stability of gold exposed to ozone and the presence of chlorides as complexing agents is investigated by online electrochemical SFC coupled to the ICP-MS (SFC-ICP-MS) and IL-SEM.Finally, the feasibility of using ozone and chlorides to recover gold from E-waste is demonstrated in a pre-pilot reactor.

Results and Discussion
To check the state of the electrode/electrolyte interface, cyclic voltammograms (CVs) were taken after each electrochemical cleaning procedure.Reproducible and well-defined features were obtained for the polycrystalline Au electrode and the Au NPs (Au black).Considering the difference between the polycrystalline electrode and NPs, all results are normalized to the electrochemical active surface area (ECSA).The determination of the ECSA was conducted by integrating the oxide reduction peaks during the negative-going part of CV, estimating the oxide formed up to 1.7 V, potential at which the electrode is presumed to be covered by a single monolayer of adsorbed oxygen.The obtained charge derived from integrating the cathodic peaks is then divided, assuming a single monolayer charge of 400 μC cm À2 (Figure 1). [54]n the past, noble metal leaching using ozone and the transient dissolution process was successfully demonstrated for Pt and Pd.In those experiments, ozone/carbon monoxide cycles were utilized to oxidize and reduce the surface of Pt or Pd, resulting in relatively high transient (depending on the rate of oxidation/reduction) dissolution. [49]The current study applies a similar approach to accelerating Au leaching.We start with estimating the Au open-circuit potential (OCP) in the presence of ozone.The OCP represents the metal/solution potential difference in equilibrium when both cathodic and anodic net reactions have the same rate, resulting in a net current of zero.The potential was measured overtime in SFC by applying a zero current value to the working electrode while continuously purging ozone into the solution.To ensure that the electrolyte in contact with the working electrode was adequately saturated with ozone in SFC, separate measurements were performed using a conventional bulk cell (Figure S1, Supporting Information).These measurements verified that the electrolyte used in the SFC setup was well saturated, and the potentials achieved were comparable (1.47 V in the SFC setup vs. 1.51V in a bulk cell).It is important to note that all experiments presented in the following figures were conducted using SFC unless otherwise specified.
After proving that the electrolyte can be saturated with ozone in SFC and that the Au working electrode potential can be moved into the anodic potential region, Au dissolution was studied in the SFC-ICP-MS setup.A polycrystalline gold foil was used as a working electrode, and constant ozone pulses for 5, 10, and 15 min were applied to investigate the effect of oxidation treatment time on the transient gold dissolution.The potential is held at 0.4 V for 300 s between pulses to return the surface to its initial reduced state.To understand if Au dissolution is driven solely by potential imposed on the electrode by ozone redox processes, the presence of ozone in the electrolyte, or both, the same protocol was also used to control the surface potential using a potentiostat mimicking the ozone-generated OCP (1.47 V) in the ozone-saturated electrolyte.With the two protocols, it is possible to study how the extent of dissolution is affected by the variation in surface potential, as shown in Figure 2. Interestingly, if the ozone redox processes set the  electrode potential (Figure 2a), gold dissolves more readily for all treatment lengths than when the potential is controlled with the potentiostat (Figure 2b).It is important to mention that the associated dissolution to the cleaning step and the first two cyclic voltammetry is presented for the sake of completeness, but the relevant parts and to what the analysis of the results is focused are the following pulses, especially the ones lasting 10 and 15 min.The first 5 min pulse is subject to lower experimental reproducibility because the well-controlled conditions of the experiment are not reached after some times of ozone exposure to the SFC system.
A possible explanation for the enhanced dissolution in the presence of ozone is the generation of highly reactive oxygen species such as hydroxyl radicals (OH•). [55]It was demonstrated that hydroxyl radicals could selectively attack gold electrodes and contribute to the strongly preferential dissolution of the rough parts of the gold surface. [56]Further explanation follows by additional experimental results later.
Despite the overall higher dissolution in the ozone-saturated electrolyte, it can be seen that during the first 200-300 s, the dissolution rate increases, but there is an abrupt decrease at extended exposures, most likely due to the passivation of the surface of the gold.Dissolution remains diminished until the surface is again reduced and reoxidized.No cathodic peaks exist in the dissolution profiles in Figure 2. The reason is that the quick reduction of the gold electrode leads to the redeposition of the dissolved Au ions back onto the surface of the electrode.This leads to forming a less coordinated structure on the surface of the electrode. [57]Previous work demonstrated the presence of low-coordinated atoms on the Au surface due to surface restructuring under oxidation/reduction during/after O 3 treatment, [58] which agrees with our observation.It is important to remark that the redeposited amounts we are referring to here as small are just enough to avoid observing the cathodic dissolution associated with the reductive jump.As ozone saturation was not achieved during the first 5 min of pulse, we only discuss the 10 and 15 min pulses for the following figures.The entire protocol for each figure is shown in Supporting Information.
A third controlling protocol combining potentiostatic control in the presence of ozone demonstrates that ozone specifically affects the oxidation behavior of the gold surface and does not act simply by increasing the dissolution rate since, in the latter case, the dissolution rate should be the same in both cases (Figure 2c).Conversely, the combination of ozone and potentiostatic control leads to reduced dissolution rates.The potentiostatimposed potential promotes gold oxide formation, which decreases dissolution.Figure 3a,b presents the electrochemical protocols and dissolution profiles for ozone and imitation of the potential response with the potentiostat.A linear sweep voltammogram (LSV) was implemented at the end of each hold.The polarization curves for the LSV are depicted in Figure 3c.A comparison of these two polarization curves makes it evident that a smaller quantity of oxide forms with the exclusive use of ozone since oxidative currents can still be measured at the beginning of the ramp, thereby enabling more gold dissolution.Additionally, the oxide formed during the ozone hold begins to be reduced almost immediately at more positive potentials and is fully reduced at 43 mV higher potential.
The quantity of oxide was calculated with same method employed to determine the ECSA, i.e., by integrating the current charge overtime and dividing the resultant figure by 400 μC cm À2 . [59]During the LSV after ozone, a small amount of oxide is formed due to the ramp.This amount was integrated and subtracted from the total quantity to determine the oxide formed solely due to ozone.For simplicity, only the 15 min pulse is displayed here; the complete protocol, including the LSVs for the two shorter pulses, can be found in Figure S3, Supporting Information.The integration curves and the total amounts are demonstrated in Figure S3, Supporting Information.Notably, when LSVs are applied, a cathodic dissolution peak is also visible. [57]omplexing agents like halide anions promote gold dissolution. [13]While chlorides, bromides, and iodides were initially considered in this work, both bromide and iodine were rejected due to safety concerns.Even at low concentrations, these halide anions can be oxidized at lower potentials than chlorides and released in significant amounts, such as bromine and iodine, which are hazardous. [60,61]The amount of Cl 2 produced in the case of chlorides is negligible and therefore proven not dangerous. [49]We chose chlorides to investigate their effect as a complexing agent in light of these observations.
Figure 4 displays the outcomes of the ozone pulse protocol conducted on gold crystal with two varying chloride concentrations, 20 and 200 μM, in the SFC-ICP-MS setup.Figure 4a illustrates gold dissolution using ozone without chloride for comparison purposes.The presence of chloride as a complexing agent demonstrates two critical benefits for polycrystalline gold's dissolution process (see Figure 4b,c).First, the initial dissolution rate following the start of the ozone pulse is notably higher in the presence of chloride than without it.This confirms that chloride intermixed with ozone aids in accelerating the dissolution process.The second advantage is that the passivation effect, which refers to the decrease in dissolution rate overtime, is substantially reduced in the presence of chloride.The results align with earlier online experiments that examined the impact of chloride concentrations as low as 10 μM using a potentiostat. [43]The entire protocol is shown in Figure S4, Supporting Information.
The main aim of the present work is to demonstrate the practicality of using ozone and chlorides for recycling gold from E-waste.Unfortunately, creating and optimizing a working electrode for online dissolution measurements would be challenging due to the E-waste's inhomogeneous and variable composition.Therefore, we used commercial gold NPs to represent the gold properties in E-waste.Consequently, we conducted additional experiments with the NPs, which included a 30 min hold with ozone, both with and without chlorides, shown in Figure S5, Supporting Information.
The experiment results on Au/poly and Au/NP dissolution in the absence and presence of chlorides are shown in Figure 5 and S5, Supporting Information, with the extended protocol.The electrochemical profiles displayed in gray alongside dissolution profiles in color for each measurement offer further insights into the electrochemical behavior of NPs.It has been observed that NPs begin dissolving at a lower potential, with an initial peak, but experience a delay in further dissolution until a steady potential is reached (see Figure 5b,d).Previous research has correlated that smaller gold NPs initiate dissolution at lower potentials. [48]ven if the average NP size is above 100 nm, some smaller NPs are still present and start dissolving first, followed by a delay in the dissolution of larger NPs.This hypothesis can be further supported by the IL-SEM images in Figure 7c,d, which clearly show some smaller NPs' disappearance, highlighted by red circles.In contrast, when using a potentiostat potential control, the dissolution profiles for Au/poly and Au/NP electrodes are nearly identical, and the currents obtained during the holds are similar.An initial peak in the onset is followed by a steady decline, as shown in Figure S6, Supporting Information.
Based on Figure 5d, it can be observed that the dissolution profiles of Au/NP initially display lower dissolution amounts.Nevertheless, they ultimately achieve higher values overtime due to the continuous rise in dissolution in the presence of chlorides.To simplify the comparison, the total dissolved quantities for polycrystalline gold and NPs are shown in Figure 6 and are nearly identical up to a 15 min hold.However, the trend of the dissolution profiles suggests that gold NPs will continue to dissolve overtime, with a continuous increase in the presence of chlorides, as seen in both the 15 min hold and further confirmed with a 30 min hold, as shown in Figure S5, Supporting Information.In contrast, polycrystalline gold experiences a steady decline due to effective oxide passivation.
Figure 6 plots the total gold dissolved, quantified by integrating the area beneath each pulse during hold times.In experiments without chloride, a 30 min hold leads to the dissolution of 560 ng cm À2 of gold from nanoparticles (Au NPs).Contrastingly, introducing chlorides markedly increases gold dissolution, with totals reaching 2870 ng cm À2 for a 15 min hold and 8610 ng cm À2 for a 30 min hold.This pronounced increase is likely due to the continual exposure of fresh gold surfaces on the nanoparticles, which hinders long-term passivation and sustains the dissolution process.Supporting this hypothesis in the case of abscess of chlorides, the dissolution amount of 560 ng cm À2 in chloride-free electrolytes corresponds to just under 1.5 monolayers of gold.[36,62] This calculation is based on dividing the dissolved amount by 410 ng cm -2 , representing the mass of one monolayer of gold.Ozone-induced oxygen uptake on the gold surface is limited to forming a single monolayer rather than completely oxidizing the gold. [63,64]This suggests that new surface areas are exposed upon the dissolution of the nanoparticles.Furthermore, an increase in the surface area of the nanoparticles is observed following inhomogeneous dissolution, as revealed by uneven roughness development on individual NPs in the SEM images in Figure 7.
The most positive conclusion that can be extracted from the previous results is that when Au NPs are used, which are closer to the morphology of real-world E-waste, and in the presence of chlorides, the dissolution of gold is constant or it even keeps increasing with time when the solution is saturated with ozone.Applying 0.4 V after the time during oxidative OCP by the presence of ozone was necessary to revert the gold surface to a reduced state conducive for subsequent oxidative holds at OCP.The point of doing successive pulses of different time length was to investigate the viability of using an oxidationreduction approach to increase gold leaching, similarly to what was presented in a previous work for Pt. [49]Although this approach would be interesting for the case of leaching by ozone without chlorides since in this case the dissolution profile decreases with time and it can only be recovered by a reduction step, the danger of using CO as reducing agent and the fact that in the presence of chlorides, and especially for Au NPs, dissolution does not decrease with time, the oxidation-reduction approach was discarded in favor of a constant pulse method.Hence, imposing oxidation-reduction cycles is unnecessary, as performed for Pt and Pd using CO in the previous work. [49]ycling with CO poses a safety risk and would require additional infrastructure for safe implementation.Hydrogen, which could be used as a reducing agent, also presents hazards since it should not be mixed with ozone.Using liquid-reducing agents could be an alternative, [50] but this would increase costs and waste output.Since for the Au NPs in the presence of ozone, the dissolution rate does not decrease with time, it can be proposed that the recovery through oxidation-reduction cycles is unnecessary and adds complexity to the process, making it less sustainable and, therefore, a constant ozone saturation approach is preferable and will be used in the following sections.
To observe the degradation of the Au NPs catalyst locally, SEM and IL-SEM imaging of the samples before and after have been performed (Figure 7a,c and b,d, respectively).SEM imaging of the pristine sample reveals the presence of spherical Au particles, with diameters mainly in the range between 100 nm and 1 μm.Particles were primarily spherical, while also, in some random places, different shapes, such as triangles and rods, were observed.In most cases, particles were grouped in a kind of agglomerated structure.
SEM imaging on the random locations confirmed that Au NPs suffer dissolution during exposure to an ozone-saturated electrolyte containing chlorides.Again the irregularly shaped and etched particles appear in the sample due to the inhomogeneous dissolution of Au NPs (see Figure 7b).That this continues to occur at a longer timescale agrees with the non-passivating dissolution of Au NPs observed by ICP-MS.
For the following IL-SEM imaging, the stability of the film posed a challenge, as some parts of it physically detached from the GC surface due to the ozone treatment.To secure the film's  S8 and S9, Supporting Information.Red circles pinpoint areas undergoing notable gold leaching.In contrast, regions under green circles, though seemingly occupied by residual NPs, have been identified through energy-dispersive X-Ray spectroscopy (EDS) analysis (refer to Figure S9, Supporting Information) as carbon residues, not intact gold NPs.Despite the degradation, the Nafion binder's persistence is noticeable around the gold NP clusters.This presence, coupled with carbon remnants, might suggest an understated leaching effect at a cursory glance.However, EDS mapping clearly illustrates that materials in the central region and smaller clusters, initially perceived as undissolved gold, are indeed Nafion and carbon constituents.
Until now, investigations into gold dissolution have been carried out under well-defined conditions using a three-electrode assembly, where the catalyst used in these experiments has been either a crystal or NPs immobilized on the working electrode within a flow-type electrochemical cell.This experimental setup has aided in analyzing and understanding the fundamental dissolution processes.To understand the effects of an increased concentration of chlorides coupled with ozone on surface potential, we initiated a preliminary screening in a bulk cell, employing a three-electrode rotating disk electrode (RDE) setup with a goldtip electrode.An observed decrease in the OCP corresponded with increased chloride concentration, as demonstrated in Figure 8.The reactions that would control the OCP are indicated in section The OCP values spanned from 1.41 V at 100 μM Cl À to just above 0.9 V at 1 M Cl À , signifying that steady-state gold dissolution can transpire within this range, which proposes a more industrially viable approach that we subsequently tested in a lab-scale reactor.The next part of this work applies ozone in conjunction with chlorides for dissolving gold to a practical E-waste sample (refer to Figure 9).
Owing to the inhomogeneity commonly found in E-waste, multiple samples underwent microwave-assisted aqua regia digestion.Gold concentration, determined via ICP-MS, was approximately 1 wt%.Before beginning the precious metal recovery process, a prewashing step was conducted on the sample.A 0.5 M hydrochloric acid solution with air sparging and mechanical agitation was utilized to remove non-noble metals, as seen in Figure 9b.The resulting blue solution indicates the high percentage of copper initially present in the E-waste.This selective recovery enables the extraction of gold and provides a secondary revenue source, as copper is a highly sought-after resource.The prewashing stage involves sparging of 0.1 M HCl with an addition of NaCl to reach a total of 0.5 M Cl À solution with air.The transition to a blue suspension indicates copper being leached into the electrolyte.Once the prewashing stage is complete, the electrolyte, retaining most non-noble metals, is removed.Following this, a fresh 0.1 M HCl electrolyte is introduced, and the chloride concentration of the solution is adjusted to 1 M Cl.Upon ozone sparging, the noticeable transition to a yellow-colored solution indicates gold leaching (Figure 8c).This change suggests the formation of aurochloride complexes.As depicted in Figure 9d, gold is almost entirely leached from the nonconductive support, with a recovery rate of 86% AE 7% (Figure S10, Supporting Information), which can be increased by extending the treatment or by further engineering optimization of the reactor and its treatment conditions.However, as a proof of concept, the experiments served the purpose.The measurement took the course over 24 h, and samples were taken at the end.
As done previously with platinum recovery, [49] comparing the experimental conditions and results from model systems is crucial to understanding how these parameters can recover gold from an applied sample.The gold dissolution research conducted using a three-electrode assembly with and without potential control provided insights into the fundamental dissolution processes and mechanisms, which were then applied to the practical system involving NaCl and HCl concentrations.The successful gold recovery in the application-relevant system demonstrates that the underlying principles and understanding from the fundamental studies apply to real-world scenarios, allowing for effective and efficient extraction of gold from Ewaste materials.

Conclusions
To summarize, the online ICP-MS experiments with electrochemical SFC using ozone and different chloride concentrations showcase the applicability of a method using constant ozone saturation in the case of gold NPs and the positive effect of using chloride as a complexing agent.These results indicate that lower chloride concentrations enhance the dissolution process and minimize the passivation effect on polycrystalline gold.This work further expands on how new ways of recycling can be developed using a flow-type electrochemical cell-coupled ICP-MS.With the SFC-ICP-MS investigations, a better understanding of the dissolution under recycling conditions was obtained, allowing for optimization and real-world application.Gold exposure to ozone leads to more significant dissolution than at the same simulated potential in the absence of ozone in the solution.The imposed potentiostatic potential prompts the formation of gold oxide, thereby decreasing dissolution.Adding chlorides enhances the dissolution and for the polycrystalline Au slows the passivation, however for the NPs prevents the passivation.The obtained SEM and IL-SEM images of NPs support this hypothesis since, at the initial stage before exposure to ozone and chlorides, the NPs exhibit a round shape.After the dissolution procedure, they show sharp edges with multiple small holes in the surface, indicating that the exposed Au surface after could even increase with time.Significant aspect is that Au NPs and polycrystalline gold dissolution profiles are a direct dichotomy.For polycrystalline gold, even with the addition of chlorides, there  is still a decline in the dissolution with time.In the case of NPs, dissolution steadily increases in the presence of chlorides in the system.This work suggests that the recovery of gold from practical samples using a constant purging of ozone in the presence of chlorides can be feasible, and oxidation-reduction cycles, which would complicate the system to a great extent due to the safety concerns of the common reducing agents, would not be necessary.The trends observed using the coupled SFC ICP-MS system for gold NPs have extrapolated and demonstrated that nearly 100% recovery is possible in large-scale reactors with a more sustainable process than currently used.Further engineering improvements will be tested in future works, such as introducing a novel technique for favoring the presence of microporous water (hydrophobically tuned nanocrystalline solids like zeolites and MOFs for creating microporous liquids capable of enhanced gas absorption compared to traditional solvents) near the metal surface, which would increase the solubility of ozone and allow the use of higher concentrations of complexing agents. [65]

Experimental Section
Preparation of Polycrystalline Gold Electrodes: A polycrystalline gold foil (99.99%,MaTecK, Germany) was used as the working electrode.The geometric surface area of the electrode exposed to the electrolyte was 0.01 cm À2 .Before each measurement, the foil was mechanically polished.Struers-MD polishing cloths and the corresponding diamond suspension DiaPro Nap B, 0.25 μm were used.An electrochemical cleaning/activation step, composed of 30 cycles up to 1.85 V RHE (potential against an RHE) at 200 mV s À1 , was utilized to ensure data reproducibility between different measurements.
Preparation of Au NPs Thin-Film Electrodes: NPs (gold powder, spherical, APS 0.5-0.8μm, 99.96þ% metals basis) were used to prepare thin films of gold on a glassy carbon (GC) substrate (SIGRADUR G, HTW 5 Â 5 cm 2 ) by drop-casting.Before this step, all GC substrates were ground with SiC paper (mounted on an MD-Gekko disc), applying 300 N force and 200 rpm rotation speed.The sample holder was counter-rotated during this procedure at 150 rpm.This protocol was repeated twice using four different grain sizes: 220, 1000, 2000, and 4000 (corresponding to 68, 18, 10, and 5 μm grain sizes, respectively).Afterward, the GC electrode was polished with the same device on an MD-Mol (catalog number: 40 500 079) polishing pad using DiaPro MD-Mol paste (water-based diamond suspension; particle size of 3 μm, Struers) with the following parameters: 150 N, 200 rpm, and the sample holder was also counter-rotated with 150 rpm for 5 min.Flat GCs were cleaned with Kimwipes and isopropanol.Finally, the GC plates were rinsed with Milli-Q water, dried, and stored under ambient conditions overnight before drop-casting of the catalyst inks.For the ink preparation, 5 mg catalyst was dispersed in 3 mL ultrapure water and isopropanol at a 1:4 ratio, respectively, and sonicated for 20 min with 25% intensity using a sonicating horn (Branson SFX 150).Sonication was on for 4 s, interrupted by a 2 s break.The dispersion was placed in an ice bath during sonication.Immediately after sonication, 0.2 μL aliquots were pipetted and drop-cast on the GC substrate reaching approximately 10 μg cm À2 metal loading.The spots dried under air at room temperature.The diameter and quality of each spot were determined using a laser scanning microscope (Keyence VK-X250) and ranged between 800 and 1000 μm.Two SFC cells were employed for polycrystalline and NP measurements with 0.01 and 0.03 cm 2 geometric areas, respectively.All electrochemical and dissolution data were normalized to the ECSA precisely determined for each measurement.
Potential-Resolved Dissolution Measurements: Stability measurements were performed with a custom-built SFC coupled to an ICP-MS (Nexion 350X, PerkinElmer).The average flow rate of electrolyte through the cell was 3.25 AE 0.05 μL s À1 using a peristaltic pump (MP2, Elemental Scientific).A GC rod (HTW Sigradur G) and a commercial Ag/AgCl (3 M KCl, Metrohm) were used as counter and reference electrodes, respectively.Measured potentials concerning the Ag/AgCl reference electrode, E Ag/AgCl , were all corrected to the RHE scale by measuring the OCP at the beginning of experiments in an H 2 -saturated 0.1 M H 2 SO 4 electrolyte using a platinum crystal (99.99%,MaTecK) working electrode.The working electrode was placed in a three-direction translation stage (Physik Instrumente M-403).A Gamry Reference 600 potentiostat was used for all the measurements.The electrolyte was freshly prepared by diluting concentrated H 2 SO 4 (Merck, Suprapur, 96%) with ultrapure water (Merck Millipore, Milli-Q, 18.2 MΩ cm).Electrolytes containing chloride anions had 20 and 200 μM concentrations of KCl (Merck, min.99.5%).These concentrations were chosen due to the technical limitations of the ICP-MS.The ICP-MS was calibrated daily for 197 Au by a four-point calibration curve (0, 0.5, 1, and 5 μg L À1 ) from freshly prepared standard solutions (Merck Certipur). 187Re at a concentration of 10 μg L À1 served as an internal standard to ensure the proper performance of the system during the whole measurement.The electrolyte was constantly purged with Ar or O 3 in a separate reservoir and transported to the SFC via a peristaltic pump via the Tygon tubing system (Ismatec, Reglo ICC).Ozone was generated on-site by 4.2 g h À1 by an ozone generator (Innotec OGVi-8G Lab) and purged with a flow of 1 L min À1 .All instruments of the SFC, including the potentiostat, were controlled via custom-designed LabVIEW software.Regarding how the amounts of dissolution were determined, we first built a calibration curve from standard solutions as stated in Experimental Section.The units of the y axis in this calibration curve were ng L À1 .It is important to remark that during the online dissolution experiments, what is measured is the amount of dissolved gold that reaches the ICP-MS in a certain instant.Therefore, the values in the y axis of the dissolution were the concentration of dissolved gold at that time, that is, in that instant.Since it was a flow cell, the electrolyte was constantly washed away from the electrode surface, so there was not accumulation of dissolved gold in the solution near the electrode.The counts measured at a certain time were converted to ng L À1 using the calibration curve previously prepared.Finally the dissolved amounts were converted to ng s À1 cm À2 by using the solution flow rate and the ECSA of the gold electrode.More details about the SFC-ICP-MS technique could be consulted elsewhere, [66][67][68] and a scheme for helping for the visualization can be found in Figure S11, Supporting Information.
Il-Sem: All IL-SEM micrographs were obtained using a field-emission SEM (Zeiss SUPRA 35VP) with an accelerating voltage of 4 kV and an InLens detector.For IL-SEM imaging, GC RDEs 5 mm in diameter were used as the working electrode and substrate for the deposition of the gold NPs.The same ink formulation described before was used to produce films of Au NPs on GC RDEs.GC RDEs were mounted directly into SEM using a homemade holder, which provides electrical contact between the GC tip and the SEM stage.The pristine film of Au NPs was imaged at several locations.In the following step, the degradation experiment was conducted in a classical three-electrode setup, with a carbon rod and RHE as the counter and reference electrodes.At the same time, the electrolyte was ozone-saturated 0.1 M H 2 SO 4 (Merck, Suprapur, 96%) þ 1 mM Cl À (added using an appropriate amount of HCl 37%, Merck, analytical grade).The Au NPs sample was exposed to the ozone-saturated electrolyte over 2.5 h, and OCP was monitored.After the experiment, the GC electrode with Au film was removed from the electrolyte, rinsed, dried, and inserted again into SEM to observe changes caused by dissolution.
Lab-Scale Reactor Tests: The experiments were carried out in a 5 L capacity, single-column jacketed reactor.The reactor features an automated temperature control system (Lauda E300-Germany), an ozone generator (MP-8000-USA), a mechanical stirrer with torque control, an electrolyte interchange system, and a gas sparger.The system also included a vacuum-assisted ultrafiltration system, enabling simple liquid phase extraction and switching between electrolyte compositions and sparging gases.An inert gas purging step was employed after the oxidative phase to avoid explosive mixtures.
The features of the reactor allowed for straightforward switching of electrolytes and gases to selectively leach different metals in a specific desired sequence.The process involved two steps: 1) a prewashing step with an electrolyte consisting of 0.1 M hydrochloric acid and sodium chloride, totaling a 0.5 M Cl À concentration and air, and ii) the recovery of gold with an electrolyte consisting of 0.1 M hydrochloric acid and sodium chloride, totaling a 1 M Cl À concentration and ozone.This process enabled effective and selective leaching of gold from various materials.
Equations: The following are the reactions that would determine the OCP against the standard hydrogen electrode (SHE) in the presence of ozone [69] Au !Au þ þe À • • • E 0 0 ¼ 1:692 V vs: SHE (1) The OCP value would be determined by the mixed potential resulting from the equations presented before in the studied conditions.
When combining Equation ( 1) and (2) with Equation ( 5), the following general equations for gold dissolution by ozone were obtained When chlorides are present in the solution, the formation of the gold complexes has to be taken into account [70,71] Au By combining Equation ( 6) and ( 8) and Equation ( 7) and ( 9), the general equations for gold dissolution when both ozone and chlorides are present are obtained [52] Au In addition, the chlorine evolution reaction from chlorides could also affect the value of the OCP in these conditions [69] 2Cl À !Cl 2 þ 2e À E 0 0 ¼ 1.395 V vs: SHE (12)

Figure 2 .
Figure 2. Au-poly dissolution in the presence and absence of ozone in the electrolyte.a) Time-resolved ozone-induced open-circuit potential (OCP) (grey curve) and corresponding time-resolved gold dissolution (blue curve).The potential was controlled by potentiostat before and in between the OCP pulses.b) Gold dissolution in the ozone-free electrolyte induced by a potentiostat (green curve) and the applied potential profile mimicking the conditions achieved with ozone saturation (grey curve).c) Same as (b) but in the ozone-saturated electrolyte.In all cases, the upper potential limit (UPL) is ≈1.47 V, with equal time durations of 5, 10, and 15 min.The base electrolyte is 0.1 M H 2 SO 4 .

Figure 3 .
Figure 3. Au-poly dissolution in the presence and absence of ozone in the electrolyte.a) Time-resolved ozone-induced OCP (grey curve) and corresponding time-resolved gold dissolution (blue curve).The potential was controlled by potentiostat before and in between the OCP pulses.b) Gold dissolution in the ozone-free electrolyte induced by a potentiostat (green curve) and the applied potential profile mimicking the conditions achieved with ozone saturation (grey curve).c) Polarization curves ozone (blue) and potentiostat (green) of linear sweep voltammograms (LSVs) applied after 15 min pulses.The base electrolyte is 0.1 M H 2 SO 4 .

Figure 4 .
Figure 4. Time-resolved ozone-induced OCP and corresponding timeresolved gold dissolution for a polycrystalline gold electrode in different chloride concentrations.a) Gold dissolution is induced by ozone-saturated 0.1 M H 2 SO 4 in the absence of chlorides (light blue curve), b) with 20 μM Cl À (blue curve), and c) with 200 μM Cl À (dark blue curve).The potential was controlled by potentiostat before and in between the OCP pulses.The potential profiles display the OCP values in these conditions (grey curves).The UPL changes with the chloride concentration.The holds have equal time durations of 10 and 15 min.

Figure 5 .
Figure 5.Comparison of the gold dissolution behavior between polycrystalline Au and Au NPs, a,b) in ozone-saturated 0.1 M H 2 SO 4 solutions and c,d) in ozone-saturated 0.1 M H 2 SO 4 solutions with 200 μM Cl. a,b) The results for polycrystalline Au (light blue curve) and Au NPs (light red curve), respectively.c) Polycrystalline Au (dark blue curve) and d) the results for Au NPs (dark red curve).The potential was controlled by potentiostat before and in between the OCP pulses.The potential profiles display the OCP values in these conditions (grey curves).The holds have equal time durations of 10 and 15 min.

Figure 6 .
Figure 6.Dissolved gold amounts indicated in monolayers in a) the absence of chlorides and b) with 200 μM Cl À in ozone-saturated 0.1 M H 2 SO 4 for polycrystalline Au (blue) and Au NPs (red).

Figure 7 .
Figure 7. Imaging of the Au NPs before and after exposure to the ozone-saturated electrolyte.a) The scanning electron microscopy (SEM) image displays the Au NPs prior to ozone exposure, while b) shows the NPs post-exposure, with both images having scale bars equivalent to 200 nm.Additionally, c,d) depict identical-location scanning electron microscopy (IL-SEM) images of the NPs before and after exposure to the ozone-saturated electrolyte, respectively, with scale bars set at 2 μm.Blue circles highlight areas showing the morphological changes of gold particles, characterized by irregular shapes; red circles identify specific sites where significant gold leaching is taking place, indicating substantial dissolution of the particles; and green circles denote areas that, while appearing to contain residual NPs, are occupied by carbon residues.Electrolyte: 0.1 M H 2 SO 4 þ 200 μM Cl À .

Figure 8 .
Figure 8. Changes in the OCP on a gold electrode surface in the presence of increasing chloride ion concentrations and ozone exposure performed in a bulk cell.

Figure 9 .
Figure 9. Gold recovery from electronic waste (E-waste) using ozone and chlorides in a pre-pilot reactor.Images of the a) nonconductive silica wafer shards with gold coating; b) suspension after prewashing where copper leaching can be observed from the blue coloration (O 2 þ 0.5 M Cl À ), and c) suspension after removal exchange of the prewashed solution with fresh electrolyte and after O 3 sparging where gold leaching can be seen from the yellow coloration (O 3 þ 1 M Cl À ), gold leaching; d) Physical vapor deposition (PVD) wafers after gold has been leached.