Dissolution Of Au Research Articles

Biogenic Cyanide Production Promotes Dissolution of Gold Nanoparticles in Soil.

Gold nanoparticles (Au NPs) are often used to study the physiochemical behavior and distribution of nanomaterials in natural systems because they are assumed to be inert under environmental conditions, even though Au can be oxidized and dissolved by a common environmental compound: cyanide. We used the cyanogenic soil bacterium, Chromobacterium violaceum, to demonstrate that quorum-sensing-regulated cyanide production could lead to a high rate of oxidative dissolution of Au NPs in soil. After 7 days of incubation in a pH 7.0 soil inoculated with C. violaceum, labile Au concentration increased from 0 to 15%. There was no observable dissolution when Au NPs were incubated in abiotic soil. In the same soil adjusted to pH 7.5, labile Au concentration increased up to 29% over the same time frame. Furthermore, we demonstrated that Au dissolution required quorum-sensing-regulated cyanide production in soil by inoculating the soil with different cell densities and using a quorum-sensing-deficient mutant of C. violaceum, CV026. Au NP dissolution experiments in liquid media coupled with mass spectrometry analysis confirmed that biogenic cyanide oxidized Au NPs to soluble Au(CN)2-. These results demonstrate under which conditions biologically enhanced metal dissolution can contribute to the overall geochemical transformation kinetics of nanoparticle in soils, even though the materials may be inert in abiotic environments.

Effect of Particle Size on the Corrosion Behaviour of Gold in the Presence of Chloride Impurities: An EFC-ICP-MS Potentiodynamic Study

A profound understanding of the Au dissolution process is a prerequisite for optimal utilization of Au-based materials. This goes for either increasing the corrosion stability of materials in the sectors where the long-term functionality of Au is needed or decreasing the corrosion stability where the recovery of the Au component is crucial. By employing an extremely sensitive online analytical system, consisting of an electrochemical flow cell coupled to an inductively coupled plasma mass spectrometry, in situ potential-resolved dissolution of Au in the ppb range is enabled. A comparative study of two Au based materials, (i) a polycrystalline Au disk and (ii) carbon-supported Au nanoparticles, is presented. As a probe, chloride ions were used to elucidate the distinct differences in the corrosion behavior of the two analogues.

Thiourea bioleaching for gold recycling from e-waste

Recycling and reuse of electronic wastes (e-wastes) are becoming an increasingly critical strategy for securing metal resources as well as for minimizing environmental impacts. Thiourea leaching of gold (Au) from e-wastes can be considered an alternative to highly toxic cyanidation, provided that its reagents consumption can be largely reduced. While awareness of the use of biohydrometallurgical techniques in metal mining industries is increasing, the knowledge on microbiological precious metal solubilization is still limited. This led us to investigate and clarify for the first time the potential utility of microbiologically-mediated thiourea leaching (TU-bioleaching) of Au, with a special focus on reducing the reagents consumption while facilitating Au dissolution. Initial screening tests found that different Fe-oxidizing bacteria/archaea possessed varying degrees of thiourea tolerance (5–100 mM). When thiourea and PCB (Printed Circuit Boards) co-exist, Acidiplasma sp. Fv-Ap displayed the most robust Fe-oxidation. The Eh level during the reaction was first optimized by fluctuating the initial ratio of thiourea to Fe3+ (TU:Fe3+ = 2:1–40:1, by using 1 mM Fe3+ vs. 2–40 mM thiourea). The ratio precisely determined the Eh level during the TU-bioleaching and dictated the fate of thiourea decomposition and the resultant Au dissolution from PCB. Microbial contribution to Fe3+ regeneration was seen to support steady and continuous Au dissolution, enabling 98% Au dissolution while using low reagent concentrations of 1 mM Fe3+ and 10 mM thiourea under the microbial Eh control at around 490–545 mV. This novel TU-bioleaching process offers a new alternative approach for Au recycling from e-wastes and minimization of environmental hazards.

Pyridinethiol‐Assisted Dissolution of Elemental Gold in Organic Solutions

AbstractDissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine‐4‐thiol (4‐PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4‐PSH to pyridine‐4‐thione (4‐PS), coordination with Au0, and then oxidation of the Au0 thione species to AuI simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a AuI complex bearing two 4‐PS ligands and SO42− as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Pyridinethiol-Assisted Dissolution of Elemental Gold in Organic Solutions.

Dissolution of elemental gold in organic solutions is a contemporary approach to lower the environmental burden associated with gold recycling. Herein, we describe fundamental studies on a highly efficient method for the dissolution of elemental Au that is based on DMF solutions containing pyridine-4-thiol (4-PSH) as a reactive ligand and hydrogen peroxide as an oxidant. Dissolution of Au proceeds through several elementary steps: isomerization of 4-PSH to pyridine-4-thione (4-PS), coordination with Au0 , and then oxidation of the Au0 thione species to AuI simultaneously with oxidation of free pyridine thione to elemental sulfur and further to sulfuric acid. The final dissolution product is a AuI complex bearing two 4-PS ligands and SO4 2- as a counterion. The ligand is crucial as it assists the oxidation process and stabilizes and solubilizes the formed Au cations.

Reflecting on Gold Geomicrobiology Research: Thoughts and Considerations for Future Endeavors

Research in gold (Au) geomicrobiology has developed extensively over the last ten years, as more Au-bearing materials from around the world point towards a consistent story: That microbes interact with Au. In weathering environments, Au is mobile, taking the form of oxidized, soluble complexes or reduced, elemental Au nanoparticles. The transition of Au between aqueous and solid states is attributed to varying geochemical conditions, catalyzed in part by the biosphere. Hence, a global Au-biogeochemical-cycle was proposed. The primary focus of this mini-review is to reflect upon the biogeochemical processes that contribute to what we currently know about Au cycling. In general, the global Au-biogeochemical-cycle begins with the liberation of gold-silver particles from a primary host rock, by physical weathering. Through oxidative-complexation, inorganic and organic soluble-Au complexes are produced. However, in the presence of microbes or other reductants—e.g., clays and Fe-oxides—these Au complexes can be destabilized. The reduction of soluble Au ultimately leads to the bioprecipitation and biomineralization of Au, the product of which can aggregate into larger structures, thereby completing the Au cycle. Evidence of these processes have been “recorded” in the preservation of secondary Au structures that have been observed on Au particles from around the world. These structures—i.e., nanometer-size to micrometer-size Au dissolution and reprecipitation features—are “snap shots” of biogeochemical influences on Au, during its journey in Earth-surface environments. Therefore, microbes can have a profound effect on the occurrence of Au in natural environments, given the nutrients necessary for microbial metabolism are sustained and Au is in the system.

Electrochemical stability of Au-TEM grid with carbon supporting film in acid and alkaline electrolytes for identical-location TEM study

Identical-location transmission electron microscopy (ILTEM), which utilizes a labeled TEM grid for both electrochemical measurement and TEM characterization of the same location, plays an indispensable role in ex-situ study of structural evolutions of nanoscale electrocatalysts. The stability of the Au TEM grid particularly the coated carbon supporting film is a critical issue for the implementation of ILTEM techniques of electrocatalysts working at highly oxidative potentials such as oxygen evolution reaction (OER). Here, we report the electrochemical stability of Au TEM grid coated with carbon supporting film at potentials up to 1.8 V/RHE in both acid and alkaline electrolytes. While showing a higher extent of corrosion in alkaline electrolyte, the carbon supporting film coated on the Au grid generally exhibited a good stability (<10% damage ratio) when holding at constant potentials up to 1.6 V/RHE in both acid and alkaline electrolytes. The likely Au dissolution and re-deposition at the OER potentials up to 1.8 V was also studied. Our results not only support the feasibility but also provide a guidance for ILTEM studies of oxygen evolution electrocatalysts or other electrocatalysts working at oxidative potentials.

Authigenic Gold in Stale Tailings of Cyanide Leaching of Gold—Sulfide—Quartz Ores (Komsomol’skii Gold-Extracting Factory, Kemerovo Region)

Based on complex mineralogical and geochemical analysis, high concentrations of dissolved and metallic native forms of Au were registered in the tailing store near-surface horizon of the Komsomol’skii gold-extracting factory. According to the physicochemical characteristics, this layer is distinguished by contrasting variations in the environmental parameters and oxidizing medium. The processes of dissolution, growth, and reprecipitation of authigenic Au were detected, and its predominant formation at the expense of redistribution from the lower horizons of gold-bearing sediments and accumulation in the oxidation zone were supported.

Nanoforms of gold from abandoned placer deposits of Wądroże Wielkie, Lower Silesia, Poland – The evidence of authigenic gold mineralization

Being the most recently exposed, gold grains from historical tailings of underground and surface workings offer the opportunity to study supergene transformations occurring over historic periods. In this study, gold grains were collected from the abandoned, historically mined deposits of Wądroże Wielkie at Lower Silesia.All three distinct gold sub-types of detrital gold display surface morphotypes and internal textures of Au and Ag dissolution indicative of supergene gold modification, as well as authigenic Au nanoparticle formation and aggregation. The indicative morphotypes include nano-particulate semispherical or bubble-like gold and aggregates of bacteriomorphic gold that could be evidence for microbial gold biomineralization. SEM investigation of the microtopography and internal texture of gold grains revealed indications of post-depositional dissolution of Au-Ag alloys and redistribution of these components within the sediment. Some of seed and bubble-like growths are also embedded in clayey (kaolinite) masses within crevices suggesting that dissolution/re-precipitation processes occur at the gold grain interface.Authigenic gold occurs as overgrowths and aggregates of nearly pure gold on a detrital Au-Ag alloy that are readily removed during transport, but are replaced by re-precipitation occurring over historic rather than geological periods.

Fabrication of Uniformly Sized Gold Nanoparticles on Glassy Carbon By Simple Electrochemical Method

Nanoparticles (NPs) of noble metals such as gold and platinum are utilized in a wide range of applications in various fields because of their unique catalytic, electronic, and optical properties. Furthermore, NPs have large surface area to volume ratios, which leads to an increase in catalytic efficiency and also to a cost-reduction. A variety of methods using chemical reactions are currently available for synthesizing NPs in solution phase and also for controlling their shapes and surface properties. In the field of electrochemistry, platinum NPs (PtNPs) deposited on carbon materials such as graphite, glassy carbon, and carbon blacks have been extensively studied because they are useful as efficient electrocatalysts for fuel cells, water electrolysis, and so on. Nishimura et al. have developed a variable method for preparing shape-controlled PtNPs on graphite surface [1]. In the method, a constant current is applied between a Pt anode and a graphite cathode. This galvanostatic electrolysis was performed under the conditions that the water electrolysis (2H2O → 2H2 + O2) occurred in strong acid solutions such as H2SO4 and HNO3. During the electrolysis, the Pt anode dissolved to form Pt ions and the graphite cathode reduced them. The method was cost friendly because PtNPs were obtained without any reducing and stabilizing agents, and also because 72 % of dissolved Pt ions were electrodeposited as PtNPs. This simple method produced not only tetragonal nanoparticles which were enclosed by (111) planes, that is, thermodynamically stable nanoparticles, but also cubic nanoparticles enclosed by (100) planes. Interestingly, the cubic nanoparticles were preferentially obtained by controlling current densities of the cathode and anode. In our previous report [2], we have shown that gold NPs (AuNPs) can be produced by galvanostatic electrolysis using an Au anode and a graphite cathode. This electrolysis was essentially the same as the simple method that has been used to prepare PtNPs [1]. The Au anode, where oxygen evolution occurred, dissolved to form Au ions (Au → Au3+ + 3e-) and the graphite cathode, where hydrogen evolution occurred, reduced them (Au3+ + 3e- → Au). The number density of AuNPs depended on the kind of electrolyte solution: it was higher when the electrolysis was conducted in 0.5 M H2SO4 solution than in 0.5 M HNO3 solution. This was because the rate of Au dissolution (or the formation rate of Au3+), which was directly measured using quartz crystal microbalance, was higher in the H2SO4 solution than in the HNO3 solution. It is interesting that this dependence was opposite to the one for PtNPs: PtNPs were effectively obtained in the HNO3 solution compared with in the H2SO4 solution [1]. In this present work, we show AuNPs can be produced on glassy carbon (GC) by the galvanostatic electrolysis. The use of GC as cathodes facilitates us to observe AuNPs by scanning electron microscope (SEM) because the surface of GC is smoother than that of graphite. Figure 1 show SEM images of GC cathodes taken after the electrolysis. Uniformly seized AuNPs are clearly observed after the electrolysis is conducted for a long time, e.g., 120 minutes (compare panel a1 with a2), which will be discussed in the presentation. On the other hand, AuNPs can be produced without using Au anode when the electrolysis solution contains HAuCl4 (panels b1 and b2). A graphite plate is used as the anode for this electrolysis to avoid the formation of Au ions from the anode. The size of AuNPs is larger than that produced in the electrolysis without HAuCl4. The difference in the size of AuNPs will be also discussed in the presentation. REFERENCES [1] T. Nishimura, T. Nakade, T. Morikawa, H. Inoue, Electrochimi. Acta, 129 (2014) 152–159. [2] Y. Mukouyama, Y. Fukuda, T. Nishimura, ECS Trans., 80 (2017) 1425-1431. FIGURE CAPTION Figure 1. SEM images of GC cathodes taken after galvanostatic electrolysis at a cathodic current density of 10 mA cm-2. The electrolysis was conducted at 30℃ for (left) 30 minutes and (right) 120 minutes. (a1 and a2) Electrolysis solution: 0.5 M H2SO4. Anode: Au wire. Anodic current density: 2.5 mA cm-2. (b1 and b2) Electrolysis solution: 0.5 M H2SO4 + 0.1 mM HAuCl4. Anode: graphite plate. Anodic current density: 0.49 mA cm-2. Figure 1

The interfacial bilayer Cu6Sn5 formed in a Sn–Ag–Cu flip-chip solder joint incorporating Au/Pd metallization during solid-state aging

A solid-state reaction between the Sn–Ag–Cu solder and Ni metallization resulted in the formation of interfacial bilayer Cu6Sn5-based intermetallic compounds (IMCs) in a solder joint incorporating Au/Pd metallization. The layer near the Ni metallization was identified as (Cu,Ni,Au)6Sn5 containing 20.3 at.% of Ni and 1.7 at.% of Au, and the layer near the solder matrix was identified as (Cu,Au,Ni,Pd)6Sn5 containing 5.7 at.% of Au, 1.2 at.% of Ni, and 1.0 at.% of Pd. The electron diffraction analysis with high resolution transmission electron microscopy further characterized the interfacial bilayer IMCs as having the same hexagonal crystal structure with different crystal orientations. The kinetics study revealed that the (Cu,Ni,Au)6Sn5 in the interfacial bilayer reaction products was formed during the initial reflow process, while the (Cu,Au,Ni,Pd)6Sn5 layer was formed during subsequent solid-state aging. Predominant growth of the (Cu,Au,Ni,Pd)6Sn5 layer was observed during solid-state aging. In contrast, the growth of the initially formed (Cu,Ni,Au)6Sn5 layer was suppressed. The growth of the (Cu,Au,Ni,Pd)6Sn5 layer was governed by the dissolution of the (Au,Pd)Sn4 IMC that occurred during solid-state aging. The complete dissolution of (Au,Pd)Sn4 resulted in a reactant-limited chemical reaction from the conversion of (Au,Pd)Sn4 to (Cu,Au,Ni,Pd)6Sn5. The development of the interfacial bilayer IMCs in the solder joint incorporating Au/Pd metallization suppressed excessive IMC growth and unfavorable phase transformation during long-term solid-state aging. The detailed mechanism of the formation of the interfacial bilayer Cu6Sn5-based IMCs was investigated in this paper.

Electrochemical behavior of gold and its associated minerals in alkaline thiourea solutions

Electrochemical measurements were conducted to study the electrochemical behavior of gold (Au) and its commonly associated minerals in alkaline thiourea solutions. The results indicated that without addition of any stabilizer, selective dissolution of Au from stibnite and pyrite was only possible at relatively low thiourea concentrations. As Na2SiO3 was added, pyrite started to become active and an oxidation peak appeared; the oxidation peaks of arsenopyrite and chalcocite appeared earlier than that of Au. The chalcocite peak shifted in the positive direction and the peak current increased. Stibnite did not show an oxidation peak and its current was nearly zero. Adding Na2SiO3 favored the selective dissolution of Au when its minerals were associated with chalcocite and stibnite. At pH 12, the Au anode dissolution peak current increased with stabilizer concentration. At 0.38 and 0.42 V and for Na2SiO3 concentration below 0.09 M, the current density continuously increased with Na2SiO3 concentration. The Na2SiO3 concentration had to be adequate to stabilize thiourea. When the potential was higher than 0.42 V, the surface of the Au electrode started to passivate. With an additional increase in potential, the presence of Na2SiO3 could not stop the inevitable decomposition of thiourea.

Electrochemical Control of Plasmonic Metal Nanogap for Ultra-Small Light Confinement

Ultra-small nanogap of plasmonic metal dimer can confine light into molecular size. In this study, we have tried to establish the method for the control of the gap distance of Au nanodimer structures using electrochemical method. Au nanodimer structures were fabricated on a conductive glass substrate by the electron beam lithography method. Electrochemical dissolution of bridged Au nanodisk dimer was conducted under in-situ dark-field microscopy observation. The electrode potential was kept at the potential for Au dissolution. SEM images proved the formation of the gap between Au nanoparticles. From the time series scattering spectra of Au nanodimer, the drastic optical property changes has been observed at the gap formation. This optical property change can be assigned to the plamon mode change from the charge transfer mode to bonding dipolar mode due to the formation of the sub-nm gap at Au nanodimer. The present method is a useful tool for controlling the shape and the gap distance between Au nanoparticles in a single nanometer scale.

Biogeochemical cycling of gold: Transforming gold particles from arctic Finland

(Bio)geochemical cycling of gold (Au) has been demonstrated in present-day (semi)-arid, (sub)-tropical and temperate environment. Hereby biofilms on Au-bearing mineral- and Au-particle surfaces drive Au dispersion and reconcentration, thereby (trans)forming the particles. However, it is unknown if biogeochemical cycling of Au occurs in polar environments, where air temperatures can reach −40 °C and soils remain frozen for much of the year. Therefore, placer Au-particles, soils and waters were collected at two placer mining districts in arctic Finland, i.e., the Ivalojoki and Lemmenjoki goldfields. Sites were chosen based on contrasting settings ((glacio)-fluvial vs. glacial-till deposits) and depths (surface to 5 m below current surface). Gold particles were studied using a combination of tagged 16S rRNA gene next generation sequencing and electron microscopic/micro-analytical techniques. Across all sites a range of Au-particle morphologies were observed, including morphotypes indicative of Au dissolution and aggregation. Elevated Au concentrations indicative of Au mobility were detected in placer particle bearing soils at both districts. Typically Au-particles were coated by polymorphic biofilm layers composed of living and dead cells embedded in extracellular polymeric substances. Intermixed were biominerals, clays and iron-sulfides/oxides and abundant secondary Au morphotypes, i.e., nano-particles, micro-crystals, sheet-like Au, branched Au networks and overgrowths and secondary rims. Biofilms communities were composed of Acidobacteria (18.3%), Bacteroidetes (15.1%) and Proteobacteria (47.1%), with β-Proteobacteria (19.5%) being the most abundant proteobacterial group. Functionally, biofilms were composed of taxa contributing to biofilm establishment, exopolymer production and nutrient cycling, abundant taxa capable of Au mobilization, detoxification and biomineralization, among them Cupriavidus metallidurans, Acinetobacter spp. and Pseudomonas spp., were detected. In conclusion, these results demonstrate that placer Au-particle transformation and Au dispersion occur in cold, arctic environments. This corroborates the existence of biogeochemical Au cycling in present-day cold environments.

Impact of silver sulphides on gold cyanidation with polymetal sulphides

Abstract Gold leaching was influenced in association with silver and polymetal sulphide minerals. A packed bed was adopted to single out the galvanic and passivation effects with four sets of minerals: pyrite–silica, chalcopyrite–silica, sphalerite–silica and stibnite–silica. Pyrargyrite enhanced Au recovery to 77.3% and 51.2% under galvanic and passivation effects from pyrite (vs 74.6% and 15.8%). Pyrargyrite in association with sphalerite also enhanced Au recovery to 6.6% and 51.9% (vs 1.6% and 15.6%) under galvanic and passivation effects from sphalerite. Pyrargyrite associated with chalcopyrite retarded gold recovery to 38.0% and 12.1% (vs 57% and 14.1%) under galvanic and passivation effects. Accumulative silver minerals enhanced Au recovery to 90.6% and 81.1% (vs 74.6% and 15.8%) under galvanic and passivation impacts from pyrite. Silver minerals with sphalerite under galvanic and passivation effects enhanced Au recovery to 71.1% and 80.5% (vs 1.6% and 15.6%). Silver minerals associated with chalcopyrite retarded Au recovery to 10.2% and 4.5% under galvanic and passivation impacts (vs 57% and 14.1%). Stibnite retarded Au dissolution with pyrargyrite and accumulative silver minerals. Pyrargyrite and accumulative silver enhanced gold dissolution for free gold and gold associated with pyrite and sphalerite. Gold dissolution was retarded for gold and silver minerals associated with chalcopyrite and stibnite.

Effect of Gold Addition on the Microstructure and Mechanical Properties of Sn–3.8Ag–0.7Cu Lead-Free Solder Alloy

The effect on mechanical properties and microstructure of SAC 387 lead free solder alloys by addition of 1 wt% Au nanoparticles was studied. Also the effect on tensile properties at different temperatures and strain rates was investigated. The powder metallurgy process followed to make the SAC 387 + 1 wt% Au alloy was found to result in mechanical alloying and complete dissolution of Au in the β-Sn matrix resulting in the formation of fine AuSn4 intermetallics. The addition of 1 wt% Au to the base alloy SAC 387 resulted in an increase in yield (YS) and ultimate tensile strengths (UTS) at all temperatures and strain rates without any decrease in the total elongation (TE). Both YS and UTS were found to decrease with increase in temperature at any given strain rate for the base alloy SAC 387 as well as SAC 387 + 1 wt% Au alloy. On the other hand, for both alloys, YS and UTS were found to increase with increase in strain rate at any given test temperature. Although the trends in change of uniform elongation (UE) and TE with temperature and strain rate were not as monotonic as in the case of YS and UTS, in general, the UE and TE increased with increasing temperature and decreased with increasing strain rate. The results were explained based on the likely deformation mechanisms operative in these alloys at different strain rates and temperatures.

Mineralogical and metallurgical study of supergene ores of the Mike Cu[sbnd]Au([sbnd]Zn) deposit, Carlin trend, Nevada

The undeveloped Mike CuAu(Zn) deposit on the Carlin trend of Nevada, USA, currently held by Newmont Mining, hosts an unusual suite of oxide Cu and Au minerals. This study evaluates the possibilities for economic recovery by 1) comparing the Cu and Au extraction achieved from the supergene ores by six different lixiviants, and 2) identifying which minerals cause low recovery for each lixiviant and why. Gold is present as auricupride and electrum and copper exists as conichalcite (CaCu arsenate), native Cu, cuprite, chrysocolla, and minor malachite, with locally significant copper in jarosite, hematite, and goethite. The reagents tested were sulfuric, sulfurous, and methanesulfonic acids to recover Cu, cyanide and thiourea to recover Au, and glycine to recover both. No reagent recovered both Cu and Au effectively, although sulfuric and methanesulfonic acids recovered most of the Cu and cyanide and thiourea recovered most of the Au. Glycine recovered nearly all of the Au and >50% of the Cu from the cuprite- and native-Cu-dominated ore type, but did not recover much Cu or Au from any of the other ore types. This supports the hypothesis that Cu2+ catalyzes Au dissolution in glycine, and thus effective Au leaching by glycine from oxide CuAu ores may be contingent upon the solubility of the Cu minerals in glycine.Post-leaching QEMSCAN analysis indicates that conichalcite and Cu-bearing Fe oxides failed to dissolve completely in all reagents and were the principal causes of low recovery. In addition, native Cu dissolved only partly in thiourea, and chrysocolla dissolved only partly in thiourea, glycine, or cyanide. Other observed mineralogical changes include the total loss of dolomite and partial loss of alunite and iron oxide from all samples, with apparent gains in alunite and jarosite.

Enhancement of gold and silver recovery from discarded computer printed circuit boards by Pseudomonas balearica SAE1 using response surface methodology (RSM).

Two-step bioleaching was applied using a cyanogenic bacterium Pseudomonas balearica SAE1 to recover gold (Au) and silver (Ag) from the computer printed circuit boards (CPCBs) via central composite design of a response surface methodology (CCD-RSM). To enhance Au and Ag recovery, factors like pH level, pulp density, temperature and glycine concentration were optimized and their interactions were studied. CCD-RSM optimization resulted in 73.9 and 41.6% dissolution of Au and Ag, respectively, at initial pH 8.6, pulp density 5g/L, temperature 31.2°C, and glycine concentration 6.8g/L, respectively. Two quadratic models were proposed by RSM which can be utilized as an efficient tool to predict Au and Ag recovery through bioleaching. The experimental results are in line with the predicted results, indicating reliability of RSM model in enhancing the Au and Ag recovery from CPCBs. The increased bioleaching yield of Au and Ag from discarded CPCBs has its importance in industrial e-waste recycling and safe disposal.

Bioleaching of Gold and Silver from Waste Printed Circuit Boards by Pseudomonas balearica SAE1 Isolated from an e-Waste Recycling Facility.

Indigenous bacterial strain Pseudomonas balearica SAE1, tolerant to e-waste toxicity was isolated from an e-waste recycling facility Exigo Recycling Pvt. Ltd., India. Toxicity tolerance of bacterial strain was analyzed using crushed (particle size ≤150µm) waste computer printed circuit boards (PCBs)/liter (L) of culture medium. The EC50 value for SAE1 was 325.7g/L of the e-waste pulp density. Two-step bioleaching was then applied to achieve the dissolution of gold (Au) and silver (Ag) from the e-waste. To maximize precious metal dissolution, factors including pulp density, glycine concentration, pH level, and temperature were optimized. The optimization resulted in 68.5 and 33.8% of Au and Ag dissolution, respectively, at a pH of 9.0, a pulp density of 10g/L, a temperature of 30°C, and a glycine concentration of 5g/L. This is the first study of Au and Ag bioleaching using indigenous e-waste bacteria and its analysis to determine e-waste toxicity tolerance.

The Effect of the Redox Potential of Aqua Regia and Temperature on the Au, Cu, and Fe Dissolution from WPCBs

Constant growth in waste electrical and electronic equipment (WEEE) levels necessitates the development of new, commercially viable recycling processes. Waste printed circuit boards (WPCBs) are a sub-group of WEEE that are of increasing interest due to their relatively high level of valuable metal content including Au, Ag, and platinum group metals (PGMs). Currently, precious metals like gold are mainly recycled from WEEE streams through copper smelting/refining; however, the possibility to peel gold from WPCBs prior to smelting, could offer advantages for recycling. In this study, the suitability of aqua regia for selective or partially selective gold leaching from un-crushed WPCBs was investigated. The redox potential of aqua regia solutions and the dissolution efficiencies of Au, Cu, and Fe from WPCBs were investigated at different temperatures (40–80 °C) and concentrations (2–32%) in batch leaching tests. The redox potential of aqua regia solution was found to depend on concentration and temperature. It is suggested that Au dissolution in aqua regia requires dissolved Cu2+ ions originating from the WPCB material to work. Au extraction (&gt;50%) was shown to require a redox potential &gt;700 mV with [Cu2+] &gt; 2500 ppm, as a potential &gt;850 mV alone was insufficient without cupric ions. Significant amounts of Au and Cu could be dissolved with only minor Fe dissolution at ≥8% aqua regia at 80 °C. Results suggest that leaching of uncrushed WPCBs in 8% aqua regia (T = 80 °C) can provide the opportunity for partial Au recovery prior to further processing.