Clean Gold Research Articles

Mitigating the negative effects of clay minerals on gold flotation by a lignosulfonate-based biopolymer

The objective of this study was to identify various mechanisms responsible for the poor flotation of a high-clay-content gold ore and then develop a remedial solution. Compared to the flotation of a clean gold ore with a low clay content, obtained from the same flotation plant, the flotation of the high-clay-content gold ore produced much lower gold grade and gold recovery. The degree of entrainment (ENT) was calculated and it was found that a high mechanical entrainment of gangue minerals occurred in the flotation of the high-clay-content gold ore, contributing to the low gold grade in flotation. Electrochemical impedance spectroscopic (EIS) measurements were conducted to detect a possible clay slime coating on gold surface during flotation. It was observed that a clay slime coating did not occur on gold surface in the flotation of the high-clay-content gold ore, and therefore slime coating was not the reason for the poor gold flotation performance. Rheological measurements show that the high-clay-content gold ore slurry exhibited a non-Newtonian behaviour with a high apparent viscosity, contributing to the low gold recovery in flotation. Based on the previous studies, a lignosulfonate-based biopolymer (DP-1777) was used to mitigate the negative effects of clay minerals in the flotation of the high-clay-content gold ore. It was found that DP-1777 improved both gold grade and gold recovery significantly through the reduction of mechanical entrainment and pulp viscosity, respectively.

Galvanic interaction of grinding media with arsenopyrite and pyrite and its effect on gold cyanide leaching

Galvanic interactions between different types of grinding media and arsenopyrite and pyrite, during the grinding was quantified and the effect of galvanic interactions on gold cyanide leaching was investigated. The synthetic ores were prepared by mixing arsenopyrite or pyrite with clean gravity gold concentrate and quartz. In the presence of arsenopyrite, the results showed that when the grinding of the ore was performed using the 30% chromium or ceramic grinding media, the gold leaching was more efficient than that when the forged steel media was used. Based upon the measurement, it was suggested that the galvanic interactions between the forged steel grinding media and sulfide minerals resulted in the formation of more iron hydroxide than that when the 30% chromium or ceramic grinding media was used. As a result, free cyanide was converted to ferrocyanide thereby lowering the cyanide available for the gold leaching. Additionally, the galvanic interactions also reduced the dissolved oxygen content and thus cyanide gold leaching was more difficult particularly at the early stages of the leaching process because the dissolved oxygen is required to dissolve gold. In contrast, when the grinding was performed with the 30% chromium or ceramic media, a very small amount of free cyanide was converted into by-products. Cyanide gold leaching was the most efficient when the 30% chromium media was used during the grinding of the ore for both pyrite and arsenopyrite. In addition, in the presence of pyrite, due to the lower galvanic current between pyrite and grinding media in comparison with arsenopyrite, the amount of iron hydroxide was lower, and thus more free cyanide was available for the gold extraction. These results indicated that more gold was extracted in the presence of pyrite rather than the arsenopyrite for a given media type used. More precisely, in the presence of pyrite, when the 21% chromium grinding media was used, 70% of gold was extracted while it was 60% in the case of arsenopyrite in 24 h. The selection of grinding media appears to be significant when cyanide reagent consumption is evaluated.

Electrocatalytic activity of Chemisorbed Hydroxyl ion towards Oxidation of Hydrogen peroxide anion in Alkaline media

In situ chemisorbed hydroxyl ion (OH-) onto clean polycrystalline gold (Au (poly)) electrode showed an extraordinary enhanced electrocatalytic activity towards oxidation of hydrogen peroxide anion (HO2-) in alkaline media. The oxidation of HO2- at the fabricated electrode was found to be a diffusion controlled process. The anodic transfer coefficient of the oxidation reaction was estimated as 0.47. Chloride ion (Cl-) replaced the chemisorbed OH- from the surface of the modified Au (poly) (OH-)Au (poly)) electrode leading to diminishing the observed electrocatalytic activity. The optimized concentration of KOH solution for the highest catalytic activity was found to be ca. 0.1 M. A linear calibration curve for various concentrations of HO2-? in the range of 0.5 to 5 mM was obtained having sensitivity equal to 1.028 A cm-2 M-1.Journal of Bangladesh Academy of Sciences, Vol. 41, No. 1, 29-36, 2017

Tunichrome-Inspired Gold-Enrichment Dispersion Matrix and Its Application in Water Treatment: A Proof-of-Concept Investigation.

Tunicate, a filter-feeder in seawater, is able to accumulate high amount of metals using intracellular polymer matrices. The woven pyrogallol structures of tunichrome, a small peptide contained in tunicate's blood cells, is believed to be responsible for selective metal sequestration in tunicates from seawater. However, the intriguing tunichrome matrix is difficult both to harvest from the tunicate and to synthesize massively due to the extreme oxidation sensitivity of the pyrogallol moiety which limits the study scope. Here, we succeeded to mimic tunichrome by conjugating two cheap and naturally occurring components: pyrogallol-5-carboxylic acid (gallic acid) and chitin nanofiber. A tunicate-mimetic infiltration matrix of surface-tailored chitin nanofibers with pyrogallol moieties (CGa) demonstrated the versatility of this strategy in generation of ingenious filtration material, especially for unprecedented fine and clean gold recovery inside of the tunicate-mimetic infiltration matrix (>99%, 533 mg gold per gram weight), which exceeds that of the presently most popular materials. Complexation between pyrogallol on the nanofiber and gold was similar to that of a tunichrome's metal sequestration. Extended X-ray absorption fine structure (EXAFS) spectroscopy and data-fitting elucidated the decreased coordination numbers for Au-Au nearest neighbors, demonstrating that gold coordinated to pyrogallol units, followed by an intramolecular association of Au0. A catalytic reduction of 4-nitrophenol mediated by the tunicate-mimetic matrix with harvested gold revealed excellent recyclability up to 30 cycles (∼95% reduction), which together with methylene blue reduction and antimicrobial performances indicates the versatile characteristics of sustainable processes by the tunichrome mimetics. This strategy opens the door for fast-developing new biomimetic alternatives for precious metal recovery, which is not restricted to gold and can offer a tool for multifaceted soft/hard nanomaterials.

Barefoot sampling in San Juan de Limay, Nicaragua: remediation of mercury pollution from small scale gold mining tailings

Author Summary: The increasing population on planet Earth has many impacts—one is a strong influence on the amount of mercury released to the environment. Burning of coal in power plants, particularly in China, has tripled several times during the last century and so has the use of coal for cooking for the ever-increasing population of India and elsewhere. These sources account for the second largest release of mercury to the environment. But the worst sinner is the rapidly increasing number of small-scale gold miners in Asia, Africa, Central and South America, who presently provide food on the table for 10s of millions of households. Small-scale gold miners use vast amounts of mercury for capturing the gold and much of this mercury is released directly to the environment. A large part evaporates to the atmosphere and the rest is transported downstream in rivers ending up in the oceans. The amount of mercury released is phenomenal, an estimated 3000 tons of mercury is released annually by small-scale gold miners alone. A vast proportion enters the food chain in fish and sea mammals, as well as in rice polluted by spillage waters which enters irrigation pathways. Human consumption of polluted fish and/or rice already today has a very severe impact on human health, and this will have even more severe consequences if the current situation is not changed radically. It is of particular concern if mercury-intoxicated women become pregnant because the foetus extracts mercury from the mother. The human foetus is much more sensitive to mercury intoxication and thus has a high risk of being born with brain damage as well as physical disabilities. Over time this will cause reduced intelligence for exposed children in the next generation. This grim outlook has prompted a group of concerned researchers to teach small-scale gold miners to work without the use of mercury and simultaneously to find ways to clean mercury-polluted gold mining tailings, which are one of the main polluting agents. Here we report on one specific part of this endeavour where the Theory of Sampling (TOS) was needed in order to secure reliable estimates of gold and mercury contents in dispersed mining tailings.First published in TOS Forum Issue 7, 30 (2017)

How “clean gold” came to matter

Recently an ancient resource—gold—gained new importance in Sudan. To inquire into the making of values and their negotiation at a time of national economic reorientation, I explore the emergence of a resource category, so-called clean gold, and link this to the introduction of a new prospecting technology—metal detectors. I follow the translation of metal detectors to Sudan and explore them in relation to a broader techno-economic infrastructure of artisanal gold mining that enables the extractive practice and circulates new forms of moral reasoning. Not only has a new resource category emerged but I argue that these devices and the moral worlds they have coconstituted have also enabled younger men to test and challenge the economic and moral authority of an older generation of men.

Low-energy electron-induced dissociation in condensed-phase L-cysteine II: a comparative study on anion desorption from chemisorbed and physisorbed films

Due to its multifunctional structure, cysteine is becoming an ideal model molecule for investigating the complex interactions of proteins with metallic surfaces such as gold nanoparticles. We report herein the results of low-energy electron induced degradation of L-cysteine films, chemisorbed on a gold substrate via the thiol group or physisorbed into a clean gold surface. The data were recorded under ultra-high vacuum conditions at room temperature. Anion yields desorbed from these films by the impact of 0.5 to 19 eV electrons provide clear evidence of the efficient decomposition of this amino acid via dissociative electron attachment (i.e., from dissociation of intermediate transient anions located between 5 and 14 eV). The peaks in the desorbed-anion yield functions, associated with DEA, are superimposed on a continuously rising signal attributed to dipolar dissociation. Similar to the results previously observed from physisorbed films, light anionic species, with masses lower than 35 amu, have been detected. In addition, we measured for first time fragments at 14 amu (CH2 -) and 15 amu (CH3 -) desorbing from physisorbed films, as well as heavier fragments of mass 45 and 46 amu desorbing from chemisorbed films.

Surface clean gold nanoflower obtained by complete removal of capping agents: an active catalyst for alcohol oxidation

Morphological stability and catalytic activity of Au nanoflowers (NFs) were improved by using γ-Al2O3 support and water extraction procedure. Formation rate of acetophenone on Au NFs/γ-Al2O3 was ten-fold higher than that on spherical Au NPs/γ-Al2O3.

A Novel Method of Immobilizing Antigens on Gold Electrode for Immunosensing

An antigen modified gold electrode based on a self-assembled lipoic acid monolayer has been developed. The contact angle measurement and cyclic voltammetry confirm the formation of a dense self-assembled monolayer on gold from 100 mM lipoic acid in ethanol. The electrochemical behavior shows a stable activity in a range of potential from -0.2 V to 0.65 V vs. Ag/AgCl. By applying a potential of 1 V vs. Ag/AgCl the monolayer is completely removed by oxidative desorption and a clean gold surface re-established. This allows for an easy renewal of the gold surface and recycling of modified immunosensor chips. This lipoic acid monolayer was covalently functionalized with a small molecule antigen synthesized from progesterone-3-(O-carboxymethyl)oxime and 4,7,10-trioxa-1,13 tridecanediamine to form a well ordered, low unspecific binding, antigen layer for an antibody-antigen interaction study.The effectiveness of antigen - antibody binding reaction was demonstrated by fluorescence imaging using a fluorescence-labeled anti-progesterone antibody as the binding partner. The new approach represents a way of improving the surface chemistry of electrochemical, surface-plasmon resonance and QCM-based immunosensors for which reusability, simplicity and sensitivity in flow-injection mode are required.

Synthesis, Electrochemistry, and Single-Molecule Conductance of Bimetallic 2,3,5,6-Tetra(pyridine-2-yl)pyrazine-Based Complexes.

The ligands 4'-(4-(methylthio)phenyl)-2,2':6',2″-terpyridine (L(1)), 4'-((4-(methylthio)phenyl)ethynyl)- 2,2':6',2″-terpyridine (L(2)), and bis(tridentate) bridging ligand 2,3,5,6-tetra(pyridine-2-yl)pyrazine (tpp) were used to prepare the complexes [Ru(L(1))2][PF6]2 ([1][PF6]2, [Ru(L(2))2][PF6]2 ([2][PF6]2), [{(L(1))Ru}(μ-tpp){Ru(L(1))}][PF6]4 ([3][PF6]4), and [{(L(2))Ru}(μ-tpp){Ru(L(2))}][PF6]4 ([4][PF6]4). Crystallographically determined structures give S···S distances of up to 32.0 Å in [4](4+). On the basis of electrochemical estimates, the highest occupied molecular orbitals of these complexes fall between -5.55 and -5.85 eV, close to the work function of clean gold (5.1-5.3 eV). The decay of conductance with molecular length across this series of molecules is approximately exponential, giving rise to a decay constant (pseudo β-value) of 1.5 nm(-1), falling between decay factors for oligoynes and oligophenylenes. The results are consistent with a tunnelling mechanism for the single-molecule conductance behavior.

Rh and Au deposited on ultrathin TiO~1.2 film formed on Rh(111) facets and the effects of CO exposure

Abstract This STM study deals with the structure and reactivity of Rh and Au deposited on a “pinwheel” TiO~ 1.2 ultrathin film (w-TiO-UTO) formed on Rh nanoparticles supported on TiO2(110). The deposition of a monolayer (ML) Au at 500 K disturbed the order of the w-TiO-UTO layer and led to the formation of 3D adparticles. The deposition of Rh (~ 1 ML) at 500 K and annealing up to 900 K led exclusively to the formation of 2D adparticles. It was found that 3D Au nanoparticles are gradually sintering in the range of 500–800 K. Upon a further raise of the temperature, a 3D→2D transformation takes place. The main difference is that on the top facet of the 2D Rh particles a w-TiO-UTO film forms, while the top facets of the 2D Au particles exhibit a clean gold metal structure. CO exposure of max. 10 min at 10 Pa causes no remarkable change of the encapsulated Rh particles, however, the 2D gold nanoparticles change significantly. The disintegration of Au particles and the spillover of Au atoms to the UTO surface accelerate at a CO pressure above 10− 1 Pa. This morphological process is reversible as an effect of annealing in UHV.

Surface-enhanced Raman spectroscopic detection and differentiation of lung cancer cell lines (A549, H1229) and normal cell line (AT II) based on gold nanostar substrates

In this paper, a non-invasive, simple and efficient approach is constructed to make clean gold nanostar substrates with higher enhancement effect, homogeneous surface-enhanced Raman scattering (SERS) activity and high reproducibility, which are the assembly of gold nanostars based on the electrostatically assisted APTES-functionalized surface-assembly method. The obtained substrates enable us to successfully compare and distinguish two lung cancer cell lines [human lung adenocarcinoma epithelial cell line (A549), human non-small cell lung cancer cell line (H1229)] and one normal lung cell line [human type II alveolar epithelial cell line (AT II)] through SERS spectra. The difference spectra (A549 – H1229, A549 – AT II and H1229 – AT II) among the three cell lines are studied to better interpret the spectral differences and gain insight into the biochemical variation. Furthermore, principal component analysis (PCA) score plots of the SERS spectra are also used to better view the differences among these three cell lines. This sensitive, efficient and non-invasive detection method, based on gold nanostar substrates, will have great potential for the differentiation of living cells and play an important role in the early diagnosis and treatment of cancers.

How reliable is the ReaxFF potential for describing the structure of alkanethiols on gold? A molecular dynamics study

The structures of self-assembled monolayers (SAMs) of short (methyl) and long (hexyl) chain alkyl thiols on the clean gold(111) surface were modeled using for the Au – S interactions either the reactive ReaxFF potential or the well-known nonreactive Morse potential, while for the Au – Au interactions either the ReaxFF potential or an embedded-atom method (EAM). Analysis of the molecular dynamics (MD) trajectories of possible SAM structures suggests that disordering of interfacial Au atoms is definitely driven by the gold–sulphur interactions. Our MD results reveal a novel structure where two methanethiol molecules are bound to a gold adatom that has been lifted from the surface at 300 K, and the same kind of RS– Au –SR motif was also observed for hexanethiol at 600 K but not at 300 K. What is more, the above motif is only observed for the reactive ReaxFF potential. Moreover, these results are in clear agreement with recent experiments and more costly first principles-based MD simulations. These findings strongly support the use of reactive potentials such as ReaxFF for gathering an accurate description of Au – S interactions in inexpensive classical MD simulations.

Polymer inclusion membranes composed of long alkyl chain alcohols for clean gold (III) recovery

AbstractBACKGROUNDIn addition to monetary and symbolic functions, gold has many practical uses in dentistry, electronics, textile and spacecraft industries, plating, and medicine. Consequently, gold recycling is an important activity. In this work, extraction and transport of gold (III) from concentrated HCl media by long chain alcohols were studied using model systems with octanol, decanol, and dodecanol in a two‐ (liquid and solid extraction) or three‐phase (polymer inclusion membrane, PIM) configurations.RESULTSNumerical data analysis revealed HAuCl4 extraction by a solvation mechanism. Regarding polymer inclusion membranes (PIMs), the efficiency factors (permeability, selectivity, and stability) of the systems were determined. Through the analysis of extraction results, permeability profiles as a function of the content of the components, thermal analyses and reported evidence of extractant aggregation, the gold (III) transport mechanism was characterized as flow‐type.CONCLUSIONGold (III) extraction and transport was satisfactorily accomplished with excellent separation from Pd(II), Pt(IV), Cu(II) and Fe(III) in the membrane system. Dodecanol‐based PIMs presented the highest stability and permeability (≈ 0.32 cm min‐1). The work extends previous analyses of metal transport mechanisms in PIMs toward systems in which the extremely high extractant/co‐extractant concentrations allow structuration of the media with consequent formation of conducting channels for solute migration. © 2014 Society of Chemical Industry.

Atomic Friction at Electrodes: Anisotropy Effects at Regularly Stepped Gold Electrodes

Friction under ambient conditions often involves wet surfaces, and thus electrochemical interfaces. Yet measurements of friction at an atomic scale under electrochemical conditions are scarce though accessible by combination of AFM with an electrochemical cell. For HOPG, a change of friction forces at steps edges was observed with a change of potential.[1] Our interest has been in the influence of potential, Cu UPD on Au(111) and organic adsorption on atomic-scale friction.[2] We will present friction measurements performed on Au(111) single crystal electrodes and the effect of adsorbates on atomic stick-slip resolution. The range of normal force necessary to observe stick-slip resolution depends on the type of ionic adsorbates. At the small normal load of 15 nN transition to atomic stick-slip resolution is observed, when sulfate is adsorbed on the surface. The reason for that transition could be a penetration of the tip in to the double layer. At the pzc on clean gold stick-slip behaviour not shows any transition or penetration effects. The adhesion on the force-distance curve disappear with a copper adsorption. However, with a monolayer of Cu on Au(111) a transition to stick-slip behavior is present, but without atomic resolution. We found 3 different types of stick-slip pattern on copper monolayer in diapason of normal loads between 0 and 100 nN; slip length is equal 4,6; 9,2; and 13,8 Å. The slope of stick combines the stiffness of tip and sample. That stiffness is independent from adsorbat or normal load and always have the same value at about 12 nN/nm, which is much smaller than torsion spring constant of cantilever 190 nN/nm.Friction is increased at step edges due to the the Schwoebel barrier. This effect is reduced when steps are decorated by Pd on Au. Due to this effect of steps on friction, vicinally stepped surfaces show a large anisotropy for friction, although atomic resolution is obtained even when scanning perpendicular to the steps.[1] E. Weilandt, A. Menck, O. Marti, Surface and Interface Analysis 1995, 23, 428; B. Schnyder, D. Alliata, R. Kötz, H. Siegenthaler, Applied Surface Science 2001, 173, 221.[2] M. Nielinger, H. Baltruschat, Physical Chemistry Chemical Physics 2007, 9, 3965.; F. Hausen, M. Nielinger, S. Ernst, H. Baltruschat, Electrochimica Acta 2008, 53, 6058.

Tuning the Stability of Surface Intermediates Using Adsorbed Oxygen: Acetate on Au(111).

Selective oxidative reactions promoted by gold depend critically on controlling the coverage and stability of adsorbed intermediates, as well as promoting specific bond activations of those intermediates. We demonstrate that acetate, a common intermediate in the oxidation of olefins, aldehydes, and alcohols, is destabilized by 7-10 kcal/mol by coadsorbed oxygen relative to its stability on the clean gold surface. The amount of destabilization depends on the oxygen coverage. Peak temperatures of products indicative of oxygen-assisted and clean-surface bond activation differ by up to 130 K. Experiments with d3-acetate show a kinetic isotope effect of 6.9 at 400 K, indicating that the rate-limiting step of the low temperature oxygen-assisted reaction is γ-CH bond breaking. This clearly demonstrates that coadsorbed oxygen activates γ-CH bonds on gold and suggests that an oxygen-assisted activation may also occur for β-CH bonds crucial in oxygen-assisted alcohol coupling on metallic gold catalysts, as predicted by theory.

How reliable is the ReaxFF Potential for Describing the Structure of Alkanethiols on Gold? A Molecular Dynamics Study

The structures of self-assembled monolayers (SAMs) of short (methyl) and long (hexyl) chain alkyl thiols on the clean gold (111) surface were modelled using for the Au-S interactions either the reactive ReaxFF potential or the well known non-reactive Morse potential, while for the Au-Au interactions either the ReaxFF potential or an embedded-atom method (EAM). Analysis of the MD trajectories of possible SAM structures suggests that disordering of interfacial Au atoms is definitely driven by the gold-sulphur interactions. Our MD results reveal a novel structure where two methanethiol molecules are bound to a gold adatom that has been lifted from the surface at 300 K, and the same kind of RS-Au-SR motif was also observed for hexanethiol at 600 K but not at 300 K. What is more, the above motif is only observed for the reactive ReaxFF potential. Moreover, these results are in clear agreement with recent experiments and more costly first principles-based MD simulations. These findings strongly support the use of reactive potentials such as ReaxFF for gathering an accurate description of Au-S interactions in inexpensive classical MD simulations.

Surface assembly and nanofabrication of 1,1,1-tris(mercaptomethyl)heptadecane on Au(111) studied with time-lapse atomic force microscopy

The solution self-assembly of multidentate organothiols onto Au(111) was studied in situ using scanning probe nanolithography and time-lapse atomic force microscopy (AFM). Self-assembled monolayers (SAMs) prepared from dilute solutions of multidentate thiols were found to assemble slowly, requiring more than six hours to generate films. A clean gold substrate was first imaged in ethanolic media using liquid AFM. Next, a 0.01 mM solution of multidentate thiol was injected into the liquid cell. As time progressed, molecular-level details of the surface changes at different time intervals were captured by successive AFM images. Scanning probe based nanofabrication was accomplished using protocols of nanografting and nanoshaving with n-alkanethiols and a tridentate molecule, 1,1,1-tris(mercaptomethyl)heptadecane (TMMH). Nanografted patterns of TMMH could be inscribed within n-alkanethiol SAMs; however, the molecular packing of the nanopatterns was less homogeneous compared to nanopatterns produced with monothiolates. The multidentate molecules have a more complex assembly pathway than monothiol counterparts, mediated by sequential steps of forming S–Au bonds to the substrate.

Adsorption and dissociation of H2O on Au(1 1 1) surface: A DFT study

Periodic density functional theory (DFT) calculations using plane waves were performed to systematically investigate the stable adsorption of H2O on Au(111) surface and the corresponding dehydrogenated reaction. The equilibrium configuration on top, bridge, and hollow (fcc and hcp) site were determined by relaxation of the system. The adsorptions of H2O on top site and OH on bridge site are favorable on Au(111) surface, while the adsorption of O on hollow(fcc) site is preferred. The adsorbates are adsorbed on the gold surface due to the interaction between p orbital of adsorbates and d orbital of gold atoms, and the interaction is more evident on the first layer than that on any others. We also investigated the dissociation reaction of H2O on clean and oxygen-covered gold surface. The results show that the hydrogen atom dissociating from H2O need the energy of 255.54kJ/mol on clean gold surface, while the dissociating energy decreases to 44.61kJ/mol with the aid of adsorbed O atom (Oads). These phenomena indicate that the dehydrogenated reaction energy barrier on the pre-covered oxygen gold surface is lower and Oads can promote the dehydrogenation of H2O.

Identifying Molecular Species on Surfaces by Scanning Tunneling Microscopy: Methyl Pyruvate on Pd(111)

The structures of low coverages of methyl pyruvate on a Pd(111) surface at 120 K were studied using scanning tunneling microscopy in ultrahigh vacuum. The experimentally observed images were assigned to adsorbate structures using a combination of density functional theory calculations and by simulating the images using the Bardeen method. Two forms of methyl pyruvate were identified. The first, previously found using reflection–absorption infrared spectroscopy, was a flat-lying, keto form of cis-methyl pyruvate. It was characterized by elongated, two-lobed images with the long axes of the images oriented at ∼0 and ∼30° to the close-packed directions. The structure was simulated using clean, CO- and methyl-functionalized gold tips, and the simulated images agreed well with those found experimentally. The simulated structures were not strongly dependent on the tip structure or tip bias. This approach was used to identify the nature of the second species as the enol form of cis-methyl pyruvate with the carbo...