Palladium and gold removal and recovery from precious metal solutions and electronic scrap leachates by Desulfovibrio desulfuricans

Biomass of Desulfovibrio desulfuricans was used to recover Au(III) as Au(0) from test solutions and from waste electronic scrap leachate. Au(0) was precipitated extracellularly by a different mechanism from the biodeposition of Pd(0). The presence of Cu²⁺ (∼2000 mg/l) in the leachate inhibited the hydrogenase-mediated removal of Pd(II) but pre-palladisation of the cells in the absence of added Cu²⁺ facilitated removal of Pd(II) from the leachate and more than 95% of the Pd(II) was removed autocatalytically from a test solution supplemented with Cu(II) and Pd(II). Metal recovery was demonstrated in a gas-lift electrobioreactor with electrochemically generated hydrogen, followed by precipitation of recovered metal under gravity. A 3-stage bioseparation process for the recovery of Au(III), Pd(II) and Cu(II) is proposed.Victoria S. Baxter-Plant – Deceased     

A Whole-Cell Biosensor for the Detection of Gold

Geochemical exploration for gold (Au) is becoming increasingly important to the mining industry. Current processes for Au analyses require sampling materials to be taken from often remote localities. Samples are then transported to a laboratory equipped with suitable analytical facilities, such as Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) or Instrumental Neutron Activation Analysis (INAA). Determining the concentration of Au in samples may take several weeks, leading to long delays in exploration campaigns. Hence, a method for the on-site analysis of Au, such as a biosensor, will greatly benefit the exploration industry. The golTSB genes from Salmonella enterica serovar typhimurium are selectively induced by Au(I/III)-complexes. In the present study, the golTSB operon with a reporter gene, lacZ, was introduced into Escherichia coli. The induction of golTSB::lacZ with Au(I/III)-complexes was tested using a colorimetric β-galactosidase and an electrochemical assay. Measurements of the β-galactosidase activity for concentrations of both Au(I)- and Au(III)-complexes ranging from 0.1 to 5 µM (equivalent to 20 to 1000 ng g⁻¹ or parts-per-billion (ppb)) were accurately quantified. When testing the ability of the biosensor to detect Au(I/III)-complexes_(aq) in the presence of other metal ions (Ag(I), Cu(II), Fe(III), Ni(II), Co(II), Zn, As(III), Pb(II), Sb(III) or Bi(III)), cross-reactivity was observed, i.e. the amount of Au measured was either under- or over-estimated. To assess if the biosensor would work with natural samples, soils with different physiochemical properties were spiked with Au-complexes. Subsequently, a selective extraction using 1 M thiosulfate was applied to extract the Au. The results showed that Au could be measured in these extracts with the same accuracy as ICP-MS (P<0.05). This demonstrates that by combining selective extraction with the biosensor system the concentration of Au can be accurately measured, down to a quantification limit of 20 ppb (0.1 µM) and a detection limit of 2 ppb (0.01 µM).     

Zinc finger proteins as templates for metal ion exchange: Substitution effects on the C-finger of HIV nucleocapsid NCp7 using M(chelate) species (M = Pt, Pd, Au)

The interactions of monofunctional [MCl(chelate)] compounds (M = Pt(II), Pd(II) or Au(III) and chelate = diethylenetriamine, dien or 2,2′,2″-terpyridine, terpy) with the C-terminal finger of the HIV nucleocapsid NCp7 zinc finger (ZF) were studied by mass spectrometry and circular dichroism spectroscopy. In the case of [M(dien)] species, Pt(II) and Pd(II) behaved in a similar fashion with evidence of adducts caused by displacement of Pt-Cl or Pd-Cl by zinc-bound thiolate. Labilization, presumably under the influence of the strong trans influence of thiolate, resulted in loss of ligand (dien) as well as zinc ejection and formation of species with only Pd(II) or Pt(II) bound to the finger. For both Au(III) compounds the reactions were very fast and only “gold fingers” with no ancillary ligands were observed. For all terpyridine compounds ligand scrambling and metal exchange occurred with formation of [Zn(terpy)]²⁺. The results conform well to those proposed from the study of model Zn compounds such as N,N′-bis(2-mercapto-ethyl)-1,4-diazacycloheptanezinc(II), [Zn(bme-dach)]₂. The possible structures of the adducts formed are discussed and, for Pt(II) and Pd(II), the evidence for possible expansion of the zinc coordination sphere from four- to five-coordinate is discussed. This observation reinforces the possibility of change in geometry for zinc in biology, even in common “structural” sites in metalloenzymes. The results further show that the extent and rate of zinc displacement by inorganic compounds can be modulated by the nature (metal, ligands) of the reacting compound.     

Synthesis, characterization and biological activity evaluation of Pt(II), Pd(II), Co(III) and Ni(II) complexes with N-heteroaromatic selenosemicarbazones

Five new complexes of Pt(II), Pd(II), Co(III) and Ni(II) with 2-pyridine(quinoline)carboxaldehyde selenosemicarbazones were synthesized and characterized. Crystal structures of Pt(II) complex with the pyridine derivative and Co(III) complex with the quinoline derivative were determined. In all complexes the ligands were coordinated through N₂Se donor atom set forming either square-planar (Pt, Pd) or octahedral (Co, Ni) geometry. All complexes showed biological activity.     

Adsorption of gold(III), platinum(IV) and palladium(II) onto glycine modified crosslinked chitosan resin

The adsorption of Au(III), Pt(IV) and Pd(II) onto glycine modified crosslinked chitosan resin (GMCCR) has been investigated. The parameters studied include the effects of pH, contact time, ionic strength and the initial metal ion concentrations by batch method. The optimal pH for the adsorption of Au(III), Pt(IV) and Pd(II) was found to range from 1.0 to 4.0 and the maximum uptake was obtained at pH 2.0 for Au(III), Pt(IV) and Pd(II). The results obtained from equilibrium adsorption studies are fitted in various adsorption models such as Langmuir and Freundlich and the model parameters have been evaluated. The maximum adsorption capacity of GMCCR for Au(III), Pt(IV) and Pd(II) was found to be 169.98, 122.47 and 120.39mg/g, respectively. The kinetic data was tested using pseudo-first-order and pseudo-second-order kinetic models and an intraparticle diffusion model. The correlation results suggested that the pseudo-second-order model was the best choice among all the kinetic models to describe the adsorption behavior of Au(III), Pt(IV) and Pd(II) onto GMCCR. Various concentrations of HCl, thiourea and thiourea-HCl solutions were used to desorb the adsorbed precious metal ions from GMCCR. It was found that 0.7M thiourea-2M HCl solution provided effectiveness of the desorption of Au(III), Pt(IV) and Pd(II) from GMCCR. The modification of glycine on crosslinked chitosan resin (CCR) was studied by Fourier transform infrared spectrometry (FTIR) and scanning electron microscopy (SEM).     

Tannin‐Adsorbents for Water Decontamination and for the Recovery of Critical Metals: Current State and Future Perspectives

Biosorption is known as an effective way to clean‐up water from organic and inorganic contaminants and has also emerged as a promising technology to recover critical substances. Tannins are renewable materials, coming from multiple vegetable sources. A variety of biosorbents have been developed from tannins, including tannin resins, rigid foams, composites with mesoporous silica, cellulose, collagen, and magnetic adsorbents. These materials have shown an excellent ability to uptake heavy‐metal cations (Cd(II), Cu(II), Pb(II), Ni(II), Cr(III)), owning to the chelating ability provided by the plentiful adjacent hydroxyl groups. In addition, tannin‐adsorbents have shown exceptional ability to remove Cr(VI), and to uptake Au(III) and Pd(II) from strong acidic solutions, which has evident application in the recovery of precious metals from e‐wastes leaching. The fact that tannin‐adsorbents can reduce the oxidation state of these adsorbates to Cr(III) and to elemental species of Au and Pd is interesting. Adsorption of dyes, surfactants, pharmaceuticals and antimony is also feasible, but the removal of certain metalloid species, such as arsenic and phosphate, seems to be limited even after applying chemical modifications. This article presents a systematic review on the preparation of tannin‐adsorbents and their application in water decontamination and in the recovery of critical metals.     

Oxidative DNA damage induced by HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid) buffer in the presence of Au(III)

Oxidative DNA damage was investigated by free radicals generated from HEPES (2-[4-(2-hydroxyethyl)-1-piperazinyl]ethanesulfonic acid) buffer, which is widely used in biochemical or biological studies, in the presence of Au(III). The effect of free radicals on the DNA damage was ascertained by gel electrophoresis, electron spin resonance (ESR) spectroscopy and circular dichroism (CD) spectroscopy. ESR results indicated the generation of nitrogen-centered cationic free radicals from the HEPES in the presence of Au(III) which cause the DNA damage. No ESR spectra were observed for phosphate, tris(hydroxymethyl)aminomethane (Tris-HCl) and acetate buffers in the presence of Au(III) or for HEPES buffer in the presence of other metal ions such as Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Pd(II) or [Au(III)(TMPyP)](5+) and [Pd(II)(TMPyP)](4+), where [H(2)(TMPyP)](4+) denotes tetrakis(1-methylpyridium-4-yl)porphyrin. Consequently, no DNA damage was observed for these buffer agents (e.g., phosphate, Tris-HCl or acetate) in the presence of Au(III) or for HEPES in the presence of other metal ions or the metalloporphyrins mentioned above. No detectable inhibitory effect on the DNA damage was observed by using the typical scavengers of reactive oxygen species (ROS) ()OH, O(2)(-) and H(2)O(2). This non-inhibitory effect indicated that no reactive oxygen species were generated during the incubation of DNA with HEPES and Au(III). The drastic change in CD spectra from positive ellipticity to negative ellipticity approximately at 270 nm with increasing concentration of Au(III) also indicated the significant damage of DNA. Only HEPES or Au(III) itself did not damage DNA. A mechanism for the damaging of DNA is proposed.     

Gold and palladium burden from dental restoration materials.

From 81 volunteers (16 without dental restorations, 65 with gold crowns or inlays) samples of saliva before and after chewing gum, blood, serum, urine and faeces were taken and analysed for gold (Au) and palladium (Pd). The Au concentration in all analysed biomonitors correlates significantly to the number of teeth with gold restorations. For Pd the correlations were still significant, but weaker than for Au. Persons with gold restorations show maximal Au and Pd concentrations, 10(2)-10(3) higher than the background burden. The calculated maximal daily Au load in saliva (1.38 mg Au per day) reaches the range of an oral Au therapy for rheumatoid arthritis with 6 mg Auranofin (= 1.74 mg Au per day). During this therapy severe and frequent side effects are reported. In contrast, the Au concentration in serum maximally reached from Au restorations, amounts to only approximately 1/20 of the Au level during arthritis therapy. But even under subtherapeutic doses of 1 mg Auranofin/day severe side effects have been reported (4 out of 56 cases). The mean Au blood concentration from 1 mg Auranofin daily was only 3 times higher than our maximum value. A toxicological classification of the Pd values is difficult, because no toxicological threshold limit has been established, especially for the low-level long-term burden with Pd.     

N-(2-Pyridyl)acetamide complexes of palladium(II), cobalt(II), nickel(II), and copper(II)

N-(2-Pyridyl)acetamide (aapH) complexes of palladium(II), cobalt(II), nickel(II), and copper(II) have been studied by means of magnetic susceptibilities, and infrared, electronic, and PMR spectra. In the octahedral complexes M(aapH)₂X₂(M = Co, Ni, Cu; X = Cl, Br, NCS, NO₃), bidentate aapH is chelated through the pyridine-N and amid-O atomes, whereas in the square-planar Pd(aapH)₂X₂ (X = Cl, Br) unidentate aapH is coordinated through the pyridine-N atom alone. Under alkaline conditions aapH is deprotonated in the presence of palladium(II) to form Pd(aap)₂·4H₂O, aap being an anionic bidentate ligand and chelating through the pyridine-N and amide-O atoms.     

In vitro toxicity of palladium(II) and gold(III) porphyrins and their aqueous metal ion counterparts on Trypanosoma brucei brucei growth

The trypanocidal effects of aqueous gold(III) and palladium(II) and their metalloporphyrin derivatives on Trypanosoma brucei brucei growth in culture have been studied using an Alamar Blue indicator assay. All the experiments were conducted in the dark. As previously described for mercury(II), cadmium(II) and lead(II) porphyrins [Chem.-Biol. Interact. 139 (2002) 177], the toxicity of the metalloporphyrin complex of palladium(II) to T. b. brucei parasites was much higher compared to the aqueous free palladium(II) and free base porphyrin. Palladium(II) porphyrin, free palladium(II), and the free base porphyrin were trypanocidal to T. b. brucei at concentrations >1.5 x 10(-6), >6.1 x 10(-6) and >1.9 x 10(-5) M, respectively. While gold(III) porphyrin was effective against the parasites at concentrations >4.8 x 10(-6) M, its aqueous gold(III) was toxic at concentrations as low as 2.0 x 10(-7) M due to the generation of free radicals in the presence of this metal ion which enhanced its toxicity to the T. b. brucei parasites. Although some cell division was observed in some of the cells treated with palladium(II) porphyrin, some dividing cells had no nucleus due to unequal division and delivery of the nuclei into the daughter cells. As a result, the rate of cell division decreased with time and cell death occurred within 24 h. Interestingly, trypanosomes treated with metalloporphyrin complexes displayed different morphological features from those cells treated with free base porphyrin or metal ions. Of all the porphyrins and free metal ions tested, only mercury(II) porphyrin and aqueous gold(III) ion were toxic to the trypanosomes in the 10(-7) M range. The chemotherapeutic potential of these observations is discussed.     

Selective recovery of precious metals by persimmon waste chemically modified with dimethylamine

Persimmon waste was chemically modified with dimethylamine (DMA) to obtain a tertiary-amine-type gel, named DMA persimmon waste gel (DMA-PW). It was found to be effective for the adsorption of Au(III), Pd(II), and Pt(IV) in hydrochloric acid medium. In contrast, base metals such as Cu(II), Zn(II), Fe(III), and Ni(II) were not practically adsorbed. The formation of ion pairs of the metal chloro complex anions with the protonated adsorption gels was proposed as the main adsorption process. The gel exhibited selectivity only for precious metals with a remarkably high capacity for Au(III), i.e., 5.63 mol/kg dry gel and comparable capacities, i.e., 0.42 and 0.28 mol/kg for Pd(II) and Pt(IV), respectively. According to the kinetic and electrochemical studies, the adsorption rate of Au(III) was greatly enhanced by the chemical modification. Also, its excellent adsorption characteristics for the precious metals were confirmed by adsorption and elution tests using a column packed with the DMA-PW gel.     

Reductive Precipitation of Gold by Dissimilatory Fe(III)-Reducing Bacteria and Archaea

Studies with a diversity of hyperthermophilic and mesophilic dissimilatory Fe(III)-reducing Bacteria and Archaea demonstrated that some of these organisms are capable of precipitating gold by reducing Au(III) to Au(0) with hydrogen as the electron donor. These studies suggest that models for the formation of gold deposits in both hydrothermal and cooler environments should consider the possibility that dissimilatory metal-reducing microorganisms can reductively precipitate gold from solution.     

Spectral and thermodynamic properties of Ag(I), Au(III), Cd(II), Co(II), Fe(III), Hg(II), Mn(II), Ni(II), Pb(II), U(IV), and Zn(II) binding by methanobactin from Methylosinus trichosporium OB3b

Methanobactin (mb) is a novel chromopeptide that appears to function as the extracellular component of a copper acquisition system in methanotrophic bacteria. To examine this potential physiological role, and to distinguish it from iron binding siderophores, the spectral (UV–visible absorption, circular dichroism, fluorescence, and X-ray photoelectron) and thermodynamic properties of metal binding by mb were examined. In the absence of Cu(II) or Cu(I), mb will bind Ag(I), Au(III), Co(II), Cd(II), Fe(III), Hg(II), Mn(II), Ni(II), Pb(II), U(VI), or Zn(II), but not Ba(II), Ca(II), La(II), Mg(II), and Sr(II). The results suggest metals such as Ag(I), Au(III), Hg(II), Pb(II) and possibly U(VI) are bound by a mechanism similar to Cu, whereas the coordination of Co(II), Cd(II), Fe(III), Mn(II), Ni(II) and Zn(II) by mb differs from Cu(II). Consistent with its role as a copper-binding compound or chalkophore, the binding constants of all the metals examined were less than those observed with Cu(II) and copper displaced other metals except Ag(I) and Au(III) bound to mb. However, the binding of different metals by mb suggests that methanotrophic activity also may play a role in either the solubilization or immobilization of many metals in situ.     

The coordination chemistry of N-(2-aminophenyl)- and N-(3-aminophenyl)quinoline-2′-carboxamide; two potentially tridentate ligands containing secondary amide and N-donor groups

New complexes of the general formulae Co(o-LH)₂X₂ (XCl, NCS), Co(o-LH)₂Br₂·EtOH (EtOHethanol), M(o-LH)(NO₃)₂ (MCo, Ni), Ni(o-LH)₂X₂ (XCl, Br, NCS), Cu(o-L)X (XCl, Br), Zn(o-LH)X₂ (XCl, Br), Pd(o-L)Cl, Pt(o-LH)₂Cl₂·H₂O, M(m-LH)Cl₂·nH₂O (MCo, Ni, Pd; n=0, 0.5, 1), Cu(m-LH)Cl₂·EtOH, M(m-LH)₂Cl₂·nH₂O (MCo, Zn, Pt; n=0, 1), M(m-LH)Br₂ (MCu, Zn), M(m-LH)₂Br₂ (MCo, Ni), Co(m-LH)(NCS)₂ and Co(m-LH)₂(NCS)₂, where o-LH=N-(2-aminophenyl)quinoline-2′-carboxamide and m-LH=N-(3-aminophenyl)quinoline-2′-carboxamide, have been prepared. The complexes were characterised by elemental analyses, conductivity measurements, X-ray powder patterns, thermogravimetric analyses, magnetic moments and spectral (¹H NMR, IR, and electronic) studies. Copper(II) and palladium(II) promote amide deprotonation at nearly acidic pH on coordination with o-LH. A variety of stereochemistries is assigned for the complexes prepared. The deprotonated copper(II) and the nickel(II) and palladium(II) complexes of m-LH appear to be polymeric. The neutral amide group of the ligands is coordinated to the metal ions through oxygen, while N(amide)-coordination is observed for the deprotonated complexes. Coordination of the secondary amide group is not observed for Zn(m-LH)₂Cl₂, Pd(m-LH)Cl₂·0.5H₂O and platinum(II) complexes. The neutral ligand o-LH shows bidentate N(ring), O-behaviour, while the anion o-L⁻ exhibits tridentate N,N,N-coordination. m-LH acts as a monodentate, bidentate and tridentate ligand depending on the metal ion, the anion and the preparative conditions.     

Appended 1,2-naphthoquinones as anticancer agents 1: synthesis, structural, spectral and antitumor activities of ortho-naphthaquinone thiosemicarbazone and its transition metal complexes

Copper(II), nickel(II), palladium(II) and platinum(II) complexes of ortho-naphthaquinone thiosemicarbazone were synthesized and characterized by spectroscopic studies. In both solution (NMR) and solid state (IR, single-crystal X-ray diffraction determination) the free ligand NQTS exists as the thione form. The Pd complex (X-ray) crystallizes as the H-bonded dimer, [Pd(NQTS)Cl]₂ · 2DMSO, where palladium(II) coordinates in a square planar configuration to the monodeprotonated, tridentate thiosemicarbazone ligand. The nickel(II) complex shows 1:2 metal to ligand stoichiometry while the other complexes exhibit 1:1 metal-ligand compositions. In vitro anticancer studies on MCF7 human breast cancer cells reveal that adding a thiosemicarbazone pharmacophore to the parent quinone carbonyl considerably enhances its antiproliferative activity. Among the metal complexes, the nickel compound exhibits the lowest IC₅₀ value (2.25 μM) suggesting a different mechanism of action involving inhibition of topoisomerase II activity.     

Metal ion-purine interactions: Preparation and study of some metal complexes of 8-ethyl-xanthine and 8-ethyl-3-methyl-xanthine

The interactions of 8-ethyl-xanthine (8EH) and 8-ethyl-3-methylxanthine (3MEH) with Cu(II), Pd(II), Ag(I) and Au(III) ions in aqueous medium were studied, and the isolated complexes characterized by means of ¹H NMR and IR as well as elemental analyses. Reactions occur over a wide pH range, with the purine bases acting as a monoanion, in molecular or protonated forms.     

Microbially supported synthesis of catalytically active bimetallic Pd-Au nanoparticles

Bimetallic nanoparticles are considered the next generation of nanocatalysts with increased stability and catalytic activity. Bio-supported synthesis of monometallic nanoparticles has been proposed as an environmentally friendly alternative to the conventional chemical and physical protocols. In this study we synthesize bimetallic bio-supported Pd-Au nanoparticles for the first time using microorganisms as support material. The synthesis involved two steps: (1) Formation of monometallic bio-supported Pd(0) and Au(0) nanoparticles on the surface of Cupriavidus necator cells, and (2) formation of bimetallic bio-supported nanoparticles by reduction of either Au(III) or Pd(II) on to the nanoparticles prepared in step one. Bio-supported monometallic Pd(0) or Au(0) nanoparticles were formed on the surface of C. necator by reduction of Pd(II) or Au(III) with formate. Addition of Au(III) or Pd(II) to the bio-supported particles resulted in increased particle size. UV-Vis spectrophotometry and HR-TEM analyses indicated that the previously monometallic nanoparticles had become fully or partially covered by Au(0) or Pd(0), respectively. Furthermore, Energy Dispersive Spectrometry (EDS) and Fast Fourier Transformation (FFT) analyses confirmed that the nanoparticles indeed were bimetallic. The bimetallic nanoparticles did not have a core-shell structure, but were superior to monometallic particles at reducing p-nitrophenol to p-aminophenol. Hence, formation of microbially supported nanoparticles may be a cheap and environmentally friendly approach for production of bimetallic nanocatalysts.     

Structural and solution chemistry, protein binding and antiproliferative profiles of gold(I)/(III) complexes bearing the saccharinato ligand

A series of new gold(I) and gold(III) complexes based on the saccharinate (sac) ligand, namely M[Au(sac)₂] (with M being Na⁺, K⁺ or NH₄⁺), [(PTA)Au(sac)], K[Au(sac)₃Cl] and Na[Au(sac)₄], were synthesized and characterized, and some aspects of their biological profile investigated. Spectrophotometric analysis revealed that these gold compounds, upon dissolution in aqueous media, at physiological pH, manifest a rather favourable balance between stability and reactivity. Their reactions with the model proteins cytochrome c and lysozyme were monitored by mass spectrometry to predict their likely interactions with protein targets. In the case of disaccharinato gold(I) complexes, cytochrome c adducts bearing four coordinated gold(I) ions were preferentially formed in high yield. In contrast, [(PTA)Au(sac)] (PTA = 1,3,5-triaza-7-phosphaadamantane) turned out to be poorly effective, only producing a mono-metalated adduct in very low amount. In turn, the gold(III) saccharinate derivatives were less reactive than their gold(I) analogues: K[Au(sac)₃Cl] and Na[Au(sac)₄] caused moderate protein metalation, again with evidence of formation of tetragold adducts. Finally, the above mentioned gold compounds were challenged against the reference human tumor cell line A2780S and its cisplatin resistant subline A2780R and their respective cytotoxic profiles determined. [(PTA)Au(sac)] turned out to be highly cytotoxic whereas moderate cytotoxicities were observed for the gold(III) complexes and only modest activities for disaccharinato gold(I) complexes. The implications of these results are thoroughly discussed in the light of current knowledge on gold based drugs.     

AM1* parameters for palladium and silver

We report the parameterization of AM1* for the elements palladium and silver. The basis sets for both metals contain one set each of s-, p- and d-orbitals. AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Zr, Mo, Pd, Ag, I and Au. The performance and typical errors of AM1* are discussed for Pd and Ag and compared with the PM6 Hamiltonian.