The effect of iron-oxidising bacteria on the stability of gold (I) thiosulphate complex

An acidophilic, iron-oxidising bacterial consortium was collected from Rio Tinto near Berrocal, Spain. This primary enriched culture was used to examine the effect of acidophilic iron-oxidising bacteria on the stability of soluble gold (I) thiosulphate. Stationary phase cultures and separate components of the cultures (i.e., aqueous ferric iron, iron oxyhydroxide precipitates and non-mineralised bacterial cells) were exposed to gold (I) thiosulphate solutions forming different experimental-gold systems. These experimental systems rapidly removed gold from solutions containing 0.002 mM–20 mM gold thiosulphate. Scanning and transmission electron microscopy demonstrated that the different culture fractions immobilised gold differently: the entire bacterial culture-gold systems precipitated 100 nm-size gold colloids; aqueous ferric iron–gold systems precipitated colloidal gold sulphide that ranged in diameter from 200 nm to 2 μm; iron oxyhydroxide-gold systems precipitated 5 nm-size gold sulphide colloids; and the bacteria-gold systems precipitated gold colloids ~ 2 nm in size along the bacterial cell envelope. Aqueous and solid ferric iron was critical in the destabilisation of the gold (I) thiosulphate complex. Analysis of the entire bacterial culture-, aqueous ferric iron- and iron oxyhydroxide-gold systems exposed to 2 mM gold using X-ray absorption near edge spectroscopy demonstrated that Au+ was immobilised from solution as gold sulphide (Au2S). The reaction between iron-oxidising bacteria and their ferric iron by-products with gold (I) thiosulphate demonstrated that thiosulphate ions would be an unstable gold complexing ligand in nature. Gold (I) thiosulphate is intuitively transformed into nanometer-scale gold sulphide or elemental gold within natural, acidic weathering environments with the potential to precipitate gold in jarosite that can subsequently be preserved in gossans over geological time

Multinuclear Complex Formation between Ca(II) and Gluconate Ions in Hyperalkaline Solutions

Alkaline solutions containing polyhydroxy carboxylates and Ca(II) are typical in cementitious radioactive waste repositories. Gluconate (Gluc(-)) is a structural and functional representative of these sugar carboxylates. In the current study, the structure and equilibria of complexes forming in such strongly alkaline solutions containing Ca2+ and gluconate have been studied. It was found that Gluc(-) significantly increases the solubility of portlandite (Ca(OH)(2)(s)) under these conditions and Ca2+ complexes of unexpectedly high stability are formed. The mononuclear (CaGluc(+) and [CaGlucOH](0)) complexes were found to be minor species, and predominant multinuclear complexes were identified. The formation of the neutral [Ca(2)Gluc(OH)(3)](0) (log beta(213) = 8.03) and [Ca(3)Gluc(2)(OH)(4)](0) (log beta(324) = 12.39) has been proven via H-2/Pt-electrode potentiometric measurements and was confirmed via XAS, H-1 NMR, ESI-MS, conductometry, and freezing-point depression experiments. The binding sites of Gluc- were identified from multinuclear NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms were proved to be the most probable sites for Ca2+ binding. The suggested structure of the trinuclear complex was deduced from ab initio calculations. These observations are of relevance in the thermodynamic modeling of radioactive waste repositories, where the predominance of the binuclear Ca2+ complex, which is a precursor of various high-stability ternary complexes with actinides, is demonstrated

Improved Low-Temperature CO Oxidation Performance of Pd Supported on La-Stabilized Alumina

Simulated diesel oxidation catalysts (DOCs) consisting of 2.5% Pd were prepared on γ-Al₂O₃ and lanthana-stabilized γ-Al₂O₃; it was found that the La-containing catalyst had higher CO conversion and lower onset temperature for CO oxidation (∼100 °C). Aberration-corrected STEM showed that the La–alumina support helped to stabilize Pd in smaller particles and clusters, increasing dispersion from 17 to 26%. The higher dispersion was responsible, in part, for the improved CO oxidation rate; at 140 °C, the turnover frequency (TOF) was improved from 0.0019 to 0.0095 s^–1 with the addition of La. This TOF increase appears to be tied to facile redox behavior of the Pd/La–alumina catalyst, which was evident in the results of in situ X-ray absorption spectroscopy (XAS) and FTIR spectroscopy. In these experiments, both catalysts were calcined at 500 °C to form PdO and then reduced to Pd metal at 140 °C in the presence of CO. When the CO-covered catalyst was exposed to CO oxidation reaction conditions at 140 °C, the 2.5% Pd/Al₂O₃ catalyst remained nearly fully reduced, and the surface coverage of CO did not change, indicating irreversible CO adsorption and very low reactivity toward oxygen. On the other hand, the more active 2.5% Pd/La–Al₂O₃ catalyst was more reactive toward oxygen, with a portion of the Pd becoming oxidized when the gas phase was switched from pure CO to the reaction mixture. There was a drop in surface coverage of CO when switching from pure CO to the reaction mixture on the Pd/La–alumina. The results suggest that the role of the La–alumina support is 2-fold, increasing the dispersion of Pd by forming small, stable Pd particles and allowing a portion of the Pd to exhibit facile redox behavior at low temperatures, making the Pd less susceptible to poisoning by CO. This work provides insights into factors that could lead to improved low-temperature CO oxidation reactivity in Pd-based automotive exhaust catalysts

XANES measurements of sulfur chemistry during asphalt oxidation

Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy is used to measure how the speciation of sulfur compounds evolves within a warm-mix asphalt as a consequence of the Rolling Thin-Film Oven (RTFO) and Pressure Aging Vessel (PAV) oxidative aging procedures. Identifying the types of sulfur compounds present is important for quantifying the growth in polar sulfur-containing species that can alter the asphalt\u27s mechanical properties over time. Elemental analysis indicates that the sulfur content of the asphalt holds constant at 5 wt% during aging. XANES analysis indicates that thiophenic sulfur compounds are most prevalent (62%), followed by sulfide and elemental sulfur compounds. RTFO and PAV aging cause smaller and larger shifts, respectively, from sulfide to sulfoxide. The amount of unreacted sulfide remains larger than the amount of sulfoxide, even with PAV aging. The XANES spectra lack features that would be expected if engine oil additives indicative of recycled engine oil bottoms were present. The results indicate the importance of including thiophene, sulfide, and sulfoxide chemistries within molecular asphalt models

Is social capital associated with HIV risk in rural South Africa?

The role of social capital in promoting health is now widely debated within international public health. In relation to HIV, the results of previous observational and cross-sectional studies have been mixed. In some settings it has been suggested that high levels of social capital and community cohesion might be protective and facilitate more effective collective responses to the epidemic. In others, group membership has been a risk factor for HIV infection. There have been few attempts to strengthen social capital, particularly in developing countries, and examine its effect on vulnerability to HIV. Employing data from an intervention study, we examined associations between social capital and HIV risk among 1063 14 to 35-year-old male and female residents of 750 poor households from 8 villages in rural Limpopo province, South Africa. We assessed cognitive social capital (CSC) and structural social capital (SSC) separately, and examined associations with numerous aspects of HIV-related psycho-social attributes, risk behavior, prevalence and incidence. Among males, after adjusting for potential confounders, residing in households with greater levels of CSC was linked to lower HIV prevalence and higher levels of condom use. Among females, similar patterns of relationships with CSC were observed. However, while greater SSC was associated with protective psychosocial attributes and risk behavior, it was also associated with higher rates of HIV infection. This work underscores the complex and nuanced relationship between social capital and HIV risk in a rural African context. We suggest that not all social capital is protective or health promotive, and that getting the balance right is critical to informing HIV prevention efforts

Can social capital be intentionally generated? a randomized trial from rural South Africa.

While much descriptive research has documented positive associations between social capital and a range of economic, social and health outcomes, there have been few intervention studies to assess whether social capital can be intentionally generated. We conducted an intervention in rural South Africa that combined group-based microfinance with participatory gender and HIV training in an attempt to catalyze changes in solidarity, reciprocity and social group membership as a means to reduce women's vulnerability to intimate partner violence and HIV. A cluster randomized trial was used to assess intervention effects among eight study villages. In this paper, we examined effects on structural and cognitive social capital among 845 participants and age and wealth matched women from households in comparison villages. This was supported by a diverse portfolio of qualitative research. After two years, adjusted effect estimates indicated higher levels of structural and cognitive social capital in the intervention group than the comparison group, although confidence intervals were wide. Qualitative research illustrated the ways in which economic and social gains enhanced participation in social groups, and the positive and negative dynamics that emerged within the program. There were numerous instances where individuals and village loan centres worked to address community concerns, both working through existing social networks, and through the establishment of new partnerships with local leadership structures, police, the health sector and NGOs. This is among the first experimental trials suggesting that social capital can be exogenously strengthened. The implications for community interventions in public health are further explored

Kinetic Pathway of Palladium Nanoparticle Sulfidation Process at High Temperatures

A significant issue related to Palladium (Pd) based catalysts is that sulfur-containing species, such as alkanethiols, can form a PdS_<i>x</i> underlayer on nanoparticle surface and subsequently poison the catalysts. Understanding the exact reaction pathway, the degree of sulfidation, the chemical stoichiometry, and the temperature dependence of this process is critically important. Combining energy-filtered transmission electron microscopy (EFTEM), X-ray diffraction (XRD), and X-ray absorption spectroscopy experiments at the S <i>K-</i>, Pd <i>K</i>-, and <i>L</i>_2,3-edges, we show the kinetic pathway of Pd nanoparticle sulfidation process with the addition of excess amount of octadecanethiol at different temperatures, up to 250 °C. We demonstrate that the initial polycrystalline Pd-oleylamine nanoparticles gradually become amorphous PdS_<i>x</i> nanoparticles, with the sulfur atomic concentration eventually saturating at Pd/S = 66:34 at 200 °C. This final chemical stoichiometry of the sulfurized nanoparticles closely matches that of the crystalline P₁₆S₇ phase (30.4% S), albeit being structurally amorphous. Sulfur diffusion into the nanoparticle depends strongly on the temperature. At 90 °C, sulfidation remains limited at the surface of nanoparticles even with extended heating time; whereas at higher temperatures beyond 125 °C, sulfidation occurs rapidly in the interior of the particles, far beyond what can be described as a core–shell model. This indicates sulfur diffusion from the surface to the interior of the particle is subject to a diffusion barrier and likely first go through the grain boundaries of the nanoparticle

The immobilization of gold from Au(III) chloride by a halophilic sulphate-reducing bacterial consortium

A consortium containing halophilic, dissimilatory sulphate-reducing bacteria was enriched from Basque Lake #1, located near Ashcroft, British Columbia, Canada to evaluate the role these bacteria have on the immobilization of soluble gold. The consortium immobilized increasing amounts of gold from gold (III) chloride solutions, under saline to hypersaline conditions, over time. Gold (III) chloride was reduced to elemental gold in all experimental systems. Salinity did not affect gold immobilization. Scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold (III) chloride was immobilized as c. 3-10 nm gold colloids and c. 100 nm colloidal aggregates at the fluid-biofilm interface. The precipitation of gold at this organic interface protected cells within the biofilm from the 'toxic effect' of ionic gold. Analysis of these experimental systems using X-ray absorption near-edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes. The immobilization of gold by halophilic sulphate-reducing bacteria highlights a possible role for the biosphere in 'intercepting' mobile gold complexes within natural, hydraulic flow paths. Based on the limited toxicity demonstrated in this experimental model, significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably 'non-toxic' to the biosphere