Mobility and microbially mediated mobilization of gold and arsenic in soils from two gold mines in semi-arid and tropical Australia

The mobility and microbially mediated solubilization of Au and As in regolith materials from two Au mines in Australia, i.e., the Peak Hill Gold Mine in semi-arid New South Wales and the Hit or Miss Gold Mine in tropical northern Queensland, was studied using a combination of geochemical and microbiological techniques. Gold is highly mobile in both environments, the mobility of Au increases with increasing degree of weathering of host materials, and the resident microbiota are capable of mediating its solubilization. The results of the microcosm experiments demonstrate that the activity of microorganisms needs to be taken into account when studying the mobility and solubilization of Au in the Australian regolith. In primary, unweathered mineralization material from the Hit or Miss mine 99 wt% of Au was extracted only in the strongest final step of the sequential extractions, in concentrated aqua regia. In alteration zone material from the Peak Hill Gold Mine 80 wt% of Au was associated with the operationally defined Mn and Fe oxides. In contrast, in auriferous soils overlying mineralization at both sites 90-95 wt% of Au was associated with the operationally defined exchangeable, clay-bound and organic fractions. Microcosm experiments were incubated biologically active and inactive (sterilized) in 1:4 (w/v) aqueous slurries at 25 °C in the dark for up to 95 days. In biologically active microcosms with soils from the Peak Hill- and the Hit or Miss Gold Mines approximately 55 wt% (907 ng g⁻¹ d.w. soil) and 20 wt% (233 ng g⁻¹ d.w. soil) of the total Au, respectively, was solubilized during the incubation. In contrast, no or significantly lower Au concentrations were observed in biologically inactive microcosms. The mobility and microbially mediated release of As was limited at both sites and appears to be mostly controlled by abiotic adsorption and desorption on Mn- and Fe-oxides. Arsenic has a low solubility in the more mobile fractions and is mostly associated with Mn- and Fe-oxides and the residual fraction. The release of As was not elevated in biologically active compared to inactive microcosms from the Peak Hill Gold Mine. In contrast, in biologically active microcosms with samples from the Hit or Miss Mine elevated concentrations of As were detected in solution compared to the biologically inactive experiments.     

Electron paramagnetic resonance characteristics of the humic acids from a podzol

A study has been made of solid and solution electron paramagnetic resonance (EPR) spectra of humic acids from different horizons in a podzolic soil. Hyperfine splitting was observed in the solution spectra of humic acids from all horizons and depended on the strength of alkali and the period of dissolution. The upper organic horizons L, F and O₁ contained humic acids with some spectral characteristics in common with lignin. Humic acid from the lower horizons showed different spectra. At least 5 different radical signals were present.     

A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-80 °C

The mobility and transport of gold in low-temperature waters and brines is affected by the aqueous speciation of gold, which is sensitive in particular to pH, oxidation and halide concentrations. In this study, we use UV-Vis spectrophotometry to identify and measure the thermodynamic properties of Au(III) aqueous complexes with chloride, bromide and hydroxide. Au(III) forms stable square planar complexes with hydroxide and halide ligands. Based on systematic changes in the absorption spectra of solutions in three binary systems NaCl-NaBr, NaCl-NaOH and NaBr-NaOH at 25 °C, we derived log dissociation constants for the following mixed and end-member halide and hydroxide complexes: [AuCl₃Br]⁻, [AuCl₂Br₂]⁻, [AuBr₃Cl]⁻ and [AuBr₄]⁻; [AuCl₃(OH)]⁻, [AuCl₂(OH)₂]⁻, [AuCl(OH)₃]⁻ and [Au(OH)₄]⁻; and [AuBr₃(OH)]⁻, [AuBr₂(OH)₂]⁻ and [AuBr(OH)₃]⁻. These are the first reported results for the mixed chloride-bromide complexes. Increasing temperature to 80 °C resulted in an increase in the stability of the mixed chloride-bromide complexes, relative to the end-member chloride and bromide complexes. For the [AuCl_(4−n)(OH)_n]⁻ series of complexes (n = 0-4), there is an excellent agreement between our spectrophotometric results and previous electrochemical results of Chateau et al. [Chateau et al. (1966)]. In other experiments, the iodide ion (I⁻) was found to be unstable in the presence of Au(III), oxidizing rapidly to I₂(g) and causing Au to precipitate. Predicted Au(III) speciation indicates that Au(III) chloride-bromide complexes can be important in transporting gold in brines with high bromide-chloride ratios (e.g., >0.05), under oxidizing (atmospheric), acidic (pH < 5) conditions. Native gold solubility under atmospheric oxygen conditions is predicted to increase with decreasing pH in acidic conditions, increasing pH in alkaline conditions, increasing chloride, especially at acid pH, and increasing bromide for bromide/chloride ratios greater than 0.05. The results of our study increase the understanding of gold aqueous geochemistry, with the potential to lead to new methods for mineral exploration, hydrometallurgy and medicine.     

The Kwakshua Watersheds Observatory,central coast of British Columbia,Canada

The Kwakshua Watersheds Observatory (KWO) is an integrative watersheds observatory on the coastal margin of a rain-dominated bog-forest landscape in British Columbia (BC), Canada. Established in 2013, the goal of the KWO is to understand and model the flux of terrestrial materials from land to sea – the origins, pathways, processes and ecosystem consequences – in the context of long-term environmental change. The KWO consists of seven gauged watersheds and a network of observation sites spanning from land to sea and along drainage gradients within catchments. Time-series datasets include year-round measurements of weather, soil hydrology, streamflow, aquatic biogeochemistry, microbial ecology and nearshore oceanographic conditions. Sensor measurements are recorded every 5 min and water samples are collected approximately monthly. Additional observations are made during high-flow conditions. We used remote sensing to map watershed terrain, drainage networks, soils and terrestrial ecosystems. The watersheds range in size from 3.2 to 12.8 km², with varying catchment characteristics that influence hydrological and biogeochemical responses. Despite local variation, the overall study area is a global hotspot for yields of dissolved organic carbon, dissolved organic nitrogen and dissolved iron at the coastal margin. This observatory helps fill an important gap in the global network of observatories, in terms of spatial location (central coast of BC), climate (temperate oceanic), hydrology (very high runoff, pluvial regime), geology (igneous intrusive, glacially scoured), vegetation (bog rainforest) and soils (large stores of organic carbon).     

A STATISTICAL PROCEDURE TO EVALUATE CLEAN-UP STANDARDS

The object of this paper is to develop a suitable statistical procedure to evaluate clean-up standards athazardous waste sites.Under the assumptions that contaminant masses at a site follow a gammadistribution and that the data from the pre-remediation baseline sample as well as from the interim orfinal sample taken after a certain period of operation are both distributed as gamma with the same shapeparameter but different scale parameters,we derive a uniformly most powerful unbiased test of thehypothesis that a specified percentage of contaminant mass has been reduced.A large-sampleapproximation of the exact test procedure and a comparison with the likelihood ratio test are provided.     

A spectrophotometric study of aqueous copper(I)-chloride complexes in LiCl solutions between 100 °C and 250 °C

Copper transport and deposition in highly saline hydrothermal fluids are controlled by the stability of copper(I) complexes with ligands such as chloride and hydrosulphide. However, our understanding of the behavior of copper(I)-chloride complexes at elevated temperatures and in highly saline brines is limited by the conditions of existing experimental studies where the maximum chloride concentration is 2 m. This paper presents the results of a study of copper(I)-chloride complexes at much higher chloride concentrations, 1.5 m to 9.1 m, using a UV spectrophotometric method. The UV spectra of copper(I)-bearing LiCl solutions were measured at temperatures between 100 °C and 250 °C at vapor-saturated pressures and quantitative interpretation of the spectra shows that CuCl₂⁻, CuCl₃²⁻, and CuCl₄³⁻ were present in the experimental solutions. The fitted logarithms of formation constants (log K) for CuCl₂⁻ are in good agreement with the previous results of solubility experiments reported by Xiao et al. (1998) and Liu et al. (2001). The log K values for CuCl₃²⁻ also agree with those of Liu et al. (2001) and theoretical estimates of Sverjensky et al. (1997). This study presents the first experimentally determined formation constants for CuCl₄³⁻, at temperatures greater than 25 °C, and indicates that this complex predominates at chloride concentrations greater than 5 m. Based on the new log K values generated from this study, the calculated chalcopyrite solubility in NaCl solutions indicates that in addition to cooling, fluid mixing (dilution of saline fluids) may be an important factor controlling the deposition of copper minerals from hydrothermal solutions.     

Effect of resident microbiota on the solubilization of gold in soil from the Tomakin Park Gold Mine, New South Wales, Australia

The processes influencing the solubilization and observed mobility of Au in soil were studied using a combination of geochemical and microbiological techniques. In this study, we demonstrate for the first time that biotic processes mediated by the resident microbiota are likely to control the mobilization of Au in auriferous soils and other regolith materials. Microcosms with auriferous soils from the Tomakin Park Gold Mine in temperate south eastern New South Wales, Australia, were incubated under biologically active versus inactive (sterilized) conditions. The soils were incubated oxic and anoxic, unamended and Au pellet- or cycloheximide amended for 70 days in a 1:4 (w:v) aqueous slurry at 25 °C in the dark. In biologically active unamended Ah- and B-horizon microcosms up to 80 wt.% of total Au was detected in solution after 45 days of incubation. In biologically active Au pellet amended microcosms Au was liberated from the soil and also from added Au pellets. Scanning electron microscopy and nucleic acid staining combined with confocal stereo laser microscopy revealed the presence of bacterial biofilms on Au pellets incubated in the biologically active microcosms. The biologically inactive microcosms displayed no or significantly reduced Au solubilization. After 40-50 days of incubation Au was generally re-adsorbed to the solid soil fractions. The results of sequential extractions conducted with dried slurry samples collected from the biologically active Ah-horizon microcosms after 0, 10, 20, 30, 40, and 68 days of incubation indicated a continuous microscale solubilization and re-adsorption of Au. In samples taken after 40 days of incubation more than 80 wt.% of the Au was extracted from the operationally defined organic fraction, which appears to act as a final re-adsorption site for Au in the soil. In samples taken after 10 days of incubation from microcosms amended with 100 μg g⁻¹ (d.w. soil) of Au as AuCl₄⁻ 95 wt.% of the Au was associated with the organic fraction. To establish a mechanistic link between Au dissolution and re-adsorption with the activity of the heterotrophic bacterial community, analysis of the community structure based on carbon utilization patterns using was conducted. The bacterial community structure changed from a carbohydrate- and polymer-utilizing to a carboxylic- and amino acid utilizing community concurrently with the change from Au solubilization to re-adsorption. The bacterial community in the early stages of incubation (0-30 days) apparently produced an excess of amino acids, which are known to form stable amino acid Au complexes. The bacterial community in the later stages of incubation (after 40-50 days) metabolized these Au complexing ligands and Au, which apparently became unstable in the solution, was re-adsorbed to the solid soil fractions.