This study aimed to acquire proof of concept validity, at the laboratory and theoretical levels, of an integrated remediation strategy involving sulfate-reducing bacteria (SRB) and composted phytostabilization plant biomass. A model mine site presenting both historic mine waste dumps and acid mine drainage (AMD) was chosen to test the integrated bioremediation scheme. A microbial consortium from a SRB bioreactor treating AMD was adapted to grass compost as the sole carbon and energy source. It was then used to assess the feasibility of feeding SRB with the AMD of a former tin mine and compost prepared with phytostabilization plant biomass from the site, inducing SO4 reduction and the precipitation of metals (Fe, Co, Ni). A 7-week-old grass compost used at 1–5

This study aims to evaluate the feasibility of an integrated remediation strategy for a mine site presenting both historic mine waste dumps and AMD. A microbial consortium from an SRB bioreactor treating a real AMD using glycerol as an electron donor was adapted to grass compost as the sole energy source. It was then used as inoculum to assess the feasibility of treating the AMD of the studied site with compost as an energy source: increasing pH and precipitating metals (Fe, Co, Ni), and arsenic. The first experiments using garden grass compost showed a slow but effective reduction of sulfate (800 mg/L in one month) and validated the ability of the SRB consortium to use grass compost as an energy and carbon source in the presence of AMD containing 200 mg/L Fe and presenting acidic pH (pH 2.5). Subsequent experiments aim to test compost obtained with the aerial parts of plants from an onsite phytostabilization pilot plot. The following parameters will help determine the efficiency/feasibility of the approach: mass balance of metals, arsenic, carbon, and sulfur in liquid and solid phases.

Pyrite-bearing coal wastes are responsible of the formation of acid mine drainage (AMD), and their management to mitigate environmental impacts is a challenge to the coal mine industry in Europe and worldwide. The European CEReS project sought to develop a generic co-processing strategy to reuse and recycle coal wastes, based on removal of AMD generating potential through bioleaching. Chemolitoautotrophic iron- and sulfur-oxidizing microbial consortia were enriched from a Polish coal waste at 30 °C and 48 °C, but not 42 °C. Pyrite leaching yield, determined from bioleaching tests in 2-L stirred bioreactors, was best with the 48 °C endogenous consortium (80%), then the 42 °C exogenous BRGM-KCC consortium (71%), and finally the 30 °C endogenous consortium (50%). 16S rRNA gene-targeted metagenomics from five surface locations on the dump waste revealed a microbial community adapted to the site context, composed of iron- and/or sulfur-oxidizing genera thriving in low pH and metal rich environments and involved in AMD generation. All together, the results confirmed the predisposition of the pyritic coal waste to bioleaching and the potential of endogenous microorganisms for efficient bioleaching at 48 °C. The good leaching yields open the perspective to optimize further and scale-up the bioleaching process.

Most coal mines produce waste which has the potential to generate acid mine drainage (AMD). If not properly managed, this can cause environmental damage through contamination of ground and surface waters and soils for hundreds of years. At the same time, the pace of technological development means that most electrical and electronic equipment becomes obsolete within a matter of years, resulting in the generation of vast quantities of electronic waste (e-waste). Where this cannot be recycled, it must be discarded. The CEReS concept is a co-processing approach for both waste streams to produce metals and other valuable products, and to reduce or eliminate the their environmental impact. This brings together two waste streams from opposite ends of the supply chain; turning each into a novel resource in a single, coherent ‘grave-to-cradle’ process. This industrial ecology approach is key to supporting a circular economy whilst securing the sustainable supply of critical raw materials. The project successfully elaborated a novel co-processing flow-sheet comprising: (i) the accelerated bioweathering of AMD-generating coal production wastes to generate a biolixiviant; (ii) the pyrolysis and catalytic cracking of low-grade PCBs to produce hydrocarbon fuel, a halogen brine and a Cu-rich char; (iii) the leaching of base metals from the char using the biolixiviant; (iv) the reuse of stabilised coal wastes; and (v) the full or partial (as enriched substrates) recovery of valuable metals. These process units were demonstrated individually at lab-pilot scale. The data were then used to validate the entire flow-sheet in an integrated process simulator and determine the economic balance. Finally, an LCA approach was used to demonstrate the environmental benefits of the CEReS process over the status quo.

In an era of unprecedented environmental change, there have been increasing ecological and global public health concerns associated with exposure to anthropogenic pollutants. While there is a pressing need to remediate polluted ecosystems, human intervention might unwittingly oppose selection for natural detoxification, which is primarily carried out by microbes. We test this possibility in the context of a ubiquitous chemical remediation strategy aimed at targeting metal pollution: the addition of lime-containing materials. Here, we show that raising pH by liming decreased the availability of toxic metals in acidic mine-degraded soils, but as a consequence selected against microbial taxa that naturally remediate soil through the production of metal-binding siderophores. Our results therefore highlight the crucial need to consider the eco-evolutionary consequences of human environmental strategies on microbial ecosystem services and other traits.

: In order to develop an optimised coal production waste bioleaching unit, a sampling campaign was carried out on the existing waste dump. Eleven samples from various surface locations of the heap were used for enrichment cultures at three temperatures (30°C, 42°C and 48°C). Interestingly, none of the 11 cultures at 42°C grew well (low cell numbers and poor pyrite dissolution). Bioreactors inoculated with pooled enrichments showed the same results, indicating that the waste heap harboured organisms capable of growth and mineral oxidation at 30°C and 48°C but not 42°C. Analysis of the 48°C culture used for subsequent reactor tests using a variety of molecular microbiology methods (CE-SSCP, T-RFLP and high-throughput sequencing) revealed the culture was dominated by Sulfobacillus thermosulfidooxidans , Acidithiobacillus caldus and an unknown member of the Acidimicrobiaceae. Further tests at 42°C using a well-characterised bi-oleaching consortium, BRGM-KCC, performed well in initial reactors. However, during each batch the relative abundance of Leptospirillum ferriphilum decreased, and its numbers declined gradually with each subsequent subculture indicating some form of accrued inhibition. Coupled with the absence of a 42°C culture from the dump it was theorised that the waste was inhibitory to Leptospirillum spp. In order to further explore the possible reasons for the absence of Leptospi-rillum in the enrichments and bioreactors, a detailed analysis of the waste heap was undertaken using a range of geochemical and molecular biology tools. Results suggest the role of an as-yet unidentified organic carbon compound from the coal itself.

The FAME (Flexible and Mobile Economic Processing Technologies) project targets the development of flexible and economic processing technologies for small and low-grade European ore deposits with complex mineralogy, targeting greisen, skarn and pegmatite ores. Amongst the valuable elements to be recovered are W, Sn, Li and minor constituents like In, Ge, Ga, Nb or Ta. To improve the processing of by-product sulfides to recover critical elements like In or Ga and to develop innovative processing strategies for raw materials, biohydrometallurgical technologies are investigated. There are different approaches in FAME for the biohydrometallurgical recovery of valuable metals from low grade ores: 1) the extraction of Li from zinnwaldite and lepidolite, 2) the heap leaching of low grade sulfide ore unsuitable for conventional processing to recover Zn and In, and 3) the bioleaching of sulfide concentrates in a two-stage tank process for recovery of Zn and Cu. So far the most promising results were achieved for heap-leaching of low-grade Zn-In ores achieving 7.4 ppm In in the leaching solution and for Li extraction (28%) from zinnwaldite.

Extremely acidophilic microorganisms (pH optima for growth of ≤3) are utilized for the extraction of metals from sulfide minerals in the industrial biotechnology of "biomining." A long term goal for biomining has been development of microbial consortia able to withstand increased chloride concentrations for use in regions where freshwater is scarce. However, when challenged by elevated salt, acidophiles experience both osmotic stress and an acidification of the cytoplasm due to a collapse of the inside positive membrane potential, leading to an influx of protons. In this study, we tested the ability of the halotolerant acidophile Acidihalobacter prosperus to grow and catalyze sulfide mineral dissolution in elevated concentrations of salt and identified chloride tolerance mechanisms in Ac. prosperus as well as the chloride susceptible species, Acidithiobacillus ferrooxidans. Ac. prosperus had optimum iron oxidation at 20 g L-1 NaCl while At. ferrooxidans iron oxidation was inhibited in the presence of 6 g L-1 NaCl. The tolerance to chloride in Ac. prosperus was consistent with electron microscopy, determination of cell viability, and bioleaching capability. The Ac. prosperus proteomic response to elevated chloride concentrations included the production of osmotic stress regulators that potentially induced production of the compatible solute, ectoine uptake protein, and increased iron oxidation resulting in heightened electron flow to drive proton export by the F0F1 ATPase. In contrast, At. ferrooxidans responded to low levels of Cl- with a generalized stress response, decreased iron oxidation, and an increase in central carbon metabolism. One potential adaptation to high chloride in the Ac. prosperus Rus protein involved in ferrous iron oxidation was an increase in the negativity of the surface potential of Rus Form I (and Form II) that could help explain how it can be active under elevated chloride concentrations. These data have been used to create a model of chloride tolerance in the salt tolerant and susceptible species Ac. prosperus and At. ferrooxidans, respectively.

Mine wastes and control soils from twelve sites around Cornwall and West Devon (UK) were analysed for microbial community structure, pH and readily extractible metals (an indication of mobility). About 70% of total diversity observed was constituted by six phyla (Acidobacteria, Chloroflexi, Planctomycetes, Proteobacteria, Verrucomicrobia, and candidate division AD3). Microbial community structure revealed patterns of distribution that mostly differed between waste samples and controls. Cluster analysis of the geochemical data (pH and readily extractible metals) indicated the presence of four groups; two groups of controls and two of samples defined by similar features. pH appeared to correlate with the portion of shared community.

Previous agglomerate-scale heap bioleaching studies have outlined the variations in cell numbers of the liquid and attached phases during colonisation of sterilised ore by a pure culture. In this study, a mixed mesophilic culture was used in agglomerate-scale columns containing non-sterilised low-grade copper ore. Over a six - month period, columns were harvested at various intervals to provide snapshots of the metal distribution and the quantity, location, and ecological variations of mineral-oxidizing microbes within the ore bed. The initial colonisation period in this experiment was dissimilar to previous work, as the indigenous community was retained within the ore-bed throughout acid agglomeration. The overall colonisation phase lasted for approximately 1,000 hours until cell concentrations stabilised. In each column, less than 0.05% of the total cells were found in the leachate, 15-20% in the interstitial phase and the remaining ~80% were attached to the mineral surface. Once cell numbers had stabilised, interstitial cell concentrations were approximately 2,000× greater than those in the leachate. This difference persisted for the duration of the experiment. Copper concentrations in the two liquid phases generally decreased over time, but were on average 50× higher in the interstitial phase. Iron concentrations were more stable, but again were 30× higher in the interstitial phase. This demonstrates that that the difference in cell concentration between the leachate and interstitial phases cannot be explained through diffusion gradients within the system as it is much greater than those observed for the dissolved metals. It also shows that the specific environmental conditions of the interstitial and attached cells are very different to those inferred through analysis of leachates alone.

A model was developed to describe microbial growth and transport in the flowing bulk solution and ore-associated phases within a mineral bioleaching heap. The retention of micro-organisms was assumed to be a function of microbial transport between the ore surface and the bulk solution, as well as growth in each of these phases. Transient variations in the corresponding microbial concentrations are presented together with predicted microbial growth, transport and oxidation kinetics within the agglomerate-scale, whole ore environment. The transport model presented in this paper was developed under the assumption that the microbial concentration gradient between the identified phases was the driving force for microbial transport. Further the population balance model was super-imposed to account for available reaction surface. The model was able to predict the change in microbial concentrations in both the bulk solution and ore-associated phase. The resulting microbial transport rates to and from the ore-associated phase were found to be significantly lower than the maximum specific microbial growth rates presented, suggesting that microbial transport is not governed by the microbial concentration difference. These findings confirm the value of the modelling approach in which the population balance model is included, while demonstrating that concentration gradient as the driving force is not the main contributor to microbial transport.

The hydrothermal liquefaction (HTL) of algal biomass is a promising route to viable second generation biofuels. In this investigation HTL was assessed for the valorisation of algae used in the remediation of acid mine drainage (AMD). Initially the HTL process was evaluated using Arthrospira platensis (Spirulina) with additional metal sulphates to simulate metal remediation. Optimised conditions were then used to process a natural algal community (predominantly Chlamydomonas sp.) cultivated under two scenarios: high uptake and low uptake of metals from AMD. High metal concentrations appear to catalyse the conversion to bio-oil, and do not significantly affect the heteroatom content or higher heating value of the bio-oil produced. The associated metals were found to partition almost exclusively into the solid residue, favourable for potential metal recovery. High metal loadings also caused partitioning of phosphates from the aqueous phase to the solid phase, potentially compromising attempts to recycle process water as a growth supplement. HTL was therefore found to be a suitable method of processing algae used in AMD remediation, producing a crude oil suitable for upgrading into hydrocarbon fuels, an aqueous and gas stream suitable for supplementing the algal growth and the partitioning of most contaminant metals to the solid residue where they would be readily amenable for recovery and/or disposal.

Recent understanding of microbial retention within heap bioleaching systems has highlighted the importance of quantifying microbial growth and activity in both the bulk flowing solution and in the ore-associated phases. Typically, industrial heap bioleaching operations report variations in process conditions such as inoculum preparation and concentration and elevated copper concentrations in the recycled irrigation solution. In this paper, a mini-column reactor system containing pre-constructed and agglomerated, low-grade ore samples representing grab samples from a larger heap, were used to investigate the effect of a selection of physico-chemical and operating conditions on microbial growth, colonisation and substrate utilisation kinetics, considering both the planktonic and sessile populations of Acidithiobacillus ferrooxidans. The factors studied included inoculum size, inoculum cultivation conditions, availability of ferrous iron in the bulk flowing solution and copper concentration in the bulk flowing solution. The microbial population in the interstitial phase, i.e. associated with, but not bound to, the ore, remained the most abundant within the heap under all physico-chemical conditions considered. A comparison of the tests with different inoculum sizes found that a smaller inoculum size resulted in an increased delay in microbial growth and ferrous iron oxidation, but similar apparent maximum specific growth rates and iron oxidation rates. In contrast to the microbial culture grown on pyrite, a delay in microbial activity was observed for the culture grown on ferrous iron. However, greater microbial cell densities were reached, in the interstitial and attached phases compared with the pyrite-grown culture. The introduction of 6 g L-1 cupric ions into the feed solution containing 0.2 g L-1 ferric iron resulted in decreased microbial growth rate in the interstitial phase but not in the attached phase. Where the pyrite culture was pre-exposed to cupric ion, the microbial growth rate in the interstitial and attached phases was significantly enhanced. Nevertheless, the presence of cupric ion in the irrigation solution resulted in a decrease in microbial ferrous iron oxidation rate, irrespective of pre-culture conditioning. This study emphasises the important role played by the stagnant interstitial phase during the colonisation of a low-grade heap, particularly under adverse conditions for microbial growth and activity. It also highlights the role of inoculum culture conditions on the potential trade-off between increased heap colonisation and increased lag periods in microbial activity during heap start-up. (C) 2015 Published by Elsevier Ltd.

•Design of multiple, mini-column reactors to simulate grab samples from a heap bioleach.•Sacrifice of a mini-column in test represents a time point in data series for microbial growth.•Independently quantify microorganisms in flowing and stagnant solution and on ore.

Electronic waste (e-waste) contains a wide range of elements, many of which are highly toxic to environmental and human health. On the other hand e-waste represents a significant potential source of valuable metals. This study used microbial oxidation of pyrite to generate a biolixiviant. Its efficiency in the dissolution of metals from printed circuit boards (PCBs) was evaluated as well as the effects of metals and PCB concentrations on microbial activity. The addition of elemental metals (Cu, Cr, Ni, Sn, Zn) had an immediate inhibitory effect on pyrite oxidation, though leaching recovered after a period of adaptation. Bioleaching was inhibited initially by the addition of 1 % (w/v) ground PCB, but recovered rapidly, whereas pulp densities of ≥5 % had sustained negative impacts on culture activity and viability. The loss of culture viability meant that only abiotic copper dissolution occurred at ≥5 % PCB. Final copper recoveries declined with increasing PCB pulp density. The relatively high content of elemental iron caused a lag period in copper solubilisation possibly due to displacement reactions. Leptospirillum ferriphilum was primarily responsible for pyrite oxidation, and most affected by both the pure metals (particularly Ni and Cu) and PCB.

The quantification of microbial colonisation and growth rate kinetics is essential in the characterisation of bioleaching systems for modelling heap performance. An experimental system, designed to simulate heap bioleaching conditions at the agglomerate scale, was used to quantify the microbial growth kinetics of a pure culture of Acidithiobacillus ferrooxidans in the PLS, the interstitial phase and attached to the mineral ore. Conventional methods for the quantification of microbial growth rate kinetics associated with the ore and in the flowing PLS have been found to be inadequate in the characterisation of whole ore systems owing to microbial transport between these regions. Growth within the whole ore system was dominated by the microbial communities associated with the ore. Two models were used to estimate the true growth rate kinetics of Acidithiobacillus ferrooxidans on whole low grade ore as a function of growth and transport between the identified phases. The “hydrodynamics” model assumed that microbial transport was promoted by fluid flow dynamics whilst the “biomass balance” model assumed that the microbial concentration gradient across identified phases was the driving force for transport.

When presented with ferrous ions and tetrathionate in growth media, individual archaeal species, Acidianus brierleyi, Metallosphaera hakonensis and Sulfolobus metallicus, exhibited different patterns of substrate utilisation and cell growth. Total cell numbers for A. brierleyi were about 60% of those for M. hakonensis and S. metallicus after complete substrate utilisation, and specific growth rates were affected by substrate concentration and cell growth history. High iron(II) concentrations inhibited S. metallicus growth. In relation to mixed microbial communities in high-temperature bioleaching reactors, the results are consistent with the hypothesis that specific growth rates of different species grown on different substrates strongly influence community structure. The combination of T-RFLP with substrate analysis and cell counts is a valuable analytical tool for the discrimination and semi-quantitative estimation of species in mixed cultures that can be used to gain better understanding of community structures and population dynamics in complex bioleaching systems.

The reliable prediction of acid rock drainage risks associated with sulfide mine wastes plays an important role in planning for their disposal. This paper explores the application of a novel biokinetic test, in combination with conventional static chemical tests, for the rapid and effective characterisation of the acid generating potential of a number of mine wastes from the processing of both hard-rock ores and coal. Findings also highlight the need to consider microbial colonisation and the effects of such on the relative kinetics of the acid forming and neutralising reactions.