Solid State Phenomena Vol. 262

Abstract: In this paper we report an approach for the structural analysis of mineral-collector interfaces of (bio) flotation systems by means of attenuated total reflection Fourier-transform infrared spectroscopy (ATR FT-IR). The extraction of rare earth metals from electronic waste materials is an important challenge for the recycling industry. In a current project bacteriophage are used as biocollectors to develop a bioflotation model system for the separation of lanthanum phosphate doped with cerium and terbium (LaPO₄:Ce³⁺,Tb³⁺) from mixed fluorescent phosphors. As an initial analytical concept fluorescence microscopy was successfully applied to investigate particles of spent fluorescent lamp powders and to visualize the bacteriophage on the surface of the waste material. However, due to the restrictions of this technique we are not able to identify the molecular interactions of the bacteriophage with the recycled material. ATR FT-IR was found to be an effective tool to detect the major coat protein of the bacteriophage biocollectors on the surface of the LaPO₄:Ce³⁺,Tb³⁺ and sense their specific bonding interaction opening the gates for the high level chemical characterization of the interface.

Abstract: The aim of this work was to use electrochemical methods, capable to follow the corrosion of minerals, in order to monitor the progressive attack of the bacteria on the mineral. The assay was performed in a three electrode cell, with pyrite as the working electrode. The tests were performed in the absence and presence of iron; when present it was in low concentration. In order to compare the bacterial attack with other conditions, the study was conducted in three systems: live bacteria in culture media, dead bacteria in culture media, and sterile culture media, used as a control. The initial bacterial concentration was 10⁶ bact.mL^-1. To follow the process, current and corrosion potential were calculated. Live bacteria system showed a continuous increase of current with respect to the incubation time, being up to 4-times higher in the condition with iron (the corrosion current is related to the increase of the mineral area, produced by the bacterial attack, which was corroborated identifying by SEM the bacterial fingerprint on the mineral). Dead bacteria and sterile culture medium showed no current increase with respect to time. In addition, voltammetric studies shown that in live bacteria system the surface area increased when the biofilm was present. Whereas, in the dead bacteria system only the presence of some organic compounds interacting with the mineral was detected. The controls (sterile culture medium) showed the presence of iron hydroxides complexes and elemental sulfur, product of chemical leaching and the initial phase of a passivation process. With this study we demonstrated that the leaching process can be monitored by electrochemical methods, where the process of bacterial-mineral interaction could be followed, and also simultaneously identifying the initial processes of passivation. Our work can be useful for the development of a device for the in situ monitoring of biomining processes.

Abstract: X-ray diffraction (XRD) is a commonly used technology to identify crystalline phases. However, care must be taken with the combination of XRD configuration and sample. Copper (most commonly used radiation source) is a poor match with iron containing materials due to induced fluorescence. Magnetite and maghemite are analysed in different configurations using copper or cobalt radiation. Results show the effects of fluorescence repressing measures and the superiority of diffractograms obtained with cobalt radiation. Diffractograms obtained with copper radiation make incontestable phase identification often impossible. Cobalt radiation on the other hand yields high quality diffractograms, making phase identification straightforward.

Abstract: Biogenic minerals possess particular characteristics, such as high specific surface area and high reactivity, which lead to interesting properties useful in different fields (adsorbents, catalysts, oxidants or reductants). The treatment of effluents charged with heavy metals is attracting growing interest because of environmental and sanitary problems. The anaerobic bioreduction of soluble Fe(III) compounds by a natural consortium from an abandoned mine originates an iron containing precipitate. The aim of this study is the evaluation of the adsorption capacity of the biogenic compounds to treat diluted solutions containing arsenate, chromate and zinc after characterization by Scanning Electron Microscopy and X-ray diffraction analysis.

Abstract: Arsenic (As) is a major impurity contaminated in metal refinery wastewaters. To immobilize As ions, we have previously reported microbial scorodite (FeAsO₄·2H₂O) crystallization using the thermo-acidophilic iron-oxidizing archaeon, Acidianus brierleyi. In order to extend the applicable range of As (III)-bearing metal refinery wastewaters (especially for dilute As (III) concentrations of 250–1500 ppm), this study investigated the effect of several factors possibly affecting the bioscorodite crystallization efficiency; (i) [Fe (II)]_ini/[As (III)]_ini molar ratio at different target As (III) concentrations, (ii) initial pH, and (iii) seed scorodite with different morphologies. The [Fe (II)]_ini/[As (III)]_ini molar ratio strongly affected the bioscorodite crystallization efficiency at each target As (III) concentration. Whilst the [Fe (II)]_ini/[As (III)]_ini molar ratio of 1.4 was most effective at 500–1500 ppm As (III), the optimal molar ratios for treating more dilute concentrations (< 500 ppm) were shown to be relatively higher. However, further increasing the [Fe (II)]_ini/[As (III)]_ini molar ratio resulted in formation of unwanted potassium jarosite (KFe₃(OH)₆(SO₄)₂) together with scorodite. Lowering the initial pH from 1.5 to 1.2 resulted in earlier scorodite nucleation, but lesser overall As immobilization. Feeding chemical-and bio-scorodite seed crystals differently affected the reaction speed and the stability of newly-precipitated bioscorodite. The TCLP test indicated that scorodite formed on bioscorodite seeds is more stable than that formed on chemically-synthesized scorodite seeds.

Abstract: Using microorganisms to mediate crystallisation of metals and minerals in open-culture bioreactors has potential to recover recyclable materials from dilute aqueous streams, but also to prevent their emission to the environment. Although this potential is already exploited in practice to some extent, biological crystallization for metal recovery is still largely a black box technology with limited understanding of the role of the microorganisms in the crystallization, and the differences with chemical crystallisation. Using biocrystallisation of scorodite (FeAsO₄.2H₂O) and sphalerite (ZnS) as examples we propose that the role of microorganisms strongly depends on established saturation state of the solution. For scorodite, microorganisms are used to exert control over the crystallization as their ferrous iron-oxidizing activity keeps the solution slightly oversaturated. Also, the oversaturation level is kept homogeneous because of continuous biological formation of the reactant ferrous iron throughout the solution. In continuous bioreactor experiments on which we reported previously, scorodite crystal sizes still increased after 72 days of bioreactor operation indicating that indeed crystal growth was favored over nucleation. On the other hand, in our experiments with zinc sulfide, crystallization proceeded in highly oversaturated solutions in a continuous sulfate reducing bioreactor fed with a zinc sulfate solution and H₂/CO₂ as electron donor and carbon source. The high oversaturation likely resulted in dominant primary nucleation in the bulk solution, with little or no control over crystal growth, even though agglomeration may still have occurred. This was exemplified by particle sizes which decreased in the bioreactor experiment and remained stable after already about 2 weeks of operation.

Abstract: We have focused on the metal-reducing bacterium, Shewanella algae that are able to reduce and deposit platinum group metals (Pt (IV), Pd (II) and Rh (III)) and gold (Au (III)) in neutral solutions at room temperature under anaerobic conditions. When processing the aqua regia solution of spent automotive catalysts, the solution pH was adjusted to the optimal range for S. algae activity between pH 4 and 7. After this pH adjustment, the S. algae cells were able to rapidly and selectively reduce and accumulate the platinum group metal ions from the leaching solution into the bacterial cells as metallic nanoparticles. The biotechnological procedure also has the potential to allow the recovery of Au (III) ions from the leaching solution of electronic waste. We also found that the S. algae cells were also applicable to the adsorption of rare metal ions from acidic solutions. We achieved selective adsorption of indium (In (III)) ions on the bacterial cells from the leaching solution of waste liquid crystal displays by adjusting its pH, because the pH range necessary for S. algae to act as an effective adsorbent differs for different metal ions. Our proposed microbial methods enable the rapid and highly efficient recovery of precious and rare metals sourced from post-consumer products.

Abstract: Pumped groundwater in the lignite open-cast mines in Lusatia, Germany, contains a high level of ferrous iron (up to 1000 mg/L) at an initial pH of about 5. In recent R&D projects G.E.O.S. developed an innovative water treatment process for ferrous iron oxidation using the autochthonous microbial consortium in the mine water. The pilot plant is operated in the Nochten open-pit mine in cooperation with the LEAG and produces 5 – 10 t of schwertmannite per year. Extensive research work was carried out in parallel to utilize the produced schwertmannite. Pigment production proved to be technically feasible but difficult due to economic and market constraints. However, the high affinity of schwertmannite to oxy-anions provides the suitability for utilization as adsorbent to remove arsenate, antimonate, chromate, molybdate, vanadate or phosphate from mine water or industrial effluents. In the R&D project SURFTRAPII two kinds of filter-stable sorption materials were developed 1) by compacting schwertmannite or 2) by adhesive curing using an organic polymer, respectively. The produced filter-stable adsorbents were tested under technical conditions in cooperation with potential end users to remove arsenate, molybdate and phosphate from mine and industrial water and to concentrate valuable metals. The results showed a better performance of the material compared to other commercially available iron hydroxide adsorbents.

Abstract: Phosphogypsum waste, originating from phosphoric acid production from apatite ores, is well known for its high production rate and possible release of sulphate-rich seepage waters. In addition to negative environmental impacts, phosphogypsum waste heaps are also remarkable secondary sources of Rare Earth Elements (REE); in the phosphoric acid production process a majority of REE, occurring in apatite, are precipitated to the phosphogypsum waste. Therefore, a method treating both sulphate-rich waters and recovering REE from phosphogypsum heaps and seepage waters would offer both economic and environmental benefits. In this ongoing study, seepage waters from a phosphogypsum heap are treated with Sulphate Reducing Bacteria (SRB) and ethanol as a substrate. Sulphate is first reduced to hydrogen sulphide, which then precipitates REE as sulphides. The main challenge, low concentration of REE in seepage waters (e.g. 2.87 μg/l La, 5.13 μg/l Ce, 0.67 μg/l Y and 3.32 μg/l Nd), is overcome by utilizing continuous mode, semi-passive and cost effective column apparatus, requiring no agitation and performing both sulphate reduction and REE recovery in a single reactor. The SRB method results in a sulphate reduction rate of 40-80 % (from app. 1400 mg/l to 276-844 mg/l sulphate in the effluent) and efficient REE recovery from seepage water. The concentrate obtained from the column consists of a mixture of anaerobic sludge and precipitated REE, with respective REE concentrations of 202 mg/kg La, 477 mg/kg Ce, 49 mg/kg Y and 295 mg/kg Nd.

Abstract: Two different species of acidophilic micro-algae were grown in axenic culture, biomass harvested and injected into a low pH sulfate-reducing bioreactor, to act as a substrate for biosulfidogenesis. The hydrogen sulfide generated was used to precipitate copper in an off-line vessel, and the bioreactor pH was maintained by automated addition of a pH 2.5 feed liquor, to compensate for protons consumed by biosulfidogenesis. Results demonstrated the potential for using algal biomass for this purpose, precipitating about 1.5 mg Cu²⁺ L^-1h^-1, though rates of sulfidogenesis were considerably slower that when glycerol and yeast extract were used as organic feed-stocks.