Lenka Soukupová, Zvětrávání arsenopyritu v lesních půdách v acidifikovaném prostředí SUMMARY The weathering of arsenopyrite (FeAsS) has been studied at the experimental site Načetín in the Ore Mountains, Czech Republic. There were chosen three areas with different vegetation (beech, spruce a unforested areas) at this site. The arsenopyrite samples were placed in all soil horizons (litter, horizons A, B and C for forest areas; horizons A, B and C for unforested area), where they were exposed to ambient conditions for one year. After one-year weathering, the newly formed secondary minerals were identified and the rate of surface oxidation was determined, both depending on the environment of oxidation. Although physical-chemical parameters and content of main and trace elements of the studied soils varied, the only detected crystalline secondary mineral of arsenic was scorodite (FeAsO4∙2H2O). Nevertheless, this differences affected amount of formed scorodite. The highest concentrations were determined on the surface of the arsenopyrite grains that oxidized in the beech stand, conversely the lowest concentrations were determined on the arsenopyrite grains from the unforested area.

Hardpans, or cemented layers, form from precipitation and subsequent cementation of secondary minerals in mine tailings and can act as both physical and chemical barriers. During precipitation, metals in the tailings are sequestered, making hardpan a potentially viable method of natural attenuation. At Montague Gold Mines, Nova Scotia, tailings are partially cemented by the iron (Fe) arsenate mineral scorodite (FeAsO4•2H2O). Scorodite is known as a phase that can effectively limit aqueous arsenic (As) concentrations due to its relatively low solubility (<1 ppm, pH 5) and high As content (~30 wt.%). However, scorodite will not lower As concentrations from waters to below the Canadian drinking water guideline (0.010 ppm). To identify current field conditions influencing scorodite precipitation and dissolution and to better understand the mineralogical and chemical relationship between hardpan and tailings, coexisting waters and solids were sampled to provide information on tailings-water interactions. Hardpan cement compositions were found to include Fe arsenate and Fe oxyhydroxide in addition to scorodite. End-member pore water chemistry was identified based on pH and dissolved concentration extremes (e.g. pH 3.78, As(aq) 35.8 ppm) compared to most other samples (avg. pH 6.41, As(aq) 2.07 ppm). These end-member characteristics coincide with the most extensive and dispersed areas of hardpan. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity and dissolved As+5 and Fe+3 for scorodite precipitation. A proposed model of progressive arsenopyrite oxidation suggests localized As cycling involving scorodite is occurring but is dependent on sulfide persistence. Therefore, permanent As sequestration is not expected. Remediation efforts would have to consider the possibility of scorodite dissolution after complete sulfide consumption or as a consequence of applying certain technologies, such as a cover. Conversely, if scorodite stability were maintained, the hardpan could be considered as a component in remediating the tailings at Montague. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2008-12-22 09:36:08.157

Many water treatment technologies for arsenic removal that are used today produce arsenic-bearing solid residuals (ABSR), which are disposed in mixed solid waste landfills. It is now well established that many of these residuals will release arsenic into the environment to a much greater extent than predicted by standard regulatory leaching tests and, consequently, require stabilization to ensure benign behaviour after disposal. Conventional waste stabilization technologies, such as cement encapsulation and vitrification, are not suitable for ABSR applications due to their lack of effectiveness or high cost, thus creating a need for a more effective and low-cost treatment technology for ABSR. Arsenic Crystallization Technology (ACT) is a proposed arsenic stabilization method that involves in converting the ABSR into arsenic-bearing minerals that resemble natural materials and have high arsenic capacity, long term stability, and low solubility compared to untreated ABSR. Three arsenic minerals, scorodite, arsenate apatite and ferrous arsenate, have been investigated in this research for their potential application as ACT for ABSR stabilization. Among the three minerals, ferrous arsenate is demonstrated to be the most suitable arsenate mineral for safe arsenic disposal due to its low arsenic solubility and ease of synthesis. An innovative treatment procedure has been developed in this research for stabilization of ABSR to a stable phase of ferrous arsenate using zero-valent iron (ZVI) as the reducing agent. The procedure works at ambient temperature and pressure, and neutral pH. In addition, a modified four-step sequential extraction method has been developed as a means to determine the proportions of various arsenic phases in the stabilized as well as untreated ABSR matrices. This extraction method, as well as traditional leach and solubility tests, show that arsenic stability in the solid phase is dramatically increased after formation of crystalline ferrous arsenate.

Mining and mineral processing generate a wide range of As-rich minerals, including scorodite (FeAsO4•2H2O), and arsenical ferrihydrite, which are common secondary minerals found in mine tailings. Scorodite and arsenical ferrihydrite are relatively stable under a wide range of physico-chemical conditions which makes them suitable arsenic sinks in mining environments. However, bacteria can reduce these minerals and release arsenic into the aqueous environment. Two dissimilatory iron and arsenic reducing bacteria, Shewanella sp. ANA-3 and Shewanella putrefaciens CN32, were used to investigate their effects on the reductive dissolution of scorodite and arsenical 6-line ferrihydrite in a chemically defined medium containing low phosphate concentrations representative of the natural environment. Analysis of the aqueous phase of all biotic reduced samples found mainly As(III), the more toxic form of As, while very little As(V) was reduced in the abiotic samples. Solid state analysis of the scorodite biotic post-reduction minerals identified scorodite, biogenic Fe(II)-As(III) compounds, parasymplesite and tooeleite, while in the biotic reduced arsenical six-line ferrihydrite, biogenic Fe(II)-As(III) compounds, hematite, akaganeite and unconfirmed magnetite were identified as secondary reduction products. Results from this research add to the body of literature on As and Fe biogeochemistry and provide very useful information for future assessments of the long term stability of As-rich minerals. L’activité minière et la transformation du minerai génèrent divers minéraux riches en arsenic, tels la scorodite (FeAsO4•2H2O) et la ferrihydrite riche en arsenic, lesquels sont des minéraux secondaires communs des résidus miniers. Comme la scorodite et la ferrihydrite riche en arsenic sont relativement stables sous une grande gamme de conditions physico-chimiques, ces minéraux peuvent potentiellement être utilisés pour stocker de façon permanente l’arsenic dans les environnements miniers. Cependant, certaines bactéries peuvent réduire ces minéraux, ce qui entraine la solubilisation de l’arsenic. Deux bactéries capables de réduire l’arsenic et le fer, soit Shewanella sp. ANA-3 et Shewanella putrefaciens CN32, ont été utilisées afin de déterminer leurs effets sur la réduction microbienne de la scorodite et de la ferrihydrite riche en As dans un milieu de culture contenant de faibles concentrations de phosphate. Les analyses de la phase aqueuse ont démontré que dans tous les systèmes biotiques, As(V) a été réduit en As(III), alors que dans les systèmes contrôles abiotiques, peu de As(V) a été réduit. L’analyse des minéraux secondaires présents à la fin réduction dans les systèmes biotiques contenant de la scorodite indique que la scorodite est encore présente, ainsi que des composés organiques riches en Fe(II) et As(III), de la parasymplésite et de la tooéleite, alors que dans les systèmes biotiques contenant de la ferrihydrite riche en As, des composés riches en Fe(II) et en As(III), de l’hématite, de l’akaganéite et de la magnétique ont été identifiés comme minéraux secondaires. Les résultats de cette étude enrichissent la littérature sur le cycle biogéochimique du Fe et de As et fournissent de l’information importante pour l’évaluation de la stabilité à long terme de minéraux riches en As.