CONICET | Buscador de Institutos y Recursos Humanos

Gold mining operations result in solid tailings with deleterious environmental footprintdue to arsenic trioxide. The magnitude of the problem can be exemplified by the resultsof five decades of operations at Giant Mine in Yellowknife (Canada), which have created amassive environmental liability. The environmental footprint of this site encompasses 950ha, comprising 8 open pits, 4 tailing ponds, 325,000 m3 of contaminated soils, andseveral buildings. Gold in Giant Mine ore is encased within larger grains of minerals,including arsenopyrite, and thus the roasting process prior to cyanide leaching producestwo major off-gases: sulphur dioxide and arsenic vapor. Initially, the roaster off-gaseswere vented directly to the atmosphere, but gas-cleaning equipment was installed in1951 that lead to the capture of arsenic off-gas and subsequent disposal of this gas byproductas arsenic trioxide dust and tailings slurry. The tailings slurry was discharged intoone of the four un-remediated tailings ponds at the mine. Approximately 16 million tonsof tailings are nowadays stored in four above ponds covering a combined area ofapproximately 95 ha. In addition, an estimated 237,000 tons of arsenic trioxide arestored in underground mine stopes that threaten groundwater. The current state of thetailings and sludge containment areas are a key concern because of the potential forphysical contact of arsenic waste by humans and wildlife resulting from dam seepage,surface overflow, and dusting from the tailings surfaces.Arsenic is considered a poison for most biological organism. Arsenic ingestion inhuman beings is associated with cancers of the kidneys, bladder, liver, lung, and otherorgans. It can also affect the vascular system, including vascular disease, which in itsmost severe form results in gangrene. Inorganic arsenic is found in two valence states:the more toxic arsenite (AsIII) which prevails in groundwater and arsenate (AsV), atleast an order of magnitude less toxic, usually associated with surface waters. Currently,the proposed method of isolating the arsenic trioxide dust in the mine stopes is bycreating a ?frozen block?, monitoring it in perpetuity, and maintaining isolation byperiodic re-freezing. This is a containment method and does not eliminate the risk offuture release of the arsenic trioxide into the surrounding ground water. Additionally, thismethod requires a continuous supply 4 MW of electricity to support the frozen blocktechnology. This energy requirement is neither a sustainable strategy, nor a realisticsolution in the long term given the limitation of energy production in the Yellowknifemarket.To overcome these drawbacks, we designed a nanotechnology-based solution,supported by our previous experience with oxidation and immobilization of arsenic bynanoiron [1]. Development and implementation of environmentally beneficialnanotechnologies for treatment and remediation has experienced noticeable growth inrecent years. In terms of site remediation, the effective application of nanotechnology forcontaminant destruction has already taken place. Several nanotechnology basedalternatives for site remediation and wastewater treatment are currently in the researchand development stages. Nanotechnology represents an extremely broad field, whichencompasses a number of materials and technologies spanning multiple disciplines.Currently a wide variety of potential remedial tools employing nanotechnology are beingexamined for use in waste water and soil remediation. However, one emergingnanotechnology, nanosized zero valent iron and its derivatives, has reached thecommercial market for field-scale remediation [2], which is the reason why we chose thisspecific material.From a practical point of view, mature applications of nanoiron in environmentalremediation can be classified into two broad types: use of nanoparticles as a catalyst anduse of nanoiron as a reactant, the latter being the case for arsenic mitigation. Thecapacity of nanoparticles (and particularly those based on iron) for chemical reduction ?oxidation reactions, has aroused the interest of the academic community due to thepotential to use these properties for the treatment of a wide range of contaminants foundin effluent waters as well as subsurface water and soils impacted by spills of chemicalsand petroleum derivatives [3]. The usual form in which these nanomaterials are used isthe creation of permeable reactive barriers designed to intercept and remediatecontamination plumes. Unlike conventional systems, based on extraction of contaminatedmaterial, its treatment and subsequent deployment on the site, these techniques forpassive mitigation in situ proved to be less costly in operative terms than ex situtreatments. However, the most important of the distinctive characteristics of thisapproach was the avoidance of health and environmental risks derived of disseminationof unwanted substances outside the original site due to eventual spills duringtransportation of contaminated material. These barriers have been used for themitigation of different kinds of contaminants [4], including some recalcitrant tobioremediation such as chlorinated organic compounds, aromatic nitro compounds,polychlorinated biphenyls [5], pesticides, and even metals and metalloids such ashexavalent chrome and arsenic.Thus, the proposed nanotechnology is concerned with the in-situ or closed systemoxidation of arsenic trioxide to the much less hazardous arsenic pentoxide, which issimultaneously immobilized over nanoparticles of iron, thus preventing future eventualleakages by leaching processes. The versatility of nanoiron for decontamination is basedon is extremely small particle size, relatively large surface area, and high reactivity. Inaddition, functionalized nanoiron can remain in suspension, and thus it can be injected assub-colloidal metal particles into contaminated solids and aquifers. The mechanism ofAsIII removal is achieved by rapid adsorption on nanoiron over a wide range of pH andanion environments and subsequent oxidation to AsV followed by adsorption on the ironcorrosion phases promoted by reactive oxygen species present in the environment. Theactual reactive surface site on iron nanoparticles includes species such as metastableferrous and ferric oxides as well as non-stoichiometric corrosion products. This process ofadsorption and redox reactions immobilizes the arsenic and drastically reduces its toxicitywhile simultaneously the proprietary polymer acting as a carrier generates aconsolidation and waterproofing of arsenic dust, which prevents leaching of contaminantsto soils and aquifers. To reach potential clients for this novel technology at least a proofof concept testing was required in order to assess efficacy, safety and absence ofbyproducts derived from the treatment. In order to ensure unbiased results, actualtesting was performed by an independent third party located at Kelso, Washington(https://www.alsglobal.com/). The experimental design was provided by us as a detailedguideline or protocol that relied on the application of a proprietary aqueous slurry of ironnanoparticles, namely nanoFe?, developed by Nanotek SA (http://www.nanotek.ws ).Testing consisted in preparation of samples from a certified reference material (CRM)AsIII (CAS Number: 1327-53-3; >99.5% pure), placing samples in adequate containers,ensuring homogenization of nanoFe? slurry by vigorous shaking, dosage of 0.3% of thissuspension to the arsenic samples to be treated and comparing results with untreatedsamples of the same material. Both treated and untreated samples were capped andtumbled for 24 hours using a tumbler model #3740-12-BRE. Finally all samples oftreated and untreated materials were extracted and analyzed by EPA 1636 ModifiedMethod in triplicate.Table 1 summarizes results, in which the inorganic arsenic analyte represents totalarsenic (AsIII plus AsV) in the samples.Analyte Sample Preparation Method Average results (ug/g)Arsenic (III) untreated 1632A Modified 87,200Total Inorganic Arsenic untreated 1632A Modified 654,000Arsenic (III) treated 1632A Modified 27,200Total Inorganic Arsenic treated 1632A Modified 624,000Table 1: Lab scale proof of concept for nanoFe? mitigation of arsenicAs can be seen from Table 1, the remaining AsIII after only 24 hours of treatmentunder a dosage of just 0.3% of nanoiron slurry, dramatically dropped by 69%, from87,200 ug/g to 27,200 ug/g (average from triplicates).Total inorganic arsenic was essentially unchanged, because the target of thisnanotechnological treatment was to modify substantially the ratio of highly toxic AsIII tothe much more benign oxidized species AsV, thus minimizing health risks in the eventualcase of leakage to groundwater.The proof of concept testing briefly described above demonstrate that the arsenicoxidation process based on nanoiron for the treatment of arsenic trioxide contaminationin solid matrixes successfully achieved significant AsIII oxidation into AsV in quite a shortlapse of time after the application of a very small percentage of nanoFe?.Further investigation will require replacement of synthetic material for contaminatedsamples gathered from sites impacted by real gold mining operations. This second stageof research should also include assessment of an overall solution. This will include notonly the adsorption, oxidation and immobilization process described in this work, but alsoprocedures for encapsulation and consolidation of arsenic containing dust.References[1] M. Morgada, I. Levy, V. Salomone, S. Faría, G. D. López, M. I., Litter, (2009), Arsenic (V)removal with nanoparticulate zerovalent iron: Effect of UV light and humic acids. Catalysis Today,2009, Volume 14, 261-268[2] G.D. López, Nanoiron for Site Remediation: Bench Scale Assessment and Field Applications,chapter 7 in Iron Nanomaterials for Water and Soil Treatment, M. I. Litter, N. Quici & M. MeichtryEditors, 2018, Pan Stanford Publishing[3] P.G. Tratnyek, R.L.Johnson, (2006), Nanotechnologies for environmental cleanup, Nanotoday,1, 44-48.[4] W. X. Zhang, (2003), Nanoscale iron particles for environmental remediation: An overview,Journal of Nanoparticle Research, 5, 323-332.[5] G. D. López, Nanohierro cerovalente para remediación in situ de compuestos organocloradosrecalcitrantes: estudio de caso, Anales SAM-CONAMET 2014 (e-book