MINERALS OF ENVIRONMENTAL RELEVANCE IN ACID MINE DRAINAGE CONTAMINATED SYSTEMS: PROPERTIES AND REACTIVITY AT THE NANOSCALE
- Universidade do Minho(líder)
- Laboratório Ibérico Internacional de Nanotecnologia(parceiro)
- Universidade de Évora(parceiro)
Summary
Acid mine drainage (AMD) occurs by oxidative dissolution of sulfide minerals, releasing strong acidity, sulfate and
metals. Moreover, the evolution of AMD generates high amounts of colloids, i.e., AMD-precipitates with nanoscale
dimensions. It is a worldwide environmental problem, causing complete degradation of the ecosystems. The current
strategy of restarting mineral exploitation in Europe highlights the relevance of the AMD issues.
The general idea of the project focuses on the formation and evolution of colloids, since they are one of the main
processes of mobilization of toxic elements over great distances. With such an emphasis, long term goal is to elucidate
the properties and role of such colloids in the fate and transport of metals and arsenic. The characterization of the
nanoparticles will also allow understanding their role at different levels regarding toxicity and participation in natural
attenuation processes.
Characterization of these natural nanoparticles is a very complex task due to heterogeneity, low crystallinity of the
phases and small dimensions. Thus, the novel character of the project relies on their mineralogical identification and
detailed chemical and morphological characterization. The results will guide further research in two main directions:
synthesis of nanoparticles and evaluation of potential applications.
Hence, the strategy includes the study of the AMD-precipitates, mainly iron oxides-hydroxides and iron
hydroxysulfates. Specific tools for executing the research strategy will include XRD, SEM, TEM, FTIR, and ICP. Also,
laboratory experiments will be conducted to examine the sorption capacity of distinctive types of AMD- nanoparticles.
Application of surface sensitive spectroscopy or microscopy techniques continues to be a challenging analytical
problem. Existing knowledge about AMD-precipitates and their surface behavior is insufficient and complex natural
mixtures further limit the applicability of first-principles of predictions. To overcome this complexity gap, model minerals
will be acquired or synthesized as reference materials.
Modern nanotechnology research has produced extensive literature about applications of nanoparticles, but typically
with a focus on synthetic materials. The novelty of the expected results of the current project will be an enhanced
understanding of natural AMD-precipitates. Consequently, it will be possible to evaluate potential applications,
including in remediation of contaminated sites, highlighting another novel aspect and relevance of the project.
The expected results will be accomplished by the establishment of strong collaborative synergies between the
partners. The University of Minho uses its long term experience in AMD contamination and AMD-precipitates, while
INL is at the forefront of the synthesis and characterization of nanoparticles. University of Évora will be dedicated to
chemical characterization of solutions and nanoparticles.
Goals, activities and expected/achieved results
Goals
The problem under study is in the innovative domain of nanogeocience, which has natural nanoscale minerals
(nanoparticles) as the main focus of interest. Although nanoparticles may present considerable mineralogical diversity,
many include ferric iron oxyhydroxides and oxyhydroxysulfates, which deserve the attention of the present project.
They are formed as result of processes such as weathering, neutralization and precipitation reactions. The importance
of a research field dedicated to naturally formed nanoparticles is stressed by their participation in the biogeochemical
cycles that take place in water and soil reservoirs. Nanoparticles have high relevance because of their high surface
areas and correspondingly high activity and reactivity. Consequently, processes such as adsorption and coprecipitation
are linked to the environmental impact of nanoparticles in geochemical reservoirs, like river waters, aquifers, soils,
and sediments.
The specific problem to be investigated concerns the nanoparticles that occur in acid mine drainage (AMD), which is
one of the most important and pernicious environmental problems in the world. AMD occurs by oxidative dissolution
of sulfide minerals, liberating acidity, sulfates, metal(oid), and huge amounts of colloids, most of them composed of
nanoparticles of typical ferric iron minerals, here denoted as AMD-precipitates or as AMD-nanoparticles. This is not
only an inherited problem from mining legacy, but also a present issue related with the revival of metallic mines in
Europe. Therefore, it is a challenge that all those involved in the mining activity, past and present, have to face.
The main objective of the project is to discover the detailed properties of the AMD-nanoparticles, including elucidation
of their role in the natural attenuation of AMD. The research challenge arises from the difficulty of characterizing
the properties and reactivity of these natural nanoparticles. The environmental role of AMD-nanoparticles is a both
interesting and important research subject, with relevance in several areas:
- A legacy of decades of mining activity, producing AMD, liabilities from which need to be remediated;
- Huge amounts of AMD-nanoparticles, that largely consists of nanoparticles, in past and present mining sites,
including in AMD treatment plants;
- Ability of nanoparticles to influence the fate and transport of pollutants;
- Knowledge gap regarding the processes of retention and/or mobilization of AMD pollutants from the surface of the
nanoparticles;
- Potential transposition of the most novel developments in engineered nanomaterials, to the field of natural AMDnanoparticles,
namely for remediation of contaminated sites;
- Scientific and technological difficulties for the characterization of AMD-nanoparticles, mainly due to their low size,
heterogeneity, and low crystallinity, resulting in poor understanding of their surface activity and reactivity in natural
environments.
Activities
The evolution of the project will comprise the following steps corresponding to project activities, which will be detailed
below:
Activity 1: Selection of study areas
Activity 2: Sampling of AMD systems (water and AMD-precipitates);
Activity 3: Characterization of the AMD solutions and risk assessment;
Activity 4: Separation and characterization of AMD-precipitates
Activity 5: Acquisition, synthesis, modification and surface characterization of model AMD-precipitates
Activity 6: Potential valorization of AMD- nanoparticles
Results
1. Elucidation about the nature and role of acid mine drainage precipitates, specifically the ochreous iron minerals discharged in water reservoirs and nonacidic rivers. It allowed the
acquisition of important knowledge regarding acid mine drainage nanoparticles, that will be subject to research in the present proposal.
2. It provided an inventory of AMD precipitates in the Spanish sector of the Iberian pyrite belt as well as information about their environmental relevance in a semi-arid climate. It
allow to acquire experience and knowhow regarding sampling and characterization of AMD precipitates.
3. It provided information regarding AMD geochemistry. Specifically, there were important results about partitioning between dissolved and particulate matter of colloidal nature
4. It allowed acquiring experience and know-how in the domain of the preparation and analysis of geological materials. As in the present proposal, chemical composition depends
on the operation of ICP-OES.
5. The project provided results regarding the use of zero valence iron for remediation of a contaminated mining site. Such knowledge will be applied to analyze the potential application
of AMD-nanoparticles for site remediation.
