Elsevier

Chemosphere

Volume 138, November 2015, Pages 340-347
Chemosphere

Natural attenuation of arsenic in the environment by immobilization in nanostructured hematite

https://doi.org/10.1016/j.chemosphere.2015.05.101Get rights and content

Highlights

  • Identification of As-bearing Al-hematite in environmental samples.

  • A mechanism of As fixation into Al-hematite mesocrystals by oriented attachment.

  • Microscopy techniques analyses of trace element distribution in environmental samples.

  • Significance of these findings for the management of As-contaminated mine sites.

Abstract

Iron (hydr)oxides are known to play a major role in arsenic fixation in the environment. The mechanisms for long-term fixation into their crystal structure, however, remain poorly understood, especially arsenic partitioning behavior during transformation from amorphous to crystalline phases under natural conditions. In this study, these mechanisms are investigated in Fe–Al-oxisols exposed over a period of 10 years to a sulfide concentrate in tailings impoundments. The spatial resolution necessary to investigate the markedly heterogeneous nanoscale phases found in the oxisols was achieved by combining three different, high resolution electron microscopy techniques – Nano-Beam Electron Diffraction (NBD), Electron Energy-Loss Spectroscopy (EELS), and High Resolution Transmission Electron Microscopy (HRTEM). Arsenic (1.6 ± 0.5 wt.%) was unambiguously and precisely identified in mesocrystals of Al-hematite with an As/Fe atomic ratio of 0.026 ± 0.006. The increase in the c-axis (c = 1.379 ± 0.009 nm) compared to standard hematite (c = 1.372 nm) is consistent with the presence of arsenic in the Al-hematite structure. The As-bearing Al-hematite is interpreted as a secondary phase formed from oxyhydroxides, such as ferrihydrite, during the long-term exposure to the sulfide tailings. The proposed mechanism of arsenic fixation in the Al-hematite structure involves adsorption onto Al-ferrihydrite nanoparticles, followed by Al-ferrihydrite aggregation by self-assembly oriented attachment and coalescence that ultimately produces Al-hematite mesocrystals. Our results illustrate for the first time the process of formation of stable arsenic bearing Al-hematite for the long-term immobilization of arsenic in environmental samples.

Introduction

The release of As into the environment from geogenic sources is ultimately controlled by the chemical stability of As-bearing mineral phases. Therefore, the thorough structural and chemical characterization of these phases is necessary to fully assess As mobility in different environmental matrices. The chemical stability of solid As-bearing phases in aqueous systems is conventionally established using extraction methods designed to measure the degree of mobilization under specified environmental conditions, such as the Toxicity Characteristic Leaching Procedure – TCLP (Ghosh et al., 2004, Jong and Parry, 2005), and sequential extraction methods and bioaccessibility tests (Cappuyns et al., 2002, Guo et al., 2007, Anawar et al., 2008, Karak et al., 2011, Larios et al., 2012a, Larios et al., 2012b). These procedures, however, provide only indirect evidence for As association with its host mineral phases while the actual nature of these phases remains unknown.

The structural characterization of As-bearing phases by traditional analytical techniques is not trivial, and in view of these difficulties, synchrotron-based analytical techniques combined with theoretical molecular modeling or with other spectroscopic techniques have been increasingly applied to investigate As distribution, speciation, and bonding characteristics, with advances in our understanding of As stability (Paktunc et al., 2008, Morin et al., 2009, Chakraborty et al., 2011, Duarte et al., 2012a). Transmission electron microscopy (TEM) techniques are especially useful for the characterization of solid phases at the nanoscale, with better spatial resolution than any other technique. However, few TEM investigations of As-bearing minerals have been described in the literature (Carlson et al., 2002, Ouvrard et al., 2005, Paktunc et al., 2008, Morin et al., 2009, Kendall et al., 2013). The difficulties in the accurate characterization of As compounds in environmental samples by conventional techniques lie in the markedly particles heterogeneity and small grain size. To overcome these difficulties we combined different TEM techniques such as Nano-Beam Electron Diffraction (NBD), Energy Dispersive X-ray Spectroscopy (EDS), Electron Energy-Loss Spectroscopy (EELS), and High Resolution Transmission Electron Microscopy (HRTEM), which give the highest spatial resolution needed in the analyses of phases at nanoscale.

In this work we used HRTEM, NBD, EDS and EELS to investigate the As-bearing phases and the distribution of As in Fe–Al-oxisols used as liners in sulfide tailings disposal facilities. A recent study from our group (Duarte et al., 2012b) investigated As speciation in the same oxisol liners as well as in the tailings after 10 years of disposal. The bulk analysis were done using a sequential extraction procedure (SEP), X-ray diffraction (XRD) and X-ray Absorption Spectroscopy Near Edge Structure (XANES) analyses. Arsenic was found mainly as As(V) in the oxisol samples and as arsenopyrite in the tailings. Up to 69% of the total As content in the oxisol was found associated with Fe and Al crystalline phases, which were not dissolved even under strongly acidic and reduction conditions. This crystalline fraction of the Fe–Al-oxisol liners is investigated in this present study, and it will be demonstrated that As is immobilized in the structure of secondary crystalline Al-hematite.

Section snippets

Sample description

Tailings of As-sulfide concentrates from the hydrometallurgical processing plant of a gold mine in Minas Gerais, Brazil were stored for more than 10 years in three disposal facilities sealed with an enriched Fe and Al oxisol. In 2011 and 2012 the tailings of two of these facilities (namely F2 and F3) were excavated and reprocessed, and samples of the oxisol liners (OL) were recovered for analysis. A sample from the local natural oxisol (LNO) was also investigated. This sample represents the

Natural oxisol

The chemical analysis showed that the As content in the natural oxisol was 12 + 7 mg kg1. The main phases identified by XRD analysis (diffractogram not shown) were quartz, kaolinite, gibbsite, muscovite, hematite, goethite, and goethite. TEM analysis showed the presence of ferrihydrite, goethite and hematite (data shown in S1). Structural Al was identified in all these phases by the EDS analysis. Arsenic was not detected by EDS.

Oxisol liner

The As content in the oxisol liners ranged from 1950 mg kg1 in sample

Environmental implications

The proposed mechanism for As fixation into the Al-hematite structure accounts for As immobilization in Fe–Al-(hydr)oxides present in the oxisols, and can be viewed as a natural remediation process. The high concentrations of Al in oxisols enhance As uptake by increasing the surface area of ferrihydrite, and may improve the long-term stability of As-bearing hematite under reducing conditions. An intermediate Al-goethite phase is likely to form at some stage of the process, but hematite is the

Conclusions

The mechanism of As immobilization in natural Fe–Al-oxisol was investigated. An oxisol liner was exposed to As-bearing sulfide tailings for over 10 years in a gold mine site. Although the role of Fe–Al-(hydr)oxides in As fixation is well established, our work showed for the first time significant amounts of As (1.6 ± 0.5 wt.%) distributed in crystalline Al-hematite with an As/Fe atomic ratio of 0.026 ± 0.006. The presence of As in the Al-hematite structure is consistent with a slight increase of the c

Acknowledgments

The authors are grateful to the Brazilian government agencies – CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), Fapemig (Fundação de Amparo a Pesquisa do Estado de Minas Gerais) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for financial support. Kinross Brasil Mineração is gratefully acknowledged for supplying the samples used in this investigation. The authors also acknowledge the Science Without Borders program for the PVE fellowship to M.

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