title: Natural attenuation of arsenic in the environment by immobilization in nanostructured hematite.
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.
authors.affiliation: Centre of Microscopy, Universidade Federal de Minas Gerais, Belo Horizonte 31270901, Brazil; National Institute of Science and Technology on Minerals Resources, Water and Biodiversity, INCT-Acqua, Brazil. Electronic address: ericotadeu@ufmg.br.
authors.forename: Erico T F
authors.initials: ET
authors.lastname: Freitas
authors.affiliation: Department of Chemistry, Universidade Federal de Minas Gerais, Belo Horizonte 31270901, Brazil; National Institute of Science and Technology on Minerals Resources, Water and Biodiversity, INCT-Acqua, Brazil. Electronic address: luciano.montoro@qui.ufmg.br.
authors.forename: Luciano A
authors.initials: LA
authors.lastname: Montoro
authors.affiliation: School of Earth Sciences, The University of Queensland, Brisbane 4072, Australia; National Institute of Science and Technology on Minerals Resources, Water and Biodiversity, INCT-Acqua, Brazil. Electronic address: m.gasparon@uq.edu.au.
authors.forename: Massimo
authors.initials: M
authors.lastname: Gasparon
authors.affiliation: Department of Metallurgical Engineering, Universidade Federal de Minas Gerais, Belo Horizonte 31270901, Brazil; National Institute of Science and Technology on Minerals Resources, Water and Biodiversity, INCT-Acqua, Brazil. Electronic address: ciminelli@demet.ufmg.br.
authors.forename: Virginia S T
authors.initials: VS
authors.lastname: Ciminelli
mesh_terms: D000327:Adsorption; D001151:Arsenic / Q000737:chemistry / Q000302:isolation & purification*; D003460:Crystallization; D004785:Environmental Pollutants / Q000737:chemistry / Q000302:isolation & purification*; D052918:Environmental Restoration and Remediation; D005290:Ferric Compounds / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission; D008956:Models, Chemical; D049329:Nanostructures / Q000737:chemistry*; D044367:Phase Transition; D046590:Spectroscopy, Electron Energy-Loss; D013499:Surface Properties
chemical_list: D004785:Environmental Pollutants; D005290:Ferric Compounds; C000499:ferric oxide; C092844:ferric oxyhydroxide; D001151:Arsenic
keywords: Arsenic fixation; As partitioning; As speciation; Long-term stability