Defects induced by Al substitution enhance As(V) adsorption on ferrihydrites.

An original rationale is proposed to explain the controversial role of aluminum, a common substitutive element in ferrihydrite (Fh), on arsenic adsorption. The adsorption of arsenic on synthetic Al-for-Fe substituted Fh (AlFh) with up to 20 mol% Al was investigated at pH 5 and 8. The reduced interplanar spacings observed by selected area electron diffraction show that all AlFh samples are isomorphically substituted up to 20 mol% Al. A 15 mol% Al incorporation increases the arsenic uptake by 28%. In contrast, the Langmuir binding constants decrease, suggesting weaker bonds. Arsenic uptake reduces by 50% as pH rises from 5 to 8. The Al-for-Fe substitution in ferrihydrite causes structural defects, proton-compensated by OH groups, as indicated by the Vegard rule deviation. X-ray photoelectron spectroscopy demonstrates the increase in the relative amount of surface M-OH sites (45% to 77%) with Al concentration (AlFh-0 to AlFh-20), respectively. The enhanced As(V) uptake was ascribed to the insertion of hydroxyls on the Fh structural defects. Fourier-transformed-infrared spectroscopy showed that the sites modified by Al introduction are involved in As adsorption. These findings help to understand aluminum's role in arsenic adsorption, fixation, and fate in the environment.

Effects of aluminum and soil mineralogy on arsenic bioaccessibility.

A comprehensive characterization was performed to investigate the composition and mineralogy of soils from a gold mining region and their correlation with arsenic (As) total concentration and its bioaccessible fraction. The arsenic bioaccessible (BAC) fraction was determined through in vitro test and calculated as the ratio between the amounts of As released and the total As concentration in the soil sample. Among the minor constituents of environmental concern, only arsenic is significantly higher (median of 748.0 mg kg-1) than the national guidelines (agricultural, 35 mg kg-1 and residential, 55 mg kg-1). All the other trace elements showed concentrations below the investigation values established for residential areas. The mean bioaccessible As was 7.0 mg kg-1, with a median value of 4.4 mg kg-1, and a median As BAC percentage of 0.7%. The Brunauer-Emmett-Teller (BET) surface area showed a consistent increase with the increase of the acid-soluble Al content in the soil samples. The distribution of As in the soil samples is not correlated with the abundance of As-minerals and the fraction of adsorbed As. Arsenic was shown to be trapped in oriented aggregates of crystalline (Al-)Fe-(hydr)oxides nanoparticles (the main metalloid reservoirs), as demonstrated by scanning and transmission electron microscopy analyses. This unique pattern supports the significant difference between total As concentration and the bioaccessible amount. There was a positive correlation between soluble Al (within the Fe-(hydr)oxides phases and minor gibbsite) and As concentration in the soil samples, and a negative correlation with bioaccessible As. Therefore, although Al in the soil is associated with high As levels, it also makes the metalloid less bioaccessible. The risk to human health from As exposure to these soils is low.

Synergistic effect of a spinel ferrite on the adsorption capacity of nano bio-silica for the removal of methylene blue.

The synergistic effect of CoFe2O4 on the capacity of bio-silica extracted from rice husk for the removal of methylene blue (MB) was investigated. The novel composite of cobalt ferrite/nano bio-silica was prepared by dispersing cobalt and iron salt in ratio 1:2 in a solution containing bio-silica, calcined at 700°C and characterized. The adsorption capacity of the composite (253.6 mg g-1) was higher than that of bio-silica (52.6 mg g-1), and the process was exothermic and spontaneous. Langmuir and Freundlich models were applicable to explain the adsorption isotherm, while pseudo-second-order and Elovich are best applicable for the kinetics mechanism. The amount of MB that was removed, increased with an increase in ionic strength due to dimerization of MB. Regeneration and reusability of the adsorbents showed that they are economically viable. Energy-filtered transmission electron microscopy (EFTEM) and Fourier transformed infrared (FTIR) analysis of MB-loaded adsorbent confirmed the adsorption of MB.

Gastric/lung bioaccessibility and identification of arsenic-bearing phases and sources of fine surface dust in a gold mining district.

Bioaccessibility (BAC) of fine surface dust (FSD, particle size ≤10 µm) and surface dust samples (particle size ≤250 µm) collected from a gold mining district was used as a tool to determine the portion of arsenic that would be available via simulated lung and gastrointestinal (G.I) fluids. BAC was considered low for both tests (lung 2.7 ±â€¯1%, n = 5 and G.I 3.4 ±â€¯2%, n = 14 for residential surface dust samples). An analytical procedure was developed to further identify arsenic-bearing phases found in FSD samples and analyze the main components that regulate arsenic solubility. Up to five different arsenic-bearing phases were identified among a total of 35 minerals surveyed by scanning electron microscopy-based automated image analysis (Mineral Liberation Analyzer - MLA). Arsenic-bearing Fe oxy-hydroxides and mixed phases comprised the main arsenic phases encountered in FSD samples, thus likely being responsible for regulating arsenic bioaccessibility. Transmission electron microscopy showed that the mixed phases comprised a mix of oriented nanostructure aggregates formed by hematite and goethite entangled with phyllosilicates. The main As-bearing phases identified in FSD samples are similar to those reported in soil samples in the same region. The predominant arsenic-bearing phase encountered in the ore was arsenopyrite, mostly in large particles (>10 µm in size), and therefore unlikely to be found in residential dust. Arsenic intake from both inhalation and ingestion were minimal when compared to total arsenic intake (considering food and water ingestion), which itself was <7% of the value established by the Food and Agriculture Organization of the United Nations Benchmark Dose Lower Confidence Limit (BMDL0.5) of 3.0 µg per kg-1 body weight per day. These results indicated that the relative risks associated with arsenic exposure by inhalation and oral ingestion in this region are low.

Low arsenic bioaccessibility by fixation in nanostructured iron (Hydr)oxides: Quantitative identification of As-bearing phases.

A new analytical protocol was developed to provide quantitative, single-particle identification of arsenic in heterogeneous nanoscale mineral phases in soil samples, with a view to establishing its potential risk to human health. Microscopic techniques enabled quantitative, single-particle identification of As-bearing phases in twenty soil samples collected in a gold mining district with arsenic concentrations in range of 8 to 6354 mg kg-1. Arsenic is primarily observed in association with iron (hydr) oxides in fine intergrowth with phyllosilicates. Only small quantities of arsenopyrite and ferric arsenate (likely scorodite) particles, common in the local gold mineralization, were identified (e.g., 7 and 9 out, respectively, of app. 74,000 particles analyzed). Within the high-arsenic subgroup, the arsenic concentrations in the particle size fraction below 250µm ranges from 211 to 4304 mg kg-1. The bioaccessible arsenic in the same size fraction is within 0.86-22 mg kg-1 (0.3-5.0%). Arsenic is trapped in oriented aggregates of crystalline iron (hydr)oxides nanoparticles, and this mechanism accounts for the low As bioaccessibility. The calculated As exposure from soil ingestion is less than 10% of the arsenic Benchmark Dose Lower Limit - BMDL0.5. Therefore, the health risk associated with the ingestion of this geogenic material is considered to be low.

Arsenic entrapment by nanocrystals of Al-magnetite: The role of Al in crystal growth and As retention.

The nature of As-Al-Fe co-precipitates aged for 120 days are investigated in detail by High Resolution Transmission Electron Microscopy (HRTEM), Scanning TEM (STEM), electron diffraction, Energy Dispersive X-Ray Spectroscopy (EDS), Electron Energy-Loss Spectroscopy (EELS), and Energy Filtered Transmission Electron Microscopy (EFTEM). The Al present in magnetite is shown to favour As incorporation (up to 1.10 wt%) relative to Al-free magnetite and Al-goethite, but As uptake by Al-magnetite decreases with increasing Al substitution (3.53-11.37 mol% Al). Arsenic-bearing magnetite and goethite mesocrystals (MCs) are formed by oriented aggregation (OA) of primary nanoparticles (NPs). Well-crystalline magnetite likely formed by Otswald ripening was predominant in the Al-free system. The As content in Al-goethite MCs (having approximately 13% substituted Al) was close to the EDS detection limit (0.1 wt% As), but was below detection in Al-goethites with 23.00-32.19 mol% Al. Our results show for the first time the capacity of Al-magnetite to incorporate more As than Al-free magnetite, and the role of Al in favouring OA-based crystal growth under the experimental conditions, and therefore As retention in the formed MCs. The proposed mechanism of As incorporation involves adsorption of As onto the newly formed NPs. Arsenic is then trapped in the MCs as they grow by self-assembly OA upon attachment of the NPs. We conclude that Al may diffuse to the crystal faces with high surface energy to reduce the total energy of the system during the attachment events, thus favouring the oriented aggregation.

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

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.