Mecanismo inédito de imobilização de arsênio baseado em agregação orientada de nanopartículas cristalinas de (Hidr)óxidos de ferro e alumínio: evidências por microscopia eletrônica de transmissão

Abstract

Iron and aluminium (hydr)oxides play a major role in arsenic fixation in the environment due to their capacity for arsenic uptake by adsorption or co-precipitation. Nevertheless, the mechanisms responsible for the arsenic long-term immobilization in Al- and Fe- (hydr)oxides, the phase transformations and their effects on arsenic partition, and the role of aluminium in the all steps of these processes are not yet elucidated, being studied in the present work. The investigated samples are Al- and Fe- rich Oxisols, synthesized Al-free and Al-containing goethite (-FeOOH), magnetite (Fe3O4) and ferrihydrite (Fe2O3.0.5H2O), aged for up 120 days at ambient temperature. The results showed that the presence of aluminum into the crystal structure of ferrihydrite (i) retards further phase transformations, thus stabilizing the original phase; (ii) favours the formation of mesocrystals produced by the mechanism of oriented aggregation and (iii) increases the average charge density of iron in the crystal lattice, which may favour anions adsorption. The As-containing phases are predominantly oriented aggregates of nanoparticles (OAN), where the arsenic is mainly found in the interstices of nanoparticles-building-units of ferrihydrite mesocrystals. The arsenic content associated to pure ferrihydrite was 5.4±0.6 wt% As (54.000 60.000 mg kg-1). The ferrihydrite sample with larger aluminium substitution (21±4% mol mol-1) presented 6.9±0.3 wt% As (66.000 72.000 mg kg-1). In the Oxisols, the arsenic is predominantly associated to OAN of Al-hematite (-Fe2O3), in concentration of 1.6±0.5 wt% As (11,000 21,000 mg kg-1). A positive correlation between arsenic and aluminium was observed in nanostructured Al-hematite. The arsenic concentration up to 1.10 wt% As (11,000 mg kg-1) was observed in OAN of Al-magnetite. The arsenic content in Al-magnetite is relatively larger than in pure magnetite, and decreases with an increase of Al-for-Fe substitution in the range (Al/(Al+Fe)) 3.5 to 11.4% mol mol-1, likely due to the increase of Fe2+ sites relatively to Fe3+ substituted by Al3+. This might contribute to weaken the interaction between arsenic anions and iron. Oriented aggregates Al-goethite nanoparticles with 13 to 32% mol mol-1 Al presented arsenic content approximately equal or less than 0.1 wt%. The proposed mechanism for arsenic incorporation in nanostructured Al-Fe-(hydr)oxides relies on a non-classical aggregation based crystal growth. The adsorbed arsenic on ferrihydrite is trapped upon the oriented aggregation of nanoparticles. The non-desorbed arsenic is irreversibly incorporated in the net-structure as the aggregates evolve to a more ordered structure. The combination of several transmission electron microscopy techniques made possible the investigation of markedly heterogeneous samples, typical of soils, at nanometre scale, thus bringing about a novel understanding of the mechanism for As incorporation in nanostructured materials. This mechanism, shown for the first time in this work, accounts for the process of long-term immobilization of arsenic in samples of environmental concern and highlights arsenic stability within nanostructured metal oxides. This understanding intends to help further developments of remediation technologies for arsenic uptake from wastewater.