Effectiveness and stability of aluminium and iron oxides nanoparticles for arsenate adsorption

• Ano de defesa: 2008
• Autor(a) principal: Silva, Juscimar da
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa; BR; Fertilidade do solo e nutrição de plantas; Gênese, Morfologia e Classificação, Mineralogia, Química,; Doutorado em Solos e Nutrição de Plantas; UFV
• Palavras-chave em Português: Arsenic; Water treatment; Stability; Contamination; Arsênio; Tratamento de água; Estabilidade; Contaminação; CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::CIENCIA DO SOLO
• Link de acesso: http://locus.ufv.br/handle/123456789/1589

The geochemical fates of arsenic and iron are closely correlated that methods of arsenic removal from water are based on the high affinity of this metalloid with Fe (hydr)oxides nanominerals. Nevertheless, in anoxic environment dissimilatory iron reducing bacteria play a fundamental role in catalysing the redox transformations that ultimately control the mobility of As in aquatic environment. Aluminium nanominerals are ubiquitous and also have great affinity for arsenic. Additionally, under reducing conditions, Al is rather stable and its presence in the Fe (hydr)oxides framework enhance their stability, as well reported in the literature. Thus, by associating the higher binding affinity of Fe (hydr)oxides for arsenic and the higher stability of Al under anoxic conditions can be an advantageous alternative for removing arsenic from water. In this study, we investigated the influence of structural Al in the Raman vibrational stretching modes of goethite and arsenate phases formed on its surface and on other Al and Fe (hydr)oxides, as well as their potential in adsorbing arsenic. The stability of arsenic retained by aluminium and iron (hydr)oxides under anoxic conditions in the presence of S. putrefaciens cells, and phosphate or carbonate competing anions was also investigated. Poorly crystalline aluminium hydroxide [Al(OH)3], gibbsite (Gb), 2-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and three Al-substituted goethites (AlGt) containing 13, 20, and 23 cmol mol-1 of Al were synthesised and characterised chemically and physically. These adsorbents without and with arsenate were investigated by X-ray diffraction, diffuse reflectance, and Raman spectroscopy. Adsorption kinetics at two different solid:solution ratios, 2.0 and 5.0 g L-1, and adsorption isotherms were obtained after equilibrating the samples with arsenate solution under constant shaking. As(V) adsorption maxima was measured at different pH ranging from 3 to 9. The adsorbents were anaerobically incubated under N2 atmosphere and supernatants were periodically sampled to evaluate the contents of soluble As. Presence of structural Al increased the specific surface area and the As adsorption capacity of the Gt. The general effects of the structural Al were to reduce Gt crystallinity and displace spectral lines. Such structural disorder was clearly identified by Raman spectroscopy and X-Ray diffraction. Changes in vibrational frequencies and linewidths due to structural Al resulted in loss and overlap of many Gt active bands. These effects increased as the degree of substitution increased. Raman technique also confirmed the co-occurrence of magnetite in AlGt13 sample, as indicated by XRD. As-O vibrational bands were visualised on all Raman spectra, except for pure Gt probably due to its lowest content of adsorbed As(V). Positions of As-O vibrational band suggested that As(V) was strongly retained on the minerals as innersphere surface complexes. In spite of the fast equilibrium, the increase in solid concentration limited the efficiency and velocity of arsenic adsorption. The As(V) adsorption maxima decreased in the following order: Al(OH)3 > Fh > AlGt13 > AlGt20 > AlGt23 > Gb > Hm > Gt. Nevertheless, by calculating adsorption capacities in terms of surface area, Gb, Gt, and Hm showed higher As(V) loading capacity. This suggest that available reactive sites were not fully occupied by arsenate on the amorphous and Alsubstituted (hydr)oxides. No relationship was observed between medium particle size and maxima adsorption. This suggests re-aggregation of the particles during the particle size measurement, or imperfections on the surface of the particles increasing their net charge, resulting in high adsorption density. The behaviour of all samples was strongly dependent on pH, and the maximum adsorption was achieved in slightly acidic conditions. In general, Al hydroxides were more efficient than Fe (hydr)oxides to remove As(V) from water. The presence of structural Al enhanced considerably the efficiency of the goethites which showed to be promising as adsorbents to remove arsenic from contaminated water. We found that S. putrefaciens cells were able to bind on mineral surfaces and utilise both noncrystalline and crystalline iron (hydr)oxides as electron acceptor releasing arsenic into solution. Al-substituted goethites presented a decrease in the fraction of soluble iron and mobilised arsenic as structural Al increased. The expected relationship between specific surface area and reductive dissolution of Fe and As was also affected by the increment in structural Al. Phosphate and carbonate affected the kinetics of iron reduction due to precipitation of soluble iron as metastable mineral phases (e.g. vivianite and siderite). It seems that analogous mineral phases of phosphates served as a sink for As limiting its mobilisation. Phosphate competed strongly with arsenate and its efficiency seemed to be governed by the nature of the binding mechanism between As and adsorbent surface. Higher fraction of arsenic was desorbed by phosphate from gibbsite followed by AlGts. Conversely, only Gb showed significant amounts of arsenate displaced by carbonate. In spite of low crystallinity, Al(OH)3 was the most efficient in retaining arsenate on its surface followed by Fh and Hm.