Adsorção e disponibilidade de arsênio em solos com diferentes composições mineralógicas

• Autor(a) principal: Assis, Igor Rodrigues de
• Data de Publicação: 2010
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/1604
• topic: Biodisponibilidade; Sorgo; Crotalaria; Bioavailability; Sorghum; Crotalaria; CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::CIENCIA DO SOLO

Heavy metals and toxic metalloids presence in different environments (air, water, soil, etc.) have caused constant environmental and public health concern. Arsenic (As) is a metalloid with great toxicity potential and its overall concentrations on natural soils is about 5 to 6 mg kg-1. However, physical, chemical and mineralogical soil characteristics play a fundamental hole an As mobility and transfer to the food chain. The aim of this study was to characterize the adsorption of As and evaluate the potential of different chemical extractants to predict the As availability in soils with different mineralogical compositions. The experiment was set up using five soils type: three Latossols, one Argisol and one Cambisol, presenting clay or heavy clay texture. The soils received five doses of As, ranging from 0 to 1,060 mg dm-3, which was corresponding to 0, 0.05, 0.1, 0.2 and 0.4 times the maximum adsorption capacity of As (CMAAs). The soils were cultivated with sorghum (Sorghum bicolor) and crotalaria (Crotalaria spectabilis). These species were selected trying to contrast the sensitive species with tolerant species to As. The available As in the soil was evaluated by five extractants: Mehlich 1, Mehlich 3, Ca(H2PO4)2 (Ca(H2PO4)2 7.43 mmol L-1 with acetic acid), DTPA and anion exchange resin (AR-103 QDP). Clay samples were submitted to X-ray diffraction and thermogravimetric (TGA) analysis, allowing, together with data from chemical characterization, to estimate the soil mineralogical composition. The CMAAs was estimated adjusting the data to the Langmuir isotherm. Before the cultivation, the soil pH´s was adjusted to 5.5 and As applied in the form of As2O3, properly prepared to obtain exclusively +5 valence. Three months after the planting time, all plant material was collected (root, stem and leaf) for determination of biomass production and the As content (ICP-OES). The soils showed a large clay proportion, reflecting the CMAAs, which varied from 1.21 to 2.65 g kg-1, with higher values belonging to Latossols. There was also wide variation in the amounts of hematite (Hm), goethite (Gt), gibbsite (Gb) and kaolinite (Ka). The CMAAs correlated significantly with the Gb and Ct amount, being positive for the first mineral and negative for the second. Hm and Gt minerals alone did not show significant correlation, however the sum of these minerals showed the best correlation with CMAAs. The soil attribute that best predicts the CMAAs was the content of iron extracted by ditionite- citratebicarbonate. By assessing the chemical methods to estimate the available As, there was a significant correlation between the available content by different extractants and the As-sorghum content. For that plant, all the chemical extractants showed significant correlation, being the highest correlation coefficient obtained for Ca(H2PO4)2 and the smallest for resin. The available As content by DTPA and resin, however, were close to zero even at the highest levels of As applied where expressive As contents were observed in the plant. Therefore, these extractants showed no ability to predict the soil available As. Adjusting regression equations between available As content and applied doses, greater sensitivity to different chemical, physical and mineralogical soil could be observed by the Ca(H2PO4)2 extractor, having greater sensitivity to arsenate buffer capacity of the soils. Thus, the predictive ability of available As in the soils follows this order: Ca(H2PO4)2> Mehlich 3> Mehlich 1. The total As soil content that leads to a 50% of biomasss reduction (T50) was about 420 mg dm-3 for crotalaria and 634 mg dm-3 for sorghum. These values correspond to the content available by Ca(H2PO4)2 extractor, 29.05 and 51.27 mg dm-3 for the crotalaria and sorghum, respectively. Therefore, available As content that leads to 50% of biomass reduction is approximately 7.5% of the total As in the soil. This shows that the use of total content, legislation adopted by Brazil, is very conservative, respecting the principle of precaution.