Mecanismo de acumulação de ferro e arsênio em biomassa vegetal fibrosa

Abstract

The general aim of the present work is to study iron and arsenic sorption in dried and ground lettuce leaves, in order to investigate in detail the mechanism of arsenic accumulation on lettuce. Several spectroscopic techniques were used to characterize the biomass as well as to investigate the oxidation states and neighborhood of the adsorbed iron and arsenic species. The results indicate that arsenic is sorbed only when the biomass is previously loaded with iron. The removal rate of iron and arsenic by the biomass is high, with 80 to 90% of the total amount of metal being removed within 60 minutes, suggesting very active surface phenomena on the biomass. The maximum adsorption capacity for As(V) in iron loaded biomass (0.22mmol/g) is 0.13mmol/g, comparable to the highest values found in the literature for sorbents containing iron. The kinetic tests for As(III) were carried out at pH=3.0 and pH=10.0 but the sorption was not effective (0.03mmol/g) under these conditions. Esterification of the biomass provokes a decrease of 84% in iron loading, indicating preferential iron binding to carboxyl groups. XANES and Mössbauer spectroscopy results show that iron and arsenic are oxidized during sorption. Resin speciation tests showed that arsenic oxidation does not occur in the solution. Raman spectra showed the presence of nitrate, suggesting that this anion could be responsible for iron and arsenic oxidation. EXAFS results indicate that iron is coordinated by 6 oxygen (Fe-O distance of 2.0Å) and 2 carbon (Fe-C distance of 2.81Å) atoms in a bidentate mononuclear form and arsenic is coordinated by 4 oxygen atoms (As-O distance of 1.69Å) in a tetrahedral geometry. The important role of iron in arsenic sorption suggests that AsO4 tetrahedra are connected with the FeO6 octahedra inside the biomass in a monodentate form. These results may explain the arsenic accumulation pattern shown by lettuce leaves grown in mine regions where the soils and the waters are rich in iron.