Extração e caracterização de frações da matéria orgânica de solos de climas úmido e semiárido de Alagoas: contribuição na complexação dinâmica de Pb(II)

Abstract

Soil represents one of the largest carbon reservoirs with important participation in climate regulation and biogeochemical cycles. The characterization of carbon compounds present in the soil allows measuring their vulnerability to climatic changes, handling and bioavailability process of contaminants. Extraction is an important step for studies involving soil organic matter and should minimize structural changes of the parent compounds. Pb(II) is a potentially toxic metal and its availability to the environment depends on the interaction with natural organic ligands. In the present study, extraction and characterization of soil organic matter fractions were carried out to evaluate the influence of these fractions on the dynamic complexion of Pb(II). Soil samples were collected in two cities of the state of Alagoas (Arapiraca and Maceió), located in semi-arid and humid climates; both samples were physicalchemically characterized. The soil with a semi-arid climate had 4.5% organic matter and a value of 10 for the C/N ratio because it is a soil used in agriculture, while humid climate soil had 11.5% organic matter and 16 for the C / N ratio due to the hydromorphic conditions of the sample, being more humidified. The extraction of the fractions was performed with: i) NaOH ii) NaNO3 and iii) simulated rainfall. The characterization of the fractions was carried out by UV-vis spectroscopy to obtain E4/E6, E2/E4 and E2/E3 parameters and molecular fluorescence to obtain A465 and A4/A1 humification parameters; and acid-base titration which is indicative of the contents of carboxylic and phenolic groups. The extractions led to the obtaining of 10 fractions, being five for each soil, denominated humic substances, humic acids, fulvic acids, soil solution and solution leached from the soil. These fractions presented different Total Organic Carbon (TOC) values (between 9,023 and 1177 mg L-1) and different characteristics. The chemometric analysis, using the MINITAB 14, software made it possible to observe the similarity of the fractions and the formation of three distinct groups based on the parameters obtained in the characterization. The group of humic acids (79.40% of similarity), presenting higher aromatic condensation, molecular size, degree of humification, contents of carboxylic and phenolic groups. The group corresponding to soil solutions, leached solutions and fulvic acids (62.24% of similarity) presented lower aromatic condensation, molecular size, lower degree of humification and contents of carboxylic and phenolic groups. This group did not present similarity with humic acids and 38.54% of similarity with the group of humic substances, which presented intermediate characteristics, forming a group with 86.16% similarity. The complexing capacity and the constant conditions for the complexes formed were determined via AGNES and at pH 5 and 6. The results showed that the different fractions have Pb(II) affinity forming stable complexes, with complexing capacity between 0.81-61.8 mg of Pb (II) g-1 COT at pH 5 and 0.86-63.2 mg of Pb (II) g-1 COT at pH 6. Humic acids had higher complexing capacity and conditional constants than the humic substances and other fractions. At pH 6 complexation capabilities and conditional constants were higher as a result of conformational changes with increasing pH and of the greater dissociation of carboxyl groups and less competition of the H+ ion and the metal at the complexation sites. So, it can be supposed that the extraction procedure influences the characteristics of the extracted organic material, due to the selectivity of the extractor, and the interaction of this material with Pb (II), which is important from the environmental point of view, since it leads to obtaining of more realistic parameters for the most reactive fraction of Soil Organic Matter (SOM) and its contribution in the bioavailability of Pb(II) to the environment, for risk assessments and molecular modeling.