Physical-chemical study of the interactions of aluminum(III) ion with fine decomposed peat of Arroio Silva, Santa Catarina, Brazil

The interactions of the fine decomposed peat (FDP) of Arroio Silva, SC, Brazil, with the Al(III) ion were studied. The infrared (IR) spectrum confirmed the presence of the characteristic functional groups of peat. Kinetic studies revealed that, with a high correlation coefficient, the pseudo-second-order model shows a good agreement between the experimental and calculated q_e values. This indicates that in the adsorption of Al(III) ion the determining step is a chemical adsorption, involving chelation. The process is described by the Langmuir adsorption model, which is limited to the formation of a monolayer of metallic ions. According to this model, the maximum adsorption capacities at pH 1.0, 2.5 and 4.0 were, respectively, 5.42, 5.89 and 6.09 ​mg of aluminum per gram of peat. The suggested adsorption mechanism involves the complexation of the Al(III) ion with the oxygenated groups present in the peat. The electron microscopy analysis showed a rough and porous surface and the determination of the point of zero charge (PZC) indicates that the interactions of the Al(III) ion with the peat are much stronger than simple electrostatic interactions. The molar amounts of the most important functional groups present and the equilibrium constants of Al(III) ion interactions with these groups were calculated and the distribution curves were obtained. At pH values up to pH ​= ​4.4, the Al(III) ion is preferably coordinated with the phthalic group. At higher pH values several interactions occur, for instance, between: a monohydroxide and the phthalic group, Al(OH)(Pht); a dihydroxide, a phthalic and a catechol group, which predominates at pH values of 4.5 to 8.2, [Al(OH)₂(Pht)(Cat)]^3-; and a monohydroxide bisphtalic, [Al(OH)(Pht)₂]^2-, a dihydroxide, a catechol and a salicylate group, [Al(OH)₂(Cat)(Sal)]^3-, which competes with the aluminate ion, Al(OH)₄^- . The linear relationship of Stern-Volmer suppression of the fluorescence of the aromatic groups present in the peat with the Al(III) ion confirms the equilibrium results.