Mecanismo de biossorção seletiva de arsênio (III) em rejeitos ricos em proteinas fibrosas

Abstract

Based on biochemical and toxicological fundamentals that explain arsenic toxicity, a novel approach for the treatment of arsenic-containing waters was developed. The approach involved the use of an abundant cysteine-rich waste biomass for the selective removal of As (III) from waters. The trivalent species is considered the most mobile andtoxic hydrosoluble arsenic species. Following the selection of the waste biomass for the present investigations - chicken feathers - the material was evaluated under different conditions of sorbent preparationand S/L ratio, pH (2-10.5) and the presence of competitor ions (phosphate and As (V)). The experiments demonstrated that the biomass is highly selective for the As (III) species with no significant effect of the studied competitor ions on adsorption capacity and the maximum uptake (170 molAs/g biomass) is obtained under acidicconditions. These findings were explained by a unique adsorption mechanism based on the results obtained by the combination of two experimental techniques, X- ray Absorption spectroscopy (EXAFS and XANES) and RAMAN spectroscopy, with theoretical modeling by Density Functional Theory - DFT calculations. XAS measurements indicated inner-sphere complexation of As (III) by the biomasssulfur atoms. The adsorption structure was represented by a trigonal, pyramidal geometry, with an As/S ratio of 1:3 and an As-S interatomic distance of pproximately2.24-2.26 A. Arsenic uptake leads to the release of 3 water molecules produced by the reaction of the OH groups in the neutral As(OH)3 species, which predominates under pH <9 , with the hydrogen of the biomass SH group. As pH is increased to 10.5, the As(OH)3 species depronates while uptake is reduced to approximately 30% of thatobtained under acid conditions. The analyses of the surface complexes formed at pH 10.5 pointed to a partial oxidation of the monovalent As (III) anion associated with the occurrence of a less ordered surface complex. Under those conditions, arsenite is adsorbed as an oxy-anion, with only one oxygen being replaced by sulfur, i.e. As/S ratio of 1:1. The As-S interatomic distance is increased to 2.31 Å. By using RamanSpectroscopy it was possible to obtain two main informations: (i) the -sheet structure was identified as the main secondary structure of the keratin biomass and (ii) the involvement of cysteine sulfur atoms during As (III) complexation was clearly indicatedby the vibrational modes of As-S bands combined with the reduction of other bands assigned to S-S bonds. The theoretical modeling provided by DFT calculations has corroborated the conclusions obtained by XAS and RAMAN spectroscopy. The results obtained in the present work confirmed the initial hypothesis that a cysteinerich biomass would be capable of adsorb the trivalent arsenic species, and provide the fundamentals for the development of a selective biosorbent for As (III) removal from contaminated waters.