Mechanisms of Cr(III) biosorption onto residual brewer's yeast

The knowledge and understanding of metal-biomass interactions is crucial to develop and maximize biosorption processes potential to the concentration, removal and recovery of heavy metals from dilute solutions, as well as to define strategies to regenerate and reuse biosorbent in multiple cycles (Ahluwalia and Goyal, 2007; Volesky, 2001). It also turns possible biosorption optimization at a molecular level, including biomass genetic modification to induce changes in morphologic and physiologic characteristics in order to increase metal uptake (Srinath et al., 2002; Volesky, 2001). To reach that purpose, and considering that several mechanisms may contribute to the overall metal uptake depending on the metal and the biosorbent used, environmental factors, and the cell metabolic activity (Dhankhar and Hooda, 2011), it is necessary to study in detail the interactions established in each biosorption system. Heavy metal biosorption involves a combination of several passive accumulation processes, that may include: i) physical adsorption; ii) chemisorption, including ion exchange, coordination, complexation and chelation; iii) and inorganic precipitation (Ahluwalia and Goyal, 2007; Wang and Chen, 2006). According to Gadd (2004), metal-microorganism interactions may be seen as natural strategies to remove, recover or diminish metal toxicity in organic or inorganic forms. Different organisms exhibit different responses to the exposure to toxic ions, varying from transport through cellular membrane, biosorption onto cell walls, entrapment in extracellular structures, precipitation, complexation and redox reactions