Biosorption metal-binding mechanisms

Biosorption is a rather complex process affected by several factors that include different binding mechanisms (Figure 10.4). Most of the functional groups responsible for metal binding are found in cell walls and include carboxyl, hydroxyl, sulfate, sulfhydryl, phosphate, amino, amide, imine, and imidazol moieties.4 90 The cell wall of plant biomass has proteins, lipids, carbohydrate polymers (cellulose, xylane, mannan, etc.), and inorganic ions of Ca(II), Mg(II), and so on. The carboxylic and phosphate groups in the cell wall are the main acidic functional groups that affect directly the adsorption capacity of the biomass.101... 

FIGURE 10.4 Binding mechanisms involved in metal biosorption. 

Metal ion binding during biosorption processes has been found to involve a complex mechanism, such as ion-exchange, complexation, electrostatic attraction or micro precipitation. 

Various metal-binding mechanisms have been postulated to be active in biosorption process and presented in Figure 3.2. 

Some confusion has prevailed in the literature regarding the use of terms "biosorption" and "bioaccumulation". Biosorption (or bioadsorption) is a passive immobilization of metals by the biomass. Mechanisms of cell surface sorption in contrast to bioaccumulation are independent of cell metabolism they are based upon physicochemical interactions between metal and functional groups of the cell wall. The microorganism s cell wall mainly consists of polysaccharides, lipids and proteins, which have many binding sites for metals. This process is independent of the metabolism and metal binding is fast. Bioaccumulation, in contrast, is an intracellular metal accumulation process which involves metal binding on intracellular compounds, intracellular precipitation, methylation and other mechanisms (Mohan and Pittman, 2007). 

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