## 2 Selectivity  
Generally, metal recovery or removal from solution may involve the following pathways: (i) The binding of metal cations to cell surfaces, or within the cell wall, where microprecipitation may enhance uptake; (ii) Translocation of the metal into the cell, possibly by active (metabolic energy dependent) transport; the active uptake or concentration of metal by living microbial cells; (iii) The formation of metal-containing precipitates, by reaction with extracellular polymers or microbially produced anions such as sulfide or phosphate (\"bioprecipitation\"); (iv) The volatilization of the metal by biotransformation--see [PERSON] and [PERSON] [11]. The metal binding properties of Gram-positive bacteria, including the actinomycetes, according to [PERSON] [12] are largely due to specific anionic polymers in the cell wall structure. Equilibrium and kinetic analyses classically lead to the appropriate rate expressions, characteristic of the possible reaction mechanism(s). The results of titration experiments suggested the presence of carboxylic and phosphate groups on the bacterial cell wall. These types of acidic sites can remove metallic ions, usually cations (cadmium), from aqueous solutions through the application of different mechanisms, such as cell surface sorption (complexation, surface precipitation _etc._), as well as by extracellular and intracellular accumulation [13]. As for the removal of hexavalent chromium oxyanions (by _Aeromonas caviae_) at low pH values may be attributed to their attraction/affinity toward positively charged surface groups of biomass or reduction of Cr(VI) to Cr(III), followed by its bonding to the negatively charged biosorbent. The characterization of our biomass, _Aeromonas caviae_, including zeta-potential measurements, supports clearly this argument [5]. Thousand tons of residual biomass are produced each year from fermentation industries and also from biological wastewater treatment plants; hence, it could be considered as suitable biosorbents, perhaps with certain modification or treatment [14]. A selectivity factor \"f\" was defined [15], when considering two metals, A and B, as:  
\[f=\frac{\left(C_{\mathrm{A}}\,/\,C_{\mathrm{B}}\,\right)_{\mathrm{bisorbed}} }{\left(C_{\mathrm{A}}\,/\,C_{\mathrm{B}}\,\right)_{\mathrm{remaining}}} \tag{1}\]  
in order to describe the effectiveness of biosorption as a separation process, similar to the one used in ion exchange or solvent extraction processes. For the system Zn/Cd, where some selective separation was apparent, the value off reached around 40 in the pH range 8 to 9. Selectivity is of interest specifically if recovery of metals is sought from a mixture of metal ions, which most of the practical cases of wastes are. Selectivity was said to be a problem generally with biosorption systems, but this may be minimized depending on the relative concentrations of the various metals present [16]. As also discussed by [PERSON] _et al._[7], the biosorption results for copper did worth mentioning, showing a selectivity of the process for copper; it is further noted that this selective separation was obtained in only one stage operation. A good explanation for that can be found in the literature [17]. A two cycles operation was attempted with alternative addition of dodecylamine surfactant; both the metals (copper, nickel, zinc) removal due to biosorption on (zetag-modified) _Streptomyces rimosus_ industrial biomass, and also flotation behavior of the system, was investigated as a function of solution pH. Again a pronounced indication for possible selective recovery of copper (at a pH value of around 6.5) was observed [18]. The selective removal of Cu(II), Cr(III) and Ni(II) by strains of exopolysaccharide-producing cyanobacteria was assessed and the interaction of sorption in solutions with multiple-metals was investigated [19].