Concurrent sorption of Zn(II), Cu(II) and Co(II) by Oscillatoria angustissima as a function of pH in binary and ternary metal solutions

This paper reports biosorption of Zn(II), Cu(II) and Co(II) onto O. angustissima biomass from single, binary and ternary metal solutions, as a function of pH and metal concentrations via Central Composite Design generated by statistical software package Design Expert® 6.0. The experimental design revealed that metal interactions could be best studied at lower pH range i.e. 4.0–5.0, which facilitates adequate availability of all the metal ions. The sorption capacities for single metal decreased in the order Zn(II) > Co(II) > Cu(II). In absence of any interfering metals, at pH 4.0 and an initial metal concentration of 0.5 mM in the solution, the adsorption capacities were 0.33 mmol/g Zn(II), 0.26 mmol/g Co(II) and 0.12 mmol/g Cu(II). In a binary system, copper inhibited both Zn(II) and Co(II) sorption but the extent of inhibition of former was greater than the latter; sorption values being 0.14 mmol/g Zn(II) and 0.27 mmol/g Co(II) at initial Zn(II) and Co(II) concentration of 1.5 mM each, pH 4.0 and 1 mM Cu(II) as the interfering metal. Zn(II) and Co(II) were equally antagonistic to each others sorption; Zn(II) and Co(II) sorption being 0.23 and 0.24 mmol/g, respectively, at initial metal concentration of 1.5 mM each, pH 4.0 and 1 mM interfering metal concentration. In contrast, Cu(II) sorption remained almost unaffected at lower concentrations of the competing metals. Thus, in binary system inhibition dominance observed was Cu(II) > Zn(II), Cu(II) > Co(II) and Zn(II) Co(II), due to this the biosorbent exhibited net preference/affinity for Cu(II) sorption over Zn(II) or Co(II). Hence, the affinity series showed a trend of Cu(II) > Co(II) > Zn(II). In a ternary system, increasing Co(II) concentration exhibited protection against the inhibitory effect of Cu(II) on Zn(II) sorption. On the other hand, the inhibitory effect of Zn(II) and Cu(II) on Co(II) sorption was additive. The model for metal interactions was found to be valid within the design space.