Biosorption of Zn by dried biomass of Euglena gracilis from aqueous solutions

Abstract

The aim of this work was to evaluate the possibility to utilize the dried biomass of Euglena gracilis var. bacillaris as a biosorbent applied for the removal of Zn from aqueous solutions. For these purposes, experiments involving solutions spiked by ⁶⁵ZnCl₂ were carried out under the conditions of batch systems. The prepared biosorbent in the form of dried biomass of E. gracilis (< 300 μm) was characterized in terms of the presence of functional groups (COOH, PO₃H₂, OH, and NH₂), the concentration of binding sites c_An and the value of pH_zpc = 6.6 using the modelling program ProtoFit. From the kinetics of Zn biosorption, it can be assumed that the Zn removal was a rapid process, in which the concentration equilibrium [Zn]_solution : [Zn]_biosorbent was stabilized in the first 5 min of interaction. In individual experiments, the effect of solution pH, initial biosorbent or Zn concentration were evaluated. Based on the MINEQL+ speciation modelling program, we predicted the decrease in the abundance of free Zn²⁺ cations in the presence of different concentrations of EDTA as a complexing agent in the solution. It was found that the Zn biosorption decreased linearly with the decreasing the proportion of Zn²⁺ in solution. The biosorption data expressed as equilibrium values of the remaining Zn concentration in solution C_eq (µmol.dm^-3) and equilibrium values of specific adsorption Q_eq (µmol.g^-1; d.w.) were well fitted to the Langmuir model of adsorption isotherm in comparison with the Freundlich model. The maximum adsorption capacity of the dried biomass of E. gracilis for the biosorption of Zn reached the value Q_max = 0.53 ± 0.05 mmol.g^-1or 34.7 ± 3.4 mg.g^-1 (d.w.), respectively.