Chromatographic method for pre-concentration and separation of Zn(ii) with microalgae and density functional optimization of the extracted species

Abstract

A novel wild strain of microalgae, Phormidium luridum containing Gloeothece rupestris and Chlorococcum infusionum (99 : 0.08 : 0.02), was studied for its ability to remove and retrieve Zn(II) from aqueous solutions in the presence of some commonly occurring ions (Na⁺, K⁺, Cl⁻, SO₄²⁻, ClO₄⁻, NO₃⁻) in their natural contamination concentration range (50–300 mg L⁻¹). The algae, which were previously collected from the river basin (Ajay), were grown on naturally occurring gravels in a glass column of nutrient enriched raw water media. Systematic studies of the sorption of Zn(II) (0.02 mg mL⁻¹) over a pH range of 4.5–7.5 identified a maximum removal extent of 104 μM g⁻¹ at neutral pH, mainly by adsorption at the surface layer. Zn(II) was retrieved by selective elution with 5 × 10⁻³ M HNO₃ solution. Initially, [Zn(H₂O)(OH)]⁺ (η_{[Zn(OH)(H2O)]⁺} = 1.25 eV) is adsorbed at the surface of the algae, which is built up of polysaccharides (η_[glucose] = 6.34 eV), before moving inside by the formation of a more stable complex with Phycocyanobilin2, which has similar hardness (η_{[Phycocyanobilin]} = 2.37 eV). The complex is stabilized by −52 195.48 eV mol⁻¹ through the formation of two strong intramolecular hydrogen bonds (–OH⋯O = 163.54 pm; HOH⋯O = 129.71 pm). Density functional theory optimization corroborates a stable [Zn(H₂O)(OH)]⁺–Phycocyanobilin2 tetrahedral complex.