Competitive adsorption of copper and zinc ions by Tetradesmus obliquus under autotrophic and mixotrophic cultivation

Abstract

The competitive adsorption mechanism of Tetradesmus obliquus on copper and zinc ions was explored under varied cultivation conditions. Removal efficiency of Tetradesmus obliquus for these ions was evaluated during autotrophic and mixotrophic cultivation. The results revealed that Tetradesmus obliquus demonstrated achieved removal efficiencies of 74.30 %, 86.48 %, 84.55 %, and 65.33 % for 16 mg/L of Zn2+, Cu2+, and a mixture of Cu2+ and Zn2+, respectively, under autotrophic conditions. Conversely, under mixotrophic conditions, Tetradesmus obliquus demonstrated improved removal efficiencies of 86.90 %, 92.31 %, 92.30 %, and 71.94 % for Zn2+, Cu2+, and a Cu-Zn mixture, along with adsorption capacities of 38.57 mg/g, 43.86 mg/g, 23.15 mg/g, and 15.24 mg/g, respectively. This represents an increase of 46.65 %, 31.16 %, 29.47 %, and 43.50 % compared to the autotrophic group, respectively. The second-order kinetic equation and Langmuir isotherm model, as proposed, successfully fitted the experimental data for kinetics and isotherms of Zn2+ and Cu2+ in the monometallic system. The maximum adsorption capacities were determined to be 43.22 mg/g and 69.41 mg/g, respectively, under mixotrophic cultivation conditions. The Extended Langmuir isotherm model best fit all experimental data in the mixed system. The presence of Cu2+ markedly reduced Zn2+ adsorption, whereas the presence of Zn2+ had no effect on Cu2+ adsorption. The 3D fluorescence results further confirmed that glucose addition stimulates algal cells to produce additional extracellular polymers, consequently enhancing metal adsorption by microalgae. These research findings confirm that culturing Tetradesmus obliquus in a mixed environment enhances heavy metal removal during water treatment, which is of significant importance for environmental remediation and sustainable water purification.