Sulfate induced surface modification of Chlorella for enhanced mercury immobilization

In this study, living Chlorella sorokiniana FK with an elevated concentration of cell envelop sulfur sites, induced by adding excess sulfate to the medium, was used as biosorbents for the removal of Hg(II) from aqueous solution. The cellular surface sulfur-based sites density of the obtained biosorbents was determined through selective sulfhydryl site-blocking and potentiometric titration. The sorption was conducted as terms of pH and initial Hg(II) concentration, and characterized by SEM, AFM, HRTEM, FT-IR, XRD and XPS. The results showed that Chlorella possessed the highest sulfhydryl site concentration and showed the largest capacity for Hg(II) detoxification in the 40 mg/L sulfur culture (termed B-40) with 74.2% sulfate consumption for the growth of algae. The equilibrium data for the studied biosorbents was adequately represented by the Langmuir isotherm and B-40 exhibited the highest sorption capacity for Hg(II), 2 times greater than that of no sulfate culture Chlorella at the optimized pH 6. Mercury biomineral particles (HgS, HgSO4 and Hg2SO5) were formed on the cellular envelop which induced mainly by sulfur-based groups (-SH, -OSO3-) for enhanced Hg(II) immobilization. These results suggested that surface sites concentration of algae can be manipulated by the available nutrients, and therefore cells with induced high densities of sulfhydryl sites represented a promising and low cost biosorbent for the effective removal and immobilization of Hg and possibly other heavy metals.