Immobilization of Hg(II) on high-salinity Spirulina residue-induced biochar from aqueous solutions: Sorption and transformation mechanisms by the dual-mode isotherms.

Removal of Hg(II) by biochar (BC) is a promising remediation technology. The high-salinity Spirulina residue (HSR) is a hazardous waste generated during extracting the pigment phycocyanin under high salinity conditions. Although HSR-derived BC (HSRBC) exhibited the excellent sorption capacity of Hg(II), the involved mechanisms have been rarely studied. In this study, we investigated the specific properties and Hg(II) sorption mechanisms of HSRBCs. Chloride and calcium minerals were formed in HSRBCs. Increments in carbonization temperature (from 350 to 700 °C) or time (from 90 to 540 min) led to the enhancement of aromaticity, porosity, and positive charge, but cracked oxygen-containing groups and C-N bonds. Further increase in carbonization temperature or time decreased the sorption of Hg(II). At environmentally relevant concentration of Hg(II) (2-4 mg/L), the sorption capacity (6.1-12.7 mg/g) obtained in HSRBC350 was comparable to activated carbon. Based on dual-mode isotherm, surface sorption accounted for 75-88% uptake, while precipitation accounted for 12-25% uptake. In addition, the C-O, CO, and CC groups were responsible for the monodentate/bidentate complexation and reduction, while Cl triggered HgCl precipitation. Overall, this study provided a new insight in creating an excellent Hg(II) sorbent from hazardous waste, and revealed the sorption mechanisms for Hg(II) uptake.