Simultaneous sequestration of cadmium and lead in brackish aquaculture water by biochars: A mechanistic insight

•Salinity alters AC and BB surface chemistry influencing Cd and Pb fractions.•DO has less effect on Pb and Cd adsorptions.•Cd is more strongly adsorbed on bamboo biochar than AC.•Multilayer adsorption on AC caused Cd desorption in column using canal water. Coastal aquaculture faces metal pollution challenges, particularly from lead (Pb) and cadmium (Cd). This study examined the synergistic effects of salinity and dissolved oxygen (DO) on Pb and Cd interactions with biochars from bamboo (BB), mangrove (MB) and palm shell (PSB) in batch experiments. The performance of biochars was compared to activated carbon (AC) in single (Pb or Cd) and bi-solute (Pb + Cd) lab-scale brackish water systems. Adsorption studies showed that Pb and Cd adsorptions onto biochars and AC followed the Freundlich adsorption model, except for BB, which followed the Langmuir model at 15 ppt salinity. The increase in DO slightly facilitated the adsorption of Pb and Cd by influencing biochar surface charge, whereas increases in salinity negatively affected adsorption. Among the biochars, BB exhibited the highest Pb and Cd adsorption capacity. BB and AC were selected for column experiments with synthetic brackish water and canal water. The adsorption data fitted the Clark model, emphasizing the role of ion exchange and the multilayer pattern of adsorption. Increased salinity decreased the exchangeable fraction while increasing carbonate-bound, reducible, and oxidizable fractions, suggesting ion exchange and oxide interactions. SEM-EDS and XRF analyses confirmed the presence of adsorbed Pb and Cd on both BB and AC. BB demonstrated to be more effective than AC in removing Pb and Cd from canal water with no desorption observed and it can be a cost-effective alternative to sequester Pb and Cd from shrimp nursery ponds. [Display omitted]