Efficient removal of Pb(II) and Cd(II) from aqueous solutions by mango seed biosorbent

Abundant fruit waste materials can be utilized as sustainable biosorbents for heavy metal removal from aqueous solutions. In this study, mango seed (MS) comprising seed kernel and shell showed the maximum adsorption capacities of 263.4 mg-Pb/g at pH 5.0 and 93.7 mg-Cd/g at pH 7.5. The best fitting of the adsorption isotherms to the Redlich-Peterson model indicated multilayer adsorption of Pb(II) and Cd(II) on the heterogeneous surface of MS. The adsorption was rapid with 93% of Pb(II) and 78% of Cd(II) adsorption accomplished within 10 min. More than 99% of the sequestered Pb(II) and 88% of the sequestered Cd(II) were recovered by desorption using 0.2 M HNO3, and the regenerated MS maintained 96-97% of the adsorption capacities. A combination of physicochemical and spectroscopic approaches showed that carboxyl, hydroxyl, amine, and ether groups were the binding sites; electrostatic attraction, microprecipitation, complexation, and ion exchange (with Ca2+ and Mg2+) jointly contributed to the adsorption. The seed kernel which has a more amorphous structure and a higher ion exchange capability favored higher Pb(II) and Cd(II) adsorption compared with the seed shell which contains a higher content of crystalline cellulose. These findings imply the application potential of MS for heavy metal removal and provide important information for the (bio)sorbent development.