Unveiling the transformation of divalent heavy metal cations in oxic environment containing iron sulfides: Kinetics, speciation and mechanisms

Divalent heavy metals are common in natural waters. Under anoxic conditions they are immobilized by iron sulfides via surface complexation and chemical precipitation. However, limited knowledge exists about the removal of divalent heavy metal cations by iron sulfide in aquatic environment under oxic conditions. The results showed that divalent heavy metal cations are immobilized and then remobilized in oxic FeS system. Cd(II) removal was most efficient, reaching equilibrium at ~99.99%, exceeding Zn(II), Co(II), and Ni(II). FeS dosage and solution matrix promote the immobilization and release of Cd(II). CdS(~96.31%) and Cd(II)(~3.69%) adsorbed on lepidocrocite are the main Cd species generated. Aqueous Cd(II) is chosen as the representative to investigate the removal mechanisms. Aqueous Cd(II) precipitates with S(-II) from FeS as CdS, forming a FeS/CdS galvanic cell that conserves residual FeS. Aqueous Cd(II) can also be adsorbed on the surface of or incorporated into lepidocrocite formed by oxygenation of the Fe(II) released from solid FeS. As reaction proceeds, residual FeS and lepidocrocite-adsorbed Fe(II) activate oxygen generating hydroxyl radicals (•OH), triggering CdS dissolution and releasing <0.1% of total Cd into the aqueous solution. The findings of this study can provide new insight into environmental behaviors of divalent heavy metal cations in oxic environments rich in iron sulfide and the remediation of heavy metal-contaminated waters. • Divalent heavy metal cations are immobilized and then remobilized in oxic FeS system. • FeS dosage and solution matrix promote the immobilization and release of Cd(II). • CdS and Cd(II) adsorbed on lepidocrocite are the main Cd species generated. • It involves chemical precipitation, galvanic effect and •OH-induced remobilization.