Abstract:Several Mn oxide minerals commonly existent in soils were synthesized by the modified or optimized methods. Their morphologies, structures, compositions and surface properties were characterized. Adsorption and redox reactions of heavy metals on them and effects of mineral structures and surface properties were also investigated. The synthesized birnessite, todorokite, cryptomelane and hausmannite are all single-phase minerals and have the typical morphology, as indicated by analyses of XRD and TEM/ED. The PZCs of the synthesized birnessite, todorokite and cryptomelane are 1.75, 3.50 and 2.10, respectively. The magnitude order of their surface variable negative charges is birnessite ≥ cryptomelane > todorokite. The PZC of the synthesized hausmannite is much higher than PZCs of other minerals, and it has the least surface variable negative charge. Among the tested Mn oxide minerals, birnessite has the greatest adsorption capacity for heavy metals such as Pb2+ , Cu2+, Co2+, Cd2+and Zn2+ , whereas hausmannite has the weakest capacity. Except for hausmannite, these minerals have the greatest adsorption capacity for Pb2+ . Hydration constants of the heavy metals and the surface variable charges of the Mn minerals remarkably affect the adsorption. Both of the factors affect the hydration induced by surfaces of the minerals and species of the adsorbed ions. Their ability in Cr (Ⅲ) oxidation and concomitant release of Mn2+varies greatly depending on the structure, composition, surface property and crystallinity. The maximum amounts of Cr (Ⅲ) oxidized by the Mn oxide minerals are in order of (mmol/kg) birnessite (1 330.0) > cryptomelane (422.6) > todorokite (59.7) > hausmannite (36.6).
