Macroscopic and modeling evidence for nickel(II) adsorption onto selected manganese oxides and boehmite

Ni(II) polluted water becomes a significant issue in recent years. Excessive levels of Ni(II) are detrimental to plant, animal and human life, so it is important to reduce the amount of Ni(II) that enters the environment from polluted water. Manganese oxides, found in soils are important in determining the availability of Ni(II) in soil systems as they absorb Ni(II) onto their surface. Therefore, they may be able to be used to remove Ni(II) from wastewater so it is important to understand the interaction mechanisms of Ni(II) with manganese-containing minerals. As a consequence, Ni(II) adsorption onto birnessite, pyrolusite, hausmannite, manganite, boehmite, and Mn-Al binary oxide were investigated as a function of solution pH using an initial Ni(II) concentration in 1 mM NaNO₃ solution at 22 ± 2 °C. The adsorption data were modeled using Langmuir, Freundlich and extended constant capacitance model (ECCM) approach to predict the possible adsorption reactions. The accumulation process was strongly dependent on pH, initial Ni(II) concentration, and the nature of the mineral. The position of adsorption edges (pH₅₀) varied significantly between the minerals investigated. Birnessite exhibited maximum Ni(II) uptake, while pyrolusite exhibited minimum uptake. The Langmuir equation provided a reasonable correlation of data. ECCM results indicate that Ni(II) formed outer-sphere complexes at low pH, and inner-sphere complexes and surface precipitation at higher pH. Proton stoichiometry suggests that more than one reaction involved in the overall Ni(II) adsorption process. This study on Mn-oxides as sorbents for Ni(II) demonstrates that MnOs may be effective in removing Ni(II) pollutants from wastewater.