Adsorption behavior and mechanism of loess soil towards manganese ions

Environmental pollution accidents frequently induced by heavy metals have aroused broad interest of many researchers. Manganese is a trace element and toxic effect will be caused by the excessive intake of manganese. The factors including the concentration of Mn(II), reaction temperature, soil water ratio and pH value are studied to investigate the adsorption behavior of loess soil towards Mn(II). The adsorption capacity of Mn(II) on loess soil is determined to be 7.84 mg/g, and the adsorption test results can be well interpreted by three isotherm models (i.e., Langmuir, Freundlich and D-R models). The thermodynamic analysis indicates that the adsorption process of Mn(II) on loess soil is spontaneous, and higher temperature benefits the adsorption process. The removal efficiency of Mn(II) from aqueous solution can be raised by increasing solid-liquid ratio, but the unit adsorption amount of Mn(II) on loess soil has an optimum value. Higher initial concentration of Mn(II) leads to smaller optimum adsorption amount of loess soil, which is reached at higher solid-liquid ratio. pH value has great influence on the removal efficiency of Mn(II) from aqueous solution, and Mn(II) can be nearly completely removed when pH>10.7. X-ray diffraction spectra and Fourier transform-infrared spectra are used to further discuss the adsorption mechanism of Mn(II) on loess soil, and the analysis indicates that the clay minerals and the organic matter in loess play important roles in Mn(II) removal.