Natural manganese oxide: Combined analytical approach for solid characterization and arsenic retention

To understand the retention of As on a natural manganese sand, the structural, textural and chemical properties of the solid were first investigated by combining scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron-energy-loss spectroscopy (EELS), X-ray diffraction (XRD), BET N2 gas adsorption, diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. Manganese sand could be mainly described as a mixture of a phyllomanganate, lithiophorite [(Al,Li)MnO2(OH)2], and pyrolusite (MnO2). Iron particle, kaolinite and gibbsite type-phases were also observed. Particles organization led to the presence of a mesoporosity with pore diameters ranging from 100 to 200 Å and a specific surface area of 23 m2 g−1. Contact with an As(V) solution (0.67 mmol L−1) led to an average fractional surface coverage of 0.4. Both As (V) and (III) were present on the surface of the sand in a 1:1 ratio. As(V) was sorbed on lithiophorite-type particles through surface complexation type reaction. As(III) was thought to result from As(V) reduction mechanism on iron particles.