Effect of γ-manganite particle size on Zn2+ coordination environment during adsorption and desorption

Abstract The nanostructured manganese oxyhydroxides are great importance in controlling the environmental pollution. Here we investigated the properties of γ-manganite (γ-MnOOH) having two different particle sizes and their effects on Zn 2+ coordination during adsorption and desorption, using a combination of X-ray diffraction analysis, thermal gravimetry, pore structure analysis, high-resolution transmission electron microscopy, acid-base titration and X-ray absorption spectroscopy. The small-particle-sized (SPS) γ-MnOOH with short aging time has a low degree of crystallinity, a large specific surface area, a low point of zero change and a low structural stability relative to the big-particle-sized (BPS) one. Comparison of natural sample with synthetic γ-MnOOH by atom distances indicates that the growth force along [11-1] direction is larger than that along [101] direction, although the particle size along the former direction is much smaller. Regarding the adsorption and desorption of Zn 2+ , the SPS γ-MnOOH has a higher adsorption density and lower desorption percentage than the BPS one under the same condition. Based on the desorption of Zn 2+ adsorbed in corner-sharing linkage on γ-MnOOH surface, the linear combination fitting results indicate that, Zn 2+ is mainly desorbed as Zn 2+ four-fold coordination (ZnO 4 ) complexes for low initial concentration of Zn 2+ ([Zn] initial ) and as Zn 2+ six-fold coordination (ZnO 6 ) complexes for high [Zn] initial ; SPS γ-MnOOH has a higher ZnO 4 percentage than the BPS one. The compensating charge capacity of ZnO 6 and ZnO 4 complexes in γ-MnOOH may increase in the linkage order: ZnO 6 -corner-sharing 4 -corner-sharing 6 -edge-sharing 4 -edge-sharing. These results indicate that the SPS γ-MnOOH can better capture and fix Zn 2+ during adsorption and desorption process.