Highly Efficient Removal Of Phosphate From Aqueous Solution By Oxides Of Low Valence States Of Manganese

A manganese oxide adsorbent was prepared for phosphate removal using simulta neous oxidation and precipitation method. The adsorbent was characterized by scan-ing electron microscope (SEM), X-ray diffraction (XRD), fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and zeta potential. The results showed that adsorbent was amorphous micron-sized particles and mainly made up of divalent and trivalent manganese oxyhydroxides with a isoelectric point around6.6. The effect of conditions such as adsorption time, dose of absorbent, initial pH, electrolyte strength, coexisting ions on phosphate adsorption as well as the kinetics and isotherm studies were investigated. Results showed that:(1)24h was found to be sufficient for the equilibrium adsorption;(2) The sorbent dose of0.20g/L was optimal for high removal rate and adsorption capacity;(3) The absorbent showed little pH dependence in the pH range from6to11with the removal rate maintaining above92%, attaining the maximum removal rate of97.5%at pH7;(4) The ionic strength had negligible effect on the adsorption, demonstrating that the phosphate adsorption was specific adsorption, mainly by forming inner-sphere complexes on the surface of adsorbent by the strong forces;(5) The pseudo-second-order kinetics model provided a better fit for phosphate adsorption onto the adsorbent, indicating that the adsorption process might be chemical sorption;(6) The maximum adsorption capacity estimated from the Langmuir equation was108.7mg/g at pH7.0, which was among the highest reported values in the literature;(7) Thermodynamic analyses indicated that the phosphate adsorption was endothermic and spontaneous in nature;(8) NO3-could promote the adsorption, while other coexisting ions such as K+, Ca2+, SO42, CO32-, SiO32-inhibited phosphate adsorption on adsorbent with diferent degree;(9) The replacement of surface hydroxyl groups (M-OH) played a great role in phosphate adsorption on the adsorbent via the formation of inner-sphere complexes on the surface. The adsorbent displayed a good specific affinity towards phosphate and was promising for phosphate removal from natural waters and municipal wastewaters.