| Two series of activated carbons modified by Fe(Ⅱ) and Fe(Ⅲ), respectively, were used as adsorbents for the removal of phosphate in aqueous solutions. The synthesized AC-Fe adsorbent materials were characterized by BET analysis, scanning electron microscopy (SEM) techniques, X-ray diffraction (XRD) analysis and infrared spectroscopy (FTIR) analysis. The adsorption of phosphate on activated carbons had been studied in kinetic and equilibrium conditions taking into account the adsorbate concentration, temperature and solution pH as major influential factors. Adsorption isotherms had been modeled by Langmuir, Freundlich and Redlich-Peterson equations and their constants were determined. Pseudo-first-, second-order and intra-particle diffusion equations were used to describe the adsorption rate of phosphate and adsorption rate constant was calculated. A mechanism involving three stages, viz.; external surface adsorption, intra-particle diffusion and final equilibrium had been proposed for the adsorption of phosphate onto adsorbent material. Thermodynamic parameters such asΔG0,ΔH0 andΔS0 were calculated in order to understand the nature of sorption.The preferential preparation condition was the following:nitric acid concentration 5 mol/L, iron-loaded pH 2, iron salt solution concentration 0.2 mol/L and heating temperature 110℃, respectively. The SEM analysis revealed that iron oxide particles was almost uniformly deposited on the surface of the PAC/N-FeⅡ, while that of PAC/N-FeⅢwere less uniformly. The surface of PAC/N-FeⅢwas much more coarse. The physical properties suggested that the BET surface area and microporous volume are affected by the presence of iron oxide in the composites, especially for PAC/N-FeⅢ. The major peaks of processed XRD spectra of PAC/N-FeⅡmatched well with the crystalline iron species akaganeite (β-FeOOH); while, the PAC/N-FeⅢXRD spectrum implied that significant amounts ofβ-FeOOH existed. After Fe modification, new peaks were detected at 903 cm-1,843 cm-1 and 680 cm-1, which might be due to the bending vibration of Fe-OH and stretching vibration of Fe-O.The optimal pH values for phosphate removal ranged from 3.78～6.84 for both PAC/N-FeⅡand PAC/N-FeⅢ. At pH 3.78, the maximum percentage of phosphate removal by PAC/N-FeⅡand PAC/N-FeⅢwas 93.32% and 72.28%, respectively. At higher pH values the phosphate removal by the sorbents was found to decrease. PAC/N-FeⅡhad a higher phosphate removal capacity than AC/N-FeⅢ. The removal percent of phosphate onto PAC/N-FeⅡdecreased from 95.99% to 62.89% as phosphate initial concentrations increased from 11.82 mg/L to 42.96 mg/L, while the latter removed 36.01%-87.26%. Competitive ions, such as Cl- and NO3- had little effect on the adsorptive efficiency of P(V), while the existence of SO42- reduced the removal efficiency obviously. Compared to other reagents, 0.1mol/L NaOH was the best desorption reagent.The influence of temperature on phosphate adsorption onto activated carbon was examined and it was found that although the difference was not great, the adsorption rate and the adsorption capacity increased along with the increase of temperature. Under three different test temperatures of 15℃,25℃and 35℃, the data for P(V) were well fitted to the Langmuir adsorption isotherm. The adsorption of P(V) on activated carbon was spontaneous and endothermic. The activation energy (Ea) for adsorption were calculated to be 22.23 and 10.89 KJ/mol for PAC/N-FeⅡand PAC/N-FeⅢ, respectively. Considering that these values are in the typical activation energy range for chemical adsorption, one can conclude that phosphate adsorbs on activated carbon mainly chemically.Kinetic studies showed that the adsorption data for P(V) could be well described by pseudo-second order rate model. The adsorption rate and equilibrium adsorption of phosphate increased as the temperature rose. It was observed that the intra-particle diffusion modeling of phosphate adsorption on PAC/N-FeⅡand PAC/N-FcⅢboth are not linear form over the whole time range and the plots obtained present multi-linearity, with a initial linear portion followed by an intermediate linear portion and a plateau, indicating that two or more steps occur during the adsorption process. The multilinearity of the curves suggested that both surface adsorption and intra-particle diffusion were simultaneously occurring during the process and contribute to the adsorption mechanism. A larger C value means a greater effect of the boundary layer. The C values increased with the temperature, which indicated an increase of the thickness of the boundary layer and therefore internal mass transfer is favored over external mass transfer.It was found that P(V) mainly chemically adsorbed on activated carbon. Hydrogen bonding, electrostatic interaction and chemical bounding mechanism were involved. Chemical bounding mechanism played an important role in the adsorption.
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