| Ammonia is the main form of nitrogen existence in aqueous solution, and is regarded as an important pollutant on eutrophication and aqueous pollution. Activated carbon is widely used as an ideal adsorbent for pollutants removal from water. Two kinds of Na+ impregnated modified activated carbon samples, denoted as AC/N-Na and AC/HCl-Na, were used as adsorbents to remove NH4+-N from aqueous solution. Compared with raw activated carbon (denoted as AC), Na+ impregnated samples in this study efficiently improved the NH4+-N adsorption capacity. AC/N-Na showed a higher adsorption capability (qe=1.18mg/g) than that of AC/HCl-Na. The preferential preparation condition was the following: nitric acid concentration 5 mol/L, hydrochloric acid concentration 5 mol/L, sodium chloride 0.5 mol/L for Na+ impregnation, and temperature at 25℃, respectively.After pre-treatment with acid and Na+ impregnation, the surface became more smooth and homogeneous than AC. The BET surface and micro-porous volume decreased after acid pretreatment and Na+ impregnation especially for AC/N-Na. The grains of carbon could be seen on the surface of carbon layer, where small salt particles were uniformly deposited on the surface. Major peaks of FT-IR analysis matched well with sodium acetate hydrate, proving the existence of sodium ions on the surface of carbon layer probably by means of ionic bonds. Oxidation treatment via nitric acid produced more oxygen groups containing the C=O moiety, and the increase of the relative concentration of the peak 1000-1200 cm-1 for AC/HCl-Na indicated an increase for single-bonded oxygen functional groups after HC1 treatment. Both oxidation and acid treatment increased the amount of available functional groups on the surface, which was beneficial for NH4+-N adsorption.All the adsorbents showed the trend that the increase in the initial ammonia concentration led to an obvious decrease in removal efficiency. Among these adsorbents, AC/N-Na had the highest removal capacity. The removal efficiency of NH4+-N increased gradually in the pH range of 3.33 to 6.43, and dropped from pH 6.43 to 9.00. The maximum removal efficiency was observed at pH 6.43, representing the fact that a neutral condition was preferred for NH4+-N adsorption. All the adsorbents showed the trend that the removal efficiency increased with reaction time, and finally reached at the maximum when the adsorption process finished.Four isotherms were investigated to fit the results including Langmuir, Freundlich, Dubinin-Radushkevich and Redlich-Peterson isotherms. The correlation coefficients indicated that all four models could be used to fit the data and estimate adsorption parameters. The adsorption process could be better explained by Langmuir, indicating that the adsorption type onto AC involves monolayer coating and available adsorption sites were distributed uniformly rather than exponentially. In our study, the Dubinin-Radushkevich equation did not fit well with the results compared to the Langmuir and Freundlich isotherm equations. Both the pseudo second-order model and intra-particle diffusion model could be used to simulate the NH4+-N adsorption kinetics on AC. According to the R2 values, the pseudo second-order equation better described the whole adsorption process, while intra-particle diffusion model revealed that three main steps occurred during the whole adsorption process. The multi-linearity of the curves suggested that both surface adsorption and intra-particle diffusion simultaneously occurred and contributed to the adsorption mechanism.It is concluded that the main mechanism of NH4+-N adsorption onto activated carbon samples is ion exchange. Oxidation and ion impregnation treatment of Na+ enhanced and broadened the chemical adsorption. Additionally, the introduction of Na+, which can exchange with H+, created a proper pH value environment under experimental conditions, and prevented the accumulation of H+which otherwise would hinder the NHt+-N adsorption. |
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