| Due to its stable nature, high toxicity and non-biodegradability, Pb(Ⅱ) pollution causes a serious threat to environmental and public health. The development of efficient water treatment materials and technology, which aimed at the removal of Pb(Ⅱ), is one of the important topics of the field of water purification. In this study, a mesoporous hydrous manganese dioxide (MHMO) was synthesized by redox of potassium permanganate (KMnO4) and hydrogen peroxide (H2O2), using the cetyltrimethylammonium bromide (CTAB) as the template. The newly synthesized mesoporous materials were used as adsorbents for Pb(Ⅱ) removal and the adsorption behaviors of Pb(Ⅱ) were evaluated. In addition, the adsorption mechanism was studied. The main conclusions are as follows:The MHMO was synthesized under the conditions of pH=4.0,298K and the molar ratio of KMnO4, H2O2and CTAB was4:6:1.1. The XRD result of MHMO was in agreement with the characterization of the synthetic amorphous δ-MnO2.In the range of P/Po=0.3-0.9, the isotherms of MHMO exhibited an obvious N2hysteretic loop. The pore size distribution analysis indicated that the MHMO had a narrow range (2-40nm) of pore sizes (peak at2.45nm).The specific surface area, pore volume and BJH pore diameter of the MHMO were79.31m2/g,0.07cm3/g and3.38nm, respectively. After the adsorption of Pb(II), N2hysteretic loop disappeared, the pore size distribution in the range of0to20nm (peak=1.73nm), indicated that the mesoporous structure has been destroyed. The external surface area and mesopores area both decreased from66.36and12.95m/g to4.75and1.95m2/g, respectively, which indicated that the adsorption of Pb(II) was mainly on the external surface of MHMO.The adsorption isotherms indicated that the adsorption capacity of Pb(II) on MHMO was increased with the increase of temperature. Langmuir isotherm equation could fit well for the experimental results. The maximum equilibrium adsorption capacity of MHMO for Pb(II) were352.49, 427.19and478.65mg/g at298,308and318K, respectively. The negative values of△G and the positive values of△H and AS indicated the adsorption process of MHMO for Pb(II) was spontaneous and endothermic in the environment.The adsorption kinetics indicated that the percentage of Pb(II) removal on MHMO increased rapidly up to approximately80%in4h. The pseudo second-order equation could best fit the adsorption data and the calculated qe were in accordance with the qexp at298K.The intraparticle diffusion might control the rate of adsorption along with the surface diffusion process. The value of the calculated activation energy (Ea) for Pb(Ⅱ) adsorption onto the MHMO was38.23kJ/mol. The Pb(II) removal efficiency of MHMO increased with the increase of pH and only negligible decreased with the increase of ionic strength. This suggested that the Pb(Ⅱ) adsorption on MHMO might involve chemisorption.The competitive adsorption showed that under the coexistence condition of Pb(Ⅱ), Cu(Ⅱ) and Cd(Ⅱ), the adsorption capacity of MHMO for coexisting ions were in the order of Pb(Ⅱ), Cu(Ⅱ), Cd(Ⅱ).The results of XPS showed that the uptake of Pb(II) by MHMO was correlated with increasing surface hydroxyl group content and the main adsorbed speciation was PbOH+.The final chemical state of Pb(II) on the surface of MHMO was similar to PbO. |
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