| Influenced by the native geological conditions and human activities, micro-polluted water often contains excessive amount of iron and manganese ions, which not only has an impact on industrial and agricultural development, but also has an impact on human health. China has abundant groundwater resources in the Sanjiang Plain, simple and stable distribution terrain; it has large capacity of groundwater resources that can be exploited. However, due to excessive Fe(II) and Mn(II) contents in the groundwater of Sanjiang Plain, direct usage of the water for industrial and agricultural purposes has been impossible, thereby constraining the regional economic development of the Sanjiang Plain. In addition, consumption of the Fe(II) and Mn(II) contaminated groundwater by humans can cause loss of appetite, vomiting, and diarrhea. Therefore, groundwater contaminated with high iron and manganese ions must undergo purification treatment before drinking. This study utilized industrial and agricultural waste rice husk combustion products- rice husk ash, as biosorbent for the removal of Fe(II) and Mn(II) ions in groundwater, using dynamic adsorption method of rice husk ash to study the best conditions for reactor operation; as well as determined dynamic adsorption model. Scanning electron microscopy, X-ray diffraction, X-ray fluorescence spectroscopy and other methods were used to analyze the adsorption mechanism of Fe(II) and Mn(II) onto rice husk ash. Finally, regeneration of the adsorbent was studied. The main results of this study are as follows:Adsorption saturation time of Fe(II) and Mn(II) by rice husk ash decreased as the flow rate and inlet of Fe(II) and Mn(II) concentration increased; and as the height of the packed adsorbent increased, the concentration of co-existing ions in solution also increased, and the more easily it could reach saturation adsorption. The optimum operating parameters for dynamic adsorption process of Fe(II) and Mn(II) by rice husk ash were as follows: the reactor flow rate was 15 m L/min, the initial concentration of Fe(II) and Mn(II) were 20 mg/L and 15 mg/L, respectively, the height of the rice husk ash was 22.5 cm(75% of the adsorption column height), initial p H were 5 and 6, respectively. The adsorption capacity of Fe(II) and Mn(II) onto rice husk ash were 2.56 and 3.55 mg/g, adsorption ability of Fe(II) was higher than Mn(II) for the same quality of rice husk ash.Scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FT-IR), Xray diffraction(XRD), X- ray fluorescence spectrometer(XRF) were used to analysis the surface structure and composition of rice husk ash. The results showed that rice husk ash was porous, and can be used as an adsorbent. after the adsorption of Fe(II) and Mn(II), crest of Fe and Mn appeared on the scanning electron microscopy chart, which also showed that the rice husk ash was able to adsorb iron and manganese ions. Rice husk ash contains large amounts of Si and O, which mainly exist in the form of amorphous silica. the results showed that electrostatic adsorption reaction could occur between functional groups which correspond to elements of the Si, O and Fe(II), Mn(II).-OH, aromatic hydrocarbons, Si-O-Si and other functional groups, which can react with the metal ion by complexation. The elemental analysis of rice husk ash was performed, metal elements of Na, Mg, K, and Ca reduced or disappeared which may be due to ion exchange in dynamic adsorption process. After adsorption reaction, contents of Fe and Mn ions were higher than before adsorption, which proved the feasibility of adsorption of Fe(II) and Mn(II) by rice husk ash.Bohart-Adams model can be used to fit the adsorption process of Fe(II) to the rice husk, the value of fitting coefficient R2 were from 0.73 to 0.94; the value of the concentration of metal ions adsorption saturation N0 increased as the water flow rate and Fe(II) concentration increased, and decreased as the height of adsorbent column packing increased. Kinetic constant kAB decreased with the increasing of Fe(II) concentration and increased with the increasing of the water flow rate. Thomas model can well reflect the dynamic adsorption process of Mn(II) on the rice husk ash: the value of fitting coefficient R2 were from 0.71 to 0.95; the value of model constant k Th increased as the Mn(II) concentration in solution increased, but decreased with the increasing of water flow rate and height of column packing with the adsorbent. Dynamic adsorption capacity of Mn(II) by rice husk ash q0 were also increased with the increasing of Mn(II) concentration, and reduced with the increasing of water flow rate and height of packing with the adsorbent.The adsorption-desorption cycle test of Fe(II) and Mn(II) by rice husk ash indicated, when H2SO4 was used as desorption reagent, the optimum cycle time was 6, the Fe(II) adsorption capacity on rice husk ash was up to 2.56 mg/g, the minimum being1.51 mg/g; the Fe(II) adsorption quantity was significant higher compared to when HNO3 and HCl were used as desorption agents; when HCl and HNO3 were used as desorbents, adsorption amount of Fe(II) before desorption was significant higher than adsorption quantity after desorption, indicating that adsorption of rice husk ash after desorption had a tendency to reduce. the Mn(II) adsorption capacity on rice husk ash was up to 3.49 mg/g, the minimum was 0.28 mg/g, which was slightly higher than adsorption capacity when HNO3 and HCl were used as desorption agents on the adsorption of Mn(II). When HCl and HNO3 were used as desorption agents, secondary adsorption ability of rice husk ash was unsatisfactory. Therefore, H2SO4 was the best desorbent on the test of adsorption and desorption of Fe(II) and Mn(II) by rice husk ash. |
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