| According to the pollution caused by the radionuclide and the associated heavy metals from the large number of decommissioned nuclear mine tailings in China and the smelting process of uranium, our research used the natural rice husk(RRH), which is a common agricultural waste, as the raw material. Then under the guidance of mature preparation technique, we utilized sodium carbonate(Na2CO3) as the expanding agent to prepare a novel biosorbent, expended rice husk(ERH), with the mechanical expanding processing technique. The adsorption properties and mechanism of this novel adsorbent for uranium and the associated heavy metals, including cooper, lead, cadmium and chromium, were studied in batch experiments and the comparative adsorption experiments with the NRH. Meanwhile, we have investigated the application processing parameters of ERH, and the results were discussed as follow:(1) In order to prepare a structurally loose-and-cracking adsorption material, the ERH, with high specific surface area and moisture content, as well as high pore volume wettability and high thermal stability, the processing conditions in our research were optimized as follow: water content of ray material(30.00%), temperature of feed and discharge(100℃ and 250℃), concentration of the expanding agent(6.00%). The results of X-ray Fluorescence Spectrometer(XRF), X-ray Photo-electron Spectroscopy(XPS), Energy Dispersive Spectrometer(EDS) and X-ray Diffraction(XRD) showed that the expanding modification have promoted the exposure of nexus, such as Si O2, in rice husk, and increased the fine fiber, leading to an increase of micro-porous structure on the surface. When hemicellulose and lignin filled in the micro-porous, the content of-OH、-CH2-and-CO- would increase, which helped increase the free hydroxyl groups and improve the chemisorption properties.(2) The result of the comparison experiments on NRH and ERH showed that the maximum adsorption capacity of ERH would enhanced with increasing initial concentration, contact time and the temperature of U6+ and its associated heavy metals. Additionally, it took a very short time to reach the equilibrium adsorption. Except for the U6+, the other four target ions reached their equilibrium adsorption in less than 30 min, But U6+ ions reached their equilibrium adsorption in 60 min, which indicated a rapid adsorption progress. The static adsorption isotherms of ERH for U6+ in the nuclear wastewater could be described with Freundlich adsorption isotherm model, and the maximum adsorption capacity was 18.461 mg/g while the static adsorption isotherms of ERH to the other four associated heavy metals fitted Langmuir adsorption isotherm model very well, the maximum adsorption capacities of ERH for those four heavy metals were 18.52 mg/g, 41.67 mg/g, 5.084 mg/g, 24.39 mg/g, respectively. The pseudo-second-order model could well consistented with adsorption, the static adsorption kinetics behavior of ERH and RRH for those five target ions, the whole adsorption procedure was controlled by membrane diffusion and intraparticle diffusion. The diffusion rate of intraparticle diffusion is the key.(3) The adsorption pattern of ERH to the five target ions fitted Bohrat-Adams model perfectly: the adsorption capacity(N0) of ERH for U6+, Cu2+, Pb2+, Cr3+ and Cd2+ reached 470.353 mg/L, 509.546 mg/L, 589.838 mg/L, 631.956 mg/L, 552.018 mg/L, respectively. Thomas model and Yoon-Nelson model could well describe the adsorption procedure of ERH dynamic column for those five target ions, the fitting parameters provided reliable designing parameters for the industrial application of ERH. What’s more, the optimal conditions of dynamic column could be as follow: low influent concentration, slow flow rate and high packing layer height.(4) The result of desorption experiments by utilizing hydrochloric acid(HCl) suggested that the desorption rate of HCl to radionuclide U6+ reached 89.046%, Cu2+, 96.43%, Pb2+, 84.18%, Cr3+, 92.21%, and Cd2+, 83.46%. At the same time, after the experiments of 20 regeneration cycles, the adsorption rate of those five target ions could still achieve 32.98%, 41.41%, 25.66%, 33.23% and 30.43% of the first total adsorption, respectively.(5) The quantitative investigation on proportion of ion exchange and acid-alkali precipitation in the process of adsorptive removal of uranium and it associated heavy metals, which are copper, lead, cadmium and chromium, showed that the ion exchange adsorption acid-alkali precipitation of ERH on U6+, Cu2+, Pb2+, Cr3+ and Cd2+ took up 19.83%, 30.73%, 23.18%, 31.6% and 45.01%, respectively.That’s to say, the adsorption of ERH for Cu2+, Cr3+ and Cd2+ was mainly chemisorption, while U6+ and Pb2+, physisorption. Moreover, the adsorption mechanism was mainly ions exchange, which took place between the target ions and the functional groups, such as –OH,-CH2- and –CO-, of the sorbent. Meanwhile, the target ions could also have chemical reduction with the surface acidic oxygen-containing functional groups. However, the adsorption of U6+ relied mainly on the multi-molecules adsorption that generated by chemical adsorption, while the other four target ions were decided by single molecules adsorption.The theoretical analysis and the batch experiments proved that the novel biomass adsorption material, ERH, showed important application value in heavy metal mining environmental pollutionespecially in the wastewater treatment for radionuclide mining and the associated heavy metals, as well as the environmental remediation. |
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