| This research was funded by the Key Projects of National Defense Basic Scientific Research of China (Grant No. B3120110001). From the view of waste resource recycle and the reducing chemical pollution, sunflower straw, stem, leaf and disk were researched for the removal of uranium and strontium ions from wastewater systematically compared to coconut-shell activated carbon (C-AC). The sorptive properties for U (Ⅵ), Cu (Ⅱ) and Pb (Ⅱ) onto raw sunflower straw were evaluated in single and binary solutions using batch experiments as a function of solution pH, adsorbent dosage, temperature, contact time and initial nuclide ion concentration in the same time. And the fixed-bed column adsorption characteristics of uranium (Ⅵ) and strontium (Ⅱ) were investigated by sunflower straw in detail. The adsorption mechanism, regeneration and cyclic utilization of the target adsorbent were also studied. The results were shown as follows,1. The particle size of sunflower biomass used in this work was in a range between1and480μm inclusive of two maxima at about50and300μm, respectively. The surface area of sunflower biomass was lower than3.65m2·g-1and it was belonged to the mesopore materials. Electron micrograph of native sunflower biomass showed that its surface was porous and loose, and a large number of irregular drapes were present on the surface. FT-IR analyses showed that hydroxyl, carboxyl and amide groups were present in abundance in the sunflower biomass, which was beneficial to adsorption for radionuclide ions. The decomposition temperature was higher than212℃and the point of zero charge (pHPZC) was observed at2.0.2. The adsorption studies for U (Ⅵ) onto sunflower biomass in single ion system showed that uranium (Ⅵ) uptake was significantly depended on variations of the pH value of solution and the optimal values were in the range between3.0and5.0. Temperature had a little effect on U (Ⅵ) sorption and removal efficiency of uranium (Ⅵ) increased with rising contact temperature except disk. The kinetic data conformed successfully to the pseudo-second-order equation and equilibrium time was in the range between30and720min. The isotherm adsorption data for sunflower straw and leaf was modeled best by the nonlinear Langmuir-Freundlich equation. The sorption data for sunflower disk was modeled best by the nonlinear Langmuir equation. The equilibrium sorption capacity of sunflower straw, leaf and disk was observed to be251.5mg·g-1,153.3mg·g-1and101.91mg·g-1under optimal conditions, respectively.3. The adsorption studies for Sr (Ⅱ) onto sunflower biomass in single ion system showed that strontium (Ⅱ) uptake was remarkably depended on variations of the pH value of solution and the optimal values were in the range between4.0and7.0. Temperature had a little influence on Sr (Ⅱ) sorption and removal efficiency of Sr (Ⅱ) decreased with rising contact temperature. The kinetic data excellently followed the pseudo-second-order equation and equilibrium time was in the range between5and60min. The isotherm adsorption data for sunflower straw, stem, leaf and disk was simulated best by the nonlinear Langmuir-Freundlich equation. The equilibrium sorption capacity of sunflower straw, stem, leaf and disk was calculated to be20.84mg·g-1,17.87mg·g-1,22.31mg·g-1and30.66mg·g-1under optimal conditions, respectively. Furthermore, all the results above manifested that sunflower straw could be well applied to the treatment of radioactive wastewater containing U (Ⅵ) in higher concentration and Sr (Ⅱ) in lower concentration. And the sunflower straw possessed distinctive advantages comparing to sunflower stem, leaf, disk and C-AC.4. The adsorption studies for U (Ⅵ), Cu (Ⅱ), Pb (Ⅱ) onto sunflower straw in single ion system, U (Ⅵ)/Cu (Ⅱ) and U (Ⅵ)/Pb (Ⅱ) binary systems indicated that metal ions uptake was notably depended on variations of the pH value of solution and the optimal value was4.0. The removal efficiency was weekly affected by contact temperature. The positive enthalpy and negative free energy suggested the endothermic and spontaneous nature of U (Ⅵ) and Pb (Ⅱ) sorption both in single and binary system, respectively. The negative enthalpy and negative free energy implied exothermic and spontaneous nature of Cu (Ⅱ) sorption. Kinetic data was best described by the pseudo-second-order model for U (Ⅵ), Cu (Ⅱ), Pb (Ⅱ) sorption, implying that the chemical adsorption was the rate-limiting step. The adsorption isotherms of sunflower straw could be fitted well by the nonlinear Langmuir and Langmuir-Freundlich equation. Obviously, this novel sunflower straw exhibited an excellent selectivity toward U (Ⅵ) over Cu (Ⅱ) from the view of distribution coefficient and separation factor, which was likely up with the properties of metal itself. While the sunflower straw displayed an outstanding selectivity in the adsorption of Pb (Ⅱ) over U (Ⅵ) in the binary system at lower initial concentrations of ions and the case was vice versa at higher concentrations, which was possibly concerned with driving force of mass transfer. The maximum adsorption capacity in single system for U (Ⅶ), Cu (Ⅱ) and Pb (Ⅱ) was calculated to be251.5mg·g-1,15.70mg·g-1and67.59mg·g-1, respectively. The maximum adsorption capacity declined to be141.8and8.827mg·g-1for U (Ⅵ) and Cu (Ⅱ) in U (Ⅵ)/Cu (Ⅱ) binary system and to be114.2mg·g-1and46.83mg·g-1for U (VI) and Pb (Ⅱ) in U (Ⅵ)/Pb (Ⅱ) binary system at higher concentrations of ions. It was shown that sunflower straw could be a promising adsorbent for the removal of U (Ⅵ), Cu (Ⅱ) and Pb (Ⅱ) both in single and binary systems.5. U (Ⅵ) and Sr (Ⅱ) sorption performance of the packed column was assessed under variable operating conditions, such as, initial influent ions concentration, bed depths and flow rates. It was found that the breakthrough time and depletion time were extended with an increase in bed heights but obviously shortened with an increase in influent concentration and flow rates, respectively. The equilibrium uptake (qe(exp)) of sunflower straw increased with an increase in initial influent concentration and flow rates but decreased with increase in bed depth, respectively. The maximum adsorption capacity was calculated to be143.61mg·g-1and15.12mg·g-1for U (Ⅵ) and Sr (Ⅱ), respectively. The data in regard to the effect of bed depths was fitted well to the Bohrat-Adams model.6. Furthermore, fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope and energy dispersive X-ray revealed that U (Ⅵ) adsorption onto sunflower straw was predominantly controlled by ion exchange as well as complexation mechanism. In other words, the sorption process contained complexation involved in uranium (Ⅵ) binding with hydroxyl, carboxyl and amide groups, and ion exchange that the K and Mg metal ions of sunflower straw were exchanged for uranium (Ⅵ) ions.7. The saturated column was successfully regenerated by0.1mol·L-1nitric acid solution and the elution efficiency of U (Ⅵ) and Sr (Ⅱ) was81.06%and81.09%. The sunflower straw possessed a superior recycling ability for U (Ⅵ), and the removal efficiency could be remained more than97%after5cycles in batch mode. The TG analysis indicated that the loss ratio of sunflower straw was62.21%and85.90%, which owned prominent advantages comparing to C-AC.Based on all the experimental data and analyses, we can draw a conclusion that the sunflower straw exhibited good sorption ability and adsorption selectivity. It was shown that sunflower straw could be a potentially attractive candidate not only for commercial application in the field of radioactive wastewater treatment, but also for new approach to enhance the values of sunflower straw of Helianthus annuus. |
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