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1.
The adsorptive removal of Cr(VI) was studied using activated carbon derived from Leucaena leucocephala (ACLL). The physico-chemical properties of ACLL were determined using proximate analysis and N2 BET surface area analysis. The N2 BET surface area of ACLL was determined to be 1131 m2 g?1. The point of zero charge (pHpzc) of 5.42 indicated that ACLL surface was positively charged for pH below the pHPZC, attracting anions. The effect of experimental operating parameters such as time of contact, ACLL dose, pH, initial concentration and temperature was investigated. The optimum values of parameters such as concentration of 100 mg L?1, 300 mg of ACLL dose, time of contact of 60 min, pH of 4 indicated the maximum Cr(VI) uptake of 13.85 mg g?1. The pseudo-second-order kinetic model best fitted with the Cr(VI) adsorption data. Adsorptive removal of Cr(VI) onto ACLL satisfactorily fitted in the order of Redlich–Peterson > Freundlich > Langmuir > Temkin adsorption isotherm model. The thermodynamic parameters showed the adsorption of Cr(VI) onto ACLL was an endothermic and spontaneously occurred process.  相似文献   

2.
The adsorption capacity of raw and sodium hydroxide-treated pine cone powder in the removal of methylene blue (MB) from aqueous solution was investigated in a batch system. It was found that the base modified pine cone exhibits large adsorption capacity compared with raw pine cone. The extent of adsorption capacity was increased with the increase in NaOH concentration. Overall, the extent of MB dye adsorption increased with increase in initial dye concentration, contact time, and solution pH but decreased with increase in salt concentration and temperature for both the systems. Surface characteristics of pine cone and base modified pine cone were investigated using Fourier transform infrared spectrophotometer and scanning electron microscope. Equilibrium data were best described by both Langmuir isotherm and Freundlich adsorption isotherm. The maximum monolayer adsorption capacity was found to be 129.87 mg g?1 at solution pH of 9.02 for an initial dye concentration of 10 ppm by raw pine cone. The base modified pine cone showed the higher monolayer adsorption capacity of 142.25 mg g?1 compared with raw pine cone biomass. The value of separation factor, R L, from Langmuir equation and Freundlich constant, n, both give an indication of favourable adsorption. The various kinetic models, such as pseudo-first-order model, pseudo-second-order model, intraparticle diffusion model, double-exponential model, and liquid film diffusion model, were used to describe the kinetic and mechanism of adsorption process. Overall, kinetic studies showed that the dye adsorption process followed pseudo-second-order kinetics based on other models. The different kinetic parameters, including rate constant, half-adsorption time and diffusion coefficient, were determined at different physicochemical conditions. A single-stage bath adsorber design for the MB adsorption onto pine cone and modified pine cone has been presented based on the Langmuir isotherm model equation. Thermodynamic parameters, such as standard Gibbs free energy (ΔG 0), standard enthalpy (ΔH 0) and standard entropy (ΔS 0), were also calculated.  相似文献   

3.
The adsorption behavior study of diethyl and dibutyl phthalates was investigated onto a new activated carbon prepared from an abundant biomass “Albizia julibrissin pods,” treated chemically by H3PO4. A series of experiments were conducted in a batch system to estimate the effect of operating conditions such as the adsorbent nature, the dose of adsorbent, the contact time, the initial concentration and the temperature on the adsorption efficiency. The optimum operating conditions were found to be 0.1 and 0.05 g of adsorbent for diethyl and dibutyl phthalates, respectively, at 30 min equilibrium time, 150 mg g?1 and 293 K. The adsorption isotherms for both phthalates were fit at different temperatures using the nonlinear regression of Langmuir, Freundlich, Dubinin–Radushkevich and Redlich–Peterson. The pseudo-first order, pseudo-second order by nonlinear regression and intraparticle diffusion models were used to describe the adsorption kinetic. The results show that the intraparticle diffusion model is not the limiting step governing the adsorption mechanism. The structural and textural characteristics of adsorbent surface were investigated. FTIR analysis of unloaded and phthalates-loaded adsorbent revealed that the aliphatic groups attached to phthalate esters are involved in adsorption mechanism.  相似文献   

4.
Adsorption of Cr(VI) using native and chemically modified marine green macroalgae Codium tomentosum biomass and its adsorption kinetics were studied under specific conditions. Maximum Cr(VI) removal occurred at pH 2 for both untreated and acid-treated biomass. However, base-treated biomass exhibited maximum adsorption at pH 6 due to the hydrolysis of methyl esters present in the cellulose, hemicellulose and lignin molecules resulting in carboxyl groups (COO?) on the surface. The effect of adsorbent dose revealed that untreated and acid-treated biomass follows Henry’s linear isotherm, while base-treated biomass exhibited sigmoidal curve indicating energetic heterogeneity on the adsorbing surface. The monolayer adsorption capacity of untreated, acid-treated and base-treated biomasses was 5.032 ± 0.644, 5.445 ± 0.947, 3.814 ± 0.559 mg g?1, respectively. Adsorption was found to follow Ho and McKay’s pseudo-second-order kinetic model with decreasing pseudo-second-order rate constant (K 2, g mg?1 min?1) of 0.088 ± 0.037 (acid-treated), 0.019 ± 0.003 (untreated) and 0.012 ± 0.003 (base-treated).  相似文献   

5.
In this work, a low-cost lignocellulosic adsorbent with high biosorption capacity is proposed, suitable for the efficient removal of hexavalent chromium from water and wastewater media. The adsorbent was produced by autohydrolyzing Scots Pine (Pinus Sylvestris) sawdust. The effect of the autohydrolysis conditions, i.e., pretreatment time and temperature, on hexavalent chromium biosorption was investigated using energy-dispersive X-ray spectroscopy (EDS) and UV–visible spectrophotometry. The Freundlich, Langmuir, Sips, Radke-Prausnitz, Modified Radke-Prausnitz, Tóth, UNILAN, Temkin and Dubinin-Radushkevich adsorption capacities and the rate constant values for pseudo-first- and pseudo-second-order kinetics indicated that the autohydrolyzed material exhibits significantly enhanced hexavalent chromium adsorption properties comparing with the untreated sawdust. The Freundlich’s adsorption capacity K F increased from 2.276 to 8.928 (mg g?1)(L mg?1)1/n , and the amount of hexavalent chromium adsorbed at saturation (Langmuir constant q m) increased from 87.4 to 345.9 mg g?1, indicating that autohydrolysis treatment at 240 °C for 50 min optimizes the adsorption behavior of the lignocellulosic material.  相似文献   

6.
The present article explores the ability of five different combinations of two adsorbents (Arachis hypogea shell powder and Eucalyptus cameldulensis saw dust) to remove Pb(II) from synthetic and lead acid batteries wastewater through batch and column mode. The effects of solution pH, adsorbent dose, initial Pb(II) concentration and contact time were investigated with synthetic solutions in batch mode. The Fourier transform infrared spectroscopy study revealed that carboxyl and hydroxyl functional groups were mostly responsible for the removal of Pb(II) ions from test solutions. The kinetic data were found to follow pseudo-second-order model with correlation coefficient of 0.99. Among Freundlich and Langmuir adsorption models, the Langmuir model provided the best fit to the equilibrium data with maximum adsorption capacity of 270.2 mg g?1. Column studies were carried out using lead battery wastewater at different flow rates and bed depths. Two kinetic models, viz. Thomas and Bed depth service time model, were applied to predict the breakthrough curves and breakthrough service time. The Pb(II) uptake capacity (q e = 540.41 mg g?1) was obtained using bed depth of 35 cm and a flow rate of 1.0 mL min?1 at 6.0 pH. The results from this study showed that adsorption capacity of agricultural residues in different combinations is much better than reported by other authors, authenticating that the prepared biosorbents have potential in remediation of Pb-contaminated waters.  相似文献   

7.
A hydrophilic kapok fiber was prepared by a chemical process of the Fenton reaction and used as an adsorbent to remove Pb(II) from aqueous solution. The effects of experimental parameters including pH, contact time, Pb(II) concentration, and coexisting heavy metals were estimated as well as evaluated. The optimum concentrations of FeSO4 and H2O2 for the Fenton reaction-modified kapok fiber (FRKF) were 0.5 mol L?1 and 1 mol L?1, respectively. The adsorption kinetic models and isotherm equations of Langmuir and Freundlich were conducted to identify the most optimum adsorption rate and adsorption capacity of Pb(II) on FRKF. The FRKF displayed an excellent adsorption rate for Pb(II) in single metal solution with the maximum adsorption capacity of 94.41?±?7.56 mg g?1 at pH 6.0. Moreover, the FRKE still maintained its adsorption advantage of Pb(II) in the mixed metal solution. The FRKF exhibited a considerable potential in removal of metal content in wastewater streams.  相似文献   

8.
Acacia nilotica was used for the adsorption of Reactive Black 5 (RB5) dye from an aqueous solution. Both the raw and activated (with H3PO4) carbon forms of Acacia nilotica (RAN and ANAC, respectively) were used for comparison. Various parameters (including dye concentration, contact time, temperature, and pH) were optimized to obtain the maximum adsorption capacity. RAN and ANAC were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The maximum experimental adsorption capacities for RAN and ANAC were 34.79 and 41.01 mg g?1, respectively, which agreed with the maximum adsorption capacities predicted by the Langmuir, Freundlich, and Dubinin–Radushkevich equilibrium isotherm models. The adsorption data of ANAC showed a good fit to the isotherm models based on the coefficient of determination (R 2): Langmuir type II (R 2 = 0.99) > Freundlich (R 2 = 0.9853) > Dubinin–Radushkevich (R 2 = 0.9659). This result suggested monolayer adsorption of RB5 dye. The adsorption of RB5 dye followed pseudo-second-order kinetics. The RAN adsorbent reflected an exothermic reaction (enthalpy change, ΔH = ?0.006 kJ mol?1) and increased randomness (standard entropy change, ΔS = 0.038 kJ mol?1) at the solid–solution interface. In contrast, ANAC reflected both exothermic [?0.011 kJ mol?1 (303–313 K)] and endothermic [0.003 kJ mol?1 (313–323 K)] reactions. However, the ΔS value of ANAC was lower when the RB5 adsorption increased from 313 to 323 K. The negative values for the Gibbs free energy change at all temperatures indicated that the adsorption of RB5 dye onto RAN and ANAC was spontaneous in the forward direction.  相似文献   

9.
In this work, the effectiveness of native and chemically modified rice bran to remove heavy metal Pb(II) ions from aqueous solution was examined. Chemical modifications with some simple and low-cost chemicals resulted in enhancement of the adsorption capacities and had faster kinetics than native rice bran. Experiments were conducted in shake flasks to monitor the upshot of parameters over a range of pH, initial Pb(II) concentrations and contact times using a batch model study. The sorption capacities q (mg g?1) increased in the following order: NaOH (147.78), Ca(OH)2 (139.08), Al(OH)3 (127.24), esterification (124.28), NaHCO3 (118.08), methylation (118.88), Na2CO3 (117.12) and native (80.24). The utmost uptake capacity q (mg g?1) was shown by NaOH-pretreated rice bran. The results showed that, using NaOH-modified rice bran, the chief removal of Pb(II) was 74.54 % at pH 5, primary Pb(II) concentration 100 mg L?1 and contact time 240 min. Equilibrium isotherms for the Pb(II) adsorption were analyzed by Langmuir and Freundlich isotherm models. The Langmuir isotherm model, showing Pb(II) sorption as accessible through the high value of the correlation coefficient (R 2 = 0.993), showed a q max value of 416.61 mg g?1. The kinetic model illustrated adsorption rates well, depicted by a second order, which gives an indication concerning the rate-limiting step. Thermodynamic evaluation of the metal ion ?G o was carried out and led to the observation that the adsorption reaction is spontaneous and endothermic in nature. NaOH chemically modified rice bran was a superb biosorbent for exclusion of Pb(II) and proved to be excellent for industrial applications.  相似文献   

10.
The most appropriate method in designing the adsorption systems and assessing the performance of the adsorption systems is to have an idea on adsorption isotherms. Comparison analysis of linear least square method and nonlinear method for estimating the isotherm parameters was made using the experimental equilibrium data of Zn(II) and Cu(II) onto kaolinite. Equilibrium data were fitted to Freundlich, Langmuir, and Redlich–Peterson isotherm equations. In order to confirm the best-fit isotherms for the adsorption system, the data set using the chi-square (χ 2), combined with the values of the determined coefficient (r 2) was analyzed. Nonlinear method was found to be a more appropriate method for estimating the isotherm parameters. The best fitting isotherm was the Langmuir and Redlich–Peterson isotherm. The Redlich–Peterson is a special case of Langmuir when the Redlich–Peterson isotherm constant g was unity. The sorption capacity of kaolinite to uptake metal ions in the increasing order was given by Cu (4.2721 mg/g)?<?Zn (4.6710 mg/g).  相似文献   

11.
Pb-contaminated water is a dangerous threat occurring near metallurgic and mining industries. This circumstance produces serious environment concern, due to Pb(II) high toxic effects. Several reactive materials have been reported for Pb(II) adsorption, but not all reached final Pb(II) suitable concentrations, or they are expensive and rejected in massive remediation technologies; hence, natural materials are good options. The adsorption behavior of a volcanic scoria (two sieved fractions 1425 and <425 µm) was studied toward synthetic Pb(II) water solutions in batch experiments (170.4–912.3 mg L?1) with high removal efficiencies (97%). The Langmuir model fits both fractions with high linear correlation coefficients (0.9988 and 0.9949) with high maximum capacity values (588.23 and 555.55 mg g?1). Separation factor R L parameter varies with initial concentration, and the empirical equation predicts the limits of the material usefulness, a criterion proposed in this paper for conditions’ selection. The Lagergren pseudo-second-order analysis demonstrates chemisorption; calculated rate constant (416.66 mg g?1 min?1). Weber–Morris intraparticle model proves that the adsorption phenomena occur fast on the material surface (k inst = 72 g mg?1 min?0.5). The characterization of the volcanic material afforded the elemental composition (X-ray fluorescence), and the empirical formula was proposed. X-ray diffraction patterns verify the material structure as basalt, with a plagioclase structure that matches anorthite and albite, mostly composed of quartz. The presence of oxides on the material surface explain the high Pb(II) adsorption capacity, observed on the surface by scanning electronic microscopy. The studied volcanic scoria has potential use as a Pb(II) adsorbent in water remediation technologies.  相似文献   

12.
Adsorption characteristics of water hyacinth roots powder for the removal of Indosol Dark-blue GL dye were investigated in batch mode. Operating variables, such as initial solution pH, presence of detergent, adsorbent dosage, initial concentration and contact time, were studied. The results showed that the adsorption of dye increased with increasing the initial concentration and contact time. The adsorption is highly pH dependent and adsorption capacity increased with decrease in pH. Kinetic study revealed that the uptake of Indosol Dark-blue GL was very rapid within the first 15 min and equilibrium time was independent of initial concentration. Batch equilibrium experiments were carried out at different pH and found that equilibrium data fitted well to Langmuir isotherm model. The maximum sorption capacity of the adsorbent was found as 86 mg g?1 at pH 3 which reduced to 64 mg g?1 at pH 5. The presence of detergent reduced the sorption capacity of the adsorbent significantly. Using equilibrium and kinetic data, the forward and backward rate constants were determined from the unified approach model. Desorption study revealed that the dye can be recovered by swing the pH from low to high.  相似文献   

13.
Microwave-assisted tetrabutyl ammonium-impregnated sulphate-crosslinked chitosan was synthesized for enhanced adsorption of hexavalent chromium. The adsorbent obtained was extensively characterized using Fourier transform infrared, X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray studies. Various isotherm models such as Langmuir, Freundlich and Dubinin–Radushkevich were studied to comprehend the adsorption mechanism of hexavalent chromium by the adsorbent. Maximum adsorption capacity of 225.9 mg g?1 was observed at pH 3.0 in accordance with Langmuir isotherm model. The sorption kinetics and thermodynamic studies revealed that adsorption of hexavalent chromium followed pseudo-second-order kinetics with exothermic and spontaneous behaviour. A column packed with 1 g of adsorbent was found to give complete adsorption of Cr(VI) up to 900 mL of 200 mg L?1 solution which discerns the applicability of the adsorbent material for higher sample volumes in column studies. The effective adsorption results were obtained due to both ion exchange and ion pair interaction of adsorbent with hexavalent chromium. Greener aspect of overall adsorption was regeneration of the adsorbent which was carried out using sodium hydroxide solution. In the present study, the regenerated adsorbent was effectively reused up to ten adsorption–desorption cycles with no loss in adsorption efficiency.  相似文献   

14.
In this work, castor cake produced as a by-product in castor oil extraction was used for activated carbon production. Castor cake was chemically treated with a K2CO3 solution, and the effect of the pyrolysis temperature in the 500–900 °C range was studied. Materials were characterized by X-ray powder diffraction, thermogravimetric analysis, scanning electron microscopy and nitrogen adsorption–desorption at ?196 °C. Methylene blue adsorption was selected as a test probe to stress the removal capacity of the prepared materials. By the X-ray powder diffraction analysis, carbon obtaining in its graphite allotropic form together with other inorganic compounds was verified. Scanning electron microscopy images evidenced the generation of porosity in the thermally treated samples compared with the pristine compound. In addition, the specific surface area values augmented progressively with the thermal treatment increment achieving a value of 1015 m2 g?1 in the 900 °C calcined sample. Calcination at 800 °C and m/V = 0.003 ratio were the best parameter combination to achieve a 99.6% methylene blue uptake.  相似文献   

15.
The adsorption properties of eggshell membranes (ESM), eggshells (ES) and orange peels (OP) were studied for the removal of arsenic (total As) and selenium (total Se). The effect of chemical treatment of these adsorbents by HNO3 and NaOH was also investigated using Fourier transform infrared spectroscopy (FT-IR). Analysis of the FT-IR spectra showed that treatment with NaOH and HNO3 had an effect on the functional groups present in the materials and also on the adsorption by extension. Thermal analysis showed that ES were more thermally stable than the others with no water molecules in their matrix, which could have caused a substantial weight loss at around 70 °C. In terms of adsorption capacities, chemical treatment increased the adsorption capacities of ESM and OP achieving up to 170 μg g?1 (As) and 160 μg g?1 (Se), and 120 μg g?1 (As) and 70 μg g?1 (Se), respectively, with not much activity for ES in terms of adsorption. The two adsorbents (NaOH-treated OP and ESM) were then tested in environmental water samples and the results showed that 68.9 % of As and 74.8 % of Se, and 54.1 % of As and 47.3 % of Se were removed from domestic wastewater samples investigated using OP and ESM, respectively. Moreover, better selectivities towards the compounds of interest were achieved.  相似文献   

16.
In this study, nickel ions adsorption from zinc ingot factory wastewater by brown algae (Sargassum glaucescens) and chitosan/polyvinyl alcohol nano-fiber membrane at continuous system was studied. The continuous process included a biosorption reactor and fixed-bed reactor that were optimized by predicting two batch steps with response surface modeling, based on the Box–Behnken in the novel approach. Nano-biosorbent characterized by scanning electron microscopy, Brunauer–Emmett–Teller and Fourier transform infrared spectrometer analysis. Maximum biosorption in this continuous system was at pH 6, biosorbent doses 8 g L?1 S. glaucescens and 0.48 g L?1 nano-fiber. The study of the reaction rate showed kinetic data best fitted by pseudo-first-order model with R 2 > 0.95 than pseudo-second-order and intraparticle diffusion models. Biosorption equilibrium data were performed using Langmuir isotherm and Freundlich isotherm, Langmuir isotherm fit better with equilibrium data.  相似文献   

17.
This study investigated the removal efficiency of pharmaceuticals from aqueous solutions supported on chemically treated fly ash. The coal fly ash was supplied by the electric power station in Krakow, Poland. There are plenty of studies showing the utilization of fly ash as a low-cost adsorbent for wastewater containing heavy metals or dyes. Adsorption and immobilization of pharmaceuticals and personal care products on fly ash is a relatively new method but it is a very promising one. In this study, the adsorptive removal of diclofenac, ketoprofen, carbamazepine, bezafibrate, bisphenol A, 17α-ethinyl estradiol and estriol by HCl- and NaOH-treated fly ash was assessed. Chemical treatment of fly ash changed structures of particles and enhanced specific surface areas. HCl-treated fly ash was characterized by the highest BET specific surface area 47.9 m2 g?1 and unburned carbon content 8.1%. Isotherms for all compounds except for 17α-ethinyl estradiol (EE2) and estriol (E3) were linear. Higher linear regression coefficients (R 2) obtained for isotherms of EE2 and E3 show that the Freundlich model better describes their sorption. Adsorption coefficients K d varied between 109.5 (L kg?1) for bisphenol A and 471.5 (L kg?1) for bezafibrate. Freundlich constants (K F) for EE2 and E3 were 62.3 and 119.9 (µg1?1/n L1/n kg?1), respectively. Acid treatment of fly ash increased adsorption of diclofenac, ketoprofen, carbamazepine, bezafibrate and bisphenol A. Comparison of the octanol–water partitioning coefficients (log K OW) with the partitioning coefficients normalized on unburned carbon content (log K UC) revealed similarities but no strong correlation. The increasing of unburned carbon increased sorption of compounds to fly ash.  相似文献   

18.
The aim of this study was to evaluate the biosorption capacity of selected strains of microscopic fungi. We optimized the biosorption process and used the Freundlich isotherm for three strains: H. haematococca BwIII43, K37 and T. harzianum BsIII33 to describe the biosorption equilibrium of anthraquinone dye, Alizarin Blue Black B (ABBB) and alkali lignin (AL). In optimal conditions (1 g of mycelium biomass, pH = 7.0, 28 °C) for ABBB and AL sorption, the live biomass of H. haematococca BwIII43 was characterized by a higher sorption capacity, amounting to 247.47 and 161.00 mg g?1, respectively. The highest sorption properties toward anthraquinone dye (K F = 19.96 mg g?1) were shown for the biomass of H. haematococca K37. In the presence of alkali lignin, the highest sorption capacity and bond strength exhibited the biomass of H. haematococca BwIII43 (K F = 28.20 mg g?1, n = 3.46). Effective decolorization of ABBB and AL by the selected strains of microscopic fungi indicated that the biosorption process additionally enhanced the removal of color compounds from the solution.  相似文献   

19.
Enzymatic and alkali pretreatments were employed to improve nickel biosorption capacity of Rhizomucor pusillus biomass. Pretreatment with 0.002–80 g l?1 NaOH and 0.0001–0.1 Anson Unit (AU) g?1 protease enhanced the biosorption capacity of fungal biomass. Increasing the concentration of NaOH from 0.002 to 5 g l?1 improved nickel removal from 93.2 to 100.0 % while untreated biomass showed 64.6 % Ni(II) removal. Pretreatment with higher concentrations of NaOH, 5–80 g l?1 resulted in nearly complete removal of nickel ions. Pretreatment of the biomass with 0.0001 AU g?1 protease improved the nickel removal to over 91 %, while increasing the enzyme loading to 0.1 AU g?1 improved the removal to 93 %. Untreated biomass removed 78.4, 63.0, and 96.3 % of chromium, copper, and lead ions, respectively, from a mixture solution of the ions. Respective metal removals were increased to 100, 98.9, and 100 % after pretreatment with 0.2 g l?1 NaOH solution and to 87.8, 86.7, and 100 % after the enzymatic pretreatment with 0.1 AU g?1 protease. Scanning electron microscopy analysis indicated that alkali and enzymatic pretreatments enhanced the porosity of the biomass. Furthermore, compositional analysis showed that both of the pretreatments removed a major part of fungal proteins (2.1–95.8 % removal). Glucosamine, N-acetyl glucosamine, and phosphates were the major ingredients of the pretreated biomass.  相似文献   

20.
Asexual spores of the filamentous fungus Rhizopus arrhizus were used as the resting biomass as they tolerate chitosan gelling for mycelia growing in chitosan beads. Biosorption of lead using the dead detergent pre-treated chitosan-immobilised and grown fungal beads was performed with initial lead (II) nitrate concentrations ranging from 9.02 to 281.65 mg/L. The adsorption data were best correlated with equilibrium adsorption isotherms in the order Redlich–Peterson, Langmuir, Freundlich and Fritz–Schlünder by non-linear regression. The biosorption kinetic model of pseudo second-order (R 2 > 0.99) fitted better than pseudo first-order and modified pseudo first-order models. Among the four pseudo second-order kinetic models, the Blanchard model was the best fit for the experimental biosorption data. The rate-limiting step of biosorption of lead was shown to be intraparticle diffusion controlled according to Weber and Morris model fitting. The beads could be regenerated using 1 M nitric acid solution. This illustrated the good performance of the beads for regenerated sorption/desorption at least five cycles.  相似文献   

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