Zhi-bin Zhang Zhi-wei Zhou Xiao-hong Cao Yun-hai Liu Guo-xuan Xiong Ping Liang

Transcription

1 J Radioanal Nucl Chem (2014) 299: DOI /s Removal of uranium(vi) from aqueous solutions by new phosphorus-containing carbon spheres synthesized via one-step hydrothermal carbonization of glucose in the presence of phosphoric acid Zhi-bin Zhang Zhi-wei Zhou Xiao-hong Cao Yun-hai Liu Guo-xuan Xiong Ping Liang Received: 20 August 2013 / Published online: 20 November 2013 Ó Akadémiai Kiadó, Budapest, Hungary 2013 Abstract The novel phosphorus-rich hydrothermal carbon spheres (HCSs PO 4 ) have been synthesized via onestep hydrothermal carbonization of glucose in the presence of phosphoric acid. The textural and surface chemistry properties were characterized using Boehm titrations, scanning electron microscopy and Fourier transform infrared spectrometer. The content of oxygen-containing functional groups on the surface of HCSs increased from to mmol g -1 by phosphate group modification. The adsorption ability of HCSs PO 4 has been explored for the removal of uranium from aqueous solutions. The adsorption kinetic data were best described by the pseudo-second-order equation. Adsorption process could be well defined by the Langmuir isotherm, the adsorption capacity of HCSs increased from to mg g -1 after phosphate group modification. And Z. Zhang (&) Z. Zhou X. Cao Y. Liu G. Xiong P. Liang State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Ministry of Education, Nanchang , People s Republic of China zhangnjut@163.com Z. Zhang Z. Zhou X. Cao Y. Liu G. Xiong P. Liang Key Laboratory of Radioactive Geology and Exploration Technology Fundamental Science for National Defense, East China Institute of Technology, Fuzhou , People s Republic of China Z. Zhang Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan , People s Republic of China P. Liang State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan , People s Republic of China thermodynamic parameters indicated the adsorption process was feasible,endothermic and spontaneous. Selective adsorption studies showed that the HCSs PO 4 could selectively remove U(VI), and the selectivity coefficients had been improved in the presence of co-existing ions, Na(I), Ni(II), Sr(II), Mn(II), Mg(II) and Zn(II). Complete removal (99.9 %) of U(VI) from 1.0 L industry wastewater containing 15.0 mg U(VI) ions was possible with 12.0 g HCSs PO 4. Keywords Hydrothermal carbon spheres Phosphate group Adsorption Uranium Introduction With the development of nuclear technology, uranium contamination in the environment has become a serious problem. Uranium and its compounds, even at trace levels, may cause progressive or irreversible renal injury and in acute cases may lead to kidney failure and death [1]. Over the last few decades, a variety of technologies, such as chemical precipitation [2], membrane processes [3], ion exchange [4], solvent extraction [5] and adsorption [6], have been developed for the removal and recovery of uranium from radioactive wastes in consideration of the dual significance of the potential environmental health threat and a nonrenewable resource of nuclear energy [7]. Adsorption has been the most economical and widely used technique for heavy metals removal and offers several advantages such as low cost, ease of use, high removal capacity and flexibility in design and operation [8]. Carbonaceous materials such as activated carbon [9, 10], carbon nanotubes [11, 12], carbon fiber [13] and

2 1480 J Radioanal Nucl Chem (2014) 299: mesoporous carbon [14, 15] have been gradually applied to this area because of their higher thermal and radiation resistance than organic exchanger resins and better acid base stability compared with familiar inorganic sorbents [16]. Hydrothermal carbon spheres (HCSs), as a new member of the carbonaceous material family, are a kind of semi-carbonized material, which can be prepared from mono- and polysaccharides [17, 18], such as glucose [18], sucrose [19, 20], fructose [21], starch [22], and cellulose [23, 24] with the following features: cheap carbon sources, mild reaction conditions, and absolutely green as it excludes organic solvents, catalysts or surfactants [25]. Current researches on HCSs are focused on the design and preparation of the materials with different frameworks and various applications of raw HCSs. However, to the best of our knowledge, there are few researches on the preparation of surface functionalized HCSs and their adsorptive property towards radio-nuclide ions. There are only OH and COOH on the surface of HCSs, which limit the development of adsorption properties. Surface modification, an efficient route for improving the adsorption capacity and selectivity beyond their existing structural and functional features, has been drawing more attention in recent years. Li s group has prepared amidoxime [25] 5-azacytosine [26] and salicylideneimine [27] grafted HCSs which illustrated excellent selective sorption towards U(VI) from aqueous solution. Recently our group has also studied the efficiency of carboxylic HCSs by simply heating at lower temperature in air for removing uranium from aqueous solutions [28]. The content of carboxyl groups on HCSs increased significantly from 0.53 to 3.81 mmol g -1 after heat-treatment at 300 C, and the U(VI) sorption capacity increased from 55.0 to mg g -1 due to the carboxyl group acting as adsorption activity sites. The aim of the present investigation was to study the efficiency of the new phosphorus-containing HCSs synthesized via one-step hydrothermal carbonization of glucose in the presence of phosphoric acid. Various techniques were used to characterize the structure and textural property of HCSs, including scanning electron microscopy (SEM), Boehm titrations and Fourier transform infrared spectroscopy (FT-IR). The effect of various experimental parameters including ph of the solution, contact time, initial uranium concentration, and temperature, as well as adsorption kinetics, isotherm models, thermodynamics were studied. The selectivity of phosphorus-containing HCSs was inspected in the presence of co-existing ions, Na(I), Ni(II), Sr(II), Mn(II), Mg(II) and Zn(II). In addition, attempt for U(VI) removal by phosphorus-containing HCSs from industry wastewater was also done. Experiment Materials For the preparation of a stock uranium(vi) solution, g U 3 O 8 was put into a 100 ml beaker, and 10 ml hydrochloric acid (q = 1.18 g ml -1 ), 4 5 drops nitric acid were added. The solution was heated until it was nearly dry and then 5 ml hydrochloric acid (q = 1.18 g ml -1 ) was added. The solution was transferred to a 1,000 ml volumetric flask and diluted to the mark with distilled water to produce a uranium(vi) stock solution (1 mg ml -1 ). The uranium solutions were prepared by diluting the stock solution to appropriate volumes depending upon the experimental requirements. Synthesis of phosphorus-containing spherical carbon The phosphorus-containing carbon spheres (HCSs PO 4 ) were prepared according to Ref. [29] with minor revision. Typically, 4.0 g glucose and phosphoric acid with different molar ratios respect to glucose (1:1, 2:1 and 3:1) were transferred into a 100 ml Teflon-lined stainless steel autoclave containing 50 ml deionized water, sealed, and heated at 190 C for 24 h. When the reaction was completed, the autoclave was cooled to room temperature, and the black precipitate was collected and washed with deionized water, pure ethanol, and acetone till neutral, and the final product was dried at 80 C in vacuum for 6 h, and heated at 400 C in the atmosphere of Ar for 2 h. The resulting carbonaceous samples were labeled as HCSs PO 4-1, HCSs PO 4-2, HCSs PO 4-3, respectively. Characterization The FT-IR spectra was recorded on a Nicolet Nexus 870 Fourier transform infrared spectrometer using the KBr pellet technique with the resolution 2 cm -1. Surface morphology was characterized on a JEOL JSM-5900 SEM. The oxygen-containing functional groups (OFGs) content of HCSs was determined by Boehm titrations [30]. Adsorption experiments The adsorption of U(VI) was studied as a function of ph, contact time, initial concentration and temperature. The batch sorption was performed in a reciprocating water bath shaker with concussion speed of 200 rpm. In the experiments 0.01 g of sorbent was suspended in 50 ml solution containing different U(VI) concentration and different initial ph (adjusted with 0.1 mol L -1 HNO 3 and 0.1 mol L -1 NaOH). The concentration of U(VI) in the solution was determined by the Arsenazo III method [31]

3 J Radioanal Nucl Chem (2014) 299: Fig. 1 FT-IR spectra of pristine HCSs (a), HCSs PO 4-1 (b), HCSs PO 4-2 (c) and HCSs PO 4-3 (d) with a 721 type spectrophotometer at 650 nm. The amounts of U(VI) adsorbed per unit mass of the HCSs were calculated by Eq. 1. ð q e ¼ C 0 C e ÞV ; ð1þ W where q e is the adsorption capacity of adsorbent (mg g -1 ); C 0 and C e are the initial and equilibrium concentrations of uranyl ions (mg L -1 ), respectively, V is the volume of aqueous solution (L) and W is the mass of dry adsorbent (g). Results and discussion FT-IR The FT-IR spectra of pristine HCSs and HCSs PO 4 were shown in Fig. 1. The band at 3,440 was attributed to the O H (hydroxyl or carboxyl) stretching vibration and C OH (hydroxyl, ester, or ether) bending vibrations [32]. The two peaks at 1,704 and 1,619 cm -1 can be attributed to C=O (carbonyl, carboxyl, quinone, or ester) and C=C vibrations, respectively [33]. The spectrum of HCSs PO 4 had strong bands at 1,217 cm -1 can be attributed to the P=O group and the appearance of two new adsorption bands at 1,019 and 1,006 cm -1 belonging to P OH stretching vibrations [34, 35]. SEM The SEM images of pristine HCSs and HCSs PO 4-3 in Fig. 2 displayed that the carbon products appear to have perfect spherical shape, but slightly adhered. And the diameter of carbon spheres had been obviously increased from 0.3 to 0.5 lm after chemical modification. Effect of ph The ph of solution is one of the most crucial parameters for the sorption of metal ions which can affect the surface charge, the metal speciation and surface metal binding sites. The effect of ph on U(VI) adsorption onto HCSs and HCSs PO 4 was carried out over the ph range using 50 lg ml -1 initial uranium concentration at 298 K and the results were displayed in Fig. 3. The U(VI) uptake increased greatly from 3.0, and reached the maximum at 6.0 and 5.0 for HCSs and HCSs PO 4, respectively, and then declined subsequently. The low adsorption capacity at lower ph value could be attributed to the increasing positivity of the adsorbents and the competition of H? ions with U(VI) on the adsorptive active sites. And with the increase of ph above 5.0 the hydroxide products of U(VI) such as UO 2 (OH)?, (UO 2 ) 2 (OH) 2 and (UO 2 ) 3 (OH) 5 2? appeared and led to the decrease of the adsorption capacity [36]. The sorption capacity and content of OFGs as the function of phosphoric acid content is shown in Fig. 4. The sorption capacity and content of OFGs increased sharply from to mg g -1 and mmol g -1, respectively, with increase in molar ratio of phosphoric acid to glucose to 2:1 and then grew slowly. The increase of sorption capacity is due to the increasing of phosphate groups acting as the uranyl ions coordination sites. Effect of contact time and sorption kinetics studies Contact time is also an important factor which can reflect the adsorption kinetics. The variation of adsorption

4 1482 J Radioanal Nucl Chem (2014) 299: Fig. 2 SEM image of pristine HCSs (a) and HCSs PO 4-3 (b) Fig. 3 Effect of ph on the U(VI) sorption by HCSs and HCSs PO 4 (C 0 = 50 mg L -1, t = 180 min, V = 50 ml, T = K and W = 10 mg) Fig. 4 The sorption capacity and content of OFGs as the function of phosphoric acid content Fig. 5 Effect of contact time on U(VI) adsorption onto HCSs and HCSs PO 4-3 (C 0 = 50 mg L -1, V = 50 ml, ph = 6.0 for HCSs and ph = 5.0 for HCSs PO 4-3, T = K, W = 10 mg) capacity with contact time was studied using 50 lg ml -1 initial U(VI) concentration at 298 K and the results are shown in Fig. 5. As can be observed, the U(VI) uptake by HCSs and HCSs PO 4-3 increased sharply at the beginning, and then gradually reached equilibrium after 60 and 30 min, respectively, which was deemed sufficient to establish sorption equilibrium and used in all the subsequent experiments. The faster adsorption rate at the beginning would be due to the larger concentration gradient. In order to explain the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, two different kinetic models, pseudo-first order and pseudo-second order models were applied to test the sorption kinetics. The pseudo-first order kinetic model and pseudo-second order kinetic model may be represented by Eqs. 2 and 3.

5 J Radioanal Nucl Chem (2014) 299: Fig. 6 a Pseudo-first-order and b pseudo-second-order plots for the adsorption of U(VI) Table 1 Kinetic parameters of U(VI) adsorption onto HCSs and HCSs PO 4-3 Adsorbents q e.exp (mg g -1 ) Pseudo-first-order kinetics Pseudo-second-order kinetics q e.cal (mg g -1 ) k 1 (min -1 ) R 2 q e.cal (mg g -1 ) k 2 (min -1 ) R 2 HCSs HCSs PO lnðq e q t Þ¼ln q e k 1 t; ð2þ t ¼ 1 q t k 2 q 2 þ t ; ð3þ e q e where q e and q t refer to the amount of U(VI) adsorbed (mg g -1 ) at equilibrium time and any time (min), respectively; k 1 (min -1 ) and k 2 (g mg -1 min -1 ) are the rate constant of first and second order adsorption. The adsorption kinetic parameters in Eqs. 2 and 3 were calculated from the slopes and intercepts of the fitted curves in Fig. 6, and the results were shown in Table 1. Obviously, the correlation coefficient, R 2 of the pseudosecond-order was better than pseudo-first-order, and the calculated q e,cal value was closer to the experimental adsorption capacity q e,exp, which indicated that the pseudosecond-order kinetic model was more suitable to describe the adsorption process of U(VI) onto HCSs and HCSs PO 4-3. Therefore, the adsorption process of U(VI) on HCSs and HCSs PO 4-3 might be regarded as chemisorption, which could be the rate-controlling step [37]. Effect of initial U(VI) concentration and isotherm studies In order to study the influence of the initial U(VI) concentrations on the amount uptake on HCSs and HCSs PO 4-3, Fig. 7 Effect of initial concentrations on U(VI) adsorption onto HCSs and HCSs PO 4-3 (V = 50 ml, ph = 6.0 for HCSs and ph = 5.0 for HCSs PO 4-3, t = 60 min, T = K, and W = 10 mg) batch tests were performed at different U(VI) concentrations ranging from 10 to 130 mg L -1. As shown in the Fig. 7, the U(VI) sorption amount of the two curves displayed a continuous increase with the increase of residual U(VI) concentration until equilibrium was reached. The maximum adsorption capacities of HCS and HCSs PO 4-3 were found to be 64.9 and mg g -1, respectively.

6 1484 J Radioanal Nucl Chem (2014) 299: Table 2 Adsorption isotherm parameters of U(VI) adsorption onto HCSs and HCSs PO 4-3 Adsorbents Langmuir adsorption isotherm Freundlich adsorption isotherm K L q m (mg g -1 ) R 2 K F n R 2 HCSs HCSs PO Fig. 8 The langmuir adsorption isotherm for U(VI) sorption onto HCSs and HCSs PO 4-3 at Fig. 10 The thermodynamics for U(VI) adsorption onto HCSs and HCSs 3PO 4 Fig. 9 The Freundlich adsorption isotherm for U(VI) sorption onto HCSs and HCSs PO 4-3 at K The equilibrium adsorption isotherms are one of the essential data to understand the mechanism of the adsorption systems. Two famous isotherm models, i.e. Langmuir and Freundlich, are the most frequently used for explain the experimental results and the real sorption behavior. The Langmuir isotherm model assumes that the sorption occurred on homogeneous surface by monolayer sorption, and there are no interactions between the adsorbates on adjacent sites [26]. The linear equation of the Langmuir sorption model is expressed Eq. 4. C e ¼ 1 þ C e ; ð4þ q e q m K L q m where C e is the equilibrium concentration (mg L -1 ), q e is the amount absorbed at equilibrium (mg g -1 ), q m is the Langmuir constant, which represents the saturated monolayer sorption capacity (mg g -1 ), K L is the equilibrium constant (L mg -1 ). The linearized plots (Fig. 8) of C e /q e versus C e are obtained for the adsorbents. The Langmuir constants q m and K L can be gotten from the slope and intercept of the linear, respectively. The Freundlich isotherm model is an empirical relationship describing the sorption of solutes from a liquid to a solid surface and assumes that different sites with several sorption energies are involved, which linear equation is given by Eq. 5 [38]. ln q e ¼ ln K F þ 1 n ln C e; ð5þ where K F [(mg g -1 )(Lmg -1 ) 1/n ] and n are the Freundlich constant that can be related to sorption capacity of the sorbent and the intensity of sorption, respectively. Linear plots of lnq e versus lnc e based on the equilibrium data are given in the Fig. 9. The corresponding Freundlich and Langmuir parameters and the correlation coefficients (R 2 ) are presented in Table 2. Langmuir isotherm was found to be more suitable to describe the sorption characteristics of U(VI) on HCSs and HCSs PO 4-3 when comparing the values of correlation coefficients of both models. The result indicated that the

7 J Radioanal Nucl Chem (2014) 299: Table 3 The adsorption thermodynamic parameters for U(VI) adsorption onto pristine HCSs and HCSs PO 4-3 Adsorbents DH (kj mol -1 ) DS (J mol -1 K -1 ) DG (kj mol -1 ) 288 K 298 K 308 K HCSs HCSs PO present adsorption process on HCSs and HCSs PO 4-3 probably dominated by a monolayer adsorption rather than a multilayer one. Thermodynamic studies The effect of temperature on the adsorption capability of HCSs and HCSs PO 4-3 was investigated. The adsorption amount of U(VI) increased with the increasing of temperature gradually. To evaluate the thermodynamic feasibility and understand the nature of the adsorption process, the thermodynamic parameters such as the change of Gibbs free energy (DG), enthalpy (DH) and entropy of adsorption process (DS) were calculated with Eqs. 6 and 7 [27]. ln K d ¼ DS DH R RT ; ð6þ DG ¼ DH TDS; ð7þ where K d is the distribution coefficient (ml g -1 ), T is the absolute temperature in kelvin (K) and R is the gas constant (8.314 J mol -1 K -1 ). The values of DH and DS were calculated from the slope and intercept of the plots of lnk d versus T -1 (Fig. 10), and the values of DG were obtained using Eq. 7. All the thermodynamic parameters were listed in Table 3. The positive value of enthalpy change DH indicated that the sorption process was endothermic, and the positive value of DS showed the randomness at the solid-solution interface was increasing during the adsorption process. The negative values of DG revealed the sorption process was spontaneous, and the decrease of DG at higher temperature suggested that the higher temperature was beneficial to the U(VI) sorption. The values of DG for HCSs and HCSs PO 4-3 at 308 K were and kj mol -1, which indicated the adsorption of U(VI) on HCSs PO 4-3 is more favorable than HCSs. Effect of competing ions In order to investigate the sorption selectivity of pristine HCSs and HCSs PO 4-3 for uranium, co-existing ions such as Mg(II), Na(I), Zn(II), Mn(II), Ni(II) and Sr(II) were added into the system at the optimal conditions. The results in Fig. 11 showed that the chemical modification had Fig. 11 Selective sorption capacity of coexistent ions (ph = 6.0 for pristine HCSs and ph = 5.0 for HCSs PO 4-3, t = 60 min, C 0 = 10 mg L -1, V = 150 ml, W = 0.04 g and T = K) brought a distinct increase in adsorption capability of U(VI), but gave little change in other coexistent ions. Selectivity coefficient (S UO 2þ=Mnþ) for uranyl ions relative to competing ions was studied by the Eq. 8 2 [39]. S UO 2þ 2 =M nþ ¼ K UO 2þ 2 d K Mnþ d ; ð8þ where K UO2þ 2 d and Kd Mnþ are distribution coefficients of uranyl ion and competing cations, respectively. The relative selectivity coefficient S r is calculated according Eq. 9. S r ¼ S HCSs PO 4 3 : ð9þ S HCSs The resulting K d, S UO 2þ and S 2 =Mnþ r calculated from the batch sorption data of HCSs and HCSs PO 4-3 were listed in the Table 4. The selectivity coefficients of HCSs PO 4-3 was greater than that of HCSs, furthermore, the K d values of HCSs PO 4-3 calculated from the sorption data were up to 10, ml g -1 for U(VI) and fairly high for other coexistent ions. Therefore the selective properties of HCSs PO 4-3 toward U(VI) had been improved after chemical modification.

8 1486 J Radioanal Nucl Chem (2014) 299: Table 4 Selective adsorption properties of pristine HCSs and HCSs PO 4-3 Elements K d (ml g -1 ) S S r Pristine HCSs HCSs PO 4-3 Pristine HCSs HCSs PO 4-3 U(VI) , Mg(II) Na(I) Zn(II) Mn(II) Ni(II) Sr(II) phosphoric acid under mild condition. HCSs PO 4-3 shows a better adsorption capacity toward U(VI) as the monolayer adsorption capacity was as high as mg g -1 larger than the pristine HCSs (80.00 mg g -1 ), and the textural and surface chemical properties were characterized using Boehm titrations, SEM and FT-IR. The adsorption of U(VI) from aqueous solution on HCSs and HCSs PO 4-3 was fitted to the Langmuir adsorption isotherms and pseudo-second order kinetics models. The thermodynamic parameters (DH, DG and DS) were calculated, which demonstrate the adsorption of U(VI) on HCSs and HCSs PO 4-3 were endothermic and spontaneous. The selective adsorption properties of HCSs PO 4-3 toward U(VI) had been improved. Almost complete removal (=100 %) of U(VI) from the wastewater containing 15 mg L -1 was achieved with 12.0 g HCSs PO 4 in 1.0 L. Acknowledgments We gratefully acknowledge the financial support provided by National Natural Science Foundation of China (Grant Nos , ), National Undergraduate Training Programs for Innovation and Entrepreneurship, Key Project of Chinese Ministry of Education (Grant No ), the Young Scientists Training Program of Jiangxi Province (Grant No BCB23023), Natural Science Foundation of Jiangxi Province (Grant Nos BAB203002, 20122BAB203012), China Postdoctoral Science Foundation (Grant No ), and Project of Jiangxi Provincial Department of Education (Grant No. GJJ13452). Fig. 12 U(VI) ion removal from industry wastewater by HCSs PO 4-3 Test with simulated nuclear industrial effluent In order to demonstrate the adsorption potential of HCSs PO 4-3, experiments were performed in a simulated nuclear industrial effluent. The wastewater containing U(VI) (15 mg L -1 ), SO 4 2- (10 g L -1 ), NO 3 - (1 g L -1 ), Mg 2? (0.2 g L -1 ), Ca 2? (0.5 g L -1 ) and Fe 3? (1.6 g L -1 ) was prepared according to the composition of resin adsorption tail liquid from uranium hydrometallurgy plant. As shown in Fig. 12, the U(VI) removal increased with increasing HCSs PO 4-3 dosage, because at high dose of adsorbent, the available adsorption sites increase leading to enhanced adsorption [40]. And almost complete removal (=100 %) of U(VI) from the wastewater containing 15 mg L -1 was achieved with 12.0 g HCSs PO 4-3 in 1.0 L. Conclusion In the present work, the novel phosphorus-rich hydrothermal carbon spheres have been successfully synthesized via one-step hydrothermal carbonization of glucose and References 1. Praveen RS, Metilda P, Daniel S, Rao TP (2005) Talanta 67: Ganesh R, Robinson KG, Chu LL, Kucsmas D, Reed GD (1999) Water Res 33: Kulkarni PS, Mukhopadhyay S, Bellary MP, Ghosh SK (2002) Hydrometallurgy 64: Ladeira ACQ, Morais CA (2005) Miner Eng 18: Singh H, Mishra SL, Vijayalakshmi R (2004) Hydrometallurgy 73: Donia AM, Atia AA, Moussa EMM et al (2009) Hydrometallurgy 95: Rao TP, Metilda P, Gladis JM (2006) Talanta 68: Shawabkeh RA, Rockstraw DA, Bhada RK (2002) Carbon 40: Zhao Y, Liu C, Feng M, Chen Z, Li S, Tian G et al (2010) J Hazard Mater 176: Coleman SJ, Coronado PR, Maxwell RS, Reynolds JG (2003) Environ Sci Technol 37: Shao D, Jiang Z, Wang X, Li J, Meng Y (2009) J Phys Chem B 113: Schierz A, Zänker H (2009) Environ Pollut 157: Xu Y, Zondlo JW, Finklea HO, Brennsteiner A (2000) Fuel Process Technol 68: Wang Y, Zhang Z, Liu Y, Cao X, Liu Y, Li Q (2012) Chem Eng J 198: Nie B, Zhang Z, Cao X, Liu Y, Liang P (2013) J Radioanal Nucl Chem 295: Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J et al (1996) Science 273: Sun X, Li Y (2004) Angew Chem Int Ed 43:

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