STUDY ON THE IMPROVEMENT OF THE REDUCTION CAPACITY OF ACTIVATED CARBON FIBER

Transcription

1 STUDY ON THE IMPROVEMENT OF THE REDUCTION CAPACITY OF ACTIVATED CARBON FIBER Chen Shuixia, Zeng Hanmin Materials Science Institute, Zhongshan University, Guangzhou 51275, China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Zhongshan University, Guangzhou 51275, P. R. China Introduction Activated carbon fiber (ACF) is a novel porous materials with plentiful micropore and huge surface area, thus have high adsorption capacity. It has been widely used in recovery of organic solvents from atmosphere and in the thorough removal of trace amount of organic substances in water. Moreover, the functional groups on the surface of Activated carbon fiber also have strong reactivity. Our previous works [1~4] indicate that Activated carbon fiber can reduce Pt(IV) into lower valence ion, and reduce Pd(II), Ag(I), Au(III) ions into metallic elements. This reduction property enable activated carbon fibers to be used in the recovery of noble metal from wastewater, or in the extraction of gold or silver from ore leaching solution. In order to effectively recover or extract noble metals, it is important that to enhance the reduction capacity of activated carbon fiber and to improve the particle form of noble metal reduced and adsorbed on the surface of activated carbon fibers. The reduction adsorption of noble metal ions on activated carbon fibers is closely related to the surface chemical properties of ACF, solution properties, and some other substance present in solution. It has been reported that the presence of humic substances in water can change the surface charge density, thus affect the adsorption capacity of adsorbents. Therefore, proper surface chemistry and solution condition must be beneficial to the reduction capacity of ACFs. In this paper, three methods that may improve the reduction adsorption capacity of activated carbon fibers have been experimented. Experimental Sisal fiber was impregnated in 5%NaOH solution for 24 hours, then impregnated with 35 v% H 3 PO 4 or 35 wt% ZnCl 2 solution for 24 hours. The treated fiber was then heated under inert atmosphere to 85ºC and then kept at that temperature for a certain time; activated carbon fibers activated with phosphoric acid (HPSACF) or zinc chloride (ZCSACF) were therefore obtained. Another part of sisal fiber treated with NaOH was directly heated under inert atmosphere to 85ºC; while keeping at that temperature, a stream of steam was introduced into the oven for 9 min., another kind of activated carbon fiber activated with steam (SACF) was prepared. In order to modify the surface chemical properties, the above SACF was cut into short piece, and treated with H 2 O 2, KMnO 4, or HNO 3, respectively, and then the resulted ACFs, called modified ACFs, were washed with water to neutral ph and dried under vacuum. Nitrogen adsorption isotherms measured with ASAP adsorption meter at 77K were used to characterized the pore structure of ACFs. A Hitach S-52 scanning microscope was used to observe the surface structure of ACF and the distribution of silver particles. A RigaRu D/MAX-III X-ray diffractometer was used to study the crystalline structure of silver and ACFs. An Escablab MK II X-ray photoelectronic microscopy was used to determine the surface chemical structure. Reduction adsorption of Ag(NH 3 ) 2 A certain weights of ACFs were added into fixed amount of Ag(NH 3 ) 2 solution with ph9.7. After shaken at 298K for 24h., ACFs were filtered from the solution, the residual concentration of silver was measured by atomic absorb spectrum. The adsorption amount of silver on ACFs was calculated according to equation (1). ( ) q = c c v w (1) Coadsorption of Ag(NH 3 ) 2 and organic substances A solution containing mg/l Ag(NH 3 ) 2 and mg/l organic substance / (methylene blue, or aniline, p-nitrophenol) was prepared, a certain weight of ACF was added into fixed amount of the above solution. The adsorption procedure and silver measurement is the same as above. The residual concentration of organic substance was measured on a 756MC spectrophotometer. In all adsorption experiments, a blank without ACF was used as reference. Results and Discussion Effect of preparation process of ACFs on their reduction adsorption capacities Fig. 1 is reduction-adsorption isotherms of Ag(NH 3 ) 2 on three kinds of ACFs activated with different process. Under the same condition, the adsorption isotherm of SACF activated with steam is closed to a horizontal line with maximum reduction-adsorption 15mg Ag/g C. It means the

2 q (mg/g carbon) reduction-adsorption of silver on SACF achieved equilibrium at lower concentration. Variously, ZCSACF activated with ZnCl2 shows much greater reduction adsorption capacity than SACF, its adsorption is similar to type I isotherm with adsorption amount over 25mg Ag/g C. HPSACF activated with H3PO4 shows the strongest reduction-adsorption for Ag(NH3)2 among the three ACFs. Under same condition, its reduction adsorption amount is as high as 7mg Ag/g C, the adsorption isotherm is similar to a vertical line. Generally, SACF activated with steam has higher specific surface area than ZCSACF or HPSACF activated with chemicals. However, the adsorption of silver on ACF includes a reduction process[2-4]. Therefore, the adsorption capacity is determined not only by its surface area but also the surface chemical property. In fact, C-H and C-OH on ACF all participate in the reduction of Ag(NH3)2[5]. Those ACFs activated with chemicals may have more reactive group on their surface, and thus have much higher reduction-adsorption capacities ceq (mg/l) SACF ZCSACF HPSACF Fig. 1. Reduction-adsorption isotherms of Ag(NH3)2 on ACFs activated with different process (a) (b) (c) Fig. 2. Silver particles on (a) SACF, (b) ZCSACF, and (c) HPSACF As shown in Fig. 2, Ag(NH3)2 was reduced to metallic silver particles, and adsorbed on the surface of ACFs. From SEM photos, it can be observed that silver particles varied in size and form on ACFs activated with different process. Silver particles on SACF are much finer than those on ZCSACF or HPSACF. Silver particles on ZCSACF were brokenly distributed in larger blocky form; and silver particles on HPSACF were the biggest, and much thicker. The above results indicate that activation process with zinc chloride or with phosphoric acid not only can enhance the reduction-adsorption capacities of resulted ACFs but also can improve the physical state of silver particle on ACF surface, which will be more beneficial to the recovery of silver from solution. Effect of chemical modification of ACFs on their reduction capacities Fig. 3 shows the nitrogen adsorption isotherms on ACFs modified with H2O2, KMnO4, and HNO3. All of them are BDDT type I isotherm, which indicates all modified ACFs remain the characteristics of microporosity. Their specific surface areas were measured with Langmuir method (p/p=.5-.3) and BET method (p/p=.5-.3); total pore volume was calculated based on the nitrogen adsorption amount at relative pressure p/p=.95. The results are shown in Table 1. The results indicate that specific surface area and pore volume decrease with the increase of oxidizing power of modification agents. The concentrated HNO3 modification make ACF greatly decrease in surface area and pore volume. The surface chemical property of modified ACFs was characterized by X-ray photoelectronic spectroscopy (XPS). The C1s spectra were fitted using the non-linear least-squares algorithm assuming a Lorentz peak shape. It has been resolved into four component peaks corresponding to graphitic carbon (bonding energy BE=284.6eV), C-OH group (BE=286.1eV), C=O group (BE=287.6eV), and COOH group (BE=289.1eV). The relative content of these groups was shown in table 2. Generally, the percentage of carboxyl group increases after oxidation modification.

3 vol of N 2 adsorbed (mg/g) modified with H 2 O 2 increased to over 55mg Ag/g C. It would be concluded that the oxidizing modification has created more oxidative groups or active sites in favor of silver ion reduction. These new group increases the reduction capacities of ACF under suitable conditions P/P Fig.3 N2 desorption isotherms of ACFs at 77K SACF SACFHO SACFMn SACFMA SACFNO1 SACFNO Tab.1 Some Characteristics of modified ACFs Name modif. reagent S L S BET V t (m 2 /g) (m 2 /g) (ml/g) SACF virgin SACFHO H 2 O SACFMA KMnO H 2 SO 4 SACFMn KMnO SACFNO1 dilut HNO SACFNO6 concentr. HNO Table 2 Oxygen-containing groups on the surface of ACFs derived from XPS analysis groups from C1s fitting C/O (at%) sample C-OH -C=O O-C=O atomic ratio SACF SACFHO SACFMn SACFMA SACFNO SACFNO the O1s spectra also show the change of chemical group species on ACF surface (Fig.4). O1s peaks of ACFs treated with KMnO 4 shift to higher bonding energy (534eV); and O1s peak of ACFs treated with HNO 3 become narrow, and fall between bonding energy eV. Fig. 5 is reduction adsorption isotherms of these modified ACFs. The results show that the oxidation modification of ACF greatly enhances the reduction capacities of ACF. The maximum reduction amount of Ag(NH 3 ) 2 on virgin SACF was 25 mg Ag/g C, but the reduction adsorption amount of all modified SACFs are much higher than 25 mg Ag/g C. The reduction amount of Ag(NH 3 ) 2 on SACFNO6 modified with concentrated HNO 3 and on SACFHO q (mg Ag/g C) binding energy (ev) Fig.4 XPS spectra of O1s on the ACF surface 1 SACF, 2SACFHO, 3SACFMn, 4SACFMA, 5SACFNO1, 6SACFNO of Ag (mg/l) SACF; SACFHO; SACFMA; SACFMn; SACFNO1; SACFNO6 Fig.5 Reduction-adsorption isotherms of Ag(NH 3 ) 2 on modified ACFs Effect of organic substance on the reduction of Ag(NH 3 ) 2 on ACFs As indicated above, the reduction ability of ACFs is closely related to their surface chemical properties. Factors that determine the reduction capacities of ACFs may include the species of surface chemical groups, ph of ACF surface, electric charge density, and the interface properties of ACF surface and the solution. The modification of surface can enhance the reduction capacities, thus the loading of some organic substance onto ACFs may also change the chemical properties of ACF surface, and will affect the reduction characteristics of ACFs. Fig. 6 shows the reduction adsorption isotherms 3 4

4 of Ag(NH 3 ) 2 on SACF with and without the presence of methylene blue. The results show that the reduction capacities of Ag(NH 3 ) 2 on SACF were increased 5 times at the presence of methylene blue. Evidently, methylene blue greatly affects the reduction-adsorption tendency of Ag(NH 3 ) 2 on SACF. SACF pre-adsorbing methylene blue also shows higher reduction capacities for Ag(NH 3 ) 2 than virgin SACF. The reduction capacities of SACF that pre-adsorbed mg /g of methylene blue in advance was increased by 3 times. with methylene blue redox-ads. q (mg/g) 7 of Ag (mg/g) (a)sacf q (mg/g carbon) without methylene blue redox-ads. q (mg/g) (mg/l) of Ag (mg/g) Fig. 6. Adsorption isotherms of Ag(NH 3 ) 2 on SACF Additionally, two other organic substances, aniline and p-nitro-phenol (PNP) were loaded onto different ACFs activated with different process. Fig.7 shows their reduction adsorption isotherms. The reduction capacities of SACF loaded PNP for Ag(NH 3 ) 2 had somewhat of increase, but the increment did not excess 5 mg/g. Distinctly, HPSACF pre-adsorbed PNP had a great increase in reduction capacity, it can reduce and adsorb almost all Ag(NH 3 ) 2 in the solution, that is, the residual concentration of Ag(NH 3 ) 2 in solution is close to zero (Fig. 7b). After loading aniline, both SACF and HPSACF have great increase in reduction capacity. The reduction capacity of SACF for Ag(NH 3 ) 2 was increased from mg Ag/g C without pre-adsorbing aniline to 7 mg Ag/g C with pre-adsorbing aniline. The reduction capacity of HPSACF was also great enhanced by aniline. After reduction adsorption, the surface of HPSACF fiber was coated with a layer of silver, and whole fiber shows metallic luster. (b)hpsacf Fig. 7. Effect of organic substance on the reduction capacity of ACFs aniline PNP virgin ACF Conclusion The studies in this paper show that some proper treatment can effectively enhance the reduction-adsorption capacities, or can improve the physical state of metal particles adsorbed on ACFs. Firstly, the preparation process of activated carbon fibers has significant influence on the reduction-adsorption capacities of ACF for silver ion. Those ACFs activated with phosphoric acid or zinc chloride have much higher reduction capacities. Secondly, the surface modification of ACFs with inorganic oxidant such as nitric acid, potassium permanganate, or hydrogen peroxide can create more active groups on the fiber surface, which greatly enhance the reduction-adsorption capacity SACF activated with steam for Ag(NH 3 ) 2. Again, organic substance such as methylene blue, aniline, or p-nitro-phenol, present in solution or adsorbed on ACFs significantly increase the reduction capacities of sisal based ACF for silver ion. References 1. Zeng Hanmin, Lu Yun, Zhu Shiping, China

5 patent Ruowen Fu, Hanmin Zeng, Yun Lu, The reduction property of activated carbon fibers[j]. Carbon, 1993, 31(4), 189~ Ruowen Fu, Hanmin Zeng, Yun Lu, The reduction of Pt(IV) with activated carbon fibers-an XPS study[j]. Carbon, 1995, 33(5): 657~ Wang Qiang, Fu Ruowen, Lu Yun, Zeng Hanmin, Adsorption of Pd on activated carbon fiber. Chinese J. Materials Research 1997,11(5): 515~ Fu Ruowen, Zeng Hanmin, Investigation of redox mechanism of activated carbon fiber. Chinese Synthetic Fiber Industry, 199,13(3):31-37 Acknowledgement This work is financed by National 863 Project, and by Doctorial Education Foundation of Education Ministry.