Synthesis of Fe-MCM-41 with highly ordered mesoporous structure and high iron content and its adsorption isotherms of arsenate and arsenite

JOURNAL OF SCIENCE, Hue University, Vol. 69, No. 6, 2011<br /> <br /> SYNTHESIS OF Fe-MCM-41 WITH HIGHLY ORDERED MESOPOROUS<br /> STRUCTURE AND HIGH IRON CONTENT AND ITS ADSORPTION<br /> ISOTHERMS OF ARSENATE AND ARSENITE<br /> Le Thanh Son1 and Dinh Quang Khieu2<br /> 1<br /> 2<br /> <br /> Hue University<br /> <br /> College of Sciences, Hue University<br /> <br /> Abstract. In this paper, the synthesis of iron containing MCM-41 material (Fe-MCM-41)<br /> and its adsorption isotherms were investigated. Fe-MCM-41 materials were synthesized by<br /> direct process using K2[Fe(CN)6] or iron oxalate as iron source. The materials obtained<br /> were characterized by XRD, adsorption/desorption isotherms of nitrogen. The results<br /> showed that using K2[Fe(CN)6] as an iron source can provide Fe-MCM-41 with highly<br /> ordered mesoporous structure and high iron content with molar ratio of Si/Fe around 10.<br /> The isotherm study showed that the prepared Fe-MCM-41 sample exhibited high<br /> adsorption activity towards As(III) as well as As(V). The experimental data of adsorption of<br /> As(III) and As(V) onto Fe-MCM-41 followed the Langmuir models. The maximum<br /> monolayer adsorption capacities based on this model were 25.4 and 37.2 mg/g for As(III)<br /> and As(V), respectively.<br /> <br /> 1<br /> <br /> Introduction<br /> <br /> Arsenic is a highly toxic chemical constituent thereby posing epidemiological problems<br /> to human health. Serious arsenic pollution has been observed not only in various<br /> mineral and chemical processes but also in some sources of ground water. Most<br /> common arsenic species found in aqueous media are anionic species of arsenate and<br /> arsenite which exist as oxoanions [1]. Arsenic has been removed by the following<br /> processes: precipitation with lime, co-precipitation with ferric sulfate, alum<br /> precipitation and precipitation as the sulfide using either sodium sulfide or hydrogen<br /> sulfide. As there is no simple treatment for the efficient removal of arsentate (III), an<br /> oxidation step is necessary to provide acceptable results in arsenic elimination [2].<br /> Driehaus et al [3] reported the use of manganese dioxide as strong oxidants in the<br /> environment to remove As(III). Porous solids are used technically as adsorbents due to<br /> their high surface areas and pore spaces. The development of mesoporous molecular<br /> sieves including MCM-41, SBA-15, SBA-16… has attracted significant attention from<br /> a fundamental as well as applied perspective [4]. The extremely high surface area and<br /> tunable pore size of these materials should provide excellent adsorbents. In recent<br /> 87<br /> <br /> studies, several chelating polymers loaded with materials such as iron, copper, cerium<br /> were used as sorbents with high selectivity for the removal of arsenic anions [5]. The<br /> question arises as to whether the dispersion of nano iron particles over MCM-41 is<br /> expected to obtain the excellent adsorbent due to the connection between high surface<br /> area of MCM-41 and high affinity of iron toward ion arsenic.<br /> In the present paper, the synthesis of Fe-MCM-41 with high iron content and<br /> highly ordered mesoporous structure was performed. The adsorption isotherms of<br /> As(III) and As(V) over Fe-MCM-41 were also presented.<br /> <br /> 2<br /> <br /> Experiments<br /> <br /> Tetra ethyl orthor silicate (TEOS) and cetyl trimetyl ammonium bromide (CTAB)<br /> (Merck) were used to synthesize MCM-41. Fe(NO3)3.9H2O (Merck), acid H2C2O4.2H2O<br /> (Quangzu, China) and K4[Fe(CN)6] (Merck) were used as iron sources. As2O3 and<br /> Na2HAsO4.2H2O (Aldrich) were used as arsenic sources to investigate the adsorption.<br /> Fe-MCM-41 samples which were prepared from potassium ferocyanide<br /> K4[Fe(CN)6] had the initial gel with molar ratio of nTEOS : nCTAB : nNaOH : nH2O : nFe =<br /> 1,00 : 0,11 : 0,47 : 204,39 : x where the molar ratio of x = Si /Fe were 5, 10, 20, 50,<br /> 100. The samples obtained were denoted as FeCN5; FeCN10; FeCN20; FeCN50;<br /> FeCN100 in which the numbers indicate the value of x. Fe-MCM-41 sample which was<br /> prepared from iron oxalate had the initial gel composition with molar ratio of nTEOS :<br /> nCTAB : nNaOH : nH2O: nFe = 1,00 : 0,11 : y : 256,79 : 0,1 in which y is mole NaOH to<br /> obtain the pH value corresponding to 13,4; 13,5; 13,6; 13,7; 13,8. The pH values of the<br /> filtrates after the hydrothermal process were measured. The samples obtained were<br /> denoted as 9.5FeOx10, 10.0FeOx10, 10.4FeOx10, 12.0FeOx10, 12.2FeOx10 in which<br /> the numbers 9.5, 10, 10.4 and 12.0 indicate the value of pH and the number 10 shows<br /> molar ratio of Si to Fe.<br /> X-ray-diffraction (XRD) at low angle using 8D Advance Bruker, Germany was<br /> applied to determine mesoporous phase. The porous characterizations were estimated by<br /> nitrogen adsorption/desorption isotherms using Micromeritics. The iron in solid sample<br /> of Fe-MCM-41 was analyzed by spectrophotometer. The arsenic adsorption was carried<br /> out in batch condition. Oxoanion solution including As(III) and As(V) was prepared<br /> from As2O3 (by diluting in NaOH solution) and Na2HAsO4.2H2O. The concentrations of<br /> arsenic in these solutions were from 20-100 mg/L with pH in the range of 8-10. For<br /> each experiment, the volume of 100-200 mL oxoanion arsenic added into the amount of<br /> catalyst (100-250 mg of Fe-MCM-41) was stirred magnetically for 72 hours to reach<br /> adsorption equilibrium. The equilibrium adsorption capacity was calculated by<br /> following equation:<br /> <br /> 88<br /> <br /> qe <br /> <br /> (Co  Ce )V<br /> (mg g-1)<br /> m<br /> <br /> (1)<br /> <br /> where Co and Ce (mg.L-1) are the concentration of ion As(III) and As(V) at initial and<br /> equilibrium time; V(L) and m (g) are the volume of solution of ion As(III) or As(V), and<br /> mass of Fe-MCM-41, respectively.<br /> In the present work, two popular adsorption isotherm equations were applied,<br /> *The linear form of Langmuir adsorption isotherm is as follows [6]:<br /> <br /> 1<br /> 1<br /> 1<br /> <br /> <br /> qe qm K L Ce qm<br /> where qm is maximum monolayer adsorption capacity (mg/g);<br /> temperature (L.g-1)<br /> <br /> (2)<br /> L<br /> <br /> constant depending on<br /> <br /> *The linear form of Freudlich adsorption isotherm is as follows [7]:<br /> 1<br /> ln qe  ln K F  ln Ce<br /> n<br /> <br /> (3)<br /> <br /> where KF {(mg.g-1)(mgL-1)n} and n (dimetionless) Freudlich equation constants.<br /> The concentrations of ion As(III) and As(V) in aqueous solution were<br /> determined by Atomic Absorption Spectrometry (AAS) using Shimadzu AA6800.<br /> <br /> 3<br /> <br /> Results and Discussion<br /> <br /> The incorporation of iron into silica framework in alkaline medium possesses inherent<br /> difficulties due to rapid precipitation of iron ions disturbing the formation of<br /> mesoporous structures. Here, the retardment of precipitation of iron ions was controlled<br /> by the introduction of iron in iron complex e.g. iron oxalate or iron cyanide. Fig. 1a<br /> shows XRD patterns of Fe-MCM-41 synthesized by iron in iron cyanide with molar<br /> ratio of Si/Fe from 5 to 100. Most samples obtained exhibited the peak (100), (110), and<br /> (200) characteristic of hexagonal mesoporous structure. The ordered degree of<br /> mesoporous structure decreased steadily as the iron introduction increased. It is noted<br /> that XRD pattern of FeCN10 sample with high molar ratio of Si/Fe =10 still retains the<br /> symmetry of peak (100) indicating that FeCN10 sample possesses highly ordered<br /> mesoporous structure. It could be explained that stable iron complex bounding to CTAB<br /> dispered (dispersed?) highly onto framework. Fig. 1b shows XRD patterns of Fe-MCM41 with iron oxalate as iron source. For 10.0FeOx10 and 10.4FeOx10, the peak of (100)<br /> was observed clearly, however, peaks (110) and (200) were very weak. These results<br /> show that the samples obtained have less ordered mesoporous structure. The<br /> characteristic peaks were not observed for the remaining 9.5FeOx10; 12.0FeOx10;<br /> 12.2FeOx10. Hence, the existence of a certain range of pH is favorable for forming<br /> mesoporous phase. Outside this range of pH, the mesoporous phase could not be<br /> 89<br /> <br /> (200)<br /> <br /> (110)<br /> <br /> 200<br /> <br /> (100)<br /> <br /> obtained. In the present condition, 10 < pH < 10.4 is suitable for forming mesoprous<br /> structure.<br /> <br /> Intensity (cps)<br /> <br /> FeCN100<br /> FeCN50<br /> FeC N20<br /> FeCN10<br /> FeCN5<br /> 0<br /> <br /> 1<br /> <br /> 2<br /> <br /> 3<br /> <br /> 4<br /> <br /> 5<br /> <br /> 6<br /> <br /> 7<br /> <br /> 8<br /> <br /> 9<br /> <br /> 2  (degree)<br /> (a)<br /> <br /> 1800<br /> 1600<br /> 1400<br /> <br /> Intensity (cps)<br /> <br /> 1200<br /> 1000<br /> <br /> 12.2FeOx10<br /> <br /> 800<br /> <br /> 12.0FeOx10<br /> <br /> 600<br /> <br /> 10.4FeOx10<br /> <br /> 400<br /> <br /> 10.0FeOx10<br /> <br /> 200<br /> <br /> 9.5FeOx10<br /> <br /> 0<br /> 0<br /> <br /> 2<br /> <br /> 4<br /> <br /> 6<br /> <br /> 8<br /> <br /> 10<br /> <br /> 2 (degree)<br /> (b)<br /> <br /> Fig. 1. XRD patterns of Fe-MCM-41 syntheized from K2 [Fe(CN)6] (a) oxalate iron as iron<br /> source (b).<br /> <br /> The pH values of synthesized gels range from 13 to 14. The fact that after the<br /> hydrothermal crystallization process, the pH values increased significantly could be due<br /> to the formation of silica frameworks by condensation reaction as follows:<br /> <br /> SiO + HO SiO<br /> <br /> SiOSi<br /> <br /> + OH<br /> <br /> Hence, the pH value rises gradually during the reaction process. Simultaneously,<br /> a part of iron ions is hydrolyzed in solutions.<br /> Fe3  3H 2 O  Fe(OH )3  3H <br /> <br /> These protons provide the pH decrease in the synthesized process. If pH value of<br /> synthesized gel is too low TEOS is hydrolyzed to form H2SiO3. Whereas the pH value is<br /> too high, iron ions are precipitated rapidly to hinder the formation of highly ordered<br /> 90<br /> <br /> hexagonal mesoporous structure. According to Tuel et al [8] the less ordered<br /> mesoporous structure in iron containing mesoporous materials is because of the<br /> tetragonal of [FeO4]- consisting of two longer Fe-O bonds and two shorter Fe-O bonds.<br /> Both these factors might contribute to the disordered mesoporous structure of these<br /> materials.<br /> The porous properties of Fe-MCM-41 were investigated by the isotherms of<br /> nitrogen adsorption for MCM-41, FeCN10 và 10.4FeOX10. The parameters calculated<br /> from nitrogen adsorption isotherms are listed in Table 1. Specific surface area of parent<br /> MCM-41, SBET is 929,7 m2/g , porous volume by BJH is 0,84 cm3/g. The increasing SBET<br /> of Fe- MCM-41 in FeCN10 up to 1356,2 m2/g is due to the contribution of specific<br /> surface area of mesopore as well as very fine nano iron oxide particles. For 10.4FeOx10<br /> sample, SBET reduced significantly due to the partial collapse of mesopore. On the other<br /> hand, iron oxides blocking the porous system also caused the reducing specific surface<br /> area. All the porous volume of Fe-MCM-41 are larger than that of MCM-41. It could be<br /> explained by the fact that the pores are enlarged due to the replacement of ion Si (0,40<br /> A0) by Fe with longer radius (0,63 A0 ). Both Fe-MCM-41 samples prepared by two<br /> different iron sources possessed molar ratio of Si/Fe close to the value of 10 indicating<br /> that the iron in initial synthesized gel corresponded mainly to silica framework.<br /> Table 1. Data of d100, Dpore, SBET, Vpore, for MCM-41, FeCN10 and 10.4FeOx10<br /> <br /> Sample<br /> <br /> nSi/Fe*<br /> <br /> nSi/Fe**<br /> <br /> d100<br /> (Å)<br /> <br /> Dpore<br /> (Å)<br /> <br /> SBET<br /> (m2/g)<br /> <br /> Vpore<br /> (cm3/g)<br /> <br /> MCM-41<br /> <br /> ∞<br /> <br /> ∞<br /> <br /> 39,2<br /> <br /> 28,1<br /> <br /> 929,7<br /> <br /> 0,84<br /> <br /> FeCN10<br /> <br /> 10<br /> <br /> 9,2<br /> <br /> 40,3<br /> <br /> 31,6<br /> <br /> 1356,2<br /> <br /> 1,23<br /> <br /> 10.4FeOx10<br /> <br /> 10<br /> <br /> 9,7<br /> <br /> 34,7<br /> <br /> 57,02<br /> <br /> 115,3<br /> <br /> 0.50<br /> <br /> * Molar ratio of Si/Fe in synthesized gel; ** Molar ratio of Si/Fe in solid Fe-MCM-4;<br /> Dpore: porous radius calculated by BJH; Vpore: mesoporous volume; SBET: specific surface area<br /> by BET method.<br /> <br /> The FeCN10 sample was used for the purpose of adsorption due to better surface<br /> properties. Fig. 2 shows the isotherm modeling of arsenic adsorption by linear plots of<br /> Freundlich and Langmuir. The maximum adsorption capacities (qm) and other constants<br /> are shown in Table 2. Isortherm data are basic requirements for the design of adsorption<br /> reactors, moreover analysis of adsorption isotherm is important to develop an equation<br /> which accurately represents the results and which can be used for design purposes. The<br /> Langmuir model describes the isotherm of arsenic adsorption with high correlation<br /> coefficient (R2>0,94) and is better than the Freudlich model. In fact, the Langmuir<br /> equation is based on the assumption that maximum adsorption corresponds to a<br /> saturated monolayer of solute on the adsorbent surface, that energy of adsorption is<br /> 91<br /> <br />