Polysaccharide Hydrolysis and Metallic Impurities Removal Behavior of Rice Husks in Citric Acid Leaching Treatment

Transcription

1 Transactions of JWRI, Vol.3 (9), No. Polysaccharide Hydrolysis and Metallic Imprities Removal Behavior of Rice Hsks in Citric Acid Leaching Treatment UMEDA Jnko*, IMAI Hisashi * and KONDOH Katsyoshi ** Abstract Rice hsk is one of the representative non-eatable biomasses, and contains 7~75 mass% organics sch as celllose and hemi-celllose and 15~ mass% silica (SiO ) elements. From a viewpoint of the wide se of the biomass energy, its cost redction is strongly reqired by selecting sitable raw materials as biomass fels and optimizing the operating conditions. In the present stdy, rice hsk was selected as inpt raw fel materials becase after extracting the above organics as the biomass energy from rice hsks, their resides consisting of high-prity amorphos silica wold be an indstrial resorce with added vale, and reslted in the total cost redction of the biomass energy. For high-prification of silica materials originating in rice hsk resides, the citric acid leaching treatment and the following water rinsing process of rice hsks were sed to remove metallic imprities sch as Na, K, Ca, Mg, Fe, C, etc. In this stdy, the optimization of the operating parameters sch as the concentration and temperatre of citric acid soltion, the stirring time in the acid leaching and water rinsing treatment, and combstion temperatre were examined ot for the removal of metallic imprities and the hydrolysis reaction of the polysaccharides. XRF and ICP analysis were sed to qantitatively evalate the residal content of imprities and carbon elements. GC-MS and FT-IR analysis of rice hsks after leaching were condcted to examine their hydrolysis behavior. KEY WORDS: (rice hsk) (silica) (hydrolysis) (polysaccharide) (monosaccharide) (metallic imprity) 1. Introdction Rice hsks and straws are representative agricltral wastes, and have an annal yield of abot 17 million tons in the world in 1). At this moment, most of them are of no practical se, and case environmental problems. For example, the combstion of 1 ton of rice hsks in the field cases abot.15 kg CO gas emission. Frthermore, they release.9kg methane (CH ) gas when left and decomposed on the grond ). From a viewpoint of the rese of rice hsks and straws as fel resorces, It is reported that they contain abot 5~75 mass% polysaccharide organics sch as celllose and hemi-celllose 3), which are sitable resorces for energy generation, 5). That is, rice hsks and straws cold be sed as bio-mass materials. Frthermore, one of their featres is a large content of amorphos silica (abot 1~ mass%) 3, ), and sggests a possibility to rese sch silica originated in rice hsks as raw materials, for example fertilizer, reinforcement of concretes, and ceramics materials 7~9). Many previos stdies have developed some processes to prepare silica materials by combsting rice hsks 1~13). From a prification of silica originated in rice hsks, a strong acid soltion washing Received on December 1, 9 * Specially Appointed Researcher ** Professor process was sed to remove the alkali metal imprities (Na and K) from the hsks 1~1), becase they have a etectic reaction with SiO, casing a drastic decrease of its melting point from 19K to 1K 17, 1). As a reslt, a large amont of carbon remained in the rice hsk ashes, and the silica prity was 9~9% or less. The athors have also established an environmentally benign process to prodce rice hsk silica with completely amorphos strctres and high-prity of 99% and over by sing citric acid soltion leaching treatment before combstion 19, ). In the present stdy, from a view point of high prification of amorphos silica, the optimization of the process parameters in the acid leaching treatment and combstion of rice hsks was examined. In particlar, the effect of the citric acid concentration on the hydrolysis behavior of organics was investigated by sing GC-MS analysis, and the removal mechanism of metallic imprities was also discssed.. Experimental Inpt raw materials are rice hsks harvested in Niigata to prodce high-prity amorphos silica materials. The previos stdy clarifies that the citric acid soltion leaching treatment and air combstion of rice hsks are Transactions of JWRI is pblished by Joining and Welding Research Institte, Osaka University, Ibaraki, Osaka 57-7, Japan 13

2 Polysaccharide Hydrolysis and Metallic Imprities Removal Behavior of Rice Hsks in Citric Acid Leaching Treatment sefl for the removal of the metallic imprities, in particlar alkali metal elements, contained in rice hsks 19, ). 3g of rice hsks were pt into 5ml citric acid soltion in a Griffin beaker. In the present stdy, the concentration and temperatre of the citric acid soltion, and stirring time in the soltion were selected as the operating parameters. This is becase the chelate reaction 1) between carboxyl grops (-COOH) and metallic imprities contained in hsks strongly depends on the above parameters. The hydrolysis of polysaccharides sch as celllose and hemi-celllose is also dependent on them. The concentration was controlled from 1% to 7% by changing the mixing ratio of citric acid powders (Kishida Chemical Co., prity; 99.5%) and distilled water. The beaker was placed on the stirrer (AS ONE, hot stirrer, HS-5BHSD), and the soltion temperatre was kept at 9K ~ 353K. The rotating speed of the magnetic stirrer bar in the soltion was controlled at 9 rpm, and the stirring time was 9 s~7. ks. After the acid leaching process, the water rinsing treatment of the rice hsks was carried ot in distilled water at 93K for 9s to remove the citric acids from the hsks. The stirrer was also sed in the water rinsing, and the magnetic bar was rotated nder 9 rpm speed. The materials were dried at 373K for 3. ks in the mffle frnace in air atmosphere, and then brned at 173K for 1. ks in the same frnace, where the air flow rate was controlled at. ml/s by sing a small air-compressor. The brning temperatre of 173K was applied in this stdy to prevent the crystallization of amorphos silica contained in the hsks 1). The content of each metallic oxide of rice hsk ashes was measred by X-Ray Florescence Spectroscopy Table 1 Qantitative analysis of silica (SiO ) and imprities contents inclded in rice hsk ashes by employing citric acid leaching with varios concentrations (leaching temperatre; 33K, soaking time; 3.ks, water rinsing time; 9s, air-brning conditions; 173K 1.ks). (XRF, PANalytical, X-ray spectrometer PW), and carbon analyzer (HORIBA, EMIA-9V) was employed to measre the carbon content of the ashes. Forier Transform-Infrared Spectrometer (FT-IR, Nicolet, MagnaIR-5 with Dra-ATR, Sens-IR) analysis was applied to the ashes to investigate the strctres remaining from the organic elements and silica materials. In order to qantitatively evalate the hydrolysis behavior of the polysaccharides of rice hsks by the citric acid leaching treatment, Gas Chromatograph Mass Spectrometer (GC-MS, Agilent Technologies, Agilent-5973N) was applied to the acid-leached specimens after air drying at 373K. The heat treatment was carried ot at 73 K for 3 s before GC-MS analysis ). The hydrolysis behavior of the organics cold be investigated by comparing the identification of the mass spectra and their intensities, and the effect of the citric acid concentration on the polysaccharide hydrolysis was discssed. 3. Reslts Table 1 shows the chemical compositions of rice hsk ashes via the citric acid soltion leaching treatment with different concentrations. The soltion temperatre of 33K and stirring time of 3. ks were sed in the above leaching process. % indicates the se of hot distilled water rinsing at 33K instead of the citric acid soltion. As shown in Table 1, when employing the citric acid soltion with a concentration of 1% and over, the silica (SiO ) prity of each ash is approximately 99.5% or more. It means the prification of this silica material obviosly progresses compared to that with a prity of 97.5% via the hot distilled water leached treatment. Concerning the content of alkali metal elements (Na and K) remained in the ashes, the se of the citric acid soltion is remarkably effective to redce them. In particlar, Na O has been completely removed from the rice hsks by sing the Concentration of citric acid soltion 1 1 (a) %-5 1 Frfral Retention time, t /min 1 1 (b) 1%-5 1 Frfral Levoglcosan Retention time, t /min Frfral (c) 3%-5 1 Frfral (d) 7%-5 Levoglcosan Levoglcosan Retention time, t / min Retention time, t / min Fig. 1 GC-MS analysis reslts of rice hsks via citric acid soltion leaching treatment sing varios soltion concentrations of % (a), 3% (b), 5% (c), and 7% (d) (soltion temperatre; 33K, pre-heating condition; 73K for 3s). 1

3 Transactions of JWRI, Vol.3 (9), No. Specific ion intensity of f rfral, mg - 1 ( 1 ) Pre-heating temperatre of specimens; 73K Table Qantitative analysis of silica (SiO ) and imprities contents inclded in rice hsk ashes by employing citric acid leaching with varios temperatres (citric acid soltion concentration; 5%, soaking time; 3.ks, water rinsing time; 9s, air combstion conditions; 173K 1.ks). Citric acid soltion temperatre Concentration of citric acid soltion (%) Fig. Specific ion intensity of frfral spectrm in GC-MS reslts of rice hsks via citric acid leaching treatment as a fnction of acid soltion concentration. citric acid soltion leaching with 1% or more concentration. The content of MgO and MnO also is extremely redced, and the redction rate is 5~93% by comparing the remained content via citric acid soltion and hot distilled water leaching treatment. It is also obvios that the citric acid soltion leaching treatment is necessary for the removal of Ca elements from rice hsks. However, its redction rate of abot 5~7% is smaller than those of MgO and MnO as mentioned above. Figre 1 indicates GC-MS analysis reslts of air-dried rice hsks via the citric acid leaching and the following water rinsing treatment. The concentration of the citric acid soltion sed in this analysis is % (a), 1% (b), 3% (c) and 7% (d). The spectrm corresponding to frfral, which is a monosaccharide formed from hemi-celllose elements via the hydrolysis and dehydration reaction ), is detected in the citric acid leached hsks (b) ~ (d) except for the hot water leaching (a). It also reveals no obvios difference of the spectrm intensity between 1% and 7% concentration. Figre shows the dependence of the ion intensity of frfral spectrm on the concentration of the citric acid soltion. When sing 1% citric acid soltion, the ion intensity is.1 1 (mg -1 ), and abot times that in the se of the hot distilled water shown in Fig.1 (a). It slightly increases with increasing the concentration from 1% to 7%. These reslts sggest that the citric acid soltion leaching treatment with 1% concentration is enogh for hydrolysis of hemi-celllose contained in rice hsks to frfrals of monosaccharides. The previos stdy shows that the metallic imprities sch as Na, K, Ca, Mg, Fe, Al and Mn are contained in the rice hsk organics consisting of hemi-celllose and celllose 3). When hydrolysis reaction of the polysaccharides progresses by the citric acid leaching treatment, the simple strctres of monosaccharides (frfral and levoglcosan) are formed in the rice hsks ). As a reslt, this reaction brings carboxyl grops into contact with the above imprities, and their metal complexes are synthesized and discharged into the soltion. Therefore, as shown in Table 1, the content of each metallic imprity oxide of the ashes are drastically redced by the citric acid soltion leaching treatment. In addition, it is obvios the redction of alkali metals of Na and K is effective to decrease the carbon content of the ashes. This is becase a decrease of their imprities prevents a etectic reaction of SiO accompanied by the drastic redction of SiO melting temperatre from 19K to 1K, and no melting of silica of rice hsks happens dring combstion at 173K. In conseqence, the organics do not remain in the silica materials, and most of them are completely brned. Table shows the chemical compositions of rice hsk ashes via the citric acid soltion leaching treatment nder different soltion temperatre. The soltion concentration of 5% and stirring time of 3. ks were sed. With increase in the temperatre, most of the imprities are removed, and the prification of rice hsk silica (SiO ) with 99.77% takes place via the citric acid soltion leaching at 353K. In the case of Na O, even the room temperatre leaching treatment is enogh to remove Na imprities from rice hsks. On the other hand, CaO content of the ashes gradally decreases with increasing the soltion temperatre. Figre 3 (a) indicates the Ca content in the sed citric acid soltion after the leaching treatment as a fnction of the soltion temperatre. The CaO content of the ashes shown in Fig.3 (b) is originates from Table 3. The content of Ca elements discharged into the acid soltion dring acid leaching process gradally increases with increasing the temperatre. By sing these measrements, a discharged rate of Ca element from rice hsks by the citric acid leaching is calclated. In this calclation, it is assmed that Ca is removed by only the acid leaching treatment, not the 15

4 Polysaccharide Hydrolysis and Metallic Imprities Removal Behavior of Rice Hsks in Citric Acid Leaching Treatment Ca content in citric acid soltion, mg litter -1 CaO content of ash (mass%) (a) Citric acid concentration; 5% Stirring time; 3. ks (b) Citric acid soltion temperatre, T / K Fig. 3 Ca content in citric acid soltion (a) and CaO content of ash (b) dependence on citric acid soltion temperatre (citric acid soltion concentration; 5%, stirring time; 3. ks, air combstion temperatre; 173K). Table 3 Dependence of discharged rate of Ca from rice hsks into citric acid soltion on soltion temperatre (concentration of citric acid soltion; 5%, stirring time; 3. ks). Temperatre, T/ K Discharged rate (%) following water rinsing process. That is, the original Ca content of rice hsks is the sm of the Ca content in the brned ashes and the citric acid soltion. As shown in Table 3, the discharged rate of Ca element is 3% nder the ambient temperatre acid leaching treatment (9K). It sddenly increases to 7% when the hot acid soltion of 313K is employed. Frthermore, an increase in the soltion temperatre remarkably increases the discharged rate of Ca elements. This means that both of the Ca complexes formated by the chelate and hydrolysis reaction of polysaccharides progress more effectively in the hot citric acid soltion, and reslt in discharging the complexes from the monosaccharides. The effects of the stirring time dring the citric acid leaching and the following water rinsing treatment on the CaO content of the rice hsk ashes was evalated. The 5% citric acid soltion of 33K was sed in the acid leaching process. As shown in Fig., both the above parameters are sefl to redce the CaO imprity content. In particlar, the citric acid leaching time is mch dominant on the redction of Ca element. Accordingly, not only the chelate reaction between Ca elements and carboxyl grops, bt also the polysaccharide hydrolysis dring the acid leaching treatment dominate the prification of SiO of the rice hsk ashes. CaO content of ash (mass%) Stirring time in citric acid leaching, T acid leaching = 3s 9s 1.ks 3.ks 7.ks Stirring time in water rinsing treatment, t / ks Fig. CaO content of rice hsk ashes via citric acid leaching with varios stirring time as a fnction of water rinsing time (citric acid soltion; 5% and 33K, water rinsing at 9K, air combstion conditions; 173K 1.ks). Carbon content of ash (mass%) Citric acid leaching conditions - Concentration; 5% - Temperatre; 33K - Stirring time; 3. ks Water rinsing conditions - Temperatre; 9K - Stirring time; 9s Combstion temperatre, T / K Fig. 5 Dependence of carbon content of rice hsk ashes on combstion temperatre (citric acid soltion concentration; 5%, temperatre; 33K and stirring time; 3. ks). The previos stdies have mentioned the effect of the combstion temperatre on the crystal strctre of the silica materials contained in rice hsk ashes, 5). In general, the temperatre of 973K and over in brning rice hsks cases the crystallization of SiO in the ashes when raw rice hsks withot any leaching treatments are sed as starting materials. However, in the se of those via the citric acid leaching treatment, SiO crystallization temperatre drastically increases from 973K to 133K. This is becase a etectic phenomenon of SiO is prevented by redcing alkali metal imprities of the starting materials as mentioned above. At the same time, the combstion temperatre also affects the remaining 1

5 Transactions of JWRI, Vol.3 (9), No. Absorbance (a) 3 171cm cm cm -1 Rice hsk silica Mineral silica 15 Wave nmbers (cm -1 ) 1 Absorbance.3... (b) 151cm -1 Si-O-Si -CH.1 -OH 9cm cm cm -1. 5cm Wave nmbers (cm -1 ) 795cm -1 1 Absorbance (c) 159cm cm -1 9cm -1 13cm -1 Fig. FT-IR spectra of rice hsk ash brned at 173K (a) and 73K (b), compared to commercialized mineral silica particle and raw materials (c) Wave nmbers (cm -1 ) 1 carbon content of the rice hsk ashes. The dependence of the carbon content of the ashes on the combstion temperatre is shown in Fig.5. The content was measred by ICP analysis. 5% citric acid soltion was sed, and the stirring temperatre and time dring the acid leaching treatment were 33K and 3. ks, respectively. It is obvios that the remaining carbon content remarkably decreases from.17 mass% to.~.3 mass% by changing the combstion temperatre from 73K to 173K and over. In additions, there is no significant difference of the carbon content by brning the hsks over 173K. The previos stdy indicates the exothermic heat of DTA profiles of rice hsks de to the organics sch as celllose and hemi-celllose is completely finished at 973K when the rice hsks via the citric acid leaching treatment is sed. That is, the thermal resoltion of most of the organics progresses over 973K. Figre indicates the reslts of FT-IR analysis of rice hsk ashes combsted at 173K (a) and 73K (b). The infrared spectrm of the commercial mineral silica powders and raw rice hsks is also shown in (a) and (c), respectively. The infrared spectra at 151 cm -1 and 791 cm -1 de to Si-O-Si stretching modes ) are detected in (a), and correspond to those of the conventional mineral silica materials. No spectrm of abot 337~5 cm -1 de to the polysaccharides, 7) of raw rice hsks is observed. This reslt means very little carbon originating from the celllose and /or hemi-celllose elements remains in the ashes after combstion at 173K. This corresponds well to the carbon measrement by ICP analysis shown in Fig.5. On the other hand, the specimen brned at 73K shown in (b) indicates not only 151 and 791 cm -1 spectra (Si-O-Si bond) bt also small spectra at 335, 9 and 5 cm -1 corresponding to OH stretching mode and CH stretch vibrations 7), which are also detected in the raw rice hsks (c). This means a small amont of the original organics still remains in the ashes after combstion at 73K, and reslts in the increase of the carbon content at 73K as shown in Fig.5. Accordingly, the combstion temperatre of 173~173K is sitable for the preparation of high-prity amorphos silica materials originated from rice hsks via the citric acid leaching treatment.. Conclsion For a high prification of amorphos silica originating in rice hsks, optimization of the operating parameters in the citric acid leaching treatment and air combstion of rice hsks was examined. When sing the citric acid soltion with a concentration of 1% or more, ICP analysis indicated that the alkali metal imprities cold be remarkably redced from rice hsks by the acid leaching treatment. In particlar, alkali metal elements of Na and K were completely removed. GC-MS analysis showed the progress of the hydrolysis reaction of their hemi-celllose dring the leaching process. As a reslt, the remaining carbon content in rice hsk ashes was remarkably redced to.~. mass% after combstion at 173~173K. The removal ability of Ca element of rice hsks dring the citric acid leaching treatment depended on not only the concentration bt also the soltion temperatre and stirring time. This was 17

6 Polysaccharide Hydrolysis and Metallic Imprities Removal Behavior of Rice Hsks in Citric Acid Leaching Treatment becase the monosaccharide formation via hydrolysis of carbohydrates cased a reactivity of Ca elements with carboxyl grops to synthesize their metal complexes dring the hot acid leaching treatment. The combstion temperatre at 173K or more was enogh to thermally remove the original carbohydrates from rice hsks, and reslted in a very small carbon content of.~.3 mass%. On the other hand, when the combstion temperatre of 73K was employed, FT-IR analysis showed CH and OH stretching modes with 335~5 cm -1 spectra originated in the carbohydrates of the rice hsks, and their ashes with.17 mass% carbon were obtained. Under the sitable acid leaching and air combsting conditions, high-prity amorphos silica materials with 99.5~99.77 mass% were prodced from rice hsks. Acknowledgements A part of this stdy was financially spported by The Japan Secrities Scholarship Fondation in. References 1) The state of food and agricltre in Asia and the Pacific region, Regional Office for Asia and the Pacific, Food and Agricltre Organization of the United Nations (FAO). ) D. L. Klass, Biomass for Renewable Energy, Fels, and Chemicals, Academic Press (199). 3) Aristos Aristido and Merja Penttilä, Metabolic engineering applications to renewable resorce tilization, Biochemical engineering, 11 (), ) Ankr Jain, T. Rajeswara Rao, S. S. Sambi, P. D. Grover, Energy and chemicals from rice hsk, Biomass and Bioenergy, 7 (199) ) M. Fang, L. Yang, G. Chen, Z. Shi, Z. Lo, K. Cen, Experimental stdy on rice hsk combstion in a circlating flidized bed, Fel Processing Technology, 5 () ) N. Yalçin, V. Sevinç, Stdies on silica obtained from rice hsk, Ceramics International, 7 (1) ) V. P. Della, I. Kühn, D. Hotza, Rice hsk ash as an alternate sorce for active silica prodction, Materials Letters, 57 () 1-1. ) Gemma Rodrígez de Sensale, Strength development of concrete with rice-hsk ash, Cement and Concrete Composites, () ) R.V. Krishnarao, M.M. Godkhindi, Distribtion of silica in rice hsks and its effect on the formation of silicon carbide, Ceramics International, 1 (199) ) A. Chakraverti, P. Mishra and H. D. Banergee, Investigation of combstion of raw and acid-leached rice hsk for prodction of pre amorphos white silica, Jornal of Materials Science, 3 (19) ) U. Kalapathy, A. Proctor and J. Shltz, A simple method for prodction of pre silica from rice hll ash, Bioresorce Technology, 73 () ) M. Patel, A. Karera, and P. Prasanna, Effect of thermal and chemical treatments on carbon and silica contents in rice hsk, Jornal of Materials Science, (197) ) M. Rozainee, S.P. Ngo, A.A. Salema, K.G. Tan, M. Ariffin, Z.N. Zainra, Effect of flidising velocity on the combstion of rice hsk in a bench-scale flidised bed combstor for the prodction of amorphos rice hsk ash, Bioresorce Technology, 99 () ) Tzong-Horng Lio, Shao-Jng W, Characteristics of microporos/mesoporos carbons prepared from rice hsk nder base- and acid-treated conditions, Jornal of Hazardos Materials, 171 (9) ) A. Chakraverti, P. Mishra and H. D. Banergee, Investigation of combstion of raw and acid-leached rice hsk for prodction of pre amorphos white silica, Jornal of Materials Science, 3 (19) 1-. 1) I. A. Rhaman, J. Ismail and H. Osman, Effect of nitric acid digestion on organic materials and silica in rice hsk, Jornal of Materials Chemistry, 7 (1997) ) Elena Yazhenskikh, Klas Hack, and Michael Mller, Critical thermodynamic evalation of oxide systems relevant to fel ashes and slags. Part 1: Alkali oxide silica systems, Compter Copling of Phase Diagrams and Thermochemistry, 3 () ) Werner E. Haltera and Bjbrn O. Mysen, Melt speciation in the system Na O SiO, Chemical Geology, 13 () ) J. Umeda, K. Kondoh and Y. Michira, Process Parameters Optimization in Preparing High-Prity Amorphos Silica Originated from Rice Hsks, Materials Transactions, (7) ) J. Umeda, K. Kondoh, High-prity amorphos silica originated in rice hsks via carboxylic acid leaching process, Jornal of Materials Science, 3 () ) N. Norman and A. Earnshaw, Chemistry of the Elements (nd ed.), Oxford, Btterworth-Heinemann (1997). ) Badal C. Saha, Hemicelllose bioconversion, Jornal of Indstry of Microbiology Biotechnology, 3 (3) ) Lio, Tzong-Horng.. Preparation and Characterization of Nano-Strctred Silica from Rice Hsk, Materials and Science Engineering A3 () ) D. M. Ibrahim, M. Helmy, Crystallite growth of rice hsk ash silica, Thermochimica Acta, 5 (191) ) Y. Shinohara and N. Kohyama, Qantitative analysis of tridymite and cristobalite crystallized in rice hsk ash by heating, Indstrial Health, () ) Eliane Ayres; Wander Liz Vasconcelos; Rodrigo Lambert Oréfice, Attachment of inorganic moieties onto aliphatic polyrethanes, Materials Research, 1 (7), ISSN , doi: 1.159/S ) S. Wang, C. Sng, Florescence and IR Characterization of Cre in Polyrea, Polyrethane, and Polyrethane-Urea, Macromolecles, 35 () 3-. 1