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ABSTRACT Giant reed (Arundo donax) is a perennial, gramineous plant that has high biomass production potential. In this study, we focused on its applicability as a raw material for the production of adsorbents for the purification of cadmium-contaminated water. Charcoals were prepared from the stalk of the giant reed at various temperatures (400 - 700°C) under nitrogen stream. Analysis of pore size distribution based on methanol vapor adsorption revealed that the mesopores within the range of 20 to 100 Å were abundant in charcoals prepared under 400°C and 500°C. Using these charcoals, the removal capacity of cadmium from aqueous solution was investigated. As a result, high removal capacity under low concentration of cadmium was observed. High yield of cadmium with 0.01 N hydrochloric acid treatment was also clarified. X-ray diffraction (XRD) analysis revealed that Cd(OH)2 existed on the surface of the cadmium-adsorbed charcoal. The possible mechanism of the “apparent adsorption” was discussed
Journal of Water and Environment Technology, Vol. 8, No.4, 2010 Address correspondence to Masaki Sagehashi, Center of Education for Leaders in Environmental Sectors, Tokyo University of Agriculture and Technology, Email: sagemasa@cc.tuat.ac.jp Received April 28, 2010; Accepted July 24, 2010. - 305 - Removal of Cadmium from Aqueous Solutions by Charcoals Prepared from Giant Reed (Arundo donax) Masaki SAGEHASHI*, Takao FUJII**, Hong Ying HU***, Akiyoshi SAKODA** *Center of Education for Leaders in Environmental Sectors, Tokyo University of Agriculture and Technology, Tokyo 184-8588 Japan **Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505 Japan ***Environmental Simulation and Pollution Control State Key Joint Laboratory, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, PR China ABSTRACT Giant reed (Arundo donax) is a perennial, gramineous plant that has high biomass production potential. In this study, we focused on its applicability as a raw material for the production of adsorbents for the purification of cadmium-contaminated water. Charcoals were prepared from the stalk of the giant reed at various temperatures (400 - 700°C) under nitrogen stream. Analysis of pore size distribution based on methanol vapor adsorption revealed that the mesopores within the range of 20 to 100 Å were abundant in charcoals prepared under 400°C and 500°C. Using these charcoals, the removal capacity of cadmium from aqueous solution was investigated. As a result, high removal capacity under low concentration of cadmium was observed. High yield of cadmium with 0.01 N hydrochloric acid treatment was also clarified. X-ray diffraction (XRD) analysis revealed that Cd(OH) 2 existed on the surface of the cadmium-adsorbed charcoal. The possible mechanism of the “apparent adsorption” was discussed. Keywords: cadmium, charcoal, giant reed, water treatment. INTRODUCTION The shortage of freshwater and its declining quality have been one of the serious issues at present. On the other hand, the progress of global warming as well as the depletion of fossil fuels are other important environmental concerns. Thus, the development of energy-saving water purification methods is essential. Constructed wetlands to purify eutrophicated water (Sagehashi et al., 2009a; 2009b), or water containing harmful compounds and heavy metals are examples of such methods because they are based fundamentally on natural energy (e.g. solar energy), and potentially produce biomass resources. Giant reed (Arundo donax), a perennial, gramineous plant having high biomass production potential, can be employed in constructed wetlands. The harvested giant reed biomass can be used as a non-wood raw material for pulp (Shatalov and Pereira, 2002), and, recently attracted attention as an energy crop (Angelini et al., 2009). To use such characteristics, we proposed a sustainable energy and material cycles system using giant reed (Sagehashi et al., 2009a). Meanwhile, the adsorptive purification of polluted water using inexpensive adsorbents is an energy and cost-saving method. A number of researches have been conducted regarding the preparation of adsorbents from low-cost materials such as agricultural wastes. Wetland plants including giant reed can be used as raw materials for the production of adsorbents. This leads to the rational, on-site utilization of the biomass produced in constructed wetland Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 306 - of which the contaminated water is nearby. The important factors to be considered are the preparation of the adsorbent, and its removal performance for the contaminants. Among harmful heavy metals, cadmium was selected and used as a pollutant in this study, because of its toxicity. Many studies on cadmium removal with constructed wetlands were reported (e.g., Pimpan and Jindal, 2009). Moreover, cadmium can be removed with various adsorbents (e.g., Yadanaparthi et al., 2009). Basso and Cukierman (2006) reported the adsorption of nickel on an adsorbent prepared from giant reed. However, there are only a few studies regarding cadmium adsorption on charcoal prepared from giant reed with simple methods. In this study, to estimate the ability of a simple carbonization process, charcoals were prepared from the stalk of giant reed under various temperatures with nitrogen stream without adding any activating reagents. Physical characteristics of these charcoals were clarified as well as their cadmium removal capacity. MATERIALS AND METHODS Carbonization Fertilized giant reed grown at a managed area in the Kanto Plain, Japan, was used for experiments. The stalk was milled by a mortar, and sieved into a fraction of 20 to 100 μm. The fraction was carbonized in a quartz tube-equipped electrical furnace under nitrogen stream for 1 hour. The electrical furnace was set at a temperature between 400 and 700ºC. Charcoals were then obtained and used in the following tests. Physicochemical analysis of charcoals The vapor-phase methanol adsorptions on the charcoals were measured using the quartz-balance adsorption measurement equipment (Fujii and Sakoda, 2008). The pore size distribution was estimated based on the methanol adsorption (Urano et al., 1970). The BET specific surface area was calculated by the methanol adsorption results. The elemental component (i.e., carbon, nitrogen and hydrogen) was measured with a CHN analyzer (model 2400-II, Perkin-Elmer, USA). For the mineral analysis, the following methods were employed. The stalk was incinerated under 550ºC, and washed with 1 N hydrochloric acid (HCl). The concentrations of Na, K, Ca, and Mg in the HCl solution were measured by an atomic absorption analyzer (AAnalyst 200, Perkin-Elmer, USA), and the contents of these metals were calculated. The X-ray diffraction (XRD) analysis was performed with an X-ray diffractometer (RINT2000, Rigaku, Japan) for the estimation of the form of Cd on the charcoal. Adsorption of Cadmium Two-hundred milliliters (for adsorption rate estimation; results are shown in Fig. 2) or 100 mL (for adsorption capacity measurement; results are shown in Fig. 3) of cadmium chloride solutions (20 mg-Cd/L) and a prescribed amount of the carbonized stalk (charcoal) of giant reed (10 - 100 mg in each) was placed in an Erlenmeyer flask. The flask was kept under 25ºC and shaken at 350 rpm by a rotary shaker (NR-20, TAITEC, Japan). After the prescribed contact time, the solution was filtrated (Millex GV, pore size = 0.22 μm, MILLIPORE, USA), and the concentration of cadmium was measured by the atomic absorption analysis described above, and the amount adsorbed was calculated. The variation of pH in the solution was also measured for the adsorption rate Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 307 - estimation (PH1500, EUTECH INSTRUMENTS, Singapore). Desorption of Cadmium An adsorption experiment using the charcoal prepared under 700ºC was performed basically in the same manner as mentioned above, in which the initial concentration of cadmium was 10 mg-Cd/L, the volume of solution was 50 mL, and the amount of charcoal was 20 mg. Three days from the start of contact (concentration of cadmium was 1 mg-Cd/L, amount of cadmium adsorbed was 22 mg/g), the charcoal with adsorbed cadmium was transferred into a 0.01 N HCl solution. After 6 hours of contact time, the concentration of cadmium in solution was measured and the amount desorbed was evaluated. RESULTS AND DISCUSSION Components of the charcoal Minerals contained in giant reed are shown in Table 1. The minerals contained in the charcoal were calculated with reference to the weight of the giant reed used for the carbonization. Especially, the potassium (K) content was high. The chemical components of a charcoal prepared under 700ºC are shown in Table 2. Pore structure of the charcoals The pore size distributions obtained by the methanol adsorption are shown in Fig. 1. The BET specific surface areas calculated from the methanol adsorption are also shown in the figure as well as the differential pore size distribution. This figure indicated that the relatively large pores, i.e. the mesopores ranging from 20 to 100 Å, were abundant in charcoals prepared under 400°C and 500°C. Such adsorbent can be applied for the adsorption of large molecules, and it is one of the important features of charcoals prepared from giant reed. With the increase in preparation temperature, the mesopores were diminished, and charcoals with abundant micropores were obtained. In this study, the carbonization time was not changed but was fixed at 1 hour, and the heating rate was not controlled since temperature dependency was not the focus of the study. Meanwhile, Xia et al. (2000) demonstrated the effects of heating rate on the yield, specific surface area, pore volume, and adsorption capacities of the carbonized sugarcane baggase under nitrogen stream. Therefore, further research about the effects of heating rate and carbonization time on the yield and characteristics of the fabricated charcoal might improve the preparation methodology of the carbonization materials from giant reed. Table 1 - Ash components of the charcoal prepared under 700ºC Na K Ca Mg Contents [mg/g-giant reed] 0.68 3.32 1.19 1.29 Table 2 - Carbon, hydrogen, nitrogen and ash in the charcoal prepared under 700ºC C H N Ash ratio [wt%] 72.4 1.1 0.5 3.1 Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 308 - 0 100 200 300 0 20406080100 細孔半径 [Å] 累積細孔容積 [ml/g] 400 500 600 700 0.3 0.2 0.1 0 Pore radius [Å] Cumulative pore volume [mL/g] preparation temperature [ºC] specific surface area [m 2 /g] 530 230 160 150 0 1 2 3 4 5 020406080100 細孔半径 [ Å ] dV/dr [μl/g Å ] 400 500 600 700 preparation temperature [ºC] Differential pore size distribution Pore radius [Å] Fig. 1 - Pore structures of the charcoals prepared from the stalk of giant reed (Left: cumulative pore volume, Right: differential pore size distribution) 4 5 6 7 8 9 10 0 20 40 60 0 24 48 72 96 120 144 168 0 10 20 30 Calculated Amount Adsorbed [mg-Cd/g] Concentration [mg-Cd/L] pH Time elapsed [hr] 4 5 6 7 8 9 10 0 20 40 0 24 48 72 96 120 144 168 0 10 20 30 Concentration [mg-Cd/L] Time elapsed [hr] Calculated Amount Adsorbed [mg-Cd/g] Fig. 2 - Time courses of pH (◇) in the solution, cadmium concentration (○), and calculated cadmium adsorbed on charcoal (▲) (The charcoal prepared under 700ºC was used and the dosages of charcoal were 316 mg/L (Left) and 250 mg/L (Right)) Removal of cadmium from aqueous solutions For estimating the removal rate, time courses of cadmium concentration in aqueous solution and calculated amount adsorbed were measured as well as the pH value of the solution for two cases with different dosage of adsorbent, whereas the initial concentration of cadmium and the volume of solution were the same. The results are shown in Fig. 2, indicating that the pH value was rapidly increased with time in each case at the beginning, and the concentration of cadmium in the solution was decreased. The alkaline metal oxides and alkaline-earth metal oxides such as K 2 O, Na 2 O, CaO, and MgO, which were produced by the carbonization, were considered as the cause of this sudden elevation of pH. Meanwhile, when pH was decreased as shown on the left side of Fig. 2, the cadmium concentration in the solution increased with the decrease in pH. The decrease in pH observed still needs to be clarified in further investigations. However, it is elucidated that the amount adsorbed was strongly dependent on the pH of the solution. The existing form of cadmium in aqueous solution is different with its pH value (Babić dV/dr [(µL/g) Å] Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 309 - et al., 2002). At low pH range, cadmium mainly exists as Cd 2+ . With the increase in pH, the dominant existing form is changed to the following order: Cd(OH) + , Cd(OH) 2 , and Cd(OH) 3 - . According to Babić et al. (2002), at a concentration of 0.01 mmol-Cd/L, almost all cadmium ions existed in the form of Cd 2+ in pH < 5, while the abundance ratios of Cd 2+ / Cd(OH) + / Cd(OH) 2 / Cd(OH) 3 - were 44 / 56 / 0 / 0, and 7 / 91 / 2 / 0 at pH 8 and 9, respectively. Therefore, there is a possibility that desorption of cadmium observed in Fig. 2 (especially on the left side) might be caused by this change in existing form. The apparent adsorption isotherms of cadmium on the charcoals are shown in Fig. 3. The amounts adsorbed on the charcoals after washing with acid (1N HCl) are also plotted in the figure. In this experiment, the amounts adsorbed were determined after three days from the start of contact. Furthermore, the amounts adsorbed on other adsorbents found in the literature are included in the figure. As compared to other adsorbents, the cadmium adsorption capacity of the charcoal of giant reed is significantly large at low concentration range. As described in Fig. 2, it should be noted that desorption was caused by the increase in pH. However, it can be stated that charcoals prepared from the stalk of giant reed are high-performance adsorbents for cadmium removal. Moreover, in the desorption experiment, 97% of cadmium was eluted into the HCl solution. It indicated that the adsorbed cadmium can be recovered efficiently. Therefore, it can be concluded that the charcoal prepared from giant reed is useful for cadmium removal. On the other hand, the amounts adsorbed on acid washed (i.e., mineral dissipated) charcoals were very low compared with non-acid washed charcoals. It suggested that the minerals included in the charcoal, which can be removed by acid wash, should play important roles for the “apparent” adsorption of cadmium. The alkali metal hydroxides react with cadmium ions in water to form Cd(OH) 2 , which is insoluble and adhered on charcoal. Fig. 4 shows the results of the XRD analysis, indicating that the crystallized Cd(OH) 2 was observed at the surface of the cadmium-adsorbed charcoals. Ichinose et al., (2004; 2005) reported that cadmium hydroxide nanostrands were abundantly observed in dilute Cd(NO 3 ) 2 solutions containing a small amount of NaOH (pH = 7.0 - 8.5). The nanostrand was extremely positively charged (Ichinose et al., 2004). Therefore, it was suggested that precipitation of cadmium including such positively charged nanostrand occurred on the negatively charged surface of charcoals. Moreover, we confirmed that the removal of manganese also made a very insoluble hydroxide, Mn(OH) 2 , with the charcoals (data not shown). We suppose that this mechanism also took place in the removal of manganese. This is why “apparent” adsorption isotherms is used to describe Fig. 3. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 310 - 0.1 1 10 100 0.01 0.1 1 10 100 Cd [mg/l] q [mg/g] 400 ℃ 500 ℃ 600 ℃ 700 ℃ literature values acid wash peat moss 2) anthracite 3) carbon nanotube 4) activated carbon 1) activated carbon cloth 5) seaweeds 7) roasted coffee beans 6) charcoal (giant reed) Amount of adsorbed cadmium [mg-Cd/g] Concentration of cadmium in water [mg-Cd/L] 1)Ferro-García et al., (1988); 2)Seki et al., (2006); 3)Petrov et al., (1992); 4)Li et al., (2003) 5)Babić et al., (2002); 6)Minamisawa et al., (2002); 7)Suzuki et al., (2005) Fig. 3 -Apparent adsorption isotherms for the charcoals prepared and other adsorbents 0 200 400 600 800 0 20406080 2 θ [deg.] Intensity [cps] 0 200 400 600 800 1000 Intensity [cps] charcoal with cadmium adsorbed Cd(OH) 2 unused charcoal Fig. 4 - XRD results of the charcoal with cadmium adsorbed (Black plots: charcoal with cadmium adsorbed, and Cd(OH) 2 (left axis); Gray plots: unused charcoal (right axis)) CONCLUSIONS Removal of cadmium from aqueous solutions by the charcoals prepared from the stalk of giant reed was investigated based on the beneficial utilization of giant reed. As a result, its high removal capacity was confirmed. The cadmium retained in the charcoals was easily desorbed with hydrochloric acid, revealing that the regenerated charcoals can be used for various purposes such as solid fuels. In addition, the pore size can be controlled upon changing the carbonization temperature. Especially, mesopores were developed in low carbonization temperature, suggesting that it can be applied for the adsorption of large molecules. Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 311 - ACKNOWLEDGEMENTS This work was partly supported by Japan Science and Technology Agency (JST) -National Natural Science Foundation of China (NSFC) Joint Program (Strategic International Cooperative Program). REFERENCES Angelini L. G., Ceccarinia L., Nassi o Di Nasso N. and Bonari E. (2009). Comparison of Arundo donax L. and Miscanthus x giganteus in a long-term field experiment in Central Italy: Analysis of productive characteristics and energy balance, Biomass and Bioenergy, 33, 635-643. Babić B. M., Milonjić S. K., Polovina M. J., Ćupić S. and Kaludjerović B. V. (2002). Adsorption of zinc, cadmium and mercury ions from aqueous solutions on an activated carbon cloth, Carbon, 40, 1109-1115. Basso M. C. and Cukierman A. L. (2006). Wastewater Treatment by Chemically Activated Carbons from Giant Reed: Effect of the Activation Atmosphere on Properties and Adsorptive Behavior, Separ. Sci. Technol., 41, 149–165. Ferro-García M. A., Rivera-Utrilla J., Rodríguez-Gordillo J. and Bautista-Toledo I. (1988). Adsorption of zinc, cadmium, and copper on activated carbons obtained from agricultural by-products, Carbon, 26(3), 363-373. Fujii T., and Sakoda A. (2008). Adsorption Characteristic of Commercial Bamboo Charcoals and Estimate of for Pore Structure, Waste Manag. Res., 19(3), 191-196. (In Japanese) Ichinose I., Kurashima K., and Kunitake T. (2004). Spontaneous Formation of Cadmium Hydroxide Nanostrands in Water, J. Am. Chem. Soc., 126(23), 7162-7163. Ichinose I., Huang J. and Luo Y.-H. (2005) Electrostatic Trapping of Double-Stranded DNA by Using Cadmium Hydroxide Nanostrands, NANO LETTERS, 5(1), 97-100. Li Y.-H., Wang S., Luan Z., Ding J., Xu C. and Wu D. (2003). Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes, Carbon, 41, 1057-1062. Minamisawa M., Nakajima S., Mitsue Y., Miyazawa K., Ueno K., Hatori A., Miyajima S., Hoshino M., Yoshida S. and Takai N. (2002). Removal of Copper(II) and Cadmium(II) in Water by Use of Roasted Coffee Beans, J. Chem. Soc. Japan, No. 3, 459-461. (In Japanese) Petrov N., Budinova T. and Khavesov I. (1992). Adsorption of the ions of zinc, cadmium, copper, and lead on oxidized anthracite, Carbon, 30(2), 135-139. Pimpan P. and Jindal R. (2009). Mathematical modeling of cadmium removal in free water surface constructed wetlands, J. Hazard. Mater., 163(2-3), 1322-1331. Sagehashi M., Kawazoe A., Fujii T., Hu H.-Y. and Sakoda A. (2009a). Analysis of Phosphorus Behavior in the Giant Reed for Phytoremediation and the Biomass Production System, J. of Water and Environ. Technol., 7(2), 143-154. Sagehashi M., Zhou S., Naruse T., Osada M. and Hosomi M. (2009b). Nitrogen Dynamics and Biomass Production in a Vertical Flow Constructed Wetland Cultivated with Forage Rice and their Mathematical Modeling, J. of Water and Environ. Technol., 7(4), 251-266. Seki H., Yu K., Maruyama H. and Suzuki A. (2006). Biosorption of Heavy Metals onto Sphagnum Peat Moss, Kagaku Kogaku Ronbunshu, 32(5), 409-413. (In Japanese) Shatalov A. A. and Pereira H. (2002). Influence of stem morphology on pulp and paper Journal of Water and Environment Technology, Vol. 8, No.4, 2010 - 312 - properties of Arundo donax L. reed, Ind. Crops Prod., 15, 77-83. Suzuki Y., Kametani T. and Maruyama T. (2005). Removal of heavy metals from aqueous solution by nonliving Uiva seaweed as biosorbent, Wat. Res., 39, 1803-1808. Urano K., Mizusawa H. and Kiyoura R. (1970). Determination of Pore-Size Distribution from Methanol Adsorption, J. Chem. Soc. Japan, Ind. Chem. Sec., 73(9), 1911-1915. (In Japanese) Yadanaparthi S. K. R., Graybill D. and von Wandruszka R. (2009). Adsorbents for the removal of arsenic, cadmium, and lead from contaminated waters, J. Hazard, Mater., 171, 1-15. Xia J., Noda K., Wakao N. and Kagawa S. (2000) Production and Properties of Porous Carbon from Bagasse of Sugarcane Grown in Brazil. NIPPON KAGAKU KAISHI, 4, 273-280. (In Japanese) . Agriculture and Technology, Tokyo 184 -85 88 Japan **Institute of Industrial Science, The University of Tokyo, Tokyo 153 -85 05 Japan ***Environmental Simulation. 6 7 8 9 10 0 20 40 60 0 24 48 72 96 120 144 1 68 0 10 20 30 Calculated Amount Adsorbed [mg-Cd/g] Concentration [mg-Cd/L] pH Time elapsed [hr] 4 5 6 7 8 9