Ind. Eng. Chem. Res. 2007, 46, 2657-2660
2657
RESEARCH NOTES Hydrothermal Synthesis of a Novel Tobermorite-Based Porous Material from Municipal Incineration Bottom Ash Zhenzi Jing,* Fangming Jin,† Nakamichi Yamasaki, and Emile Hideki Ishida Graduate School of EnVironmental Studies, Tohoku UniVersity, Aoba 6-6-20, Aoba-ku Sendai 980-8579, Japan, and School of EnVironmental Science and Engineering, Tongji UniVersity, 1239 Siping Road, Shanghai 200092, China
A novel tobermorite-based porous material has been synthesized from municipal incineration bottom ash (MIBA), using a hydrothermal processing method under saturated steam pressure (1.0 MPa) at 453 K for 12 h. Without additional vesicants, NaOH solution that is incorporated into the starting material acts as both a reaction solvent and a vesicant. As a reaction solvent, the NaOH solution first promoted tobermorite formation to form a huge number of small pores during hydrothermal processing; then, as a vesicant, it was released as vapor from the solidified specimens to form large pores after drying. The porous material possessed a pore diameter distribution of 0.01-10 µm, a BET specific surface area of 60 000 m2/kg, and a bulk density of 500-700 kg/m3. Moreover, the leaching tests showed that the concentration of heavy metals dissolved from the solidified specimen became very low after hydrothermal processing. Introduction More than six million tons of incineration ash from municipal waste is generated in Japan annually.1 Only a small proportion of the municipal incineration ash (MIA) has been utilized, whereas most of it is still disposed directly by landfilling. Because of the shortage of landfill sites and tighter environmental regulations, new ways of treating/utilizing MIA are greatly needed. Recently, MIA has been used to produce zeolites.2 Zeolites have many applications, e.g., they have been used as cation exchangers in wastewater treatment to remove heavy-metal pollutants.3-6 Tobermorites, on the other hand, have been discovered as a new family of cation exchangers,7,8 and have been shown to have high potential applications in cation exchange and nuclear and hazardous wastewater treatment.9,10 Furthermore, Komarneni11 reported that tobermorites, both synthetic and natural, were extremely efficient in the removal of all the heavy metals, except hexavalent chromium (Cr6+), and are superior to zeolites. Tobermorite is very rare naturally but can be synthesized under hydrothermal processes. The hydrothermal process is a very useful way to activate reactions whose mechanisms are considered to be via dissolution/precipitation. Here, we first report that a novel tobermorite-based porous material has been synthesized from MIA through a hydrothermal processing method at low temperature (453 K). The porous material possessed a huge number of pores within the diameter range of 0.01-10 µm, a Brunauer-Emmett-Teller (BET) specific surface area of 60 000 m2/kg, and a bulk density of 500-700 kg/m3. Possibly because of the formation of tobermorite, the heavy metals that are dissolved from the solidified porous material have reduced remarkably, below the regulatory levels * To whom correspondence should be addressed. Tel./Fax: +8122-795-4398. E-mail address:
[email protected]. † Tongji University.
for the environmental quality standards of Japan. As such, the tobermorite-based porous material may have high potential in regard to applications in cation exchange and in nuclear and hazardous wastewater treatment. Experimental Section The raw material used in this study was municipal incineration bottom ash (MIBA) produced from a stoker-type incineration facility at Kochi, Japan. The incinerator has a yearly processing capacity of 30 000 tons. The MIBA was first dried at 383 K and then ground with a ball mill to obtain a BET specific surface area of 11 000 m2/kg (ChemBET 3000, Quantachrome). The particle size distribution determined by laser diffraction technology (Model X100, Microtrac), and the chemical composition of the MIBA, as determined by X-ray fluorescence (XRF) (Model RIX3100, Rigaku) are shown in Figure 1 and Table 1, respectively. The MIBA powder, mixed with slaked lime (CaO/ SiO2 molar ratio of 0.95), was used as the starting material. NaOH solution (2 M) was added to the MIBA to promote tobermorite formation, because the hydrothermal reaction between silica and lime for tobermorite formation is mainly controlled by the solubility of silica, and a higher pH leads to a higher solubility of silica. The starting material (15 g) was first mixed manually in a mortar with 40 mass % NaOH solution (2 M) of 6 mL, and then the mixture was compacted using a compaction pressure of 1-5 MPa in a disk-shaped mold (30 mm diameter × 120 mm height). The demolded specimens were subsequently autoclaved under the saturated steam pressure (1.0 MPa) at 453 K for 12h. The polytetrafluoroethylene (Teflon, PTFE)-lined stainless steel autoclave shown in Figure 2 was used for hydrothermal treating the demolded specimens. After autoclaving, all the solidified specimens were dried at 343 K for 24 h before testing. The solidified disk-shaped specimens (30 mm diameter × 20 mm height) were used to measure the tensile strength, using
10.1021/ie070016z CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
2658
Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007
Figure 1. Particle size distribution of municipal incineration bottom ash (MIBA).
Figure 3. X-ray diffraction (XRD) patterns for the starting material and solidified specimen.
Figure 4. Effects of compaction pressure on (0) bulk density and (b) tensile strength.
Figure 2. Hydrothermal apparatus used to cure compacted specimens. Table 1. Chemical Composition of the Municipal Incineration Bottom Ash (MIBA) component
content
SiO2 CaO Al2O3 MgO Fe2O3 K2O Na2O loss on ignition, LOI Cd Pb Cr (Cr3+ + Cr6+) As Hg Se
44.0 mass % 21.1 mass % 11.3 mass % 4.2 mass % 7.0 mass % 2.6 mass % 8.2 mass % 1.6 mass % 0.3 mg/kg 1430 mg/kg 158 mg/kg 5.5 mg/kg
