Adsorption Characteristics of Tannin for Heavy Metal Ions

Tannins gel was investigated as an adsorbent for heavy metal ions, such as lead, cadmium, copper, and others. The gel used in the present work was made by Mitsubishi Nuclear Fuel Co. Ltd. (MNF). It was found that the gel showed high adsorption capacity of heavy metal ions and the adsorption isotherms were correlated well by the Langmuir equation. The maximum adsorption capacity was influenced by some factors, as following: initial pH of solutions, kinds of heavy metal ion species and counter ion species. These results suggest that the gel can capture a few heavy metal ion species selectively from their mixture. INTRODUCTION Heavy metal ions, such as lead, cadmium and copper have been detected at high concentrations in wastewater of many industries and soil. It has been serious problems because they exist as contaminants and are accumulated in living organisms. It has been strictly prohibited to discharge these ions in many countries. There are various methods, such as precipitation, ion exchange and membrane filtration, used to remove these ions from wastewater. However, those approaches are not efficient, because their treatment cost is comparatively high in the case of low concentrations and it is difficult to clear legal limits. Therefore, it needs to use low-cost materials for removal of those ions from waste water. There is tannin to leaves and bark in many kinds of plant. Tannin is usually extracted from them, it is used as a tanning agent in leather industry and a raw material in manufacturing of adhesives. Other than application above-mentioned, it investigated to apply tannin as an adsorbent to capture various kinds of chemical species. Randall et al. (1974, 1975, 1976, 1977 and 1978) found that a certain bark including tannin adsorbed some heavy metal ions selectively and they were able to improve the adsorption ability by putting some process. The use of some bark including tannin have been reported: Coniferous wood (Fuji et al.,1988), peanut inner skin (Nakajima et al., 1990), pinus pinaster (Vazquez et al., 2002), pinus radiata (Palma et la., 2003) and pinus sylvestris (Taty-Costodes et al., 2003 ). Yamaguchi et al. (1992) produced a resin from condensed tannin and found that copper ion (Cu) was adsorbed on the resin by physical adsorption. Nakano et al. (2000) reported that a gel made by gelation of natural condensed tannin shown in Fig. 1 is capable of adsorbing hexavalent chromium ions (Cr). Zhan et al. (2001, 2003) developed an adsorbent synthesized from condensed tannin including sodium ions (Na) and found two Na ions in the gel were exchanged by one lead ion (Pb), as shown in Fig. 2. Mitsubishi Nuclear Fuel Co., Ltd. (MNF) (1997) produced a certain tannin gel (TANNIX) from cross-linking “wattle tannin” (condensed tannin) and formaldehyde and developed a liquid waste treatment system to remove uranium and the gel. MNF reported the gel had some following characteristics; during incineration, discharged elements were carbon, hydrogen and oxygen, the volume was dramatically reduced and the gases generated were harmless to the environment. The properties and the SEM photographs of the gel are shown in Table 1 and Phot. 1, respectively. In present study, a tannin gel including Na ions was used and the adsorption characteristics by the gel for removel of heavy metal ions, such as lead, cadmium, copper and zinc were investigated. R1 = H for resorcinol ; R1 = OH for phloroglucinol ; R2 = H for pyrocatechol ; R2 = OH for pyrogallol Fig. 1 An example of molecular formula of condensed tannin HO O