Development of System for Phytoextraction and Recovering
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Development of system for phytoextraction and recovering valuable metals from contaminated soil On-line Number 119 Fumihisa Kobayashi,1 Teruya Maki,1 and Yoshitoshi Nakamura1 1 Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kakuma-cyo, Kanazawa, Ishikawa 920-1192, Japan E-mail: [email protected] ABSTRACT Phytoremediation, a new plant-based technology for the removal of toxic contaminants from soil and water, is a potentially attractive approach. Though a number of plants have been identified as hyperaccumulators for the phytoextraction of a variety of metals and some has been used in field applications, no study on recovery and recycle of heavy metals from contaminated soil has been investigated. Recently, Athyrium yokoscense has been discovered as an efficacious heavy metallic hyperaccumulating fern plant. In this work, the purification of contaminated soil with heavy metals using A. yokoscense and the development of system for recovering valuable metals by steam explosion and Wayman’s extraction were investigated. After the harvest of A. yokoscense adsorbed heavy metals from contaminated soil, it was treated by steam explosion and separated into extractive components, i.e. holocellulose, water-soluble material, Klason lignin, and methanol-soluble lignin, using Wayman’s extraction method. The distribution of metals in the components was clarified using ICP emission spectrometry. KEYWORDS Phytoremediation, hyperaccumulating fern plant, recovering valuable metals, steam explosion INTRODUCTION The soil contaminated with harmful metals arise a serious environmental problem in the world (Asami, 2001). The soil contaminated with metals, i.e., vacant lots of iron works or abandoned mine areas, is one of the major sources of watersupply and the drinking water sometimes contains high concentrations of metals (National Research council, 1999; Welch et al., 2000). The concentration of metals in cereals, vegetables, and fruits is directly related to the level of metals in contaminated soil. Though the remediation of the soil contaminated with metals is an important and timely issue, cost-effective remediation techniques are not currently available. Phytoremediation using a hyperaccumulating plant, such as fern and grass, for the removal of toxic contaminants from soil is a potentially attractive approach (Fiorenza, 2000; Dahmani-Muller et al., 2000; Ma et al., 2001; Zang et al., 2002). This technique has received much attention lately as a cost-effective alternative established treatment method used at hazardous waste sites. However, very few papers on system for phytoextraction and recovering valuable metals from contaminated soil have been published. 1 We have investigated the pretreatment and effective utilization of plant biomass such as wood, rice straw, potato, and bamboo using steam explosion (Nakamura et al., 1989, 1991; Kobayashi et al., 1998, 2004; Nakamura and Kobayashi, 2004a). Steam explosion was an effective pretreatment method for degrading and removing lignin that covered holocellulose (cellulose and hemicellulose) and steam-exploded product was converted easily into sugar and ethanol by using enzymes and yeast. It means that steam explosion is an useful pretreatment for liquefaction and separation of components in plant biomass. In this work, the phytoextraction and recovering valuable metals from contaminated soil using fern (Athyrium yokoscense), steam explosion were examined. The components of steam-exploded A. yokoscense fern, such as holocellulose, waster-soluble material, Klason lignin (high-molecular-weight lignin), and methanol-soluble lignin (low-molecular-weight lignin), were extracted and weighed. Amount of metals in the extractive components were measured and the development of system for recovering metals were established for practical use of phytoremediation. MATERIALS AND METHODS Plant sample Fern plants (Athyrium yokoscense) were collected from Ogoya abandoned mine area (Komatsu City, Ishikawa, Japan) as shown in Figure 1. The dry plants were separated into roots (containing stalks) and leaves, and used in this study. For measuring metals concentration in the contaminated soil, 10 g of sand soil was took samples in Ogoya abandoned mine area near A. yokoscense. Figure 1. Photograph of fern plants (Athyrium yokoscense) in Ogoya abandoned min area. 2 Pretreatment method The plant samples were hydrolyzed using a steam explosion method. The apparatus for the steam explosion (Japan Chemical Engineering and Machinery, Osaka, Japan) consisted of a steam generator, a high-pressure reactor, a receiver, and a condenser with a silencing action (Kobayashi et al., 1998, 2004) as shown in Figure 2. The capacity of the reactor was 1.2 L, the highest pressure was 5.49 MPa, and the highest temperature was 275 oC. The solid and liquid products of the exploded fern were recovered in cyclone in the bottom of receiver, and the gaseous products passed from the top of the receiver into the condenser. Steam explosion were conducted under a steam pressure of 2.55 MPa (225 oC) and steaming times of 1 min. Figure 2. Steam explosion apparatus. Extraction method The components of steam-exploded fern, such as holocellulose, waster-soluble material, Klason lignin (high-molecular-weight lignin), and methanol-soluble lignin (low-molecular- weight lignin), were extracted and weighed by the Wayman’s extraction method (Chua and Wayman, 1979). Figure 3 shows the outline of Wayman’s extraction method. After water in the steam-exploded A. yokoscense was sublimated by freeze dry method, 5 g of steam-exploded dry A. yokoscense and 300 ml of distilled water mixed for 12 h at room temperature and filtered. The filtrate was dried and this solid was water-soluble material. The residue was dried rapidly and 1 g of dry residue was extracted by a Soxhlet extractor with 100 ml of methanol for 12 h. The dry component extracting by methanol was methanol-soluble lignin (low-molecular-weight lignin). The saccharide containing the residue of methanol extraction was removed using a sulfuric acid treatment. Klason lignin (high-molecular-weight lignin) was obtained as a residue after the sulfuric acid treatment. The filtrate was a holocellulose. 3 Figure 3. Flow-chart of Wayman’s extraction method. Analysis For the measurement of metals in extractive components and the soil contaminated with metals, all organic compounds were removed. Their samples were dried at 90 oC for 12 h and heated at 500 oC for 6 h using oven (SH-OMT, Nitto Kagaku Co. LTD., Tokyo, Japan). The solid samples without organic compounds were acidified with 10 % (v/v) HNO3 solution. The metal concentrations of the filtrate were determined by an ICP (inductively coupled plasma) emission spectrometry OPTIMA 3300XL (Perkin Elmer, Massachusetts, USA). RESULTS AND DISCUSSION Components of the steam-exploded A. yokoscense In order to recover valuable metals from A. yokoscense, the components, such as holocellulose, waster-soluble material, Klason lignin (high-molecular-weight lignin), and methanol-soluble lignin (low-molecular-weight lignin), were extracted from steam-exploded A. yokoscense. Table 1 shows the amount of components to the dry weight of steam-exploded A. yokoscense in leaves and roots. Comparing between the amounts of leaves and roots, the amount of water-soluble material in leaves was higher than that in roots and Klason lignin (high-molecular-lignin) in roots was higher than that in leaves. This reason 4 depended on the fact that leaves contain many chloroplasts and roots contain lignin as the same as many woody biomass. It is interesting to note that the amount of Klason lignin was very high, i.e. 0.417 g in unit per gram of steam-exploded dry A. yokoscense. Table 1. The amount of components to 1 g of the dry weight of steam exploded A. yokoscense in leaves. Part Holocellulose Waster-soluble Klason lignin Methanol-soluble [g] material (high-molecular- lignin (low- [g] weight lignin) molecular- weight [g] lignin) [g] Leaves 0.328 0.321 0.262 0.089 Roots 0.367 0.140 0.417 0.076 Recovering valuable metals Table 2 shows the concentrations of Cu, Fe, Pb, and Zn in unit per gram of components of leaves in steam-exploded A. yokoscense. The concentrations of Cu, Fe, Pb, and Zn were high in water-soluble material, Klason lignin (high-molecular weight lignin), methanol-soluble lignin (low-molecular-weight lignin), and methanol-soluble lignin (low-molecular-weight lignin), respectively. The existence of metal element varied based on the extractive component. Though this reason was unknown in detail, it was thought that the protein, such as plant hormone, combined with metal element varied based on the extractive component. Table 2. Concentrations of Cu, Fe, Pb, and Zn in unit per gram of components of leaves. Metals [mg g-conponent-1] Components Cu Fe Pb Zn Holocellulose N.D. 0.248 N.D. N.D. Waster-soluble material 41 0.127 N.D. 0.205 Klason lignin 0.74 2.6 0.63 0.42 (high-molecular-weight lignin) Methanol-soluble lignin 0.252 0.16 6.1 2.9 (low- molecular- weight lignin) (N.D.: No Detected) Table 3 shows the concentrations of Cu, Fe, Pb, and Zn in unit per gram of components of roots in steam-exploded A. yokoscense. In the case of roots, the concentration of metals was high in Klason lignin (high-molecular-weight lignin). Since Klason lignin in roots is the complex polymer, many metals seem to be entrapped in this polymer. In cases of both leaves and roots, it found that concentrations and distributions of metals varied based on the extractive component. 5 Table 3. Concentrations of Cu, Fe, Pb, and Zn in unit per gram of components of roots. Metals [mg g-conponent-1] Components Cu Fe Pb Zn Holocellulose N.D. N.D. N.D. N.D. Waster-soluble material N.D. 0.429 0.773 1.24 Klason lignin 15 4.2 5.8 0.74 (high-molecular-weight lignin) Methanol-soluble lignin 0.215 0.192 6.79 0.147 (low- molecular- weight lignin) (N.D.: No Detected) Mass balance for phtoextraction and recovering valuable metals Next, we evaluated the system of the remediation of the soil contaminated with metals and recovering valuable metals of Cu and Fe.