Published online 26 April 2006
Published in J Environ Qual 35:912-917 (2006)
DOI: 10.2134/jeq2005.0318
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Waste Management
A Novel Extractant for Removal of Hazardous Metals from Preservative-Treated Wood Waste
Tomo Kakitania,
Toshimitsu Hataa,*,
Takeshi Kajimotob and
Yuji Imamuraa
a Research Institute for Sustainable Humanosphere (RISH), Kyoto University, Uji, Kyoto 611-0011, Japan
b Industrial Technology Center of Wakayama Prefecture, Wakayama, 60 Ogura, Wakayama-shi 649-6261, Japan
* Corresponding author (hata{at}rish.kyoto-u.ac.jp)
Received for publication August 13, 2005.
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ABSTRACT
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The purpose of this study was to develop a one-step metal extraction process that would effectively remove hazardous elements from wood powder or chips of western hemlock [Tsuga heterophylla (Raf.) Sarg.] treated with chromated copper arsenate (CCA) preservative. In addition, we tested this method for wood treated with other copper-based preservatives such as ammoniacal copper quaternary (ACQ) and copper, boron, and azole (CuAz). A bioxalate solution consisting of 0.125 M oxalic acid adjusted to pH 3.2 with sodium hydroxide was tested for its ability to extract chromium, copper, and arsenic from wood treated with CCA and copper from ACQ, CuAz, or a mixture of CCA-, ACQ-, and CuAz-treated wood in single step. The extraction proceeded efficiently with 6 h of treatment, and was insensitive to the differences in chemical characteristics, including solubility of individual elements. After 6 h of treatment, approximately 90% of chromium, copper, and arsenic were effectively removed from wood treated with CCA or a mixture of CCA, ACQ, and CuAz and 90% of copper from ACQ- and CuAz-treated wood. These results demonstrate that the solvent extraction technique using pH-adjusted bioxalate solution with sodium hydroxide is a promising method for pollution minimization by various types of wastes contaminated with heavy metals and arsenic.
Abbreviations: ACQ, ammoniacal copper quaternary • CCA, chromated copper arsenate • CuAz, copper, boron, and azole
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INTRODUCTION
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GENERALLY, wood for exterior use or for use in damp conditions is chemically treated to prevent attack from fungi and insects. Treatment with preservatives can expand the potential uses of wood by prolonging its service life. Southern yellow pine (Pinus elliottii Engelm.) treated with chemical preservatives is a commonly used in Europe and America, while treated western hemlock is used in Japan. Chromated copper arsenate (CCA) is one of the most commonly used waterborne wood preservatives in the world. It is mixture of chromium trioxide (CrO3), copper oxide (CuO), and arsenic pentoxide (As2O5). The wood is impregnated with liquid in a vacuum followed by pressure treatment and is then dried for conditioning. Generally, the CCA preservative is impregnated into wood that has a solid mass of approximately 3.5 kg/m3. When the gravity of wood is assumed to be 0.4, 1.0 g of wood contains a minimum of approximately 8.8 mg of CCA metals. During the treatment process, the CCA preservative is dispersed and fixed in the wood matrix in the form of sparingly soluble metal complex, such as CuCrO4 and CrAsO4 (Pizzi, 1982).
Recently, the production of CCA-treated wood has been regulated in many countries, because of the serious disposal problem associated with CCA-treated wood waste (Dobbs and Grant, 1978; Kakitani et al., 2004a, 2004b; McQueen and Stevens, 1998; Solo-Gabriele et al., 2002). To date, only a few promising solutions for disposing or recycling of CCA-treated wood waste have been proposed (Helsen et al., 1998; Peek, 2006). In Japan, the market share of CCA-treated wood has been approximately 80% of all treated-wood products for several decades. However, the use of CCA-treated wood has been drastically reduced, and currently the market share of wood treated with alternatives, such as ammoniacal copper quaternary (ACQ) and copper, boron, azole (CuAz), has already reached over 60% (Japan Wood Preserving Association, 2002). Although these alternatives are believed to be safer than the conventional CCA preservative, copper-containing preservatives can cause a new disposal problem when the products treated with these preservatives are taken out of service. So, there is a need to develop a proper disposal or a recycling technology not only for CCA-treated wood but also for ACQ- and CuAz-treated wood products to prevent environmental contamination in the future.
Removing elements from CCA-treated wood by solvent extraction has been considered a promising approach (Cooper and Ung, 1992; Gezer et al., 2003; Kazi and Cooper, 1998). However, such extractions are complicated by the decomposition of wood components during acid exposure (Kanjo et al., 1994), the strong bonding between chromium and lignin (Pizzi, 1990a, 1990b), and the low solubility of CCA salts. Therefore, it is necessary to develop and optimize an effective method for extraction of hazardous elements from these materials.
Based on the efforts of previous studies (Kakitani et al., 2004a, 2006; Kazi et al., 1998; Kanjo et al., 1994), we propose that the following conditions must be met for an effective extraction of elements from CCA-treated wood:
- (i) Acid solvents are necessary to break the strong bonds between chromium and lignin and to dissolve the sparingly soluble CCA salts. The pH of the extractant should be below 5.0 because above that pH, Cu(II) and Cr(III) precipitate as insoluble metal hydroxides as controlled by the solubility products, Ksp, for:
- (ii) Strong acidic conditions must be avoided to prevent the hydrolysis of carbohydrates and lignin. The decomposition of wood components is undesirable for both recycling of wood fiber and wastewater treatment.
- (iii) As an alternative to strong acidic conditions, sparingly soluble metals can be solubilized in solution by the addition of complexing agents. The formation of the dissolved complex as follows:
where M is a metal and L is a ligand (or complexing agent), increases the efficiency of metal extraction. This reaction becomes less favorable under strong acidic conditions.
To date, only a few promising chelating agents have been proposed for extraction of metals from CCA-treated wood. Multidentate ligands, such as ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), have been used in one-step extraction of CCA-treated wood (Kartal and Kose, 2003). Although it would be possible to combine different complexing agents in a multi-step extraction process (Kakitani et al., 2006), a one-step extraction process is more advantageous.
This paper is a continuation of the previous report by Kakitani et al. (2006). The purpose of the present study was to develop a one-step extraction process that would remove hazardous elements from CCA-treated wood. In addition, we examine the application of this technique to wood treated with the copper-based preservatives like ACQ and CuAz. In this experiment, we tested the effect of bioxalate solution for one-step, simultaneous extraction of chromium, copper, and arsenic from CCA-treated wood and copper from ACQ- and CuAz-treated wood. Preliminary experiments were conducted with wood powder and further experiments were conducted with chips of CCA-, ACQ-, and CuAz-treated wood and their mixture.
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MATERIALS AND METHODS
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Sample Preparation
Western hemlock treated with CCA-III preservative (chromium as CrO3, 45–51%; copper as CuO, 17–21%, arsenic as As2O5, 30–38%) and ACQ-II (copper as CuO, 62–71%; dimethyl-ammonium-chloride, 29–38%) supplied by Koshii Mokuzai Co. (Osaka, Japan) and CuAz (copper as CuO, 45–59%; boron as H3BO3, 40–46%; azole, 1.6–2.4%) supplied by Xyence Co. (Tokyo, Japan) was used in the study. Wood samples were treated using a full cell process including a 30-min vacuum (0.08 MPa) followed by a 1-h pressure period (0.6–1.0 MPa). Samples were removed from the tank and drained of the excessive solution. They were allowed to cure at room temperature for 3 mo to assure fixation of metals in the wood. Then, treated samples were converted into small chips (30 mm long) and a portion of chips were milled to powder (20-mesh) for extraction of metal. Also, CCA-, ACQ-, and CuAz-treated wood chips were mixed thoroughly at a weight ratio of 1:1:1 and then used for the extraction of metals from this mixture.
Solvent Extraction
Bioxalate solution (pH adjusted to 3.2) was prepared using 0.125 mol of oxalic acid with distilled water. The pH of the solution was adjusted to 3.2 by adding sodium hydroxide solution and changes in pH were monitored with a basic pH meter made by Denver Instrument (Denver, CO). The ratio of the solid (wood powder or chips) to the liquid (extraction solvent) was fixed at 1 g to 20 mL and the extraction temperature was adjusted to 75°C for rapid extraction. The duration of the extraction was varied from 1 to 6 h. The solvents were gently stirred into the samples using a magnetic stirrer in a glass reactor and the samples and solvents were removed from the reactor after 1, 2, 3, 4, 5, and 6 h. The removed samples were rinsed using distilled water and filtered through glass-fiber filter paper (Advantec-GA100; Toyo Roshi Kaisha Ltd., Nikata, Japan) using an aspirator. The experiment was repeated twice. The samples removed from the test were milled to a fine powder and used for metal analysis.
Chemical Analysis
The 100 mg of wood powder was put into Erlenmeyer flasks and digested in 65% HNO3 (pure reagent supplied by Nacalai Tesque, Kyoto, Japan) by gently heating the flasks in a sand bath to avoid loss of elements through volatilization during digestion. Nitric acid was added repeatedly until the samples were completely dissolved. The elements in the liquid solution were analyzed using XRF spectrometry (JSX-3220 Element Analyzer; JEOL Ltd., Tokyo, Japan) and the calibration curve method, in which the concentration of each element is determined according to the intensity of the fluorescent radiation with comparison to standard solutions of arsenic, copper, and chromium. The results presented in figures represent the mean value of three samples, and the standard deviations are shown in figures. The amount of copper, chromium, and arsenic elements in wood were determined by the difference between the initial concentrations in wood minus the concentration in wood after extraction. The changes in metal content in the liquid and mass loss of wood were not measured.
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RESULTS AND DISCUSSION
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Preliminary Experiment
In this study, the solvent extraction of CCA, ACQ, and CuAz wood powder with bioxalate solution was performed first as a preliminary experiment. Then, metal extraction from treated chips and a mixture of three preservatives (1:1:1 w/w) was conducted based on the powder extraction method as described in Metals Extraction from Chips, below.
CCA-Treated Wood Powder
Figure 1a shows changes in copper, chromium, and arsenic contents during bioxalate extraction of CCA-treated wood powder, where the residual percentage of elements in the wood was defined as: (element content in the wood after extraction)/(element content in the wood before extraction) x 100.
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Fig. 1. Changes of percentage residual amount of arsenic, copper, or chromium in preservative-treated wood powder during sodium oxalate solvent extraction. The percentage residual elements content in the wood was defined as (element content in the wood after extraction)/(element content in the wood before extraction) x 100. (a) Percentage residual amount of arsenic, copper, and chromium in chromated copper arsenate (CCA)–treated wood powder during sodium oxalate solvent extraction. (b) Percentage residual amount of copper in ammoniacal copper quaternary (ACQ)–treated wood powder during sodium oxalate solvent extraction. (c) Percentage residual amount of copper in copper, boron, and azole (CuAz)–treated wood powder during sodium oxalate solvent extraction.
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As shown in Fig. 1a, copper, chromium, and arsenic were effectively removed compared to the initial concentration, but extraction efficiency of chromium was slightly lower compared to copper and arsenic. This is probably due to strong bonding between chromium and lignin (Pizzi, 1990a, 1990b). The strong chromium–lignin affinity has reduced the extraction percentage of CCA elements with conventional chelating agents. Previous researchers have attempted to break the bonds by mechanical or thermo-chemical pretreatment, such as steam explosion or pyrolysis (Clausen and Smith, 1998; Kakitani et al., 2004a, 2004b; Shiau et al., 2000).
On the other hand, copper and arsenic (in particular arsenic) were removed rapidly compared to chromium at the beginning of extraction (1–3 h) with bioxalate solution with higher leaching rate. This is an obvious advantage, because arsenic is released easily during pyrolysis, whereas copper and chromium are highly stable (Dobbs and Grant, 1978; Shiau et al., 2000; Hirata et al., 1993). Therefore, higher leaching rate of arsenic is favorable for the purpose of the pre-treatment of CCA-treated wood that will be then be burned for energy. The copper leaching rate at the beginning (1–3 h) was approximately 80%; cumulative copper removal did not increase with increased duration of extraction. Higher levels of copper leaching show that this extraction technique could be applicable to the other copper-based preservatives like ACQ and CuAz.
Kazi and Cooper (1998) evaluated the effect of 10% oxalic acid (pH 0.6 by calculation) on metal extraction from CCA-C-treated wood powder at 50°C keeping the solid-to-liquid ratio of 1 g to 20 mL. CCA is composed of copper as CuO, chromium as CrO3, and arsenic as As2O5. There are three types of CCA produced in the world: CCA-A, CCA-B, and CCA-C. The results showed that the extraction percentage of copper was lowest among the three elements. In addition, the percentage of copper extracted decreased as the duration of extraction progressed and the metal removal rate was approximately 70, 60, and 55% after 1, 3, and 6 h of extraction, respectively. This phenomenon may have been the result of accumulation of insoluble copper oxalate in the sample. Recently, Clausen (2000) examined a dual remediation process for metal extraction from CCA-treated wood. The CCA-treated wafers were soaked in various concentrations of oxalic acid as a pre-treatment before exposing then to a bacterial culture of Bacillus licheniformis. After 18 h soaking, 55% of chromium and 65% of arsenic were removed, but in case of copper, only 18% was removed by 1.0% oxalic acid at pH 1.4. It was also reported that the leachability of copper is constant, regardless of an increase in the concentration of oxalic acid from 0.02 to 1.00%. This indicates that oxalic acid is not suitable for the extraction of copper and it is considered to be an ineffective chelating agent for extraction of copper. However, it may be a good chelater for chromium due to its stronger inductive effect on lignin, and it may form soluble complex with chromium, which was assumed to be as follows:
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where M is chromium, H is a proton, and Ox is an oxalate ligand.
In this experiment, adding sodium hydroxide to oxalic acid to increase the pH of the solution might have restrained the accumulation of copper quite effectively. Based on the literature (Nakahara, 1997), copper oxalate is known to dissolve alkaline oxalate solution as a chelate. Therefore, we hypothesized that the mechanism of copper extraction by pH-adjusted bioxalate solution is ascribed to the formation of a water-soluble complex due to the effect of pH adjustment. After 6 h of extraction, only 8.3% of chromium, 9.5% of copper, and 0% of arsenic were presented in the extracted samples. Precipitation of copper was not observed regardless of prolonging the duration of extraction. We can conclude that pH-adjusted bioxalate solution is able to remove not only chromium but also copper and arsenic from CCA-treated wood using a one-step extraction process, in contrast to conventional oxalic acid or some other chelating extraction process.
ACQ-Treated Wood Powder
Figure 1b shows changes in copper content during bioxalate extraction of ACQ-treated powder. Metals were easily removed from ACQ-treated powder and copper extraction levels reached approximately 90% at the end of 6 h, but some variability was observed at some time intervals. The reason for the variability can be derived from the decomposition of powder sample. The sample tended to be decomposed mechanically or chemically during stirring, particularly for the powdered sample, which may have contributed to the variability. Therefore, it was desirable to measure the mass loss of sample or the elements in the extractant directly instead of measuring residual metals in the wood samples. The complexation and elemental leaching appeared to be completed in the later stages of extraction. During the extraction, the behavior of copper in ACQ-treated samples was quite similar to that of CCA-treated samples. Considering the solubility characteristics of copper in ACQ, there can be two reasons for the absence of accumulation. One is the effect of pH, which was adjusted by sodium hydroxide, on restraining the formation of an insoluble copper complex, and the other is the effect of amine, which is originally contained in ACQ reagent, forming some sort of soluble copper, amine, and oxalate complex. The present results demonstrate that bioxalate solution is highly effective not only for the purification of CCA-treated wood but also for ACQ-treated wood powder.
CuAz-Treated Wood Powder
Changes in copper content during bioxalate extraction of CuAz-treated wood powder are shown in Fig. 1c. As seen from the figure, copper was extracted rapidly during the first hour of leaching with over 90% of the total copper coming into solution. This demonstrates that the behavior of copper in the CuAz-treated sample was quite similar to that of CCA- and ACQ-treated samples. The reprecipitation of copper was not observed, regardless of increase in the duration of the extraction. These results prove that pH-adjusted bioxalate solution is highly effective not only for the extraction of CCA and ACQ, but also for CuAz-treated wood.
From the results of a series of experiments, we conclude that one-step extraction of metals from CCA, ACQ, and CuAz wood powder with bioxalate solution shows a great promise. Therefore, the application of the same method for the extraction of metals in chips of CCA-, ACQ-, and CuAz-treated samples was studied in detail in the following section.
Metals Extraction from Chips
CCA-Treated Wood Chips
Figure 2a shows changes in the chromium, copper, and arsenic contents during extraction of metals from wood treated with CCA. The results demonstrate that extraction of CCA elements by bioxalate solution proceeded rapidly and effectively. In particular, copper appeared to be extracted at the highest levels within the first hour of exposure to the solution. This level decreased to 30% of the initial amount thorough the course of extraction. Thus, copper was reconfirmed to have the highest mobility among the three elements in the extractant. Generally, longer duration of the extraction led to enhanced removal of chromium and arsenic. The order of extractability among the three elements was estimated to be copper, chromium, and arsenic, at the first stage, from 1 to 3 h. As with powder extraction, the accumulation of copper was also restrained in the chip extraction. The percentage removal of chromium, copper, and arsenic reached 88.0, 95.5, and 89.4 in 6 h, respectively. The percentage of extractability of three elements was almost same at the second stage, from 4 to 6 h. Although approximately 10% of residual elements remained in the sample even after rinsing with distilled water, we have conducted a preliminary experiment with hot water at 75°C and confirmed that the residue could be removed easily by washing with hot water (data not presented).
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Fig. 2. Changes of percentage residual amount of arsenic, copper, or chromium in chromated copper arsenate (CCA)–treated wood chips during sodium oxalate solvent extraction. (a) Percentage residual amount of arsenic, copper, and chromium in CCA-treated wood chips during sodium oxalate solvent extraction. (b) Percentage residual amount of copper in ammoniacal copper quaternary (ACQ)–treated wood chips during sodium oxalate solvent extraction. (c) Percentage residual amount of copper in copper, boron, and azole (CuAz)–treated chips during sodium oxalate solvent extraction. (d) Percentage residual amount of arsenic, copper, and chromium in the mixture of CCA-, ACQ-, and CuAz-treated wood chips during sodium oxalate solvent extraction.
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ACQ-Treated Wood Chips
Figure 2b shows the changes in copper content during extraction of wood chips treated with ACQ. The chemical composition of ACQ is different from that of CCA, in particular the solubility of ACQ salts may differ from that of CCA salts. For this study, the behavior of copper was similar to that of CCA-treated wood during extraction. As observed in CCA extraction, copper was highly mobile in the solution. It is notable that the rate of extraction from chips was only slightly different from powder, indicating that bioxalate solution can remove copper regardless of the geometric size of the sample.
CuAz-Treated Wood Chips
Figure 2c shows the changes in copper content during extraction of wood treated with CuAz, which showed that the behavior of copper in CuAz-treated wood is similar to that of CCA- and ACQ-treated wood chips. Thus, the pH-adjusted bioxalate chelating agent effectively complexed with CCA and ACQ and also with CuAz metal salts. From these results, we deduced that the superior ability of pH-adjusted bioxalate solution in removing copper is independent of the type of preservative (CCA, ACQ, and CuAz), indicating that the solvent can be a multipurpose chelating extractant for the purification of wood contaminated with various kinds of toxic metals.
Mixture of CCA-, ACQ-, and CuAz-Treated Wood Chips
The mixture of CCA-, ACQ-, and CuAz-treated chips served as model of mixture of preservative-treated wood waste. Copper may be easily extracted from ACQ- or CuAz-treated wood with citric acid or possibly other chelating agents (Cooper and Ung, 1992; Kakitani et al., 2004a; Kartal and Kose, 2003), but it is difficult to separate CCA, ACQ, and CuAz from the real preservative-treated wood wastes at a demolition site, waste treatment site, or incineration site, and separation of treated waste wood from the mixtures is not cost effective. Therefore, we prepared a mixture of these treated wood chips with a weight ratio of roughly 1:1:1 as a model mixture. The purpose of the present section is to confirm a multi-chelating ability of bioxalate extractant from the mixture of CCA-, ACQ-, and CuAz-treated wood for practical application.
Figure 2d shows the changes in chromium, copper, and arsenic contents during the extraction from the mixture of CCA-, ACQ-, and CuAz-treated wood chips. The behavior of copper was quite similar to the results of the previous experiments. On the other hand, chromium and arsenic were removed more rapidly from the mixture than from CCA-treated wood chips. Consequently, there was a minor change in the leachability of copper in the mixture of CCA, ACQ, and CuAz. One of the reasons for the minor change in the leachability of copper was the difference in the initial copper content of the sample. The initial elemental content in the CCA sample was 8.32 mg/g for chromium, 5.10 mg/g for copper, and 4.64 mg/g for arsenic, and the copper contents of the ACQ and CuAz samples were 2.69 and 1.63 mg/g, respectively. In this case, elemental content in the mixture of chips was 2.96 mg/g for chromium, 3.04 mg/g for copper, and 1.60 mg/g for arsenic. The chromium and arsenic contents in the mixture of CCA, ACQ, and CuAz were much lower than those of CCA-treated wood, but there was only a minor difference in the content of copper. The present results have shown that the lower content tends to make the extraction easier for chromium and arsenic, while the relatively higher level of copper may make extraction difficult.
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DISCUSSION
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There have been many studies to purify CCA-treated wood waste by chelating extraction. Kartal and Imamura (2003) compared the removal rates of copper, chromium, and arsenic by various extractants, particularly complexing agents. Those results were partly modified and some more information has been added in the present study (Table 1). The extraction of metals from treated wood at elevated temperatures (compared to room temperature) has rarely been reported. As seen in Table 1, the reactivity of copper, chromium, and arsenic with complexing agents varies widely and this makes a one-step solvent extraction difficult. The wood species used in previous studies was yellow pine (Pinus sylvestris L.), and the extraction was typically conducted at 25°C (except for the present experiment). In previous studies, the solid to liquid ratio of extraction was fixed at 1 g to 10 mL for Kartal and Clausen (2001) and Kartal and Kose (2003), 1 g to 20 mL for Shiau et al. (2000), and in the present study it was 1 g to 20 mL. Other studies did not report the wood to liquid ratio. Also, the experimental conditions in previous studies were wide ranging, thus it is difficult to compare the experimental results between the studies. However, the present study demonstrated that pH-adjusted bioxalate solution solubilized these elements efficiently regardless of their different characteristics. It is advantageous that the pH-adjusted bioxalate extraction proceeds under atmospheric pressure, and no special apparatus or machine (e.g., autoclave) is required. The required agents are only oxalic acid and sodium hydroxide, with current market prices of approximately 3.73 and 0.22 US$/kg (1 US$ = 110 Japanese yen), respectively, and both agents are commonly available. The pH of the solvent used for the extraction process is not strongly acidic, which may prevent the decomposition of wood chips or the corrosion of the apparatus. Finally, purified wood can be recycled into a variety of products such as composite panels (Clausen et al., 2000).
Solvent extraction of copper, chromium, and arsenic from preservative-treated wood was conducted with bioxalate solution at pH adjusted to 3.2 with sodium hydroxide. All the elements were removed efficiently and the extracted percentage of metals reached about 90% in 5 to 6 h. The present results confirmed the effectiveness of bioxalate solution at pH adjusted to 3.2 with sodium hydroxide and further research on optimization of extraction conditions, such as pH, temperature, concentration, duration of extraction, and various kinds of alkaline metals is needed to make the process more efficient.
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ACKNOWLEDGMENTS
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We would like to thank Dr. S.N. Kartal, visiting scientist from Istanbul University of Turkey at the Research Institute for Sustainable Humanosphere (RISH), Kyoto, Japan, for his helpful advice during the experiment, and Dr. Koichi Yamamoto, Forest and Forest Products Research Institute, Tsukuba, Japan, for providing the samples and for his helpful advice during the experiment.
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REFERENCES
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ABSTRACT
INTRODUCTION
