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Adsorptivity of heavy metals CuII, CdII, and PbII on woodchip-mixed porous mortar
Chemical Engineering Journal 215–216 (2013) 202–208
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Adsorptivity of heavy metals CuII, CdII, and PbII on woodchip-mixed porous mortar Masanobu Mori a,⇑, Yoshimasa Sekine a, Naoko Hara a, Ken-ichiro Nakarai b, Yuji Suzuki b, Haruka Kuge a, Yusuke Kobayashi c, Akira Arai c, Hideyuki Itabashi a a
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan Department of Civil and Environmental Engineering, Graduate School of Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan c Kobayashi Kogyou, Inc., 2-11-8 Omote-cho, Maebashi, Gunma 371-0024, Japan b
h i g h l i g h t s " Woodchip-mixed porous mortar (WPM) was developed. " Cation exchange capacity of WPM is twice as large as that of wood chips. II
" WPM can adsorb 23 times the amount of Cd as can wood chips. " Speciation of Cu, Cd, and Pb adsorbed to WPM allows control of elution behavior. " WPM maintains high adsorptivity for Cu, Cd, and Pb even after immersion for 1 year.
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 12 October 2012 Accepted 13 October 2012 Available online 10 November 2012 Keywords: Woody biomass Adsorption capacity Fractionation Elution Immersion
a b s t r a c t Here, we developed a woodchip-mixed porous mortar (WPM) by mixing wood chips obtained from thinned Japanese cedar wood with porous mortar aggregates and evaluated the adsorption properties of the heavy metal ions CuII, CdII, and PbII on the WPM. The cation-exchange capacity (CEC) of cup-shaped WPM was approximately twice as large as that for wood chips. The saturation adsorption capacity of WPM for the heavy metal ions CuII, CdII, and PbII, determined by passing an aqueous solution of heavy metal ions, was 6, 23, and 7 times as much, respectively, as wood chips alone. Furthermore, the elution behavior of heavy metal ions adsorbed by WPM was studied by fractionation, wherein the metal speciation was divided into five classes by a sequential extraction method. The leached percentage of Fractions 1 and 2 combined, corresponding to the more easily eluted species of CdII, was higher than those of CuII and PbII. Finally, we found that the immersion of flat WPM in pure water and 3% sodium chloride for 1 year did not degrade the sample, which maintained its high adsorption capacity for heavy metal ions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction With the revitalization of industrial activities over recent years, scrap wood has emerged as a major form of waste that is discharged into Japan’s environment, with approximately 1500 t generated annually. Scrap wood can be classified as: (1) wood chips and bark generated from sawmills, (2) residual wood materials such as packaging materials generated from plants, (3) discarded wood materials generated in the construction of new buildings and the demolition of existing structures, and (4) unused wood chips generated from the thinning and felling that is related to forest maintenance [1]. To make use of this scrap wood, a multistage (cascade) procedure has been recommended in which wood waste is first put to ⇑ Corresponding author. Tel.: +81 277 30 1275; fax: +81 277 30 1271. E-mail address: [email protected] (M. Mori). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.097
one use and then the remainder is used for another purpose. For example, disassembled scraps from lumber products and wood scraps are converted into chips, which are then used to manufacture particle board [1] and bark compost [2,3]. In addition, such scraps could be carbonized for use in controlling moisture under the floors of houses, eliminating odors in a room, or adsorbing hazardous chemical substances [4]. The remaining degraded materials can then be converted into pellets [5] and fuels such as bioethanol [6], completing the multistage recycling process. In this context, one approach to effectively utilizing unused wood waste is to convert it into an adsorbent for heavy metal environmental pollutants. For example, the application of aspen and maple sawdust as heavy metal adsorbents has been investigated, utilizing the metal adsorption properties of lignin and cellulose contained in the wood [7–12]. We have investigated the heavy metal ion adsorption properties of wood chips generated from the wood waste of Japanese cedar
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grown in Gunma Prefecture. Wood chips and sawdust have large surface areas and numerous sites capable of forming complexes with heavy metal ions, such as phenolic hydroxyl groups and aromatic carboxyl groups in lignin, which is one of the principal components of wood. There have been reports on the use of industrial wastes for adsorption of heavy metals. These include the woody biomass of peat and activated char [13], rice hulls, wheat husks, straw [14–19], brewer’s yeast [20], brewer’s refuse [21], and coffee beans [22–25]. As these technologies have low environmental loads and allow industrial wastes to be returned to the earth, they are useful for cleaning soil that is heavily contaminated with heavy metals. However, direct spraying of the waste makes it difficult to recover post-adsorption, presenting problems to the multistage procedure described above. In this study, we developed a woodchip-mixed porous mortar (WPM) for utilization near an area in Gunma Prefecture (Fig. S1 in Supplementary Material), where it has been used on walking trails in hot spring resorts and in filters for acidic hot springs, to achieve easier collection of waste that adsorbs heavy metals. We fabricated cylindrical WPM and determined its cation exchange capacity (CEC), adsorption ratio, and saturation adsorption capacity for CuII, CdII, and PbII ions. Furthermore, to predict the potential elution of heavy metal ions adsorbed by WPM, the sequential extraction method [26] was used to identify five classes of heavy metal ions based on the ease with which they were leached out. We then determined the abundance ratio of each fraction to predict the speciation of heavy metal ions adsorbed by the WPM. Finally, a flat WPM plate was immersed in pure water and aqueous sodium chloride for an extended period of time (1 year), resulting in the leaching of calcium ions. The variations in the adsorption properties of WPS for heavy metal ions within the immersion period were investigated. 2. Experiments 2.1. Reagents Water purified with a water distillation apparatus (ASK-2DS, Iwaki) was used to prepare aqueous solutions and to test the adsorption of heavy metal ions. All reagents were purchased from Wako Pure Chemical Industries. All stored solutions containing metal ions were acidified with nitric acid to pH < 1. Separate stock solutions were prepared by measuring a precise amount of copper (II) sulfate pentahydrate, cadmium (II) acetate dihydrate, and lead (II) acetate such that they contained 1 104 M of each heavy metal ion. The pH was adjusted using 0.1 M solutions of sodium hydroxide and nitric acid. Stock solutions of 0.1 M barium chloride dihydrate, 0.2 M magnesium sulfate heptahydrate, and 10 mg L1 lanthanum nitrate hexahydrate were prepared for use in determining the CEC of the WPM samples.
to determine the concentration of heavy metal ions. 0.01 M 3morpholinopropanesulfonic acid was added as a buffer to the solution, and the pH was adjusted within the range of 2.0–7.0 by a portable pH meter (F-22 pH meter, Horiba). To determine the saturation adsorption capacity of wood chips for heavy metal ions, Eq. (1) for a Langmuir adsorption isotherm was modified to Eq. (2), in which 1/qe on the left-hand side was plotted against 1/Ce, yielding a linear plot that was extrapolated to intersect the y-axis and provide the intercept:
Q 0 bC e 1 þ bC e
ð1Þ
1 1 1 1 þ ¼ qe Q 0 b C e Q 0
ð2Þ
qe ¼ and
where qe is the amount of the heavy metal adsorbed per unit weight of the adsorbent (mol g1), Ce is the equilibrium concentration of the heavy metal bulk solution (mol L1), Q0 is the monolayer adsorption capacity (mol g1), and b is the constant related to the free energy or net enthalpy of adsorption (b / eDH/RT). Accordingly, Q0 calculated from Eq. (2) indicates the saturation adsorption capacity. 2.3. Preparation of WPM River sand (particle size: 0.2–1.0 mm), wood chips (maximum particle size: 5 mm), cement, and tap water were mixed, stirred, and fed into a mold for fabrication into a flat plate 300 300 60 mm in size using a vibration press. This mixture was cured at ambient temperature to form the WPM. Subsequently, 100-mmlong and 50-mm-thick flat plates were cut after the water content of the cement reached approximately 30%. The chemical compositions of the cement were determined by X-ray fluorescence analysis. The compositions were 60% Ca, 23% Si, 5.0% Al, 2.2% S, 2.0% Fe and 1.9% Mg. The flat WPM plate was then cut into a filter cup 10 cm in both diameter and height (Fig. S1). The tested materials were sand:woodchip:cement in the ratios of 4:0:1, 0:4:1, and 2:2:1, as mentioned in Supplementary Materials. Control samples composed of concrete only (sand and cement), of cement and wood chips, and of sand, cement, and wood chips were defined as Samples 1, 2, and 3, respectively. 2.4. Cation exchange capacity The CEC values of the samples were determined based on the Standard for Soil Quality Testing (ISO 11260) recommended by the Japanese Geotechnical Society. The details are described in Section 2 in the Supplementary Material. 2.5. Adsorption of heavy metals by WPM
2.2. Adsorption of heavy metal ions by wood chips Japanese cedar wood chips generated in forest maintenance operations were supplied by Gunma Prefecture’s Forestry Cooperative and sifted with a sieve to collect particles with a diameter less than 5 mm as test specimens. For the heavy metal ion adsorption tests for wood chips, 50 mL of solution containing 2 106 to 1 104 M of heavy metal ions and 0.1 g of wood chips was added to a 100 mL Erlenmeyer flask. The flask was sealed with a stopper and immersed in a temperature-controlled water bath (thermoregulated water bath, Iwaki CTR-330, Iwaki) at 30 °C for 60 min and stirred with a magnetic stirring bar. The filtrate of the mixture was then collected and analyzed by a polarized Zeeman effect flame atomic absorption spectrophotometer (AAS; Z-5310, Hitachi)
A sample was placed on the experimental apparatus illustrated in Fig. S2. Five hundred milliliters of 1 105 M heavy metal ion solution, prepared by mixing aqueous CuII, CdII, and PbII solutions, was introduced to the top of the sample such that the aqueous solution was allowed to pass through. The filtrate was collected, and the concentration of heavy metals was determined by AAS. The heavy metal absorption ratio was calculated using Eq. (3):
Adsorption ratio ð%Þ ¼
C0 Cr C0
100
ð3Þ
where C0 is the initial concentration of heavy metal and Cr is the concentration of heavy metal remaining in solution after passing through the WPM test samples.
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2.6. Effects of initial pH on adsorption of heavy metal ions
3. Results and discussion
The acidity of a 1 105 M aqueous solution containing CuII, Cd , and PbII ions was adjusted within the range of pH 2–6 by adding either nitric acid or sodium hydroxide. Hundred milliliters of this solution was passed through the WPM sample, and the filtrated was collected and analyzed by AAS to determine the concentration of heavy metals.
3.1. Adsorption of heavy metals by wood chips
II
2.7. Saturation adsorption capacity of heavy metal ions by WPM Aqueous solutions of heavy metal ions were continuously passed through the WPM sample while determining their final concentration of heavy metal ions. The initial concentration of heavy metal ions was 1.0 103 M, the pH of the solutions was adjusted to 5 using an acetate buffer, and the flow rate was set at 50 mL min1. This was done until the adsorption of heavy metal ions reached saturation. The amount of heavy metal ions adsorbed by the WPM sample was then plotted against the flow volume of aqueous solution. As the curve flattened out, the adsorption reached saturation, thus giving the saturation adsorption capacity of WPM for heavy metal ions.
2.8. Elution of heavy metal ions After heavy metal ion adsorption, the cup-shaped WPM samples were air-dried at ambient temperature for 2–3 d, pulverized with a mortar and pestle, and then passed through a sieve with 2 mm diameter holes for sampling. To estimate the ease of elution of heavy metal ions from the WMP samples, the heavy metals were classified into five fractions, depending on the metal speciation in the soil and in accordance with the sequential extraction method of Tessier et al. [26]. The detailed operational procedure is given in Section 4 in the Supplementary Material. The five fractions obtained by sequential extraction were as follows. Fraction 1 (F1) was generated by stirring a mixture of the sample and 1 M MgCl2 (pH 7). Fraction 2 (F2) was generated by combining the residue of F1 and a 1 M solution of CH3COONa–CH3 COOH (pH 5). Fraction 3 (F3) was generated by combining the residue of F2 and 0.04 M NH2OH–HCl solution (25% CH3COOH v/v). Fraction 4 (F4) was generated by combining the residue of F3, 0.02 M HNO3, and 5 mL of 30% H2O2. Fraction 5 (F5) was generated by adding 46% HF and 60% HClO4 to the residue of F4. The extracts were centrifuged at 10,000 rpm for 30 min, and the remaining residue after extraction processing was washed with 8 mL of water, followed by centrifugation similar to that for the extracts. The extracts obtained were diluted with water to a volume of 25 mL and the heavy metals were quantified by AAS.
We first investigated the effects of pH on the amount of adsorbed heavy metal ions per unit weight of wood chips. As illustrated in Fig. 1, it was found that the extent of adsorption depended heavily on the pH. In the pH range 2–4, the amount adsorbed was less than 1 105 mol g1 in all cases. The exchange reactions of protons with heavy metal ions and the formation of complexes between metal ions and oxo groups are likely to be involved in the adsorption mechanism. Protons compete with metal ions to occupy the adsorption sites when there is a decrease in pH, resulting in a drastic decrease in the amount of adsorbed heavy metal ions. The heavy metal ion adsorption was maximized at pH 6–7, a level at which there was reduced competition with protons for binding sites but which was still acidic enough to prevent precipitation of heavy metals as hydroxides, which occurs at higher pH values. Fig. 2 shows the Langmuir plots for the adsorption of heavy metal ions by wood chips, revealing the relationship between the amount adsorbed and the concentration of heavy metal ions in solution. The adsorption isotherm in this plot yields a straight line for all heavy metals. Use of its intercept to estimate the saturation adsorption capacity at pH 6 resulted in capacities of 6.7 105 mol g1 for CuII, 4.4 105 mol g1 for CdII, and 6.7 105 mol g1 for PbII. The saturation adsorption capacity of the metal ions was found to be in the order of CuII = PbII > CdII. To compare with the saturation adsorption capacity of wood chips for heavy metal ions, commercially available lignin (Lignin Organosolv, Sigma-Aldrich) was used to repeat this experiment. The saturation adsorption capacity of lignin was 2.0 105 mol g1 for CuII, 1.4 105 mol g1 for CdII, and 2.1 105 mol g1 for PbII, which are all smaller values than those determined for the wood chips (Fig. S4). These results suggest that the wood chips have more available trapping sites for heavy metal ions as compared to lignin. Differences in the saturation adsorption capacity between heavy metal ion types is presumably caused by differences in the hydrated ion radii in solution [27,28] or by differences in electronegativity [27], but the exact details remain unclear at the present time. The adsorptivities of heavy metals on wood chips were also compared with those of several other adsorbents obtained from
2.9. Long-term immersion of flat WPM plate In a long-term immersion test of WPM, a flat WPM plate molded to a size of 300 100 60 mm was immersed in liquid for accelerated leach testing. A schematic view of the accelerated leaching test is provided in Fig. S3. Deionized water and salt water were used for immersion. The salt concentration was 3%, which is similar to that of sea water. The WPM plate was withdrawn on days 30, 90, 180, and 365 after immersion, dried, and cut to a cube of size 50 50 50 mm. Each side of the cube was wrapped with aluminum tape to prevent leakage of the solution containing heavy metal ions. Heavy metal adsorption tests similar to those described in Section 2.5 were then performed.
Fig. 1. Adsorption of heavy metals by wood chips as a function of initial solution pH.
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sorbed CdII and PbII almost quantitatively and 91% of CuII. Sample 3 (WPM), containing sand, cement, and wood chips, adsorbed CdII and PbII almost quantitatively but only 87% of CuII. The flow rate for the solutions passed through Samples 2 and 3 was varied in order to determine the adsorption ratio of CuII, which was found to increase with decreasing flow rate, resulting in almost quantitative adsorption at a flow rate of 50–60 mL min1 for both samples. Conversely, an increased flow rate significantly reduced the adsorption ratio of CuII, resulting in virtually no adsorption at a flow rate of 200 mL min1 (see Fig. 3). Similar results were observed for Sample 2. The adsorption ratios of CdII and PbII were nearly 90% even at flow rates of 200–300 mL min1, suggesting that WPM adsorbed CdII and PbII faster than it did CuII. 3.4. Effects on adsorptivity of initial pH of solutions containing heavy metal ions
Fig. 2. Langmuir plots for the amount of adsorbed CuII, CdII, and PbII by wood chips 1 against their concentrations (plots of q1 e vs: C e ).
industrial wastes [7,10,12,14,21,23–25]. As summarized in Table S1, the saturation adsorption capacities of wood chips for heavy metals were smaller than those of several of the adsorbents, most likely because the particle size of the wood chips used (<5 mm) was larger than that of the other adsorbents (approximately <1 mm). In general, the biomass adsorbents adsorbed CuII and PbII more strongly than CdII except for coffee residue as reported by Boonamnuayvitaya et al. [23]. Next, in order to identify the adsorption sites for heavy metal ions in wood chips, Fourier transform infrared (FT-IR) spectroscopy was used to analyze the samples before and after heavy metal ion adsorption. A decrease in the absorption peak intensity around 1700 cm1 was observed upon heavy metal ion absorption, which was assigned to the carboxyl group, thus confirming involvement of the carboxyl group in the adsorption of heavy metal ions [29] (Fig. S5). Similar results have been reported in studies using date palm seeds and lignin, and the involvement of the carboxyl groups in lignin as adsorption sites of heavy metal ions has been previously proposed [30,31]. 3.2. Cation exchange capacity (CEC) of WPM
Aqueous heavy metal ion solutions with initial pH values adjusted to lie within the range of 2–6 were passed through WPM to determine the heavy metal ion adsorption ratio. Based on the results presented in Section 3.3, the flow rate was set at 50 mL min1. As illustrated in Fig. 4, the adsorption ratio of heavy metal ions by WPM increased with increasing solution pH. The heavy metals were adsorbed almost quantitatively at an initial pH of 4. The pH of the solution after having passed through the WPM was found to be 7.6–8.2. This result indicates that the alkalinity remained inside the WPM, presumably because not only adsorption but also precipitation of heavy metal ions as hydroxides occurred. The adsorptivity of heavy metal ions was lowered when the initial pH was lowered to 2–3. Compared to the results in Fig. 1, the lower adsorption ratio of heavy metal ions at a low initial pH is likely to be caused by competition between protons and heavy metal ions for cation exchange groups (e.g., carboxyl groups) present in the wood chips or cement in WPM. 3.5. Saturation adsorption capacity of WPM for heavy metals The saturation adsorption capacity of WPM (Sample 3) for heavy metals was investigated by determining the relationship between the flow volumes of the heavy metal ion solutions to their ratios of adsorption by WPM. Heavy metal solution (1.0 103 M, pH = 5) was passed over the WPM at 50 mL min1. The determined saturation adsorption capacities for heavy metal ions were 4.0 104 mol g1 for CuII, 9.9 104 mol g1 for CdII, and
The CECs of Samples 1, 2, and 3 were determined as an index for the adsorptivity of heavy metals. The CEC of Sample 1 (sand and cement), Sample 2 (wood chips and sand), and Sample 3 (WPM) were 16.2 cmol kg1, 55.0 cmol kg1, and 36.6 cmol kg1, respectively. Samples 2 and 3 adsorbed approximately three and two times the amount of heavy metal ions, respectively, as a typical concrete product such as Sample 1. Sample 2, exhibiting the highest CEC, contained almost twice as many wood chips as Sample 3, suggesting that the CEC of the samples was dependent upon the amount of wood chips present. 3.3. Adsorption of heavy metals by WPM To study the adsorption properties of each sample as they related to heavy metals, 1 105 M solutions of CuII, CdII, and PbII were first passed through the samples to determine their adsorption ratios. The pH and flow rate of the solutions were 5 and 80 mL min1, respectively. Sample 1, a typical concrete product, adsorbed the three heavy metal ions almost quantitatively. On the other hand, Sample 2, composed of wood chips and cement, ad-
Fig. 3. Adsorption ratio of CuII, CdII, and PbII by WPM as a function of flow rate.
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Fig. 4. Adsorption ratio of heavy metals by WPM as a function of initial solution pH.
4.8 104 mol g1 for PbII, indicating a significant increase in the saturation adsorption capacity of WPM. Specifically, these values are nearly two, seven, and twenty-three times the amounts of CuII, PbII, and CdII as those determined for the wood chips alone. The saturation adsorption capacities determined for Sample 1 (cement and sand) were 1.7 104 mol g1 for CuII, 3.2 104 mol g1 for CdII, and 2.8 104 mol g1 for PbII, suggesting that the adsorption effects of WPM for CdII are firmly maintained through complex interactions within the mixed system of wood chips, cement, and sand. In addition, the capacities of WPM were similar to those of other adsorbents, such as coffee residue or papaya wood, as shown in Table S1. This led us to conclude that the adsorptivity of heavy metals on wood chips was enhanced by mixing the wood chips with cement.
The content of heavy metals originally present in WPM and their abundance ratios in each fraction were studied. The total content of heavy metals was defined as the sum of the concentration of heavy metals determined for the operations involving F1–F5. The content of heavy metals was found to be 21 lg g1 for Cu, 1.7 lg g1 for Cd, and 19 lg g1 for Pb. As illustrated in Fig. 5A, the abundance ratios determined for heavy metals by the sequential extraction method were 83–97% of all heavy metals in F3 to F5. However, the abundance ratio of Cd in F2 was 13%, which was higher than those of Cu and Pb. Similar analysis has been carried out for sediments, sludge, and incinerator ash, and in all cases it has been reported that the abundance ratio of Cd is relatively high in more easily eluted species (F1 and F2), while those of Cu and Pb are high in less-easily eluted species (F3–F5). These results indicate that this elution behavior of heavy metals was not unique to WPM [32–38]. The total content of heavy metals in the WPM after filtering solutions containing 1.0 103 M heavy metal ions was 162 lg g1 Cu, 104 lg g1 Cd, and 196 lg g1 Pb, confirming that WPM adsorbed the heavy metals from solution. The heavy metal ions adsorbed by the WPM were divided into five fractions by the sequential extraction method in order to determine their abundance ratios. As illustrated in Fig. 5B, the abundance ratios of Cu, Cd, and Pb showed the highest ratio in F3. As heavy metal ions were dissolved in solution, the abundance ratio of heavy metals in more easily eluted species (F1 and F2) was likely to be higher than their abundance ratio in the WPM raw materials. However, the sum of the abundance ratios of heavy metals in less easily eluted species (F3 to F5) was 85% for Cu and 95% for Pb. These results suggest that both metals form insoluble hydroxides as well as inactive organic complexes bonded to the lignin of wood chips in WPM. When the results in Figs. 4 and 6B are considered together, it is apparent that the Cu ions were slowly adsorbed by WPM and firmly retained after adsorption. On the other hand, as approximately 40% of the total amount of Cd was fractionated in F1 and F2, the Cd ions were quickly adsorbed by WPM but were held less tightly and eluted relatively easier. 3.7. Adsorption of heavy metals by a flat WPM plate immersed in pure water and salt water
3.6. Elution of heavy metals from WPM by sequential extraction The elution behavior of heavy metals adsorbed by WPM was investigated. This method can classify the various heavy metals in soil according to their speciation [26]. The major speciation of each fraction is as follows: chlorides for F1, carbonates for F2, oxides for F3, sulfides or complexes with colloidal organic matter for F4, and silicates for F5. This method has been applied not only to soil but also to lake sediments [32], sewage sludge [33–36], and incinerator ash [37,38].
WPM molded into a flat shape for use as roadbed material was soaked in solutions of pure water and salt water (3% sodium chloride). The sample was withdrawn from the solution and examined at intervals of 0, 30, 90, 180, and 365 days during the immersion period in order to determine the adsorption ratio of the heavy metal ions for each WPM plate. No degradation of any WPM plates was observed during the period of immersion. 16% and 25% of calcium ions were leached from the WPM plates in pure water and in salt water, respectively.
Fig. 5. Fractionation of heavy metals adsorbed to WPM by sequential extraction procedure. WPM (A) before absorbing heavy metals, and (B) after absorbing heavy metals.
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aggregate of wood chips and sand maintained a higher degree of adsorptivity for heavy metal ions than a control sample prepared from sand alone. It was also found that the flow rate of the solution passing through WPM played an important role in the adsorption of heavy metal ions. The CEC values of concrete and WPM were determined to be 16.2 and 36.6 cmol kg1, respectively. The larger CEC of WPM is due to the aromatic carboxyl groups in the wood chips functioning as adsorption sites. Furthermore, to assess the potential elution of heavy metals adsorbed by WPM, the sequential extraction method for heavy metals in soil was applied for classifying the metal speciation. It was found that the abundance ratio of CdII immobilized by WPM was elevated in F2, indicating potential elution of CdII adsorbed by WPM by rain water and acidification of soil. Based on these results, we determined that WPM can maintain a high level of adsorptivity with regards heavy metal ions, even in the degraded state generated by long-term immersion in running water. WPM is also able to maintain its form, resisting the elution of adsorbed heavy metals. Therefore, it has been determined that WPM, which is fabricated from wood waste, is useful as an adsorption material for heavy metals. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research (23656276) provided by the Japan Society for the Promotion of Science (JSPS). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.10.097. Fig. 6. Effects of long-term immersion on the adsorption ratio of heavy metal ions by a flat WPM plate (300 100 60 mm) immersed in (A) pure and (B) salt water (3% sodium chloride).
The adsorption of heavy metal ions by these WPM samples undergoing long term immersion was tested. The pH of the heavy metal solutions was adjusted to 5 and the flow rate was kept to within the range of 50–60 mL min1. As illustrated in Fig. 6, the flat WPM plate immersed in pure water quantitatively adsorbed heavy metal ions after one year of immersion, and the pH of the solution passed through the WPM plate was stable (between 11 and 12). On the other hand, the adsorption ratio of heavy metal ions by the WPM plate immersed in salt water decreased with increasing duration of immersion and, after one year of immersion, decreased by approximately 10% for all heavy metals. The pH of the solutions passed through the WPM plate showed little change, suggesting that immersion in salt water caused an excessive coordination of sodium ions to the adsorption sites in WPM, thus reducing the adsorption effects of WPM for heavy metal ions. The adsorption ratio of heavy metal ions by the WPM plate immersed in salt water for one year was lower than that of the WPM sample described in Section 3.6, despite the fact that the calcium ion elution ratio was 25% for both samples. This decrease may be due to interference from sodium ions as well as a decrease in the efficiency of contact with heavy metal ions from the elution of calcium ions. 4. Conclusions In the present study, woodchip-mixed porous mortar (WPM) was prepared and the adsorptivity of heavy metal ions for this material was studied. It was found that WPM prepared from an
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Adsorptivity of heavy metals CuII, CdII, and PbII on woodchip-mixed porous mortar Masanobu Mori a,⇑, Yoshimasa Sekine a, Naoko Hara a, Ken-ichiro Nakarai b, Yuji Suzuki b, Haruka Kuge a, Yusuke Kobayashi c, Akira Arai c, Hideyuki Itabashi a a
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan Department of Civil and Environmental Engineering, Graduate School of Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma 376-8515, Japan c Kobayashi Kogyou, Inc., 2-11-8 Omote-cho, Maebashi, Gunma 371-0024, Japan b
h i g h l i g h t s " Woodchip-mixed porous mortar (WPM) was developed. " Cation exchange capacity of WPM is twice as large as that of wood chips. II
" WPM can adsorb 23 times the amount of Cd as can wood chips. " Speciation of Cu, Cd, and Pb adsorbed to WPM allows control of elution behavior. " WPM maintains high adsorptivity for Cu, Cd, and Pb even after immersion for 1 year.
a r t i c l e
i n f o
Article history: Received 18 August 2012 Received in revised form 12 October 2012 Accepted 13 October 2012 Available online 10 November 2012 Keywords: Woody biomass Adsorption capacity Fractionation Elution Immersion
a b s t r a c t Here, we developed a woodchip-mixed porous mortar (WPM) by mixing wood chips obtained from thinned Japanese cedar wood with porous mortar aggregates and evaluated the adsorption properties of the heavy metal ions CuII, CdII, and PbII on the WPM. The cation-exchange capacity (CEC) of cup-shaped WPM was approximately twice as large as that for wood chips. The saturation adsorption capacity of WPM for the heavy metal ions CuII, CdII, and PbII, determined by passing an aqueous solution of heavy metal ions, was 6, 23, and 7 times as much, respectively, as wood chips alone. Furthermore, the elution behavior of heavy metal ions adsorbed by WPM was studied by fractionation, wherein the metal speciation was divided into five classes by a sequential extraction method. The leached percentage of Fractions 1 and 2 combined, corresponding to the more easily eluted species of CdII, was higher than those of CuII and PbII. Finally, we found that the immersion of flat WPM in pure water and 3% sodium chloride for 1 year did not degrade the sample, which maintained its high adsorption capacity for heavy metal ions. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction With the revitalization of industrial activities over recent years, scrap wood has emerged as a major form of waste that is discharged into Japan’s environment, with approximately 1500 t generated annually. Scrap wood can be classified as: (1) wood chips and bark generated from sawmills, (2) residual wood materials such as packaging materials generated from plants, (3) discarded wood materials generated in the construction of new buildings and the demolition of existing structures, and (4) unused wood chips generated from the thinning and felling that is related to forest maintenance [1]. To make use of this scrap wood, a multistage (cascade) procedure has been recommended in which wood waste is first put to ⇑ Corresponding author. Tel.: +81 277 30 1275; fax: +81 277 30 1271. E-mail address: [email protected] (M. Mori). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.097
one use and then the remainder is used for another purpose. For example, disassembled scraps from lumber products and wood scraps are converted into chips, which are then used to manufacture particle board [1] and bark compost [2,3]. In addition, such scraps could be carbonized for use in controlling moisture under the floors of houses, eliminating odors in a room, or adsorbing hazardous chemical substances [4]. The remaining degraded materials can then be converted into pellets [5] and fuels such as bioethanol [6], completing the multistage recycling process. In this context, one approach to effectively utilizing unused wood waste is to convert it into an adsorbent for heavy metal environmental pollutants. For example, the application of aspen and maple sawdust as heavy metal adsorbents has been investigated, utilizing the metal adsorption properties of lignin and cellulose contained in the wood [7–12]. We have investigated the heavy metal ion adsorption properties of wood chips generated from the wood waste of Japanese cedar
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grown in Gunma Prefecture. Wood chips and sawdust have large surface areas and numerous sites capable of forming complexes with heavy metal ions, such as phenolic hydroxyl groups and aromatic carboxyl groups in lignin, which is one of the principal components of wood. There have been reports on the use of industrial wastes for adsorption of heavy metals. These include the woody biomass of peat and activated char [13], rice hulls, wheat husks, straw [14–19], brewer’s yeast [20], brewer’s refuse [21], and coffee beans [22–25]. As these technologies have low environmental loads and allow industrial wastes to be returned to the earth, they are useful for cleaning soil that is heavily contaminated with heavy metals. However, direct spraying of the waste makes it difficult to recover post-adsorption, presenting problems to the multistage procedure described above. In this study, we developed a woodchip-mixed porous mortar (WPM) for utilization near an area in Gunma Prefecture (Fig. S1 in Supplementary Material), where it has been used on walking trails in hot spring resorts and in filters for acidic hot springs, to achieve easier collection of waste that adsorbs heavy metals. We fabricated cylindrical WPM and determined its cation exchange capacity (CEC), adsorption ratio, and saturation adsorption capacity for CuII, CdII, and PbII ions. Furthermore, to predict the potential elution of heavy metal ions adsorbed by WPM, the sequential extraction method [26] was used to identify five classes of heavy metal ions based on the ease with which they were leached out. We then determined the abundance ratio of each fraction to predict the speciation of heavy metal ions adsorbed by the WPM. Finally, a flat WPM plate was immersed in pure water and aqueous sodium chloride for an extended period of time (1 year), resulting in the leaching of calcium ions. The variations in the adsorption properties of WPS for heavy metal ions within the immersion period were investigated. 2. Experiments 2.1. Reagents Water purified with a water distillation apparatus (ASK-2DS, Iwaki) was used to prepare aqueous solutions and to test the adsorption of heavy metal ions. All reagents were purchased from Wako Pure Chemical Industries. All stored solutions containing metal ions were acidified with nitric acid to pH < 1. Separate stock solutions were prepared by measuring a precise amount of copper (II) sulfate pentahydrate, cadmium (II) acetate dihydrate, and lead (II) acetate such that they contained 1 104 M of each heavy metal ion. The pH was adjusted using 0.1 M solutions of sodium hydroxide and nitric acid. Stock solutions of 0.1 M barium chloride dihydrate, 0.2 M magnesium sulfate heptahydrate, and 10 mg L1 lanthanum nitrate hexahydrate were prepared for use in determining the CEC of the WPM samples.
to determine the concentration of heavy metal ions. 0.01 M 3morpholinopropanesulfonic acid was added as a buffer to the solution, and the pH was adjusted within the range of 2.0–7.0 by a portable pH meter (F-22 pH meter, Horiba). To determine the saturation adsorption capacity of wood chips for heavy metal ions, Eq. (1) for a Langmuir adsorption isotherm was modified to Eq. (2), in which 1/qe on the left-hand side was plotted against 1/Ce, yielding a linear plot that was extrapolated to intersect the y-axis and provide the intercept:
Q 0 bC e 1 þ bC e
ð1Þ
1 1 1 1 þ ¼ qe Q 0 b C e Q 0
ð2Þ
qe ¼ and
where qe is the amount of the heavy metal adsorbed per unit weight of the adsorbent (mol g1), Ce is the equilibrium concentration of the heavy metal bulk solution (mol L1), Q0 is the monolayer adsorption capacity (mol g1), and b is the constant related to the free energy or net enthalpy of adsorption (b / eDH/RT). Accordingly, Q0 calculated from Eq. (2) indicates the saturation adsorption capacity. 2.3. Preparation of WPM River sand (particle size: 0.2–1.0 mm), wood chips (maximum particle size: 5 mm), cement, and tap water were mixed, stirred, and fed into a mold for fabrication into a flat plate 300 300 60 mm in size using a vibration press. This mixture was cured at ambient temperature to form the WPM. Subsequently, 100-mmlong and 50-mm-thick flat plates were cut after the water content of the cement reached approximately 30%. The chemical compositions of the cement were determined by X-ray fluorescence analysis. The compositions were 60% Ca, 23% Si, 5.0% Al, 2.2% S, 2.0% Fe and 1.9% Mg. The flat WPM plate was then cut into a filter cup 10 cm in both diameter and height (Fig. S1). The tested materials were sand:woodchip:cement in the ratios of 4:0:1, 0:4:1, and 2:2:1, as mentioned in Supplementary Materials. Control samples composed of concrete only (sand and cement), of cement and wood chips, and of sand, cement, and wood chips were defined as Samples 1, 2, and 3, respectively. 2.4. Cation exchange capacity The CEC values of the samples were determined based on the Standard for Soil Quality Testing (ISO 11260) recommended by the Japanese Geotechnical Society. The details are described in Section 2 in the Supplementary Material. 2.5. Adsorption of heavy metals by WPM
2.2. Adsorption of heavy metal ions by wood chips Japanese cedar wood chips generated in forest maintenance operations were supplied by Gunma Prefecture’s Forestry Cooperative and sifted with a sieve to collect particles with a diameter less than 5 mm as test specimens. For the heavy metal ion adsorption tests for wood chips, 50 mL of solution containing 2 106 to 1 104 M of heavy metal ions and 0.1 g of wood chips was added to a 100 mL Erlenmeyer flask. The flask was sealed with a stopper and immersed in a temperature-controlled water bath (thermoregulated water bath, Iwaki CTR-330, Iwaki) at 30 °C for 60 min and stirred with a magnetic stirring bar. The filtrate of the mixture was then collected and analyzed by a polarized Zeeman effect flame atomic absorption spectrophotometer (AAS; Z-5310, Hitachi)
A sample was placed on the experimental apparatus illustrated in Fig. S2. Five hundred milliliters of 1 105 M heavy metal ion solution, prepared by mixing aqueous CuII, CdII, and PbII solutions, was introduced to the top of the sample such that the aqueous solution was allowed to pass through. The filtrate was collected, and the concentration of heavy metals was determined by AAS. The heavy metal absorption ratio was calculated using Eq. (3):
Adsorption ratio ð%Þ ¼
C0 Cr C0
100
ð3Þ
where C0 is the initial concentration of heavy metal and Cr is the concentration of heavy metal remaining in solution after passing through the WPM test samples.
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2.6. Effects of initial pH on adsorption of heavy metal ions
3. Results and discussion
The acidity of a 1 105 M aqueous solution containing CuII, Cd , and PbII ions was adjusted within the range of pH 2–6 by adding either nitric acid or sodium hydroxide. Hundred milliliters of this solution was passed through the WPM sample, and the filtrated was collected and analyzed by AAS to determine the concentration of heavy metals.
3.1. Adsorption of heavy metals by wood chips
II
2.7. Saturation adsorption capacity of heavy metal ions by WPM Aqueous solutions of heavy metal ions were continuously passed through the WPM sample while determining their final concentration of heavy metal ions. The initial concentration of heavy metal ions was 1.0 103 M, the pH of the solutions was adjusted to 5 using an acetate buffer, and the flow rate was set at 50 mL min1. This was done until the adsorption of heavy metal ions reached saturation. The amount of heavy metal ions adsorbed by the WPM sample was then plotted against the flow volume of aqueous solution. As the curve flattened out, the adsorption reached saturation, thus giving the saturation adsorption capacity of WPM for heavy metal ions.
2.8. Elution of heavy metal ions After heavy metal ion adsorption, the cup-shaped WPM samples were air-dried at ambient temperature for 2–3 d, pulverized with a mortar and pestle, and then passed through a sieve with 2 mm diameter holes for sampling. To estimate the ease of elution of heavy metal ions from the WMP samples, the heavy metals were classified into five fractions, depending on the metal speciation in the soil and in accordance with the sequential extraction method of Tessier et al. [26]. The detailed operational procedure is given in Section 4 in the Supplementary Material. The five fractions obtained by sequential extraction were as follows. Fraction 1 (F1) was generated by stirring a mixture of the sample and 1 M MgCl2 (pH 7). Fraction 2 (F2) was generated by combining the residue of F1 and a 1 M solution of CH3COONa–CH3 COOH (pH 5). Fraction 3 (F3) was generated by combining the residue of F2 and 0.04 M NH2OH–HCl solution (25% CH3COOH v/v). Fraction 4 (F4) was generated by combining the residue of F3, 0.02 M HNO3, and 5 mL of 30% H2O2. Fraction 5 (F5) was generated by adding 46% HF and 60% HClO4 to the residue of F4. The extracts were centrifuged at 10,000 rpm for 30 min, and the remaining residue after extraction processing was washed with 8 mL of water, followed by centrifugation similar to that for the extracts. The extracts obtained were diluted with water to a volume of 25 mL and the heavy metals were quantified by AAS.
We first investigated the effects of pH on the amount of adsorbed heavy metal ions per unit weight of wood chips. As illustrated in Fig. 1, it was found that the extent of adsorption depended heavily on the pH. In the pH range 2–4, the amount adsorbed was less than 1 105 mol g1 in all cases. The exchange reactions of protons with heavy metal ions and the formation of complexes between metal ions and oxo groups are likely to be involved in the adsorption mechanism. Protons compete with metal ions to occupy the adsorption sites when there is a decrease in pH, resulting in a drastic decrease in the amount of adsorbed heavy metal ions. The heavy metal ion adsorption was maximized at pH 6–7, a level at which there was reduced competition with protons for binding sites but which was still acidic enough to prevent precipitation of heavy metals as hydroxides, which occurs at higher pH values. Fig. 2 shows the Langmuir plots for the adsorption of heavy metal ions by wood chips, revealing the relationship between the amount adsorbed and the concentration of heavy metal ions in solution. The adsorption isotherm in this plot yields a straight line for all heavy metals. Use of its intercept to estimate the saturation adsorption capacity at pH 6 resulted in capacities of 6.7 105 mol g1 for CuII, 4.4 105 mol g1 for CdII, and 6.7 105 mol g1 for PbII. The saturation adsorption capacity of the metal ions was found to be in the order of CuII = PbII > CdII. To compare with the saturation adsorption capacity of wood chips for heavy metal ions, commercially available lignin (Lignin Organosolv, Sigma-Aldrich) was used to repeat this experiment. The saturation adsorption capacity of lignin was 2.0 105 mol g1 for CuII, 1.4 105 mol g1 for CdII, and 2.1 105 mol g1 for PbII, which are all smaller values than those determined for the wood chips (Fig. S4). These results suggest that the wood chips have more available trapping sites for heavy metal ions as compared to lignin. Differences in the saturation adsorption capacity between heavy metal ion types is presumably caused by differences in the hydrated ion radii in solution [27,28] or by differences in electronegativity [27], but the exact details remain unclear at the present time. The adsorptivities of heavy metals on wood chips were also compared with those of several other adsorbents obtained from
2.9. Long-term immersion of flat WPM plate In a long-term immersion test of WPM, a flat WPM plate molded to a size of 300 100 60 mm was immersed in liquid for accelerated leach testing. A schematic view of the accelerated leaching test is provided in Fig. S3. Deionized water and salt water were used for immersion. The salt concentration was 3%, which is similar to that of sea water. The WPM plate was withdrawn on days 30, 90, 180, and 365 after immersion, dried, and cut to a cube of size 50 50 50 mm. Each side of the cube was wrapped with aluminum tape to prevent leakage of the solution containing heavy metal ions. Heavy metal adsorption tests similar to those described in Section 2.5 were then performed.
Fig. 1. Adsorption of heavy metals by wood chips as a function of initial solution pH.
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sorbed CdII and PbII almost quantitatively and 91% of CuII. Sample 3 (WPM), containing sand, cement, and wood chips, adsorbed CdII and PbII almost quantitatively but only 87% of CuII. The flow rate for the solutions passed through Samples 2 and 3 was varied in order to determine the adsorption ratio of CuII, which was found to increase with decreasing flow rate, resulting in almost quantitative adsorption at a flow rate of 50–60 mL min1 for both samples. Conversely, an increased flow rate significantly reduced the adsorption ratio of CuII, resulting in virtually no adsorption at a flow rate of 200 mL min1 (see Fig. 3). Similar results were observed for Sample 2. The adsorption ratios of CdII and PbII were nearly 90% even at flow rates of 200–300 mL min1, suggesting that WPM adsorbed CdII and PbII faster than it did CuII. 3.4. Effects on adsorptivity of initial pH of solutions containing heavy metal ions
Fig. 2. Langmuir plots for the amount of adsorbed CuII, CdII, and PbII by wood chips 1 against their concentrations (plots of q1 e vs: C e ).
industrial wastes [7,10,12,14,21,23–25]. As summarized in Table S1, the saturation adsorption capacities of wood chips for heavy metals were smaller than those of several of the adsorbents, most likely because the particle size of the wood chips used (<5 mm) was larger than that of the other adsorbents (approximately <1 mm). In general, the biomass adsorbents adsorbed CuII and PbII more strongly than CdII except for coffee residue as reported by Boonamnuayvitaya et al. [23]. Next, in order to identify the adsorption sites for heavy metal ions in wood chips, Fourier transform infrared (FT-IR) spectroscopy was used to analyze the samples before and after heavy metal ion adsorption. A decrease in the absorption peak intensity around 1700 cm1 was observed upon heavy metal ion absorption, which was assigned to the carboxyl group, thus confirming involvement of the carboxyl group in the adsorption of heavy metal ions [29] (Fig. S5). Similar results have been reported in studies using date palm seeds and lignin, and the involvement of the carboxyl groups in lignin as adsorption sites of heavy metal ions has been previously proposed [30,31]. 3.2. Cation exchange capacity (CEC) of WPM
Aqueous heavy metal ion solutions with initial pH values adjusted to lie within the range of 2–6 were passed through WPM to determine the heavy metal ion adsorption ratio. Based on the results presented in Section 3.3, the flow rate was set at 50 mL min1. As illustrated in Fig. 4, the adsorption ratio of heavy metal ions by WPM increased with increasing solution pH. The heavy metals were adsorbed almost quantitatively at an initial pH of 4. The pH of the solution after having passed through the WPM was found to be 7.6–8.2. This result indicates that the alkalinity remained inside the WPM, presumably because not only adsorption but also precipitation of heavy metal ions as hydroxides occurred. The adsorptivity of heavy metal ions was lowered when the initial pH was lowered to 2–3. Compared to the results in Fig. 1, the lower adsorption ratio of heavy metal ions at a low initial pH is likely to be caused by competition between protons and heavy metal ions for cation exchange groups (e.g., carboxyl groups) present in the wood chips or cement in WPM. 3.5. Saturation adsorption capacity of WPM for heavy metals The saturation adsorption capacity of WPM (Sample 3) for heavy metals was investigated by determining the relationship between the flow volumes of the heavy metal ion solutions to their ratios of adsorption by WPM. Heavy metal solution (1.0 103 M, pH = 5) was passed over the WPM at 50 mL min1. The determined saturation adsorption capacities for heavy metal ions were 4.0 104 mol g1 for CuII, 9.9 104 mol g1 for CdII, and
The CECs of Samples 1, 2, and 3 were determined as an index for the adsorptivity of heavy metals. The CEC of Sample 1 (sand and cement), Sample 2 (wood chips and sand), and Sample 3 (WPM) were 16.2 cmol kg1, 55.0 cmol kg1, and 36.6 cmol kg1, respectively. Samples 2 and 3 adsorbed approximately three and two times the amount of heavy metal ions, respectively, as a typical concrete product such as Sample 1. Sample 2, exhibiting the highest CEC, contained almost twice as many wood chips as Sample 3, suggesting that the CEC of the samples was dependent upon the amount of wood chips present. 3.3. Adsorption of heavy metals by WPM To study the adsorption properties of each sample as they related to heavy metals, 1 105 M solutions of CuII, CdII, and PbII were first passed through the samples to determine their adsorption ratios. The pH and flow rate of the solutions were 5 and 80 mL min1, respectively. Sample 1, a typical concrete product, adsorbed the three heavy metal ions almost quantitatively. On the other hand, Sample 2, composed of wood chips and cement, ad-
Fig. 3. Adsorption ratio of CuII, CdII, and PbII by WPM as a function of flow rate.
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Fig. 4. Adsorption ratio of heavy metals by WPM as a function of initial solution pH.
4.8 104 mol g1 for PbII, indicating a significant increase in the saturation adsorption capacity of WPM. Specifically, these values are nearly two, seven, and twenty-three times the amounts of CuII, PbII, and CdII as those determined for the wood chips alone. The saturation adsorption capacities determined for Sample 1 (cement and sand) were 1.7 104 mol g1 for CuII, 3.2 104 mol g1 for CdII, and 2.8 104 mol g1 for PbII, suggesting that the adsorption effects of WPM for CdII are firmly maintained through complex interactions within the mixed system of wood chips, cement, and sand. In addition, the capacities of WPM were similar to those of other adsorbents, such as coffee residue or papaya wood, as shown in Table S1. This led us to conclude that the adsorptivity of heavy metals on wood chips was enhanced by mixing the wood chips with cement.
The content of heavy metals originally present in WPM and their abundance ratios in each fraction were studied. The total content of heavy metals was defined as the sum of the concentration of heavy metals determined for the operations involving F1–F5. The content of heavy metals was found to be 21 lg g1 for Cu, 1.7 lg g1 for Cd, and 19 lg g1 for Pb. As illustrated in Fig. 5A, the abundance ratios determined for heavy metals by the sequential extraction method were 83–97% of all heavy metals in F3 to F5. However, the abundance ratio of Cd in F2 was 13%, which was higher than those of Cu and Pb. Similar analysis has been carried out for sediments, sludge, and incinerator ash, and in all cases it has been reported that the abundance ratio of Cd is relatively high in more easily eluted species (F1 and F2), while those of Cu and Pb are high in less-easily eluted species (F3–F5). These results indicate that this elution behavior of heavy metals was not unique to WPM [32–38]. The total content of heavy metals in the WPM after filtering solutions containing 1.0 103 M heavy metal ions was 162 lg g1 Cu, 104 lg g1 Cd, and 196 lg g1 Pb, confirming that WPM adsorbed the heavy metals from solution. The heavy metal ions adsorbed by the WPM were divided into five fractions by the sequential extraction method in order to determine their abundance ratios. As illustrated in Fig. 5B, the abundance ratios of Cu, Cd, and Pb showed the highest ratio in F3. As heavy metal ions were dissolved in solution, the abundance ratio of heavy metals in more easily eluted species (F1 and F2) was likely to be higher than their abundance ratio in the WPM raw materials. However, the sum of the abundance ratios of heavy metals in less easily eluted species (F3 to F5) was 85% for Cu and 95% for Pb. These results suggest that both metals form insoluble hydroxides as well as inactive organic complexes bonded to the lignin of wood chips in WPM. When the results in Figs. 4 and 6B are considered together, it is apparent that the Cu ions were slowly adsorbed by WPM and firmly retained after adsorption. On the other hand, as approximately 40% of the total amount of Cd was fractionated in F1 and F2, the Cd ions were quickly adsorbed by WPM but were held less tightly and eluted relatively easier. 3.7. Adsorption of heavy metals by a flat WPM plate immersed in pure water and salt water
3.6. Elution of heavy metals from WPM by sequential extraction The elution behavior of heavy metals adsorbed by WPM was investigated. This method can classify the various heavy metals in soil according to their speciation [26]. The major speciation of each fraction is as follows: chlorides for F1, carbonates for F2, oxides for F3, sulfides or complexes with colloidal organic matter for F4, and silicates for F5. This method has been applied not only to soil but also to lake sediments [32], sewage sludge [33–36], and incinerator ash [37,38].
WPM molded into a flat shape for use as roadbed material was soaked in solutions of pure water and salt water (3% sodium chloride). The sample was withdrawn from the solution and examined at intervals of 0, 30, 90, 180, and 365 days during the immersion period in order to determine the adsorption ratio of the heavy metal ions for each WPM plate. No degradation of any WPM plates was observed during the period of immersion. 16% and 25% of calcium ions were leached from the WPM plates in pure water and in salt water, respectively.
Fig. 5. Fractionation of heavy metals adsorbed to WPM by sequential extraction procedure. WPM (A) before absorbing heavy metals, and (B) after absorbing heavy metals.
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aggregate of wood chips and sand maintained a higher degree of adsorptivity for heavy metal ions than a control sample prepared from sand alone. It was also found that the flow rate of the solution passing through WPM played an important role in the adsorption of heavy metal ions. The CEC values of concrete and WPM were determined to be 16.2 and 36.6 cmol kg1, respectively. The larger CEC of WPM is due to the aromatic carboxyl groups in the wood chips functioning as adsorption sites. Furthermore, to assess the potential elution of heavy metals adsorbed by WPM, the sequential extraction method for heavy metals in soil was applied for classifying the metal speciation. It was found that the abundance ratio of CdII immobilized by WPM was elevated in F2, indicating potential elution of CdII adsorbed by WPM by rain water and acidification of soil. Based on these results, we determined that WPM can maintain a high level of adsorptivity with regards heavy metal ions, even in the degraded state generated by long-term immersion in running water. WPM is also able to maintain its form, resisting the elution of adsorbed heavy metals. Therefore, it has been determined that WPM, which is fabricated from wood waste, is useful as an adsorption material for heavy metals. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research (23656276) provided by the Japan Society for the Promotion of Science (JSPS). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.10.097. Fig. 6. Effects of long-term immersion on the adsorption ratio of heavy metal ions by a flat WPM plate (300 100 60 mm) immersed in (A) pure and (B) salt water (3% sodium chloride).
The adsorption of heavy metal ions by these WPM samples undergoing long term immersion was tested. The pH of the heavy metal solutions was adjusted to 5 and the flow rate was kept to within the range of 50–60 mL min1. As illustrated in Fig. 6, the flat WPM plate immersed in pure water quantitatively adsorbed heavy metal ions after one year of immersion, and the pH of the solution passed through the WPM plate was stable (between 11 and 12). On the other hand, the adsorption ratio of heavy metal ions by the WPM plate immersed in salt water decreased with increasing duration of immersion and, after one year of immersion, decreased by approximately 10% for all heavy metals. The pH of the solutions passed through the WPM plate showed little change, suggesting that immersion in salt water caused an excessive coordination of sodium ions to the adsorption sites in WPM, thus reducing the adsorption effects of WPM for heavy metal ions. The adsorption ratio of heavy metal ions by the WPM plate immersed in salt water for one year was lower than that of the WPM sample described in Section 3.6, despite the fact that the calcium ion elution ratio was 25% for both samples. This decrease may be due to interference from sodium ions as well as a decrease in the efficiency of contact with heavy metal ions from the elution of calcium ions. 4. Conclusions In the present study, woodchip-mixed porous mortar (WPM) was prepared and the adsorptivity of heavy metal ions for this material was studied. It was found that WPM prepared from an
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