Environ. Sci. Technol. 2006, 40, 4281-4285
Removal of Formaldehyde by Hydroxyapatite Layer Biomimetically Deposited on Polyamide Film TAKAHIRO KAWAI,† CHIKARA OHTSUKI,† M A S A N O B U K A M I T A K A H A R A , * ,† MASAO TANIHARA,† TOSHIKI MIYAZAKI,‡ YOSHIMITSU SAKAGUCHI,§ AND SHIGEJI KONAGAYA§ Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka, 808-0196, Japan, and Toyobo Research Center Co., Ltd., 2-1-1 Katata, Otsu, Shiga 520-0292, Japan
Some harmful volatile organic compounds (VOCs), such as formaldehyde, are regulated atmospheric pollutants. Therefore, development of a material to remove these VOCs is required. We focused on hydroxyapatite, which had been biomimetically coated on a polyamide film, as an adsorbent and found that formaldehyde was successfully removed by this adsorbent. The amount of formaldehyde adsorbed increased with the area of the polyamide film occupied by hydroxyapatite. The amount of adsorbed formaldehyde and its rate of adsorption were larger for hydroxyapatite deposited on polyamide film than for the commercially available calcined hydroxyapatite powder. This high adsorption ability is achieved by the use of nanosized particles of hydroxyapatite with low crystallinity and containing a large number of active surface sites. Therefore, hydroxyapatite biomimetically coated on organic substrates can become a candidate material for removing harmful VOCs such as formaldehyde.
Introduction Some harmful volatile organic compounds (VOCs) have been used as components of coating or adhesive agents for building materials (1). Prominent among these is formaldehyde, which irritates the respiratory system, and has been widely used in the past. Such harmfulness leads to a problem known as the “sick house syndrome.” Hydroxyapatite is of considerable interest as an adsorbent for removal of harmful pollutants since it shows both the ability to adsorb organic substances on its surface (2-6) and to uptake ionic spices within its crystal lattice (7-10). Use of a thin coating of hydroxyapatite is required to achieve efficient adsorption of the target organic substances from the surrounding environment. An organic polymer coated with hydroxyapatite on its surface would be a useful countermeasure against pollutants because it can be formed into any shape without losing its activity. Coating of a robust surface with hydroxyapatite can be achieved by * Corresponding author phone: +81-743-72-6122; fax: +81-74372-6129; e-mail:
[email protected]. † Nara Institute of Science and Technology. ‡ Kyushu Institute of Technology. § Toyobo Research Center Co., Ltd.. 10.1021/es050098n CCC: $33.50 Published on Web 06/08/2006
2006 American Chemical Society
FIGURE 1. Structural formula of aromatic polyamide, containing carboxyl groups. use of a plasma spray or the sputtering technique but these techniques cannot be used to coat an organic substrate since the operating conditions are too severe. Some studies have been reported where biomimetic processing, using a simulated body fluid and its related solutions (11-13), was used to fabricate a coating of hydroxyapatite on an organic substrate. This process can be carried out under mild conditions, e.g., neutral aqueous solution and room temperature. Miyazaki et al. successfully coated a hydroxyapatite layer on an aromatic polyamide film containing carboxyl groups and calcium chloride through immersion in a solution called 1.5SBF that has 1.5 times the ionic concentration of inorganic species to those of human blood plasma (14). In this process, nanosized hydroxyapatite particles with low crystallinity can precipitate on the polymer substrates. Hydroxyapatite layers formed in this way may be very effective as an adsorbent of formaldehyde but nothing has been reported. In this study, we evaluated the capability of hydroxyapatite biomimetically deposited on a polyamide film as an adsorbent of formaldehyde.
Materials and Methods Preparation of Polyamide Film Coated with Hydroxyapatite. One gram of the aromatic polyamide, containing carboxyl groups as shown in Figure 1 (15), was dissolved in 10 mL of N,N-dimethylacetamide (Wako Pure Chemical Industries Ltd., Japan) together with 0.67 g of CaCl2 (Nacalai Tesque Inc., Japan). The mixture was stirred for 24 h to obtain a homogeneous solution. The solution was coated on a glass plate using a bar coater and dried under 133 Pa at 60 °C for 8 h to obtain a film. The film was removed from the glass plate and cut into pieces 30 mm × 30 mm in size. The thickness of the film obtained was between 12 and 18 µm. The film was soaked in 270 mL of 1.5SBF (Na+ 213.0, K+ 7.5, Mg2+ 2.3, Ca2+ 3.8, Cl- 221.7, HCO3- 6.3, HPO42- 1.5, and SO42- 0.8 mol‚m-3), which has ion concentrations 1.5 times those of a simulated body fluid (SBF) (11, 16), for various periods under shaking at 120 strokes‚min-1 using a water bath shaker (Personal Lt-10F, TITEC, Japan) at 36.5 °C. The pH of 1.5SBF was buffered at 7.40. The 1.5SBF solution was renewed daily. After soaking for a given period, the film was taken out of the solution, gently washed with ultrapure water, and dried in air for 24 h. The weight of the film was then measured and compared with that of a film soaked in ultrapure water for 2 h, which is known to remove all the CaCl2 incorporated into the film. The film was characterized by thin film X-ray diffraction (TF-XRD; RINT2200V/PC-LR, Rigaku Co., Japan) using CuKR radiation. The angle of the incident beam was fixed at 1° against the surface of the specimen. The surface of the film was observed by scanning electron microscopy (SEM; S-3500N, Hitachi Ltd., Japan). In the SEM observations, an Au thin film was sputtered onto the surfaces of the specimens. Testing the Adsorption of Formaldehyde. Formaldehyde gas was prepared by stirring 1 mL of 37% formaldehyde VOL. 40, NO. 13, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. TF-XRD patterns of surfaces of polyamide films before and after soaking in 1.5SBF for various periods. FIGURE 2. SEM images of surfaces of polyamide films before and after soaking in 1.5SBF for various periods. solution (Wako Pure Chemical Industries Ltd., Japan) in a 500 mL glass bottle at 23 °C for 24 h. All the adsorption experiments were carried out at 23 °C, which was selected as one of the room temperatures. A piece of polyamide film, 30 mm × 30 mm in size and coated with hydroxyapatite, was put into a Tedlar bag (SANSYO Co., Ltd., Japan). Three liters of highly pure N2 gas (purity > 99.999 vol %) and gaseous formaldehyde was then injected into the bag. The initial concentration of formaldehyde was adjusted by collecting an appropriate volume of the formaldehyde gas. The initial concentrations that we applied were 2, 5, 25, and 40 ppm. After holding the specimen in the bag for 0.5, 1, 2, and 6 h, the concentration of formaldehyde in the bag was examined by a detecting tube (no. 91L, GASTEC, Japan). The ability of a commercial calcined hydroxyapatite powder (Wako Pure Chemical Industries Ltd., Japan) and an activated charcoal made from coconut shell (Nacalai Tesque Inc., Japan) to adsorb formaldehyde was also tested, as comparable materials, by the method described above. Morphological Observation and Measurement of Specific Surface Area. Morphological observation at high magnification was carried out for typical specimens using a field emission scanning electron microscope (FE-SEM; S-4800 Hitachi Ltd., Japan). Measurement of the specific surface area was carried out using an automatic vapor-adsorption isotherm apparatus (BELSORP-18SP FMS-LP-100, BEL Japan, Inc., Japan). The specific surface areas of examined specimens were determined from N2 adsorption isotherms at 77 K under 0-0.4 relative pressure (equilibrium pressure to saturation pressure) range using the BET equation.
Results Characterization of Hydroxyapatite Deposited on Polyamide Film. Figures 2 and 3 show SEM photographs and TFXRD patterns of the surfaces of the polyamide films before and after soaking in 1.5SBF for 1, 2, and 3 days. Spherical particles were observed adhering to the film surface after soaking (Figure 2). Peaks at 2θ ) 26° and 32°, which are assigned to hydroxyapatite, were absent before soaking and increased in magnitude with the time of soaking (Figure 3). These results confirm that hydroxyapatite deposits form on the surface of polyamide film within 1 day and increase with increasing soaking period. Table 1 gives the amount of deposited hydroxyapatite per unit area after soaking in 1.5SBF for various periods. This was calculated by dividing the amount of the formed hydroxyapatite by the area of the polyamide film (18 cm2). The amount of the deposited hydroxyapatite increased with increasing soaking period and the thickness of the hydroxyapatite layer increased to a few micrometers. 4282
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FIGURE 4. Changes in concentration of formaldehyde after exposure of polyamide with different amounts of hydroxyapatite to formaldehyde.
TABLE 1. Amount of Hydroxyapatite (HAp) Deposited Per Unit Area on Polyamide Films after Soaking in 1.5SBF for Various Periods soaking period (day)
weight of HAp (mg‚cm-2)
1 1.5 2 3
0.11 0.28 0.61 1.08
Removal of Formaldehyde by Hydroxyapatite on Polyamide Film. Figure 4 shows changes in the concentrations of residual formaldehyde after the exposure to formaldehyde of various amounts of hydroxyapatite deposited on polyamides. Films with deposits of 0, 0.11, 0.28, 0.61, and 1.08 mg‚cm-2 of hydroxyapatite were selected for this examination. Samples labeled as blank and 0 mg are blanks: the former was a procedural blank (nothing in the bag) and the latter had only the pure polyamide film in the bag. The concentration of residual formaldehyde decreased with increasing amount of hydroxyapatite up to the specimens with 0.61 mg‚cm-2 of hydroxyapatite. The adsorption of formaldehyde on the specimen with 1.08 mg‚cm-2 was similar to that with 0.61 mg‚cm-2. This result indicates that the total amount of adsorbed formaldehyde and the rate of its adsorption depend on the amount of hydroxyapatite deposited on the polyamide films while they are saturated at 0.61 mg‚cm-2. The concentration of formaldehyde due to exposure of the hydroxyapatite on polyamide film at 0.61 mg‚cm-2 was examined three times using three independent samples. The error rate (standard deviation/mean) was less than 8% at any period. This indicates that there was no significant difference among these three samples, and the reproducibility was confirmed.
TABLE 2. Constants from Langmuir Plots for Two Kinds of Hydroxyapatite specimen
A (µg‚mg-1)
K (Pa-1)
hydroxyapatite on polyamide hydroxyapatite powder
6.94 0.83
2.70 0.78
equation can be rearranged into the following form (eq 2).
p/N ) p/A + 1/(AK)
(2)
From this Langmuir plot, the constants A and K can be obtained. Figure 7 shows the Langmuir plot of adsorption FIGURE 5. Changes in concentration of formaldehyde after exposure of various specimens to formaldehyde. The polyamide film with hydroxyapatite at 1.08 mg‚cm-2 was used, while the weights of both the calcined hydroxyapatite powder and the activated charcoal were 20 mg. Comparison of Hydroxyapatite Deposited on Polyamide Film with Other Materials. Figure 5 shows changes in the concentration of residual formaldehyde after exposure of a commercial calcined hydroxyapatite powder and an activated charcoal made from coconut shell to formaldehyde for various periods, in comparison with hydroxyapatite deposited on polyamide at 1.08 mg‚cm-2. The weights of both the calcined hydroxyapatite and activated charcoal were 20 mg. A decrease in concentration of formaldehyde was seen for every specimen. The decrease was larger in the order calcined hydroxyapatite < activated charcoal < hydroxyapatite deposited on polyamide. This indicates that the total amount was larger, and the rate of adsorption of formaldehyde was greater for hydroxyapatite on polyamide than those for the calcined hydroxyapatite and activated charcoal. Figure 6 shows changes in the concentration of formaldehyde due to exposure of the hydroxyapatite on polyamide film at 0.61 mg‚cm-2 (left) and 20 mg of calcined hydroxyapatite powder (right) to formaldehyde with different initial concentrations. At any initial concentration, larger decreases in formaldehyde concentration were observed for the hydroxyapatite on polyamide film than for the calcined hydroxyapatite powder. When an adsorption isotherm is categorized as a Langmuir type, the amount of adsorbate (N) is represented as a function of the equilibrium pressure of adsorbate (p) as shown below (eq 1).
N ) AKp/(1 + Kp)
(1)
In this equation, A and K are constants that indicate the saturated adsorption amount and the ratio of the rate constants of adsorption to desorption, respectively. This
FIGURE 7. Langmuir plots of adsorption isotherms of formaldehyde at 296 K for (a) hydroxyapatite deposited on a surface of polyamide film at 0.61 mg‚cm-2 and (b) commercial hydroxyapatite powder. isotherms of formaldehyde at 296 K in the two cases where hydroxyapatite deposited on polyamide at 0.61 mg‚cm-2 and a commercial hydroxyapatite powder were adsorbents. Every point was plotted by calculating the equilibrium pressure of formaldehyde and the total consumption of formaldehyde after exposure to formaldehyde at different initial concentrations. The equilibrium pressure was calculated from the residual concentration of formaldehyde, which was estimated by the regression of the curve of decreasing formaldehyde concentration at an infinite period in Figure 6. The amount of adsorbed formaldehyde was determined from the difference between the initial concentration and the residual concentration estimated above, per unit weight of hydroxyapatite. Both these plots were linear. The constants A, which was normalized for weight of samples, and K were obtained from the Langmuir plots, the values being shown in Table 2. The slope [1/A] and intercept [1/(AK)] of the regression line in the Langmuir plots for the hydroxyapatite deposited on the polyamide film and those for the commercial hydroxyapatite are significantly different (p < 0.05). Figure 8 shows FE-SEM photographs of the hydroxyapatite deposited on polyamide after soaking in 1.5SBF for 2 days and commercial hydroxyapatite powder at high magnifica-
FIGURE 6. Changes in concentration of formaldehyde due to exposure of hydroxyapatite on polyamide film at 0.61 mg‚cm-2 (left) and 20 mg of calcined hydroxyapatite powder (right) to formaldehyde with different initial concentrations. VOL. 40, NO. 13, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. FE-SEM photographs of (a) hydroxyapatite deposited on surface of polyamide film at 0.61 mg‚cm-2 and (b) commercial hydroxyapatite powder. tion. The hydroxyapatite deposited on the polyamide film formed nanosized particles that were observed to aggregate and form a porous network, while the commercial hydroxyapatite powder was formed from hexagonal crystal rods that grew along the c-axis to a length of a few micrometers. The specific surface areas of hydroxyapatite deposited on polyamide and powdered hydroxyapatite were 226 and 7 m2‚g-1, respectively. That of hydroxyapatite deposited on polyamide after soaking for 3 days was 190 m2‚g-1 and this value being almost the same as that after soaking for 2 days. These results show that much finer particles of hydroxyapatite can be formed by the biomimetic process than by calcination. Although the activated carbon had a much larger specific surface area (891 m2‚g-1) than the hydroxyapatite deposited on polyamide film, it showed lower adsorption ability.
Discussion The amount of formaldehyde adsorbed was larger for hydroxyapatite deposited on polyamide film than for the calcined powder. The Langmuir plots of formaldehyde at 296 K for hydroxyapatite on polyamide and calcined powder were linear, indicating that formaldehyde is adsorbed on the surface of hydroxyapatite as a monolayer within the examined range of the concentration of formaldehyde (0-40 ppm). Both the constants A and K for hydroxyapatite on polyamide are much larger than those for calcined powder. This indicates that the hydroxyapatite on polyamide has a larger number of surface sites available for adsorption of formaldehyde and a larger rate of adsorption than the calcined powder. FESEM observation shows that hydroxyapatite deposited on polyamide consists of much smaller (nanosized) particles, with a porous network, so it necessarily has a larger specific surface area than the calcined powder. The morphology of the hydroxyapatite formed biomimetically contributes to the presence of a large number of sites available for adsorption of formaldehyde. Moreover, it is known that hydroxyapatite formed biomimetically shows low crystallinity and has many defects (17). This structure might give a high rate of adsorption of formaldehyde. These characteristic features of hydroxyapatite formed biomimetically were effective in the adsorption of formaldehyde. When the amounts were normalized for the specific surface area (SSA) about the adsorbed formaldehyde, that for biomimetically deposited hydroxyapatite (31 µg‚m-2) was smaller than that for calcined hydroxyapatite (119 µg‚m-2). We estimate that an effective surface area of biomimetically deposited hydroxyapatite for adsorption of formaldehyde is less than 226 m2‚g-1 because only the hydroxyapatite near the surface could adsorb formaldehyde, as shown in Figure 4. Moreover, even if the amount normalized for SSA is small in biomimetically deposited hydroxyapatite, biomimetically deposited hydroxyapatite has a significantly larger surface area and this large surface area results in a high capacity to adsorb formaldehyde. In the biomimetic process, we can obtain hydroxyapatite consisting of very fine particles. This gives a significantly large surface area of hydroxyapatite, and we want to emphasize not only the low crystallinity of the 4284
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hydroxyapatite but also this high surface area. To emphasize this effect, we normalized the adsorbed formaldehyde for weight. The amount of formaldehyde adsorbed on a commercial activated carbon was less than that of hydroxyapatite on polyamide, although the activated carbon had a larger specific surface area than hydroxyapatite. This might be due to a difference in the number of activated sites available for the adsorption of formaldehyde. It is known that formaldehyde is easily adsorbed by adsorbents possessing a polar surface. In fact, it has been reported that the ability of activated carbon to adsorb formaldehyde increased with an increase in the number of amino groups on the surface of the activated carbon after amination, independent of the specific surface area (18, 19). Other studies report that the capability of activated carbon to adsorb harmful gases is dependent on the carbonizing temperature because acidic functional groups, such as carboxyl groups, assisting the activated site to adsorb formaldehyde are removed at appropriate temperatures (20, 21). The amount of adsorbed formaldehyde depended not on the thickness of the apatite layer but on the area of the film covered by hydroxyapatite. This indicates that only the hydroxyapatite on the top surfaces can interact with formaldehyde molecules and adsorb them. When the specimens contain more than 0.61 mg‚cm-2 of hydroxyapatite, the whole surfaces of the polyamide films are covered with hydroxyapatite, and therefore, they have the maximum ability to remove formaldehyde. Consequently, it was found that the nanosized hydroxyapatite layer coated biomimetically on organic substrates is expected to be a candidate for a novel and excellent adsorbent effective for removal of formaldehyde.
Acknowledgments This work was conducted under the auspices of the research project, Technology Development for Medical Materials Merging Genome Information and Materials Science, in the Kansai Science City Innovative Cluster Creation Project, supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Received for review January 15, 2005. Revised manuscript received April 4, 2006. Accepted May 1, 2006. ES050098N
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