Impact of Temperature on the Ratio of Initial Emittable Concentration

Article pubs.acs.org/est

Impact of Temperature on the Ratio of Initial Emittable Concentration to Total Concentration for Formaldehyde in Building Materials: Theoretical Correlation and Validation Shaodan Huang,† Jianyin Xiong,*,‡ and Yinping Zhang† †

Department of Building Science, Tsinghua University, Beijing 100084, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China

‡

S Supporting Information *

ABSTRACT: The initial emittable concentration (Cm,0) is a key parameter characterizing the emission behaviors of formaldehyde from building materials, which is highly dependent on temperature but has seldom been studied. Our previous study found that Cm,0 is much less than the total concentration (C0,total, used for labeling material in many standards) of formaldehyde. Because Cm,0 and not C0,total directly determines the actual emission behaviors, we need to determine the relationship between Cm,0 and C0,total so as to use Cm,0 as a more appropriate labeling index. By applying statistical physics theory, this paper derives a novel correlation between the emittable ratio (Cm,0/C0,total) and temperature. This correlation shows that the logarithm of the emittable ratio multiplied by power of 0.5 of temperature is linearly related to the reciprocal of temperature. Emissions tests for formaldehyde from a type of medium density fiberboard over the temperature range of 25.0−80.0 °C were performed to validate the correlation. Experimental results indicated that Cm,0 (or emittable ratio) increased significantly with increasing temperature, this increase being 14-fold from 25.0 to 80.0 °C. The correlation prediction agreed well with experiments, demonstrating its effectiveness in characterizing physical emissions. This study will be helpful for predicting/controlling the emission characteristics of pollutants at various temperatures.

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INTRODUCTION The presence of hazardous chemicals in the indoor environment, for example, formaldehyde and volatile organic compounds (VOCs), can cause poor indoor air quality.1,2 Formaldehyde, which is classified as a human carcinogen, is of particular concern.3,4 Over the past 20 years, China has produced and consumed 1/3 of the world’s formaldehyde, most of which is used to synthesize resin, which is frequently found in wood-based materials.5 Consequently, a great deal of research has concentrated on understanding the emission behaviors of formaldehyde and VOCs from these materials so as to realize effective source control. Three emission characteristic parameters, the initial emittable concentration (Cm,0), the diffusion coefficient (Dm), and the partition coefficient (K), are found to be useful for describing formaldehyde and VOC emissions from building materials.6−13 It should be noted that these three emission parameters are dependent on the physical properties of the material−pollutant combinations as well as environmental conditions (e.g., temperature, relative humidity). At present, methods to measure Cm,0, Dm, and K are well developed under certain conditions. However, the impact mechanisms on these parameters are still poorly understood, thus hindering a deep understanding of the emission behaviors of formaldehyde and © 2015 American Chemical Society

VOCs from building materials under different environmental conditions. Without this understanding, it is difficult to control the emissions. Previous studies have shown that temperature has a significant impact on the emission behaviors of formaldehyde and VOCs from building materials. Research on this topic can be divided into two categories. The first category is direct research into the impact of temperature on the emission rate (or chamber/room concentration). Emission rate is a function of the three characteristic parameters. Because the emission rate can be easily measured it is considered to be a good indicator of indoor pollution. An increase in the emission rate with increasing temperature had been frequently reported in the literature.14−18 Myers15 observed an increase in the emission rate of formaldehyde from particleboard by a factor of 5.2 in the temperature range 23−40 °C. Lin et al.16 demonstrated that the VOC emission rate and concentration increased 1.5−12.9 times in the temperature range of 15−30 °C. The above studies are all based on experimental investigations. Later, Xiong et al.19 Received: Revised: Accepted: Published: 1537

October 22, 2014 December 19, 2014 January 6, 2015 January 6, 2015 DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

Article

Environmental Science & Technology

molecules increases with temperature, it follows that there will be a corresponding increase in Cm,0. For the convenience of analysis, a new parameter, P (called the emittable ratio), is defined to represent the ratio of Cm,0 to C0,total for formaldehyde emission at different temperatures:

derived a theoretical correlation between the emission rate and temperature. This correlation has proven to be appropriate for three different emission scenarios: formaldehyde emissions; standard reference emissions; and semivolatile organic compound (SVOC) emissions. The second category is concerned with the impact of temperature on the three emission characteristic parameters (Cm,0, Dm, and K). Zhang et al.20 established a theoretical correlation between K and temperature. Deng et al.21 presented a theoretical correlation between Dm and temperature. Xiong and Zhang22 reported that Cm,0 of formaldehyde increases significantly with increasing temperature in the range of 25.0−50.0 °C. However, a theoretical analysis of this important phenomenon is lacking, so it is difficult to determine Cm,0 at temperatures other than those used in the experiments. In addition, Cm,0 at temperatures higher than 50.0 °C has not been investigated. For Cm,0, sensitivity analysis7,23 shows that among the three characteristic parameters, it is the most sensitive one to the emission behaviors. Because temperatures indoors, and particularly in vehicles, may reach relatively high temperatures (∼70 °C) on hot summer days,24,25 theoretical study as well as experimental work at high temperature on Cm,0 is urgently needed. In the Chinese National Standard GB/T17657−1999,26 GB/ T18580−200127 and the European Standard EN120,28 it is suggested that total formaldehyde concentration (or content) in wood-based boards be measured by the perforator method at high temperature (about 110.8 °C), and this total concentration should be used as a parameter for labeling building materials. The total formaldehyde concentration (C0,total) includes both the emittable part (Cm,0) and the bonded part. In many cases, the total concentration does not correspond to the emittable concentration, which means that a high C0,total does not necessarily result in a high Cm,0.29 Because Cm,0 and not C0,total determines the actual emission behaviors of formaldehyde from building materials, the perforator method may lead to labeling mistakes. Considering that the perforator method is widely used, it would be very useful to compare the results of Cm,0 at different temperatures with C0,total, and obtain a theoretical relationship between them, so as to be able to make some reasonable suggestions for the standard revision. In addition, such work is also helpful for developing low emission building materials. Considering the above observations, the main objective of this paper is to derive a theoretical correlation between the ratio of Cm,0 to C0,total and temperature from physics. In addition, we measure the Cm,0 of formaldehyde from building material over a broad temperature range (25.0−80.0 °C), and this provides a substantial amount of data to prove the theoretical correlation.

P=

Cm,0 C0,total

(1)

We use statistical physics theory to quantitatively address the association between P and temperature. We made two assumptions to simplify the analysis of this problem. First, the effect of the energy barrier mentioned above can be understood as a potential well that prevents molecules with low energy from escaping. Nakayama et al.’s observation indicates that the potential well remains constant for N2O in the temperature range of 243−353 K.32 In addition, the energy barrier is analogous to the heat of adsorption for pollutant adsorption onto materials. The heat of adsorption is generally regarded as independent of temperature in a certain range (e.g., 17−100 °C for some pollutants).33−35 In light of that, the present study makes an assumption that the energy barrier (ε0) is a constant for a given material−formaldehyde combination over the temperature range studied. Second, chemical reactions are ignored and only physical interactions between formaldehyde and material molecules are considered. The kinetic energy distribution of the ideal gas, g(εk), is applied to describe the formaldehyde molecule kinetic energy distribution (Figure 1). It can be represented by the following equation:36 g (εk) =

2 (kBT )−3/2 e−εk / kBT εk π

(2)

Figure 1. Schematic of the kinetic energy distribution and the emittable ratio.

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THEORETICAL CORRELATION The emission of formaldehyde and VOCs from building materials is mainly a physical process for short-term emissions (it will be validated in the Results and Discussion section). From the microcosmic perspective, the formaldehyde and VOC molecules are bonded to the material surface by adsorption, and a molecule is emittable only when the kinetic energy of this molecule is high enough to overcome the bonding forces, that is, to overcome an energy barrier, the adsorption energy (ε0).30,31 The sum of these molecules constitute the initial emittable concentration (Cm,0), which is obviously less than the total concentration (C0,total). Therefore, the cutoff point of Cm,0 can be defined as a collection of molecules whose kinetic energy is higher than ε0. Because the average kinetic energy of

where, kB is the Boltzmann constant, J/K; T is the temperature, K; εk is the kinetic energy of the ideal gas, J. It should be pointed out that although the state of a certain formaldehyde molecule changes constantly when it travels from the interior to the surface of the material, the kinetic energy distribution of all the formaldehyde molecules does not change at a certain temperature but still follows the statistical average distribution according to statistical physics theory. The emittable ratio P equals the shaded area in Figure 1. The figure indicates that P decreases with ε0, and P can be expressed as P=1− 1538

∫0

ε0

g (εk )dεk

(3) DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

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Environmental Science & Technology

Figure 2. Schematic of the experimental system.

extent. By comparing eq 6 with experimental data, we can validate the rationality of this substitution. If we combine eqs 1 and 6, we derive the relationship between Cm,0 and temperature:

Combining with eq 2, and then performing integration on the right side, we get: P=

Cm,0 C0,total

=1−

∫0

=2+

ε0

2 π

2 (kBT )−3/2 e−εk / kBT εk dεk π ⎛ 2ε ε0 −ε0 / kBT 0 − 2ϕ⎜⎜ e kBT k T ⎝ B

Cm,0 =

⎞ ⎟⎟ ⎠

where ϕ is the standard normal cumulative distribution function. It should be noted that ε0 can be taken as a constant for the temperature range of interest, which is just related with the physical properties of the material−formaldehyde combination. When (2ε0/kBT)1/2 is larger than 3, the value of ϕ((2ε0/ kBT))1/2 is close to 1. To further simplify, we assume ((2ε0/ kBT)1/2 > 3) (this will be validated in the context that follows) eq 4 can thus be reduced to 2 π

ε0 −ε0 / kBT e kBT

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EXPERIMENTAL SECTION To validate the derived theoretical correlation, we need to measure Cm,0 at different temperatures. The ventilated chamber C-history method37 has proved to be a rapid and accurate method for determining the three emission characteristic parameters (Cm,0, Dm, K), thus is applied in this study. The principle behind this method is briefly introduced on Page S1 of the Supporting Information (SI). The experimental system consists of the environmental chamber, a temperature control system, a humidity control system, and a sampling system (Figure 2). A 30L stainless steel chamber was used, with a fan in the top to accelerate the mixing of pollutants in the air. The chamber has three ports: inlet, outlet, and sampling port. A stainless shelf is placed in the chamber upon which the building materials can be arranged. The temperature is controlled using a water-bath jacket (CS501-SP, HONGRUI Co.), and the relative humidity is controlled using a three-way valve. A kind of medium density fiberboard (MDF) widely used for decoration was chosen for the test. The size of the MDF sample

(5)

After multiplying by √T and taking the logarithm of both sides of eq 5, we get: ln(P T ) = −

A +B T

(7)

where, C = C0,totalexp(B). This theoretical correlation is similar to the empirical correlation that was obtained by analyzing the experimental data of formaldehyde emissions.19 The derived correlations (eqs 6 and 7) quantitatively establish the relationship between P, Cm,0, and T for formaldehyde emissions from building materials. When the parameters A and B (or C) in eqs 6 and 7 are obtained from available results, the derived correlations can be used to predict the P and Cm,0 at other temperatures. This is very helpful and should be useful for engineering applications.

(4)

P=

⎛ A⎞ C exp⎜ − ⎟ ⎝ T⎠ T

(6)

where, A = ε0/kB; B is a constant. It should be pointed out that the parameters, A and B, are not dependent on temperature but are only associated with the physical properties of the materialformaldehyde combination. Strictly speaking, B is equal to ln(2(A/π)1/2). However, considering that the formaldehyde molecules are confined to the surface of the material, the formaldehyde molecules will probably deviate from the ideal molecular kinetic energy distribution. The substitution of ln(2(A/π)1/2) with B can overcome this problem to a certain 1539

DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

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Environmental Science & Technology is 10.0 cm × 10.0 cm × 0.3 cm. The edges of the MDF were sealed during the test, and only the front and back of the sample were exposed to air. According to symmetry, emissions from the front and back can be considered to be the same onedimensional emission in two directions with half thickness and double emission area. Such treatment helps to increase the emission rate and reduce the test time. The MDF tested in this study did not contain any formaldehyde scavengers. For each temperature point, air was sampled at the outlet of the ventilated chamber using a pump (PCXR8, SKC Co.), with a sampling rate of 0.2L/min and sampling time of 5 min. The sampled air was then pulled through a tube containing a 5 mL of MBTH (3-methyl-2-benzothiazolinone hydrazone) aqueous solution.31 Finally, the formaldehyde concentrations in the samples were detected by the MBTH method with the spectrophotometer (723N, INESA Analytical Co.).38 Formaldehyde was selected because it is a major pollutant from wood-based boards and a typical indoor air pollutant in many countries, especially in China. Generally speaking, the MBTH method is an unspecific method used in the determination of all aldehydes. In our experiment, the concentrations of other aldehydes are very low and could only be detected at the initial period of the ventilated condition. These aldehydes can hardly be analyzed by the ventilated chamber C-history method due to the sparse data. In addition, these aldehydes with relatively low concentrations will not pose significant risks to human health. Taking the above into consideration, we used the MBTH method for detecting formaldehyde. In cases where the effect of interfering aldehydes must to be quantified to ensure the given formaldehyde concentrations (e.g., emissions from OSB panels), the HPLC method should be used. The chamber was initially airtight until equilibrium was reached, and it was then ventilated with clean air. Because no sampling is needed during the initial airtight phase, this phase is called a pretreatment procedure. At the beginning of the airtight phase, the relative humidity in the chamber was 50.0 ± 5.0% obtained by maintaining the background air at this level. For airtight tests, this method has been applied in many studies.39−41 The equilibrium state for the airtight test is defined as the point where the relative deviation of the mean formaldehyde concentration during the subsequent hour to that of the preceding hour is less than 1%.37 Generally speaking, the equilibrium time will vary for different material−formaldehyde combinations. We see in the literature that the equilibrium time under airtight conditions for some common material−formaldehyde combinations was less than 24 h.29,39,42 For the sake of safety, a pretreatment time of 36 h is selected so as to reduce the measurement error. Once equilibrium was reached in the airtight condition, clean air with 50.0 ± 5.0% relative humidity and (5.0 ± 0.1) LPM (liter per minute) flow rate was supplied to ventilate the chamber. During the experiments, only the chamber air temperature was measured. However, given that the temperature of the material is the factor directly influencing the emission characteristic parameters, we performed another experiment to investigate the temperature difference between the chamber air and the solid material. We measured both temperatures using thermocouples (WZP-321, CEPAI GROUP Co.). During the test, three thermocouples were fixed at different locations on the material surface using insulating tape, and the average value from the three points was taken as the material temperature. A fourth thermocouple was used to measure the air temperature. SI Figure S1 shows the results of the air and

material temperatures when the chamber air temperature is controlled at 50.0 ± 0.5 °C. The difference between them is no more than 0.2 °C when the material temperature reaches steady state after about 0.5 h. For other temperatures, the results are similar and are thus not included here. From these results, we assume that it is reasonable to take the air temperature as the material temperature, which is convenient for the experiments. The geometrical dimensions of the tested building material and experimental conditions are shown in Table 1. Table 1. Experimental Conditions of the Tested Building Material temperature (°C)

humidity (%)

dimensions (cm × cm × cm)

number of pieces

± ± ± ± ± ± ± ±

50.0 ± 5.0

10.0 × 10.0 × 0.3

4 4 4 4 2 1 1 1

25.0 29.0 35.0 42.0 50.0 60.0 70.0 80.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

To investigate the relationship between Cm,0 (or P) and temperature, we carried out experiments at eight different temperatures: 25.0 °C, 29.0 °C, 35.0 °C, 42.0 °C, 50.0 °C, 60.0 °C, 70.0 °C, 80.0 °C. Duplicate experiments were performed to reduce measurement error. Under ventilated conditions, the experimental time was no more than 12 h, and about 10 samplings were performed at regular intervals.

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RESULTS AND DISCUSSION Determination of the Characteristic Parameters at Different Temperatures. By applying equation (S3) in the Supporting Information (SI), we show the linear curve fitting results of the first and second experiments at 35.0 and 42.0 °C in Figure 3 (Cequ is the equilibrium concentration under airtight condition; Ca is the hourly concentration under ventilated condition). The results obtained at other temperatures are shown in SI Figure S2. On the basis of the SL and INT of the regression curve, the three emission characteristic parameters (Cm,0, Dm, K) of formaldehyde at different temperatures can be determined using the ventilated chamber C-history method. The results of the determined emission characteristic parameters and the coefficient of determination (R2) at temperatures 25.0−80.0 °C are summarized in Table 2. High R2 values indicate good measurement accuracy according to ASTM Standard D5157-97.43 The relative deviations of Cm,0, Dm, and K (designated by RDC, RDD and RDK) are calculated for different temperatures and mostly found to be less than 25%, with the exception of RDD at 25.0 °C (26.6%). These results imply good test repeatability and prove that the measurement method is acceptable. Results in Table 2 also show that Cm,0 of formaldehyde changes drastically with temperature. The Cm,0 at 25.0 °C is only 5.52 × 106 μg/m3, but when the temperature increases to 80.0 °C, it becomes 8.03 × 107 μg/m3, which is about 14-fold greater than that at 25.0 °C. Generally speaking, very few indoor environments are likely to experience temperatures as high as 50.0 °C. The measurement of Cm,0 for formaldehyde emissions from MDF at high temperatures (>50.0 °C) is based on two considerations. First, 1540

DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

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Environmental Science & Technology

reference for vehicular material tests even though the materials in the two environments may be different. That is to say, the findings in this study can also be applied to vehicular environments where relatively high temperatures (∼70.0 °C) can be experienced during hot summer days. The measured emission characteristic parameters obtained by the ventilated chamber C-history method should be validated to further prove its reliability. We calculate the chamber formaldehyde concentration under ventilated conditions using an analytical model based on the determined parameters given in Table 2, and then we compare these results with the experimental data. The good agreement between the simulated results and experimental data (duplicate experiments) at 35.0 and 42.0 °C (Figure 4) as well as at other temperatures (SI Figure S3) convincingly demonstrates the effectiveness of the measured parameters. Validation of the Theoretical Correlation. The C0,total for the MDF was determined using the perforator method recommended by the Chinese National Standard GB/ T17657-1999,26 GB/T18580-2001,27 and the European Standard EN120,28 and the result was 1.38 × 108 μg/m3, which is much higher than Cm,0. C0,total is a surrogate for the emission potential of a given product over a long period of time. In addition to the perforator method, some other methods are also used for the labeling of wood-based products, such as the desiccator method JIS A 1460,44 and the chamber method EN 717-1.45 These two standard methods both test for emissions over a short period of time. This seems to imply that the shortterm emissions of formaldehyde from wood-based boards can also be a good indicator for labeling products. Therefore, the Cm,0 based on short-term emissions can or may also be useful for product labeling purposes. Formaldehyde and VOC emission rate in the indoor environment decreases with time. So, the short-term effect may play a significant role in human exposure levels due to the relatively high formaldehyde and VOC concentrations during this period. The discrepancy between Cm,0 and C0,total at room temperature indicates the perforator method may lead to problems when applying C0,total as an index for labeling material in short-term emissions because Cm,0 and not C0,total determines the actual emission behaviors of materials. As pointed out by Risholm-Sundman et

Figure 3. Linear relationship between ln(Ca/Cequ) and time at 35.0 and 42.0 °C by fitting the experimental data.

it can provide substantial data to prove the derived theoretical correlation, as described below. Second, because the emission mechanisms of pollutants in indoor and vehicular environments are similar, the results of indoor material tests can provide a

Table 2. Determined Emission Characteristic Parameters Based on Ventilated Chamber C-History Method at Different Temperatures temp (°C) 25.0 ± 0.5 29.0 ± 0.5 35.0 ± 0.5 42.0 ± 0.5 50.0 ± 0.5 60.0 ± 0.5 70.0 ± 0.5 80.0 ± 0.5

times

Cm,0 (μg/m3)

1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd

5.40 × 10 5.64 × 106 5.67 × 106 5.93 × 106 6.48 × 106 5.94 × 106 10.00 × 106 9.52 × 106 1.71 × 107 1.33 × 107 3.05 × 107 3.75 × 107 5.67 × 107 5.80 × 107 8.88 × 107 7.17 × 107 6

Cm,0 (μg/m3) 5.52 × 10

RDC (%)

6

2.1

5.80 × 106

2.3

6.21 × 106

4.3

9.76 × 106

2.5

1.52 × 107

12.5

3.40 × 107

10.3

5.74 × 107

1.1

8.03 × 107

10.6

Dm (m2/s) 8.81 5.11 7.34 5.96 6.64 9.09 1.11 1.28 1.80 2.12 9.25 7.24 5.64 7.27 7.76 6.13

× × × × × × × × × × × × × × × ×

−11

10 10−11 10−11 10−11 10−11 10−11 10−10 10−10 10−10 10−10 10−11 10−11 10−11 10−11 10−11 10−11 1541

Dm (m2/s)

RDD (%)

K

6.96 × 10

26.6

6.65 × 10−11

10.4

7.87 × 10−11

15.5

1.20 × 10−10

7.5

1.96 × 10−10

8.2

8.24 × 10−11

12.3

6.45 × 10−11

12.6

6.95 × 10−11

11.7

× × × × × × × × × × × × × × × ×

−11

2.36 2.00 1.97 1.78 1.46 1.35 1.39 1.34 1.08 0.87 1.27 1.47 1.94 1.69 2.47 1.62

K̅ 3

10 103 103 103 103 103 103 103 103 103 103 103 103 103 103 103

RDK(%)

R2

3

2.18 × 10

8.5

1.88 × 103

5.1

1.41 × 103

3.8

1.36 × 103

1.8

0.98 × 103

11.0

1.37 × 103

7.2

1.81 × 103

6.8

2.04 × 103

20.7

0.98 0.99 0.97 0.99 0.92 0.97 0.99 0.98 0.91 0.97 0.96 0.97 0.98 0.97 0.98 0.94

DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

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Environmental Science & Technology

Figure 5. Relationship between P and temperature.

Figure 4. Comparison between the simulated results and experimental data at 35.0 and 42.0 °C.

Figure 6. Linear regression results by treating the experimental data with theoretical eq 6 and its comparison with eq 4

al., “the correlation between the desiccators JIS A 1460 and the chamber and perforator methods respectively is, however, not convincing”.46 Therefore, it is suggested to take Cm,0 as a classification index when we focus more on the short-term health hazards caused by building materials. Certainly, much more work is necessary to determine if it is suitable or not in the future. Taking the measured Cm,0 in the temperature range of 25.0− 80.0 °C, we can obtain the P values at these temperatures. The emittable ratio is only 4.0% at 25.0 °C, meaning that only a few formaldehyde molecules in building materials can emit at room temperature. However, when the temperature increases to 80.0 °C, 58.2% of formaldehyde in building materials becomes emittable (Figure 5). The determined P values at different temperatures are used to validate the derived correlation. Given that eq 7 is derived from eq 6, we will only validate eq 6 in this section. Linear curve fitting by applying eq 6 to the experimental data of formaldehyde emissions at different temperatures, as well as data from previous research,22 is shown in Figure 6. The figure indicates that both R2 are about 0.97, demonstrating a high accuracy. From the slope and intercept of the regression line, A and B can be obtained, with the values of 5780.8 and 18.8 for the tested MDF, and 6884.1 and 22.7 for Xiong and Zhang’s material.22 Using the determined A and B values, P is calculated by eq 4. These results are represented by dotted lines in Figure

6. We can see that the results are all very close to the linear regression results from eq 6, demonstrating the validity of the derived correlation and the simplification. Because A is equal to ε0/kB, it is easy to calculate the energy barrier ε0 as 48.0 kJ/mol for the tested MDF and 57.2 kJ/mol for the material in the reference.22 So, (2ε0/kBT)1/2 is in the range of 5.7 to 6.8, which is greater than 3 thus validating the previously applied assumption ((2ε0/kBT)1/2 > 3). The energy barrier of formaldehyde emission is analogous to the heat of adsorption, as mentioned previously. Srisuda and Virote47 investigated the adsorption of formaldehyde in some amine-functionalized mesoporous silica materials, and found that the heat of adsorption was in the range of 36−160 kJ/mol, which is of the same magnitude as the energy barrier measured in this study. According to the derived correlation, the ratio of Cm,0 to C0,total remains constant for a given material−formaldehyde combination at a certain temperature. If the material is pretreated by heating, the emission rate will increase with the increasing temperature, and the resulting C0,total of formaldehyde in the material will be lower after this pretreatment. There will also be a corresponding decrease in Cm,0. This explains why heating pretreatment is beneficial for IAQ control. It should be noted that the source of formaldehyde emitted from building materials, especially wood-based boards, is generally complicated. The majority of the formaldehyde originates from the residual formaldehyde trapped in the 1542

DOI: 10.1021/es5051875 Environ. Sci. Technol. 2015, 49, 1537−1544

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Environmental Science & Technology

This material is available free of charge via the Internet at http://pubs.acs.org.

wood, and the emission of this formaldehyde can be regarded as a physical process. Formaldehyde can also be formed by hydrolytic reactions and aging of urea−formaldehyde bonds. The emission of this formaldehyde can be viewed as a chemical process, and the extent to which this formaldehyde is generated depends on factors such as temperature, the moisture content of the product, relative humidity, pH of the media, and time. For short-term emissions (several months), when the temperature and other factors are not too high, the generated formaldehyde may not be extensive, and the overall emissions can be approximately described by physical mechanisms. For long-term emissions (several years), the formaldehyde generated by chemical reactions (hydrolysis, aging) may comprise a considerable percentage of the total emitted. Considering that chemical reactions are often complicated and are difficult to quantitatively analyze, our study focuses solely on the physical emissions. This is appropriate for a short-term formaldehyde emission process. This treatment has been widely applied in previous research6−10,39,42,48 because the models derived in these studies are all based on physical mechanisms (internal diffusion, interface partition, and external convection). These models result in acceptable accuracy when predicting formaldehyde and VOC emissions from building materials. As for this study, although the increased Cm,0 at elevated temperature in the chamber tests may be partially due to the effect of accelerated hydrolysis, the good agreement between the correlation prediction and the experimental data means that such an effect does not make an observable contribution to Cm,0. This further demonstrates the effectiveness of regarding the short-term emissions as a physical process. For long-term emissions, however, chemical reactions may contribute significantly to Cm,0. In these cases the physical model will introduce grave errors and additional terms that consider the chemical reactions need to be included in the model. Moreover, the existing measurement methods for determining the characteristic parameters cannot distinguish which part comes from the physical process and which part refers to the chemical process. Improvements in these models and measurement methods merit systematic research. In this paper, we apply statistical physics theory to theoretically investigate the impact of temperature on the emittable ratio (initial emittable concentration by total concentration), and we derive a theoretical correlation between them. Experimental results from temperatures in the range of 25.0−80.0 °C validate the effectiveness of the correlation. Once the parameters in the correlation are determined by more than two sets of experiments, the emittable ratio (or initial emittable concentration) at other temperatures can be obtained, facilitating engineering applications. Future research will focus on improving the existing physical models and measurement methods by considering chemical reactions for some specific emission scenarios, and investigating the impact of relative humidity and time on the long-term formaldehyde emission characteristics.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 10 68914304. Fax: +86 10 68412865. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 51106011, No. 51476013, No. 51136002), the 12th 5 Year Key Project, Ministry of Science and Technology of China (No. 2012BAJ02B01).

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on introduction of the principle of ventilated chamber C-history method (Page S1); comparison of the air and material temperatures (Figure S1); linear relationship between ln(Ca/Cequ) and the emission time by fitting the experimental data at other temperatures (Figure S2); validation of the determined results at other temperatures (Figure S3). 1543

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Article

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