Volatilization characteristics of organic solutes in stirred solution

Journal of Environmental Management 90 (2009) 3422–3428

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Volatilization characteristics of organic solutes in stirred solution Huan-Ping Chao* Department of Bioenvironmental Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 March 2008 Received in revised form 20 April 2009 Accepted 16 May 2009 Available online 23 June 2009

The effects of turbulence intensity (velocity gradient, G (s1)), Henry’s law constant (H), and molecular weight (M) on the volatilization rates of organic compounds are examined using changes in the mass transfer coefficients (KOL (cm/min)) under specific liquid-mixing intensities. The selected compounds were divided into three groups according to their H values (mole in gas/mole in liquid, dimensionless), which ranged from 102 to 105. The relationship of the KOL relative to G, H and M was obtained via multiple regression. The obtained values of these parameters indicate that the primary factor affecting the KOL values of the high H compounds is their M values. The effects of the H values on the KOL values of the high H compounds can be neglected. On the other hand, the H value is the major factor determining the KOL values of the low H compounds. The changes in the KOL values of the different H compounds exhibit different profiles as the liquid-mixing intensity increases. The M and H values of middle H compounds possibly affect their KOL values. The effects of the liquid-mixing intensity on the KOL values of the organic compounds increase with increasing H values. The variation in the KOL values might be a result of the concentration of the organic compounds at the interface between the liquid and gas films. The empirical relationship between KOL and some selected parameters, G, H and M, is examined in this study. The obtained results can help to estimate volatilization loss of organic solutes in wastewater treatment facilities. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Henry’s law constant Organic solutes Transfer coefficients Turbulence Volatilization

1. Introduction The volatilization of organic compounds from water to the atmosphere is an important process for determining their fates in the environment. Some emitted organic compounds may be toxic to human beings, aquatic organisms, or plants. A number of investigators have studied the emission of organic compounds from various water bodies (Mackay and Yeun, 1983; Parker et al., 1993; Corsi et al., 1995; Lee et al., 2004; Chao et al., 2005a). The results indicate that the volatilization rates of organic solutes are governed by the properties of the solution and the properties of the organic compounds. The two-film model has traditionally been used to estimate the volatilization loss of a solute from water into air (Liss and Slater, 1974). In this model it is assumed that there is a transition layer through which chemicals pass by molecular diffusion, to build an interface between the liquid and gas films. The resistance of a chemical to transfer at the liquid and gas interface is related to its respective mass transfer coefficient. The mass flux of the volatilization can be expressed as

* Tel.: þ886 3 2654914; fax: þ886 3 2654949. E-mail address: [email protected] 0301-4797/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2009.05.022

    Q ¼ kL CL  CL* ¼ kG CG*  CG ;

(1)

where Q is the mass flux of the volatilization (mass area-time1); kL is the liquid-phase transfer coefficient (length time1); kG is the gas-phase transfer coefficient (length time1); CL is the concentration of the bulk liquid (mass volume1); CL* is the concentration on the liquid side of the interface (mass volume1); CG* is the concentration on the gas side of the interface (mass volume1); and CG is the concentration in the bulk air (mass volume1). The kL and kG in Eq. (1) cannot in general be individually determined because CL* and CG* cannot be measured; With the assumption that CL* is equal to equilibrium concentration in liquid, CL* can be replaced with CG/H. Because CG is close to 0, Eq. (1) can be expressed as (Mackay and Leinonen, 1975)

Q ¼ KOL ðCL  CG =HÞyKOL CL ;

(2)

with

1 1 1 ¼ þ ; KOL kL HkG

(3)

where H (mole in gas/mole in liquid, dimensionless) is the Henry’s Law constant; KOL is the overall mass transfer coefficient. The conversion of units used for the Henry’s law constants can be

H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

obtained from literature (Altschuh et al., 1999). The approach, however, cannot be used to estimate the volatilization rates of organic solutes under different environmental conditions. Thus, the concept of the reference compound is applied to obtain the KOL values of the selected chemicals by the KOL values of the reference compounds for a specific condition (Smith et al., 1980; Rathbun and Tai, 1987; Rathbun, 1990; Chao et al., 2005b). It is assumed that the ratio of the volatilization rate constant of an organic solute to that of a reference substance is independent of the liquid-mixing conditions in water, i.e.,

kOrg ¼ 4kRef ;

(4)

where kOrg is the volatilization rate constant of an organic compound; kRef is the volatilization rate constant for a reference substance; and 4 is a reference-substance parameter, which is assumed to be independent of the liquid-mixing conditions. Moreover, if the chemical structure and physical–chemical properties of the organic solutes are similar, it could indicate that 4 will remain constant, even as the liquid-mixing level increases (Chao et al., 2005b). Although the approach is an alternative to the estimation of the volatilization loss of organic solutes in a complete-mixing tank, the KOL values of organic compounds with similar structure and properties have to be known. It is useful for estimating the emissions of organic solutes from wastewater treatment facilities if the KOL value of an organic compound under liquid turbulence can be obtained directly. For all organic compounds under liquid-mixing conditions, the dominant factors for determining the KOL values are assumed to be the Henry’s Law constants (H), the diffusion coefficients (D) of the chemicals, and the liquid-mixing intensity (Peng et al., 1995). It is also known that in a stirred solution the volatilization rates of the high H organic solutes will be greater than those of the low H organic solutes (Lee et al., 2004). If we could establish the relationship between the above factors, it would facilitate the estimation of the volatile loss of the organic solutes. Generally, the calculation to find the D of the organic solutes in a solution is complicated. Thus, when examining the volatilization characteristics of the high H organic compounds the D is frequently replaced by the molecular weight (M). The velocity gradient (G) can be used to describe the turbulence intensity in a wastewater treatment facility. The G value can be expressed as (Tchobanoglous and Burton, 1991)

G ¼ ðP=VmÞ1=2

(5)

P ¼ 1=2CD rAVr3

(6) 3

where P is power (W); V is the volume of tank (m ); m is solution dynamic viscosity ¼ 0.000900(N. m s1) at 25  C; CD is drag coefficient; CD is considered as 1.8; A is the area of blade (m2); Vr is velocity of the blade relative to water (m s1); r is solution density (1000 kg m3). In this study, the KOL values of organic compounds are regarded as a function of G, H and M. The tested chemicals are divided into three groups based on their H values. The relationship between the KOL values and the three parameters, G, H and M, are then defined. Moreover, the volatilization characteristics of the different H compounds are also elucidated. The results are expected to be a good reference for predicting the emissions of organic solutes from the wastewater treatment facilities. 2. Experiments All the volatilization experiments were conducted in the laboratory. Since organic compounds with different H values exhibit

3423

different volatilization characteristics, organic compounds need to be discussed based on their H values. All tested organic compounds possessing H values (dimensionless) of more than 0.05, are classified as high H compounds. Organic compounds with H values between 0.05 and 6.0  104 are classified as middle H compounds. Organic compounds with H values less than 6.0  104 are classified as low H compounds. The important physical–chemical properties of the tested organic compounds relative to the estimated KOL values are listed in Table 1. All the organic compounds, except for the pesticides, which were obtained from Chem Service, were purchased from the Fluka Co, and had high purities (>98%). The initial solutions of the chemicals were prepared with concentrations at 50% of the water solubility. For relatively highly soluble (>1000 mg L1) compounds, the initial concentrations of the chemicals were limited to 500 mg L1. The selected compound was added to a 100 mL vessel. The vessel was then closed with a Teflon-lined screw cap and equilibrated for 24 h in a reciprocating shaker at 120 rpm.

Table 1 Physical–chemical properties of the selected compounds. Compounds

M

Water solubility

Ha

n-Heptane n-Pentane n-Hexane Methylcyclohexane Methylcyclopentane Cyclohexane Tetrachloroethene Ethylbenzene p-Xylene m-Xylene Propylbenzene Toluene Fluorobenzene 1,2,4-Trimethylbenzene Benzene o-Xylene m-diChlorobenzene Trichloromethane Bromobenzene o-diChlorobenzene Bromoform 1,1,2,2-Tetrachloroethane Naphthalene 1.2-diBromoethane Ethyl acetate N,N-Dimethyaniline 1-Methylnaphelene Phenanthrene Methyl benzoate 1-Heptanol Heptachlor epoxide 1-Hexanol 1-Octanol 2-Chloroaniline p,p-DDE a-Endosulfan p,p-DDD Endrin Lindane 3-Toluidine 2-Toluidine Aldrin Aniline 1-Nitronaphthalene Dieldrin 3-chloroaniline Phenol m-Cresol Dimethylphthalate

100.21 72.15 86.17 98.19 84.16 84.16 165.83 106.2 106.2 106.2 120.2 92.13 96.11 120.2 78.11 106.2 147.01 119.4 157.02 147.01 252.8 167.85 128.19 187.9 88.1 121.2 142.2 178.23 136.1 116.2 389.4 102.2 130.2 127.6 318 406.9 320.1 380.9 290.8 107.2 107.2 364.9 93.1 173.2 372.9 127.6 94 108.1 194.2

2.93 38.5 9.5 14 42 55 140 152 185 162 55 515 1553 57 1780 175 123.2 7900 410 145.2 3033 3000 34.4 1696 80400 1105 28.5 1.18 2100 1740 0.275 5600 540 3765 0.04 0.51 0.09 0.25 7.3 15000 16300 0.017 36200 9.18 0.195 5400 75000 25000 5000

92.2 68.2 50.1 14.7 14.7 7.8 1.18 0.321 0.285 0.281 0.281 0.269 0.253 0.237 0.225 0.201 0.144 0.129 0.084 0.077 0.025 0.019 0.017 0.013 5.07E-3 4.61E-3 3.5E-03 1.60E-03 1.31E-3 9.78E-04 8.51E-04 7.58E-04 6.30E-04 5.29E-4 3.39E-04 2.88E-04 2.69E-04 2.57E-04 1.28E-04 1.23E-4 1.14E-4 1.10E-04 8.73E-5 7.24E-05 4.37E-05 4.07E-05 2.84E-05 2.84E-05 1.13E-5

a

Cited from Mackay and Shiu (1981).

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H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

A Jar test machine was utilized to produce the turbulence intensity in the solution. The experimental layout has been described elsewhere (Chao et al., 2005b). The liquid depth was around 2.2 cm. The impeller is 6 cm in height and 1.5 cm in width. The turbulent intensity of the Jar test machine could be set as revolutions per minute (rpm). The selected rpms included 0, 20, 50, 80, 100, 120, 150, 180, 200, 220, 250 rpm, which can be expressed as the G value using the appropriate calculation. The specific rpms relative to the G values are listed in Table 2. For measurement of the KOL values of the solutes, the solutions were maintained at 25  C by the circulation of thermostatically controlled water. The solution was held in a glass dish 8.0 cm in diameter and 4.0 cm in height. The liquid depth was maintained at 2.2 cm with a liquid volume of about 100 mL. Measurement periods varied with the half-lives of the solutes. For the high H compounds, the experiments were carried out for 1 or 2 h. For the middle H compounds, the measurement periods were from 6 to 12 h. For the low H compounds, the experiments were run for 12–24 h. The solute concentrations in water were analyzed by taking the 1-mL sample at the end of every run. After being sampled and extracted with 1 mL of hexane (for the chlorinated pesticides) or carbon disulfide (for the other compounds) the extracted samples were analyzed using a Hewlett-Packard Model 5890A gas chromatograph equipped with a flame ionization detector (FID) and an electron capture detector (ECD) according to the analyzed species. The organic compounds were separated with the HP 5890A that has a capillary column (J&W DB-5) with 30  0.53 mm ID and a film thickness of 3 mm film. The operating temperatures for the injection and the detector were set at 200 and 250  C, respectively. The temperature of the oven was ranged from 60  C to 180  C, varying with the different chemicals. The detailed analytical method has been described in elsewhere (Lee et al., 2004; Chao et al., 2005a). Generally, the volatilization of organic solutes from water is regarded as a first-order kinetic and the variation of the concentration with time can be expressed as (Peng et al., 1995; Dewulf et al., 1998; Chao et al., 2000)

CL ¼ C0 expðktÞ:

(7)

where C0 is the initial concentration in the bulk-water phase; and k is the volatilization rate constant (time1). The residual concentration CL of organic solutes in the solution versus time t is plotted on a semi-log scale. The k values can be obtained via the slope. Moreover, the relationship between k and KOL can be expressed as

k ¼

KOL ; L

(8)

where L is the depth of the solution in a container with a uniform cross section. In this study, the KOL value is estimated from the experimentally determined k value. The volatilization process is regarded as a first-order kinetic. In these experiments the difference between the initial concentrations and recoveries for the tested organic compounds was negligible. 3. Results and discussion It is well-known that stirring a solution can effectively increase the volatilization rates of organic solutes. In a previous

Table 2 Rpm of the Jar test equipment relative to G value (s1). rpm

20

50

80

100

120

150

180

200

220

250

G

31

121

245

344

451

630

828

970

1119

1355

investigation, it was found that the volatilization characteristics exhibited by organic compounds with similar physical–chemical properties are identical (Chao et al., 2005b). This indicates that the KOL ratios of the chemicals for a given group might approach a constant under the given liquid-mixing conditions. Moreover, the changes in KOL values of the selected compounds in the respective group for the same liquid-mixing intensity might be small. Tables 3–5 lists the KOL values of high, middle and low H compounds under the various liquid-mixing intensities, and the average KOL values and standard deviations are also indicated in the Tables 3–5. Although the H value is an important parameter to determine the KOL values of the organic compounds, the KOL values of the organic compounds cannot be directly obtained via their H values. Admittedly, the H value is frequently incorporated into empirical equations to estimate volatilization loss of organic compounds from wastewater treatment facilities. Thus, the H values of the organic compounds should be thought of as a dominant factor for determining their volatilization rates. As expected, the compounds in the high H groups possess relatively higher KOL values. It can be also found that there is a significant increase in the volatilization rates of the high H compounds as the solution is stirred. In other words, when the H values of the tested compounds decrease, the KOL values of the tested compounds under the liquid-mixing condition increase slowly. The result is consistent with other investigations (Lee et al., 2004; Chao et al., 2005a). The reason for this is ascribed to that high H compounds possess a low concentration at the liquid–gas interface, as has been published elsewhere (Chao et al., 2005a). Traditionally, diffusion is regarded as an important mechanism for emissions of the high H compounds from water. In addition to the H values, there are other properties of organic solutes, such as the molecular weight (M), which is contributed to the volatilization process. This generates a somewhat high deviation in the average KOL values for the three groups. If the KOL values of organic compounds can be normalized, the volatilization characteristics of organic solutes in a stirred solution can be understood more clearly. Fig. 1 indicates the average KOL ratios of stirred and static conditions for the three groups of compounds under the given turbulence. It can be seen in this figure that the trend of the average KOL ratios of the high H compounds is different from those of the middle and low H compounds. The profile for the high H compounds has a concave upward curve. This profile corresponds to that found in another investigation (Dewulf et al., 1998). The profiles for the middle and low H compounds however, exhibit concave downward curves. This result could be due to volatilization resistance and the organic compound concentration at the interface. For the high H compounds, the major resistance to mass transfer occurs in the liquid phase. Conversely, for the low H compounds, volatilization resistance is concentrated in the gas-phase. Although stirring the solution allows the low H solutes to maintain a higher equilibrium concentration at the interface, the relatively higher gas resistances make it difficult for emissions to leave the solution surface. The volatilization rates of the high H compounds are limited by the concentration at the interface. Stirring the solution enhances the concentrations of the organic compounds at the interface. If the gas resistance is constant, stirring can effectively increase the volatilization rates of the high H solutes. Another interesting finding is that high volatile organic solutes are frequently defined via their H values. According to other studies, the H values of organic compounds exceed 0.1 (dimensionless), which is considered as highly volatile solutes (Mackay and Leinonen, 1975). The kL values for these compounds are equal to their KOL values. Although the method is able to determine the volatility of organic solutes, investigators run into

H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

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Table 3 KOL values (cm min1) for the high H chemicals under the different liquid-mixing intensity (rpm). 0

20

50

80

100

120

150

180

200

220

250

n-Heptane n-Hexane n-Pentane methylcyclohexane methylcyclopentane cyclohexane Tetrachloroethylene Ethylbenzene p-Xylene Propylbenzene m-Xylene Toluene Fluorobenzene Benzene o-Xylene 1,2,4,-Trichlorobenzene m-Dichlorobenzene Trichloromethane Bromobenzene o-Dichlorobenzene

0.0139 0.0145 0.0166 0.0014 0.0152 0.0146 0.0071 0.0096 0.0091 0.0087 0.0093 0.0101 0.0099 0.0103 0.0089 0.0049 0.0073 0.0083 0.0086 0.0067

0.037 0.039 0.042 0.034 0.040 0.038 0.018 0.031 0.030 0.024 0.031 0.040 0.035 0.044 0.029 0.016 0.020 0.022 0.021 0.019

0.057 0.060 0.070 0.050 0.063 0.059 0.032 0.053 0.052 0.038 0.052 0.064 0.054 0.068 0.050 0.031 0.035 0.041 0.033 0.032

0.078 0.083 0.090 0.074 0.085 0.081 0.046 0.072 0.076 0.052 0.071 0.084 0.077 0.089 0.068 0.041 0.048 0.054 0.042 0.039

0.094 0.096 0.112 0.091 0.101 0.098 0.053 0.098 0.091 0.064 0.094 0.120 0.100 0.111 0.083 0.052 0.061 0.071 0.058 0.056

0.120 0.128 0.144 0.115 0.133 0.130 0.067 0.114 0.118 0.081 0.113 0.135 0.124 0.140 0.106 0.063 0.074 0.090 0.070 0.069

0.169 0.172 0.196 0.165 0.189 0.181 0.093 0.153 0.154 0.113 0.153 0.187 0.162 0.180 0.147 0.089 0.096 0.130 0.092 0.094

0.254 0.265 0.294 0.259 0.280 0.261 0.141 0.215 0.213 0.153 0.209 0.237 0.222 0.255 0.199 0.140 0.145 0.185 0.142 0.137

0.310 0.325 0.348 0.336 0.369 0.336 0.168 0.279 0.279 0.192 0.276 0.303 0.284 0.352 0.269 0.179 0.184 0.238 0.182 0.175

0.400 0.413 0.436 0.439 0.430 0.417 0.220 0.337 0.332 0.257 0.320 0.386 0.360 0.417 0.316 0.217 0.234 0.296 0.226 0.224

0.559 0.569 0.582 0.577 0.579 0.571 0.296 0.464 0.463 0.314 0.457 0.486 0.454 0.534 0.450 0.295 0.303 0.379 0.313 0.288

Average KOL Standard Deviation

9.75E-03 3.71E-03

3.05E-02 8.89E-03

4.96E-02 1.28E-02

6.76E-02 1.74E-02

8.51E-02 2.14E-02

1.07E-01 2.71E-02

1.46E-01 3.69E-02

2.10E-01 5.27E-02

2.69E-01 6.78E-02

3.34E-01 8.17E-02

4.47E-01 1.12E-01

compounds with similar physical–chemical properties exhibit stable changes in KOL ratios under the liquid-mixing conditions. Although the KOL values of the high H compounds are customarily determined according to their D (or M), the H values may still affect the KOL values of compounds in a stirred solution. In order to examine the effects of M and H on the KOL values, we plot the increasing trend of the KOL values of the high H compounds relative to their H and M when stirred at speeds from 0 to 250 rpm; see Fig. 2. From Fig. 2 it can be seen that the organic compounds with low M values show a dramatic increase in the KOL values. This result indicates the great importance of M for the high H compounds. It also demonstrates that diffusion is the primary mechanism for emissions of high H compounds. In fact, the above-mentioned result has been widely reported in the literature (Smith et al., 1980; Mackay and Yeun, 1983; Rathbun, 1990). However, the effects of the H values on the emissions of the high H compounds have rarely been described or taken into account in past studies. The results in Fig. 2 clearly show the slight effect of the H values on the KOL values of high H compounds in a turbulent solution. Traditionally, the H values of organic solutes can be used to determine their concentration at the interface. According to Eq. (3), for high H compounds, the H values are independent of the kL or KOL. Other investigators have postulated that the KOL values of the high H compounds need to be thought of as a function of H and M (Peng et al., 1995). The

trouble when the H values of the chemicals approach 0.1. For example, o-dichlorobenzene and m-dichlorobenzene possess similar physical–chemical properties and have the same molecular weight. It has been recognized that the volatilization characteristics of o-dichlorobenzene are identical to those of mdichlorobenzene. However, according to the above-mentioned principle, o-dichlorobenzene (H ¼ 0.077) and m-dichlorobenzene (H ¼ 0.144) are classified as having different volatilities. When the volatility of the selected solute is determined according to profile of KOL values for the solute during stirring, the data obtained have a relatively higher accuracy. On the other hand, an examination of Fig. 1 also shows that the range of values between high and low volatile compounds for the KOL ratio (around 1 w 60) is lower than that for the KOL values (span four orders). Among three groups, middle H compounds generate a relatively higher standard deviation. The chemicals in the middle H group lack the dominant factor to determine the volatilization rates. As described above, the KOL values of high H compounds are proportional to the D values of the organic solutes, that is, inversely proportional to the M of the organic solute. The KOL values of the low H compounds are controlled by their H values. The relatively higher H compounds generally possess higher KOL values. Therefore, the changes in the KOL values of these compounds remain stable as the liquid-mixing intensities increase. As for middle H compounds, both H and M could affect their KOL values. Only

Table 4 KOL values (cm min1) for the middle H chemicals under the different liquid-mixing intensity (rpm). 0

20

50

80

100

120

150

180

200

220

250

Bromoform 1,1,2,2-Tetrachloroethane 1-Methylnaphelene Naphthalene Phenanthrene 1.2-Dibromoethane N,N-Dimethyaniline Methylbenzoate n-Octanol n-Heptanol n-Hexanol

5.2E-03 4.3E-03 6.0E-03 7.6E-03 5.1E-03 5.7E-03 7.1E-03 6.5E-03 3.5E-03 3.4E-03 3.2E-03

1.6E-02 1.3E-02 1.7E-02 2.3E-02 1.4E-02 1.8E-02 1.4E-02 9.9E-03 6.2E-03 5.7E-03 5.3E-03

2.7E-02 2.4E-02 2.9E-02 3.9E-02 2.3E-02 3.1E-02 2.0E-02 1.4E-02 8.1E-03 7.7E-03 7.5E-03

3.7E-02 3.5E-02 3.9E-02 5.3E-02 3.5E-02 4.5E-02 2.4E-02 1.8E-02 9.9E-03 9.5E-03 8.8E-03

4.4E-02 4.3E-02 4.6E-02 6.5E-02 4.1E-02 5.5E-02 2.7E-02 2.0E-02 1.2E-02 1.1E-02 1.1E-02

5.1E-02 5.0E-02 5.4E-02 7.5E-02 5.0E-02 6.4E-02 3.0E-02 2.2E-02 1.4E-02 1.3E-02 1.1E-02

6.2E-02 6.0E-02 6.4E-02 8.8E-02 5.9E-02 7.6E-02 3.2E-02 2.4E-02 1.6E-02 1.5E-02 1.3E-02

7.2E-02 6.6E-02 7.6E-02 1.0E-01 6.9E-02 9.1E-02 3.5E-02 2.6E-02 1.7E-02 1.7E-02 1.5E-02

7.7E-02 7.3E-02 8.0E-02 1.1E-01 7.3E-02 9.8E-02 3.7E-02 2.7E-02 1.8E-02 1.8E-02 1.6E-02

8.1E-02 7.7E-02 8.5E-02 1.2E-01 7.9E-02 1.0E-01 3.8E-02 2.9E-02 2.0E-02 1.9E-02 1.7E-02

8.7E-02 8.4E-02 8.9E-02 1.2E-01 8.4E-02 1.1E-01 3.9E-02 3.0E-02 2.1E-02 2.1E-02 1.8E-02

Average KOL Standard Deviation

5.25E-03 1.52E-03

1.28E-02 5.54E-03

2.09E-02 1.05E-02

2.86E-02 1.54E-02

3.42E-02 1.89E-02

3.94E-02 2.24E-02

4.62E-02 2.69E-02

5.33E-02 3.23E-02

5.70E-02 3.45E-02

6.04E-02 3.66E-02

6.46E-02 3.93E-02

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H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

Table 5 KOL values (cm min1) for the low H chemicals under the different liquid-mixing intensity (rpm). 0

20

50

80

100

120

150

180

200

220

250

Heptachlor epoxide 2-Chloroaniline p,p-DDE a-Endosulfan p,p-DDD Endrin 2-Toluidine 3-Toluidine Lindane Aldrin Aniline 3-chloroaniline 1-nitronaphthalene Dieldrin Dimethylphthalate phenol m-cresol

1.3E-03 3.0E-03 2.0E-03 1.1E-03 1.9E-03 9.3E-04 8.8E-04 8.5E-04 4.3E-04 1.4E-03 6.8E-04 5.4E-04 4.6E-04 2.9E-04 9.2E-05 1.2E-04 1.2E-04

1.7E-03 3.9E-03 2.6E-03 1.4E-03 2.5E-03 1.2E-03 1.1E-03 1.1E-03 5.6E-04 1.8E-03 8.3E-04 6.6E-04 5.9E-04 3.6E-04 1.1E-04 1.5E-04 1.4E-04

2.0E-03 4.7E-03 3.3E-03 1.7E-03 3.0E-03 1.4E-03 1.3E-03 1.3E-03 7.1E-04 2.2E-03 9.7E-04 7.7E-04 6.8E-04 4.1E-04 1.3E-04 1.7E-04 1.6E-04

2.4E-03 5.6E-03 4.0E-03 2.0E-03 3.8E-03 1.7E-03 1.6E-03 1.5E-03 8.7E-04 2.7E-03 1.2E-03 9.4E-04 8.1E-04 5.0E-04 1.6E-04 2.1E-04 1.9E-04

2.9E-03 6.6E-03 4.7E-03 2.4E-03 4.4E-03 1.9E-03 1.8E-03 1.8E-03 1.0E-03 3.2E-03 1.3E-03 1.1E-03 9.4E-04 5.8E-04 1.8E-04 2.3E-04 2.2E-04

3.3E-03 7.3E-03 5.1E-03 2.6E-03 4.9E-03 2.1E-03 2.0E-03 1.9E-03 1.1E-03 3.5E-03 1.4E-03 1.1E-03 9.9E-04 6.3E-04 1.9E-04 2.5E-04 2.4E-04

3.4E-03 7.7E-03 5.5E-03 2.8E-03 5.1E-03 2.2E-03 2.1E-03 2.0E-03 1.2E-03 3.8E-03 1.6E-03 1.2E-03 1.1E-03 6.7E-04 2.1E-04 2.8E-04 2.6E-04

3.6E-03 8.0E-03 5.7E-03 2.9E-03 5.7E-03 2.3E-03 2.2E-03 2.1E-03 1.3E-03 3.9E-03 1.6E-03 1.3E-03 1.1E-03 6.9E-04 2.1E-04 2.8E-04 2.7E-04

3.7E-03 8.3E-03 5.9E-03 3.0E-03 5.9E-03 2.4E-03 2.3E-03 2.2E-03 1.3E-03 4.0E-03 1.7E-03 1.3E-03 1.2E-03 7.2E-04 2.2E-04 3.0E-04 2.8E-04

3.8E-03 8.5E-03 6.0E-03 3.1E-03 6.0E-03 2.5E-03 2.4E-03 2.3E-03 1.4E-03 4.3E-03 1.7E-03 1.4E-03 1.2E-03 7.3E-04 2.3E-04 3.1E-04 3.0E-04

4.0E-03 8.9E-03 6.3E-03 3.2E-03 6.3E-03 2.6E-03 2.5E-03 2.4E-03 1.4E-03 4.4E-03 1.8E-03 1.4E-03 1.3E-03 7.7E-04 2.4E-04 3.3E-04 3.2E-04

Average KOL Standard Deviation

9.43E-04 7.83E-04

1.22E-03 1.03E-03

1.46E-03 1.25E-03

1.77E-03 1.52E-03

2.07E-03 1.79E-03

2.27E-03 2.00E-03

2.42E-03 2.10E-03

2.54E-03 2.21E-03

2.62E-03 2.28E-03

2.71E-03 2.34E-03

2.82E-03 2.44E-03

Fig. 1. Changes in the KOL ratios of the different H compounds at different mixing intensities.

results obtained from this study confirm that the H values have only a slight influence on the emissions of the high H solutes. For the low H compounds, the H values of the chemicals are generally thought of as a primary factor in determining their KOL values. Some investigators have presented the solute concentration at the interface can affect the volatilization or adsorption of the solute from air–water interface (Sadiki et al., 2003, 2005; Chao, 2009). The H values of organic compounds can determine their concentration at the interface. Thus, the H values of the organic compounds were regarded as a major parameter to determine the solute concentration at the interface. However, it is difficult to examine the relationship between the KOL values and the H values for the low H compounds, because precise H values are difficult to be obtained. Another characteristic of these compounds is that water volatilization needs to be modified during the experimental process. For organic compounds with H values less than 0.0001, the volatilization rates with respect to water are especially low, which makes the relationship between H and KOL difficult to discuss. Fig. 3 shows the characteristics of the KOL values correlated with their H and M under turbulent intensities from 0 to 250 rpm. As expected, the H values are the primary factor for determining the KOL values. The characteristics of the organic compounds with the different H values under the mixing conditions can be also interpreted with the Eq. (3). In Eq. (3), the second term on right side is negligible if the H value is the relatively higher. The result represents the effects of the H values on the KOL values of the high H compounds is the relatively lower. On the contrary, the effects of H values on the KOL values of the low H compounds need to be taken into account. Thus, the H value is an important factor to determine the volatilization rates of the low H compounds. Traditionally, it is assumed that the low H compounds can maintain a relatively higher concentration at the interface. The low H compounds always possess high water solubilities. The H value of an organic compound can be expressed as the ratio of its vapor pressure to its water solubility at a given temperature (Mackay and Shiu, 1981). The high affinity of these compounds to water means it is difficult to volatilize from water. Thus these compounds are thought to volatilize together with water (Chao et al., 2005b). Some organic compounds possess low H values and low water solubilities. These compounds also have low vapor pressure and high M. The effects of diffusion on the KOL values still need to be considered although the expected effect is small. It can be seen in Fig. 3 that the M values of the organic

H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

Fig. 2. Dependence of the KOL values of the high H solutes on their H and M.

compounds also slightly affect the KOL values. This confirms that the above assumption is reasonable. It is recognized that the KOL values of the organic solutes are a function of the turbulence intensity (G), H and M. If the relationship of the KOL values relative to the above parameters can be determined, it would make it convenient to estimate volatilization loss of organic compounds in wastewater treatment facilities. Fig. 1 shows that under the given liquid-mixing intensities the different H compounds exhibit different volatilization characteristics. This can be expressed empirically as follows:

KOL fGp Hq Mr :

G>0

(9)

Then,

Log KOL ¼ a þ p Log G þ q Log H þ r Log M;

(10)

3427

where a, p, q and r are empirical constants; and G, H and M are defined as above. It has been known that the organic compounds with the different H values exhibit the different volatilization characteristics. The tested organic compounds are divided three groups according to their H values to discuss their volatilization characteristics. The characteristics of these empirical constants vary with the H values. The constants can be observed via multiple regression. The obtained results are listed in Table 6. The high R-square values were observed indicating a high relativity between these parameters and the KOL values. The values of the empirical constants, p, q and r, can be used to elucidate the volatilization characteristics of the given compounds. Among these constants, only r is negative for all groups of compounds. The results show that M is inversely proportional to the KOL values. It is demonstrated that the turbulence increases the volatilization rate so positive p values are expected. The positive q values also provide proof that organic compounds with the high H values might possess higher KOL values. After the positive or negative values of these constants offer an indication for determining the volatilization effects, these values need to be discussed further. The turbulence intensity has an effect on the KOL value. The obtained p values indicate that the high H compounds have relatively higher p values. This means that the liquid-mixing intensity leads to a more significant increase in the KOL value for the high H compounds. It is necessary to note however, that for selected low H compound (p ¼ 0.231), the liquidmixing still effectively increases the KOL values. As mentioned above, the volatilization rates of organic compounds with an H of less than 104 cannot be measured unless the water volatilization is modified. It is difficult for this type of organic compound to volatilize from water under the liquid-mixing condition. The KOL values for some organic compounds in the low H compound group with an H of more than 104 still increase as the liquid-mixing intensities increases. Thus, the p value can reach 0.231. It can be expected that the smaller p values will be found for the organic compounds with the lower H values. Traditionally, the H values of the high H compounds have been regarded as a less-important factor on the KOL values while on the contrary, the KOL values of the low H compounds are easily affected by their H values. Figs. 2 and 3 show the demonstrated results. According to an examination of the q values of the selected compounds, the q values of the high H compounds are far lower than those of the other compounds. For the high H compounds, the selected H range is relatively lower. It can be deduced that the H values only slightly affect the KOL values for all of the high H compounds. Since diffusion is regarded as the major mechanism for volatilization, the M values are frequently utilized to estimate the KOL values under specific liquid-mixing intensities. The obtained r values indicate that the M values are a dominant factor for determining the KOL values of the high H compounds under the liquidmixing conditions. For the middle H compounds, the obtained r value (0.340) representing the effects of the M values of these compounds on the KOL values cannot be neglected. The r value of the selected low H compounds is 0.106, which indicates that the M values of these compounds have only a relatively weak effect on their KOL values. Similarly, for the organic compounds with lower H

Table 6 Empirical constants of KOL relative to G, H and M.

Fig. 3. Dependence of the KOL values of the low H solutes on their H and M.

High H Compounds Middle H Compounds Low H Compounds

a

p

q

r

R2

0.833 0.736 0.260

0.713 0.409 0.231

0.012 0.480 0.883

0.947 0.340 0.106

0.907 0.917 0.877

3428

H.-P. Chao / Journal of Environmental Management 90 (2009) 3422–3428

values, the effects of the M values on the KOL values gradually become negligible. In a previous investigation, it was found that organic compounds with similar physical–chemical properties exhibited similar volatilization characteristics under a variety of environmental conditions, including liquid or air turbulence or both at once (Chao et al., 2005b). Although the major objective of this study is to explore the effects of H and M on the volatilization rates of different organic compounds in stirred solutions, the obtained results (see Table 6) can also be utilized to estimate the volatilization loss of organic compounds in the given H range in wastewater treatment facilities. 4. Conclusion In this study, we selected forty-eight organic compounds and examined the changes in their KOL values under different liquidmixing intensities. Based on their H values, these compounds were divided into three groups, and the effects of G, H and M on the KOL values were examined. The obtained results indicate that compounds with different H values exhibit different trends in terms of increased KOL values under liquid-mixing conditions. Thus, it is suggested that the volatility of organic solutes be classified based on the profile of the KOL values relative to the liquid-mixing intensities. Moreover, it can be concluded that H values of high H compounds have only a weak effect on their KOL values; the effects are negligible. On the contrary, for the low H compounds, the H value is the primary factor for determining the KOL values. The effect of the M values on the KOL values of the organic compounds increases with increasing H values. When the H values of the organic compounds are less than 105, the effects of M on the KOL values should be negligible. The relationship of KOL relative to G, H, and M and according to the H range, can be obtained by multiple regression. The results can be utilized to estimate the volatilization loss of organic compounds with a similar H range from wastewater treatment facilities. Acknowledgements The support of this work by the National Science Council, ROC, under Grant NSC 96-2221-E-033-012 – MY3 is highly appreciated.

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