A Remediation Study of Soils Contaminated by Chlorinated Hydrocarbons with Mechanical Soil Aeration Yi SHI1,a, Fasheng LI2,b, Xiaoming DU2, Zhu XU2, Yan MA2.3, Zheng LI2, Jidun FANG2, Chunming ZHANG2 and Qunhui WANG1,c 1

Key Laboratory y of Educational Ministry for High Efficient Mining and Safety in Metal Mine, University of Science and Technology Beijing, Beijing 100083, China

2

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China 3

Engineering Research Center of Groundwater Pollution Control and Remediation, Ministry of Education, Beijing Normal University, Beijing 100875, China

a

[email protected], [email protected], [email protected](corresponding author)

Key words: Mechanical soil aeration; VOCs; Soil remediation

Abstract. Mechanical soil aeration is an effective and low cost ex-situ remediation technique suitable for large areas of volatile organic contaminated sites. To understand the effectiveness of the remediation technique, the current study remediated an abandoned industry site for a typical chlor-alkali chemical using this technology. The results showed that the technology is effective in the remediation of volatile organic compounds with a pass percentage greater than 90%. The results also showed that a lower vapor pressure or higher molecular weight with the similar concentration of pollution resulted in higher residual concentrations, which need increasing agitation frequency or machine power to promote the volatilization of pollutants. In addition, we found that the more pollution or water or organic matters (more than 1%) contained, the higher concentration of residual. These type of soils should be remediated not only by physical agitations but also by other strengthen measures and long period. The present study aims to promote the remediation of contaminated sites, especially large areas contaminated by volatile contaminants. Introduction Volatile organic compounds (VOCs) are one of the most abundant chlorinated industrial products. They are introduced into the environment through their use as chemical intermediates in the synthesis of a number of other chlorinated hydrocarbons [1,2]. They have toxic effect on the human central nervous system, respiratory system, kidneys and liver as potential mutagens and carcinogens [3]. Because of its persistence toxicity, potential bioaccumulation and toxic effects on environment, there is a growing interest in the technologies for the removal of them. At present, there are already many proven remediation methods of this type of contaminated sites. The main technologies of remediating contaminated sites such as soil vapor extraction [4], soil flushing [5], thermal desorption [6], phytoremediation [7], bioremediation [8-9], microwaves [10-12] and bioventing [13]. These methods are effective on the removal of chlorinated volatile organic compounds, but the high cost goes against the economic requirements during remedial projects, especially not suitable for large areas of contaminated sites. Mechanical Soil Aeration is a growing low cost ex-situ remediation technique for VOCs contaminated sites. It agitates contaminated soil by tilling or other means to volatilize contaminants while collecting and disposing the released gas. It is suitable to VOCs with simple and low cost

 

processes. Refer to the “Treatment Technologies for Site Cleanup in Annual Status Report”, from 1982-2005, currently, the technology is still in the test phase without related theory or laboratory research and there is quite a few sites using this technology in the United States. However, up to now, no such treatment in either practice or theory has showed up yet in China. Limited research has been conducted on Mechanical Soil Aeration of VOCs contaminated soil. In the purpose of learn the efficiency of mechanical soil aeration and the effect of the condition of soils, including the type of contaminants, initial concentration and soil texture, we carried on a case study with this technology on site which was contaminated by VOCs based on the previous laboratory studies [14]. The main goal of the work is to provide a technical and theoretical support for this site and analogies in China, especially the large area of volatile organic contaminated sites. Materials and methods Contaminated samples. The experiment site is an abandoned chlor-alkali chemical industry site with 50 years of producing history and had been discontinuing for four year, which mainly produced alkali, polyvinylchloride (PVC), hydrochloric acid, liquid chlorine and other basic chemical materials. The soil of this site contained vinyl chloride, chloroform and other volatile halogenated hydrocarbons, which are found seriously polluted in former investigation. The main pollutants in the soil and their remedial target values are showed in Table 1. Table 1 The main contamination in soil Contamination Benzene Vinyl chloride 1,1–dichloroethylene 1,1–dichloroethane Carbon tetrachloride 1,2–dichloroethane Trichloroethylene 1,1,2–trichloroethane Tetrachloroethylene 1,1,2,2-tetrachloroethane Chloroform

Concentration [mg/kg] 0.25Y*-11.2 0.25 Y* -160.61 0.25 Y* -39.7 0.025 Y* -131 0.025 Y* -53 0.025 Y*-16900 0.025 Y* -8.61 0.025 Y* -99 0.025 Y* -36.5 0.025 Y* -1.19 0.025 Y* -197

Remedial target value [mg/kg] 2.4 0.5 29.1 130 0.97 0.82 5.19 2.8 22 1.21 0.82

Detection limit [mg/kg] 0.05 0.5 0.5 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

*: according to the environmental statistical requirements, concentrations below the detection limit expressed by half of the detection limit and marked Y.

There are four types of soil in site: the first layer (0-2.5m) of soil is backfill, the second layer (2.5-5.0m) of soil is silt clay, the third layer (5.0-9.0m) of soil is stay clay, and the forth layer (9.0-18.5m) of soil is sand. The properties and characteristics of soil in this site are shown in Table 2 and Figure 1. Table 2 The properties and characteristics of soil Type of soil (depth) Backfill （0-2.5m）

Silt clay （2.5-5.5m）

Average Maximum Minimum Amount Average Maximum Minimum Amount

Silt[%]

Clay[%]

Sand[%]

18.3 33.9 5.6 15 9.4 23.0 3.1 17

66.6 89.8 53.4 15 71.4 92.4 48.2 17

15.1 38.4 2.0 15 19.3 46.3 4.5 17

 

Type of soil (depth)

Silt[%]

Clay[%]

Sand[%]

69.3 90.7 40.6 20

7.5 39.0 1.0 20

Average 23.2 Maximum 43.7 Minimum 8.2 Amount 20 silt: d < 0.005[mm]；clay: d = 0.005 ~ 0.05[mm]；sand: d > 0.05[mm] Stay clay （5.5-9.0m）

0.00 1.00

0.25

of

silt

0.50

y cla

Ra tio

of

0.50

0.75

1.00 0.00

tio Ra

0.75

0.25

0.25 Backfill

0.00 0.50 0.75 1.00 Ratio of sand Silt clay Stay clay

 

Fig. 1 Typical particle size distribution of soil

Process of remediation. The remediation was carried on in an air closed shed which made of multi-layer plastic film with ring beam foundation design in bottom to keep the shed sealed and assembled with aluminum fixed fixtures, inflatable fan and devices for control. The process of remediation with mechanical soil aeration is shown in Figure 2. Excavated soil was carried to the shed and then agitated by overturned machines at a room temperature and a setting frequency (12times/day) to strengthen the volatilization of VOCs from soil during 5-7days with the treatment capacity of 1000～1500m3/h. The restored soil was inspected by Photo Ionization Detector (PID) first, and then piled as 4m high trapezoidal soil stacks for inspection in shed. To prevent secondary environmental pollution and physical hazards caused by contamination volatilizing from the soil, the shed were always kept under negative pressure through air flow control during the experiment and exhausted via the treatment of activated carbon adsorption system.

VOCs

excavation

conveyance

Protection o f operators

VOCs

Remediation in closed shed

Meet the requirement

Backfilled or disposal inspection

Fig. 2 Process of remediation with mechanical soil aeration

activated carbon adsorption system

 

Samples collection and analysis. Semi-circular drilling was used to collect the undisturbed samples of the piles with horizontal depth of 0.75m. At each sampling point, the sample about 5g for VOCs analysis was collected by a hand-hold VOCs sampling tube while 250ml was collected for moisture content analysis at the same place. The VOCs samples were kept in a 40ml closed glass vessel pre-stored with methanol. All samples were stored in a fridge at 4℃ before analyzing. The concentration of VOCs in soil was measured by gas chromatograph (Agilent Technologies 5973) equipped with a mass spectrometer (Agilent Technologies 5975C) using the US-8260C method. A capillary column (DB-VRX 60m×0.25mm ID×1.4µm) was used. The GC was operated with a helium-carrier-gas flow rate of 1.2ml/min and the maximum oven temperature is 230℃, the same as the temperature of the injector. The detection limit is 0.05mg/kg. Relative percentage error (RPD) of replicate samples was less than 30%. Results and discussion Removal efficiency. After treatment, majority of VOCs in soil samples was removed under the setting temperature and agitation frequency. The results are illustrated in Table 3 and Figure 3. All the concentration of contaminants after remediation is below the remedial target values and the pass rates above 90%. The results showed that mechanical soil aeration was effective, but have differences results of different contaminants. In this study, 1,2–dichloroethane has the lowest passing rate of 93.8% and the highest removal rate of 99.44%. All the other contaminants have the same pass rates (100%) but different removal rates because of low initial concentration. Thus, it is not sensible to judge the remediation results only by removal efficiency. Table 3 The removal efficiency of main contamination in soil Contamination 1,1,2,2-tetrachloroethane 1,1–dichloroethylene Trichloroethylene Benzene Vinyl chloride Carbon tetrachloride Tetrachloroethylene Chloroform 1,1,2–trichloroethane 1,1–dichloroethane 1,2–dichloroethane

Initial concentration a [mg/kg] 0.03 0.34 0.06 0.06 0.62 0.11 0.17 0.52 0.41 0.72 30.59

Concentration after remediation a [mg/kg] 0.025Yb 0.22 0.025Yb 0.025Yb 0.25Yb 0.025Yb 0.026 0.032 0.025Yb 0.025Yb 0.17

Remedial target value [mg/kg] 1.21 29.1 5.19 2.4 0.5 0.97 22 0.82 2.8 130 0.82

The average of removal rate [%] 16.67 35.29 58.33 58.33 59.68 77.27 84.71 93.85 93.90 96.53 99.44

Pass rate [%] 100 100 100 100 100 100 100 100 100 100 93.8

a: the concentration is average value; b: according to the environmental statistical requirements, concentrations below the detection limit values expressed by half of the detection limit and the marked Y.

Mechanical soil aeration is a process to promote volatilization and transformation of contaminants in soil by physical disturbing, which affected both by environment and contaminants itself including the initial concentration [15], characteristics [16-17,27], organic matters [18] and moisture content [19-25] that would influence the remediation efficiency.

 

100

Remedial target value

log(concentration)

10

1

de le 1, t e r oe n e 2- tr th d i ac a n T c hl e 1 , ri h lo o r i 1, ch ro de 2- l o e t r 1 , T t ri o e t ha n 1 , et c h h y e 2, r a l o le 2 - c h ro n e t e lo e t tr r o h a a c e t ne h l hy or l e oe ne Ch tha lo n ro e fo rm hy

ri

ch

lo

ro

Ca

rb

1-

1,

on

di

lo

l ny

ch

Vi

di 11,

et

lo

ch

Be

nz

en

e

0.1

Fig.3 Contamination in soil after remediation with mechanical soil aeration

Effect of initial concentration. According to the initial concentration, the contaminants were divided into three levels: 10-2 (including 1,1,2,2-tetrachloroethane, trichloroethylene and benzene), 10-1(including carbon tetrachloride, tetrachloroethylene, 1,1–dichloroethylene, 1,1,2–trichloroethane, chloroform, vinyl chloride and 1,1–dichloroethane), 101(including 1,2-dichloroethane). The effect of initial concentration on removal efficiency was showed in Table 4. The average of removal rate for different concentration levels were 44.4%, 77.3% and 99.4% respectively. It was obviously that the more pollution contained the higher concentration of residual and removal rate. The amount of volatilization was increased when the concentration was high, which was similar to the results of Zhang Xiaohui [26]. The average of residual concentration in Table 4 showed that all concentration of contaminants dropped below the detection limit except 1,2-dichloroethane which is 0.17mg/kg in soil and 2.4 times above the detection limit. The higher of initial concentration, the more amount of the residual concentration was. When the concentration of chlorinated volatile organic compounds in the soil is high, most contaminants are likely to be adsorbed in pores between soil particles, which is difficult to remove. Under this situation, we need strengthen the remediation and prolong the remedial period. In field applications, we should focus on not only on the removal rate but also the residual concentration which has close relationship with the remedial target value.

 

Table 4 The remedial results of different initial concentrations Level 10-2

10

-1

101

Contamination

Initial concentration a [mg/kg]

Residual concentration a [mg/kg]

1,1,2,2-tetrachloroethane

0.03

0.025Y b

Trichloroethylene

0.06

0.025Y b

Benzene

0.06

0.025Y

b

Carbon tetrachloride

0.11

0.025Y b

Tetrachloroethylene

0.17

0.026

1,1–dichloroethylene

0.34

0.22

1,1,2–trichloroethane

0.41

0.025Y b

Chloroform

0.52

0.032

Vinyl chloride

0.62

0.25Y b

1,1–dichloroethane

0.72

0.025Y b

1,2-dichloroethane

30.59

0.17

Removal rate a [%] 44.4

77.3

99.4

a: the data is average value; b: according to the environmental statistical requirements, concentrations below the detection limit values expressed by half of the detection limit and the Y mark.

Effect of characteristics of concentration. After the treatment of mechanical soil aeration, residual concentration was relatively low. But there are still differences because of the characteristics of contamination. Physical and chemical properties are main factors affect the behavior of contaminants, especially the vapor pressure for VOCs [27]. Different vapor pressure would result in different evaporation rate [28]. In this study, contaminants with the same concentration were selected to exclude the impact of the initial concentration, including carbon tetrachloride, 1,1–dichloroethane, 1,1,2–trichloroethane, tetrachloroethylene and chloroform. The molecular weight, vapor pressure and residual concentration removing the data below the detection limit was shown in Table 5. Tetrachloroethylene with the lowest vapor pressure and highest molecular weight got the highest residual concentration as 0.140mg/kg after treatment. On the contrary, 1,1-dichloroethane had the lowest residual concentration as 0.066mg/kg with higher vapor pressure and smaller molecular weight. These findings illustrate that molecular weight and vapor pressure have a significant effect on the residual concentration of VOCs in soil during mechanical soil aeration. Higher VOCs losses occurred in samples of porous media with higher vapor pressure and smaller molecular weight. Table 5 The molecular weight, vapor pressure and residual concentrations of contaminants Contamination

Molecular weight

Vapor pressure [kPa]

residual concentration [mg/kg]

1,1–dichloroethane

98.97

15.33

0.066

1,1,2–trichloroethane

133.42

5.33

0.135

Chloroform

119.39

21.28

0.007

Carbon tetrachloride

153.84

13.33

0.092

Tetrachloroethylene

165.82

2.11

0.140

The vapor pressure of the liquid (or solid) is a pressure on the surface of liquid (or solid). Compounds with high vapor pressure mean more volatilization and easier to volatilize from soil. On the other hand, contaminant with complex molecular structure has relatively high molecular weight and strong van der Waals force. It is not easy for them to escape from soil particles. For soil contaminated by low vapor pressure or great molecular weight contaminants, increasing the frequency of agitation or enhancing the operation conditions may have a beneficial effect on remediation.

 

Effect of characteristics of soil. Soil texture is one of the physical properties of soil. It has a closing relationship with soil aeration, water retention and fragmentation. Thus it’s a main factor of the operation of mechanical soil aeration by influencing the state of contaminants in soil. Soil texture includes organic matter content and moisture content, etc. To understand the effect of them on remediation by mechanical soil aeration, contaminated soils in the same area but different depths were compared as they have similar operation conditions. The concentration of 1,2-dichloroethane after treatment in different soil was shown in Table 6. Table 6 concentration of 1,2-dichloroethane after treatment in different soil Depth

Type of soil

0-2.5m 2.5-5.5m 5.5-9.0m

Backfill Silt clay Stay clay

Detection rate

Moisture content

Organic matter

Minimum

Average

Maximum

[%]

[%]

[%]

[mg/kg]

[mg/kg]

[mg/kg]

23.3 6.2 7.6

18.7 15.5 24.3

4.4 2.2 3.2

0.025Y* 0.025Y* 0.025Y*

0.06 0.03 0.03

0.80 0.06 0.17

*: according to the environmental statistical requirements, concentrations below the detection limit values expressed by half of the detection limit and the Y mark.

1.5

5

1.5 25 1.2

0.0

ay St

ay

cl

ll Ba

ck

fi

cl St

lt Si

ay

cl

ll fi Ba

10

ay

1

ay

0.0

0.3

ay

2 0.3

15

0.6

cl

0.6

lt

3

20 0.9

Si

0.9

organic matter（%）

4

moisture content（%）

1.2

ck

concentration of 1,2-dichloroethane（mg/kg）

After treatment of mechanical soil aeration, the maximum concentration of 1.2-dichloroethane in three types of soil was all below the remedial target value (0.82mg/kg) with a little difference, which illustrated that the remediation for dealing with VOCs in different types of soil was effective but had different results. 1,2-dichloroethane had the highest detection rate as 23.3% in the first layer of soil (0-2.5 m) but lowest rate as 6.2% in the second layer of soil. It can be explained that as a volatilization compound, 1,2-dichloroethane is likely to volatilize than migrate downward when the concentration is low (