The interactions between trichloroethylene (TCE) and clay

Transcription

1 Hydrvlogical, Œemicat and Bioh^calPmcess^ofTransformation and Transport of Contaminants inaquatic Environments (Proceedings of the Rostov-on-Don Symposium, May 1993). IAHS Publ. no. 219, The interactions between trichloroethylene (TCE) and clay C. TANG Geography Department, Zhongshan University, Guangzhou , China S. SHESfDOU & H. OHASHI Department of Earth Science, Chiba University, Chiba 263, Japan Abstract Groundwater pollution caused by chlorinated volatile organic compounds (VOCs) has become very serious in many countries. It is found that concentrations of VOCs are often high above a clay layer in an aquifer. Experiments were conducted with TCE and montmorillonite to investigate the relationship between trichloroethylene (TCE) and clay. X-Ray diffraction was used to monitor the changes in clay structure before and after the clay was treated with TCE. Changes in the hydraulic conductivity of the clay were tested. From the mineralogical point of view, clay can swell in liquid, because it enters the inner structure of the clay. Experiments indicate that the degree of clay swelling is different at various concentrations of TCE. Trichloroethylene can intercalate into the clay and thereby change the clay hydraulic conductivity. As a result, TCE will move slowly in clay. Finally, the effects of the clay and NaCl solute on the TCE are discussed. INTRODUCTION Any consideration of migration process of a given solute in groundwater necessitates an understanding of its interactions with aquifer materials. Clay plays a very important role in water movement and mass transport in groundwater systems (Lee et al., 1990; Boyd et al, 1988a, 1988b; Mortland et al, 1986). It is commonly thought that, if the hydraulic conductivity of the clay is low and if there are no structural defects, the clay layer will provide an adequate barrier between the waste and the underlying hydrogeologic domain. Johnson et al (1989) have evaluated the diffusion of contaminants in a natural clay. They determined vertical profiles of organic contaminants in several leachates by analysing core samples taken from an impervious, unweathered, water-saturated clay deposit beneath a waste landfill. They suggested that the downward transport of the organic contaminants into the clay was the result of simple Fickian diffusion. On the other hand, the hydraulic conductivity is dependent on the soils that make up the aquifer. The swelling of the clay has an effect on its hydraulic conductivity. Green et al (1983) reported interactions of clay soils with water and organic solvents. Their data showed that the degree of clay swelling may depend on its interactions with the chemical solutes in the aqueous phase. Such interactions could increase the retardation factor of non-ionic organic solutes (NOS) in porous media that contain little or even no natural organic matter. The interactions could also potentially alter the contaminant hydrology of NOS in subsurface systems.

2 100 C. Tang et al. The purpose of this work is to investigate the migration of TCE through clay. Therefore, this paper evaluates the surface-chemical interactions of trichloroethylene (TCE) and the clay in aqueous solutions, as well as their effects on the hydraulic conductivity of the clay. MATERIALS AND METHODS Clay used in our study was Japanese clay, a type of acid clay, mostly constituted of montmorillonite. The soil physical features are shown in Table 1. Two kinds of experiments were carried out: hydraulic conductivity analysis and batch experiments. All the experiments were done at room temperature (T = C). Table 1 Characteristics of the clay used in the experiments. Clay Montmorillonite Organic carbon content (%) 0.01 Cation exchange capacity (CEC), in meq 100g" Hydraulic conductivity analysis In order to investigate the change of hydraulic conductivity of clay, leaching experiments were carried out with water and a TCE aqueous solution. The column (10 cm in length and 2 cm in diameter) was constructed with materials selected to minimize sorption losses. They included a glass column, Teflon O-rings to seal the end parts, a Teflon cap on the downstream end and stainless steel fittings. The column design allows easy assembly or disassembly and can easily be adjusted to various lengths using different length of glass tubes. Before and after the experiments, the hydraulic conductivity was measured. The degree of clay swelling, in dry form and in water, TCE aqueous solution (in 1100 mg l" 1 ) and pure TCE, was also tested by X- ray diffraction. Batch experiments In order to clarify the effects of ionic strength and ph on the concentration of TCE in aqueous solution, batch experiments were conducted with clay (2 g) and TCE aqueous solution (in 1045 mg l" 1 ) in increasing concentrations of NaCl (0, 10, 50, 100, 500 and 1000 mg l" 1 ). The ratio of clay and solution in the experiments was 2 g:20 ml. Preliminary studies with Japanese clay indicated that an 8-h shaking on a platform shaker at low speed was sufficient to attain equilibrium. However, samples used in our experiments were shaken for two days. Na + and CI" were analysed by ion chromatography. Trichloroethylene was analysed by gas chromatography with FID.

3 RESULTS AND DISCUSSION Effects of swelling and shrinkage of clay The interactions between TCE and clay 101 In general, clay swells or shrinks under both wet or dry conditions. When a liquid enters into the clay, it changes the inner structure. The results of average layer distances within the clay structure as measured by the X-ray method are listed in Table 2. The degree of clay swelling was dependent on the water content and the concentrations of TCE, and decreased in the order of water > TCE aqueous solution > TCE and dry clay (Table 2). Table 2 Comparison of inner layer distances and hydraulic conductivity of clay treated with water and TCE solutions. Treatment Average inner layer Standard deviation Hydraulic distance (A) (A) conductivity (cm s" 1 ) Dry clay TCE x 10" 8 Aqueous TCE solution (1100 mgl" 1 ) x 10' 5 Water x 10" 5 The hydraulic conductivity was tested before and after the leaching experiments. After the leaching experiment with TCE, the hydraulic conductivity was reduced to half of its initial value. It could be considered that as TCE enters the clay layers, it changes the hydraulic conductivity of the clay. The change of the hydraulic conductivity of the clay under different conditions is also shown in Table 2. The hydraulic conductivity of the clay in water is twice that in TCE aqueous solutions. From the results of the hydraulic conductivity experiments, it seems that the hydraulic conductivity of the clay is related to the characteristics of the liquid which flows through it. The presence of water in the clay soils suggests that swelling occurs as a result of the penetration of an organic liquid (or water) into the soil, and that the shrinkage occurred as a result of the exchange of pore water with organic liquid in the surrounding bulk liquid. It can thus be expected that molecules of liquids that hydrogen bond with water or that interact with water through dipole-dipole forces will have a tendency to become soluble in the soil pore water, and therefore, will cause bulk clay expansion. Apolar, hydrophobic liquids, on the other hand, should have little tendency to move into and be solubilized within the pore water region. Compared with water, these liquids will cause only slight swelling in most cases and may cause the clay soil to shrink. The shrinkage seems to be the result of the exchange of water with organic solute in the bulk liquid. A suitable parameter for correlating shrink-swell behaviour of clay soils with the hydrophobic or hydrophilic character of the solute is the octanol-water partition coefficient (K ow ). K ow is a measure of the degree to which solute molecules are apt to partition themselves between n-octanol and water (Green et al., 1983). It also is possible to use K ow, or log K ow, as an index of molecular transfer from the solvent

4 102 C. Tang et al. itself into water, that is, as an index of aqueous solubility. The fact that clay soils swell in solvents with large negative log K ow values (hydrophilic) and may shrink in solvents having large log K ow values (hydrophobic) is consistent with the view that shrink-swell behaviour is related to solvent solubility in the soil pore water. It seems that if the different organic liquids are percolated through the clay sequentially, the clay will swell or shrink according to the composition of liquid. In practice, the clay layer in the groundwater system is covered by the aquifer, the hydraulic conductivity of the clay will change responding to the composition of liquid passing through not to the swelling and shrinking itself. Therefore, K ow may be used as an index to estimate the hydraulic conductivity of clay in different organic solutions. Effects of the electrolyte solutions and ionic strength To investigate the effects of the ionic strength and its role in the partitioning of TCE, solubility experiments were conducted at different concentrations of NaCl solute. During the experiments, ph was kept at about 7. The results of ph versus concentrations of TCE are shown on Fig. 1. The concentration of TCE decreased with the increase of NaCl concentrations. In general, partition of TCE can be altered by the effects of ionic strength of inorganic solute on its aqueous activity, which is observed as the "salting out" effect on the hydrophobic partitioning. Combined effects of ph and NaCl Because the content of organic carbon in the soil used here was less than 1 %, the TCE absorbed by organic carbon could be ignored. We only consider how the presence of clay and inorganic ions can affect on the behaviour of TCE. Figure 2 shows the results of batch experiments with clay, NaCl and TCE. The TCE concentration decreased with the increase of NaCl concentrations. At the same PH «TCE ph Cl" concentration of (ng/i) 1000 Fig. 1 Variations of TCE in batch experiments mixed by NaCl. C and C 0 are respectively the concentration of TCE in the experiments and its initial value.

5 The interactions between TCE and clay i.o O S 0.8 o I a o I 0.2 ^ CI"" concentration of (nig/1) 1000 Fig. 2 Variations of TCE in batch experiments mixed by clay and NaCl. C and C 0 are respectively the concentration of TCE in the experiments and its initial value. time, the ph in the solution also decreased from 5.5 to 4.5, because the Japanese clay is an acid clay which released H + into water. The presence of NaCl could destroy the clay structure, then release aluminum and ferrous ions into water, resulting in a decrease in ph. The results of the batch experiments show that the presence of NaCl can affect the solubility of TCE by changing ph and. ionic strength. The principal conclusions are: (a) The analysis of data obtained in the experiments shows that different liquid intercalation into the clay causes changes in clay volume. It seems that the change of clay volume responds to the change of its hydraulic conductivity. In fact, the hydraulic conductivity of clay will depend on the characteristics of previous and present passing liquids. (b) When a different liquid moves into the clay, the intercalation occurs. Under the field condition, hydraulic conductivity will change because of swelling or shrinkage of clay. Therefore, it is expected that change of hydraulic conductivity due to different liquids may depend on the condition of the clay as well as the K ow coefficient of the liquid which passes through the clay. (c) The data presented here suggest that ionic strength plays an important role in the partitioning of TCE. The presence of NaCl causes a decrease in both the TCE concentration and the ph by reacting with the clay. The overall effect of this process is a function of ph and ionic strength. Although this investigation was performed with TCE, the results may be extended to other VOCs. Acknowledgements We acknowledge valuable advice given by Dr M. Hirata and Dr S. Yashuhara of Japan Environmental Research Institute. We gratefully acknowledge the assistance provided by Professor K. Sakura of Chiba University. REFERENCES Boyd,S. A., Shaobai.S., Lee, J. F. &Mortland, M. M. (1988a) Pentachlorophenol sorption by organic clay. Clays Clay Miner. 36,

6 104 C. Tang et al. Boyd, S. A., Mortland, M. M. & Chiou, C. T. (1988b) Sorption characteristics of organic compounds on hexadecyl trimethylammonium smectile. Soil Sci. Am. J. 52, Green, W. J., Lee, G. F., Jones, R. A. & Pâlit, T. (1983) Interaction of clay soils with water and organic solvents. implications for the disposal of hazardous wastes. Environ. Sci. Technol. 17(5), Johnson, R. L., Cherry, J. A. & Pankow, J. F. (1989) Diffusive contaminant transport in natural clay, a field example and implications for clay-lined wasted disposal sites. Environ. Sci. Technol. 23(3), Lee, L. S., Rao, P. S. C, Nkedi-Kizza, P. & Delfino, J.J. (1990) Influence of solvent and sorbent characteristics on distribution of pcntachlorophenol in octanol-water and soil-water systems. Environ. Sci. Technol. 24(5), Mortland, M. M., Shaobai, S. & Boyd, S. A. (1986) Clay organic complexes as adsorbents for phenol and chlorophenols. Clays Clay Miner. 36,