Ind. Eng. Chem. Res. 1997, 36, 2435-2439
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Feasibility Study on Salting-in of Organic Pollutants To Enhance the Effectiveness of Ex-Situ Soil Washing† Sao-Ling Chiu and Nilufer H. Dural* Department of Chemical Engineering, Cleveland State University, Cleveland, Ohio 44115
The feasibility of using a salting-in effect for leaching organic pollutants from subsurface materials was investigated. Two clay samples with contrasting properties and three wellcharacterized soil samples were spiked with nitrobenzene, a priority contaminant, and experimental desorption equilibrium studies were conducted. By using aqueous solutions of benzoic acid ammonium salt with a concentration range from zero to 72% of saturation, desorption isotherms were generated. The impact of salting-in on the leaching efficiency and the influence of soil properties on the process were examined. Qualitative criteria for feasibility were established. Results showed that the presence of salt caused a substantial increase in the amount of nitrobenzene leached from subsurface materials with low ion-exchange capacity, and the positive trends become more pronounced with increasing levels of salt concentration. In the case of subsurface materials with high ion-exchange capacity, however, solute-solid interaction was the controlling mechanism. Consequently, improvement in leaching efficiency due to increased liquid-side solubility was lessened or diminished. Both physical and chemical characteristics of soil samples had an impact on the levels of leaching achieved. Clay type and its percentage were the dominant factors. They were followed by organic matter content and surface properties. No significant level of salt adsorption on soil was observed. Introduction Soil contamination is one of the most critical environmental issues currently being addressed throughout the United States. It is difficult to describe a decontamination technique for all types of contaminants, and a general approach to solve the problem is not feasible. Effective solvent extraction of organic pollutants from soils is one option currently being explored (O’Neil et al., 1993; Jones, 1992; Aronstein et al., 1991; Hall et al., 1990). Soil washing or leaching is based on a desorption phenomenon that results when the solutesolvent interaction overcomes the adsorbate-adsorbent interaction. Solubility of the solute in the solvent is the key element for leaching efficiency. Common soil washing solvents include both organic and aqueous systems. Aqueous systems are always preferable since they do not pose potential contamination of the soil through the adsorption of the solvent. However, water is not an ideal solvent for hydrophobic compounds, which constitute the majority of organic pollutants having water solubilities of less than a few parts per million (Karickhoff et al., 1979). Therefore, increasing the water solubility of an organic pollutant may increase the efficiency of soil washing. The solubility of a nonelectrolyte in water changes by the addition of an electrolyte. The result is called the salting effect, which is commonly expressed by the Setschenow equation (Das and Ghosh, 1983):
log fc ) log(S0/S) ) KsCs
(1)
where fc is the activity coefficient of the nonelectrolyte, S0 and S are the solubilities in pure solvent and in salt solution, respectively, Ks is the salting constant, and Cs is the salt concentration. A positive salting constant indicates that when the salt is added, the solubility of * To whom correspondence should be addressed. † Paper presented at the AIChE 1995 Annual Meeting, Nov 12-17, 1995, Miami Beach, FL. S0888-5885(96)00571-4 CCC: $14.00
the solute in the solution decreases due to the increased activity. This effect is called salting out, which has been recently explored to its benefits in renovation of aqueous waste streams (Kan et al., 1994). On the other hand, a negative salting constant is an indication of a saltingin effect, which causes an increase in the solubility of the solute. Both cations and anions contribute to the salting effect. In general, cations lead to salting-out and anions lead to salting-in. The final result depends on the effect that predominates. An investigation on the adsorption capacity of soils for various fertilizers has shown that the presence of NaCl and MgSO4 has significantly increased the adsorption through salting-out the organic material from the solution and displacing cations from the soil (Hamaker and Thompson, 1972). Work on the effect of anionic materials associated with soil sorption has not been reported yet. However, the activity of anions in the solution would be reduced, which, in turn, would tend to decrease the adsorption. Consequently, it could be expected that the leaching efficiency would increase if an appropriate salt (a salt that would cause a saltingin effect and would not be adsorbed by the soil) was added into a soil washing system. The present work was undertaken to investigate the feasibility of salting-in of organic pollutants from contaminated soils into aqueous solutions. Equilibrium desorption of nitrobenzene, a priority contaminant, from two clay samples with contrasting properties and three soil samples with different characteristics into pure water and aqueous solutions of benzoic acid ammonium salt (0-72% saturation) was studied. The impact of the presence of salt on the leaching efficiency and the influence of soil properties on the process were examined. Nitrobenzene was the target organic pollutant due to its abundance in the subsurface environment and the inconsistencies associated with its degradation in soils. Experimental Section Materials. Nitrobenzene was obtained from Mallinckrodt Chemical Works with a purity of 99.8%+. Based © 1997 American Chemical Society
2436 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
on the limited published information about the salts that would lead to salting-in of nitrobenzene in water (Das and Ghosh, 1984; Long and McDevit, 1952), benzoic acid ammonium salt (NH4C6H5CO2) was deemed to be an appropriate one because of its salting coefficient, Ks ) -0.21, and its negligible and reversible adsorbability on various subsurface materials. It was supplied by Sigma Co. The adsorbents were Kaolinite and Bentonite clays and three different soil samples. Kaolinite clay (specific surface area, 10-20 m2/g; cation-exchange capacity, 3-15 mequiv/g) and Bentonite clay (specific surface area, 700-840 m2/g; cation-exchange capacity, 80-150 mequiv/g) were chosen due to their contrasting properties. The soil samples which will be referred to as soil I (11.4% sand, 52.7% silt, 33.4% clay-Kaolinite, 2.4% organic matter; specific surface area, 44.14 m2/g), soil II (45.1% sand, 35.2% silt, 21.7% clay-Bentonite, 1.7% organic matter; specific surface area, 25.33 m2/g), and soil III (91.7% sand, 6.3% silt, 2.0% clay-Kaolinite, 1.7% organic matter; specific surface area, 23.48 m2/g) were well characterized prior to the experiments. Procedure. Batch-type equilibrium desorption experiments were conducted according to the EPA procedure (Roy et al., 1991). A 40 mL amber glass serum bottle with Teflon-lined septa was used as the experiment container. Typically, 10-15 g of adsorbents was added to each serum bottle and spiked with various levels of nitrobenzene. After 48 h, 35 mL of deionized water or 0.5-1.0 M salt solutions was added to each bottle, and the bottles were left for equilibration in a constant-temperature reciprocating shaker bath at 25 °C. The equilibration time was determined from kinetic batch studies. Following equilibration, the samples were directly centrifuged in serum bottles, and the supernatant was removed and taken for concentration measurement. Equilibrium concentration of nitrobenzene was measured by using a gas chromatograph/flame ionization detector. A spectrophotometer was used to determine salt concentrations (at 271 nm for maximum absorbancy). Desorption equilibrium uptake was calculated from the difference between the amount of nitrobenzene spiked and the final solution concentration. Results and Discussion The desorption equilibrium data describing the binding tendency of nitrobenzene on clays and soils as pure water and water-benzoic acid ammonium salt solutions being the extracting solvents were measured at 25 °C. The results are presented as desorption isotherms, plotted in the standard manner, namely, the amount of nitrobenzene retained after desorption in milligrams per kilogram of clay/soil versus the equilibrium concentration of nitrobenzene in the liquid phase in milligrams per liter. The reproducibilities of the isotherms were examined by conducting two sets of experiments for each pair. Several data points were reproduced within the limits of experimental error. Because Kaolinite clay is the most common clay type found in soils, the equilibrium isotherms describing desorption of nitrobenzene from Kaolinite clay into pure water and into 0.53-1.00 M salt solutions were generated first. A concentration of 1.00 M salt solution, which is about 72% of saturation, was the highest concentration used due to solubilty limitations at 25 °C. The results were evaluated for the impact of salt concentration on the isotherm shift. Then, equilibrium desorption
Figure 1. Desorption of nitrobenzene from Kaolinite clay.
Figure 2. Desorption of nitrobenzene from Bentonite clay.
of nitrobenzene from Bentonite clay into water and into 1.00 M salt solution was studied. The influence of clay type on the extraction improvement was investigated. The final step was the construction of isotherms for desorption of nitrobenzene from soils I, II, and III into water and into a 1.00 M benzoic acid ammonium salt solution. This led to the analysis of the effect of soil properties on the leaching efficiency as well as the improvement achieved through salting-in, with reference to leaching by pure water. The possibility of benzoic acid ammonium salt adsorption on the test soils and clays was examined by measuring the salt concentration before and after each experiment. Results showed that salt adsorption was negligible, varying from zero to 1.94% for different subsurface materials. This implies that benzoic acid ammonium salt primarily changes the activity of nitrobenzene in the solution, but it does not significantly alter the charge density on soil adsorptive sites to cause substantial salt adsorption. The maximum salt adsorption occurred on Bentonite clay, which possesses the highest ion-exchange capacity among the test samples investigated. This indicates ion displacement from the clay ion-exchange matrix due to the presence of benzoic acid ammonium salt. Adsorption of salt was completely reversible upon soil washing at 25 °C. In Figures 1 and 2, desorption of nitrobenzene from Kaolinite and Bentonite clays into pure water and into salt solutions is presented. When the desorption into pure water is examined, it is clear that the leaching efficieny is much lower with the Bentonite clay than with the Kaolinite clay. The equilibrium uptake of Kaolinite is about 11% of the uptake of Bentonite. In a concentration range from 200 to 1400 mg/L, the percent recovery of nitrobenzene is from 77% to 81% for the Kaolinite clay, while within the same range, the percent recovery from the Bentonite clay varies from 72% to 80%. These results can be explained by the differences in the clay structures. Bentonite is an expanding clay with a very high internal surface area and cation-
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Figure 3. Improvement on nitrobenzene leaching from Kaolinite clay due to the presence of benzoic acid ammonium salt.
exchange capacity, which depends on the clay minerals, when it is exposed to water. It has a high negative surface charge and produces an electric field in the soil solution by double layer formation to attract sorbate. Kaolinite, on the other hand, is a nonexpanding clay with much lower surface area and cation-exchange capacity. This limits its capability to retain large masses of organic solute and facilitates the desorption. In the presence of salt, desorption of nitrobenzene from the Kaolinite clay significantly increases (Figure 1). The removal efficiencies at 800 mg/L are 94% and 95% for 0.53 and 1.00 M solutions, respectively. At 1400 mg/L, they are 89% (0.53 M) and 93% (1.00 M). In order to quantify the impact of salt on leaching, extraction improvement was determined as follows:
improvement ) {(Uw - Us)/Uw} × 100%
Figure 4. Desorption of nitrobenzene from soil I.
Figure 5. Desorption of nitrobenzene from soil II.
(2)
where Uw and Us are the equilibrium uptakes (mg/kg) in pure water and salt solution, respectively. Improvement percentages observed during the leaching of nitrobenzene from Kaolinite clay using different benzoic acid ammonium salt concentrations are shown in Figure 3. The average improvements over the concentration range investigated are 58% and 81% for 0.53 and 1.00 M solution, respectively. Considering that clay, as a soil constituent, is the the most difficult one to clean up, one can conclude that salting-in can be effectively used for soils with Kaolinite clay. However, the results from the Bentonite clay exhibit a totally different trend. As shown in Figure 2, the retention capacity of the clay in a salt solution is much higher than the one in pure water. These results indicate that an increased liquid solubility is not the only consequence of the presence of the salt in the system. The increase in the uptake is due to the high level cation exchange on the mineral surface of the Bentonite clay. In the presence of weak hydrated cations, such as NH4+, the distribution coefficient increases with decreasing free energy of hydration of the cation (Haderlen et al., 1993). During the experiments, when NH4+ was introduced into the process, the desorption was positively affected by the increased solubility, while at the same time, it was negatively influenced by cation exchange. The overall result depends on the competition between the liquid solubility, which makes the solute move to the liquid phase, and the cation exchange, which increases the affinity between the solute and the sorbent. Cation-exchange capacity of Bentonite clay is very high (80-150 mequiv/ 100 g) when compared to Kaolinite clay (3-15 mequiv/ 100 g). Thus, the solute-solid interaction was the dominant mechanism during desorption from Bentonite
Figure 6. Desorption of nitrobenzene from soil III.
clay, which increased the desorption uptake. In Kaolinite clay, slight cation exchange could not overcome the interaction between the solute and the solvent, and the overall process was controlled by the liquid side. Consequently, this led to an increased leaching efficiency. In Figures 4-6, desorption of nitrobenzene from different soils into water and a 1.00 M salt solution is presented. The best-fit Freundlich parameters which were used for interpolation of the experimental data for recovery and improvement estimations are given in Table 1. A comparison of desorption from different soils into pure water shows that the retention capacities of the soils for nitrobenzene are different and, overall, they are in the following order:
soil I > soil II > soil III Up to 800 mg/L equilibrium concentration, the percent recovery ranged from 100% to 82%, 85%, and 91% for soils I, II, and III, respectively. At around 1400 mg/ L, the recoveries were 69%, 67%, and 69% for the same soils. These results can be interpreted by the combined effect of physical/chemical soil properties.
2438 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 Table 1. Best-Fit Parameters of the Freundlich Isotherm adsorbent
salt solution (M)
K
1/n
R2
Kaolinite Kaolinite Kaolinite Bentonite Bentonite soil I soil I soil II soil II soil III soil III
0.00 0.53 1.00 0.00 1.00 0.00 1.00 0.00 1.00 0.00 1.00
0.422 0.0018 9.18 × 10-5 0.657 0.524 0.0187 2.86 × 10-15 0.0180 6.08 × 10-5 0.000176 4.88 × 10-7
11.223 56.660 120.628 21.954 40.228 56.570 316451.7 56.285 305.859 215.665 881.049
0.964 0.947 0.966 0.952 0.984 0.956 0.897 0.927 0.954 0.898 0.954
Figure 7. Nitrobenzene recovery from soil I.
Soil’s organic matter is claimed to be the most important component influencing its sorption capacity in the presence of a solvent. Soils I, II, and III have 2.4%, 1.7%, and 1.6% organic matter, respectively. Their sorption capacities are in the same order; i.e, the order is positively correlated with their organic matter content (Figures 4-6). Because organic matter and clay provide a significant portion of the soil’s specific surface area, this result is not surprising. However, if the organic matter was the dominant factor, soil I should have had a much higher retention capacity when compared to the other two. This implies a combined effect of additional properties, especially clay content and clay type. Soil I, II, and II have 33.4%, 21.7%, and 2% clay contents, respectively. Accordingly, the order of their retention capacities is the same as the order of their clay percentages. However, clay content alone does not satisfactorily explain the close retention values obtained for soils I and II, while the results from soil III are substantially different. Soils I and III have the same type of clay, which is Kaolinite, but soil II has Bentonite clay. As discussed earlier, Bentonite clay has a much higher sorption capacity than Kaolinite clay; thus, its impact on desorption from soil II is significant. The presence of Bentonite clay in soil II can also explain the fact that soils II and III have similar organic matter content (1.7% and 1.6%), but their desorption behaviors are quite different, while soils I and II with 2.4% and 1.7% organic matter contents desorb similarly. On the other hand, the different retention capacities of soils I and III are directly related to their organic matter and clay content since they both have the same type of clay but in different proportions. An analysis of the desorption of nitrobenzene into salt solution shows that, for all soil types, the uptake of sorbent in salt solution is much lower than the one in pure water (Figures 4-6). At 800 mg/L, the removal efficiencies obtained with 1.00 M solution are 98% (soil I), 84% (soil II), and 96% (soil III). At 1400 mg/L, they are 96%, 75%, and 93%. In Figures 7-9, nitrobenzene
Figure 8. Nitrobenzene recovery from soil II.
Figure 9. Nitrobenzene recovery from soil III.
recoveries from different soils using pure water and salt solutions are presented as a function of the initial soil concentration. At an initial soil nitrobenzene concentration of 10 000 mg/kg, solute recovery from soil I increases from 68% to 98% when 1.00 M salt solution is used instead of water. Under the same conditions, recovery increases from 70% to 81% for soil II and from 85% to 95% for soil III were observed. As can be seen from Figures 4-6, the presence of salt changes the retention capacities of the soils but the degree of change for each case is different. Unlike the results obtained with pure water, the equilibrium capacities of the soils for nitrobenzene in the presence of salt are in the following order:
soil II > soil III > soil I The influencing factors for the change in the trend include solubility, ion exchange, and other soil properties. It is reasonable to assume that the soil’s organic matter has little or no impact on this change. Since solvated ions do not interfere or compete with the penetration of nonpolar organic compounds into natural organic matter, salt affects the distribution coefficient primarily through the activity term. In the presence of benzoic acid ammonium salt, the activity of nitrobenzene in water should decrease according to the saltingin characteristics of the salt; thus, the solubility increases. Finally, the retention capacity decreases. If the organic matter content was the dominant factor in this process, the order of retention capacity would be the same as the one observed in pure water desorption. The change in the uptake order is directly related to the soil’s clay type and clay content. In soil II, the presence of salt causes a strong cation-exchange reaction on the surface of the Bentonite clay (21.7%), which, in turn, produces a negative effect on desorption. At the same time, it also increases the solubility of nitrobenzene in the liquid side, favoring the desorption. In light
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organic contaminants such as benzene, aniline, paminophenol, and p-nitrophenol can be considered as viable candidates. Results presented herein clearly demonstrate the significant impact of the soil composition on the leaching improvement levels. What remains to be investigated in the next phase is the quantification of improvement levels with different contaminants. Combined results will ultimately lead to the formulation of complex interactions among soil, contaminant, and salt and parameter estimation for the design of a generalized system. Figure 10. Improvement on nitrobenzene leaching from different soils due to the presence of benzoic acid ammonium salt.
of the results presented in Figure 2, one may conclude that the overall result of this competition would clearly be dependent on the clay content. Soil II has a lower uptake in salt solution than in pure water (Figure 5), but not as low as soils I and III, which both have Kaolinite clay. In Figure 10, improvement levels on leaching due to the salting-in effect are presented for different soils. Soil I delivered the highest improvement on the leaching efficiency, which is higher than 90% up to an equilibrium concentration of 1400 mg/L. The overall improvement with soil I is 98%, and it is followed by soil III with 86% improvement. Soil II allowed for the lowest improvement, only 60%, because of the impact of the Bentonite clay with its high ion-exchange capacity. The results of the present work show that use of the salting-in effect to enhance aqueous soil washing of organic contaminants is a promising approach because of its potential to significantly increase the leaching efficiency and its suitability for integration into the existing soil washing facilities. The improvement levels summarized in Figure 10 correspond to a single-stage equilibrium contact operation. Based on the desorption equilibrium isotherms presented in Figures 4-6, increased leaching levels can be obtained to effectively meet any clean-up criterion by engaging in a countercurrent multistage equilibrium contact operation. From the equilibrium data, it is clear that the presence of salt will decrease the number of theoretical stages required for the separation, which, in turn, decreases the required equipment size and involved operational cost. The process is directly applicable to existing ex-situ soil washing facilities with few additional equipment including mixing tanks and related auxiliaries. Therefore, the additional cost involved in integrating the process into the existing facilities is anticipated to be insignificant. The actual design, economic evaluation, and implementation of a generalized system will require intermediate steps including experimental quantification with other organic contaminants, pilot-scale tests, and preliminary design studies. The method can be applied to other organic contaminants provided that an appropriate salt is added into the soil washing system, i.e., a salt that will cause notable salting-in effect and will not contaminate the soil through significant and/or irreversible adsorption. Based on published information (Das and Ghosh, 1984; Long and McDevit, 1952), several
Conclusions Feasibility of the use of the salting-in effect in conjunction with soil washing was investigated. Experimental studies to analyze the desorption of nitrobenzene from clays and soils into pure water and benzoic acid ammonium salt solutions were conducted. Results showed that the salting-in effect caused a substantial increase in the amount of nitrobenzene leached from subsurface materials with low ion-exchange capacity, and the positive trend became more pronounced with increasing levels of salt concentration. In the presence of high levels of ion exchange, solutesolid interaction was the controlling mechanism; thus, the improvement in leaching efficiency due to the increased liquid-side solubility was lessened. Clay type and its percentage in the soil were the dominant factors. Salt adsorption on soil was reversible and negligible, with a maximum of 1.94%. Literature Cited Aronstein, B. N.; Calvillo, Y. M.; Alexander, M. Environ. Sci. Technol. 1991, 25, 1728. Das, B.; Ghosh, R. J. Chem. Eng. Data 1983, 28, 45. Das, B.; Ghosh, R. J. Chem. Eng. Data 1984, 29, 137. Haderleln, S. B.; Schwarzenbach, R. P. Environ. Sci. Technol. 1993, 27, 316. Hall, D. W.; Sandrin, J. A.; McBridge, R. E. Environ. Prog. 1990, 9, 98. Hamaker, J. W.; Thompson, J. M. Adsorption. In Organic Chemicals in the Soil Environment; Goring, C. A. I., Hamaker, J. W., Eds.; Marcel Dekker: New York, 1972; Vol. 1, Chapter 2. Jones, R. G. Environ. Prog. 1992, 11, 223. Kan, A. T.; Fu, G.; Thomson, M. B. Environ. Sci. Technol. 1994, 28, 859. Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241. Long, F. A.; McDevit, W. F. Chem. Rev. 1952, 51, 119. O’Neil, M. W.; Symons, J. M.; Lazaridu, M. E.; Park, J. B. Environ. Prog. 1993, 12, 12. Roy, W. R.; Krapac, I. G.; Chou, S. F. J.; Griffin, R. A. Batch Type Adsorption Procedures for Estimating Soil Attenuation of Chemicals. EPA/530-SW-87-006. U.S. EPA, Cincinnati, OH, 1991.
Received for review September 16, 1996 Revised manuscript received March 10, 1997 Accepted March 15, 1997X IE960571T
Abstract published in Advance ACS Abstracts, May 1, 1997. X
