Enhanced Removal of Organic Contaminants by Solvent Flushing

Chapter 18

Enhanced Removal of Organic Contaminants by Solvent Flushing 1

D. C. M . Augustijn and P. S. C. Rao

Downloaded by FUDAN UNIV on January 25, 2017 | http://pubs.acs.org Publication Date: November 9, 1995 | doi: 10.1021/bk-1995-0607.ch018

Soil and Water Science Department, University of Florida, Gainesville, FL 32611

The use of cosolvents to enhance the remediation of contaminated soils is based on four observations: (1) enhanced mobilization of a residual N A P L phase; (2) increased solubility; (3) reduced sorption or retardation; and (4) increased mass-transfer rates. The theoretical basis for each of those mechanisms is briefly reviewed. Solvent washing technologies for treatment of excavated soils are already commercially available. The potential use of organic cosolvents for in situ remediation (solvent flushing) is illustrated by a review of several studies that showed an enhanced removal of contaminants upon addition of a cosolvent. Possible problems that may arise when cosolvents are used for in situ remediation are indicated, which should be helpful for planning further research and to develop solvent flushing into a viable remediation technique.

With a growing experience in site remediation, it becomes more evident that many conventional technologies are not able to restore contaminated sites to required clean up levels in a reasonable time frame. This fact has stimulated the search for alternative technologies that are able to clean up contaminated sites in a timely and cost-effective way. Several new technologies are being developed, among which the use of various adjuvants is receiving considerable attention. There are essentially two types of adjuvants: (1) those that enhance the release and mobility of contaminants (e.g., cosolvents, surfactants), and (2) those that enhance the transformation of contaminants (e.g., nutrients, ozone, chemical reaction agents). This paper will focus on the first type of adjuvants, in particular on the use of cosolvents for enhanced remediation of source areas at waste disposal sites. Current address: Department of Civil Engineering and Management, University of Twente, P.O. Box 217, 7500 A E Enschede, Netherlands

0097-6156/95/0607-0224$12.00/0 © 1995 American Chemical Society Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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Cosolvency Theory The use of cosolvents (e.g., alcohols) for soil remediation is based on four principles. First, cosolvents decrease the interfacial tension between the solution phase and a nonaqueous phase liquid (NAPL), inducing mobilization of a trapped N A P L phase. Second, cosolvents increase the solubility of non-polar organic chemicals, enhancing the release of organic constituents of an immobile N A P L phase. Third, cosolvents reduce sorption, facilitating faster transport of dissolved contaminants. Fourth, masstransfer rates generally increase in the presence of cosolvents. These characteristics all favor an enhanced removal of organic chemicals from contaminated soils, and therefore indicate a potential application of cosolvents for remediation of contaminated sites. In the following sections, the cosolvency theory will be briefly reviewed. Mobilization. Many industrial waste disposal/spill sites are contaminated with immiscible nonaqueous phase liquids (NAPLs). If the N A P L is present as a free moving phase, a part of it can be recovered hydraulically. What is left behind is a residual saturation of discontinuous ganglia trapped by capillary forces in the pore spaces. The ratio of viscous and capillary forces acting on the residual N A P L is expressed by the capillary number:

where μ is the dynamic viscosity of the solution phase, ν is the groundwater velocity, and φ is the interfacial tension between the solution and N A P L phase^ In general, residual N A P L is mobilized as the capillary number is greater than 2x10 and virtually all N A P L is displaced when the capillary number exceeds 5x10 (7). Equation 1 indicates that mobilization of the N A P L globules can be established by increasing the groundwater velocity, decreasing the NAPL-water interfacial tension, or a combination of both. Given the practical limitations on groundwater pumping rates, reducing the interfacial tension by adding cosolvents or surfactants to groundwater may be a more viable option for remediating sites contaminated with a residual N A P L phase. Cosolvents may enhance the mobilization of a residual N A P L phase also as a result of the partitioning of the cosolvent into the NAPL, which depends on the hydrophobicity of the N A P L as well as the cosolvent. When a cosolvent partitions preferentially into the N A P L phase, the residual N A P L globules may swell considerably. The swollen N A P L globules may become a relative continuous phase, making them much easier to displace. In addition, the swelling will reduce the density of the N A P L phase, which increases the buoyancy force acting on submerged N A P L globules. This makes it easier to displace the residual N A P L in an upward direction and reduces the potential for further downward migration (2).

Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT V

Dissolution. NAPLs may consist of one or more components. Chlorinated solvents like tri- and perchloroethylene (TCE, PCE) are prominent examples of singlecomponent NAPLs. Gasoline, motor oil, diesel, creosote, and coal tar are typical examples of multi-component NAPLs. Due to the limited solubility of most organic constituents in water, removal of the N A P L by dissolution in water, as in pump-andtreat techniques, often requires years or even decades (5). By adding an organic cosolvent to water, the polarity of the solvent mixture decreases, resulting in an increased solubility of non-polar organic chemicals. The solubility of a non-polar organic solute in a binary solvent mixture (S ) increases in a nearly log-linear manner with increasing volume fraction of cosolvent (f ) (4- 6). m

c

\ogS =\ogS +fiaf Downloaded by FUDAN UNIV on January 25, 2017 | http://pubs.acs.org Publication Date: November 9, 1995 | doi: 10.1021/bk-1995-0607.ch018

m

w

(2)

c

where σ=1ο |^

(3)

8

β is an empirical coefficient that accounts for water-cosolvent interactions, S is the solubility in the neat organic solvent, S is the solubility in water, and σ is the cosolvency power of the organic solvent for the solute of interest. When it is assumed that the cosolvent effects are additive, equation 2 can be generalized as follows for a mixture of water and several cosolvents: c

w

Ν

\ogS =\ogS ^i^fcj m

W

w

/=1 th

where the subscript / designates the values for the 7 cosolvent, and Ν the number of cosolvents in the mixture. The cosolvency power (σ) is an important parameter in the log-linear model since it indicates the capacity of a cosolvent to increase the solubility of an organic solute. The cosolvency power will increase with increasing hydrophobicity of the solute as well as the solvent. Since the degree of hydrophobicity for solutes is much more variable than for solvents, σ values can be estimated from polarity or hydrophobicity indices for the solute, such as the octanol-water partition coefficient (6). Non-ideal behavior due to water-cosolvent interactions generally lead to positive deviations from the log-linear model. For small deviations, the empirical coefficient β (> 1) can be used to predict the solubility over a range of cosolvent fractions. However, when water-cosolvent interactions are significant, more sophisticated methods should be used to predict activity coefficients in the solvent mixture (7, 8). Also, when the N A P L is completely dissolved in the mixed solvent, particular at high cosolvent fractions, the log-linear approximation needs to be replaced by a more general approach based on phase diagrams. For groundwater in contact with a single-component N A P L , the equilibrium concentration is equal to the aqueous solubility. The increase in solubility, and hence equilibrium concentration, upon addition of a cosolvent is predicted by equation 4.


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The equilibrium concentration ( Q of component / in a solution which is in contact with a multi-component N A P L can be predicted by:

c = yxs


i

i

i

(5)

i

where γ, is the activity coefficient of component / in the N A P L phase, X, is the mole fraction in the N A P L phase, and is the aqueous solubility of the pure component in its liquid state (i.e., for solids, the solubility of a hypothetical, super-cooled liquid is considered). In many cases the activity coefficient of an organic constituent in an organic N A P L can be assumed unity, simplifying equation 5 to Raoult's law. Since the solubility increases in a log-linear manner with increasing cosolvent content, it follows that C, obeys the same log-linear model. Thus, cosolvent addition to a multi-component waste mixture can be expected to result in an exponential increase in solution-phase concentrations of organic constituents of the waste. This prediction is consistent with the data reported by Lane and Loehr (9) for the dissolution of several polycyclic aromatic hydrocarbons from tar-contaminated soils into binary mixtures of alcohols and water. It should be noted, however, that the activity coefficient and mole fraction of N A P L components can change if a significant amount of cosolvent partitions into the N A P L phase. It is also possible to describe the dissolution of a multi-component N A P L in a solvent mixture by a single pseudocomponent (10). Sorption. Sorption of non-polar organic compounds can generally be described by a linear sorption isotherm, characterized by an equilibrium partition coefficient (K). Since sorption is inversely related to the solubility, an increase in solubility, resulting from addition of cosolvents, leads to a proportional decrease in sorption. Thus, the equilibrium sorption coefficient (K ) measured in mixed solvents decreases in a loglinear manner with increasing cosolvent content (11-13): m

log*.=logtf„-i«,M/ , e

(6)

i=l

where α is an empirical coefficient that accounts for solvent-sorbent interactions, the subscripts m and w denote mixed solvent and water, respectively, and all other terms are as defined previously. The reduction in sorption upon addition of a cosolvent will enhances the mobility of organic solutes. Non-ideal behavior due to cosolvent-sorbent interactions may result in either positive deviations (a > 1) or negative deviations (a < 1). The product αβ accounts for deviations arising from both cosolvent-water and cosolvent-sorbent interactions. Values for a and β vary with cosolvent, soil, and chemical. Since limited information is available for estimating a or Rvalues for specific cases, as a first approximation, the product αβ can be assumed equal to 1. Nonequilibrium Conditions. In many field-scale applications, equilibrium approaches prove to be inadequate in estimating the dissolution or sorption processes



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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT V

during transport since nonequilibrium conditions usually prevail. Failure of pumpand-treat, soil venting or bioremediation techniques has been attributed to nonequilibrium conditions at all spatial scales. There are essentially two types of nonequilibrium conditions. The first type results from the heterogeneous character of porous media that causes the hydraulic conductivity to be spatially variable, and results in a heterogeneous distribution of groundwater velocities. The concentration gradients created by the nonuniform velocity distribution can result in rate-limited diffusion of the solute between different flow domains, causing nonequilibrium conditions. This type of nonequilibrium is often referred to as physical nonequilibrium. The second type of nonequilibrium occurs when the contact time between phases is not sufficient to achieve an equilibrium distribution of the chemicals in all phases. Typical examples of this type of nonequilibrium conditions are dissolution and sorption nonequilibrium. In the following section, we will briefly examine the effects of cosolvents on rate-limited processes. Physical Nonequilibrium. It is known that organic solvents, such as alcohols, have the potential to increase the intrinsic permeability of soils and aquifers, especially in media with high clay contents (14, 15). This characteristic is due to shrinking and cracking of the clay as a result of the low dielectric constant of cosolvents, and may have a positive effect on the recovery of contaminants from zones of low permeability in at least two ways. First, cracking will increase the advective flow through the clay lenses which are otherwise accessed only via molecular diffusion. Second, the average diffusion path length is decreased as a result of Assuring or cracking of the clay lenses. On the other hand, the cosolvent still has to diffuse into and the contaminants have to diffuse out of the denser clay regions developed in between the cracks. Data are lacking to estimate the extent to which the enhanced permeation is offset by decreased diffusion rates. Dissolution Nonequilibrium. Dissolution processes are generally considered to be limited by diffusion of the component through a boundary layer from the interface into the bulk solution (16). The rate of dissolution from a N A P L (ami at) is often described by a first-order rate law: ^

=-0*(C.,-O

(7)

where m is the contaminant mass in the N A P L phase, t is the time, 0is the volumetric water (or mixed solvent) content, k is the first-order dissolution rate coefficient, and C is the solution-phase concentration. C is the equilibrium concentration which equals the solubility for single-component NAPLs or the equilibrium concentration based on Raoult's law (equation 5) for multi-component NAPLs. The mass-transfer coefficient for dissolution (k) is often expressed as the dimensionless Sherwood number and appears to have positive correlation with pore-water velocity, N A P L saturation, and fluid properties such as viscosity and density (17). It is probable that interactions of the N A P L with natural soil organic matter will also modify the dissolution rates. eq


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When a cosolvent is added to a soil contaminated with a N A P L , the solubility, and hence C , will increase according to equation 4, enhancing the dissolution rate. In addition, the changes in fluid properties may also affect the dissolution rate. In viscous, multi-component NAPLs, like coal tar or crude oil, diffusive transport within the organic phase is most likely the mechanism controlling the dissolution kinetics (18). When the dissolution process proceeds, the low molecular weight components will dissolve while the large molecular weight components remain, making the N A P L phase more viscous and the diffusion even slower. The partitioning of cosolvents into the N A P L phase may prevent this to some extent. So far, very little information is available on the effect of cosolvents on dissolution kinetics. More research is needed to accurately predict the dissolution rate of NAPLs in solvent mixtures. Downloaded by FUDAN UNIV on January 25, 2017 | http://pubs.acs.org Publication Date: November 9, 1995 | doi: 10.1021/bk-1995-0607.ch018

eq

Sorption Nonequilibrium. Sorption nonequilibrium has generally been described by a bicontinuum model where sorption is considered to take place in two steps. The first step is considered to occur instantaneously in domains that are readily accessible for the sorbate (Si), while the second step is rate-limited, often attributed to a diffusion controlled process (S ). Some researches have proposed retarded intraparticle diffusion as a mechanism to describe rate-limited sorption (19, 20), while others have described sorption nonequilibrium by intra-organic matter diffusion (21, 22). The bicontinuum model can be conceptualized as follows: 2

C
S

Enhanced Removal of Organic Contaminants by Solvent Flushing

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