[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Frontal Chromatographic Concepts To Study Competitive Adsorption

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  • 31 Frontal Chromatographic Concepts To Study Competitive Adsorption Humic Substances and Halogenated Organic Substances in Drinking Water

    Ronald J. Baker and I. H . Suffet

    Environmental Studies Institute, Drexel University, Philadelphia, PA 19104

    Thomas L . Yohe

    Philadelphia Suburban Water Company, Bryn Mawr, PA 19010

    This chapter introduces the use of frontal chromatographic theory to describe the breakthrough of solutes in granular activated carbon (GAC). Depletion of adsorption sites during use of GAC contactors can be expressed in terms of changes in moving concentration profiles, or fronts. These fronts can be defined in chromatographic terms. Data from a pilot-scale carbon adsorption study are presented and used as an example of how frontal chromatographic theory can be applied. Current models for describing and predicting solute break-through from GAC columns cannot predict breakthrough of a wide variety of compounds under water-treatment conditions. Frontal chromatography theory, as applied in this chapter, is useful for un-derstanding the displacement and breakthrough phenomena in car-bon contactors.

    CxRANULAR ACTIVATED CARBON (GAC) HAS RECENTLY BEEN PROPOSED by the U.S . Environmental Protection Agency's Office of Drinking Water Quality as the best available technology for removal of several categories of synthetic organic substances, including trihalomethanes (THM) and other halogenated organic compounds (I). One consequence of this regulatory

    0065-2393/89/0219-0533$06.(X)/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 534 AQUATIC HUMIC SUBSTANCES

    activity wil l probably be increased use of G A C for halogenated-organic-substance removal. However, many low-molecular-weight organic materials, including T H M and other halogenated disinfection byproducts, are not strongly adsorbed by G A C . Considering the substantial capital and operating costs involved in G A C adsorption, optimization of this process in terms of equipment required and carbon consumption will be a high priority.

    Many models for describing and predicting solute breakthrough from G A C columns have been and are being developed (2-5). Although progress is being made, current models cannot predict breakthrough of a wide variety of compounds under water-treatment conditions, where influent concentrations and types of organic mixtures vary and most contaminants are in the nanogram- to microgram-per-liter concentration range. In water- and waste-water-treatment applications the complexity and variability of influent quality make it difficult to develop models to predict breakthrough of individual solutes.

    Because competitive effects from humic materials are difficult to predict, they greatly complicate the process of predicting specific solute breakthrough and thus of designing effective and efficient G A C installations. Site-specific water-quality differences add to the complexity. Therefore, the National Academy of Science (NAS) has recommended that each water-treatment design for G A C be piloted at that location and that only general principles be assumed to apply between plants (6). This chapter introduces ideas that may lead to a new approach for evaluation of pilot-plant column data. The approach is based on the concepts of frontal chromatography.

    Adsorption-site distribution on a G A C bed and depletion upon use of the G A C are expressed in terms of changes in moving concentration profiles, or fronts, that represent movement of specific solutes down the column. The shapes of these fronts can be defined by using frontal chromatographic theory. Data from a pilot-scale carbon adsorption study are presented and used as an example of how some aspects of frontal chromatographic theory can be applied to observed G A C behavior at specific locations. It is hoped that this concept of using pilot-plant data will enhance general understanding for the control of future full-scale operations.

    Analogy: Similarities Between GAC and Frontal C hromatography Conceptually, a G A C column can be thought of as a slow, inefficient liquid chromatographic (LC) column operated in the frontal elution mode. Table I lists operational similarities and differences between the two systems. The distribution coefficient or capacity factor, k\ in chromatographic theory (7) is analogous to points on a carbon isotherm (8), where each point on the isotherm indicates the distribution between the solid and liquid phases. In both systems diffusion is thought to control the mass-transfer rate (7, 8).

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 31. BAKER ET AL. Frontal Chromatographic Concepts 535 Table I. Comparison Between Frontal Elution Liquid Chromatography and

    Water-Treatment GAC Operation Frontal Elution Water-Treatment

    Attnbute Liquid Chromatography GAC Operations Definable distribution coefficients yes (*') yes (isotherm points) Diffusion-controlled mass-transfer

    kinetics yes yes Theoretical plate concepts applicable yes yes Competition for adsorption sites yes yes Displacement of adsorbed compounds

    by others yes yes Local equilibrium maintained

    throughout system yes yes Constant number of theoretical plates no no Time to breakthrough minutes days or months Constant influent composition yes no Irreversible adsorption at some sites no yes Homogeneity of adsorption sites preferably no Homogeneity of adsorption media yes no

    Displacement of some solutes by others through competitive adsorption also occurs in both systems. The most significant differences between G A C and frontal chromatography (FC) are time to breakthrough (minutes for F C , months for GAC) , influent variability (constant for F C , variable for GAC) , and homogeneity of the adsorbent (GAC is nonhomogeneous, most chromatographic media are homogeneous). In both F C and G A C systems the solutes compete for adsorption sites, and less strongly adsorbed species are displaced by those more strongly adsorbed.

    The humic materials that are irreversibly adsorbed onto G A C (9) effectively reduce the number of theoretical plates in the system in a nonuniform manner. It is not known what fraction ofhumic materials adsorbs irreversibly, and a priori prediction of how the humic substances as represented by nonvolatile total organic carbon (NVTOC) in a feedwater will interact with G A C is not now possible. This type of irreversible fouling would not be experienced in F C , as analysts minimize decreases in column efficiency by sample pretreatment (e.g., cleanup by base extraction (10) to remove humic substances before G C or L C analysis).

    Each system (FC and GAC) can be described as a column reactor with advection, diffusion, and reaction terms as shown in equation 1. This general equation for liquid-phase concentration as it changes with time results from a mass-balance procedure (11) and has been applied to F C (12) and G A C (7)

    total rate of change = advection + diffusion + reaction

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 536 AQUATIC HUMIC SUBSTANCES

    where C is solute concentration, t is time, V is flow velocity, D is diffusivity, is distance down the column, and r(C) is a reaction term.

    The reaction term r(C) in both systems refers to rates of adsorption and desorption. Treatment of this term varies somewhat between G A C and F C , although in both cases thin-film and intraparticle mass transfer and distribution equilibrium are considered rate- and capacity-limiting. In order to define and quantify the r(C) term, most G A C models rely on estimation of mass-transfer and competitive equilibrium parameters for each solute; then aqueous and adsorbed concentrations of the solutes are calculated as gradients through the G A C particles at different bed depths through time. Although this approach has also been used in F C , another approach is to describe the front or wave of solute concentration (solid or liquid phase or both) in terms of its statistical moments, and to project how the wave wil l change with time and distance down the column. Application of this approach to G A C should be possible.

    Another chromatographic approach that may be applicable to G A C is the coherence concept developed by Helfferich (13). In this approach solutes are considered to be interactive (competitive) in their movement through chromatographic columns, and multicomponent wave movement occurs in a predictable "coherent" manner. Rates of wave and solute movement are found as solutions to eigenvector problems. Application of the coherence theory to G A C in water treatment will be mathematically complex because of the constantly changing influent and because some simplifying assumptions that apply to many chromatographic systems (13) may not hold true for G A C with its nonconstant separation factors. This essentially means that relative solute retention times (time spent in the column by the solutes) are significantly affected by relative aqueous-phase concentrations in G A C . The effect of this phenomenon on the use of coherence theory for describing G A C breakthrough has yet to be seen.

    Some of the physical factors responsible for front shape and adsorption site depletion in column systems are shown in Figure 1. These factors apply to both frontal chromatography and G A C adsorption. For each column in Figure 1 (I-IV) a profile of bed depth versus surface loading concentration of a compound is shown for a column taken off-line after a period of loading. Each successive column shows additional physical factors that further complicate the system and reduce the column's capacity to adsorb compounds from the mobile phase.

    Column I (isothermal) in Figure 1 shows the ideal situation for maximum adsorption of a solute in a column: a single solute (no competition) and instantaneous mass transfer. The advancing front is asymptotic because the initial influent equilibrates with carbon at the column inlet and is reduced in concentration when it contacts carbon farther down the column. This reduction results in lower surface loading at points farther down the column. Because phase equilibrium exists at each point in the front, the isotherm could be constructed from the surface loading-mobile phase concentration

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 31. BAKER ET AL. Frontal Chromatographic Concepts 537

    Figure 1. Front shaping mechanisms.

    ratio at points in the front. Conversely, the front shape could theoretically be drawn from the isotherm.

    The effect of noninstantaneous mass transfer is shown in column II (Finite Mass Transfer). Here the front has a sigmoidal or half-Gaussian appearance characteristic of frontal chromatography and sometimes observed in G A C loading profiles. The slowest (rate-limiting) mass-transfer steps control the front shape in the loading profile and the breakthrough profile. Both G A C modeling theory and chromatographic theory consider diffusion as a rate-limiting step in mass transfer. Chromatographic theory additionally considers adsorption and desorption as potential rate-limiting mass-transfer steps. The current practice of not considering rates of adsorption and desorption as finite in G A C modeling (5) may be an important oversight in breakthrough prediction for low-molecular-weight, rapidly diffusing compounds. In this case, points on the frontal curve of Column would not represent isotherm points because stationary and mobile phases are not in equilibrium in the frontal region.

    Column III (Competition) shows the loss of adsorption capacity resulting from competition from other compounds. A more strongly adsorbing compound is shown concentrated at the column inlet, a weakly adsorbing species

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 538 AQUATIC HUMIC SUBSTANCES

    has its highest loading farther down the column, and there is some irreversible adsorption throughout the column, typical of some fractions of humic materials (13). A l l three of these generalized types of competition reduce the adsorption capacity for other compounds. The reduction results in earlier breakthrough in G A C and earlier front elution in F C .

    Column IV (Variable Influent) shows the effect of varying the influents of the adsorbing species. This variable poses the greatest challenge for G A C modeling, especially when the degrees of variation are not known. Mobile-phase effluent profiles can assume virtually any shape and are difficult to describe mathematically. This is the reason that site specificity for G A C treatment of different water quality is invoked (6).

    The Statistical Moment Approach Any curve can be described in terms of its statistical moments, which are mathematical descriptions of area distribution under the curve (14). The shape of a curve can be estimated and regenerated if enough moments are known. Figure 2 shows the procedure of approximating curve shape from statistical moments. If only the Oth moment is known (curve area), nothing is known about the curve shape between the boundaries. However, if the first moment is also known (center of mass), the estimated peak begins to take shape. When moments 2-4 are added, the estimated peak approaches

    Figure 2. Curve description by statistical moments.

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 31. BAKER ET AL. Frontal Chromatographic Concepts 539 the actual peak in shape. If an infinite number of moments were app...

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