Effects of Humic Background on Granular Activated Carbon Treatment

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30 Effects of Humic Background on Granular Activated Carbon Treatment Efficiency Walter J. Weber, Jr., and Edward H . Smith Environmental and Water Resources Engineering Program, The University of Michigan, Ann Arbor, MI 48109

Uncharacterized background organic matter can impair the effectiveness and complicate the design and operation of adsorption treatment processes directed at the removal of specific target organic compounds from waters and wastewaters. Mathematicalmodelscalibrated with system-specific information may facilitate process design and operation by allowing quantification of the effects of such background matter on adsorption efficiency. In this work, a two-resistance homogeneous surface diffusion adsorption model was used to simulate and predictfixed-bedadsorber breakthrough behavior for two specific solutes in background waters from various sources. Independent measurements of requisite model coefficients were made for the two target solutes directly in the presence of the background organic matter, which in turn was treated as an unspecified class of components quantified only in terms of the lumped analytical parameter of total organic carbon. This approach suitably incorporated the effects of the background matter in model forecasts offixed-bedad-

unds.

sorber performance for the target compo

ORGANIC CONTAMN IANTS OF CONCERN

in the application of adsorption processes for water and waste-treatment practice can be divided into two major classes: (1) potentially hazardous specific compounds, generally of anthropogenic origin; and (2) relatively uncharacterized background dis solved organic matter ( D O M ) , frequently of humic and fulvic character. 0065-2393/89/0219-0501$09.00/0 © 1989 American Chemical Society

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


502

AQUATIC HUMIC SUBSTANCES

Granular activated carbon (GAC) adsorption in fixed-bed reactor (FBR) sys tems constitutes an established technology for effective removal of even trace quantities of an extensive range of the first category of compounds. However, the presence of uncharacterized background D O M in the solution matrix of many waters and wastewaters complicates the design and operation of ad sorption treatment processes directed at removing specific target organic compounds. This hindrance stems from complexities introduced as a result of D O M interactions with both the adsorbent and target species present in solution. The results of prior efforts to address this problem have suggested that mathematical models and approaches, if properly formulated and struc tured, offer potential for quantifying the impact of background D O M on the adsorption of target contaminants for purposes of process design and per formance forecasts (1-6). Mathematical models incorporating film and intraparticle mass-transfer resistance terms have enjoyed broad application for describing and predicting the dynamics of F B R adsorbers. One such model, the homogeneous surface diffusion (HSD) version of the Michigan Adsorption Design and Applications Model ( M A D A M ) , requires determination of equilibrium and rate param eters that can be estimated or, preferably, measured in bench-scale labo ratory experiments that closely simulate particular systems of interest (7, 8). Mass-transport parameters have traditionally been evaluated by using data derived from completely mixed batch reactor (CMBR) measurements in combination with dimensionless-group correlation techniques. Although vir tually untested for complex mixtures of organic substances, the short-bed adsorber (SBA, defined as a bed of sufficiently short length that immediate contaminant breakthrough occurs) more closely approximates the hydro dynamics of full-scale columns and allows simultaneous determination of both film and intraparticle mass-transport parameters and thus offers poten tial as an alternative design tool (9).

Objectives and Approach The research described here was designed to (1) examine the effects of different sources and concentrations of D O M background on adsorption capacities and rates for typical target organic compounds, (2) evaluate the capabilities of relatively straightforward models such as the M A D A M - H S D for description and prediction of fixed-bed adsorber breakthrough charac teristics for these target compounds in the presence of the various D O M backgrounds, and (3) evaluate several alternative approaches for estimating the adsorption-rate parameters required for modeling the behavior of the target compounds in the systems studied. The systems selected for investigation were chosen to simulate a broad range of conditions typically encountered in field applications, from a rela tively simple background composed of a few target species typical of a ground


30.

WEBER & SMITH

Granular Activated Carbon Treatment Efficiency 503

water supply to the more complex matrix of target compounds and humictype D O M characteristic of leachates emanating from hazardous-waste dis posal or spill sites. For each compound modeled, the M A D A M - H S D approach required input relative to (1) a film diffusion parameter, kf, characterizing mass transfer of solute to the exterior carbon particle surface; (2) an intraparticle diffusion coefficient, D , quantifying diffusional transport along the interior carbon particle pore surfaces; and (3) appropriate isotherm coefficients to characterize the functional dependence of the solid-phase concentration of adsorbed solute on the solution-phase concentration at equi librium. In the modeling approach employed, these coefficients were eval uated for the target compounds directly in the presence of complex leachate material, which was considered as uncharacterized but system-specific back ground organic matter quantified only in terms of total organic carbon (TOC).


s

The rationale for this approach is that many field-scale applications of adsorption involve waters that are complex in composition. The adsor bent-solute-solution interactions associated with such complex matrices are typically site-specific, and correspondingly site-specific design criteria are needed. Further, the operation of fixed-bed adsorbers is often governed by the breakthrough pattern(s) of one or two specific compounds, either by virtue of these compounds being the only identified hazardous substances in the treatment stream or because they are the first to exceed prescribed limits in the adsorber effluent. Such an approach is easier to implement and affords reduced model input and computational requirements over method ologies that seek to identify and predict the individual adsorptive behaviors of all components present.

Experimental Details A two-stage experimental program was implemented to obtain equilibrium and masstransfer coefficients for various matrices containing two different target organic compounds and for background waters having different types and amounts of DOM measured as TOC. In Phase I, CM BR isotherm and rate studies were conducted for one- and two-component solutions of the target compounds. Phase II experiments used information from Phase I to formulate two different column-type adsorber experiments, namely, short-bed adsorber (SBA) measurements as an alternate to CM BR and correlation techniques for kinetic-parameter estimation and model calibration, and deep-bed adsorber (DBA) experiments for parameter-model verification. Activated Carbon. The adsorbent used in all these experiments was activated carbon (Filtrasorb-400, Calgon Corporation). The general physical properties of F-400 are documented elsewhere (10). For isotherm studies, the carbon was used in powdered form to facilitate rapid attainment of equilibrium and limit interferences due to biological activity. Powdered carbon was prepared by crushing random bag samples, sieving, and retaining the 200-325 U.S. standard sieve sizefraction.CMBR rate andfixed-bedadsorber experiments were conducted with a 30-40 sieve size fraction of the same carbon. The sieved carbon was washed with deionized-distilled water, oven-dried at 104 °C, and stored in airtight glass containers. Carbon for


504

AQUATIC HUMIC SUBSTANCES


immediate use was dried to a constant weight and then cooled to ambient temperature in a desiccator. Solutes. Two organic compounds, trichloroethylene (TCE) and p-dichlorobenzene (p-DCB), both designated as priority pollutants by the U.S. Environmental Protection Agency, were chosen as target solutes. These compounds were selected because they exhibit different adsorption characteristics, have been identified in contaminated surface and ground waters, are relatively straightforward to analyze, and embody a wide range of properties and behavioral characteristics. The majority of multicomponent adsorption studies to date have involved compoundsfromsimilar organic class groupings, whereas the intent here was to examine compounds having different structural and solution characteristics. TCE is a straight-chain, unsaturated aliphatic of relatively high volatility and solubility, compared to the aromatic p-DCB. Background waters included (1) a baseline solution of deionized-distilled water (DDW) containing no background organic material (this baseline solution also served as a make-up or dilution water for all other solutions); (2) a commercial humic acid (HA, Aldrich Chemical Co.); (3) a leachatefroma hazardous-waste landfill cell (HWL, Wayne Disposal, Rawsonville, MI); (4) a three-solute mixture of known organic contaminants (TRISOL); and (5) a solution composed of the commercial humic acid in conjunction with the trisolute mixture (TRISOL 4- HA). Humic acid stock solutions were prepared byfirstdissolving an appropriate mass of dried Aldrich humic acid (Lot No. 3061-KE) in DDW at pH 11. Following readjustment of the pH to 7, the solution wasfilteredthrough a prewashed glassfiberfilter(Whatman 934 AH) to remove undissolved solids. The TOC of the fil trate was then measured. A typical stock solution had a DOM concentration of 250 mg/L as TOC. Working concentrations of humic acid were obtained by dilu tion of the stock with DDW to initial background concentrations of 0, 5, 15, and 25 mg/L as TOC. Raw leachate was prefiltered through a glass-fiberfilterprior to use to remove suspended particles that might adsorb pollutants in competition with activated car bon. The leachate was characterized as a high-strength waste (TOC = 10,000 mg/L; total hardness = 2100 mg/L as CaC0 ), and its color and adsorptive char acteristics suggested the presence of significant amounts of humic material. Appro priate amounts of HWL were diluted with buffered DDW to achieve background DOM concentrations of 0, 16, 60, and 200 mg/L as TOC. (Note: HWL background with an initial concentration of XX mg/L as TOC is expressed as HWL(XX), with a similar notation for HA; for example, HWL(60), HA(15). The three solutes constituting the TRISOL mixture (lindane, tetrachloroethylene, and carbon tetrachloride) were chosen with selection criteria similar to those for the two target compounds. The concentrations of these solutes used in all studies were approximately 1000, 575, and 525 μg/L, respectively. This amount corresponds to an equimolar amount of each compound of about 3.4 μηιοΙ/L, and a total TOC of 0.37 mg/L. Identical concentrations of the TRISOL components were used in the TRISOL + HA(15) background. Working solutions consisted of background water spiked with TCE or p-DCB, prepared as methanol-based stock solutions, to the desired concentration. A slight variation of this procedure employed for application of TCE to fixed-bed adsorbers is noted in the section on column methods. Experiments were conducted at room temperature (22 ± 2 °C), and all solutions were buffered at pH 6.5 ± 0.2 with 10" M phosphate. Mass concentration determinations for TCE and p-DCB were by gas chroma tography, with a liquid-liquid extraction procedure for sample preparation and an external standard calibration procedure. Background organic analysis was performed by direct TOC measurement or by UV spectroscopy correlated with TOC measure ments. 3

4


30.

WEBER & SMITH



Data Collection and Analysis Isotherm Studies. Equilibrium data were collected by using the completely mixed batch reactor (CMBR) bottle-point technique. Varying amounts of powdered carbon were carefully measured and added to a series of 0.16-L glass vials, followed by contact with solutions containing the adsorbates. After a 5-day contact period, a sample was taken from each reactor, filtered through a prewashed glass-fiber filter to separate the carbon, and extracted or diluted according to the appropriate analytical technique. Upon measurement of the equilibrium solute concentration, C , the corresponding equilibrium solid-phase concentration, q , was calculated from a mass bal ance. Control measures employed in capacity experiments included elimi nation of headspace in reactors to limit volatilization losses and evaluation and accomodation of losses encountered in the filtration step. For single-solute systems of p - D C B and T C E , the Freundlich isotherm model was found to adequately describe the equilibrium liquid-solid-phase relationship over the concentration ranges of interest. The Freundlich equa tion is a semiempirical, nonlinear expression of the form e

e

(1) Freundlich model coefficients, K and n, were determined with a non linear geometric mean functional regression algorithm that recognizes errors encountered in measurement and calculation of both liquid- and solid-phase equilibrium concentrations (II, 12). Multicomponent adsorption equilibria were described by the ideal ad sorbed solution theory (IAST), by using an empirical modification similar to that employed by others (13, 14) to provide a more precise fit of the data. The generalized equations to be solved for the modified IAST are F

7Γ,

=

(2)

"IT

(3) 1=1

QT

32.0

14.1 26.5 >32.0

1.5 2.7 4.6 8.0

1.5 3.4 5.4 7.4

2.6 5.0 7.5 9.6

3.5 5.5 7.5 9.2

NOTE: All values are thousand bed volumes treated. They were generated by using different rate parameter determination techniques (Run DB6); background: HWL(16). °kf from best correlation; D from C M B R rate data. kffromworst correlation; D ,fromC M B R rate data. s

b

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5.00

1000

1500 20.00 25.00 50.00 BED VOLUMES (IN THOUSANDS)

55.00

4000

Figure 19. MADAM-DBA breakthrough profiles for TCE in the presence of p-DCB for varying concentrations of background HWL.

Summary and Significance G A C adsorption of two target organic compounds in the presence of various sources and amounts of background D O M was analyzed with an existing mathematical adsorption model, M A D A M . The modeling approach applied was one in which model parameters were experimentally determined and calibrated for the target compounds only, with the leachate considered as an unspecified background characterized only in terms of a lumped-param-



528

AQUATIC H U M I C SUBSTANCES

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9.00

«00

tS.OO

20.00

29.00

30.00

39.00

40.00

BED VOLUMES (IN THOUSANDS)

Figure 20. MADAM-DBA breakthrough profiles for p-DCB in the presence of TCE for varying concentrations of background HWL.

/

/A /

/

5

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/

/ / /

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^00*

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1 2 3 4

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BACKGROUND TRISOL (SINGLE) TRIS0L+HA (SINGLE) TRISOL (WfTH p-DCB) TRISOL+HA (WITH p-DCB) 25.00

BED VOLUMES (IN THOUSANOS)

30.00

35.00

Figure 21. MADAM-DBA breakthrough profiles for TCE in TRISOL and TRISOL •¥ HA(15) backgrounds for single- and bisolute systems. eter measure, T O C . Findings and conclusions derived from the study are summarized as follows. In general, the presence of leachate D O M reduced adsorption capacities and rates for both target compounds (p-DCB and T C E ) , compared to their respective values in water having no background D O M . These impacts, reflected quantitatively in equilibrium and kinetic model coefficients deter-


30.

W E B E R & SMITH


υ


ζ

°0.00

LINE

BACKGROUND

2

TRISOL TRISOL+HA

5.00

10.00 15.00 20.00 25.00 BED VOLUMES (IN THOUSANDS)

30.00

35.00

Figure 22. MADAM-DBA breakthrough profiles for p-DCB in TRISOL and TRISOL + ΗA(15) backgrounds for single- and bisolute systems.

mined by calibration of bench-scale experimental data, are proportional to the strength of the background water, as quantified by T O C . The SBA technique was verified as a useful method for determination of system-specific kinetic parameters for complex mixtures of organic com pounds. Film and surface-diffusion coefficients estimated by use of this method resulted in adequate predictions of the performance of deeper, bench-scale FBRs for single- and multicomponent cases for the range of system conditions studied. These predictions were as accurate and, in some cases, more accurate than those deriving from a more traditional method ology for estimating rate parameters through literature correlations and C M B R rate data. Discrepancies that result from application of the correla t i o n - C M B R technique are more pronounced for p - D C B than for T C E , and most are attributable to differences in values for the internal diffusion coef ficient. Values of D determined from C M B R rate data were significantly higher than those estimated from SBA analysis, presumably because of the different hydrodynamic conditions that prevail in the two reactor systems and to differences in the extent to which differences in these conditions were reflected in the corresponding kf values. Overprediction of fixed-bed ad sorber performances resulted for certain cases when the correlation-CMBR parameters were used in the M A D A M model projections. Differences in film mass-transport coefficient values for fixed-bed adsorbers result from the fact that literature correlations do not account for interactions between target compounds and background D O M , although these interactions appear to be reflected in Rvalues derived by the SBA technique. s



530


Beyond the specific conclusions summarized, the findings of this study bear on an issue of more general significance in terms of demonstrating the merits of a phenomenological modeling approach for target solutes in com plex systems: namely, the implicit incorporation of many of the impacts of adsorbent-solute, adsorbent-DOM, and s o l u t e - D O M interactions into non descript model coefficients. Independent characterization and experimental verification of all of the reactions and associated mechanisms in complex systems is an enormous and, in many cases, prohibitively expensive task. A phenomenological approach focuses the study by first identifying critical model coefficients, followed by calibration of these coefficients according to perturbations in system composition. Other advantages of this approach are that: (1) it streamlines pilot-study requirements, because it enables primary information gathering and hypothesis testing to be done on the bench scale, with the pilot plant used as more of a verification tool; and (2) it allows for further investigation, at least on a macroscopic scale, into mechanistic proc esses and their impacts on model parameters, where the modeling approach (if not the results) can be extrapolated from one case to another and the system-specific results compared to provide meaningful scientific insights. For example, future studies might explore correlations between experimen tally determined model parameters for given compounds and various sys tem-solution characteristics, and then verify the consistency of such cor relations for different types of background D O M . The current study also illuminates the need to investigate carbon ad sorption of complex mixtures in connection with other treatment processes. For instance, the removal of a weakly adsorbed compound such as T C E may be more effectively achieved by air stripping. Similarly, if the presence of humic D O M significantly reduces the performance of G A C adsorbers with respect to a particular target compound (or compounds), adjustments in the coagulation step may be warranted to effect a greater preremoval of humic substances. In addition, the potential formation of m e t a l - D O M complexes during the course of treatment may result in substances that compete with target organic compounds for adsorptive sites on activated carbons. There fore, the impact of prior treatment steps in the flow scheme on the removal of target organic species also requires examination.

Nomenclature C Ci Cf e

C d D

oi

g

equilibrium liquid-phase solute concentration (y^g/h) liquid-phase concentration of species t, IAST (μΜ) single-solute concentration of species i evaluated at the bisolute mixture spreading pressure, IAST (μΜ) initial solute concentration of species i (μΜ or μ g / L ) carbon particle diameter (cm) solute distribution parameter (dimensionless)


30. Dj D i.d. kf K L M η Ν Pi s


F

q q qf e

t

q q V v ζι e η ν IT ρ oi

T

z

W E B E R & SMITH

Granular Activated Carbon Treatment Efficiency 531 2

bulk liquid diffusivity (cm /s) surface diffusion coefficient (cm /s) internal diameter (cm) film diffusion coefficient (cm/s) Freundlich isotherm constant carbon bed depth (cm) carbon dose (mg adsorbent) Freundlich isotherm exponent (dimensionless) number of target species in solution empirical competition coefficient for species i , IAST (dimensionless) equilibrium solid-phase solute concentration fag/mg) solid-phase concentration of species i , IAST (μπιοΐ/mg) solid-phase concentration of species i in single-solute system evaluated at the bisolute mixture spreading pressure, IAST ^mol/mg) initial solid-phase concentration of species i ^ m o l / m g or μg/mg) total solid-phase concentration of solutes, IAST (μΓηοΙ/mg) liquid volume in reactor (L) interstitial flux velocity in carbon bed (cm/s) solid-phase mole fraction of species t, IAST (dimensionless) bed void fraction (dimensionless) inverse of Freundlich exponent, η (dimensionless) kinematic viscosity (cm /s) spreading pressure, IAST (kcal/cm ) carbon particle density (g/cm ) 2

2

2

3

Acknowledgments The authors express their appreciation to Brett Farver, Barbara Jacobs, and Daniel Peters for their contributions to the experimental aspects of this project. The work was supported in part by Award No. CEE-8112945 from the National Science Foundation. The contents do not necessarily reflect the views and policies of the NSF, and the mention of trade names or commercial products does not constitute endorsement.

References 1. Weber, W. J., Jr.; Pirbazari, M. J. Am. Water Works Assoc. 1982, 74, 203. 2. Frick, B. R.; Sontheimer, H. In Treatment of Water by Granular Activated Carbon, Suffet, I. H.; McGuire, M. J., Eds.; American Chemical Society: Wash ington, DC, 1983; p 247. 3. Tien, C. In Proceedings, First International Conference on the Fundamentals of Adsorption; Myers, A. L.; Belfort, G., Eds.; Engineering Foundation and American Institute of Chemical Engineers: New York, 1984; p 647.


532 4. 5. 6. 7.


Endicott, D. D.; Weber, W. J., Jr. Environ. Prog. 1985, 4, 105. Crittenden, J. C.; Luft, P.; Hand, D. W. Water Res. 1985, 19, 1537. Smith, E . H.; Tseng, S.; Weber, W. J., Jr. Environ. Prog. 1987, 6, 18. Crittenden, J. C.; Weber, W. J., Jr. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1978, 104, 1175.

8. Weber, W. J., Jr. In Proceedings, First International

Conference on the Fun

damentals of Adsorption; Myers, A. L . ; Belfort, G., Eds.; Engineering Foun dation and American Institute of Chemical Engineers: New York, 1984; p 647.

9. Weber, W. J., Jr.; Liu, Κ. Τ. Chem. Eng. Commun. 1980, 6, 49.


10. Weber, W. J., Jr.; Wang, C. K. Environ. Sci. Technol. 1987, 21, 1096. 11. Halfon, E . Environ.

Sci. Technol. 1985, 19, 747.

12. Smith, Ε. H . Ph.D. Dissertation, University of Michigan, 1987. 13. Yonge, D . R. Ph.D. Dissertation, Clemson University, 1982. 14. Thacker, W. E . ; Crittenden, J. C.; Snoeyink, V. L . J. Water Pollut. Control Fed. 1984, 56, 243. 15. Kataoka, T.; Yoshida, H . ; Ueyama, K. J. Chem. Eng. Jpn. 1972, 5, 132.

16. Weber, W. J., Jr. Physicochemical Processes for Water Quality Control; Wiley:

New York, 1972. 17. Wershaw, R. L . ; Burcar, P. J.; Goldberg, M . C. Environ. Sci. Technol. 1969, 3, 271. 18. Carter, C. W.; Suffet, I. H . Environ. Sci. Technol. 1982, 16, 735. 19. Calloway, J. Y.; Gabbita, Κ. V.; Vilker, V. L . Environ. Sci. Technol. 1982, 18, 890. 20. Weber, W. J., Jr.; Voice, T. C.; Jodellah, A. J. J. Am. Water Works Assoc. 1983, 75, 612. 21. Williamson, J. E . ; Bazaire, K. E . ; Geankoplis, C. J. Ind. Eng. Chem. Fundam. 1963, 2, 126. 22. Wilson, E . J.; Geankoplis, C. J. Ind. Eng. Chem. Fundam. 1966, 5, 9. 23. Roberts, P. V.; Cornel, P.; Summers, R. S. J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1985, 111, 891. 24. Ohashi, H . ; Sugawara, T.; Kikuchi, Κ. I.; Konno, H . J. Chem. Eng. Jpn. 1981, 14, 433. 25. Dwivedi, P. N.; Upadhyay, S. N . Ind. Eng. Chem. Process Des. Dev. 1977, 16, 157. 26. Wilke, C. R.; Chang, P. J. AIChE J. 1955, 1, 264. 27. Awuwa, A. A. Ph.D. Dissertation, Illinois Institute of Technology, 1984. 28. Fettig, J.; Sontheimer, H . J. Environ. Eng. Div. (Am. Soc. Civ. Eng.) 1987, 113, 780. RECEIVED

for review September 17, 1987.

ACCEPTED

for publication February 23,

1988.


Effects of Humic Background on Granular Activated Carbon Treatment

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