Activated-Carbon Adsorption of Organic Pollutants - ACS Publications

33 Activated-Carbon Adsorption of Organic Pollutants

Downloaded by COLUMBIA UNIV on July 26, 2012 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch033

Gerhard Zimmer, Heinz-Jürgen Brauch, and Heinrich Sontheimer Engler-Bunte-Institut, Universität Karlsruhe, 7500 Karlsruhe, Federal Republic of Germany

Interaction with organic background substances is important in the removal of micropollutants by adsorption. Adsorption analysis characterizes the organic matter in drinking water by adsorbability. The approach is applied for a comparison of varying humic substances and water-treatment processes, along with a description of pH effects. The influence of very low concentrations of natural organic matter on the removal of halogenated pollutants is shown. Organic background significantly affects the range of adsorption capacities for a particular compound between different activated carbons. The main effect could be caused by the slow kinetic properties of humic substances and a long-term preloading of the carbon in a column. Despite different water sources, organic-matter concentrations, and activated-carbon types, a single relationship is found for the maximum column capacities of a halogenated pollutant.

WATER SUPPLY FACILITIES USING GROUND WATER

in nearly all countries have encountered chlorinated hydrocarbons in their raw water within the last few years. This situation has often prompted the construction of treat ment plants that use activated-carbon filters. In Germany about 30 waterworks operate plants for the removal of chlorinated hydrocarbons. Although air stripping is used as a pretreatment for very high initial concentrations, all such plants use activated-carbon filters as the final or only treatment (1-4). In addition to the chlorinated hydro carbons, humic substances are present in these ground waters. Optimal design and operation of a carbon filter plant must allow for adsorption com0065~2393/89/0219-0579$06.00/0 © 1989 American Chemical Society

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


580

AQUATIC H U M I C SUBSTANCES

petition between a small amount of micropollutants and humic substances, which are present in a much higher concentration (5). Design and operation of activated-carbon treatment plants requires a detailed quantification of the equilibrium state between dissolved and ad sorbed substances. Isotherm measurements are necessary to determine this relationship (6). A n example for three volatile chlorinated hydrocarbons is given in Figure 1. A n adsorption isotherm describes how the solid-phase concentration, q, depends on the concentration, c, in solution after a long enough contact time. Figure 1 shows that the isotherms for these three important chlorinated hydrocarbons are fit by a linear relationship in a log-log diagram. The wellknown Freundlich isotherm equation (q = Κ · c ) is valid for these hydro carbons over a large concentration range. Κ and η in this equation are empirically determined constants. Figure 1 also indicates further a large difference in the adsorbability of the three substances, although they have similar structures. The nonpolar tetrachloroethene is the most adsorbable (K = 12.02) and 1,1,1-trichloroethane is the least adsorbable (K = 0.367). The single-solute isotherms given in Figure 1 were measured in deionized-distilled water without any additional substances present. Isotherm tests with ground water containing micropollutants and humic substances n



33.

ZIMMER E T A L .

Activated-Carbon

Adsorption

of Organic

Pollutants

581

result in a very small reduction in the solid-phase micropollutant concen tration at the same aqueous-phase concentration. This reduction is ascribed to adsorption competition by humic substances. Because of the low naturalorganic-matter content (0.4-1.5 m g / L of dissolved organic carbon) and the nearly neglectable impact on isotherm results, we did not expect a large influence on filter behavior. This assumption has been proven wrong, as demonstrated in Figure 2, where the predicted breakthrough of a micropollutant is compared with observations. Figure 2 indicates that only about 30,000 bed volumes could be reached for a 50% breakthrough, instead of the 130,000 bed volumes predicted from the isotherm measurements. In order to understand the large influence of natural organic matter on filter efficiency, the adsorption behavior of these ubiquitous substances must be investigated in more detail. However, it is important to realize that natural organic matter consists of various substances, which cannot be isolated or separately analyzed (8, 9).

Adsorption Analysis for Humic Substances Adsorption analysis is used to describe the adsorption equilibrium for mul ticomponent mixtures of unknown substances i n water (10-12). In this

TRICHLOROETHENE Influent

Prediction based upon

the

isotherm

ι / /

AC : F 100 c =0.7 0

Velocity

50

75

Bedvolumes Figure

2. Comparison

of measured

100

m g / l DOC

Bed-depth

125

10

1.30

m

m/h

150

175

(I000m /m )

and predicted

3

3

breakthrough

curves

trichloroethene.


for

582

AQUATIC H U M I C SUBSTANCES

method, described in detail by Sontheimer et al. (13), the mixture of un known substances is replaced with three to six selected components of dif ferent adsorbabilities. If we assume that the adsorption equilibria can be described by the Freundlich equation, * = IW

(1)

then the overall isotherm concentration ( C ) , measured by dissolved organic carbon (DOC), represents the summation of single concentrations (c

^Single solute system

—

12.02

η = 0.42 = const.

ίο 10

15

20

25

30

35

40

45

50

55

60

Preloading time (weeks) Figure 13. Reduction in the adsorption isotherms and Freundlich parameter Κ for tetrachloroethene by preloading with organic matter. Data are from ref 7.


33.

ZIMMER E T AL.

Activated-Carbon

Adsorption of Organic Pollutants

595

100

p-NITROPHENOL


2,4-DiCHLOROPHENOL

TRICHLOROETHENE TETRACHLOROETHENE

1,1,1

Activated 10

carbon: F 20

-TRICHLOROETHANE

100 30

40

50

60

80

Preloading time (weeks) Figure 14. Reduction in the Freundlich parameter Κ for several micropollutants as a function of preloading time with organic matter. Data are from ref. 7.

Summary The results presented can be summarized as follows. Adsorption analysis is a good method to characterize the activated-carbon adsorption of humic substances and can be used as an aid for drinking-water-treatment design. Removal of micropollutants with an activated-carbon filter depends on ad sorption competition with the humic substances that are always present in the waters to be treated. The large effect of humic substances on micropollutant adsorbability is due to the long time available for preadsorption. This preadsorption may lead to an enrichment of the organic substances on the activated carbon. This process is very slow, but leads to a large reduction of the adsorption capacity for the chlorinated hydrocarbons. The effect of preadsorption depends on the chemical structure of the micropollutant, but is nearly independent of the humic concentration and the activated-carbon type. The impact of humic substances is a general reduction of the carbon capacity, which can be adequately described by diminishing of the Freund lich parameter K. With this time dependency, the breakthrough of micropollutants in treatment plants can be calculated.


5%

AQUATIC HUMIC SUBSTANCES


References 1. Sontheimer, H . Verfahrenstechnische Grundlagen fur Anhgen zur Entfernung von Halogenkohlenwasserstoffen aus Grundwassern; Publication Series, EnglerBunte-Institut, Universität Karlsruhe (TH): Federal Republic of Germany, 1983; Volume 21. 2. McKinnon, R. J.; Dyksen, J. E . J. Am. WaterWorksAssoc. 1984, 76(5), 42-47. 3. Hand, D .W.;Crittenden, J. C.; Gehin, J. L . ; Lykins, B. W. J. Am. Water Works Assoc. 1986, 78(9), 87-98. 4. Kavanaugh, M . C.; Trussell, R. R. J. Am. Water Works Assoc. 1980, 72(12), 684. 5. Baldauf, G. Water Supply 1985, 3, 187-196. 6. Crittenden, J. C.; Luft, P.; Hand, D. W. Water Res. 1985, 19, 1537-1548. 7. Zimmer, G. Ph.D. Thesis, Universität Karlsruhe (TH), Federal Republic of Germany, 1988. 8. Schnitzer, M . ; Khan, S. U . Humic Substances in the Environment; Marcel Dekker: New York, 1972. 9. Fuchs, F.; Raue, B. Vom Wasser 1981, 57, 95-106. 10. Sontheimer, H . ; Frick, B.; Fettig, J.; Horner, G.; Hubele, C.; Zimmer, G. Adsorptionsverfahren zur Wasserreinigung; Engler-Bunte-Institut, Universität Karlsruhe (TH): Federal Republic of Germany, 1985. 11. Frick, B.; Bartz, R.; Sontheimer, H . ; DiGiano, F. A. In Activated Carbon Adsorption, Volume 1; Suffet, J. H . ; McGuire, M . J., Eds.; Ann Arbor Sciences: Ann Arbor, 1980; pp 229-242. 12. Fettig, J.; Sontheimer, H . J. Environ. Eng. (Ν.Ύ.) 1987, 113(4), 795-810. 13. Sontheimer, H . ; Crittenden, J. C.; Summers, R. S. Activated Carbon for Water Treatment; Engler-Bunte Institut, Universitä t Karlsruhe: Federal Republic of Germany; distributed in the United States by the American Water Works As sociation, 1988. 14. Sontheimer, H . ; Volker, E . Charakterisierung von Abwassereinleitungen aus der Sicht der Trinkwasserversorgung; Engler-Bunte-Institut, Universitä t Karls ruhe (TH): Federal Republic of Germany, 1987; Volume 31. 15. Brauch, H . J. Ph.D. Thesis, Universität Karlsruhe, Federal Republic of Ger many, 1984. 16. Summers, R. S. Ph.D. Thesis, Stanford University, 1986. 17. Hubele, C. Ph.D. Thesis, Universität Karlsruhe (TH), Federal Republic of Ger many, 1985. 18. Baldauf, G.; Zimmer, G. Vom Wasser 1986, 66, 21-31. 19. Zimmer, G . ; Haist, B.; Sontheimer, H . Proc. AWWA Annu.Conf.1987, Kansas City, 815-826. RECEIVED for review July 24, 1987. ACCEPTED for publication February 26, 1988.


Activated-Carbon Adsorption of Organic Pollutants - ACS Publications

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