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Chapter 15

Downloaded by UNIV MASSACHUSETTS AMHERST on October 8, 2012 | http://pubs.acs.org Publication Date: November 19, 1996 | doi: 10.1021/bk-1996-0649.ch015

The Reduction of Bromate by Granular Activated Carbon in Distilled and Natural Waters 1,3

1

2

Jennifer Miller , Vernon L. Snoeyink , and Joop Kruithof 1

Department of Civil Engineering, University of Illinois, 3230 Newmark Civil Engineering Laboratory, 205 North Mathews, Urbana, IL 61801 KIWA N.V., Groningenhaven 7, P.O. Box 1072, 3430 BB Nieuwegein, Netherlands 2

An existing model describing the reduction of free chlorine by granular activated carbon (Suidan et al., 1977a and 1977b) has been applied to the reduction of bromate by granular activated carbon. Bothfinitebatch and packed bed column studies were used to verify the model predictions. Distilled water tests were conducted under varying initial bromate concentrations, at several solution pH values, and using different carbon particle sizefractions.The effect of natural organic matter was studied by spiking solutions with a fulvic acid isolate, as well as using water obtained from the Interstate Water Company in Danville, IL. Preloaded carbon was also used for batch and column tests. It was found that the model describes bromate reduction well in distilled water, but fails to account for the cumulative effect of natural organic matter in natural waters. Recent interest in the byproducts of alternative oxidants for drinking water treatment has led to the publication of several studies of bromate reduction by granular activated carbon (GAC) in distilled water (Siddiqui et al., 1994; Miller et al., 1995). Much more research on bromate will undoubtedly be forthcoming. It would be useful to have a model to describe and extrapolate the data, both in distilled water and also in natural waters. In this paper we present batch and column test data for bromate reduction by GAC in distilled and natural waters, and describe the data with an existing model. 3

Current address: Department of Civil Engineering, W348 Nebraska Hall, University of Nebraska, Lincoln, NE 68588 0097-6156/%/0649-0251$17.75/0 © 1996 American Chemical Society In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


252

WATER DISINFECTION AND NATURAL ORGANIC MATTER

The model which will be discussed here was developed previously by Suidan et al. (1975, 1977a, 1977b, 1978) to describe the reduction offreechlorine by GAC. Although bromate and hypochlorite are both oxyanions, bromate requires more electrons for reduction, reacts more slowly, and is found in drinking water at much lower concentrations than hypochlorous acid and hypochlorite. Never the less, the main assumptions used by Suidan (1975) in developing this model, namely that the reduction reaction is limited by both mass transfer limitations and surface reaction rate limitations, appear to hold true for bromate. In this paper, we will discuss the applicability of the model developed by Suidan et al. (1975,1977a, 1977b, 1978) to bromate reduction in distilled water, as well as in waters containing natural organic matter (NOM). The success and failure of the model to predict bench top GAC column performance under different laboratory conditions will be discussed. Methods and Materials Chemicals used in these experiments were reagent grade quality. Anion solutions were prepared using sodium and/or potassium salts of the various anions. Salts were dried for twenty-four hours in a 105°C oven, and then were cooled and stored in a desiccator. Salts were weighed using a Mettler AE2000 electronic balance (Hightstown, NJ). Waterfromthe Interstate Water Company in Danville, IL, was obtainedfromthe clarifiers after lime softening. The water was stored at 4°C until use. The source water for the Interstate Water Company in Danville is Lake Vermilion. The raw water characteristics of Lake Vermillion water are given in Table 1. At the time of the experiments, the water was buffered with a phosphate buffer and the pH was adjusted. Table 1 : Raw water characteristics for Lake Vermillion Water

pH

Total Hardness mg/L as CaC0

Total Alkalinity mg/L as CaC0

90-120

40-60

3

Lake Vermillion

8-9

Turbidity Chloride, mg/L NTU

TOC, mg/L

3

0.3-5

25-30

2

The carbon used in these experiments was Ceca GAC 40, batch number B709212B. The carbon was sieved following ASTM Standard D 2862-82 (ASTM, 1988). Following sieving, the 30 χ 40 mesh sizefractionwas rinsed with distilled water to remove dust, and then dried in a 105°C oven. Dried carbon was stored in a desiccator. Preloaded carbon was loaded with natural organic matter at the Interstate Water Company, Danville, Illinois. The waterfromthefiltereffluent was passed through carbon which had been packed into glass columns. A one minute empty bed contact time was used for preloading, and the carbon was loaded for various lengths of time.

In Water Disinfection and Natural Organic Matter; Minear, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

15. MILLER ET A L

Finite batch tests were used to examine the effects of reaction variables such as solution pH, initial bromate concentration, and NOM concentration. Batch tests were conducted in a four liter pyrex glass beaker. The solution was stirred using a magnetic stirrer, turning at a speed sufficient to keep carbon suspended uniformly throughout the beaker. A phosphate buffer (approximately 0.1 mM) was used to maintain a constant pH during batch tests. The adjustment of solution pH was accomplished by adding either 1 MH S0 or 1 NNaOH. Samples were taken during batch tests using a 10 mL glass syringe with a luerlock connection, andfilteredusing afiltertip with 0.45 μπι cellulosicfilterpaper. Samples were stored in high density polyethylene screw top bottles, and were kept in a cold room (temperature 3-8°C) until analysis. Column studies were conducted in glass columns of 1.3 cm (one-half inch) or 2.5 cm (one inch) diameter, depending on carbon particle size. Carbon beds were supported on silica sand, glass beads and/or glass wool. Prior to packing, the carbon was soaked in distilled, deionized water overnight to remove air bubbles. Care was taken to insure that no air bubbles entered the carbon bed during the test. Influent solution was mixed and stored in glass carboys on a magnetic stirrer. Masterflex pumps (Cole-parmer, New Jersey) and Tygon tubing were used to pump the solution through the carbon bed. The columns were operated upflow. Samples were collected directly into sample bottles. In general, short empty bed contact times, less than one minute, were used. Standards for anion analysis were mixed according to Standard Methods for the Examination of Water and Wastewater, procedure 4110 Β (1989). A stock solution for bromate was mixed using 1.1798 g NaBr0 in distilled water; this amount of NaBr0 was calculated to give a stock solution of 1 mg Br0 " per 1 mL. The working standard for bromate was made as for the other anions. Working standards were mixed using distilled water, and were run at the beginning of each analysis session. The following concentrations were used for working standards: 0,5,10,20,50, and 100 μg/L of each anion. Analysis was not done if the linear correlation factor for the standards was less than 0.99. The method used for analysis was developed by Hans van der Jagt and co workers at KIWA, Nieuwegien, the Netherlands and is described as follows. Analysis was done on a Dionex Series 300 ion chromatograph, equipped with a gradient pump, an autosampler and an electrochemical detector. Samples were analyzed at room temperature, and the system temperature compensation factor was 1.7. The following operational parameters were used: 2


253

Reduction of Bromate by Granular Activated Carbon

4

3

3

3

Eluent 1: 0.5mMNa^,0.18mMNaHC0 Eluent 2: 4 mM N a ^ , 1.5 mM NaHC0 Régénérant: 25mMH S0 Columns: AG9-SC, AS9-SC Flowrate: 2 mL/min Conductivity detector range = 0.01 μ8 Run time=12 minutes: 4.5 minutes on Eluent 1,5.5 minutes on Eluent 2, remaining time on Eluent 1 Bromate retention time = 2.33 minutes 3

3

2

4


254


The detection limit was calculated to be 2 μg/L, based on the slope of the calibration curve and the reproducibility of analysis at low concentrations. Concentrations less than the detection limit are plotted in thefiguresas zero. Natural water samples were filtered with a silverfilter(Dionex) to remove chloride prior to analysis.


Model The details of the development of the model by Suidan et al. which describes the reduction offreechlorine by GAC can be found in previously published papers (1977a, 1977b). Here we will discuss the most important assumptions used in the development of the model which pertain to bromate reduction. The equations used in the model and a list of the variables are given in the appendix. The reduction of bromate, in this case, is assumed to be limited by both the rate of mass transfer and the rate of the surface reaction. The surface reaction occurs in two steps: a reversible adsorption/desorption step, followed by an irreversible dissociation step. The irreversible dissociation step is considered to be rate limiting. The surface reaction (forfreechlorine) can be visualized as: C* C*

+ HOC1 —> C*-HOCl reversible adsorption/desorption C*-HOCl —> Cl" + H* + C*Q irreversible dissociation step + HOC1 —> CI' + H + C*0 overall +

In this case C* represents an "active site" on the carbon surface. Voudrias et al. (1983) visualized the reduction of chlorite by GAC in an analogous manner:

c*

+ cio - —> cr + c*o 2

2

The above reaction also follows the pattern of reaction proposed in the model by Suidan et al. (1977a, 1977b), that one oxyanion molecule reacts at one "active site" on the carbon surface. Because bromate is chemically related to chlorite andfreechlorine, a similar reaction between bromate and GAC can be visualized. C*

+ Br0 " —> C*-Br0 " reversible adsorption/desorption C*-BrQ ~ —> Br + C*Om irreversible dissociation step. + Br0 - —> Br + C*0 overall 3

3

3

C*

3

(3)

The reduction of bromate by a reaction similar to thefreechlorine reaction would be pseudofirstorder with respect to bromate concentration. The reaction would not be first order over all, since bromate reduction requires the transfer of six electrons. If only one step is rate limiting, however, the reaction rate could be approximated asfirstorder. Over time, the reduction of bromate would result in a build-up of oxides on the carbon surface, which would decrease the rate of bromate reduction (i.e., the carbon behaves as a poisoned catalyst). Suidan et al. (1977a, 1977b) coupled the above assumptions with ideal reactor analyses for afinitebatch reactor and a packed bed column. To obtain the kinetic parameters for the reduction reaction, the datafroma bench scale batch reactor is analyzed. The constants are then used to predict bromate reduction in a packed bed



15. MILLER ET AL.

Reduction of Bromate by Granular Activated Carbon

255

column over time. The information needed in the model for afinitebatch reactor includes: the initial bulk concentration, the pore volume of the carbon, the difiiisivity, the "pore length" which is one sixth of the carbon particle dimeter (Levenspiel, 1962), the carbon concentration, and four kinetic constants which are found by trial and error. For the packed bed column model, the information needed includes: the influent concentration, the axial dispersion coefficient, the bed length, the cross sectional area of the packed bed, the bed porosity, the carbon density in the bed, the flow rate, and nine empirical constantsfroman algebraic rate expression developed by Suidan (1975). The determination of the kinetic parameters for the model is discussed below. Parameter B$tim^to

A large number of parameters are used in the model. For the batch test description there are four variables which were derived in the original model: SIT, K8, K9, and K10. The parameter SIT describes the number of "active sites" on the carbon surface. There is no indépendant method for determining this parameter, since we do not know exactly how bromate is reacting with the carbon surface. In general, however, SIT can be thought of as a relative measure of carbon activity. A carbon with a larger SIT value will be more effective for bromate reduction. The parameters K8, K9, and K10, are kinetic constants which were derived in the original model. Again, there is no independent method of evaluating these constants, and the constants vary for different carbons and solutions (reflecting effects on the bromate reduction reaction.) The parameter K8 is the equilibrium constant for the reversible adsorption/desorption step. The value of this constant was determined by trial and error using batch test data. Once determined, the value of 0.00015 was used for all batch tests conducted in a solution of pH 7, since the partitioning of bromate is a constant for that species. The parameter K9 is a rate constant for the formation of oxidized sites, ie. for the irreversible dissociation step. The value of K9 varied during batch tests, but was, with one exception, between 10 and 100 during this study. The remaining rate constant, K10, describes the spontaneous degradation of oxidized sites (or the regeneration of sites on the carbon surface.) There is no direct evidence for the regeneration of carbon sites after bromate reduction, however, other researchers (Kim, 1977) have noted this phenomenon when dealing with other oxidants. In the case of bromate, spontaneous regeneration of the carbon surface does not appear to be important, and K10 was held constant at 380. The variables SIT, K8, and K9 were determined for the virgin 60x80 carbon by comparing the model predictions to the batch test results and column test results. The parameters determined for the 60x80 carbon data were used to calculate values for SIT and K9 for virgin carbon of other mesh sizes. According to Suidan (1975), SIT and K9 are proportional to the square of the pore length. The pore length is equal to one sixth of the particle diameter (Levenspiel, 1962), so SIT and K9 were easily calculated. The value for K8 was held constant, since K8 does not depend on carbon particle size (Suidan, 1975). For batch tests conducted at a solution pH other than 7, the value for K8 was changed. At low pH, bromate is more reactive (Siddiqui et al., 1994). Thus, there should be less bromate adsorbed to the surface, since the bromate will decompose rapidly to form bromide and the oxidized carbon surface. In batch tests conducted in



256


low pH solutions, the value for K8 was lower compared to the value for neutral solutions, reflecting the lower amount of adsorbed bromate. Conversely, in high pH solutions, the value of K8 was higher, reflecting the build-up of unreacting bromate on the carbon surface. In batch tests with solution pH other than 7, the values for SIT and K9 were held constant since these parameters are characteristic of the carbon and are not affected by the solution pH. For batch tests conducted using preloaded carbon and in natural water solutions, SIT and K9 were found by trial and error to match the batch test data. A summary of values for the four kinetic parameters is given in Table 2. A simple optimization, such as least squaresfit,was not used to determine the values of the parameters because the four constants are considered to have a physical significance. Arbitrarily selecting the parameters would diminish this aspect of the model. Table 2: Kinetic parameters for batch test model Water

Carbon Size

pH

SIT

K8

K9

K10

Distilled

0.51mm (30x40)

7

0.0077

0.00015

41

380

Distilled

0.2 mm (60x80)

7

0.0013

0.00015

19

380

Distilled

0.16 mm (80x100)

7

0.00076

0.00015

10

380

Distilled

1.44 mm (12x16)

7

0.61

0.00015

110

380

Distilled

0.51 mm

4

0.0077

0.000015

41

380

Distilled

0.51 mm

10

0.0077

0.0015

41

380

Distilled + fulvic acid

0.51 mm

7

0.003

0.00015

25

380

Danville

0.51 mm

7

0.0007

0.00015

36

380

Preloaded

0.51 mm

7

0.0006

0.00015

35

380

The model developed by Suidan (1975) also contains an approximate rate expression which contains nine variables. The only reason for the approximate rate expression was to simplify the computer solution of the non linear differential equations. The nine rate expression constantsfitthe equation: J8 J9

R = (Jl χ C)/{(J2 + C)[l + J3exp(-J4 χ C) χ X f [1 + (J6exp(J7 χ C) χ X) ] } (1)



15.

MILLER ET AL.

Reduction of Bromate by Granular Activated Carbon 257

The rate expression constants were determined using the batch test model, after the values for SET, K8, K9, and K10 were chosen. The values for the nine rate expression constants were not chosen arbitrarily, but were based on correlations developed by Suidan et al. (1977a). According to Suidan et al. (1977a), the values for J4 and J7 are constant, 10700 and 26600, respectively. The values for J5, J8 and J9 are related by the equation: J5 + J8*J9 = 1. In addition, J8 is equal to two. Suidan et al. (1977a) was able to correlate the remaining variables, Jl, J2, J3 and J6, to the pore length of the carbon. In this study, the correlations were found not to be true, however, it was found that J3 and J6 could be held constant at the values that Suidan (1975) used in his initial work, 5900 and 100, respectively. The remaining variables, Jl, J2, J5 and J9, were found by trial and errorfitof the rate curve. The value for J5 (an exponent) ranged from 0.01 to 2, while the value for J9 (also an exponent) rangedfrom0.445 to -0.5. Results and Discussion Thç effect q{ Mtial Concentration The reduction of bromate to bromide requires the transfer of six electrons. Generally, the transfer of electrons occurs individually (Larson and Weber, 1994). Thus, bromate reduction by GAC would not be expected to be an elementary reaction. The model developed by Suidan (1975) assumes that the reduction reaction occurs in one step, after a surface complex is formed. This assumption is adequate for bromate reduction if the overall reaction is limited by the rate of one step. The rate of bromate reduction is affected by the initial concentration of bromate, as shown infigure1. The rate of bromate reduction cannot be described as having a simplefirstor second order dependence on the initial bromate concentration, since die reaction is not elementary. Moreover, as will be discussed later, the reduction of bromate by GAC is limited by pore diffusion at low concentrations. The initial concentrations of bromate for the batch test data shown infigure1 differ more than one order of magnitude. The very high initial concentration of460 ppb is not realistic of bromate concentrations encountered in water treatment, but is presented for discussion purposes. All of the data sets shown infigure1 have been described using the model developed by Suidan. The kinetic parameters (discussed above) were the same for all three cases, and only the initial concentration was varied in the model calculation. The batch data shown infigure1 were used to predict the performance of a packed bed reactor with a high initial bromate concentration. The packed bed column experiment was conducted using 30x40 mesh virgin Ceca carbon, a one minute empty bed contact time (EBCT), an average initial bromate concentration of 470 ppb in distilled water, and a solution pH of 7. The predicted breakthrough curve and the column effluent data are shown infigure2. There is good general agreement between the predicted curve and the data. The data show some scatter caused by pump and influent concentration variations over the course of the experiment, which lasted for 43 days. The carbon was not exhausted at the end of the study, but at that time 27 mg bromate per gram carbon had been reduced.




258



Figure 2: Bromate reduction in a packed bed column with a high initial bromate concentration



50

100 Time, minutes

150

200

250

300

Initial concentration = 26-29 ppb bromate Solution pH = 7 Carbon dose = 250 mg/L Distilled water

• 1.44 mm dia. (12x16), model 1.44mmdia., data •0.51 mm dia. (30x40), model 0.51 mm dia., data • 0.2 mm dia. (60x80), model 0.2 mm dia., data • 0.16 mm dia. (80x100), model 0.16 mm dia., data

Figure 3: The effect of carbon particle size on bromate reduction



15. MILLER ET AL.

Reduction ofBromate by Granular Activated Carbon

261

The effect of pore diffusion The reduction of bromate is limited by pore diffusion, which has been shown in this work. The effect of pore diffusion can be nicely shown by comparing bromate reduction by GAC of various particle sizes. Figure 3 is a comparison of batch test data obtained using four different sizefractionsof virgin Ceca carbon. With smaller GAC particles, the average pore length, defined for porous spherical particles as one sixth of the particle diameter (Levenspiel, 1962), is shorter. Thus, bromate reaches "active sites" on the carbon surface more rapidly and the overall rate of bromate reduction is more rapid. Figure 4 shows batch test data for batch tests conducted using slightly higher initial bromate concentrations. The same model parameters were used to obtain the curves in bothfigures3 and 4, with only the initial concentration being changed. Note that the kinetic parameters for the various carbon particle sizes are mathematically related (SIT and K9 are related to particle size, per Suidan et al., 1975). Once data have been obtained for one carbon sizefraction,it is simple to calculate the effect of using a different particle size for bromate reduction. The model curves for the different size fraction particles represent predictions, based on the 30x40 data. At very high initial bromate concentrations, in the mg/L range for example, bromate reduction is not limited by pore diffusion, but by the rate of the surface reaction itself. Figure 5 shows batch test data collected at very high initial bromate concentrations, using various sizefractionsof the same Ceca carbon. It is possible to estimate when diffusion limits a reaction by using the WeiszPrater criterion (Cwp). (For more information on this criterion, see Fogler, 1992). The Weisz-Prater criterion is calculated as: Cwp = -r '(obs) * *R /(D *C ) (2) where -r '(obs) is the observed rate of reaction, ρ is the particle density, R is the particle radius, D is the effective diffusivity (assumed, based on Fogler, 1992), and Ç is the initial batch test concentration. For a single batch test, an observed rate of reaction was calculated which was then used to calculate Cwp. In general, if Cwp » 1, the reaction is severely limited by internal diffusion. If, on the other hand, Cwp « 1, the reaction is not limited by diffusion. For some of the batch tests shown infigures3,4, and 5, Cwp has been calculated. A summary of these values is given in table 3. To avoid the limitation of pore diffusion, either very small carbon particles may be used, or the initial bromate concentration must be very high. 2

A

Pp

A

e

0

p

e

Table 3: Weisz-Prater criterion for several batch test experiments Initial concentration

Cwp

26 ppb

1.1

1.5 χ 10" * Co

459 ppb

0.65

2x10"** Co

10.9 ppm

0.08

Carbon

Radius, mm

Rate, observed mVsec-g^

30x40 mesh

0.255

2.5 χ ΙΟ" * Co

30x40 mesh

0.255

30x40 mesh

0.255

7

7

80x100 mesh

0.08

5.8xlO- *Co

27 ppb

0.25

PAC (p325)

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