Effect of Coagulation, Ozonation, and Biodegradation on Activated

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Downloaded by UCSF LIB CKM RSCS MGMT on September 4, 2014 | http://pubs.acs.org Publication Date: December 15, 1988 | doi: 10.1021/ba-1988-0219.ch040

Effect of Coagulation, Ozonation, and Biodegradation on Activated-Carbon Adsorption 1

2

Gregory W. Harrington , Francis A. DiGiano, and Joachim Fettig

Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, NC 27514

The ability to describe humic substance adsorption is important for the design of activated-carbonfiltersin water treatment. Humic solutions are composed of a multitude of unknown molecular species, and competitive adsorption among these species, as well as with trace anthropogenic organic chemicals, needs to be understood better. In this research, ideal adsorbed solution theory was used to describe an aquatic humic solution as a set of several pseudocomponents and to evaluate the effects of two treatment processes (alum coagulation and a combination of coagulation, ozonation, and biodegradation) on the solution's equilibrium and kinetic adsorption behavior.

OUR LIMITED UNDERSTANDN IG OF THE STRUCTURE

of aquatic humic sub stances (HS) is manifested in our poor understanding of how these materials are adsorbed on activated carbon. When measured collectively by a surrogate parameter such as total organic carbon (TOC), HS solutions are not consid ered well adsorbed by activated carbon. Nevertheless, their adsorption is important because the removal of synthetic organic chemicals may be de creased by the presence of H S . Moreover, the removal of HS by adsorption

1Current address: Malcolm Pirnie, Inc., Newport News, VA 23606 Current address: Division of Hydraulics and Sanitary Engineering, Norwegian Institute of Technology, N-7034 Trondheim-NTH, Norway

2

0065-2393/89/0219-0727$06.00/0 © 1989 American Chemical Society

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


728

AQUATIC HUMIC SUBSTANCES

may become important if more stringent maximum contaminant levels (MCLs) for trihalomethanes and other chlorination byproducts are set. Water treatment facilities will be forced to rely more on adsorption if future M C L s cannot be met by moving the point of chlorination or increasing the efficiency of coagulation. Therefore, the ability to describe H S adsorption and its effect on the adsorption of pollutants is of utmost importance in the design of activated-carbon filters. Because of the heterogeneous nature of H S solutions, this research was aimed at testing the applicability of a competitive adsorption model to de scribe equilibrium and kinetic adsorption behavior. The approach is similar to that used by Frick and Sontheimer (J) and Crittenden et al. (2), wherein an unknown mixture is described as a set of several pseudocomponents by using the ideal adsorbed solution theory (IAST) (3). This approach was used to evaluate the effects of coagulation, ozonation, and biodégradation on the adsorption behavior of H S solutions.

Preparation of Samples Raw water was obtained from Lake Drummond in southeastern Virginia. The water is highly colored, low in alkalinity (—50 m g / L as C a C 0 ) , and p H 4. Upon return to the laboratory, the water was prefiltered with 1.0μπι honeycomb filters to remove leaves and sediment. The water was then stored in a cool, dark storage area prior to treatment. The first stage of treatment used was alum coagulation. The p H of the raw water was adjusted to 6.5 by the addition of 2.4 X 10 M sodium carbonate. Alum was added at high mixing intensity and constant p H to a concentration of 205-210 m g / L as A l ( S 0 ) - 1 8 H 0 to achieve destabilization. This step was followed by 40 min of flocculation, overnight settling, and filtration with 0.45-μιη membrane filters to achieve 76% removal of U V absorbance at 254 nm and 50% removal of T O C . The coagulated water was then stored in a refrigerator for prevention of biodégradation. The coagulation stage was followed by ozonation and biodégradation. Distilled, deionized water was ozonated to a concentration of 25 mg of 0 / L and then combined with an equal volume of the coagulated sample to yield an ozone dosage of 1.15 mg of 0 / m g of T O C . The ozonated sample was placed in the reservoir of a recycle batch reactor, the column of which was previously seeded with return activated sludge from the local wastewater-treatment plant. The reactor was allowed to run until no further re duction in T O C was observed (10 days of run time) and the sample was then stored in a refrigerator. T O C values of each stage are given in Table I. 3

2

2

4

3

2

3

3

Adsorption Isotherms Equilibrium studies were performed by using the bottle point technique for determining adsorption isotherms. Granular activated carbon (GAC, F-400) In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

40.

HARRINGTON ET AL.

Coagulation, Ozonation, and Biodégradation

729


Table I. Total Organic Carbon Levels in the Humic Mixtures Tested TOC Humic Mixture (mg/L) Prefiltered 43.8 Alum coagulated 21.7 Ozonated and biostabilized 7.0 was washed, dried, stored, and ground to a 200-325 mesh size to ensure that a representative sample of carbon was used (4). In order to adequately describe the isotherms, bottles were filled with powdered carbon dosages ranging from 5 to 4000 m g / L . Samples were buffered with a 5 m M phosphate buffer to yield a p H of 6.5. Each bottle was then filled with 100 m L of sample and tumbled for 7-10 days at 23 °C. After the period of tumbling, the powdered carbon was removed by using 0.45-μιη membrane filters and the T O C of each sample was measured. Each component in a given mixture must satisfy the following mass balance equation for each bottle:

*

=

"MÂT-

ω

where q is the surface loading of component i at equilibrium, C is the bulk liquid-phase concentration of component i at equilibrium, C is the initial liquid-phase concentration of component i , and M/V is the carbon dosage. {

{

0 i

Prediction of Multicomponent Equilibrium IAST was assumed capable of describing the multicomponent nature of the resulting H S isotherms through the use of the following five equations:

= Σ

q

T

(2)

Qi

% = —

for i = 1 to Ν

(3)

QT N

ι

— = Σ—

(4)

0

C, = z,C,° TlAd,

~W

TMads =

~W

(iP d[\ll =

I

for i = 1 to Ν

(5)

0

(C, )]

ÂMtfJÏ*

1

f o r

'

=

l

t

o

N

( 6 )

Total surface loading, q , is defined as the sum of individual solute surface loadings by equation 2; the surface mole fraction, z , of an individual solute is defined by equation 3. Equation 4 defines the surface loading of T

t

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

730

AQUATIC HUMIC SUBSTANCES

the mixture as a function of single-solute surface loadings, q®, achieved when the single-solute systems adsorb at the same temperature and spread ing pressure as the mixture. By equating the chemical potentials of a solute in the adsorbed and liquid phases, one arrives at equation 5, where would be in equilibrium with q? in a single-solute system. Equations 4 and 5 are key equations in IAST, because both assume that the adsorbed phase forms an ideal solution. Finally, equation 6 equates the spreading pressure of the mixture, τ τ , with the spreading pressures of the single-solute systems, ir °. A is the adsorbent surface area per unit mass of adsorbent. The spreading pressure of a single-solute system is evaluated by the integral shown in equation 6. Each H S solution was assumed to contain a set of several individual pseudocomponents, each having a single-solute adsorption behavior that could by described by the Freundlich isotherm equation. The linearized form of the Freundlich equation is given by τ


t

a d s

j In (C °)

In (q?) = In (K.) + ^

(7)

where K and n are the Freundlich isotherm constant and exponent, re spectively, for component i. When equation 7 is substituted into equation 6, the following expression results: f

i

no iq

= n°

(8)

for j = 2 to Ν

jqj

Combining equations 1-5, 7 and 8 yields the following objective func tion, which was derived by Crittenden et al. (2): Ν

Σ F i

= 0 = C

0t

_Ι

m

υι Q

Effect of Coagulation, Ozonation, and Biodegradation on Activated

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