Natural Dissolved Organic Matter Removal and Subsequent

Chapter 17

Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch017

Natural Dissolved Organic Matter Removal and Subsequent Disinfection By-Product Formation: A Comparison of Ion Exchange and Activated Carbon 1

1,

Yongrui Tan , James E . Kilduff *, and Tanju Karanfil

2

1

Civil and Environmental Engineering, Rensselaer Polytechnic Institute, 317 MRC Building, 110 8 Street, Troy, NY 12180 Department of Environmental Engineering and Science, Clemson University, 342 Computer Court, Anderson, SC 29625 th

2

A weak base ion exchange resin and a granular activated carbon (GAC) were compared for their ability to sorb natural dissolved organic matter (DOM) and reduce specific ultra violet absorbance (SUVA). Disinfection by-product formation of water having high bromide content after separate ion exchange and carbon adsorption and subsequent chlorination was compared. The ion exchange resin showed superior dissolved organic carbon (DOC) uptake, although SUVA reduction by the two sorbents was comparable. The concentrations of total trihalomethanes (THM4) and the sum of six haloacetic acids (HAA6) were reduced by both sorbents, although the resin was capable of meeting drinking water regulations at much lower doses. The effect of ion exchange and carbon adsorption on THM4 reactivity (yield) was markedly different, and the results demonstrate that ion exchange removes reactive DOM species that GAC does not. Bromine incorporation factor (BIF) values for both sorbents increased initially and were similar at low dosages; however, as dosage increased further the BIF after resin treatment decreased rapidly and remained significantly lower than the BIF after GAC treatment.

242

© 2008 American Chemical Society In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

243


Introduction During chlorination of natural water, chlorine reacts with naturally present dissolved organic matter (DOM) to form potentially hazardous chlorinated hydrocarbons, so-called disinfection byproducts (DBPs) (1). In the presence of bromide ion, brominated DBPs are formed in parallel with chlorinated species. Research has suggested that DBP toxicity increases with increasing bromine substitution (2). Controlling DBP formation is increasingly important as the allowable concentrations of such DBPs as trihalomethanes (THMs) and haloacetic acids (HAAs) in finished water are lowered (3) and the regulation of individual DBP species, especially the brominated ones, is considered. Bromide is naturally present in drinking water sources, with concentrations ranging from Shimadzu) and UV absorbance at λ = 254 nm (Cary 100, Varian). Bromide concentrations were measured using a reagent-free ion chromatography system (ICS-2000, Dionex). The batch approach used in this research should apply directly to mixed reactors, which are being used increasingly for disinfection byproduct formation control via N O M removal by ion exchange resins. However, the results will generally not apply directly to column results, because of different mass transfer rates among components, resulting in chromatographic effects that we do not account for here.


C

Molecular Weight Distribution The molecular weight distribution (MWD) of DOM solutions was measured before and after ion exchange sorption using an HPSEC technique using UV detection at 254 nm (25). Poly(styrene sulfonate) (PSS) was used as an external standard to calibrate a Waters Protein Pak 125 column with silica based gel stationary phase operated on a HP 1100 HPLC system. An ionic strength of 0.1 M and pH 6.8 was maintained in all standard and sample solutions.

Chlorination and DBP formation Supernatants obtained from DOM isotherms were chlorinated using the Uniform Formation Conditions (UFC) protocol, as proposed by Summers et al. (26). After the chlorination period, THMs and haloacetonitriles (HANs) were extracted with hexane according to Standard Method 6232 B. HAAs were reacted with acidic methanol to yield esters, and extracted with methyl-tert-butylether (MTBE) according to EPA Method 552.2. Four T H M (chloroform (TCM), bromodichloromethane (BDCM), chlorodibromomethane (DBCM), and bromoform (TBM)), four HAN (trichloro-, dichloro-, bromochloro-, and dibromoactonitrile) and six HAA species (chloro- (MCAA), bromo- (MBAA), dichloro- (DCAA), trichloro- (TCAA), bromochloro- (BCAA), and dibromoacetic acid (DBAA)) were identified and quantified using a gas chromatography system (Agilent 6890) equipped with micro-ECD detector. THM and HAA standards were purchased (Supelco) and spiked into reagentgrade I water to prepare external standards that were then extracted and analyzed in the same manner as samples.

In Disinfection By-Products in Drinking Water; Karanfil, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

247

Results and Discussion


D O M Uptake and Effects on MWD and SUVA The uptake of TMK DOM by the weak base resin and the GAC is shown in Figure 1. The equilibrium liquid phase concentration was normalized by dosage (D) to control for any effects of initial concentration on the DOC isotherm (see, e.g., 25). It is clear that the resin exhibits higher DOM uptake from the TMK-Br water than the GAC. There was little effect of bromine on the uptake by either of the sorbents (data not shown). The MWD of DOM remaining in solution after sorption by the resin and the GAC was measured using HPSEC, to investigate which DOM components were adsorbed preferentially. As shown in Figure 2, with an increase in GAC dose, sorption reduces the total mass of DOM in solution, and adsorption preferentially removes DOM components with lower MW, especially at low carbon doses, when adsorption sites are more limited and the competition among DOM components is high. This is consistent with the results observed in previous research for activated carbon adsorption (25), and is explained by size exclusion effects. In contrast, with an increase in M43 resin dose, components having intermediate molecular weights are removed preferentially at low dosages; with an increase in dosage, higher molecular weights DOM species are then removed in preference to smaller ones. Clearly, GAC and ion exchange target different DOM components, although both are effective in reducing DOC individually. Therefore, a combination of GAC adsorption and anion exchange could cover a broad spectrum of DOM components with different properties. Selective uptake of DOM components having different SUVA values was evaluated by measuring the change in SUVA (λ = 254 nm) after sorption, as shown in Figure 3. For both the resin and the GAC, the SUVA254 values decreased with increasing sorbent dose, indicating a preferential removal of high SUVA254 components from solution. However, the pattern of SUVA removal was different. The rate of decrease in SUVA with adsorbent dose was greater for the M43 resin, up to dosages of about 200 mg/L. At higher dosages, SUVA removal by the resin was minimal, whereas removal by the GAC continued. Taken in combination with the results from Figure 2, this suggests that the smallest DOM components in TMK water do not have highest SUVA values. This is consistent with the findings of Kitis et al. (22). Because they target different DOM components, the generally similar behavior of these two sorbents with regard to SUVA removal is rather surprising, although it appears that GAC can reduce SUVA to lower levels, albeit at high dosages.



248

0.00

0.05

0.10

0.20

0.15

DOM Concentration, C /D, [mg DOC/mg sorbent] e

Figure 1. Uptake of DOMfrom TMK-Br bromide spiked water by M43 weak base resin (C = 4.2 mg/L) and TOG GAC (C = 4.7 mg/L). 0

0

ο

10

15

20

10

elution time, min

15

20

edition time, min

Figure 2. Size exclusion chromatograms of TMK DOM remaining in solution after sorption by TOG GAC (C = 4.7 mg/L), left, andM43 resin (C = 3.94 mg/L), right. 0

0


249

2

J J

ϋ

•

5

•

2.0

Ο

* •

Ο)

•

•

•

Ε

ι Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: August 5, 2008 | doi: 10.1021/bk-2008-0995.ch017

"

10 • GAC 0.5

• Resin

ηη 0

200

400

600

800

1000

Sorbent Dosage [mg/L]

Figure 3. Effect of sorbent dose on SUVA254 of TMK DOM components remaining in solution after sorption by M43 resin (C = 4.2 mg/L) and TOG GAC (C = 4.7 mg/L). 0

0

Bromide removal by M43 Resin Bromide ion removalfromTMK-Br water by the M43 resin was

Natural Dissolved Organic Matter Removal and Subsequent

Recommend Documents