[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Characteristics of Humic Substances and Their Removal Behavior in Water Treatment

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  • 29 Characteristics of Humic Substances and Their Removal Behavior in Water Treatment

    J. S. Kim1, E . S. K. Chian, F. M. Saunders, E . Michael Perdue2, and M . F. Giabbai1

    School of Civil Engineering, Georgia Institute of Technology, Atlanta, GA 30332

    The characteristics of naturally occurring aquatic humic substances and their removal behavior in water treatment were investigated in a plant operation and in alum coagulation with conventional labo-ratory jar-test experiments. Specific characteristics of humic sub-stances affected the performance of alum coagulation in removing both humic substances and turbidity. Up to 50% of the humic sub-stances were removed in both the treatment plant and alum coagu-lation. High-molecular-weight humic substances were preferentially removed. This preference resulted in substantial decreases in color intensity and trihalomethane formation potential per unit mass of humic substances. The characteristics and removal behavior by alum coagulation of commercial humic acid were significantly different from those of the source-water humic substances.

    REMOVAL OF HUMIC SUBSTANCES has long been of concern in water treatment because of their abundance in natural waters and their potential adverse effects on public health and esthetics. They impart color to water and are able to form complexes with other inorganic and organic species (1-4) that frequently prevent the removal of these pollutants in treatment processes. More importantly, humic substances are major precursors in the formation

    1Current address: HazWaste Industries, Inc., 2264 Northwest Parkway, Suite F, Marietta, GA 30067

    2Current address: School of Geophysical Sciences, Georgia Institute of Technology, Atlanta, GA 30332

    0065-2393/89/0219-0473$07.25/0 1989 American Chemical Society

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

  • 474 AQUATIC HUMIC SUBSTANCES

    of trihalomethanes (THM) upon chlorination (5, 6). The ubiquity of T H M s in drinking water (7), together with their potential carcinogenic activity (8), prompted the U.S . Environmental Protection Agency (EPA) to promulgate the maximum contaminant level of 100 g / L of total T H M s in water systems serving more than 10,000 persons (9).

    Most traditional water-treatment plants have been designed and operated to maximize removal of turbidity and pathogens through the use of coagulation-flocculation-sedimentation, filtration, and chlorination processes. Now that humic substances can be removed in water treatment, a considerable amount of effort has been focused on the optimization of existing treatment processes, especially coagulation, for effective removal of humic substances (10-17). However, there are still questions relating to the performance of these processes in removing humic substances because the nature of humic substances is not fully understood. Because the nature of humic substances can be an important factor influencing the performance of the water-treatment processes and the formation of T H M s upon chlorination, this study was initiated to investigate the specific characteristics of humic substances in natural water sources and their removal behavior in water treatment.

    Humic substances in natural waters are unresolvably complex mixtures of organic matter; their physical and chemical properties are difficult to characterize. They are generally described as yellow, acidic, chemically complex, and polyelectrolytelike materials that range in molecular weight from a few hundred to several thousands (18). Recent studies (19-21) have indicated that humic substances in natural waters from different sources are relatively similar to each other in nature, but are significantly different from the commercial model humic substances commonly used in laboratory studies.

    Most information on the removal ofhumic substances in water treatment and their impact on water quality has been obtained from studies with model humic substances (11, 13, 14). A clear distinction should be made between data acquired from model humic substances and those from natural water sources.

    In order to address this concern, the characteristics ofhumic substances and their removal behavior in water treatment were investigated at a full-scale operating plant and in alum coagulation with conventional laboratory jar tests. The Chattahoochee Water Treatment Plant (CWTP) of Atlanta, G A , was selected for this study.

    C W T P is a conventional plant that uses the Chattahoochee River as its source of raw water. The treatment sequences are coagulation-flocculation-sedimentation, filtration, and chlorination. Humic substances isolated from the C W T P source water and the model Aldrich humic acid were used in alum coagulation. The specific objectives of this study were to characterize the humic substances isolated from the source water and from

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

  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 475

    the treated effluents of each subsequent treatment process of the C W T P ; to evaluate the removal behavior ofhumic substances by alum coagulation with conventional jar-test procedures; and to compare the nature of both types of humic substances and their removal behavior by alum coagulation.

    Experimental Materials and Methods CWTP and Water Sampling. The CWTP is a conventional plant that employs

    coagulation-flocculation-sedimentation, filtration, and chlorination (Figure 1). Alum is used as a coagulant in coagulation-flocculation after pH adjustment with lime. Prechlorination is practiced in both plant-intake and rapid-mix units. The supernate from the sedimentation basins is filtered through a dual-media filter (anthracite and sand); this process is followed by postchlorination. The sludge produced is chemically conditioned and dewatered with a filter press. The resulting sludge cakes are disposed of in a sanitary landfill.

    The CWTP, designed to treat 230 106 L/day, was treating an average of 150 X 106 L/day. It uses the Chattahoochee River as a water source; general water characteristics are shown in Table I. The average color and turbidity values were 28 Pt-Co units and 27 nephelometric turbidity units (NTU), respectively. These values represent relatively low color and high turbidity, compared to many surface waters (22).

    The water-sampling points, selected for the isolation of humic substances and investigation of their characteristics and removal behavior in the CWTP, included the source water (SW), effluent from coagulation-flocculation-sedimentation (ACFS), filtration effluent (AF), and the clear well (CW). The water volumes and sampling dates taken for the isolation ofhumic substances from each sampling point are shown in Table II, along with the amount of humic substances isolated. Because a large volume of water from each sampling point was required to isolate sufficient quantities of humic substances for subsequent laboratory experiments, water samples were taken during several periods of time rather than at intervals chosen according to the hydraulic retention time of each unit process. Approximately 200-700 L of water samples per day were taken from a sampling point.

    The source-water quality and the performance of the CWTP were considered to influence the removal behavior of humic substances in the treatment processes. Therefore, various water-quality parameters were measured at each sampling point according to the hydraulic retention time of each unit process during the water-sampling period, as shown in Table II.

    Figure 2 illustrates the results of the water-quality parameters for the four sampling points at the CWTP. Instantaneous THMs were produced after prechlorination and increased to 29.1 g/L in CW. Trihalomethane formation potential (THMFP) is a measure of the maximum amount of THMs that can be formed by the reaction of free residual chlorine with humic substances. A comparison of SW THMFP (228.5 g/L) with CW THMFP (95.2 g/L) shows a 69% reduction of THM precursors. The nonvolatile TOC (NVTOC) was reduced by 50% (i.e., from 4.3 to 2.1 mg/L), whereas UV absorbance at 254 nm and color were removed by 71 and 100%, respectively, through the treatment processes.

    All water-quality parameters for humic substances decrease significantly during the treatment processes (Figure 2), especially after alum coagulation-flocculation-sedimentation. NVTOC decreased to a lesser extent than THMFP and humic substances (as measured by color). This preferential removal of THMFP and humic substances corroborates the results of Babcock and Singer (23), who observed

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

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

  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 477

    Table I. General Characteristics of CWTP Source Water Parameters Concentrations0

    Dissolved oxygen 8.6 Color (Pt-Co unit) 28.0 pH 6.8 Alkalinity (mg/L as CaC0 3) 12.0 Hardness (mg/L as CaC0 3) 10.8 Turbidity (NTU) 27.0 Suspended solids 17.6 Dissolved solids 37.5 Fluoride 0.1 Sulfate 5.4 Phosphate 0.2 Nitrate 0.04 Aluminum 0.6 Iron 0.4 Calcium 2.5 Magnesium 0.9 Sodium 2.2 Potassium 1.4 Aldrin NF f c Chlordane NF DDT NF Dieldrin NF Endrin NF Heptachlor NF Heptachlor epoxide NF Lindane NF Methoxychlor NF Toxaphene NF NOTE: Data were obtained from plant records. Values presented are averages of two determinations sampled on October 5 and No-vember 5, 1984. 'In milligrams per liter. fcNot found.

    Table H . Humic Substances in CWTP Water Water Vol. of Water Humic Sampling

    Sampling Sampled Substances Dates Points (L) Isolated (g) (1984)

    SW 3102 3.99 Nov. 4-Nov. 15 ACFS 3143 2.92 Oct. 25-Nov. 3 AF 6467 4.79 Oct. 1-Oct. 14 CW 7164 4.94 Oct. 15-Oct. 24

    that alum coagulation selectively removed those portions of organic matter most responsible for T H M production. These observations allowed us to investigate more closely the types of humic substances removed by alum coagulation. The humic substances remaining after alum coagulation are ultimately chlorinated, and chlorination results in the formation of THMs.

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  • 478 AQUATIC HUMIC SUBSTANCES

    CO J < 5 g. 2

    in CM g < 3

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    2.11 1(0.2 6).

    0.018 |(0.0 0 5J

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    CW

    Figure 2. Monitoring the water-quality parameters of the CWTP during the water-sampling period. Values presented are daily averages and those in pa

    rentheses are standard deviations.

    Isolation of Humic Substances. The humic substances in water samples from the four CWTP sampling points were isolated as a hydrophobic acid fraction of dissolved organic matter by methyl methacrylate resin (XAD-8) adsorption procedures (24) from the concentrate of reverse osmosis (RO). The overall scheme for the isolation ofhumic substances from water samples is depicted in Figure 3. The organic matter in water samples was concentrated at the plant by using the membrane process facility shown in Figure 4, which was equipped with a spiral-wound composite membrane module for seawater desalination (FT-30, FilmTec Corp., Minneapolis, MN).

    Water samples were continuously pumped to the sample reservoir at the same flow rate as the RO permeate, and the result was a concentration of organic matter in the sample reservoir. The operating pressure and feed flow rate of the RO module were maintained at 4140-4830 kPa and 910 L/h, respectively. Sodium azide (0.5 mg/L) was added to the water samples to prevent algal and microbial growth, and a stoichiometrically determined amount of sodium sulfite was added to prevent possible oxidation of organic matter by residual chlorine. The initial sample, even-

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 479

    Water Samples

    \

    RO Concentrt ion of Organics

    tPermeates

    Aqueous Triethylamine S o l u t i o n (0.15 U)

    S o l u t i o n

    RO Concentrates

    V *

    F i l t r a t i o n on 0.45 um F i l t e r s

    J.PH 1.8 with conc-HCl

    XAD-8 Columns

    E f f l u e n t s

    Desorbed Humics

    I Vacuum Evaporation 1 and Desalting

    Glass F i l t e r

    L y o p h i l i z a t i o n

    Hur.ic Substanc

    Resin + ppt 10.15 NaOH

    V. 71. S o l u t i o n Resm Reapply t o XAD-8

    Column

    Figure 3. Overall scheme for isolation of humic substances from water samples.

    tually concentrated from approximately 3000-7000 L to 20-40 L, was transferred to an 80-L Pyrex glass bottle. The final RO concentrates were filtered through 0.45- membrane filters (Gelman Sciences, Ann Arbor, MI), acidified to pH 1.8 with HC1, and passed over XAD-8 resin columns to adsorb the humic substances.

    The filtered and acidified yellow-to-brown RO concentrates were pumped to two XAD-8 resin columns connected in series at 3 bed volumes per hour. Humic-substance adsorption was monitored by measuring the effluent absorbance at 420 nm. The humic substances adsorbed on XAD-8 resins were subsequently eluted with a 0.15 M aqueous triethylamine solution until no color was apparent in the eluates.

    Three different aqueous base solutions have been commonly used to desorb humic substances from XAD-8 resins. Mantoura and Riley (32) used 2 M ammonium hydroxide solution, Perdue (33) used 0.15 M triethylammonium hydroxide solution, and Malcolm and Thurman (24) used 0.1 M sodium hydroxide solution. All three base solutions are effective in desorbing humic substances from XAD-8 resins. However, triethylammonium hydroxide solution has advantages over the other two so-

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  • 480 AQUATIC HUMIC SUBSTANCES

    Bypass

    P e r m e a t e ^ C o n c e n t r a t e

    Water Sample N a N 2

    N a 2 S f c 3

    |0 5 4,140-4,830 kPa

    900 L/day

    1 - Sample Reservoir 2 - High-Pressure Pump 3 - Pressure and Flow

    Regulator

    4 - Flow Meter 5 - Pressure Gauge 6 - R O Module

    (FT-30)

    Figure 4. Schematic of reverse osmosis system.

    lutions with respect to minimal possible reactions that may alter the chemical structures of humic substances and the ease of removal of excess base.

    The excess triethylamine in the eluates was removed by slight concentration in a rotary vacuum evaporator. The resulting triethylamine salts of humic substances were desalted with 20-40-mesh cation-exchange resins ( + -form AG 50W x 8, Bio-Rad Labs, Richmond, CA) by a batch operation. The desalted humic substances were filtered through glass filters to remove the resins and lyophilized to yield brown fluffy powders, which were readily soluble in water.

    This isolation procedure provided an 81.5% recovery ofhumic substances after RO concentration of organic matter when tested with the humic substances isolated in this study. Hereafter the humic substances isolated from SW, ACFS, AF, and CW are referred as SW-HS, ACFS-HS, AF-HS, and CW-HS, respectively.

    A commercially available humic acid (sodium salt) was supplied by Aldrich Chemical Co. (Milwaukee, WI). One gram of sodium humate was purified by dissolving it in 2 L of distilled water and filtering it through 0.45- membrane filters. The sodium humate solution was desalted with cation-exchange resins. The resulting solution was lyophilized to yield a dark brown fluffy powder. The final yield was 0.627 g ofhumic acid (i.e., a 62.7% recovery). Hereafter the purified Aldrich humic acid is referred as AL-HA.

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 481

    Characterization of Humic Substances. Because humic substances are a heterogeneous mixture of many compounds with generally similar chemical properties, they need to be characterized on the basis of several chemical and physical properties. The characterization in this study included elemental composition, color, acidic functional groups, apparent-molecular-weight (AMW) distribution, and THMFP.

    ELEMENTAL ANALYSIS. The isolated humic substances were analyzed for C, H, and by an automatic analyzer. Halogen as chlorine was analyzed by combustion followed by subsequent titration. Ash content was analyzed by crucible combustion at 850 C. Acetanilide (C, , N) and p-chlorobenzoic acid (CI) were used as standard compounds for quality assurance purposes.

    SPECTROSCOPIC ANALYSIS. Visible and UV absorbances of humic substances were measured in aqueous solution on a spectrophotometer (model 26, Beckman Instruments, Fullerton, CA) with 1-cm quartz cells. The color intensity ofhumic substances was expressed as the absorbance at 420 nm at pH 10 by adjusting with 0.1 M NaOH (Am)y and UV absorbance was measured at 254 nm without pH adjustment (A 254). IR spectra were recorded on a spectrophotometer (AccuLab 6, Beckman Instruments) by using KBr pellets.

    ACIDIC FUNCTIONAL GROUPS ANALYSIS. Carboxylic groups of humic substances were determined by direct titration (25). Accurately weighed samples of approximately 10 mg were dissolved in 100 mL of aqueous 0.1 M NaCl. The solution was titrated with 0.1 NaOH by using a digital microburet (Gilmont). A reagent blank was titrated parallel to the sample. Net titration curves were derived by subtracting reagent blank titration data from the sample titration. The carboxylic groups were calculated from the net titration curves at pH 7.0.

    Total acidity was determined by the barium hydroxide method (18), Thirty milliliters of 0.5 M barium hydroxide was reacted with 10-20 mg ofhumic substances for 24 h at room temperature. The solution was filtered through a UM05 membrane ultrafilter. The filtrates were then titrated with 0.1 N HC1. All operations were performed under nitrogen atmosphere and a blank was determined simultaneously. The total acidity was calculated as follows:

    total acidity (meq/g) = [ t i t e I ^ ~ "ter^J x x 1000 milligrams of sample

    where titer is volume (mL) of 0.1 N HC1 consumed to titrate the filtrate to pH 8.4 and ^ is normality of the standardized HC1 solution used for titration. The weak acidic groups were calculated as the difference between total acidity and carboxylic groups.

    AMW DISTRIBUTION. Both gel permeation chromatography (GPC) and membrane ultrafiltration (UF) were used to determine the AMW distribution of humic substances. GPC provided an elution chromatogram that enabled comparison of the AMW distribution of humic substances; UF gave discrete AMW fractions based on the molecular weight cutoff values of specific membranes.

    The AMW distribution by GPC was obtained by using a gel (Sephadex G-25, Pharmacia Fine Chemicals, Piscataway, NJ) with a MW exclusion limit of 500-5000 daltons by dextrans. A 1.25-cm i.d. glass column filled with preswollen gel to a bed height of 36 cm was used. A 0.5-mL sample (2.5 mg ofhumic substances) was placed on the column and eluted with a 0.01 M NaCl solution at a flow rate of 0.5 mL/min. The effluent from the column was connected to the UV detector to

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  • 482 AQUATIC HUMIC SUBSTANCES

    monitor absorbance at 254 nm. The column was calibrated with phenol (MW 94), polypropylene (MW 1000), polystyrene (MW 3600), and dextran blue (MW 2,000,000).

    Fractionation of humic substances was achieved by using two UF membranes (XM50 and UM10, Diaflo Membranes, Amicon, Lexington, MA). The XM50 and UM10 membranes have reported MW cutoff values of 50,000 and 10,000, respectively. The retention of any solute by a specified membrane can be expressed by the local rejection coefficient (R).

    = In (Cf/C0) In (V./Vf)

    where V 0 and V> are the volumes of original solution and retentate, and C D and Cf are the solute concentrations in original solution and retentate, respectively. For solute entirely excluded by the membrane, R is equal to 1; for freely permeable solute, the value is 0.

    Approximately 400 mg ofhumic substances was dissolved in 200 mL of distilled water and fractionated by XM50 and UM10 membranes in series. The solutions were concentrated by fourfold (v/v) with XM50 membrane under nitrogen in an Amicon model TCF10 cell at a pressure of 240 kPa. Repetitive dilution and concentration of the retentates were performed for maximal separation until R became 90% or more. The humic substances in the resulting retentate of the XM50 membrane are defined as an AMW fraction of >XM50 (MW >50,000). The composite XM50 filtrates were further concentrated with UM10 membrane according to the procedures used for the XM50 membrane. The UM10 retentate of the XM50 filtrates is defined as an AMW fraction of UM10-XM50 (10,000 < MW < 50,000) and the composite UM10 filtrates of the XM50 filtrates is defined as

  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 483

    Coagulation was performed by using a conventional six-place jar-test apparatus (Phipps & Bird, Richmond, VA) with 1-L beakers. Suspensions with humic-substance concentrations of 5 and 10 mg/L (2.5 and 5 mg of C / L , respectively) and kaolin concentration of 50 mg/L were prepared in distilled water containing 5 X 10 3 M NaHC0 3 . The pH was adjusted by addition of HC1 and NaOH so that subsequent addition of the coagulant would result in the desired final pH value.

    The coagulant was added during a rapid-mix period of 1 min at 100 rpm. This was followed by a slow-mix period at 25 rpm for 30 min and gravity settling for 30 min. After settling, residual turbidity was measured by an analytical nephelometer (model 2100A, Hach Chemical Corp., Ames, IA). The residual concentrations of humic substances in the settled solutions were determined by UV absorption at 254 nm after filtration with a 0.45- membrane filter.

    Results and Discussions

    Characteristics of Humic Substances. The results of the characterization of the humic substances isolated from the four sampling points of the C W T P and A L - H A are summarized in Table II. The concentration of dissolved organic carbon (DOC) in the C W T P source water was 2.1 mg/L, of which approximately 30% was humic substances. The concentrations of D O C and humic substances were decreased through the C W T P treatment process.

    The elemental compositions of the humic substances isolated from the C W T P were similar (i.e., 56.2-56.9% C, 6.0-6.1% H , 0.8-1.0% N , and 36.2-36.7% O). The values were comparable with those of surface-water humic substances (6, 20, 27), but were different from those of A L - H A and soil humic substances (28).

    Color and T H M F P ofhumic substances are the important parameters in evaluating water quality. In this study, color equals 50 X absorbance at 420 nm at p H 10. The color and T H M F P of S W - H S were 0.051 and 67.8 g of T H M / m g of humic substances (i.e., 135.6 g of T H M / m g of TOC), respectively, and were similar to those of surface-water humic substances (6). Ranges or surface-water humic substances are 0.092-0.137 for color (measured at 400 nm at p H 7 and the same unit) and 36.0-77.5 g of T H M / m g of humic substances for T H M F P . The values also decreased through the treatment processes of the CWTP. However, A L - H A showed approximately 7 times higher in color and 2 times higher in T H M F P , as compared with S W - H S .

    The existence of acidic functional groups in humic substances is evidenced by IR spectra shown in Figure 5. The carboxylic groups are responsible for the strongest IR adsorption features: an intensive broad-bend centered at 1720 cm" 1 (C = stretching) and a weak absorbance at 2500-2700 cm" 1 ( C - 0 stretching or O - H deformation). A broad and intensive absorption centered at 3400 cm" 1 indicates weak acidic groups ( O - H stretching of -bonded hydroxyls).

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 485

    Quantitative estimation of the carboxylic groups was made by the net titration curves shown in Figure 6. From the shape of the net titration curves, it is apparent that the acidic functional groups are titrated beyond p H 10. Although the curves did not have sharp inflection points, carboxylic groups were simply estimated at p H 7 because the curves were highly linear within the p H 7-9 intervals.

    S W - H S contained 3.8 and 2.6 meq/g of carboxylic and weak acidic groups (gram basis of humic substances), respectively, which are similar to

    3 5 7 9 11

    pH

    Figure 6. Net titration curves of humic substances from the four sampling points of the CWTP.

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  • 486 AQUATIC HUMIC SUBSTANCES

    those of surface-water humic substances (20). There are no significant differences between the humic substances isolated from the four sampling points of the CWTP, as shown in Table HI; however, A L - H A contained a lower acidic functional group content than S W - H S . The majority of the acidic functional groups of A L - H A were weak acidic functional groups.

    One of the most important characteristics governing the physicochem-ical properties ofhumic substances is their molecular weight. G P C and U F were used to estimate the molecular weight of humic substances. Because both techniques separate the humic substances based on molecular size rather than molecular weight, the results are expressed as A M W distribution. The G P C chromatograms of the humic substances from the C W T P are i l lustrated in Figure 7. The U V absorbance responded proportionally with the molecular weight of humic substances (Table III), and thus the chromatograms represent only a relative distribution as a function of apparent molecular weight. As in most G P C studies of humic substances (27, 29), there are generally two peaks in the chromatograms: M W >5000 (which is totally excluded by the column) and 1000 < M W < 5000 (which was totally included by the column).

    S W - H S showed the following A M W distribution: 30.3% in >XM50 ( M W >50,000), 51.2% in U M 1 0 - X M 5 0 (10,000 < M W < 50,000) and 18.5% in < U M 1 0 ( M W X M 5 0 fraction; this distribution indicates that A L - H A are composed of higher-molecular-weight compounds than S W - H S .

    The U F was reasonably successful in fractionating humic substances on the basis of their molecular weights, as evidenced by the G P C chromatograms of S W - H S and their three A M W fractions (Figure 8). However, the M W values assigned for each fraction of S W - H S by the U F method are higher than those assigned by the G P C method. The >XM50 ( M W >50,000), U M 1 0 - X M 5 0 (10,000 < M W < 50,000), and < U M 1 0 ( M W 5000, 2000 < M W < 5000, and 1000 < M W < 5000 fractions by the G P C method, respectively. Similar observations were reported by Thurman et al. (30), who concluded that the M W values determined by the U F method for humic substances are 2 to 10 times higher than those shown by the G P C method.

    Because the three A M W fractions of S W - H S obtained by U F were used in the jar-test experiments, they were also characterized, and the results are shown in Table III. Trends linking higher molecular weight with higher color and T H M F P and lower acidic functional groups were observed. No intuitively obvious reason exists for these correlations, and more studies are needed to explain these phenomena.

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  • Tab

    le I

    II. C

    hara

    cter

    istic

    s an

    d R

    emov

    al B

    ehav

    ior

    of H

    umic

    Sub

    stan

    ces

    THM

    FP1

    Aci

    dic

    Fun

    ctio

    nal

    AMW

    Dis

    trib

    uti

    on

    Hum

    ic

    DO

    C E

    lem

    enta

    l Com

    posi

    tion

    {%

    f C

    olor

    "

    (ug

    THM

    )/ G

    rou

    ps (

    meq

    /g)

    by U

    F (%

    y S

    ubs

    tan

    ces

    {mg

    CIL)

    C

    H

    Ash

    50

    - L

    /mg

    - cm

    (m

    gHS

    ) C

    OO

    H

    Ph

    enol

    ic O

    H >

    XM

    50

    UM

    10-X

    M50

    X

    M50

    0.

    117

    79.9

    3.

    3 2.

    4 U

    M10

    -XM

    50

    0.02

    9 72

    .9

    3.7

    2.5

  • 488 AQUATIC HUMIC SUBSTANCES

    Sephadex G-25 Eluant: 0.01 M NaCl

    CW-HS

    /

    //"\ .5000 3600 2000 1000 500

    Apparent Molecular Weight

    Figure 7. GPC chromatograms of humic substances from the four sampling points of the CWTP.

    Removal Behavior of Humic Substances in the CWTP. Figure 9 illustrates the removal behavior of humic substances in the CWTP. The treatment processes decreased the concentration of D O C from 2.1 to 1.5 mg of C / L and the concentration of humic substances from 0.9 to 0.5 mg/L. These concentration changes indicate that the plant is able to remove some D O C , including humic substances. Overall, 44% of humic substances were removed (i.e., from 0.9 to 0.5 mg of C / L ) through the treatment processes of the CWHTP. Specifically, 74% of the >XM50 fraction (i.e., from 30.3% of 0.9 mg of C / L to 14.1% of 0.5 mg of C / L ) and 55% of the U M 1 0 - X M 5 0 fraction (i.e., from 51.2% of 0.9 mg of C / L to 41.9% of 0.5 mg of C / L ) were removed, but the

  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 489

    >

    Sephadex C-2b Eluant: 0.01 ! Naci

    * *N. / ,

    XM50

    SW-HS / I 1 1

    >b000 3600 2000 1000 t>00 Apparent Molecular Weight

    Figure 8. GPC chromatograms of SW-HS and their three AMW fractions by

    UF

    in the acidic functional group content may have resulted from oxidation reactions of chlorine with humic substances.

    The data clearly demonstrated not only that humic substances are removed, but also that their characteristics are affected by C W T P treatment processes. The preferential removal of the high-molecular-weight fraction of humic substances through the C W T P treatment processes resulted in substantial decreases in color intensity and T H M F P per unit mass ofhumic substances. This removal behavior is attributed to the coagulation-flocculation-sedimentation, filtration, and prechlorination practiced at plant-intake and rapid-mix units. However, alum coagulation is considered to be the major process responsible for this removal because alum coagulation of S W - H S provided similar removal behavior of humic substances observed in the CWTP.

    This finding has a special significance in water-treatment practice. Because the humic substances remaining after coagulation are chlorinated, the performance of alum coagulation in removing humic substances can be an important variable in the reduction of T H M s in finished drinking water. Therefore, proper operation of the water-treatment plant for maximal removal of high-molecular-weight fractions of humic substances by alum co-

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  • 490 AQUATIC HUMIC SUBSTANCES

    <

    I

    c

    ; j < ~

    J

    r w 0.03

    u 100

    CM 10 - X>*.50

  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 491

    settling periods caused the p H to decrease up to 0.2 unit at the highest aluminum dose (0.2 meq/L) at initial p H 6.5. No efforts were made to keep the p H constant during the experiments.

    REMOVAL CHARACTERISTICS. A typical set of results obtained from alum coagulation of humic substances is shown in Figure 10. S W - H S were removed gradually, with increasing aluminum dose approaching a maximum of approximately 40% removal at high aluminum dosages. The removal behavior of A L - H A was quite different from that of S W - H S . At a 0.06 meq/L aluminum dose the removal of A L - H A is trivial, but removal is essentially complete at higher dosages ( >0.1 meq/L).

    This differential removal behavior between S W - H S and A L - H A may be caused by their different characteristics, especially A M W distribution. Most of A L - H A were found in the >XM50 fraction, although only 30.3% of S W - H S were found in that fraction. To test this hypothesis, the three A M W fractions of S W - H S were coagulated; the results are shown in Figure 11. Although the >XM50 fraction of S W - H S was not removed to the same degree as A L - H A , it was removed by 62%. In contrast, U M 1 0 - X M 5 0 and

  • 492

    1 0 0

    8 0

    e a ce

    1 6 0 9 4 0

    CO

    s 0 5 2 0 -

    AQUATIC HUMIC SUBSTANCES

    [ t ; t > [ t ; ; t .

    Concenlrotion 5 mg/L SW-HS >XM50 UM10-XM50 I I 0

    0 . 0 0 0 0 . 0 8 0 0 . 1 6 0 0 . 2 4 0 0 . 3 2 0

    Aluminum Dose (meq/L)

    Figure 11. Removal characteristics of SW-HS and their AMW fractions by alum coagulation at initial pH 6.5.

    EFFECTS OF TURBIDITY. The results of alum coagulation experiments on solutions containing 5 mg/L of S W - H S or A L - H A together with 50 mg/L of kaolin (35 NTU) are shown in Figure 12. The removal of humic substances in the presence of kaolin resembled their removal in the absence of kaolin, except for a slight increase in aluminum dose requirement for an equivalent removal ofhumic substances. In contrast, the presence ofhumic substances considerably increased the aluminum dose required for kaolin removal.

    Although the behavior of kaolin is somewhat different from that of natural turbidity, the results are of practical significance. Because alum coagulation has long been operated to maximize turbidity removal, a proper improvement of the process may increase the removal of humic substances, thus reducing the color and T H M in finished drinking water.

    EFFECTS OF pH AND STOICIUOMETRY. Because the maximum removal of S W - H S obtained with alum coagulation was 50%, the 30% removal areas of S W - H S by alum coagulation at various p H values are shown in Figure 13 in an Al(III) stability diagram based on the thermodynamic equilibria of Al(III) hydrolysis products (14). The optimum aluminum dose and p H for the removal of 5 mg /L S W - H S were approximately 0.02 meq/L (2 m g / L as alum) at p H 6 and those of 10 m g / L were 0.04 meq/L (4 mg /L as alum)

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 493

    ( > ' I I J I I I I J I

    Koolin(5 NTU) only Koolin V SW-HS 5 mg/L Kaolin AL-HA 5 mg/L

    i .*!~. . . . I . I I I I I I I I I i I I I I I I i t I I 1 I I t I

    0.000 0.040 0.080 0.120 0.160 0.200

    Aluminum Dose (meq/L)

    Figure 12. Effect of kaolin on the removal of humic substances by alum coagulation at initial pH 6.5.

    at p H 5.5. In general, the 30% removal areas follow a trend toward higher aluminum dosages and lower p H as the concentration of S W - H S increases.

    A stoichiometric relationship was observed between the concentration of S W - H S and the aluminum dose required to initiate the removal of S W - H S . This stoichiometry is consistent with the following observations. S W - H S appear to form a complex with aluminum so that Al(OH) 3(s) cannot form until the aluminum dose exceeds the available complexation sites of S W - H S . S W - H S contain 3.8 meq/g of caboxylic groups and 2.6 meq/g of weak acidic groups. If only the carboxylic groups are ionized at p H 6.5 and if an initial concentration of S W - H S of 5 mg/L is used, at least a 0.02-meq/L dosage of aluminum will be required to initiate the formation

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  • 494 AQUATIC HUMIC SUBSTANCES

    Figure 13. The 30% removal areas of SW-HS by alum coaguhtion.

    of Al(OH) 3(s). This dosage corresponds precisely to the aluminum dosage required for the removal of S W - H S shown in Figure 13.

    The 50% removal areas of A L - H A by alum coagulation are shown in Figure 14, which substantiates the differences between the removal behavior of A L - H A and that of S W - H S . Two distinct 50% removal areas were observed for 5 m g / L of A L - H A ; the two removal areas were connected to

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 495

    3 4 5 6 7 8 9 10

    P H Figure 14. The 50% removal areas of AL-HA by alum coagulation.

    form a single band with an initial A L - H A concentration of 10 mg/L. These removal areas of A L - H A correspond with the regions of removal observed in the literature (14, 31).

    Considerable insight into the removal of humic substances has been gained with studies of A L - H A . However, caution must be exercised in extending the results to water-treatment practice because the characteristics and removal behavior of A L - H A are significantly different from those of S W - H S .

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  • 496 AQUATIC HUMIC SUBSTANCES

    Conclusions Humic substances in the Chattahoochee River, which exhibited characteristics similar to those found in many surface waters, were not only removed but also transformed physically and chemically through the treatment processes of the Chattahoochee Water Treatment Plant. The major process responsible for this removal was alum coagulation. The alum coagulation process was effective in removing high-molecular-weight fractions of humic substances, and the result was substantial decreases in the color intensity and T H M F P ofhumic substances.

    The characteristics of humic substances are the important factors influencing the performance of the alum coagulation process in removing both turbidity and humic substances. The concentration of carboxylic groups of humic substances determined the alum dose necessary to bring about their removal in the process. The optimum coagulation conditions for both humic substances and turbidity removal were dictated by the presence of humic substances.

    Caution must be exercised in extending the results of the studies with Aldrich humic acid to water-treatment practice because commercial humic substances differ significantly from natural-water humic substances, both in characteristics and in removal behavior in alum coagulation.

    References 1. Reuter, J. H.; Perdue, . M. Geochim. Cosmochim. Acta 1977, 41, 325. 2. Buffle, J.; Tessier, .; Haerdi, W. In Complexation of Trace Metals in Natural

    Waters; Kramer, C. J. M.; Duinker, J. C., Eds.; Martinus Nijhoff/Dr W. Junk Publishers: The Hague, 1984; pp 301-316.

    3. Gjessing, E. T.; Berlind, L. Arch. Hydrobiol. 1981, 92, 24. 4. Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16, 735. 5. Rook, J. J. Water Treat. Exam. 1974, 23, 234. 6. Oliver, R.; Thurman, . M. In Water Chlorination: Environmental Impact and

    Health Effects; Jolley, R. L . ; Brungs, W. .; Cortruvo, J. .; Gumming, R. B.; Matrice, J. S.; Jacobs, V. ., Eds.; Ann Arbor Science: Ann Arbor, 1983; Vol. 4, p 231.

    7. Symons, J. M.; Bellar, . .; Carswell, J. K.; Demarco, J.; Kropp, K. L.; Robeck, G. G.; Seeger, D. R.; Slocum, C. J.; Smith, B. L.; Stevens, A. A. J. Am. Water Works Assoc. 1975, 67, 634.

    8. Carcinogenesis Bioassay of Chloroform; National Cancer Institute: Washington, DC, 1976.

    9. Fed. Regist. National Interim Primary Drinking Water Regulations; Control of Trihalomethanes in Drinking Water; Final Rule, Vol. 44, 1979; No. 231.

    10. Hall, E. S.; Packham, R. F. J. Am. Water Works Assoc. 1965, 57, 1149. 11. Narkis, N.; Rebhun, M. J. Am. Water Works Assoc. 1975, 67, 101. 12. Kavanaugh, M. C. J. Am. Water Works Assoc. 1978, 70, 613. 13. Edzwald, J. K. AIChE Symp. Ser. 1979, 75, 54. 14. Amirtharajah, .; Mills, K. J. Am. Water Works Assoc. 1982, 74, 210.

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  • 29. KIM ET AL. Humic-Substance Removal Behavior in Water Treatment 497

    15. Dempsey, . .; Ganho, R. M.; O'Melia, C. R. J. Am. Water Works Assoc. 1984, 76, 141.

    16. Collins, M. R.; Amy, G. L.; King, P. H. J. Environ. Eng. (.Y.) 1985, 111, 850. 17. Sinsabaugh, R. L.; Hoehn, R. C.; Knocke, W. R.; Linkins, A. E. J. Environ.

    Eng. (.Y.) 1986, 112, 139. 18. Schnitzer, M.; Kahn, S. U. Humic Substances in Environment; Marcel Dekker:

    New York, 1972. 19. Chian, E. S. K.; Giabbai, M. F.; Kim, J. S.; Reuter, J. H.; Kopfler, F. C. In

    Organic Pollutants in Water; Suffet, I. H.; Malaiyandi, M., Eds.; Advances in Chemistry Series 214; American Chemical Society: Washington, DC, 1987; p 181.

    20. Malcolm, R. L. In Humic Substances in Soil Sediment, and Water; Aiken, G. R.; McKnight, D. M.; Wershaw. R. L.; MacCarthy, P., Eds.; Wiley: New York, 1985; p 181.

    21. Malcolm, R. L.; McCarthy, P. Environ. Sci. Technol. 1986, 20, 904. 22. Edzwald, . K. In Control of Organic Substances in Water and Wastewater;

    Berger, . ., Ed.; Office of Research and Development, U.S. Environmental Protection Agency: Washington, DC, 1983; p 26.

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    -chem. 1982, 4, 27. 31. Mangravite, F. J., Jr.; Buzzell, T. D.; Cassell, . .; Matijevic, E.; Saxton, G.

    B. J. Am. Water Works Assoc. 1975, 67, 88. 32. Mantoura, R. F. C.; Riley, J. P. Anal. Chim. Acta 1975, 76, 97. 33. Perdue, . M. In Chemical Modeling in Aqueous Systems; Jenne, . ., Ed.;

    ACS Symposium Series 93; American Chemical Society: Washington, DC, pp 94-114.

    RECEIVED for review March 24, 1988. ACCEPTED for publication September 9, 1988. Dow

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    29 Characteristics of Humic Substances and Their Removal Behavior in Water TreatmentExperimental Materials and MethodsResults and DiscussionsConclusionsReferences

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