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Water Research 39 (2005) 3849–3862

www.elsevier.com/locate/watres

Coagulation of humic substances and dissolved organic matterwith a ferric salt: An electron energy loss spectroscopy

investigation

A.-V. Junga,�, V. Chanudeta, J. Ghanbajab, B.S. Lartigesa, J.-L. Bersillona

aLaboratoire Environnement et Mineralurgie (LEM), UMR 7569 CNRS-INPL-ENSG, 15 avenue du Charmois, BP 40,

54501 Vandæuvre, FrancebService commnun d’analyses (MET), Universite Nancy I, BP 239, 54 506 Vandæuvre, France

Received 5 February 2005; received in revised form 7 July 2005; accepted 11 July 2005

Available online 19 August 2005

Abstract

Transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) and energy

dispersive X-ray spectroscopy (EDXS) was used to investigate the coagulation of natural organic matter with a ferric

salt. Jar-test experiments were first conducted with a reconstituted water containing either synthetic or natural extracts

of humic substances, and then with a raw water from Moselle River (France). The characterization of the freeze-dried

coagulated sediment by EELS in the 250–450 eV range, showed that Fe-coagulant species predominantly associate with

the carboxylic groups of organic matter, and that this interaction is accompanied by a release of previously complexed

calcium ions. The variation of Fe/C elemental ratio with iron concentration provides insightful information into the

coagulation mechanism of humic substances. At acid pH, Fe/C remains close to 3 over the whole range of iron

concentrations investigated, while a much lower atomic ratio is expected from the value of optimal coagulant dosage.

This suggests that a charge neutralization/complexation mechanism is responsible for the removal of humic colloids, the

aggregates being formed with both iron-coagulated and proton-neutralized organic compounds. At pH 8, the decrease

in Fe/C around optimal coagulant concentration is interpreted as a bridging of stretched humic macromolecules by Fe-

hydrolyzed species. Aggregation would then result from a competition between reconformation of humic chains around

coagulant species and collision of destabilized humic material. EELS also enabled a fingerpriting of natural organic

substances contained in the iron-coagulated surface water, N/C elemental analyses revealing that humic colloids are

removed prior to proteinic compounds.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Coagulation; Humic substances; Natural organic matter; Electron energy loss spectroscopy (EELS)

e front matter r 2005 Elsevier Ltd. All rights reserve

atres.2005.07.008

ing author. Private domicile, 84 chemin du

350 Brunstatt, Brunstatt, France.

2464105; fax: +0033 3 89647787.

ess: av1jung@hotmail.com (A.-V. Jung).

1. Introduction

Dissolved organic matter (DOM) represents the

heterogeneous mixture of organic colloids (excluding

living organisms) and truly dissolved organic molecules

present in aquatic systems. It comprises biochemically

defined compounds such as proteins and carbohydrates,

d.

ARTICLE IN PRESSA.-V. Jung et al. / Water Research 39 (2005) 3849–38623850

operationally defined substances such as fulvic and

humic acids, and any supramolecular assembly of those

molecules and macromolecules (Leenheer and Croue,

2003). DOM has largely been studied in recent years, as

it may play a major role in the mobility and bio-

availability of anthropic contaminants in the environ-

ment (Thurman and Malcolm, 1981; Tipping, 1993). In

drinking water treatment, the main motivation for

studying DOM stems from its role as precursor of

carcinogenic chlorination by-products during the disin-

fection stage (Oliver and Visser, 1980). However, most

problems encountered in the treatment process can be

related to the presence of DOM, including reduced

adsorption capacity in activated carbon beds, taste and

odour of the finished water, and biological regrowth in

the distribution networks (Owen et al., 1995).

The removal of DOM from water supplies is carried

out in the first place by coagulation-flocculation with a

hydrolyzable metal salt such as aluminum or iron

(Dempsey et al., 1984; Edwards and Amirtharajah,

1985; Gregor et al., 1997). When added to water,

aluminum and iron coagulants yield hydrolysis products

that destabilize the organic material present in the raw

water. The destabilization mechanisms most often

quoted in the literature include charge neutralization,

enmeshment, and adsorption (Dempsey et al., 1984).

Charge neutralization preferentially applies at acidic

pH, while sweep-flocculation and adsorption are com-

monly referred to in pH-coagulant concentration

domains where abundant precipitation of metal hydro-

xide is expected (Edwards and Amirtharajah, 1985).

However, DOM-specific characteristics, such as its

molecular weight distribution, carboxylic acidity, and

humic substance content, have been shown to signifi-

cantly influence the efficiency of DOM removal (Collins

et al., 1985; Bose and Reckhow, 1998; Matilainen et al.,

2002). Recent extended X-ray absorption fine spectro-

scopy (EXAFS) at the Fe K-edge experiments also

questioned the coagulation mechanisms usually invoked

in the literature, as poorly polymerized Fe species were

evidenced in a wide range of operational conditions

(Vilge-Ritter et al., 1999a, b; El Samrani et al., 2004).

As an optimized DOM coagulation is a key to better

drinking water treatment, the interaction behavior

between organic compounds and coagulant species

should be better understood. The approach taken in

this work was to investigate, within flocs, both the

nature of DOM and the speciation of an iron-based

coagulant by using electron energy loss spectroscopy

(EELS) combined with energy dispersive X-ray spectro-

scopy (EDXS). EELS provides a quantitative analysis at

the nanometer scale of light elements (carbon, nitrogen,

oxygen) through the K-shell excitation, and of other

important elements in water treatment such as iron,

calcium, phosphorus, and sulfur, through the L-shell

excitation. It also enables the identification of organic

chemical bonds within polymeric systems (Varlot et al.,

1998), and can then be used to fingerprint the various

organic macromolecules contained in coagulated organ-

ic matter. In environmental science, EELS was first used

by biochemists to describe in situ natural materials

degradation with C/N ratio measurements (Villemin et

al., 1995; Watteau et al., 1996). It has also been used to

investigate the association of organic matter with clay

particles (Furukawa, 2000).

In this paper, we present the results obtained by C-

EELS. Fe-EELS results will be reported in a companion

paper (Jung et al., 2005a). To avoid problems associated

with changes in the quality of the river water, experi-

ments were first carried out with a reconstituted river

water containing either humic-like synthetic compounds

or natural aquagenic humic extracts. In this way, the

influence of pH on humics removal could be investi-

gated. The method was then assessed on raw Moselle

river water.

2. Experimental section

2.1. Origin of natural organic matter (NOM) and

extraction of humic substances

Three sources of organic matter were selected for this

study. A synthetic humic-like substance (SHS) was

prepared by polymerization of catechol with glycine

(Andreux et al., 1980; Jung et al., 2005b). Catechol

(6.6 g) was dissolved in 500mL of a solution of sodium

phosphate buffer NaH2PO4/Na2HPO4 at pH 8 to obtain

a 0.03M catechol solution. This was then mixed with an

equal volume of buffered glycine 0.03M solution. The

resulting mixture was then allowed to react in a 2L flask

in the dark at 25 1C and under a pure oxygen constant

pressure (0.9 bar) for 5 days. After dialysis, (Spectra/

Pors 7 membrane- 8 kDa molecular weight cut-off—

Roth-Sochiel Lauterbourg, France), the synthetic mate-

rial in a sodium form was freeze-dried and stored at 4 1C

in the dark. Extensive characterization of SHS and

relevance to natural humic substances have been

discussed in a previous work (Jung et al., 2005b).

Elemental analysis gave a carbon to nitrogen ratio (C/N)

of 18.3 (%C w/w ¼ 40.7). Potentiometric titration

showed three acidities at pKA ¼ 4.6 (2.3meq/g), 5.1

(0.8meq/g) and 7.2 (0.6meq/g).

A natural aquagenic humic substance, hereafter

referred to as NHS, was extracted from the Moselle

river suspended matter. The sampling site was located at

Richardmenil (France) in the vicinity of the pumping

facility supplying Grand Nancy Urban Community

drinking water treatment plant in surface water.

Approximately, 12m3 of river water were pumped

(1m3/h) into a flow-through centrifuge operating at

17,000 rpm. The humic substances contained in the

ARTICLE IN PRESSA.-V. Jung et al. / Water Research 39 (2005) 3849–3862 3851

suspended material were then extracted following the

procedure of Thurman and Malcolm (1981), adapted to

yield the natural extract in a sodium form. NHS was

also purified by dialysis (Spectra/Pors 7 membrane—

8kDa molecular weight cut-off) and freeze-dried.

Total exchangeable protons and C/N ratios determined

for NHS were 3.0meq/g and 9.2 (%C w/w ¼ 44.2),

respectively.

Grab samples of the Moselle river water (9.3mg/L

total organic carbon content) were also taken at the

sampling site, thus providing a raw water with

unfractionated NOM.

2.2. Coagulation procedure

Synthetic waters were prepared by dissolving 30mg of

humic material (either SHS or NHS) in 1L of Milli-Qs

ultra-pure water. 336mg of NaHCO3 (4� 10�3mol/L)

(Sigma-Aldrich—analytical grade) was also added to the

suspension to provide a carbonate alkalinity similar to

that of natural river waters. Prior to coagulant injection,

the pH of synthetic waters was adjusted to pH 6 or 8

with dropwise addition of 0.1M HCl. The surface water

was used as collected (natural pH ¼ 7.4). Jar-tests were

performed in 1L baffled reactors of known power

dissipation characteristics (Lartiges et al., 1997). Suspen-

sion was stirred at 100 rpm which corresponds to a mean

velocity gradient of 135 s�1. The coagulant, a 0.1M

ferric nitrate solution prepared from crystallized

Fe(NO3)3, 9 H2O (95% pure—Sigma-Aldrich), was

injected under agitation using a 5000mL micro-pipette

(Eppendorf, Hamburg, Germany), and mixing was

continued for 20min. The suspension pH was not re-

adjusted after coagulant addition. After 24 h of quies-

cent settling in graduated Imhoff cones, the residual

turbidity of the supernatant was measured on a HACH

Ratio XR turbidimeter. The settled volume was

recorded, and the sediment was then collected and

freeze-dried (48 h—EL105 Sentry) to perform TEM

observations, EDXS and EELS analysis.

2.3. Transmission electron microscopy (TEM)

Electron microscopy observations were performed

with a CM20 Philips transmission electronic microscope

(TEM) using a 200 kV accelerating voltage. The electron

microscope is coupled with an EDAX EDXS, and with a

GATAN 666 EELS. The freeze-dried sediment was re-

suspended in ethanol (99.5% spectroscopy grade) under

ultrasonication, and a drop of suspension was evapo-

rated on a carbon network-like holey support film

placed on a 200mesh copper grid (Euromedex—

Mundolsheim, France). EDX spectra were recorded

with a 10 nm spot diameter and a 30 s counting time.

EELS spectra were collected in the 250–450 eV region

with the same spot size (analyzed area of about 80 nm2).

The spectrometer entrance aperture, the half-angle of

collection and the dispersion were 2mm, 10mrad, and

0.2 eV/channel, respectively. The energy resolution,

determined at the full-width at half-maximum of the

zero-loss peak is 1.4 eV. The time acquisition was 2.5 s

for the low energy region and 8 s for the C K-edge

region. The relative thickness (ratio of sample thickness

to the free electronic inelastic diffusion pathway), was

kept about 0.570.2 for all measurements. The electron

dose, calculated from the Egerton relationship (Egerton,

1996), was about 101.9C/m2 which is less than the

critical value-inducing beam damage within organic

polymers (Varlot et al., 1998).

For each sample, 15 EEL spectra were recorded. All

data processing was carried out with GATAN EL/P 2.1

software. The spectra were first corrected for dark

current, gain variations, and double excitation by

applying a Fourier-ratio deconvolution with weak loss

energy spectra. Background fitting was performed with a

power law in order to extract the core-excitation peaks

for each element. The amounts of C, Ca, and N were

then obtained by integrating the surface area of a 50 eV

energy window after having defined the lower-limit in

energy for each element, that is 283.8, 346.4 and

401.6 eV for carbon (C K-), calcium (Ca L2,3-) and

nitrogen (N K-) edges, respectively. Integration results

are given by the software in atomic concentration/nm2.

Hence, Ca/C and N/C atomic ratios were directly

calculated from EELS measurements, whereas Na/C

and Fe/C ratios were inferred from the amounts in Na,

Ca, and Fe, provided by EDXS microanalyses. When

Ca was barely detectable on EEL spectra, which is the

case for most coagulation experiments carried out at pH

6, Fe/C and Na/C ratios obtained from EDXS spectra

were corrected by a proportional constant determined

from EELS and EDXS results at pH 8.

3. Results

3.1. Jar-test results

Fig. 1 presents the results obtained from jar-tests

carried out at pH 6 and 8 with SHS and NHS. All

turbidity and sediment volume curves display a classical

behavior: the residual turbidity steadily increases at low

coagulant concentration to reach a maximum, and then

steeply decreases upon further addition of ferric

coagulant. The point where the extrapolated steep

portion of the turbidity curve intersects the x-axis is

selected as the optimum coagulant concentration

(OCC), and is indicated with an arrow on the graphs.

At higher iron concentrations, the residual turbidity

remains low and then strongly increases as the

restabilization of the suspension is induced. The inverse

pattern is observed for sediment volume: at low iron

ARTICLE IN PRESS

Iron concentration (M)

0

2

4

6

8

10

12

14

16

18

0

5

10

15

20

25

0

2

4

6

8

10

12

0

10

20

30

40

50

60

0 2 10-4 4 10-4 6 10-4 8 10-4 10-3

SHS pH 8 SHS pH 6

NHS pH 8 NHS pH 6

(a)

(b)

(c)(d)

(e)

(f)(g)

(h)

(i)

(j)

(k)

(l)

(m) ( n)

(o)

(p)

(q)

Iron concentration (M)

0 2 10-4 4 10-4 6 10-4 8 10-4

Iron concentration (M)

0 2 10-4 4 10-4 6 10-4 8 10-4 10-3

Iron concentration (M)

0 4 10-4 8 10-4 1.2 10-3

Fig. 1. Evolution of residual turbidity (K) and sediment volume (’) with coagulant concentration for NHS and SHS at pHs 6 and 8.

Arrows indicate optimal coagulant concentrations. The letters refer to freeze-dried sediment examined by TEM.

A.-V. Jung et al. / Water Research 39 (2005) 3849–38623852

concentration, no settleable aggregate is observed up to

the maximum in supernatant turbidity. The sediment

volume then rapidly builds up just before the OCC, and

it levels off with further addition in coagulant. At pH 6,

the sediment volume gradually diminishes as the humic

suspension is restabilized.

OCC values, expressed in molar concentrations, and

pHs at OCC are given in Table 1. In all cases, the pH at

OCC falls within the 4.5–6 pH range for best removal of

organic substances (Dempsey et al., 1984). Surprisingly,

OCCs obtained for NHS coagulated under acid and

alkaline conditions are equivalent. As fresh stock

solutions of iron coagulant were prepared for each

experiment, this result could be due to the proteina-

ceaous nature of NHS (Jung et al., 2005b).

3.2. Morphology of coagulated humic substances

Typical micrographs of coagulated humic substances

are shown in Fig. 2. The sample preparation used in this

study precludes artifact free observations. Indeed,

freeze-drying is known to drastically modify the

structure of aggregated colloids (Hung et al., 1996),

whereas redispersion in ethanol may induce secondary

ARTICLE IN PRESS

Table 1

Optimum coagulant (iron) concentrations (OCC) determined by jar-test procedures

OCC pH 8.0 pH 6.0 pH 7.4

SHS NHS SHS NHS NOM

Molar concentration (mol/L) 8.3� 10�4 6.4� 10�4 2.2� 10�4 8.1� 10�5 1.7� 10�4

‘‘Reduced’’ concentration (mg Fe/mg Corg ) 3.83 2.69 1.01 0.34 1.02

pH at the respective OCC 6.5 6.3 5.1 5.2 5.9

A.-V. Jung et al. / Water Research 39 (2005) 3849–3862 3853

agglomeration effects upon solvent removal (Stevenson

and Schnitzer, 1982). Actually, techniques such as

atomic force microscopy in aqueous solution (Maurice

and Namjesnik–Dejanovic, 1999; Plaschke et al.,

1999) or TEM observation of ultracentrifuged

and resin-coated colloids (Wilkinson et al., 1999),

seem more appropriate to provide a description of

hydrated humic material. Nevertheless, as the prepara-

tion conditions remained identical for all samples, the

morphology of coagulated humic substances could be

compared.

Coagulated SHS appears as compact aggregates of

overlapping spheroids about 30 nm in size, regardless of

pH and iron concentration (Figs. 2a and b). The

appearance of NHS aggregated at pH 8 is similar with

smaller spheroids (10–15 nm) (Fig. 2c). In contrast,

electron micrographs of NHS coagulated at pH 6 reveal

a network of filament or sheet-like structures at the

micron scale (Fig. 2d). No significant change of the

latter morphology could be detected with coagulant

concentration. Despite the structural changes certainly

induced by the preparatory technique, our TEM

observations evidence individual spheroids which aver-

age dimension is consistent with the range of sizes

previously reported in the literature for humic colloids

(Wilkinson et al., 1999; Plaschke et al., 1999).

3.3. EELS spectra of coagulated SHS and NHS

To the authors knowledge, EELS investigations of

NOM remain relatively scarce in the literature (Watteau

et al., 1996; Furukawa, 2000). The spectra of well-

characterized compounds such as Bovine serum albumin

and Dextran (polysaccharide—MW ¼ 100; 000) were

then recorded as standards, and the identification of

EELS spectral features was conducted in agreement with

recent near-edge X-ray absorption spectroscopy (NEX-

AFS) studies (Myneni, 2002 and references herein).

EELS spectra were calibrated in energy using the

346.4 eV Ca L2,3-edge as an internal reference. This

yielded a 401.6 eV for the N K-edge energy which is

consistent with the attribution usually proposed for

nitrogen in NOM (Watteau et al., 1996). When calcium

was not detected in our samples, the nitrogen K-edge

was used instead.

An example of energy loss spectrum recorded in the

250–450 eV region (SHS coagulated at pH 8 with

[Fe] ¼ 7� 10�4mol/L), is illustrated in Fig. 3. Three

main features can be observed at the carbon edge: the

well-defined shoulder at 285 eV can be assigned to the

1s-p* transition of aromatic carbons (CQC); the

small peak around 288 eV is attributed to the 1s-p*transition of the CQO bond in carboxylic groups, while

the broad peak centered at 300 eV is attributed to

overlapping s* transitions for CQC, C–C, C–H,

CQO, and C–O bonds (Myneni, 2002). At higher

energy, the two peaks observed at 346 and 358 eV

correspond to the scattered electrons of the L2–3 shell of

calcium (Watteau et al., 1996). The presence of calcium

corresponds to a contamination brought about by the

addition of NaHCO3 ([Ca2+]�2.7� 10�4mol/L). It is

likely associated with the functional groups of SHS as

calcium has been shown to compete with trace elements

for the binding sites of humic substances (e.g., Tipping,

1993; van den Hoop et al., 1995). Finally, in accordance

with the elemental composition of SHS, a small amount

of nitrogen can be detected at the N K-edge around

401 eV.

Increasing the coagulant concentration at pH 8

significantly modifies the electron loss spectrum of

coagulated SHS (Fig. 4a). At the carbon edge, the sharp

peak at 288 eV assigned to p* (CQO) drastically

decreases, while the large peak at 300 eV broadens

further. In addition, the peak at the calcium edge rapidly

drops. A recent NEXAFS study of pedogenic humic

acid also showed a similar decrease of the p* resonance

of the carboxyl group double bond upon aggregation

with Fe3+ at pH 2 (Thieme et al., 2002). Such

observations can be readily explained by an exchange

of Fe-coagulant species with previously bonded Ca2+

ions onto carboxylic groups of SHS. The same interac-

tion mechanism has also been inferred from infrared

spectroscopy results in the case of adsorption/desorption

of NOM on iron oxide particles (Gu et al., 1994),

coagulation of sewage with ferric chloride (El Samrani et

al., 2004), and more generally, such a reaction scheme is

consistent with the strong influence of carboxylic group

ARTICLE IN PRESS

Fig. 2. Electron micrographs of freeze-dried coagulated sediment: (a) SHS pH 8—[Fe] ¼ 8.0� 10�4mol/L, (b) SHS pH 6—

[Fe] ¼ 2.0� 10�4mol/L, (c) NHS pH 8—[Fe] ¼ 7.0� 10�4mol/L, (d) NHS pH 6—[Fe] ¼ 8.0� 10�4mol/L.

A.-V. Jung et al. / Water Research 39 (2005) 3849–38623854

content on coagulant demand (Lefebvre and Legube,

1990; Randtke, 1993; Collins et al., 1986; Specht et al.,

2000).

The inset of Fig. 4a presents the evolution of EELS

spectra of freeze-dried humic material remaining in the

supernatant after settling. For each iron concentration,

the spectrum closely resembles that of a sediment

aggregated with a lower coagulant dosage, and can be

characterized by a higher calcium content and by a

slightly sharper peak at 288 eV (p*CQO). This suggests

ARTICLE IN PRESS

Nor

mal

ized

inte

nsity

(A

.U.)

270 320 370 420

Energy Loss (eV)

285 eV

1s → π∗C=C

288 eV

1s → π∗C=C

296 - 297 eV

1s → σ∗C-N, C=O, C-O

C=C, C-C, C-H

C -K

Der

ivat

ive

of n

orm

aliz

ed in

tens

ities

(A

.U.)

293-294299-300

286

285 288 297

275 310 345 380 415

C EELSbinding energies Energy Loss (eV)

Ca-L2,3 N-K

Standard 1Polysaccharide

Standard 2Bovine serum

albumin346.4

SHS401.6

Ca L2,3 -

N-K

Fig. 3. Typical EELS spectrum (SHS pH 8—[Fe] ¼ 8.0� 10�4mol/L). The inset shows the double derivative for (i) Bovine serum

albumin, (ii) Polysaccharide, (iii) SHS at pH 8 and at [Fe] ¼ 8.0� 10�4mol/L.

A.-V. Jung et al. / Water Research 39 (2005) 3849–3862 3855

that the unsettled humic material contains a lesser

amount of hydrolyzed Fe species. The EELS spectra of

NHS coagulated at pH 8 as a function of coagulant

concentration exhibit similar features (Fig. 4c): however,

the peak characterizing the 1s-p* transition of the

CQO bond in carboxylic groups is much less resolved

at low iron concentration, and it is gradually replaced by

a weak shoulder at higher coagulant dosage. Similarly,

the two peaks at the Ca L2,3 edge, clearly defined at low

coagulant concentration, rapidly weaken with an

increase in iron concentration.

The EELS spectra of SHS and NHS coagulated at pH

6 as a function of iron concentration are shown in Figs.

4b and d. In that case, the relative intensities of the

various contributions do not vary noticeably with

coagulant concentration. At the carbon edge, the 1s-p*transition of aromatic carbons (CQC) at 285 eV

remains as a distinct shoulder, whereas the contribution

at 288 eV (p* CQO) is now present as a weakly

resolved shoulder. The large dominant feature corre-

sponding to the s* transitions for CQC, C–C, C–H,

CQO, and C–O bonds has shifted around 297 eV.

Finally, a large contribution in the 325 eV region can be

evidenced, but could not be assigned. However, the

freeze-dried humic material from the supernatant yield

EELS spectra similar to those obtained at pH 8 (inset of

Fig. 4b). In particular, the p* resonance of the CQO

double bond appears as a peak at low iron concentra-

tion, and this peak decreases at higher coagulant dosage.

Likewise, the Ca edge which was virtually absent in the

coagulated sediment, can be clearly observed except at

the highest iron concentration. Such observations

suggest that the residual turbidity is formed from a less

coagulated humic material also at pH 6.

3.4. Elemental analysis of coagulated SHS and NHS

Fig. 5 shows the evolution of N/C, Ca/C, Na/C, and

Fe/C atomic ratios of coagulated SHS and NHS with

iron concentration at pHs 6 and 8. N/C remains

constant as a function of coagulant concentration and

pH, thus indicating that both the extraction procedure

and the synthesis yielded a homogeneous humic material

for coagulation experiments. N/C atomic ratios calcu-

lated from EELS spectra are equal to 0.0770.02 and

0.1770.02 for SHS and NHS, respectively. Such values

are consistent with previous N/C elemental ratios—

0.054 and 0.167—obtained from chemical analysis (Jung

et al., 2005b). Similar trends are also observed for the

evolution of Ca/C and Na/C. At pH 8, Ca/C markedly

decreases just before OCC to reach an almost constant

elemental ratio at higher coagulant concentration,

ARTICLE IN PRESS

Fig. 4. EEL spectra of freeze-dried coagulated humic substances as a function of iron concentration: (a) SHS pH 8, (b) SHS pH 6,

(c) NHS pH 8, (d) NHS pH 6.

A.-V. Jung et al. / Water Research 39 (2005) 3849–38623856

whereas Na/C remains close to 0.1 for both SHS and

NHS. At pH 6, Ca is not detected within the coagulated

sediment, whereas Na/C continuously decreases with

iron concentration in accordance with an exchange of

sodium ion for hydrolyzed Fe species onto anionic

groups of humic substances. The lower Na/C ratios

obtained at pH 8 are also consistent with an increased

deprotonation of SHS and NHS functional groups,

even though Na/C values may be modified during

freeze-drying.

The evolution of Fe/C atomic ratios as a function

of coagulant concentration follows two different

patterns according to pH. At pH 8, for both SHS

and NHS, Fe/C is about 3 before OCC, it surprisingly

drops to a value close to 1 at OCC (0.8 (c) and 1.25 (m)),

and then re-increases to an intermediate value at

higher coagulant dosage. At pH 6, Fe/C slightly

increases with iron concentration from a value of

1 at underdosage for NHS, to reach a Fe/C ratio

of about 3 at OCC for both humic compounds, and

ARTICLE IN PRESS

Fig. 5. Evolution of elemental ratios as a function of coagulant concentration: N/C (’), Fe/C *1/10 (&), Na/C *1/10 for NHS pH 8

(K) and Ca/C (J).

A.-V. Jung et al. / Water Research 39 (2005) 3849–3862 3857

values greater than 4 when the suspension is restabilized.

Interestingly, recent Fe K-edge EXAFS measure-

ments carried out on Seine river NOM coagulated

with iron chloride (Vilge–Ritter et al., 1999a, b), revealed

that (i) iron hydrolysis is hindered to at most the

trimer stage, (ii) each iron atom is surrounded at

optimum dosage by three C and 1 C at pH 5.5 and

pH 7.5, respectively. Such results would yield Fe/C

elemental ratios in the 1–3 range which is consistent with

our data.

Assuming a complete removal of humic material by

coagulation, the determination of elemental ratios from

chemical analysis predicts that, at pH 8, Fe/C should be

equal to 0.8 and 0.66 for SHS and NHS, respectively,

which is in relatively good agreement with the values

obtained from EELS and EDX spectra. In contrast, at

pH 6, the same calculations yield Fe/C atomic ratios of

0.22 and 0.7 which is much less than the measured

values. Such a discrepancy may be explained by the

aggregation behavior of humic substances with pH. The

hydrolysis of iron coagulant implies a decrease in pH;

hence, at OCC, the final pH of the suspension initially

coagulated at pH 6, was about 4.5. At the latter pH, a

small degree of aggregation of humic substances has

been clearly demonstrated from the measurement of

their diffusion coefficients by fluorescence correlation

spectroscopy (Lead et al., 2000). In situ AFM observa-

tions of adsorbed humic colloids as a function of pH

(Plaschke et al., 1999; Balnois et al., 1999), also indicate

an increased surface aggregation with a decrease in pH.

The iron-coagulated sediment should then include some

loosely attached humic colloids that may become rapidly

ARTICLE IN PRESSA.-V. Jung et al. / Water Research 39 (2005) 3849–38623858

volatilized under the electron beam; the microanalysis

being performed mainly with the iron coagulated humic

substances. The almost constant Fe/C atomic ratio

obtained at pH 6 is then in accordance with previous

studies suggesting a complexation mechanism between

oligomeric coagulant species and functional groups of

fulvic and humic colloids around pH 5 (van Breemen et

al., 1979; Jeckel, 1986; Lefebvre and Legube, 1990).

The effect of pH on Fe/C elemental ratio can be

explained by a dynamic reorganization of humic

macromolecules upon interaction with Fe coagulant

species. At pH 8, the behavior of deprotonated humic

material is generally compared with that of a flexible

anionic polymer in a stretched configuration (Gosh and

Schnitzer, 1980; Vermeer et al., 1998). In that case, at

low coagulant concentration, the partial neutralization

of humic macromocules by Fe-hydrolyzed species can

induce a reconformation of polymer chains, thus leading

to a compact structure and values of Fe/C close to 3.

Recent fluorescence spectroscopy using pyrene as a

polarity-sensitive fluoroprobe also suggests such a

rearrangement of humic macromolecules upon associa-

tion with multivalent cations (Engebretson and von

Wandruszka, 1994; Kazpard et al., 2005). At higher

coagulant dosage, the amount of hydrolyzed Fe species

increases relative to the number of humic colloids, and

aggregation between stretched polymer chains may

proceed before folding and reconformation of humic

material take place. In that case, the coagulant species

may form bridges between stretched humic chains. In

accordance with such an aggregation mechanism, the

increase in sediment volume observed with pH above

OCC, is proportional to C/Fe ratios for both SHS and

NHS. Therefore, the increase in sludge volume at pH 8

is here attributed to a change in shape factor of humic

colloids, and not to the formation of abundant iron

hydroxide precipitate as predicted by a sweep-floccula-

tion mechanism (Gregory and Dupont, 2001).

3.5. Coagulation of Moselle river raw water

Experiments with Moselle river water were run in

duplicate, similar trends being observed for the two grab

samples. Four kinds of NOM compounds were identi-

fied from TEM examination (Fig. 6): humic-like colloids

(Fig. 6a (A)), by far the most frequent compounds in the

freeze-dried coagulated sediment, were about 20 nm in

diameter with a N/C ratio in the 0.02–0.05 range;

proteinic substances (Fig. 6a (B)), appeared as large

grayish areas 300 nm in size with N/C values greater

than 0.1; polysaccharidic material was characterized by

micron size spheroids with neither nitrogen nor iron

detected, a CEEL spectrum equivalent to that of a sugar

backbone, and it was easily degraded under the electron

beam (Fig. 6b); small organic colloids intimately

associated with smectite clay material and with a sharp

299 eV peak, were also found (Fig. 6c). Such organic

matter is reminiscent of that observed by Furukawa in

Jourdan river sediments (Furukawa, 2000). It should be

noted that, for NOM, the interpretation of EELS

spectra at the carbon edge is complicated by the

presence of L2–3 peaks at 297 and 300 eV of potassium

(Henke et al., 1993).

Fig. 7 shows the jar-test results obtained with Moselle

river water coagulated with iron at natural pH (7.4). The

supernatant turbidity continuously increases with iron

concentration up to [Fe]�1.2� 10�4mol/L, then steeply

decreases at OCC ([Fe] ¼ 1.7� 10�4mol/L), slightly re-

increases above OCC, and then decreases again at higher

coagulant dosage. The evolution with coagulant con-

centration of C-EELS spectra corresponding to humic-

like colloids is illustrated in Fig. 8a. This kind of

material shows a similar reactivity to that of SHS and

NHS at pH 8, with a strong decrease of the p*CQO

peak at 288 eV with iron concentration. Fig. 8b presents

the variation of N/C, Ca/C, and Fe/C elemental ratios of

coagulated NOM with iron concentration. Fe/C in-

creases continuously with coagulant dosage to reach a

value close to 1 above OCC, which is consistent with the

results obtained with SHS and NHS. Likewise, Ca/C

elemental ratio decreases in accordance with a substitu-

tion of Ca for Fe-hydrolyzed species onto functional

groups of organic matter. N/C values reveal that NOM

removal occurs in two main stages: up to OCC, the N/C

atomic ratios are less than 0.03, thus suggesting a

preferential coagulation of humic substances; above

OCC, N/C increases significantly to reach values in the

0.1 range which indicates the presence of proteinic

matter in the coagulated sediment. Such results are

consistent with previous pyrolysis-gas chromatography-

mass spectrometry (Py-GC-MS) experiments showing

that polyhydroxyaromatics are preferentially removed

by ferric chloride (Vilge–Ritter et al., 1999a, b; Jung,

2004). More generally, the hydrophobic fraction of

NOM is known to be more easily destabilized than

hydrophilic compounds (e.g., proteinic matter) (Croue

et al., 1993; Bose and Reckhow, 1998). In particular, the

slight increase of residual turbidity observed above OCC

could correspond to the restabilization of some humic

colloids (Fig. 7).

4. Conclusion

This study shows that TEM observations combined

with EDXS and EELS analyses, represent a powerful

tool to investigate the coagulation of NOM. The

nanoscale resolution of EELS and EDXS is well-

adapted to the size range of organic colloids encoun-

tered in surface waters. In particular, a fingerprinting of

coagulated NOM can be achieved, even though further

ARTICLE IN PRESS

250 300 350 400 450Energy Loss (eV)

250 300 350 400 450Energy Loss (eV)

285

B

300

346.4

401.6

B : protein-likesubstances

BA

0.5 µm

TEM photo401.6285

A289 300

C.K EELS

A : humio-likesubstances

A

0 1

2

6 78 9

10

0 : C Ka

1' : Fe La

0.7 2.1 3.5 4.9 6.3Energy (keV)

EDXSElmt At%

1267891011

ONaPSClKCaFe

55.63.51.01.12.52.62.1

31.6

1267891011

ONaPSClKCaFe

81.63.41.11.12.62.56.11.6

Elmt At%B

0 1'2 6 7

8 9

10

0 : C Ka

1' : Fe La

11

1

0.7 2.1 3.5 4.9 6.3

250 300 350 400 450

285

300C-K EELS TEM photo

0.5 µm

EDXS

0

1Elmt (K) At%

0

1

C

0

55

45

(indicative)

0.7 2.1 3.5 4.9 6.3

C-K EELS

298299

285

346.4

300 350 400 4500.2 µm

TEM photo EDXS1

01'

234

5

678 9

10

11

0.7 2.1 3.5 4.9 6.3

0 : C Ka

1' : O La

12

67891011

ONa

P

3 Mg4 Al5 Si

SClKCaFe

58.93.43.46.69.50.50.52.81.82.2

10.5

Elmt At%

Energy (keV)

Energy (keV)Energy Loss (eV)

Energy Loss (eV) Energy (keV)250

(a)

(b)

(c)

Fig. 6. Typical NOM compounds identified by combining TEM observation with EDXS and EELS spectra: (a) Humic-like substances

(A) and protein-like material (B), (b) Polysaccharide-like substances, (c) Organo-mineral species.

A.-V. Jung et al. / Water Research 39 (2005) 3849–3862 3859

ARTICLE IN PRESS

No

rmal

ized

inte

nsi

ty (

A.U

.)

NOM natural pH 7.4

0.08

0.1

0.12

(v)

(r)

(s)

(t)

(u)

Energy Loss (eV)

250 350 400 450

(a)

0

0.02

0.04

0.06

1.1 10-4 2.3 10-41.4 10-4 1.7 10-4 2.0 10-4

Iron concentration (M)

Ele

men

tal a

tom

ic t

al

(r)

(s)

(t)

(u) (v)

(b)

300

NOM natural pH 7.4

Fig. 8. (a) EEL spectra for NOM as a function of coagulant

dosage. (b) Elemental atomic ratio: N/C (’), Fe/C *1/10 (&),

Na/C (K) and Ca/C (J).

0

1

2

3

4

5

6

7

8

9

10

0 10-4 1.5 10-4 2 10-4 2.5 10-4

NOM natural pH 7.4

(r)

(s)(t)

(u)(v)

Iron concentration (M)5 10-5

Fig. 7. Jar-test results for NOM at natural pH (7.4). (K

residual turbidity (NTU) ’ sediment volume (mL)).

A.-V. Jung et al. / Water Research 39 (2005) 3849–38623860

work is still required to improve the distinction of

natural organic substances.

EELS and EDXS examination of coagulated sediment

also provides a useful insight into the destabilization

mechanism of humic substances and NOM: Fe-hydro-

lyzed species associate with carboxylic groups of humic

colloids at both pH 6 and 8, thus inducing a release of

previously complexed calcium ions. An overall charge

neutralization/complexation mechanism seems to be

responsible for the removal of humic substances in our

experimental conditions as no Fe-hydroxyde precipitate

could be detected in the coagulated sediment. The

association of non-coagulated humic colloids within

flocs may also occur at pH 6. On the other hand, at

alkaline and neutral pH, our results suggest that a

reorganization of humic macromolecules from a

stretched to a coiled conformation takes place upon

interaction with coagulant species. Such phenomenon is

worth exploring, as the optimal coagulant dosage should

then depend on the complex interplay between the rate

of collisions of organic colloids/coagulant species and

the rate of macromolecules reconformation.

Acknowledgments

This work was achieved within the framework of

‘‘Zone Atelier du bassin de la Moselle’’, with financial

support from Region Lorraine and CNRS (Contrat de

Plan Etat-Region and Programme Environnement Vie et

Societe). Agence de l’Eau Rhin-Meuse is gratefully

acknowledged for providing the Field-centrifuge. The

authors would also like to thank T. Jacquin (Agence de

l’Eau Rhin Meuse, Rozerieules, France) for providing

technical assistance during field measurements.

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