characterization of aquatic humic substances

Characterization of aquatic humic substances

Francisco J. Rodrıguez1 & Luis A. Nunez2

1Department of Chemistry, University of Burgos, Burgos, Spain and 2Chemical Engineering Department, University of Burgos, Burgos, Spain

Keywords

humic substances; infrared analysis; molecular

weight distribution; natural organic matter

(NOM); organic acidity.

Correspondence

Luis A. Nunez, Chemical Engineering

Department, University of Burgos, Pl. Misael

Banuelos s/n, E-09001 Burgos, Spain. Email:

[email protected]

doi:10.1111/j.1747-6593.2009.00205.x

Abstract

Aquatic natural organic matter (NOM) is composed of a large variety of

compounds. In this study, NOM from Uzquiza Reservoir in Burgos (Spain)

was fractionated using a resin adsorption procedure, and three humic sub-

stances – natural fulvic and humic acids (NFA and NHA, respectively) extracted

from the mentioned reservoir and a commercially supplied humic acid (CHA) –

were characterized by molecular weight (MW) distribution, elemental compo-

sition, organic acidity and ultraviolet and infrared spectroscopy. The results

indicate that Uzquiza Reservoir NOM is mainly composed of fulvic acids (45%)

and low MW hydrophilic acids (27%). MWs obtained are in the following

order: CHA (4500 Da)4NHA (2500 Da)4NFA (1000 Da). Both humic acids

(CHA and NHA) have the highest specific UV absorbance values (SUVA, an

aromaticity indicator) whereas NFA shows a higher total and carboxylic acidity,

which is in accordance with its higher solubility in water. Infrared spectra

confirm these results.

Introduction

A range of organic compounds are found in natural

waters, from low-molecular-weight (MW) hydrophilic

acids, carbohydrates, proteins and amino acids to higher

MW compounds such as humic substances (fulvic and

humic acids) (Choudry 1984). The majority of the aquatic

organic matter is present in a dissolved form [dissolved

organic carbon (DOC)] whereas a small amount of the

organic compounds (generally around 10%) is in a colloi-

dal particle form (Thurman 1985).

The concentration of organic matter is usually low in

groundwater and seawater [around 1 mg/L total organic

carbon (TOC)] (Williams 1971), variable in surface waters

(around a few mg/L TOC, averaging in the range of

5–6 mg/L TOC) and can be relatively high in a few specific

cases, such as certain eutrophic lakes (30 mg/L TOC and

greater) (Langlais et al. 1991).

Natural organic matter (NOM) composition clearly

depends on the environmental source (Aiken & Costaris

1995). Most of the NOM found in natural waters are

humic substances (30–50%) (Thurman 1985); humic

substances result from elutriation of the surrounding soils

and from microbiological, chemical and photochemical

reactions (humification process) that occur during the

degradation and polymerization of vegetable organic

matter in water (Langlais et al. 1991; Galapate et al. 1997).

The main physicochemical properties of humic sub-

stances are the following (Langlais et al. 1991): they are

responsible for colour in natural waters (an increase in pH

causes an increase in colour intensity) and show a strong

reactivity with halogens, acting as precursors to disinfec-

tion by-products (DBP). They show a significant adsorb-

ability on activated carbon, alumina and mineral colloids

(which is likely to modify the performance of the coagu-

lation–flocculation process) and biodegrade very slowly

and persist for long periods in water. They form com-

plexes with different metal ions (particularly with Fe3+,

Al3+ and Cu2+) and are capable of absorbing many organic

micropollutants (pesticides, phthalates, PCBs, etc.), thus

increasing their solubility in water and hindering their

removal.

Humic substances are a mixture of different macro-

molecular organic acids. Fulvic and humic acids make up

the two main fractions of humic substances and they can

be distinguished by their different solubility at pH 1; the

precipitated fraction is humic acid and the part remaining

in solution is fulvic acid. Other differences between the

two fractions are the following (Langlais et al. 1991;

Andrews & Huck 1996): fulvic acids always represent the

larger fraction (the fulvic acid/humic acid mass ratio is

generally around 9 : 1) and are more soluble than humic

acids because they have a lower average MW and higher

acidity (especially carboxylic acidity) than humic acids

Water and Environment Journal 25 (2011) 163–170 c� 2009 The Authors. Water and Environment Journal c� 2009 CIWEM. 163

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(humic acids are often in colloidal form because of their

large size), whereas humic acids show more aromaticity

and UV absorbance and have more colour than fulvic

acids. Humic acids generally have a greater trihalo-

methane formation potential and are more readily coagu-

lated by aluminium and iron (III) salts than fulvic acids.

MW is one of the most important attributes of humic

substances, because certain processes, such as coagulation

and adsorption, are most effective in removing specific

MW fractions. Furthermore, trihalomethane production

upon chlorination has been noticed to vary as a function

of the MW of humic substances. Humic substances are

polydisperse systems with a specific distribution of MWs.

The First studies establish relatively high MWs for aquatic

humic substances; according to the data collected by

Rebhun & Lurie (1993), humic acids are in the range of

5000–100 000 Da, whereas fulvic acids are in the range of

500–5000 Da, and according to the data collected by

Schnitzer & Khan (1972), humic acids range from 100 to

106 Da, whereas fulvic acids range from 180 to 10 000 Da.

Recent literature values indicate that these substances are

smaller and less polydisperse than previously believed

and propose a range between 500 and 2000 Da for fulvic

acids and between 2000 and 5000 Da for humic acids

(Thurman 1985; Chin 1994).

The main objective of this paper is to investigate the

organic composition of the natural water from the Uzqui-

za Reservoir (Burgos, Spain) (Rodrıguez 1999) and the

structural characteristics of its main components (humic

substances). NOM fractionation and the analysis of the

main properties of the majority fraction, the humic sub-

stances, can yield important information regarding NOM

reactivity in water treatment processes (coagulation–floc-

culation, adsorption, biodegradation and formation of

DBP).

Materials and methods

Fractionation of NOM

The NOM fractionation scheme used in this work is based

on the resin adsorption procedure described by Andrews

& Huck (1996), which has been proved to provide satis-

factory results for this purpose. In this procedure, a series

of three different resins are used sequentially, where the

natural water is pumped through the columns at pH 2.0.

An Amberlite XAD-7 resin (a nonionic polymeric adsor-

bent based on acrylic ester; Aldrich Chemical Co., Dorset,

UK) isolates the humic substances (hydrophobic organic

matter). Fulvic and humic acids are then separated by

precipitation at pH 1.0. Humic acids precipitate whereas

fulvic acids remain in solution. After precipitating for

24 h, the sample is centrifuged (7000 g – 20 min). An

Amberlite XAD-4 resin (a nonionic polymeric adsorbent

based on polystyrene) isolates the low MW hydrophilic

acids (Croue et al. 1993; Edzwald 1993; Martin-Mousset

et al. 1997), hydroxy acids, low MW alkyl monocarboxylic

and dicarboxylic acids. An Amberlite IR-120 Plus resin (a

strongly acidic gel-type cation-exchange resin, with hy-

drogen form and sulphonic acid functionality) isolates the

hydrophilic bases (Leenheer 1981), amino acids, purines

and pyrimidines and low MW alkyl amines. The remain-

ing part, which is not adsorbed by any of the three resins

is made up of neutral hydrophilic compounds, polysac-

charides, low MW alkyl alcohols, etc. (Andrews & Huck

1996). Each isolated NOM fraction adsorbed on the resins

is individually eluted with 0.1 N NaOH and collected for

TOC analysis (TOC-5050 analyzer, Shimadzu, Tokyo,

Japan).

Humic substances analysis

An analysis of the main structural and chemical charac-

teristics of the humic substances was carried out: MW

distribution, elemental composition, organic acidity and

spectroscopic characterization (infrared and ultraviolet).

Natural fulvic and humic acids (NFA and NHA, respec-

tively) (extracted from the Uzquiza Reservoir using the

method described previously) and a commercially sup-

plied humic acid (CHA, Aldrich Chemical Co., Dorset,

UK) were the humic substances used in this study.

MW distribution of humic substances

MW of the humic substances were determined using

high-performance size-exclusion chromatography

(HPSEC), using a column packed with hydroxylated

polymethacrylate-based gel because of its minimal non-

size exclusion effects. Mobile phases used in this study

were comprised of Milli Q water buffered with phosphate

to a pH of 7.0, and sodium chloride was added to yield an

ionic strength equivalent to 0.1 mol/L NaCl. Humic sub-

stances were detected at a wavelength of 224 nm. A range

of sodium polystyrene sulphonate (PSS) macromolecules

(1 K, 5 K, 13 K and 20 K) were used as standards in this

study because some investigators have shown that globu-

lar proteins standards tend to overpredict the MW of

humic substances (Chin 1994).

Infrared analysis of humic substances

Fulvic and humic acids in alkaline solution (0.1 N NaOH)

were passed through a strong cation-exchange resin

(Amberlite IR-120 Plus) in the hydrogen-saturated form

to convert the sodium salt of the acids to its free-acid form;

the hydrogen-saturated acids are then vacuum dried at

Water and Environment Journal 25 (2011) 163–170 c� 2009 The Authors. Water and Environment Journal c� 2009 CIWEM.164

Aquatic humic substances F. J. Rodrıguez and L. A. Nunez

40 1C. The FT-IR spectra of solid samples (5% in KBr)

were then recorded between 400 and 4000 cm� 1 using a

spectrophotometer (Nicolet Impact 410; New model of

spectrophotometer: model M1200, Midac Co., Costa

Mesa, CA, USA).

Ultraviolet, colour and elemental analysis of humic

substances

UV-absorbance measurements were determined in 1 cm

pathlength quartz cells using a spectrophotometer (model

100-10, Hitachi Ltd., Tokyo, Japan) at a wavelength of

254 nm. Colour was measured in a 2.5 cm cuvette using a

spectrophotometer (model DR/2000, Hach Co., Loveland,

CO, USA), at a wavelength of 455 nm (Pt–Co units). The

elemental analysis (C, H, N) of the humic substances was

performed using a CHN analyser (model 2400, Perkin-

Elmer, Norwalk, CT, USA).

Organic acidity of humic substances

Both the carboxylic and phenolic acidities (total=

carboxylic + phenolic) of the humic substances were

determined by the potentiometric titration technique

described by Collins (Collins et al. 1986; Chandrakanth &

Amy 1996; Paode et al. 1997). Samples of 100 mL volume

(TOC � 100 mg/L) were used for the titrations (all sam-

ples and blanks contained NaCl at a concentration of

0.1 mol/L to maintain a constant ionic strength). Titra-

tions were performed in a chamber under a nitrogen

atmosphere to minimize interferences from carbon diox-

ide. The initial pH of the sample was adjusted to pH 3.0

and then titrated to pH 10.0 by stepwise additions of a

carbonate-free standard 0.01 mol/L NaOH solution. Car-

boxylic acidity was defined as the milliequivalents of base

required to titrate the sample from a pH of 3 to 8; phenolic

acidity was defined as twice the milliequivalents of base to

titrate from pH 8 to 10. Although it is possible that some

very weak humic carboxyl groups may not be deproto-

nated by pH 8, the benefit of choosing this pH value was

to minimize the contribution from phenolic groups (Kar-

anfil et al. 1996). Titration curves were recorded with an

automatic titration system (model Titralab TIM90, Radio-

meter Analytical A/S, Copenhagen, Denmark).

Results and discussion

NOM fractionation

The results of the fractionation of the NOM from the

Uzquiza Reservoir water (Burgos, Spain) are shown in

Table 1 (average values), where they can be compared

with other results reported in the literature. The results

show that humic substances constitute approximately

50% of the TOC in the Uzquiza Reservoir water (fulvic

acids representing the larger fraction). XAD-4 fraction

(low MW hydrophilic acids) is also significant (27%).

Both fractions, the humic substances and the low MW

hydrophilic acids, make up the bulk (79%) of the organic

matter present in the water. Although NOM composition

depends on the source, our data are in agreement with

the results reported by other researchers for unpolluted

waters (see Table 1).

Humic substances analysis

MW distribution

The results of the HPSEC measurements are shown in

Fig. 1. It is noted that all the humic substances were

Table 1 Natural organic matter fractionation from several natural waters

This study

Thurman

(1985)

Malcolm

(1991)

Humic substances 52% 50% 50%

Fulvic acids:

47%

Fulvic acids:

45%

Humic acids:

5%

Humic acids:

5%

Low-molecular-weight

hydrophilic acids

27% 36% 25%

Hydrophilic bases 8% 3% 4%

Neutral hydrophilic

compounds

13% 10% 15%

Neutral hydrophobic

compounds

Not

analysed

1% 6%

55

75

95

Commercialhumic acid

Naturalfulvic acid

Naturalhumic acid

1000

Da

5000

Da

13 0

00 D

a

–5

15

35

0 5 10 15 20 25 30

Res

pons

e

Time (min)

Fig. 1. High-performance size exclusion chromatogram of humic

substances.


Aquatic humic substancesF. J. Rodrıguez and L. A. Nunez

eluted from the HPSEC column as a broad, monomodal

distribution with subtle shoulders and small subpeaks.

MWs (average values) for the three humic substances

studied are in the following order:

commercial humic acid4 natural humic acid4 natural fulvic acid

ðMW � 4500 DaÞ ðMW � 2500 DaÞ ðMW � 1000 DaÞ

These results appear consistent with the hypothesis

that these substances occupy a relatively narrower size

fraction and do not possess MWs that vary by orders of

magnitude (Chin 1994). Our MW values are in good

agreement with those reported in the literature, for

instance, the weight-averaged MWs of Aldrich humic

acid (the CHA used in this study) reported by several

investigators are 4006 Da (Karanfil et al. 1996) and

4100 Da (Chin 1994), results that are very similar to our

MW value (4500 Da).

Elemental and UV analysis

The results from the elemental analysis of the three humic

substances used in this study are shown in Table 2, where

they can be compared with other results reported in the

literature. The most striking aspect of these results is

the apparent lack of nitrogen (or the presence of a

minimum content that is lower than the detection limit

of the analyser) in the composition of CHA, whereas NFA

and NHA clearly contain nitrogen (this difference may be

because of the aquatic origin of the natural acids versus

the terrestrial origin of CHA). With the exception of N,

both humic acids have a similar composition (C, H),

including the H/C atomic ratio.

NFA shows some differences from humic acids. Fulvic

acid contains more carbon and hydrogen than humic acids,

and the H/C atomic ratio is also greater for fulvic acid, in

accordance with its greater aliphatic (or less aromatic)

character. Nevertheless, NHA has a higher nitrogen content

than fulvic acid. These results are similar to those reported

in the literature.

The data summarized in Table 3 show the results of

specific UVabsorbance (SUVA) and colour analysis. SUVA,

defined as the ratio of UV absorbance at 254 nm to the

TOC (mg/L), provides insight into the aromatic character

of NOM. Aromaticity for humic acids, both natural and

commercial, is higher (higher SUVA) than for fulvic acid,

a fact which is in accordance with the greater H/C ratio for

the latter. CHA has the highest value for the SUVA

parameter and natural water, the lowest. Fulvic acid has

a higher SUVA than natural water (and 47% of NOM

from the Uzquiza Reservoir water are fulvic acids), which

means that all the remaining organic matter together

have a lower aromaticity than fulvic acid. Colour data

follow the same tendency as for SUVA, except for natural

water, which has a higher colour/TOC value than ex-

pected. This may be explained by the contribution of

several inorganic species present in the Uzquiza Reservoir

water, such as iron and manganese (Weber & Wilson

1975).

Organic acidity

The results are summarized in Table 4, along with data

from the literature. The results show that total organic

acidity is higher for fulvic acid than for both NHA and

CHA, CHA having the lowest total acidity. The higher

total acidity for fulvic acid is explained primarily by

carboxylic acidity, which is clearly higher for NFA. Phe-

nolic acidity, however, is higher for NHA than for fulvic

acid, again CHA showing the lowest value of all three

humic substances.

Carboxylic acidity is higher than phenolic acidity for

the three humic substances studied, which indicates that

carboxyl groups predominate over the phenolic groups in

the macromolecular structure of fulvic and humic acids.

Table 2 Elemental composition of several humic substances

Humic substance C (%) H (%) N (%) H/C ratio C/N ratio

Uzquiza FA (res) (this study) 55.19 6.44 0.98 1.40 65.70

Uzquiza HA (res) (this study) 51.76 5.15 1.65 1.19 36.60

Aldrich HA (com) (this study) 52.97 5.34 0.00 1.21 –

IHSS FA (com) (Langlais et al. 1991) 53.75 4.29 0.68 0.96 92.22

IHSS HA (com) (Langlais et al. 1991) 54.22 4.14 1.21 0.92 52.28

Suwanee FA (riv) (Thurman & Malcolm 1981) 54.65 3.71 0.47 0.81 136.65

Suwanee HA (riv) (Thurman & Malcolm 1981) 57.24 3.94 1.08 0.82 61.83

Biscayne FA (gw) (Thurman & Malcolm 1981) 55.44 4.17 1.77 0.90 36.54

Biscayne HA (gw) (Thurman & Malcolm 1981) 58.28 3.39 5.84 0.70 11.64

Laramie FA (gw) (Thurman & Malcolm 1981) 62.67 6.61 0.42 1.27 174.08

Laramie HA (gw) (Thurman & Malcolm 1981) 62.05 4.92 3.21 0.95 22.55

Oyster FA (riv) (Weber & Wilson 1975) 51.10 3.62 1.13 0.85 52.76

Oyster HA (riv) (Weber & Wilson 1975) 53.40 3.73 2.10 0.84 29.67

FA, fulvic acid; HA, humic acid; com, commercial; res, reservoir; riv, river; gw, groundwater.



The results shown in Table 4 are in agreement with data

reported in the literature, which show a wide range of

organic acidity values for humic substances, perhaps

because of different techniques used to measure this

parameter. Also shown in Table 4, there is a reference to

the CHA used in this study (Aldrich humic acid), showing

values slightly higher than our values, although there is

no significant difference considering the wide range of

values reported in the literature.

Because humic substances are highly substituted with

acid groups, an aqueous solution of them constitutes a

typical weak acid type of polyelectrolyte (Gamble 1970);

this behaviour cannot be explained by two specific pKa

values (one corresponding to the carboxylic acidity and

the other to the phenolic acidity) because of the large

variety of carboxyl and phenolic groups present in the

humic substances macromolecules. Nevertheless, several

authors have managed to estimate a few pKa values

(calculated by different techniques: potentiometric titra-

tion, synchronous fluorescence, etc.) that explain the

weak acid polyelectrolyte behaviour of humic substances.

For example, three pKa values (2.4, 3.6 and 5.6) have

been used by Tipping et al. (1990), four pKa values (1.8,

3.2, 4.2 and 5.7) by Ephraim(1986) and six pKa values

(2.6, 4.3, 5.5, 6.7, 8.0 and 9.6) by Gomez (1991). Esteves

& Machado (1995), who propose four pKa values (3.1,

4.8, 8.0 and 10.0), indicate that the majority (70%) of

acid–base functionalities corresponds to the two stronger

acid classes that are of the carboxylic type (pKa 3.1 and

4.8), whereas phenolic structures (pKa 8.0 and 10.0), the

weakest acid structures, contribute only with 30% of the

total acidity of a marine fulvic acid. Gamble(1970) also

proposes two types of carboxyl groups for fulvic acids:

type I (carboxyl groups on the aromatic rings, ortho to

phenolic-OH groups, that are quite acidic) and type II

(more weakly acidic carboxyl groups, which are not ortho

to phenolic-OH groups).

Infrared analysis

The FT-IR spectra of fulvic and humic acids are shown in

Fig. 2. The band assignments are summarized in Table 5

(Ricca & Severini 1993; Esteves et al. 1998).

The first observation is that there is a great similarity in

the spectra of both humic acids (commercial and natural),

whereas the NFA spectrum shows some remarkable dif-

ferences.

All the three spectra show the typical broad and intense

band centred at about 3400 cm� 1, which is attributed to

the associated OH stretching vibrations (alcohols, phenols

and carboxylic acids). The presence of carboxyl groups in

all the three humic substances is established by the

following data: the large width of the band mentioned

above, the broad and weak band centred at about

2620 cm� 1 (because of the OH stretching vibrations of

the hydrogen-bonded COOH), the band at 1720 cm� 1

(because of the C=O stretching vibration) and the band at

1410 cm� 1 (attributed to the O–H bend in the COOH

group).

Several oxygenated groups are characterized by the

bands at 1220–1260 cm� 1 (carboxylic acids, phenols and

aromatic or unsaturated ethers) and at 1030–1095 cm� 1

(alcohols and aliphatic ethers). The higher carboxylic

acidity for fulvic acid is confirmed by the larger OH

Table 3 Typical characteristics of natural water and humic substances

Sample

UV254/TOC (AU/

mg C)

Colour/TOC (Pt–Co U/

mg C)

Natural water (Uzquiza

Reservoir)

0.025 16.80

Natural fulvic acid 0.029 5.87

Natural humic acid 0.040 23.33

Commercial humic acid 0.050 35.53

AU, absorbance unit; Pt–Co U, platinum–cobalt Unit; TOC, total organic

carbon.

Table 4 Organic acidity of humic substances

Humic substance Carboxylic acidity (mEq/g C) Phenolic acidity (mEq/g C) Total acidity (mEq/g C)

Uzquiza FA (res) (this study) 9.1 4.5 13.6

Uzquiza HA (res) (this study) 7.9 4.8 12.7

Aldrich HA (com) (this study) 7.4 3.1 10.5

Oyster FA (riv) (Weber & Wilson 1975) 6.8 4.3 11.1

Oyster HA (riv) (Weber & Wilson 1975) 4.5 3.7 8.2

Jewell FA (res) (Weber & Wilson 1975) 8.1 1.5 9.6

Jewell HA (res) (Weber & Wilson 1975) 4.9 2.2 7.1

Suwanee FA (riv) (Chandrakanth & Amy 1996) 12.2 2.4 14.6

Suwanee HA (riv) (Chandrakanth & Amy 1996) 9.8 2.9 12.7

Fluka HA (com) (Weber & Wilson 1975) 4.2 2.9 7.1

Pfaltz & Bauer HA (com) (Mccreary & Snoeyink 1980) 6.9 3.5 10.4

Aldrich HA (com) (Karanfil et al. 1996) 7.9 3.6 11.5

FA, fulvic acid; HA, humic acid; res, reservoir; riv, river; com, commercial.



stretching vibration band associated at 3600–2400 cm� 1

as well as by the relatively high intensity of the C=O and

C–O stretching bands.

For the saturated groups, CH3 and CH2 groups are

characterized by the bands at 2850–2960 cm� 1 (due to

the C–H stretch), with a greater relative intensity for

humic acids. In addition, there are also bands at 1455

and 1375 cm� 1 because of the deformation vibrations of

the CH3 and CH2 groups.

In reference to the aromatic-type structures, it is inter-

esting to note that there is practically no characteristic

band at 3000–3100 cm� 1 because of =C–H groups, an

absence that indicates a high degree of substitution at the

aromatic rings; the slight presence of out-of-plane CH

deformation vibration bands also supports this conclu-

sion. The band at 805 cm� 1 is attributed to tri- and tetra-

substituted aromatic rings; the relative intensity of this

band leads to the conclusion that CHA has the highest

degree of substitution at the aromatic rings, whereas

fulvic acid has the lowest. The band at 1620 cm� 1 is

characteristic of the C=C bond (probably aromatic rings,

given the proposed structures for the humic substances)

and seems to have a relatively higher intensity for CHA,

which is in accordance with its higher aromaticity. All the

three humic substances show a very weak band at

1540 cm� 1, attributed to N–H structures, which is in

agreement with the elemental analysis results.

Conclusions

(1) The analysis of the aquatic NOM composition can

yield important information regarding its reactivity in

water treatment processes (formation of DBP, coagula-

tion–flocculation, adsorption, biodegradation, etc.).

Water from the Uzquiza Reservoir (Burgos, Spain) is

mainly composed of fulvic acids (45%) and low MW

hydrophilic acids (27%).MWs of humic substances used

in this study (measured by HPSEC) are in good agreement

with those reported in the literature. MWs obtained are in

the following order: CHA (MW � 4500 Da)4NHA

(MW � 2500 Da)4NFA (MW � 1000 Da).

Fig. 2. FT-IR spectra of humic substances.

Table 5 Main characteristics of the FT-IR spectra of humic substances

Band

(cm� 1) Assignment

3400 Associated O–H stretch (alcohols, phenols and carboxylic

groups)

2850–2960 C–H stretch (CH3 and CH2)

2620 O–H stretch (hydrogen-bonded carboxylic groups)

1720 C=O stretch (carboxylic groups)

1630 C=C stretch (alkenes and aromatic rings)

1540 N–H bend (N–H structures)

1455 C–H bend (CH3 and CH2)

1410 O–H bend (carboxylic groups)

1375 C–H bend (CH3)

1260 and

1220

C–O stretch (carboxylic groups, phenols, aromatic/

unsaturated ethers)

1095 and

1030

C–O stretch (alcohols, aliphatic ethers)

805 C–H bend (tri- and tetra-substituted aromatic rings)



(2) The analysis of the elemental composition and SUVA

of humic substances (fulvic and humic acids) provides

information about certain structural characteristics (aro-

maticity). The aquatic humic substances extracted from

the Uzquiza Reservoir contain more nitrogen than the

CHA used in this study. Humic acids have higher SUVA

values than fulvic acid, which is in accordance with the

suggested higher aromaticity for humic acids.

(3) The acidity of humic substances influences several of

their properties such as the solubility and the capacity to

form complexes with metal ions. The results indicate a

higher total and carboxylic acidity for fulvic acid, which is

in accordance with its higher solubility in water.

(4) Infrared analysis provides a considerable amount of

information about the main functional groups present

in humic substances and it allows us to reach some conclu-

sions about their macromolecular structure. Fulvic acid

is quite different from humic acids and the spectra appear

to confirm the greatest content of carboxyl groups in

fulvic acid and a higher aromaticity for humic acids. CHA

seems to have the highest degree of substitution at the

aromatic rings.

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