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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
Rev. Int. Contam. Ambie. 29 (3) 191-199, 2013
pH EFFECT ON SURFACTANT PROPERTIES AND SUPRAMOLECULAR STRUCTURE
OF HUMIC SUBSTANCES OBTAINED FROM SEWAGE SLUDGE COMPOSTING
Víctor Angel RAMÍREZ COUTIÑO
1
, Luis Gilberto TORRES BUSTILLOS
2
,
Luis Arturo GODÍNEZ MORA TOVAR
1
Ricardo Jaime GUERRA SÁNCHEZ
3
and
Francisco Javier RODRÍGUEZ VALADEZ
1
*
1
Centro de Investigación y Desarrollo en Electroquímica (CIDETEQ), Parque Tecnológico Querétaro, Sanfan-
dila, Pedro Escobedo, Querétaro, México
2
Unidad Profesional Interdisciplinaria de Biotecnología-Instituto Politécnico Nacional (UPIBI-IPN), Acueducto
s/n, La Laguna Ticomán, Gustavo A. Madero, 07340, Distrito Federal, México
3
Centro de Innovación Aplicada en Tecnologías Competitivas (CIATEC), Omega 201, Fraccionamiento Indus-
trial Delta, CP 37545, León, Guanajuato, México
*Autor responsable: frodriguez@cideteq.mx
(Recibido mayo 2012, aceptado abril 2013)
Key words: compost, sewage sludge, surface tension, size exclusion chromatography, pH, humic acid
ABSTRACT
This work evaluates the surfactant properties of humic acids (HA) obtained from sew-
age sludge composts and the production of value-added products from the residue that
is typically a disposal problem. To obtain the HA, aerobically digested sewage sludge
was composted with either wooden shavings (HACOMP1) or tezontle (HACOMP2) as
a bulking agent. HACOMP1 and HACOMP2 samples were evaluated for their effect on
surface tension and compared with a commercial vermicompost sample (HALCOMP).
The surface tension values for a concentration of 5000 mg/L, showed a decrease from
72 mN/m to 49 and 51 mN/m
for HACOMP1 and HACOMP2 respectively, while the
commercial vermicompost diminished only to 65 mN/m. The changes in surface ten-
sion could be related to the presence of functional groups, particularly COOH and a
predominance of either aliphatic or aromatic chains. This fact was conFrmed by ±TIR
analysis, COOH quantiFcation, and the E
4
/E
6
ratio. These results indicate that lower
surface tension is due to the predominance of aliphatic chains, plus lower content of
COOH groups and a higher E
4
/E
6
ratio
(short size molecule). Also, samples exhibit pH
dependence, having a lower surface tension at pH values from 3 to 4. According to size
exclusion chromatography (SEC) tests, this effect could be related to changes in the
supramolecular structure of the humic acids, in such way that at this pH value there
is an arrangement of molecules that decrease surface tension. Humic acids obtained
from sewage sludge compost affect surface tension in a way similar to other humic
acids obtained from soils and compost obtained from other organic compounds. Thus,
humic acids can be used as surfactants, with the added advantage that their use also
disposes of the hazardous waste that is the source of these compounds.
Palabras clave: compost, lodo residual, tensión superFcial, cromatografía de exclusión por tamaño, pH, ácido
húmico
F.J. Rodríguez Valadez
et al.
192
RESUMEN
Este trabajo evalúa las propiedades surfactantes de ácidos húmicos (AH) obtenidos de
compost realizados con lodos de plantas de tratamiento de agua, con el fn de obtener
un producto de valor agregado a partir de un residuo que usualmente representa un
problema de disposición. Para obtener los AH, el lodo biológico estabilizado de ma-
nera aeróbica fue descompuesto utilizando viruta de madera (HACOMP1) o tezontle
(HACOMP2) como material abultante. Se evaluó el efecto de las muestras de ácidos
húmicos obtenidos de los compost HACOMP1 y HACOMP2 sobre la tensión superfcial
y se comparó con muestras obtenidas de un vermicompost comercial (HALCOMP),
encontrando que usando una concentración de 5000 mg/L se tiene una disminución
desde valores de 72 mN/m hasta 49 y 51 mN/m
para
HACOMP1 y HACOMP2
respectivamente, en tanto que para el vermicompost comercial la tensión superfcial
disminuye solamente hasta 65 mN/m. Los cambios observados en la tensión superfcial
pudieran estar relacionados con la presencia de grupos funcionales, particularmente
el contenido de COOH así como la predominancia de cadenas alifáticas o aromáticas.
Este hecho Fue confrmado por análisis ±TIR, cuantifcación de grupos COOH y cálculo
de la relación E
4
/E
6
; los resultados obtenidos indican que la mayor disminución de
la tensión superfcial se debe a la predominancia de cadenas aliFáticas en los AH, así
como a un bajo contenido de grupos COOH y a una relación E
4
/E
6
alta (moléculas de
tamaño pequeño). Asimismo, la tensión superfcial mostró una dependencia del pH,
obteniéndose una mayor disminución a pH entre 3 y 4. De acuerdo con los resultados
de la cromatografía de exclusión molecular este efecto podría estar relacionado con
cambios en la estructura supramolecular de los ácidos húmicos, de manera que a estos
valores de pH hay un arreglo de las moléculas que disminuye la tensión superfcial.
De manera general el trabajo muestra que los ácidos húmicos obtenidos de la descom-
posición de lodos residuales de plantas de tratamiento aFectan la tensión superfcial
de manera similar a los ácidos húmicos obtenidos del suelo o de otros compuestos
orgánicos, pudiendo ser usados como surfactantes, con la ventaja de que son obtenidos
a partir de un residuo contaminante y peligroso.
INTRODUCTION
Oxidation of organic matter from plants and ani-
mals, followed by a polymerization process, leads to
the formation of humic substances (Stevenson 1994,
Schulten and Schnitzer 1997). Humic substances
are defned as heterogeneous, dark-colored reFrac
-
tory compounds, which are produced by the action
of microorganisms (MacCarthy 2001, Sutton and
Sposito 2005). These substances are classifed into
three groups according to their solubility: humic
acids (HA), which are insoluble at pH 2; fulvic
acids (HF), which are soluble at all pH values; and
humins (HU), which are insoluble in acidic and
alkaline conditions (Stevenson 1994, Wandruszca
2000, Chilom
et al.
2009).
Most reports relating to humic substances have
focused on the physicochemical characterization
of humic acids extracted from soil. However, some
studies of humic characterization have used compost
obtained from organic waste. These studies have
reported data for pH, organic matter and nitrogen
content, cation exchange capacity (CEC), elemental
analysis of C, H, N, S, as well as infrared spectra and
nuclear magnetic resonance (NMR) analysis (Rev-
eille
et al.
2002, Zbytniewski and Buszewski 2005a,
2005b, Campitelli
et al.
2006, González
et al.
2006).
Using the last two techniques it has been established
that humic acids contain aromatic, carboxylic and
phenolic functional groups (Swift 1989, Yates
et al.
1999, Sánchez
et al.
2003, Simpson
et al.
2002).
Chemical properties of humic acids are important,
particularly the presence of acidic functional groups,
because they defne their complexation, Folding, and
supramolecular properties
.
Because of their chemical structure, HA have
characteristics that promote their accumulation at
the air-water interface, which causes a decrease in
the surface tension (ST) of water (Wershaw 1993,
Wandruszka 1998, Tombácz 1999, Quadri
et al.
2008). Also, previous studies have shown that ST is
signifcantly aFFected by changes in environmental
conditions. For instance, Yates
et al.
(1999) reported
that for solutions of humic acids obtained from
pH EFFECT ON SURFACTANT PROPERTIES OF HUMIC SUBSTANCES
193
rivers and soil, surface tension decreases as pH is
lowered from 9 to 4. Moreover, surface tension also
showed an increase when pH values drop from 3
to 2. Recently, Adani
et al.
(2010) confrmed ST
reduction when pH drops to 3. This behavior may
be related to acidic group neutralization, a fact that
generates intramolecular electrostatic interactions
that determine their physical shape. At high pH and
low ionic strength, acid groups are fully ionized,
and there is a strong repulsion between charged
groups causing the molecule to expand (Tombácz
1999). This phenomenon is also related to the buffer
capacity of humic acids that has been related to the
physisorption of protons and hydroxyl ions, which
gives them a buffer capacity between pH 5.5 and
8.0 (Pertusatti 2007). In addition, there are several
studies showing that humic acids can form micelle-
like structures that act as solubilizing agents for
hydrophobic compounds (Guetzloff and Rice 1994,
Terashima
et al.
2004, Sierra
et al.
2005) in a way
similar to that of synthetic surfactants.
On the other hand, size exclusion chromatography
(SEC) has been used to gain insight to HA structure
at the molecular level under different conditions
(Piccolo
et al.
1996, Piccolo 2001, Maia
et al.
2008). For example, Piccolo
et al.
(1996) reported
that pH signifcantly aFFects the conFormation and
size of humic acids. SEC was also used to evaluate
changes in the elution volume of sodium and am-
monium humates obtained by composting (Maia
et
al.
2008). According to Pellegrino
et al.
(2005) HA
can be used to aid bioremediation techniques, since
they are natural surfactants that reduce the sorption of
organic contaminants, showing removal of pollutants
similar to that of the synthetic surfactants. Due to the
fact that the surfactant properties can be affected by
the degree of aromaticity of the molecule, the E
4
/E
6
ratio has been proposed as a measure of the degree
of condensation of the aromatic carbon network in
humic acids. Thus, while a high E
4
/E
6
ratio re±ects
a low degree of aromatic condensation, and a large
proportion of aliphatic structure, a low E
4
/E
6
ratio
reflects a high degree of aromatic condensation
(Gieguzynska
et al.
1998, Fong
et al.
2006).
This work evaluates the surfactant properties of
humic acids obtained from sewage sludge composts,
resulting in the production of a value-added product
from a residue that usually represents a disposal
problem. The composting process was carried out
by mixing aerobically digested sludge and grass, and
using either wooden shavings or tezontle as a bulk-
ing agent. HA were characterized in terms of their
chemical properties and structure, and compared with
samples of soil and HA obtained from other organic
wastes. An interpretation of the effect of HA on the
surface tension of the solution was also made. In ad-
dition, the surfactant effects of these materials were
evaluated under different pH conditions in order to
correlate their supramolecular structure with their
surfactant properties.
MATERIALS AND METHODS
The humic acids used in this work were obtained
from: 1) Compost produced from mixing 30% mu-
nicipal sewage sludge, 60% grass, and 10% wooden
shavings (HACOMP1); 2) Compost employing 30%
municipal sewage sludge, 60% grass, and 10% of
tezontle, a porous volcanic material (HACOMP2);
and 3) a commercial product named Vermilik, con-
sisting of vermicompost humic acid (HALCOMP)
produced from manure by Biotecnología Agrícola
SA de CV that contains 28% organic matter. Bulking
agents were used in order to evaluate their effect on
the surfactant properties of the humic acids obtained
in the compost process.
Composting of sewage sludge
Sewage sludge was obtained from an activated
sludge wastewater treatment plant located at the
City of San Miguel de Allende, in central Mexico.
Samples were collected after they had received
an aerobic digestion treatment and thickening in a
flter band. The 1 m
3
compost piles were produced
by mixing the sludge with the grass and the bulking
agent as indicated above.
Subsequent temperature
measurements confrmed that in both cases thermo
-
philic temperatures were obtained from the 3
rd
to
the 21
st
day. For HACOMP1 temperatures ranged
between 50 and 65 ºC, and for HACOMP2 between
40 and 55 ºC. The residual organic matter was 19%
for HACOMP1 and 21% for HACOMP2.
Humic acids isolation
Humic acids were extracted by placing 10 g
samples of the humic acid containing material, with
100 ml of Na
4
P
2
O
7
0.1 M in sealed bottles, and stirred
during 24 h. The extracts were centrifuged for 20
min at 10 000 rpm, and the supernatant was carefully
removed and passed through a 0.45 µm cellulose
syringe flter. The fltered solution was acidifed to
pH 2 using 5 M HCl and maintained at 4 ºC for 24
h. Then, it was centrifuged for 20 min at 10 000 rpm.
In this step a precipitate containing the HA fraction
was separated, which was washed twice with 0.1 M
F.J. Rodríguez Valadez
et al.
194
HCl and twice with deionized water. The HA were
further puriFed by stirring with a 0.06 M HCl solu
-
tion for 12 h, and then centrifuged and washed with
deionized water. The HA obtained was dried at room
temperature and stored for further analyses.
Fourier-transformed infrared analysis (FT-IR)
The chemical composition of the humic acids was
investigated using the FT-IR technique, with a FT-IR
Perkin Elmer 100 series spectrophotometer operat-
ing in transmission mode using potassium bromide
pellets (200 mg KBr) containing 1 mg of the relevant
freeze-dried humic acid.
Acid-base titration
Total acidity and carboxylic group quantiFcation
of the HA were determined by means of titrations
with barium hydroxide and calcium acetate respec-
tively, as previously reported by Perdue (1985). The
content of phenolic groups was calculated as the
difference between total acidity and the amount of
carboxyl groups. All experiments were run in tripli-
cate and the acid-base content of the HA was reported
as the average of three independent titrations.
UV-Vis analysis
HA samples (3 mg) were dissolved in 10 mL of
0.05 M NaHCO
3
solution, and UV-Vis absorption
spectra were recorded in the 200-800 nm window
using a Hach D/R 100 spectrophotometer. The E
4
/E
6
ratios were calculated as the ratio of absorbance of
the sample at 465 and at 665 nm (Chen
et al.
1977).
Surface tension (ST) and critical micelle concen-
tration (CMC) measurement values for HA in
solution
In order to determine the effect of humic acids on
water surface tension, solutions containing HA were
prepared by mixing concentrations between 10 and
6000 mg/L of dry powder and 0.1 M NaOH. The
solution pH was adjusted to 11.8 with 0.1 M HCl,
and stirred for 4 h. Afterward samples were Fltered
using a 0.45 µm membrane and left to stand overnight
before measurements, at 4ºC. All ST
measurements
were carried out at 25± 0.2 ºC and the reported data
are the average of three measurements. In order to
evaluate the effect of pH on surface tension, solu-
tions were prepared containing 100 mg/L of HA
dissolved in 0.1 M NaOH; pH values were adjusted
between 7 and 2
with 0.1 M HCl.
All surface ten-
sion measurements were made using the Du-Noy
ring methodology in a laboratory tensiometer (Cole
Palmer, USA). After each measurement, the platinum
ring was washed and cleaned with acetone (J.T.Baker,
Mexico). In addition, the ring was exposed to a ±ame
in order to avoid the presence of any organic material
on the surface.
Size exclusion chromatography analysis of humic
acids
Changes in molecular size and shape of the HA
under study were analyzed using an HPLC apparatus
(Agilent 1260 InFnity series) equipped with a DAD
detector (wavelength range 254-600 nm), using a
standard size exclusion column (Polymer Standard
Service, 8 x 300 mm). The injection volume was 100
µL and the mobile phase consisted on 0.001 M NaOH
at pH 10.3 and a ±ow rate 0.8 mL/min. Chromatog
-
raphy samples were prepared as follows: 2.5 mg of
HA were dissolved in 25 mL of 0.1 M NaOH; and
pH values were modiFed using a 0.1 M HCl solution
in the of 7-2 units range.
RESULTS AND DISCUSSION
FT-IR spectroscopy
FT-IR spectra of the different HA samples used
in this study (
Fig. 1
) show similarities with typical
soil HA peaks (Stevenson 1994, Senesi
et al.
1996,
Francioso
et al.
2002). In general, FTIR spectra of
the sewage sludge compost HACOMP1 and HA-
COMP2 reveal similar, intense peaks at 1430-1460
cm
–1
, which are assignable to aliphatic structures.
The absorption bands at 1720-1734 cm
–1
, 1600-1625
cm
–1
, and 1510-1516 cm
–1
were more pronounced
for HALCOMP, indicating a higher content of
C=O, COOH, and aromatic C=C groups. A broad
absorption band at 3200-3400 cm
–1
(O-H vibration
of carboxylic and alcoholic groups or N-H groups),
a 1250-1280 cm
–1
peak (C-O stretching and OH de-
formation of COOH, C-O stretching of aryl ethers),
and a 800-1150 cm
–1
signal (C-O stretching of poly-
saccharide or polysaccharide-like substances) were
also observed.
These results showed that HA obtained
from vermicompost contains aromatic groups, while
HA from the sewage sludge compost has predomi-
nantly aliphatic structures.
Chemical and spectroscopic characteristics of the
different origin-HA
Total (R-COOH+R-OH), carboxyl (R-COOH),
and OH (R-OH) acidic group contents were deter-
mined and reported in
Table I
. For R-COOH acidity,
the values range between 1.59 (for HACOMP1) and
2.96 meq/g (for HALCOMP). The HALCOMP2
pH EFFECT ON SURFACTANT PROPERTIES OF HUMIC SUBSTANCES
195
sample showed an intermediate content value of
2.12 meq/g. Sánchez
et al.
(2003) have character-
ized many HA from composts obtained from poultry
manure made from sewage sludge, and reported R-
COOH contents in the range of 2.60-5.67 meq/g. The
carboxylic acidity of HA sewage compost samples
(HACOMP1 and HACOMP2) and vermicompost
(HALCOMP) are within those reported previously.
Also, higher values of R-COOH were measured for
HALCOMP, an observation that is consistent with
FTIR analysis that showed a higher content of car-
boxylic groups.
The E
4
/E
6
ratio has been proposed as a measure
of the degree of condensation of the aromatic carbon
network in humic acids. In this way, while a high E
4
/
E
6
ratio refects a low degree oF aromatic condensation
and a large proportion of aliphatic structure, a low E
4
/
E
6
ratio refects a high degree oF aromatic condensa
-
tion (Gieguzynska
et al.
1998, Fong
et al.
2006). HA-
COMP2 and HACOMP1 samples did not show signi±
-
cant differences between them. Both were characterized
by a high E
4
/E
6
ratio (7.2 and 7 respectively), which
corresponds to a high content of aliphatic chains. The
HALCOMP commercial sample had the lowest E
4
/E
6
ratio (5.7,
Table I
), which indicates more condensation
of aromatic groups than that of the HA composts. These
results are consistent with the FTIR analysis, whereby
HACOMP2 and HACOMP1 displayed relatively higher
absorption intensity at 1460 cm
–1
(Aliphatic C-H) com-
pared to that observed for HALCOMP, which displays
a high absorption intensity at 1610 cm
-1
(aromatic C=C
and H-bonded C=0).
Surface tension (ST) as a function of humic acid
concentration
The ST values determined as a function of the con-
centration of compost and commercial HA are very
different. As can be seen from the data presented in
fgure 2
, there is a decrease in ST when the concentra-
tion of humic acids in the solution is increased. This
trend is maintained up to a particular concentration,
after which ST remains relatively constant for higher
HA concentrations. This point is known as the critical
micelle concentration for surfactants. Some authors
have identi±ed a pseudo-CMC For humic acids, since
the conformation of HA pseudo-micelles is similar
to that of real micelles (Wandruszka 2000). In this
work, the term pseudo-micelles or micelles will be
used interchangeably. At a lower dosage, HACOMP1
is the humic acid that decreases the ST value the most,
followed by HACOMP2. Their surface tension values
were 49 and 51 mN/m respectively. This suggests
that the bulking agent does not have a signi±cant
effect on the surfactant properties of humic acids
HALCOMP
HACOMP1
3500
2500
1500
500
3000
2000
Wavenumber (cm
–1
)
Transmitance (arbitrary units)
1000
HACOMP2
Fig. 1.
FT-IR of humic acids. HACOMP1, compost
produced of
municipal sewage sludge,
grass and wooden shavings;
HACOMP2, compost of municipal sewage sludge, grass
and tezontle; HALCOMP, commercial vermicompost.
TABLE I.
ACIDIC FUNCTIONAL GROUPS AND E
4
/E
6
RELATION OF HUMIC ACIDS AND SURFACE
TENSION OBTAINED IN THE CRITICAL MICELLE CONCENTRATION OF HUMIC ACIDS.
Humic acid
E
4
/E
6
Total acidity
(meq/g)
COOH
(meq/g)
OH
(meq/g)
*CMC
(mg/L)
**Surface tension
(mN/m)
HACOMP1
7.00
13.11
1.59
11.52
2000
51
HACOMP2
7.20
10.11
2.12
7.99
3000
53
HALCOMP
5.70
12.30
2.96
9.34
4800
65
*CMC = Critical micelle concentration
** Surface tension at CMC
F.J. Rodríguez Valadez
et al.
196
obtained from sewage sludge compost, so that any
of the studied materials can be used. On the other
hand, the vermicompost humic acid (HALCOMP)
showed minimal ST reduction, with an average value
of 65 mN/m at higher concentrations. Based on these
results and the values obtained for carboxylic acid-
ity (
Table I
), surface tension values are lower in the
samples that contain a large number of COOH groups
(HACOMP1 and HALCOMP). Also, the E
4
/E
6
ratio
and FTIR analysis show that the last samples obtained
from sewage sludge compost have a large proportion
of aliphatic structures, so changes in surface tension
could be related to the predominance of aliphatic
structures in the humic acid.
Plotting two lines on the experimental surface
tension points, one for the zone of continuous ST
decrease and one for the zone where ST is almost
constant, gives an intersection that corresponds
to the CMC value. The computed critical micelle
concentration value (
Table I
) indicates concentra-
tions of 2000 mg/L
for HACOMP1, 3000 mg/L
for
HACOMP2, and 4800 mg/L
for HALCOMP. Thus,
humic acids obtained from sewage sludge composts
have a lower CMC than the commercial vermicom-
post HA. Consistent with these results Quadri
et al.
(2008) reported CMC values for HA of different
samples obtained from compost of lignocelluloses
wastes that oscillated between 471 to 4040 mg/L. An-
other interesting point is the value of surface tension
at the CMC. The obtained values of 51, 53, and 65
mN/m for HACOMP1, HACOMP2, and HALCOMP,
respectively, are similar to those obtained by Quadri
et al.
(2008) who reported values between 40.8 and
50.4 mN/m for the different humic acids obtained
from lignocellulosic waste compost, and 36.1 mN/m
for Aldrich humic acid.
These results indicate that humic acids obtained
from sewage sludge compost can be used as a sur-
factant, because they decrease surface tension. Also,
those humic acids have properties similar to humic
acids obtained from soil and other organic waste
compost.
Surface tension (ST) for humic acids as a func-
tion of pH
Figure 3
shows the ST profles as a Function oF pH
for the HA samples, indicating a decrease in surface
tension when pH is lowered. For HALCOMP, when
pH is lowered from 7 to 4, its ST decreases from 69
to 65.5 mN/m. The other two HA showed similar
behavior at pH 3, and minimal surface tensions
values were determined for HACOMP1 (60 mN/m)
and HACOMP2 (61.5 mN/m). This behavior of the
surface tension for the samples obtained from sewage
sludge compost is similar to that reported previously
by Yates
et al.
(1999) for humic acids of different soil
samples. The decrease in surface tension when the
pH is changed indicates the neutralization of the acid
sites, creating amphiphilic species that migrate to the
air-water interface (Yates
et al.
1999). This phenom-
enon is consistent with the functional group density
distribution and molecule size, which allowed exten-
sive H-bonding with the aqueous solvent throughout
the experimented pH range (Engebretson and Wan-
druszka 1998). In this work, it can be observed that
around pH = 4, HA are especially effective in forming
amphiphilic molecules. In all cases, the ST increased
again at low pH, where continued neutralization pro-
duced more hydrophobic molecules. It is important to
note that for all humic acid solutions, a precipitation
phenomenon was observed (at pH between 2 and 3).
75
70
65
60
55
45
50
40
0
1000
2000
3000
HA (mg/L)
Surface tension (mN/m)
4000
5000
HACOMP1
HACOMP2
HACOMP
6000
7000
Fig. 2
. HA surface tension as a function of HA concentration.
70
68
66
64
60
62
58
234567
8
pH
Surface tension (mN/m)
HACOMP1
HACOMP2
HACOMP
Fig. 3.
Surface tension of HAs as a function of pH values. For all
HAs, precipitation was observed at pH´s between 2 and 3.
pH EFFECT ON SURFACTANT PROPERTIES OF HUMIC SUBSTANCES
197
This could be due to an increase in H
+
concentration
that causes protonation of the HA carboxylic groups,
which eventually lead to precipitation.
As can be seen, the lowest surface tensions were
obtained in composts containing more aliphatic
groups and fewer carboxylic groups, facilitating
their neutralization and leading to rearrangement of
hydrophobic and hydrophilic groups for micellar for-
mation of the molecule. These results indicate that the
tested composts have a strong amphiphilic character
compared with commercial vermicompost products
and have the same behavior as soil humic acids when
there are changes in the pH of the solution.
Supramolecular structure of HA as a function
of pH
In order to correlate functional group content,
acidity, and surface tension to the structure of the
humic acids, SEC analyses were done at different
pH. As shown in
fgure 4
, there is one absorbance
peak for each HA located near 5.8 ml of elution vol-
ume. As pH decreases, all the surveyed HA samples
showed a decrease in the height of these peaks and
slightly lower values of elution volume were ob-
served. This phenomenon has been described previ-
ously as a change in supramolecular structure of soil
humic acids (Piccolo
et al.
1996). These changes in
the intensity of peaks are related to changes in the
supramolecular structure of HA originating from the
progressive neutralization of acid groups caused by
a decrease in pH. It is important to point out that in
all tested samples the peaks disappear at pH 2, sug-
gesting that under acid conditions HA precipitates.
These results show that changes in surface tension
in the samples of humic acids obtained from sewage
sludge compost are related to their supramolecular
structure, as can be seen in previous work with
samples of humic acids obtained from soils.
Considering the results obtained in this study for
the tested samples (HACOMP1, HACOMP2, HAL-
COMP), it can be seen that all samples produced
a decrease in water surface tension. As mentioned
before, neutralization of carboxyl groups enhances
the amphiphilic character of the molecules, and the
lowest surface tension was obtained in all cases be-
tween pH 3 and 4. Also, surface tension was reduced
more in humic acids obtained from sewage sludge
compost samples (HACOMP1 and HACOMP2) that
contain more aliphatic chains than aromatic groups.
At the same time, samples with fewer carboxyl groups
(HACOMP1 and HACOMP2) produced a greater
effect on the decrease of surface tension. Thus, for
the samples used in this study, ST decreases more for
molecules containing predominantly aliphatic groups
Fig. 4
. SEC chromatograms of humic acids (100 mg/L)
at different pH. HACOMP1, compost
produced
from municipal sewage sludge, grass and wooden
shavings humic acid; HACOMP2, compost from
municipal sewage sludge, grass and tezontle humic
acid and HALCOMP, commercial vermicompost
humic acid.
HALCOMP
HACOMP2
pH7
pH6
pH5
pH4
pH3
pH2
80
100
120
140
160
60
40
20
0
45
6
456789
Elution volume (mL)
Elution volume (mL)
Intensity (mV)
78
9
pH7
pH6
pH5
pH4
pH3
pH2
300
250
200
150
100
50
350
400
450
0
45
6
456789
Elution volume (mL)
Elution volume (mL)
Intensity (mV)
78
9
HACOMP1
pH7
pH6
pH5
pH4
pH3
pH2
70
60
50
40
30
20
10
0
45
6
456789
Elution volume (mL)
Elution volume (mL)
Intensity (mV)
78
9
F.J. Rodríguez Valadez
et al.
198
and low concentrations of carboxyl groups. Also, the
drop in pH resulted in the neutralization of humic acid
COO
-
groups, which produced a reduction of intramo-
lecular electrostatic interactions causing a contraction
of the HA molecule. This fact can be observed in the
changes and disappearance of the absorption peaks in
the SEC analysis of the HACOMP1, HACOMP2 and
HALCOMP samples. Taking into consideration the
consistency of chemical and spectroscopic properties,
as well as changes in ST and SEC analysis, it can be
seen that changes in surface tension are determined
by the presence of aliphatic/aromatic groups, COOH
content in the HA molecule, and changes in the supra-
molecular structure of the humic acid.
CONCLUSIONS
Humic acids obtained from sewage sludge com-
posting can be used to modify surface tension of
aqueous solutions in a way similar to HA obtained
from soil or compost of other organic compounds. In
this study, the use of wooden shavings (HACOMP1)
or tezontle (HACOMP2) as a bulking agent in com-
post had similar results, because surface tension
decreased from 72 mN/m to 49 and 51 mN/m, so
the bulking agent had no signifcant eFFect. Changes
in surface tension can be related to the presence of
functional groups, particularly COOH. Properties of
humic acids obtained from sewage sludge compost-
ing also depend on pH. In this work it was observed
that, at pH values of 3-4, HA exhibits lower surface
tension than the commercial one, which implies that
both HACOMP1 and HACOMP2 have a lower con-
tent of COOH groups and a predominance of aliphatic
chains, which is not the case for HALCOMP. The
effect of pH on surface tension is related to changes
in the supramolecular structure of the humic acids.
These results suggest that humic acids obtained from
sewage sludge compost can be used as a natural
surfactant in a way similar to other humic acids ob-
tained from other organic compounds, but they have
the added advantage of using dangerous waste as a
source of the humic acids.
ACKNOWLEDGMENTS
V.R.C. acknowledges the Consejo Nacional de
Ciencia y Tecnología (CONACyT) for a graduate
fellowship. The authors would like to express sincere
thanks to Biotecnología Agrícola SA de CV for the
donation of vermicompost humic acid.
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