isotherm and kinetic studies on the adsorption of humic acid...
TRANSCRIPT
ORIGINAL ARTICLE
Isotherm and kinetic studies on the adsorption of humic acidmolecular size fractions onto clay minerals
Mohamed E. A. El-Sayed1,2• Moustafa M. R. Khalaf1,3
• James A. Rice1
Received: 31 October 2018 / Revised: 7 January 2019 / Accepted: 4 March 2019 / Published online: 8 March 2019
� Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2019
Abstract Humic acid (HA) can adsorb onto mineral sur-
faces, modifying the physicochemical properties of the
mineral. Therefore, understanding the sorption behavior of
HA onto mineral surfaces is of particular interest, since the
fate and transport of many organic and inorganic contam-
inants are highly correlated to HA adsorbed onto clay
surfaces. Due to the extreme heterogeneity of HA, the
extracted IHSS Leonardite humic acid (LHA) used in this
work was fractionated using an ultrafiltration technique
(UF) into different molecular size fractions (Fr1, [ 0.2
lm; Fr2, 0.2 lm–300,000 daltons; Fr3,
300,000–50,000 daltons; Fr4, 50,000–10,000 daltons; Fr5,
10,000–1000 daltons). Equilibrium and the kinetics of
LHA and fraction adsorption onto kaolinite and montmo-
rillonite were investigated. The results demonstrated that
the maximum adsorption capacity of LHA, Fr1, Fr2, Fr3,
Fr4, and Fr5 was 5.99, 13.69, 10.29, 7.02, 5.98, and 5.09 on
kaolinite while it was 8.29, 22.62, 13.17, 8.91, 8.62, and
5.69 on montmorillonite, respectively. The adsorption
equilibrium data showed that the adsorption behavior of
LHA and its fractions could be described more practically
by the Langmuir model than the Freundlich model. The
rate of humic acid fraction adsorption onto clays increased
with decreasing molecular size fraction and increasing
carboxylic group content. Pseudo-first- and second-order
models were used to assess the kinetic data and the rate
constants. The results explained that LHA and its fractions
adsorption on clay minerals conformed more to pseudo-
second-order.
Keywords Kaolinite �Montmorillonite � Leonardite humic
acid � Humic acid fractions � Kinetics � Equilibrium
1 Introduction
Sorption of humic substances (HSs) to clays is a funda-
mental process in the environment. This process modifies
surface properties and the reactivity of clay minerals (Al-
Essa and Khalili 2018). Understanding the interactions of
HSs and clay minerals is important for modeling the geo-
chemical fate and transport of nutrients and contaminants
in soil and water (Zaouri 2013).
Humic acid is a major fraction of HSs and generally
contains both hydrophobic and hydrophilic moieties, as
well as many reactive functional groups (e.g. –COOH, –
C=O and –OH) in the component molecules. The existence
of carboxylic and phenolic groups results in HA carrying a
predominantly negative charge in aqueous solutions under
normal environmental conditions (Maghoodloo et al.
2011).
The clay minerals kaolinite and montmorillonite are
highly abundant and composed of two basic building
blocks containing silicon (Si2O5-2) and aluminum
(Al(OH)6-3). Kaolinite and montmorillonite are layers of
tetrahedral Si and octahedral Al sheets. Furthermore, the
Si:Al ratio of kaolinite as nonexpanding clay is 1:1 while
for montmorillonite, the ratio as expanding clay is 2:1.
Clay minerals play the role of a natural scavenger of metals
and organic matter (OM). Clay minerals have a high
& Mohamed E. A. El-Sayed
1 Department of Chemistry and Biochemistry, South Dakota
State University, Brookings, SD 57007, USA
2 Soils, Water, and Environmental Research Institute,
Agriculture Research Center, El-Giza, Egypt
3 Chemistry Department, Faculty of Science, Minia University,
El-Minia 61519, Egypt
123
Acta Geochim (2019) 38(6):863–871
https://doi.org/10.1007/s11631-019-00330-4
specific surface area, chemical and mechanical stability,
layered structure, and high cation exchange capacity. Thus,
the clay minerals are excellent materials for adsorption
(Bhattacharyya and Gupta 2008). Moreover, nano-clay
minerals have some important features such as having
significantly wider surface areas than other particles, and
they can be combined with different chemical groups to
increase their efficacy (Derakhshani and Naghizadeh
2018).
Several authors reported different binding mechanisms
between clay minerals and HA, such as cation bridging,
ligand exchange, and van der Waals forces (Murphy and
Zachara 1995). Polydispersity and polyelectrolytic prop-
erties of HA also play a major role in the adsorption pro-
cess onto clay minerals. Thence, to better understand the
interaction mechanism and due to the chemical hetero-
geneity and polydispersity of HA, UF has been used to
reduce the chemical heterogeneity of HA by using suit-
able membrane filters. UF is a reasonably simple method
for fractionation of HA into different molecular size frac-
tions (Khalaf 2003).
The kinetics of LHA and its fractions adsorption on clay
minerals are a significant factor in determining HA
behavior in soil and aquatic systems and gives information
about the adsorption capacity and its mechanisms.
Accordingly, the objective of this study was to explore and
understand the sorption mechanisms of HAs on clay min-
erals through kinetics and fractions sorption experiments.
2 Materials and methods
2.1 Clay minerals
Kaolinite (KGa-2) and montmorillonite (SWy-2), two clay
minerals commonly found in soils, were purchased from
the Source Clay Minerals Repository, University of Mis-
souri-Columbia, Missouri. Clay minerals were pretreated
as described elsewhere (Shang et al. 2001). 25 g of clay
sample was placed in a 500 mL Erlenmeyer flask, and then
250 mL of pH 5 acetate buffer were added to remove
carbonates. The mixture was heated in a water bath (90 �C,
2 h) before centrifugation. The supernatant was discarded.
Hydrogen peroxide (30%) was added to the clay samples
and the sample was digested (12 h, 90 �C, water bath) to
oxidize organic matter. After organic matter oxidation, the
sample was suspended in citric bicarbonate buffer (pH 8.3)
at 80 �C and an appropriate amount of dithionite powder
was added to remove free iron oxides. Finally, the sample
was transferred into a 1 L plastic bottle and the clay was
shaken with 1 mol/L NaCl (6 h) before centrifugation.
Then the supernatant was discarded. The salt washing step
was repeated once. The Na-saturated clay was washed once
with distilled and deionized water and dialyzed against
water until it was free of chloride (silver nitrate test). Clay
minerals were dried in an oven at 100 �C.
BET surface area of clay minerals was obtained from
nitrogen gas adsorption/desorption isotherms at 77 K,
using a Micromeritics ASAP 2000 analyzer. Prior to
measurements, all samples were degassed to 0.1 Pa and
150 �C for 2 h. Furthermore, particle size distribution was
done by x-ray diffraction technique, and elemental com-
position was done by scanning electron microscopy/en-
ergy-dispersive X-ray analysis (SEM/EDAX) techniques.
2.1.1 Preparation of clay mineral suspensions
Clay mineral suspension (kaolinite and montmorillonite)
5 g/L stock solutions were prepared in order to obtain
consistent solid concentrations for the equilibrium
adsorption experiments. These suspensions were prepared
in 0.01 mol/L of NaCl for LHA and fraction adsorption
isotherms at pH 6 (Khalaf et al. 2009).
2.2 Humic acid and fractions
The LHA was isolated from the IHSS Leonardite by alkali
extraction (Swift 1991) and fractionated into five humic
acid molecular size fractions using the UF technique as
described below.
2.2.1 Fractionation of LHA by ultrafiltration
The LHA sample was fractionated into five size-fractions
using a cross-flow ultrafiltration technique (MinitanTM,
Millipore). Different cellulose membranes (Millipore) with
nominal molecular weight cutoffs of 1, 10, 50, 300 kDa
and 0.2 lm were used. An aqueous LHA solution at a
concentration of 1.5 g/L was prepared by dissolving an
appropriate amount of the LHA in NaOH. In brief, the
alkaline LHA solution was microfiltered using a 0.2 lm
Nylon filter and an Amicon ultrafiltration stirred cell
(model 8050) under a pressure of 2.5 bar using argon gas.
The retentate was washed with small portions of Millipore
water. The washed retentate (Fr1) was carefully pipetted
out of the UF cell. The filtrate obtained was then frac-
tionated using an Amicon UF cell (model 8400) and a
series of Amicon membranes of successively smaller pore
size to obtain HA fractions with different nominal molec-
ular sizes which were classified as Fr1 [ 0.2 lm, Fr2,
0.2 lm–300 kDa; Fr3, 300–50 kDa; Fr4, 50–10 kDa, and
Fr5, 10–1 kDa. All separations were done under the same
conditions. After each run, the membrane was removed,
rinsed with distilled water, and stored overnight in a
refrigerator at 4 �C. The complete separation process was
repeated at least three times to assess the reproducibility of
864 Acta Geochim (2019) 38(6):863–871
123
the fractionation technique. The recovery percentage of the
whole LHA fraction process was calculated at 92%. The
separated LHA fractions were freeze-dried, weighed, and
stored in the dark at 3 �C. LHA and five fractions were
characterized by UV–visible and 13C DP solid state NMR
spectroscopy.
A carbon-type distribution of LHA and the fractions
were determined by solid state 13C DP-MAS NMR using a
Bruker ASX300 spectrometer (rotation speed = 13 kHz;
recycle delays were determined for each sample and were
between 3 and 15) (Li et al. 2004).
LHA and the five fractions were characterized using a
Milton Roy UV–visible spectrometer. The degree of aro-
maticity was calculated by determining the ratio of
absorbance of LHA and its fractions at 465 and 665 nm
(E4/E6 ratio) (Purmalis and Klavins 2013; Helal et al.
2011). Aqueous solutions of LHA and each fraction were
prepared in the same manner as those used in the adsorp-
tion experiments.
2.2.2 Preparation of LHA and fraction solutions
Stock solutions of LHA and fractions were prepared by
dissolving LHA in an aqueous solution of NaOH (0.5 mol/
L) with shaking for 1 h. The adjustment pH to 6 was done
by adding HCl or NaOH. Amounts of NaCl were added to
LHA and fraction solutions to adjust to the desired ionic
strength of 0.01 mol/L. Final LHA or fraction concentra-
tions were 1 g/L.
2.3 Adsorption of LHA and fractions onto clays
A fraction of well-mixed clay suspension at the desired pH
and ionic strength was pipetted into a series of LHA or
fraction solutions at varying concentrations (0.083–0.75 g/
L) to give the final volume of 30 mL in 50 mL poly-
ethylene centrifugation tubes. The pH value checked up
through the experimental lifetime. The suspensions were
shaken for 24 h on a horizontal shaker (Lab line Instru-
ments, Melrose Park, IL, USA) to reach equilibrium
(Shaker et al. 2012). The final (kaolinite or montmoril-
lonite) concentrations for all adsorption experiments were
2.5 g/L. Preliminary experiments verified that, after 4 h, no
measurable change occurred in the adsorbed amounts. Each
sample was centrifuged for 5 min at 20,000 rpm (Damon/
IEC Division Model High-Speed Centrifuge). The amount
of the adsorbed LHA or fractions was calculated from the
difference between the total added LHA or fraction con-
centration and the LHA or fraction concentration in the
supernatant by the respective total organic carbon contents.
Total organic carbon contents were determined with a
Shimadzu TOC-VSCN with a solid-sampling module SSM
5000.
The amounts of LHA or fractions adsorbed were cal-
culated from the mass balance equation for each isotherm
bottle using Eq. (1):
qe ¼ Ci� Ceð ÞV
mð1Þ
where V is the volume of solution used in the adsorption
experiment (L), Ci and Ce are the initial and the equilib-
rium concentrations of LHA and fraction solutions (mg/L),
respectively, and m is the dry weight of the adsorbent
(g) (Komy et al. 2014).
Two models of Langmuir (1916) and Freundlich (1906)
in their related linearized expressions have been used as
Eqs. (2) and (3) respectively.
1
qe
¼ 1
KLCe
þ aL
KL
ð2Þ
where Ce is the concentration of adsorbate (mg/L) at
equilibrium, qe the amount of adsorbate at equilibrium
(mg/g), and aL (L/mg) and KL (L/g) are constants.
log qe ¼ log Kf þlog Ce
nð3Þ
where Kf ((mg/g)(mg/L)-1/n) and n are constants incorpo-
rating all factors affecting the adsorption process, including
the capacity and intensity of adsorption. If n is close to 1,
the surface heterogeneity could be assumed to be less
significant, and as n approaches 10, the impact of surface
heterogeneity becomes more significant (Noroozi et al.
2007).
2.4 Kinetic experiments
Kinetics experiments were carried out by shaking certain
amounts of adsorbents (kaolinite or montmorillonite) with
a 50 mL solution containing LHA or fraction (to achieve
435 mg/L concentration) at 25 �C. At pre-determined time
intervals for 24 h, portions of the mixture were drawn by a
syringe and then centrifuged, and LHA or fraction con-
centration was determined as described before. In adsorp-
tion kinetics, the amount of adsorption at time t, qt (mg/g)
was calculated by the following formula:
qt ¼ Ci�Ctð ÞV=w ð4Þ
where Ci is the initial concentration of LHA or fraction
solutions (mg/L), Ct is the concentration at any time t (mg/
L), V is the volume of solution used in the adsorption
experiment (L), and w is the dry weight of adsorbent (g).
These models include the irreversible pseudo-first-order
kinetics (Chang and Juang 2005) and the pseudo-second-
Acta Geochim (2019) 38(6):863–871 865
123
order (Wu et al. 2009; Asfaram et al. 2015). The linear
form of the irreversible pseudo-first-order model can be
formulated as:
lnCi
Ct
� �¼ k � t ð5Þ
where Ci (mg/L) is the initial concentration of LHA or
fraction and Ct (mg/L) is the equilibrium concentration of
LHA or fraction at time t respectively, and k (min-1) is the
rate constant. The values of k and correlation coefficients
were determined.
The linear pseudo-second-order kinetics can be formu-
lated as:
t
qt
¼ 1
k2q2e
þ t
qe
ð6Þ
where qe and qt are surface loading of LHA at equilibrium
and time t respectively, and K2 (g/mg min) is the rate
constant. The linear plots of t/qt as a function of t provided
not only the rate constant K2 but also an independent
evaluation of qe.
3 Results and discussion
3.1 Characteristics of clay minerals
The elemental analysis of kaolinite and montmorillonite
were determined by SEM/EDAX. The SEM/EDAX results
showed that the chemical composition analysis of kaolinite
was (SiO2 48.66%, Al2O3 40.80%, Na2O 3.9%, MgO
2.71%, K2O 1.55%, CaO 0.45%, Ti 0.91%) while that of
montmorillonite was (SiO2 60.41%, Al2O3 22.66%, Na2O
5.9%, MgO 4.49%, K2O 1.1%, CaO 0.35%, Ti 1.33%).
BET surface area and particle size of kaolinite and
montmorillonite properties were determined. Montmoril-
lonite and kaolinite particles were 80 ± 5 nm and
850 ± 20 nm on the nanoscale, respectively, and the BET
surface area of montmorillonite (107 m2/g) was much
greater than that of kaolinite (11.2 m2/g). BET surface area
is the measure of the accessible surface area per unit mass
of soil minerals and is equal to the external surface area.
3.2 Characteristics of LHA and fractions
LHA was fractionated by UF into five fractions. Each
fraction had relatively less heterogeneous properties than
the whole material, as described later. The fractionation
process was repeated three times. The (%TOC) distribution
of LHA fractions was 9 ± 0.01%, 4 ± 0.004%,
31 ± 0.017%, 33 ± 0.04%, and 23 ± 0.19% for Fr1, Fr2,
Fr3, Fr4, and Fr5 respectively. The fraction with 50–10 kDFig. 1 13C DP MAS NMR spectra of LHA and fractions
Table 1 A carbon type distribution based on the integration of the 13C-NMR spectra, degree of aromaticity, and carbon content percentage for
LHA and its size fractions
Sample Carbon type distribution (%) Carbon content
(%)
E465/E665
Alkyl carbon O-Alkyl carbon Aromatic carbon Carboxyl/carbonyl
0–50 (ppm) 50–108 (ppm) 108–160 (ppm) 160–220 (ppm)
LHA 11.8 12.2 62.9 13.1 54.78 5.99
Fr1 64.5 12.6 22.7 0.2 68.93 3.70
Fr2 21.8 13.1 57.8 7.3 64.05 4.04
Fr3 12.2 13.9 59.6 14.3 59.63 4.47
Fr4 9.97 13.31 60.21 16.51 57.29 7.90
Fr5 6.3 7.1 65.2 21.4 45.25 15.6
866 Acta Geochim (2019) 38(6):863–871
123
molecular weight was most obvious while fraction with
0.2 lm–300 kD molecular weight was lowest.
The solid-state 13C NMR spectra are shown in Fig. 1 for
the bulk and supernatant whole LHA samples. The
assigned peaks and the estimated peak areas obtained by
integration of the spectra are listed in Table 1. 13C-NMR
analysis of the LHA fractions revealed that the chemical
forms of carbon varied between the different size fractions.
The Fr1 fraction contained predominantly aliphatic carbon
(0–110 ppm) with lower contents of aromatic and carboxyl
carbon. By contrast, the Fr3, Fr4, and Fr5 fractions of much
smaller molecular weight had a much higher content of
aromatic (110–160 ppm) and phenolic and carboxyl car-
bons (140–160 ppm and 160–185 ppm, respectively) and
lower levels of aliphatic carbons (Nagao et al. 2009;
Tanaka 2012; Mukasa-Tebandeke et al. 2015). Further-
more, fraction Fr2 had approximately the same proportion
of aliphatic and aromatic carbon. These findings support
0 100 200 300 400 500 6000
1
2
3
4
5
6
7
8
9
10
Leo Humic acid
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
0 100 200 300 400 500 6000
5
10
15
20
25
Fr>0.2um
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
0 100 200 300 400 500 6000
3
6
9
12
15Fr=0.2um-300kD
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
0 100 200 300 400 500 6000
2
4
6
8
10
Fr=300-50kD
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
0 100 200 300 400 500 6000
1
2
3
4
5
6
7
8
9
10
Fr=50-10kD
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
0 100 200 300 400 500 6000
1
2
3
4
5
6
7
Fr=10-1kD
Cad
s (m
g/g)
Equillibrium Concentration, mg/L
Kaolinite Langmiur Freundlich Montmorillonite Langmiur Freundlich
Fig. 2 Adsorption of LHA and fractions onto kaolinite and montmorillonite with Langmuir and Freundlich model fitting
Acta Geochim (2019) 38(6):863–871 867
123
the degree of aromaticity determined by UV spectroscopy
as related in the E4/E6 ratio and the carbon content of each
fraction (Table 1). The degree of aromaticity increased
with decreasing LHA molecular size fractions while the
carbon content decreased.
3.3 Humic acid and fraction adsorption
Adsorption data for HA and fractions onto clay minerals at
pH 6 and 0.01 mol/L NaCl are shown in Fig. 2. The results
showed that the amount of LHA and fractions adsorbed
onto montmorillonite are greater than that adsorbed onto
kaolinite. This is due to the fact that the surface area and
cation exchange capacity for montmorillonite was more
than kaolinite (Tombacz et al. 2004; Khalaf et al. 2009).
When referencing the amount of LHA fraction adsorption
onto clay, it was obvious that the molecular size and the
structure of the LHA fractions played an important role in
the adsorption process. Moreover, the largest molecular
size fraction was more adsorbed while the smallest was less
adsorbed onto kaolinite and montmorillonite. These results
mean that the adsorbed amount increased with increasing
LHA molecular size. As shown from the 13C-NMR results
(Fig. 1), this observation suggests that the larger HA
molecular size fractions, which have more aliphatic carbon
(less hydrophilic), were more strongly adsorbed in com-
parison to the smaller molecular size fractions (more
hydrophilic), which have higher contents of aromatic car-
bon. Because adsorption is site-specific through the
hydroxyl functional groups on clay surfaces and the
hydrophobic moieties on siloxane sheets, only limited
numbers of the ionizable groups of HA fractions may react
with the clay surfaces. By suggesting limited surface site
availability, a lower amount of Fr5 was required to cover
the clay surfaces compared with Fr1. It is also observed
that there is a decrease in the slope of the adsorption iso-
therm in the plateau regions with decreasing the LHA
fraction size. This indicates a lower contribution of the
hydrophobic interactions due to the decrease in the ali-
phatic carbons (Fig. 1). Hence, the hydrophobicity of LHA
molecular size fractions and clay minerals also had an
important role in the adsorption process beside the func-
tional groups of LHA and clay. Similarly, previous workers
(Khalaf 2003; Dunnivant et al. 1992) have directly or
indirectly indicated that dissolved humic substances with
higher molecular size exhibited higher adsorbed amounts
on mineral surfaces compared to the smaller (more
hydrophilic) ones (El-sayed et al. 2019).
In addition, adsorption isotherm data are an essential
way of predicting the mechanisms of adsorption. The
results of the adsorption isotherm shape of LHA and
fractions, the structure of LHA and fractions, and the
highly variable adsorption amounts indicated that the
mechanism of HA adsorption onto clay minerals will be a
mix between two or more of following mechanisms: ligand
exchange, van der Waals, water bridging, anion exchange,
and hydrogen binding. The actual mix depends on the
composition and ratio of each LHA fraction. Moreover, the
ligand exchange mechanism could be the specific and
predominant mechanism due to the LHA fractions per-
centage, structure, and slight increase in pH values after the
adsorption process.
Furthermore, the results of LHA and fraction adsorption
on clay minerals, cf. Fig. 2 and Table 2, were compared
with Langmuir and Freundlich adsorption isotherm models,
where the Langmuir and Freundlich models have been
Table 2 LHA and fractions adsorption fitting by Langmuir and Freundlich equations
Humic acid
and fractions
Langmuir Freundlich
R2 Kl (L/g) al (L/mg) R2 KF(mg/g) (mg/L)-1/n n
Kaolinite LHA 0.996 0.1722 0.028 0.95 1.934 5.56
Fr1 0.98 0.1532 0.0091 0.91 1.4332 2.71
Fr2 0.99 0.1583 0.0132 0.95 1.498 3.12
Fr3 0.99 0.1622 0.0215 0.97 1.734 4.38
Fr4 0.98 0.1947 0.0306 0.95 1.5141 3.28
Fr5 0.97 0.1838 0.0336 0.94 1.96 1.57
Montmorillonite LHA 0.98 0.1364 0.01384 0.92 1.91 5.27
Fr1 0.994 0.2053 0.0069 0.94 1.34 2.13
Fr2 0.986 0.2374 0.0149 0.98 2.04 3.1
Fr3 0.992 0.16023 0.0154 0.90 1.77 3.7
Fr4 0.98 0.1455 0.0142 0.91 1.59 3.52
Fr5 0.98 0.1958 0.0332 0.97 2.01 4.91
868 Acta Geochim (2019) 38(6):863–871
123
frequently used to fit experimental data. Lists of the
parameters obtained along with R2 values are predicted in
Table 2. The results clarified that the Langmuir model
fitted to the experimental data was better than the Fre-
undlich model. This means that LHA and fraction
adsorption onto kaolinite and montmorillonite particles
occurred strongly on homogeneous surfaces.
3.4 Kinetic studies
The kinetic adsorption data were collected to understand
the dynamics of the adsorption reaction and explore the
rate constant. The kinetic parameters are generally helpful
for predicting the adsorption rate and give important
information to develop and model the adsorption processes
(Ghaedi et al. 2015).
The LHA and its fractions adsorption onto kaolinite and
montmorillonite reached equilibrium within different times
(cf. Fig. 3), meaning different mechanisms occurred for
0 20 40 60 80 100 1200
3
6
9
12
15
18
21
24
0 200 400 600 800 1000 1200 1400 16000
5
10
15
Leo Humic acid
q t mg/
gTime (min)
Kaolinite Montmorillonite
Leo Humic acid
q t mg/
g
Time (min)
Kaolinite Montmorillonite
0 50 100 150 200 250 300 3500
3
6
9
12
15
18
21
24
0 200 400 600 800 1000 1200 1400 16000
5
10
15
20
25
Fr > 0.2um
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fr > 0.2um
q t mg/
g
Time (min)
Kaolinite Montmorillonite
0 20 40 60 80 100 1200
3
6
9
12
15
18
21
24
0 100 200 300 400 500 600 700 8000
3
6
9
12
15
18
21
24
Fr=0.2um-300kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fr=0.2um-300kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
0 20 40 60 80 1000
3
6
9
12
15
18
21
24
0 100 200 300 400 500 600 700 8000
3
6
9
12
15
18
21
24
Fr=300-50kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fr=300-50kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
0 20 40 60 80 1000
3
6
9
12
15
18
21
24
0 100 200 300 400 500 600 700 8000
3
6
9
12
15
18
21
24
Fr=50-10kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fr=50-10kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
0 20 40 60 80 1000
3
6
9
12
15
18
21
24
0 100 200 300 400 500 600 700 8000
3
6
9
12
15
18
21
24
Fr=10-1kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fr=10-1kD
q t mg/
g
Time (min)
Kaolinite Montmorillonite
Fig. 3 The changing of LHA and fractions adsorption onto kaolinite and montmorillonite with time
Acta Geochim (2019) 38(6):863–871 869
123
each fraction. LHA adsorption onto montmorillonite and
kaolinite reached equilibrium within 60 and 90 min,
respectively, while Fraction [ 0.2 lm reached equilibrium
within 2 h in the case of kaolinite and montmorillonite. In
addition, Fraction from 10 to 1 kD reached equilibrium
within 10 and 15 min in case of montmorillonite and
kaolinite respectively. Results explained that the adsorption
process was highly dependent on carboxylic group content
and became faster by increasing carboxylic group content.
The values of kobs are affected by the change of the LHA
and its fractions structure where kobs values increased with
the increasing carboxylic groups’ content of LHA fractions
as shown in Table 3. The results indicated that the car-
boxylic groups’ content played a very important role in the
LHA and its fractions adsorption onto clay minerals.
Moreover, kobs values for LHA and its fractions adsorption
on montmorillonite were higher than kaolinite.
Pseudo-first-order and pseudo-second-order kinetic
models were used to evaluate the kinetic data and the rate
constants. The fitting of experimental data to the pseudo-
first-order and pseudo-second-order equations, cf. Table 4,
and the calculated correlation coefficient (R2) showed that
LHA and its fraction adsorption on montmorillonite and
kaolinite were more conforming to pseudo-second-order
kinetics model. In the pseudo-second-order kinetics model,
the rate-limiting step is the surface adsorption that involves
chemisorption due to physicochemical interactions
between the two phases. This means that the adsorption of
LHA and its fractions on montmorillonite and kaolinite is
mainly chemisorption. Additionally, qe values agree with
the experimental data. The initial adsorption rate, k2qe2, for
montmorillonite is greater than that for kaolinite. This can
be explained as due to the greater adsorption of LHA on
montmorillonite than of that on kaolinite (as described in
the adsorption section).
This result suggests varying binding mechanisms may
be accountable for different clays (Feng et al. 2005) and
some LHA properties such as molecular weight, functional
group compositions, and hydrophobicity lead the different
adsorption of LHA and fractions on clay minerals as well
as the reactive size of clay minerals (Zhang et al. 2012).
Moreover, the selectivity of mineral surfaces on LHA and
fraction adsorption by different clay minerals surface
deserve study in another paper.
4 Conclusion
In this study, LHA was fractionated into five molecular size
fractions by UF to decrease LHA heterogeneity and better
understand the LHA adsorption mechanism on clay min-
erals. The amount of LHA and fractions adsorbed onto
montmorillonite was greater than that of those adsorbed
onto kaolinite. The largest molecular size fraction was
Table 3 The relationships between clay minerals and kobs of LHA
and fractions
Kaolinite Montmorillonite
LHA and fractions kobs LHA and Fractions kobs
LHA 0.036 LHA 0.0773
Fr1 0.034 Fr1 0.067
Fr2 0.06 Fr2 0.127
Fr3 0.0613 Fr3 0.155
Fr4 0.108 Fr4 0.407
Fr5 0.219 Fr5 0.571
Table 4 Comparison of the pseudo-first-order and pseudo-second-order kinetic models for the adsorption of LHA and fractions onto clay
minerals at 25 �C
Humic acid fractions Pseudo-first-order kinetics Pseudo-second-order kinetics
R2 k1 (min-1) R2 k2 (g/min mg) qe (mg/g)(qexp)
Kaolinite LHA 0.89 0.0025 0.99 0.0257 5.99 (5.87)
Fr1 0.75 0.0004 0.97 0.0049 13.69 (13.2)
Fr2 0.59 0.00033 0.99 0.0087 10.29 (10.19)
Fr3 0.72 0.0002 0.99 0.019 7.02 (6.98)
Fr4 0.69 0.00015 0.99 0.026 5.98 (5.95)
Fr5 0.53 0.00008 0.99 0.036 5.09 (5.1)
Montmorillonite LHA 0.74 0.0022 0.99 0.0618 8.29 (8.25)
Fr1 0.52 0.0009 0.97 0.008 22.62 (22.15)
Fr2 0.53 0.0004 0.99 0.0245 13.17 (13.08)
Fr3 0.603 0.00021 0.99 0.053 8.91 (8.88)
Fr4 0.60 0.00016 0.99 0.057 8.62 (8.604)
Fr5 0.43 0.00004 1 0.131 5.69 (5.7)
870 Acta Geochim (2019) 38(6):863–871
123
more readily adsorbed than the smallest fractions. Fur-
thermore, the results showed that the Langmuir model fit-
ted to the experimental data was better than the Freundlich
model. This means that LHA and fraction adsorption onto
clay particles occurred mostly on homogeneous surfaces.
The kinetic adsorption data were collected and explained
that the LHA and its fraction adsorption onto kaolinite and
montmorillonite reached equilibrium within different
times. Moreover, the results illustrated that LHA and its
fraction adsorption on montmorillonite and kaolinite were
more conforming to pseudo-second-order kinetics model.
This result suggests varying binding mechanisms may
be accountable for different clays and some LHA proper-
ties such as molecular weight, functional group composi-
tions, and hydrophobicity lead the different adsorption of
LHA and fractions on clay minerals as well as the reactive
size of clay minerals.
Acknowledgements This work was funded by a Fulbright Visiting
Scholar fellowship to Mohamed El-sayed and performed at South
Dakota State University.
Compliance with ethical standards
Conflict of interest The authors declare that they have no conflict of
interest.
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