isotherm and kinetic studies on the adsorption of humic acid...

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

[email protected]

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)



0 100 200 300 400 500 6000

3

6

9

12

15Fr=0.2um-300kD

Cad

s (m

g/g)



0 100 200 300 400 500 6000

2

4

6

8

10

Fr=300-50kD

Cad

s (m

g/g)



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)



0 100 200 300 400 500 6000

1

2

3

4

5

6

7

Fr=10-1kD

Cad

s (m

g/g)



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)


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)


Fr > 0.2um

q t mg/

g

Time (min)


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)


Fr=0.2um-300kD

q t mg/

g

Time (min)


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)


Fr=300-50kD

q t mg/

g

Time (min)


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)


Fr=50-10kD

q t mg/

g

Time (min)


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)


Fr=10-1kD

q t mg/

g

Time (min)


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


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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