ORIGINAL PAPER

Potential application of termite mound for adsorptionand removal of Pb(II) from aqueous solutions

N. Abdus-Salam • A. D. Itiola

Received: 18 February 2011 / Accepted: 26 May 2011 / Published online: 10 January 2012

� Iranian Chemical Society 2012

Abstract The morphological and mineralogical composi-

tion of a termite mound from Ilorin, Nigeria was investigated

with a view to understand its sorption properties. The termite

hill soil was subjected to some spectroscopic analyses such as

X-ray fluorescence (XRF) and Scanning Electron Micros-

copy. The XRF results revealed that the adsorbent contains a

large fraction of Silicon, Iron and Aluminium minerals. The

organic matter (OM) content expressed as percentage carbon

was 3.45% while the high value of cation exchange capacity

of 14.0 meq/100 g is in agreement with high percentage OM,

which signifies high availability of exchangeable ions. The

maximum Pb(II) adsorption capacity of the mound was found

to be 15.5 mg/g. Batch adsorption experiments were carried

out as a function of contact time, ionic strength and pH.

Maximum and constant adsorption was observed in the pH

range of 2–5.5. The experimental results of Pb(II) adsorption

were analyzed using Langmuir, Freundlich, and Temkin

isotherms. The Langmuir and Temkin isotherms were found

to fit the measured sorption data better than Freundlich. The

constants obtained from the Langmuir model are maximum

sorption value, Qm = 18.18 and Langmuir energy of

adsorption constant, b = 0.085, while the constants of the

Freundlich model are the intensity of adsorption constant,

n = 0.134, and maximum diffusion constant, Kf = 1.36. The

adsorption data for Pb(II) was found to fit well into the

pseudo-second order model. Desorption experiment was

conducted using different concentrations of leachant and this

was repeated three times to determine the life span of the

adsorbent. It was observed that 0.2 M HCl had the highest

desorption efficiency for reuse.

Keywords Termite mound � Cation exchange capacity

(CEC) � Adsorption isotherms � Pseudo-first and second

order models � Desorption

Introduction

Termites are social ants that exert significant influence on the

physical and chemical properties of tropical and sub-tropical

soils [1–3]. The feeding habit, the food processing and

mound construction operations introduce significant modi-

fications to the soil on which the mound is built [4, 5]. Studies

have shown that termite mounds in tropical soil like Nigeria

may have lower or higher values for exchangeable Ca, Mg

and K, effective cation exchange capacity (ECEC), water

holding capacity, and water infiltration rates [6]. African

farmers are familiar with the practice of applying termite

mounds on their farms as it contains required nutrients for

plants growth [7]. In a related research, termite-mound soil

was reported to contain as much as 20% of the total nitrogen

as inorganic nitrogen, an average organic carbon content of

9.3% and 2.25 times more total P than the adjacent soils [8].

Lead is one of the major industrial pollutants of human

concern as air-particulate or in dilute aqueous medium. The

metal is associated with industries such as paints, pigments,

batteries, ceramic glazes, metal products, petroleum and

cosmetic products, cable sheathing and ammunition pro-

duction [9]. The divalent form [Pb(II)] is the stable ionic

species of lead. The metal has the tendency to form insol-

uble compounds with OH-, CO32-, and S2-. The ratio of

suspended form to the soluble form of lead may vary from

4:1 in rural streams to 27:1 in urban streams [10].

Different methods for the removal of heavy metals from

aqueous solutions have been proposed [11–14]. These

include solvent extraction, ion exchange filtration and

N. Abdus-Salam (&) � A. D. Itiola

Department of Chemistry, University of Ilorin,

P.M.B.1515, Ilorin, Nigeria

e-mail: nasalami2002@yahoo.co.uk

123

J IRAN CHEM SOC (2012) 9:373–382

DOI 10.1007/s13738-011-0047-2

membrane separation, reverse osmosis, chemical precipi-

tation and coagulation. These methods are known for one

shortcoming or the other, ranging from incomplete

removal, high energy consumption, reagents cost, disposal

of large volume of organic solvents and inefficiency when

the metal concentrations are \10 mg/l [9]. Adsorption

process is a promising alternative technique that is free

from the shortcomings of the earlier techniques. Adsor-

bents from natural or modified materials and synthetic

origin are subject of recent research efforts. These adsor-

bents include activated carbon from agricultural products

[15–17], clay and clay materials [11, 18, 19], and oxides of

iron [20].

Modifications of soil physical and chemical properties

by termite’s mounds and mechanical properties have been

reported, however, only limited studies have involved

application of the mounds in the decontamination of metal

polluted effluents. The objective of this study was to

characterize termite mound soils and determine the sorp-

tion characteristics. Langmuir isotherm equation was

employed to quantify the adsorption equilibrium. The

effects of solution pH, ionic strength and variation of time

on Pb(II) adsorption were examined.

Materials and methods

Collection of samples and pretreatment

The termite mound soil was collected within the compound

of Ilorin Grammar School, Ilorin, Kwara State. A sample of

about 2 kg was collected and air-dried in the laboratory at

room temperature for 5 days. The sample was then placed

in an oven set at 60 �C for 10 h before crushed in a mortar,

grounded and thoroughly mixed by repeated (more than 10

times) conning and quartering. The grounded mound was

then sieved into different fractions: U B 0.09 B 0.25 B

0.4 B 0.5 mm and the fraction with diameter U B 0.25 mm

was used for subsequent experiment. This fraction was pre-

treated to remove non-clay material such as carbonate and

quartz minerals in order to concentrate the active minerals

and improve the sorption property of the adsorbent. The pre-

treatment involved washing a pre-heated slurry of termite

mound at 90 �C with 0.1 M HNO3 to leach the non-clay

materials. The mixture was stirred for 2 h and then thor-

oughly washed with distilled water until the pH of the

washings remains constant and finally air-dried [21].

Soil pH was determined potentiometrically in 0.01 M

CaCl2 at a soil-solution ratio of 1:2.5 [22, 23]. The organic

carbon content of the mound sample was determined by

the dichromate oxidation, a Walkley–Black method [24].

The ammonium acetate method was used to determine the

cation exchange capacity (CEC). The CEC was obtained by

summation of the exchangeable acidity and exchangeable

cations, which are mostly Ca, Mg, Na, K (collectively

termed as bases). The K and Na were determined by flame

photometry method while Ca and Mg were determined by

titrimetric method [24]. The point of zero charge (pzc) was

determined by both mass titration and potentiometric

methods [25]. This involve measurement of pH of the

mound (0.5–2.0 g) suspension in 25 ml of different ionic

strength (10-3 to 10-1 M). The pH values at the start of

experiment and after 24 h were recorded. The pH of the

mixture was then adjusted to pH 5–6 with 0.01 M HNO3,

equilibrated for 24 h and pH measured. The pH of these

mixtures was then re-adjusted to pH 10–11 with 0.01 M

KOH, equilibrated for 24 h and the pH measured. The acid

and base adjustments were made in order to determine the

influence of pH on the pzc [25]. The termite mounds

sample was analyzed to determine its constituents and

qualities using X-ray flourescence (XRF) model Axios of

Analytical type with a 2.4kWatt Rh X-ray tube. The

scanning analysis was accomplished on a Leo 1430 VP

Scanning Electron Microscope.

Contact experiment

A 0.5 g portion of the various soil samples collected was

weighed in a 100 mL conical flask and 25 cm3 of working

standard solutions (500, 350, 250, 150, 100 and 50 ppm) of

Pb(II) prepared by serial dilution of the standard solution,

was added to the soil sample in the beaker. Each solution

was agitated on a flat orbital mechanical shaker for 7 h and

then filtered. The filtrate was analyzed using Atomic

Absorption Spectroscopy (AAS) to determine the quantity

of Pb(II) remaining in the solution. The quantity sorbed

was then evaluated using the following equation [26].

qe ¼Ci � Cf

M� V ð1Þ

where qe is the amount sorbed at equilibrium (mg/g), Ci is

the initial concentration of lead solution (mg/L), Cf is the

final concentration of lead solution (mg/L), V is the volume

of lead nitrate solution used (mL), M is the mass of soil

used (g).

The quantity sorbed was plotted against the initial

concentration to determine the equilibrium concentration

which is the concentration with the highest sorption

capacity [the adsorption capacity of the mound soil for

Pb(II)]. This equilibrium concentration was used subse-

quently for the sorption kinetics.

Effect of reaction time

The effect of reaction time was investigated using 25 mL

of 350 mg/L of Pb(II) solution and 0.5 g of the mound

374 J IRAN CHEM SOC (2012) 9:373–382

123

sample each in an 100 mL reactor flask. A total of ten

reactors were set up for time duration ranging between 5

and 420 min. The mixtures in reactors was then agitated on

a flat orbital shaker and allowed to equilibrate at different

time intervals. The flaks were removed at designed inter-

vals and content filtered. The Pb(II) in the filtrates were

analyzed using AAS and the amount sorbed were calcu-

lated from Eq. 1.

Effect of ionic strength

The influence of ionic strength on the adsorption of Pb(II)

was investigated using three different concentrations (0.1,

0.01, 0.001 M) of potassium nitrate (KNO3) solution. The

different concentration was added to the solution of

350 mg/L of Pb(II) solution and the mound sample in dif-

ferent flasks. The mixtures were equilibrated for 7 h and

then filtered; this filtrate was analyzed for Pb(II) using AAS

and the quantity adsorbed was calculated from Eq. 1 above.

Effect of pH

25 mL of 350 mg/L of Pb(II) solution was added to 0.5 g

of soil sample in a 100 mL conical flask and the initial pH

was noted. The pH of the resulting solution was then varied

between pH 2 and 7.5 using 0.1 M HNO3 and/or KOH

solution. The mixture was equilibrated for 7 h and then

filtered. The filtrate was analyzed for Pb(II) using AAS and

the quantity adsorbed calculated.

Fitness of Pb(II) adsorption data into Langmuir,

Freundlich, and Temkin equations

The equilibrium concentrations data obtained in the contact

experiment were subjected to the Langmuir, Freundlich,

and Temkin adsorption isotherms to find the equation that

best fits the data. Data were fed into Eqs. 2–4 separately

and constants were calculated.

Langmuir :Ce

Qe

¼ 1

bQm

þ Ce

Qm

ð2Þ

Freundlich : LogQe � LogKf þ1

nLogCe ð3Þ

Temkin : Qe ¼ BlnAþ BlnCe ð4Þ

where Qe is the quantity sorbed at equilibrium (mg/g), Ce is

the equilibrium concentration of adsorbate (mg/L), A and

B are Temkin isotherm constants related to adsorption

efficiency (dm3/mmol) and energy of adsorption, respec-

tively. The Langmuir adsorption isotherm constants were

determined by plotting Ce/Qe against Ce [27–29]. The

Freundlich adsorption isotherm was determined by plotting

log Qe against log Ce. The slopes and intercepts obtained

from the graphs were used to calculate the Langmuir and

Freundlich constants. In the case of Temkin, the quantity

sorbed Qe was plotted against lnCe and the constants were

determined from the slope and intercept.

Adsorption kinetics

The adsorption kinetic models are important in the process

of removal of toxic heavy metals from the environment

because it provides information on pollution flux. Both

pseudo- first and second order models were tested with the

data obtained to establish the model that best fits the data.

Pseudo-first order model [30, 31]

The equation for this reaction is

dqt

dt¼ kðqe � qtÞ ð5Þ

where qe is the quantity of solute adsorbed at equilibrium

per unit mass of adsorbent (mg/g), qt is the amount of

solute adsorbed at any given time t, (mg/g) and k is the rate

constant of first order sorption (1/min). By using the

boundary conditions t = 0 to t = t and qt = 0 to qt = qt

and simplifying, Eq. 5 becomes

log(qe � qtÞ ¼ logqe �k

2:303t ð6Þ

The plot of log (qe - qt) versus t gives the slope and

intercept from which k and qe were evaluated.

Pseudo-second order model

The pseudo-second order model [30, 31] was used for the

sorption data and the equation for this is

dqs

dt� kzðqe � qlÞ2 ð7Þ

On integration for boundary conditions when t = 0 to

t = t and qt = 0 to qt = qt then Eq. (7) can further

simplification results in the following equation

t

qt

¼ 1

k2q2e

þ 1

qe

t ð8Þ

where k2 is the rate constant of second order of sorption

(g/mg min). The plot of t/q versus t was made to evaluate

k2 and qe.

Sorption–desorption experiment

The normal sorption procedure was carried out using 1.0 g

of mound sample in 25 cm-3 of 500 ppm Pb(II) solution.

The mixture was equilibrated for 7 h and then filtered and

J IRAN CHEM SOC (2012) 9:373–382 375

123

the filtrate was analyzed for Pb(II) while the residue was

used for desorption experiment. A 25 cm-3 of different

concentrations of HCl (0.1, 0.2, 0.5 and 1.0 M) were added

to the residues in different conical flasks and each mixture

was equilibrated for 7 h and then filtered. The filtrate was

analyzed for the amount of Pb(II) released back into the

solution. This sorption and desorption processes were

repeated on the same sample for three times. The quantity

sorbed or desorbed was calculated using Eq. 1 above. A

graph of the quantity desorbed was plotted against the steps

of the desorption process for the different ionic strengths.

Results and discussion

The physical properties of the termite hill soil are summa-

rised in the Table 1. The termite hill soil obtained from

Ilorin Grammar School in Ilorin was brown with fine tex-

ture. The colour is within the variable colours of mound

hills (brownish to blackish) depending on the soil mor-

phology of the environment. The pH of the supernatant fluid

of a mixture of the termite hill soil and water was slightly

lower than neutral (pH 6.7).The termite hill soil is relatively

rich in organic matter as expressed by percentage organic

carbon (3.45%). Lower percentage organic matters were

reported for soil fractions such as utisol (1.86%) [18],

montmorillonite (2.1%) [32], and soil mound 0.8–1.32 [33].

The CEC depends greatly on the soil organic matter.

The more the soil’s organic matter content, the higher will

be its CEC. The high CEC value of 14.0 meq/100 g is in

agreement with high percentage of OM, signifying high

availability of exchangeable ions. This translates to

enhance adsorptive capability of the soil.

Spectroscopic studies

The XRF elemental analyses of virgin (A) and after sorption

of Pb(II) onto the mound are reported in Table 2. The result

showed that SiO2, Al2O3 and Fe2O3 were present as major

components as indicated from their high intensities. TiO2,

CaO, K2O, H2O and MgO were present as minor compo-

nents while P2O5, Cr2O3 and MnO were present as trace

components. This implies that the adsorbent contains a

large fraction of silicon, iron and aluminium minerals.

Sample B data also indicate an increase in the %weight of

aluminium oxides after sorption of Pb(II) and a decrease in

%weight of silicate after sorption of Pb(II) and conse-

quently a decrease in the total weight which may be due to

the presence of some minerals which were not present in the

list of the major elements used in the analysis. This trend is

attributable to the loss on ignition (LOI) which was found to

increase from 6.38 to 13.25 after sorption of Pb(II).

The result of SEM elemental and weight per cent of their

oxides for virgin and after sorption of Pb(II) onto the mound

are reported in Table 3. Although, there were variations in

the percent weights of elements obtained from the two

analyses (XRF and SEM), Si, Al and Fe remained major

elements while K, and Ti are minor elements. Figures 1 and

2 are the results of scanning electron micrographs of virgin

samples. Round crystals arranged well next to each other

and forming large round aggregates of crystals were

observed. A similar image was observed in an SEM pho-

tograph of lead hydroxide [34]. The presence of many open

pores is visible from Figs. 1 and 2. Figures 3 and 4 show the

filled pores and signify adsorption of Pb(II).

The point of zero charge (pzc) of the termite hill soil by

potentiometric method was found to be 7.8 for the different

ionic strengths which is similar to what was obtained for

natural goethite (pzc = 7.8) [35]. Comparative values

obtained for the same sample by mass titration were

between 6.1 and 6.5 range for the different ionic strengths

(0.1, 0.01, 0.001 M), but after acid adjustment, the pzc

increased proportionately to the range 6.4–6.9. After

alkaline adjustment, the pzc was observed at pH 8.2. The

variable pzc obtained from mass titration is often affected

by the nature of contaminants such as basic and acidic

elements in the sample [34]. The potentiometric method

may therefore be preferred because it is free from the

interference of acidic/basic contaminants. Therefore, the

pzc of this termite hill is 7.8. At pH below this pzc value,

the acidic water donates more protons than hydroxide

groups and so the termite hill soil will have a positive

surface charge characteristic and will therefore electro-

statically repel cations and attract anions to its surface.

Conversely, above pzc, the surface charge characteristic

will be negative [35]. The presence of ionic species or

complexing agents in the reaction medium may change this

adsorption pattern by conferring a net negative surface

charge at pH below pzc value. This explains the adsorption

of metal or hydrated metal ions onto surfaces at pH lower

than its pzc value.

Sorption capacity of termite mound

Figure 5 represents a two steps process. An initial linear

rise in the uptake of Pb(II) which is followed by a less steep

Table 1 Physico-chemical properties of termite hill soils

Properties Quantity/value

Colour Brown

Texture Fine

pH 6.7

Weight organic carbon (%) 3.45

CEC (meq/100 g) 14.0

376 J IRAN CHEM SOC (2012) 9:373–382

123

curve. As the initial concentration increases the amount

adsorbed increases proportionally until a plateau was

reached, where there was no corresponding rise in the

amount adsorbed. The favourable sites with lower

adsorption energies have been filled making the unfa-

vourable sites more difficult to fill. This is an indication of

surface saturation or a monolayer adsorption. The second

rise is a continuation of monolayer adsorption given a two-

step Langmuir curve

Influence of contact time on the adsorption of Pb(II)

Figure 6 which shows the effect of time on the adsorption

of Pb(II), can be classified into three portions according to

the behaviour of the curve. An initial decrease in the

quantity of Pb(II) (5–20 min) adsorbed due to inability of

the system to establish equilibrium. This is followed by

sharp rise in adsorption where majority of the metal ion

was adsorbed. This is a fast uptake (between 30 and

90 min) in which about 65% of Pb(II) was sorbed. The last

portion (90–600 min) represents a slow kinetics. It is

characterized by an increase in the quantity of Pb(II)

adsorbed but at a relatively slow rate compared to the

second portion of the curve. This occurs when the available

(favourable) binding sites on the adsorbent is low. The

energy of adsorption becomes relatively higher than before.

Similar trends were reported for adsorption of Pb(II) on a

natural goethite [35].

Effect of ionic strength on adsorption of Pb(II)

A steady decrease in the amount of Pb(II) adsorbed was

observed as the ionic strength increases (0–0.5 M) until it

gets to a point where a further increase in ionic strength has

no effect on the quantity of Pb(II) adsorbed. Higher ionic

strength of the reacting medium has no influence on the

adsorption of Pb(II) (Fig. 7). The decrease in the quantity of

Pb(II) sorbed between 0 and 0.2 M ionic strength is due to

modification of surface charge characteristics. As the ionic

strength increases, the net negative charge on the termite

hill surface decreases, thereby decreasing the attraction

between the metal and the surface.

Table 2 XRF analysis of Ilorin virgin termite hill soil (A) and after sorption of Pb (B)

Wt% Al2O3 CaO Cr2O3 Fe2O3 K2O MgO Na2O MnO P2O5 SiO2 TiO2 LOI H2O Total

A 11.36 0.10 0.01 6.38 0.72 0.10 bd 0.08 0.06 73.46 0.97 6.38 0.89 100.52

B 17.86 0.62 0.01 8.35 0.97 0.14 0.01 0.04 0.16 43.41 1.08 13.25 1.64 87.52

A: element analysis before sorption of Lead

B: element analysis after sorption of Lead

Bd below detection

Table 3 Scanning electron microscopy elemental and % compound results

Analysis Al (Al2O3) Si (SiO2) K (K2O) Ti (TiO2) Fe (FeO) O Total

Sample A (wt%) 18.02 (34.048) 20.38 (43.59) 0.39 (0.468) 0.75 (1.254) 16.04 (20.64) 44.42 100.00

Sample B (wt%) 16.93 (31.988) 25.04 (53.576) 0.56 (0.678) 1.29 (2.155) 9.02 (11.603) 47.15 100.00

Fig. 1 SEM photograph of termite hill soil using K91.50

magnification

Fig. 2 SEM photograph of termite hill soil after sorption using

K91.50 magnification

J IRAN CHEM SOC (2012) 9:373–382 377

123

Effect of pH on the adsorption of Pb(II)

The graphical illustration of the pH on the adsorption

characteristics of mound is shown in Fig. 8. The sorption

experiment was carried out at pH ranging from 2.0 to 7.5.

A steady increase was observed in the quantity of Pb(II)

adsorbed as the pH decreased from 7.5 until after a pH of

5.5. The quantity of Pb(II) adsorbed was highest and

practically constant between the pH of 2.0–5.5 but

decreases from 6.0 to 7.5. Although, the natural surface

charge characteristics of soil materials is positive at pHs

below the pzc but practically, adsorption of metal ions is

found to be favourable at pH less than the pzc values

because at this value, the adsorbent is in a monomeric

anion form and consequently has affinity for cations [36].

Fig. 3 SEM photograph of termite hill soil using K91.00

magnification

Fig. 4 SEM photograph of termite hill soil after sorption using

K91.00 magnification

Fig. 5 Determination of equilibrium concentration

Fig. 6 Effect of contact time on the adsorption of Pb(II)

Fig. 7 Quantity of Pb(II) sorbed versus ionic strength

Fig. 8 Quantity of Pb(II) sorbed versus pH

378 J IRAN CHEM SOC (2012) 9:373–382

123

The presence of anionic species such as nitrate ions from the

Pb(II) salt solution used confers a net negative charge on the

mound in a monomeric anionic form. This is attributed to a

higher adsorption of Pb(II) between pH 2 and 5. Above the

pzc value, the surface is positively charged and will repel

adsorption of cations. The only process that can increase

the quantity of metal adsorbed after pzc is by precipitation

of metals. Many metals precipitate out in an alkaline

medium.

Adsorption isotherms

The adsorption data obtained from the adsorption of Pb(II)

experiment were tested for fitness of data against three

common adsorption equations, Langmuir, Freundlich and

Temkin adsorption isotherms, respectively. Figure 9 is a

Langmuir adsorption isotherm for the sorption of Pb(II) by

termite hill soil. The adsorption isotherm data as analyzed

from Fig. 9 are 18.8, 0.085 and 0.993 for Qm, b and the

regression coefficient R2, respectively. The observed b

value (b � 1) shows that the mound sample prefers to bind

acidic ions and that speciation predominates on sorbent

characteristic when ion exchange is the predominant

mechanism that takes place in the adsorption of Pb(II) [28].

The observed R2 = 0.993 shows that the Langmuir iso-

therm fitted fairly well the adsorption data for Pb(II). The

favourability of this adsorption process was subjected to

the equation of separation factor RL [37] given as:

RL ¼1

1þ bCi

ð9Þ

where b is the Langmuir equilibrium constant (K), Ci is the

initial concentration.

For a favourable adsorption, 0 \ RL \ 1, while for an

unfavourable adsorption, RL [ 1 and when RL = 0,

adsorption is linear and irreversible. The RL value obtained

from this adsorption process is 0.033 which indicates that

the adsorption process is favourable.

The Freundlich adsorption constants obtained from

Fig. 10 are n = 0.134, Kf = 1.36 and the regression

coefficient R2 = 0.972. The relatively small slope, n � 1,

indicates that sorption intensity is favourable over the

entire range of concentrations studied [36, 38]. Likewise,

the high value of the intercept Kf, is indicative of a high

sorption capacity of termite hill soil. The regression coef-

ficient R2 = 0.972 shows that the Freundlich adsorption

isotherm equation fits the experimental data too. When the

R2 values are compared, the data fits into Eqs. 2 and 3,

while Langmuir predicts a physical and mono-layer

adsorption process, the possibility of two-step (Fig. 5) and

chemo-sorption may not be ruled out because of the fitness

into Eq. 3.

The adsorption data was subjected to Temkin equation

and Fig. 11 is a representation of the isotherm obtained.

The Temkin adsorption constant A, obtained from Fig. 11

is -0.122 and the regression coefficient R2 is 0.985. In

comparison, the regression coefficients, R2, for the iso-

therms are: Langmuir (0.993), Freundlich (0.972) and

Temkin (0.985). It shows that the Langmuir and Temkin

isotherms fitted the adsorption data for lead ions better than

the Freundlich isotherm.

Table 4 is a summary of isothermal constants and

regression coefficients. Comparing these data with similar

data for goethite, where the Langmuir constants (b and Qm)

obtained for Pb(II) on goethite are 4.99 and 2.40, respec-

tively [39], it can be concluded that Pb(II) is better sorbed

on to termite hill soil than natural goethite.

Adsorption kinetics

The data obtained from the influence of time on the

adsorption of Pb(II) onto the mound sample were subjected

to the pseudo-first order and pseudo-second order kinetics

(Lagergren) equations for a test of fitness of data and

the plots of which gave Figs. 12 and 13, respectively. A

negative and zero slopes were observed for pseudo-first

order model; therefore, pseudo-first order model was not

sufficient to explain the adsorption kinetics of Pb(II) on

termite hill soil. Figure 13 is the plot of t/qt versus t, theFig. 9 Langmuir isotherm curve

Fig. 10 Freundlich adsorption isotherm for the sorption Pb(II)

J IRAN CHEM SOC (2012) 9:373–382 379

123

pseudo-second order model, yields a very good plot with a

regression coefficient R2 = 0.998, pseudo-second order

rate constant k2 = 107.8 g/mg and qe = 13.6 mg/g.

Therefore, the kinetics is best described by the pseudo-

second order kinetics.

Desorption experiment

Desorption data obtained is graphically represented in

Fig. 14. From the graph, it was revealed that the quantity

desorbed for the first process was practically the same for

the various acid concentrations. There is significant differ-

ence in the quantity desorbed in the second process among

the different acid concentrations. The quantity of Pb(II)

desorbed from the second step was highest for 0.2 M HCl

concentration and has the greatest efficiency for reuse. The

adsorption efficiency of termite hill soil dropped by 52.8%

after the first desorption and by 86% after the second

desorption, the quantity desorbed between the processes is

consistent. The adsorption process is therefore reversible.

Desorption data for 1.0 M is the most irreversible, the

adsorption efficiency dropped by 88% after the first

desorption process and by 12% after the second. The

quantity desorbed between each process is inconsistent.

The desorption process using distilled water followed a

different pattern. The quantity desorbed during the first

process was very low but the efficiency increased after

each desorption process. With similar solutions for the

desorption process, it can be seen that the quantity des-

orbed decreased after subsequent desorption for all the

acidic strengths except for distilled water which increased

after subsequent desorption. The 0.5 M HCl solution has

the highest desorption efficiency. The quantity desorbed

dropped from 62 to 13% after subsequent desorption, fol-

lowed by 0.1 M HCl which dropped from 61 to 34% and

then 0.2 M HCl with 60% which also dropped to 28%. We

can deduce from this that the desorption efficiency of both

0.1 and 0.2 M are the most consistent and reversible which

can also be observed on the graph (33).

Conclusion

The mineralogical details of a Nigerian termite hill soil

were reported with the aid XRF and SEM. We have also

determined the surface charge characteristics and CEC of

this termite hill soil in order to understand its sorptive

characteristics. Both the mineralogical compositions and

Fig. 11 Temkin adsorption isotherm for the sorption of Pb(II)

Table 4 Isothermal constants and correlation coefficients

b Qm Kf n B A R2

Langmuir 0.085 18.18 – – – 0.993

Freundlich – – 1.36 0.134 – – 0.972

Temkin – – – – 1.567 -0.22 0.985

Fig. 12 Pseudo-first order plot for the sorption of lead on termite hill

soil

Fig. 13 Pseudo-second order plot for the sorption of lead on termite

hill soil. t Time (min), qt quantity adsorbed at time t (mg/g)

380 J IRAN CHEM SOC (2012) 9:373–382

123

surface characteristics suggest that the termite hill soil

material is a good adsorbent in the class of oxides of metals

or manganate–silicate.

From the adsorption kinetics, the sorptive property of

the mound was found to be dependent on contact time, pH,

and ionic strength. The equilibrium adsorption data showed

satisfactory correlation with the Langmuir adsorption data

and was also found to fit the pseudo-second order kinetics.

Termite mound (soil) has also been found to have a high

efficiency for desorption and also for reuse even at low

concentrations of leachant.

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