Improvement of Fe(Ii)-Adsorption Capacity of Feooh-Coated Brick in Solutions, and Kinetics Aspects

doi:10.4236/jwarp.2012.47054

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Journal of Water Resource and Protection, 2012, 4, 464-473

http://dx.doi.org/10.4236/jwarp.2012.47054 Published Online July 2012 (http://www.SciRP.org/journal/jwarp)

Improvement of Fe(II)-Adsorption Capacity of

FeOOH-Coated Brick in Solutions, and Kinetics Aspects

Saint Charles Dehou1, Joseph Mabingui1, Ludovic Lesven2, Michel Wartel2, Abdel Boughriet2,3*

1Chaire Unesco sur la Gestion de l’Eau, Laboratoire Hydrosciences Lavoisier,

Faculté des Sciences, Université de Bangui, Bangui City, République Centrafricaine

2Université de Lille1 Laboratoire Géosystèmes Equipe Chimie Analytique et Marine,

CNRS UMR 8217, Villeneuve d’Ascq, France

3Département de Chimie, Université Lille Nord de France, I.U.T de Béthune,

Rue de l’Université, Béthune, France

Email: *[email protected]

Received April 12, 2012, revised May 11, 2012; accepted May 30, 2012

ABSTRACT

The adsorbent, iron oxy-hydroxide coated brick, was used in the present work for removal of iron(II) from aqueous so-

lutions. The adsorption performances of this composite were significantly improved when brick pellets (as a support

material) were pre-treated in a 6 M HCl solution at 90˚C for 6 hours, when compared to untreated ones and those

pre-washed in a 1 M HCl solution at RT for 1 day. This phenomenon was attributed to larger surface areas measured for

modified brick by BET, thus enabling a better FeOOH deposition. The ability of this new composite to better adsorb

Fe2+ ions from synthetic solutions was evidenced from fixed-bed column experiments: data were compared to those

obtained from raw brick and iron oxides-coated sand columns. The adsorption mechanism followed better pseudo-sec-

ond-order reaction kinetics, suggesting a chemisorption process, and the rate constant increased with a temperature in-

crease, revealing the endothermic nature of Fe(II) adsorption. Furthermore, the equilibrium data fitted the Langmuir

isotherm model with a maximum monolayer sorption capacity Qmax = 0.669 mg/g and a Langmuir constant KL = 0.659

L/mg at room temperature. The activation energy (Ea) of Fe(II) adsorption and the changes in entropy (ΔS), enthalpy

(ΔH) and free energy (ΔG) of activation were determined, with values suggesting the involvement of an activated

chemical adsorption and an associative mechanism.

Keywords: Brick; Ferrous Ion; Iron Oxyhydroxide; Acid Activation; Adsorption; Kinetics; Activation Energy; Water

Treatment

1. Introduction

To remove soluble iron efficiently from ground waters,

various modern processes could be implemented for their

treatments. For instance, an important water-treatment

technology that is often used by drinking water supply

companies in developed countries to remove soluble iron

from ground waters consists to inject O2-rich water into

an aquifer [1]. However, industrial methods are ex-

tremely expensive for poor people in developing coun-

tries. This has led many researchers to develop low-cost

effective and economic techniques that can be easily

used in rural and remote regions. On this view, many

works demonstrated the beneficiary effects of surface

coatings of iron oxide(s)/hydroxide(s) on the sorption

behavior towards metal ions onto, for instance: 1) silica

and Al(OH)3/SiO 2 [2-8]; 2) aluminosilicate minerals such

as zeolite [9,10]; and 3) hydrated magnesium silicate

mineral, sepiolite [11]. One possible solution to this

problem is to use the brick material which is composed

predominantly of sand and clays. Recently published

works had allowed us to show that brick pellets reco-

vered with FeOOH represented an appropriate material

for removing soluble iron from ground waters [12].

The first objective of the present work had been to im-

prove the Fe(II)-adsorption capacity performances of the

brick made by craftsmen in Central African Republic.

The adsorption capacity of a new absorbent (FeOOH

deposited onto 6 M HCl-treated brick pellets) was inves-

tigated by carrying out column experiments, and data

were compared to those obtained with untreated brick

grains and ones that were previously gently treated in a 1

M HCl solution at RT before their FeOOH coating.

The second objective of this work had been to investi-

gate the adsorption kinetics and isotherm models of

iron(II) removal from aqueous solutions by this new

composite, and to elucidate the resulting Fe(II)—adsorp-

*Corresponding author.

S. C. DEHOU ET AL. 465

tion mechanism .

2. Materials and Experiments

2.1. Materials

Bricks were made by craftsmen and used for construction

activity by local people in Bangui region (Central Afri-

can Republic). Bricks makers extracted starting material

directly near their homes at ≥0.2 m below ground.

Briefly, extracted soils were mixed with water and the

obtained mud was shaped manually; Resulting air-dried

(48 h) bricks were placed in efficient stackings in order

to ensure a continuous air flow through a setting up as-

similated to a basic oven (that was built simply on

ground), heat treated with dry wood for a period of about

three days at temperatures ranging from 500˚C to 800˚C

and finally cooled progressively up to ambient tempera-

ture during two/three days. In order to increase the sur-

face area of the brick material, this latter was broken in

grains manually by using a hammer. The brick particles

were afterwards sieved with a mechanical sieves and the

fraction containing particles sizes which varied from 0.7

to 1.0 mm, was kept for our experiments. This fraction

was washed with Milli-Q water and then decanted; After

settling, the water was eliminated and brick grains were

dried at 105˚C. The dried solid particles thus obtained

were ready for two leaching procedures: the first one

consisted of washing brick pellets with a 1 M HCl solu-

tion for one day and with Milli-Q water several times,

and finally dried them at 105˚C; the second one consisted

of leaching the material in a 6 M HCl solution at a con-

stant temperature of 90˚C for six hours. Afterwards, a

deposition of FeOOH onto modified brick grains was

performed by the precipitation of a 0.25 M ferric nitrate

solution in the presence of a 6 NaOH solution followed

by a 1 M NaOH solution in order to adjust pH at 6 - 7;

and finally, the resulting pellets were washed several

times with Milli-Q water in order to eliminate the excess

of FeOOH not attached to grains.

Specific surface areas pore volumes of brick pellets

before and after chemical treatments were determined by

nitrogen adsorption isotherm analyses (BET) using Sor-

ptomatic 1990 Carlo Erba at –196˚C.

2.2. Column Experiments

All the experiments were conducted in a glass column

with an inner diameter of 1.4 cm, a height of 25 cm, and

a medium—porosity sintered—pyrex disc at its bottom in

order to prevent any loss of material. This column was

packed with an absorbent composite (14.1 g) which con-

tained either sand or brick as the support material. The

adsorption capacity of these different absorbents was in-

vestigated for a bed height of about 10 cm. The iron(II)-

containing influent ([Fe2+] = 9.6 mg/L) passed at room

temperature through the column downward using a peri-

staltic pump at a flow rate of 10 mL per minute. Before

being used in every experiment, about 5 bed volumes of

Milli-Q water were passed through the column: 1) first in

order to remove any unbound and thin particles/iron ox-

ide(s)/hydroxide(s); 2) second to check the absence of

soluble iron in the effluent by ICP-AES (Inductively

Coupled Plasma Atomic Emission Spectroscopy); and 3)

third to confirm the stability of the FeOOH coating on

brick grains. Effluent samples were collected at various

time intervals, and the concentration of soluble iron in

the effluent was analyzed with time using an ICP-AES

spectrometer model: Varian Pro axial view.

2.3. Adsorption Experiments

Kinetics studies on Fe(II) adsorption onto Bangui brick

were carried out with brick grains with 0.7 - 1.0 mm

sizes that were first pre-treated with a 6 M HCl solution

at 90˚C for six hours, and second coated with FeOOH.

Experimental data were afterwards compared to those

found for FeOOH-brick composites that were prepared

directly from raw brick grains and from ones pre-washed

in a 1 M HCl solution for 24 hours. 50 mL of a Fe2+ ions

solution [prepared from the salt: Fe(NH4)2(SO4)2, 6H2O]

at a concentration of 30 mg of iron per liter were trans-

ferred into a cell containing 4 g of brick pellets (with

average diameters varying from 0.7 to 1.0 mm). The

mixture thus prepared was shaken gently at a constant

speed of 120 rpm using a mechanical shaker (Model:

IKA Labortechnik KS 250 basic). 1 mL of the super-

natant (which was filtered through a 0.45 µm pore di-

ameter cellulose nitrate filter) was collected at various

time intervals from 0 to 30 minutes and analyzed for the

determination of iron level by using an ICP-AES spec-

trometer (Inductively Coupled Plasma Atomic Emission

Spectroscopy; Model: Varian Pro axial view). The re-

producibility of concentration measurements was ensured

by repeating three times the same experiments under

identical experiment conditions. This procedure permit-

ted us to determine average values of iron content in the

reaction solution, and standard deviations of these analy-

ses were evaluated to be within ±3%. The Fe(II) adsorp-

tion capacity of brick was calculated by using the fol-

lowing equation: Qe = (Co – Ce)V/m, where Qe repre-

sented the adsorption capacity of iron(II) on FeOOH-

coated brick (in mg/g); Co was the initial content of

iron(II) in the cell (in mg/L); Ce represented the equilib-

rium solute concentration (in mg/L); and m and V corre-

sponded to the mass of brick used (g) and the volume of

Fe(II) solution used (L), respectively.

Adsorption-isotherms studies were performed in ten

100 mL-flasks each one containing 2 g of brick pellets in

which were added 50 mL of an iron(II) solution having a

concentration ranging from 2 to 20 mg/L. These flasks

S. C. DEHOU ET AL.

466

100

120

010203040506070

Time/mn

(Ct* 100/ Co)(% )

were placed on a mechanical shaker (as mentioned above)

and gently shaked at a speed of 120 rpm. Preliminary

sorption experiments showed that a reaction time of 4

hours at a temperature of 17˚C ± 1˚C was sufficient for

the system to attain thermodynamic equilibrium. After-

wards, suspensions were filtered and the recovered solu-

tion was analyzed to determine Fe2+ ions concentrations

using ICP-AES. The quantity of iron adsorbed onto brick

pellets, noted Qe (in mg/g), was assessed from the dif-

ference between the initial and the equilibrium contents

of iron(II) in the liquid phase. It should be noted that all

these experiments were at least triplicated and data were

averaged.

(a)

(b)

0Fe% d

t tfinal

3. Results

3.1. Improvement of Adsorption Performances

of Brick

There was evidence in the literature [13-16], that clay

minerals could be chemically modified to enhance their

adsorption capacity, particularly, when amorphous meta-

kaolinite (which is a mineral present in the study brick

[12]) was thermally treated with concentrated inorganic

acids. In our case, this suggestion was clearly confirmed,

indeed, the surface area (S.A.) and pore volume (Vpore) of

crushed brick increased notably after acid leaching from:

S.A. = 31.2 cm2/g and Vpore = 0.15 cm3/g in 1 M HCl-

washed brick pellets at room temperature for 24 hours, to:

S.A = 76 cm2/g and Vpore = 0.23 cm3/g in 6 M HCl-

treated ones at 90˚C for 6 h. Recently, more exhaustive

studies (not reported here) revealed that HCl concentra-

tion and reaction temperature/time induced dramatic mor-

phological, compositional, textural, and surficial modifi-

cations of Bangui-brick grains, resulting in more micro-

porous structures in which clay minerals played a pre-

dominant role [17].

On the other hand, the effects of contact time on the

adsorption of Fe2+ ions onto FeOOH-coated brick pellets

were examined in the present work. Overall, it can been

seen in Figure 1 that Fe(II) adsorption was found to be a

more rapid process when pellets were previously pre-

treated with a 6 M HCl solution at 90˚C for 6 hours than

when they were simply washed with a 1 M HCl solution

for 1 day. This observation confirmed the importance of

medium acidity and temperature on the chemical treat-

ment of the brick in order to improve its properties as a

support material. Thus, owing to the acid pre-activation

of Bangui brick more than 80% of the reaction occurred

within the first 8 min, and the solid-liquid equilibrium

was attained in less than 25 min.

Finally, Fe(II)-adsorption capacities of different adsor-

bents in aqueous media were investigated in a fixed-bed

column. The influent with a Fe(II) concentration of 9.5

mg/L was continuously passed through a glass column at

Figure 1. Variation of Fe2+-ions concentration versus con-

tact time at room temperature during Fe(II) adsorption

onto the composite FeOOH-coated brick: (a) With brick

previously washed in a 1 M HCl solution for 24 h; (b) With

brick previously pre-activated in a 6 M HCl solution at

90˚C for 6 h. Co represented the initial concentration of

iron(II) in the medium, and Ct was the Fe(II) content at the

adsorption time, t.

a flow rate of 10 mL/mn and at a pH range 5.5 - 5.9. As

a whole, for either raw sand or iron oxides-coated sand,

barely 40% of Fe2+ ions in this influent were adsorbed by

these adsorbents at the beginning of the experiments

(Figure 2), and these quantities decreased rapidly up to

reach Fe(II) levels close to those initially measured in the

influent. As for raw brick, its iron(II) removal process

was found to be better than that observed for raw sand

and modified sand, and increased drastically when brick

pellets were previously recovered with FeOOH (Figure

2).

It was further shown that, before FeOOH coating, the

pre-treatment of the brick in a 6 M HCl solution at 90˚C

for 6 hours contributed to enhance strongly its Fe(II)-

adsorption capacity (see Figure 2). The maximum co-

lumn capacity, qmax (in mg), for a given feed concentra-

tion and flow rate was calculated from the equation: qtotal

= FCoA/105, where F was the volumetric flow rate (in

ml/min), and A represented the area under the plot of the

adsorbed Fe(II) percentage versus time that was evalu-





 [where Fe% =



ated from:

(Co – C)·100/Co, with Co = influent Fe(II) concentration

(in mg/L) and C = effluent Fe(II) concentration(in

mg/L)]. We found: qtotal = 9.9 mg for FeOOH-coated

brick, and qtotal = 22.6 mg for FeOOH-coated (pre-acti-

vated) brick (qtotal values were determined for a brick

mass of 14.1 g inside the glass column). The equilibrium

uptake (Qe) representing the amount of iron(II) adsorbed

onto brick surfaces per unit mass of dry material in the

column was calculated from the equation: Qe =

qtotal/mbrick, where mbrick (14.1 g) corresponded to the total

mass of dry adsorbent inside the column. We found: Qe =

0.7 mg/g for FeOOH-coated brick, and Qe = 1.6 mg/g for

FeOOH-coated (pre-activated) brick. These values indi-

S. C. DEHOU ET AL.

467

EFFLUENT CONC ENTRA TI ON (C ) OF I R ON(I I ) ( m g/ L

)

FeOOH-C

pre -tre a teO A TED (6M - HCl

d) BRICK

FeOO H-CO A TED BRICK

SAND

I RON OXI

SAND DES-COATED

RA W BRI CK

0 0.5 1 1.5 2 2.5 3 3.5

INFLUENT VOLUME (L)

Figure 2. Evolution of Fe(II) concentration in the effluent (in mg/L) as a function of influent volume (containing 9.6 mg of

iron per liter)) passed through a glass column (which was packed with different adsorbents at a bed height of 10 cm) at a flow

rate of 10 mL/mn: raw sand (□); iron oxides-coated sand (∆); raw brick (●); FeOOH-brick composites prepared with brick

pellets treated in a 1 M-HCl solution at room temperature for 24 hours (▲), and in a 6 M-HCl solution at 90˚C for 6 hours (■).

cated clearly that the acidic and thermal treatment of the

brick contributed to generate a better support material

(with greater surface areas, as evidenced above) for

FeOOH deposition and, as a consequence, to improve the

Fe(II)-adsorption characteristics of FeOOH-brick com-

posite.

3.2. Adsorption Kinetics Characteristics

The kinetics of Fe(II) adsorption onto Bangui brick was

studied by carrying out a batch of experiments at several

constant temperatures ranging from 17˚C to 45˚C, and by

monitoring the content of adsorbed Fe2+ ions with time.

The kinetic mechanism involved in the study system was

examined in terms of different kinetic models: 1) pseudo-

first order kinetics; 2) pseudo-second order kinetics; 3)

Elovich equation; and 4) intra-particle diffusion (for

more details, consult e.g. [18] and references therein).

The rates of Fe(II) adsorption onto FeOOH-coated brick

in aqueous media were then examined by plotting the

following curves: 1) Ln(Qe – Qt) versus time for a

pseudo-first order kinetics; 2) t/Qt versus time for a

pseudo-second order kinetics; 3) Qt versus Ln(t) accord-

ing to Elovich equation; and 4) Qt versus (t1/2) in case

Fe2+ ions were assumed to be transported from the liquid

phase towards the brick surface according to an intrapar-

ticle diffusion phenomenon [where Qe and Qt represented

the contents of Fe(II) adsorbed (mg/g) at equilibrium and

at time t (min), respectively]. In order to find the best

kinetic model, the fitting of experimental data to kinetic

equations was tested, and kinetic constants and correla-

tion coefficients were determined (Table 1). In this table,

e which represented the quantity of iron deposited

onto brick pellets was determined experimentally from

the equation [(Ci – Ce)·V]/(1000·mbrick), where Ci was the

initial concentration of Fe2+ ions in contact with brick

pellets, Ce was that obtained when the system reached the

equilibrium state, and mbrick was the mass of brick pellets

in interaction with a V(mL) volume of Fe(II) solution;

and e was calculated from the mathematiccal treat-

ment of the first-order and second-order kinetics plots

drawn.

exp

calc

As a whole, our findings showed that: 1) the pre-ac-

tivation of brick pellets with HCl contributed to enhance

significantly the rate constants for Fe2+-ions adsorption

with values increasing at room temperature from 0.078 to

0.102 (1/mn) for the first-order kinetics and from 0.060

to 0.830 g/(mg·min) for the second-order kinetics; and 2)

a rise in temperature from 20˚C to 39.5˚C led to an in-

crease of the pseudo-first order kinetic constant k1 (from

0.102 to 0.159 1/min) and the pseudo-second order ki-

netic constant k2 (from 0.830 to 1.315 g/(mg·min)). It

could also be noticed that: 1) second-order plots (t/Qt vs

time) had better linearity with correlation coefficients

0.997 ≤ R2 ≤ 0.998 than those obtained for a first-order

kinetics [Ln(Qe – Qt) vs time] : 0.907 ≤ R2 ≤ 0.994; 2) the

calculated Qe values (where Qe represented the amount of

S. C. DEHOU ET AL.

468

Table 1. Kinetics constants obtained for Fe(II) adsorption onto FeOOH-coated (pre-activated) brick pellets at different tem-

peratures.

Pseudo-first-order Pseudo-second-order

Temperature (˚C) exp

Q (mg/g) calc

Qcalc

(mg/g) K1 (1/min) R2 (mg/g) K2 (g/(mg·min)) R2

17 0.292 0.301 0.097 0.994 - - -

20 0.393 0.185 0.102 0.947 0.423 0.830 0.998

25 0.349 0.276 0.122 0.991 - - -

30 0.380 0.236 0.1 0.985 0.408 1.085 0.997

35 0.352 0.226 0.157 0.985 0.376 1.12 0.998

39.5 0.358 0.248 0.165 0.907 - - -

39.5 0.355 0.237 0.159 0.992 0.385 1.315 0.998

45 0.315 0.230 0.167 0.980 - - -

Fe2+ ions adsorbed onto brick per mass unit at equilib-

rium) which were obtained from second-order kinetics

plots, were closer to the experimental ones than the cal-

culated Qe ones found from first-order kinetics plots (see

Table 1); and 3) consequently the pseudo-second order

model exhibited lower



² values (0.0012 ≤



² ≤ 0.0020)

than those determined for the pseudo-first-order model

(0.0070 ≤



² ≤ 0.0087), where



represented a general-

ized error function, defined as:



² = (1/N)∑i=1..N

Q–e)2, with e corresponding to the experi-

mental values of adsorbate per gram of adsorbent and

e being the calculated ones. These observations sug-

gested that Fe(II) adsorption onto brick might be ap-

proximated to the pseudo-second order kinetics model.

This finding indicated that a chemisorption process or an

activated reaction occurred more predominantly in the

rate controlling step. Such a model then assumed that one

iron ion was sorbed onto two sorption sites (noted 2A) on

the brick surface according to:

exp calc

Qexp

calc

  

rick solutionbrick

2AFeA Fe





Also, in the assumption that solid surfaces in this mo-

dified brick were energetically heterogeneous, we could

apply to our pseudo-second-order kinetic system Elovich

equation defined as: Qt = [Ln(αβ)]/β + [Ln(t)]/β, where Qt

was the sorption capacity at time t; α (in mg/(g·min)) was

the initial sorption rate of Elovich equation; and the pa-

rameter β (in g/mg) was dependent upon the extent of

surface coverage and activation energy of the considered

chemisorption reaction. From our experimental data, Elo-

vich plots were drawn, and the lines were found to be

linear at different temperatures ranging from 25˚C to

45˚C (0.968 ≤ R2 ≤ 0.991). Table 2 lists the kinetic con-

stants obtained from Elovich equation under the follow-

ing experimental conditions: the initial concentration, Ci,

was 30 mg of iron per liter, and the brick mass, mbrick,

was 4 g in 100 mL of Milli-Q water.

Overall, it can be seen that the initial adsorption rate, α,

and the β coefficient do not change significantly with

Table 2. Kinetic parameters determined from Elovich equ-

ation.

Temperature (˚C) α (mg/(g·min)) β (g/mg) R2

25 0.480 15.36 0.991

35 0.445 13.40 0.978

39.5 0.434 12.58 0.973

45 0.517 16.13 0.968

temperature: α varying from 0.434 to 0.517 mg/(g·min)

and β from 12.58 to16.13 g/mg.

On the other hand, in order to check whether or not the

overall kinetics of Fe(II) adsorption reaction was con-

trolled by surface/intra-particle diffusion, the intra-parti-

cle model developed by Weber and Morris [19] was also

tested to our system by plotting Qt against t1/2 (not shown

here). The obtained curves at the beginning of the kinetic

process were roughly linear; however, their straight lines

did not pass through the origin, what meant and con-

firmed that the intra-particle diffusion was not the rate-

controlling step. Instead, we believed that the formation

of solid solutions as: Fe(II)xFe(III)y(OH)z at the external

surface of brick pellets might be responsible for this ad-

sorption, thus preventing the sorbate molecules to move

into the interior of sorbent particles (FeOOH coatings),

and thereby limiting any binding of Fe2+ ions on / with

interior sites of the sorbent by diffusion.

3.3. Adsorption Isotherms Characteristics

In order to gain more information about Fe(II) adsorption

mechanism and surface characteristics and affinities of

Bangui brick, three adsorption isotherm models were

applied to our system (for more details, see e.g. [20] and

references therein): 1) Ce/Qe versus Ce (Langmuir adsor-

ption isotherm); 2) Log(Qe) versus Log(Ce) (Freundlich

adsorption isotherm); and 3) Qe versus Ln(Ce) (Temkin

adsorption isotherm). In this latter case, the plots ob-

tained (not shown here) were badly linear with correla-

S. C. DEHOU ET AL. 469

tion coefficients R2 ≤ 0.85, and this model was therefore

excluded.

Feundli ch m odel

y = 0.460 9x - 1.3047

= 0.9502

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0. 00

0.00 0.200.40 0.60 0.801.00 1.20

LogC

As for Freundlich and Langmuir models, all the plots

were found to be linear with correlation coefficients R2 ≥

0.95 (see Figure 3 and Table 3).

The slope and intercept of these curves were used to

calculate relevant parameters: 1) first Freundlich ones,

KF and n, which are two constants indicative of the rela-

tive adsorption capacity and adsorption intensity, respec-

tively; and 2) second Langmuir ones, KL being the equi-

librium adsorption constant and Qmax representing the

maximum amount of Fe2+ ions per unit mass of brick.

Considering Freundlich adsorption model data, the n

values were found to be higher than 1, and further the

Freundlich constant KF increased with temperature. All

these findings suggested that: 1) Fe(II) ions interacted

favorably with brick according to a chemisorption me-

chanism and 2) this process was endothermic. The iso-

therm plots of Log(Qe) vs Log(Ce) at room temperature

showed clearly that, when attaining the equilibrium state,

FeOOH-coated (pre-activated) brick possessed better

adsorption characteristics of Fe(II) ions than those ob-

served in original (untreated) brick: indeed the extent of

adsorption in percentage and the Fe(II) content adsorbed

per unit mass of modified brick pellets (Qmax) were found

to increase appreciably (see Table 3).

LogQe

Thus, from the linear Freundlich plots obtained for

no-activated brick (with correlation coefficients 0.95 ≤

R2 ≤ 0.98), the Freundlich isotherm capacity, KF, varied

from 0.036 to 0.050 mg/g and the adsorption intensity, n,

ranged from 1.581 to 2.170 (see Table 3); whereas from

the linear Freundlich plots obtained for activated brick

(with R2 ≈ 0.88), KF and n were found to be: 0.348 and

4.531, respectively (see Table 3).

Langmuir isotherm plots found for our system had also

good linearity with regression coefficient, R2, ranging

from 0.990 to 0.993, indicating strong monolayer chemi-

sorption. It is also worth noting that the Langmuir equili-

brium coefficient, KL, was indicative of the way in which

the presumed equilibrium reaction was displaced:

brick + Fe(II)(in aqueous phase) <=> brick – Fe(II)

As a whole, when brick pellets were pre-treated by

HCl at 90˚C, larger KL values were found and confirmed

better association between brick and Fe(II) ions. Also,

acid activation was found to have further positive effect

on the maximum adsorption capacity, Qmax increasing

from 0.179 - 0.264 mg/g to 0.669 mg/g by enhancing the

Langmuir capacity of brick by about 300%. This phe-

nomenon can be explained by the generation of a great

number of additional adsorption sites through the HCl

treatment of brick pellets preceding their FeOOH coat-

ing.

The essential features of Langmuir isotherm can be

expressed by the dimensionless constant separation fac-

Langmuir m odel

y = 17.704 x + 5.5932

= 0.99

0.00 0.501.00 1.50

1/ Ce(L/mg)

1/ Qe(g/m g)

Figure 3. Typical Langmuir and Freundlich isotherms ob-

tained for Fe(II) adsorption on to FeOOH-coated (pre-ac-

tivated) brick pellets at room temperature. The initial con-

centration of Fe(II) solution was 30 mg of iron per liter; and

the amount of brick grains, mbrick, in the medium was 4 g in

100 mL.

Table 3. Adsorption parameters determined from Lang-

muir and Freundlich isotherms.

Langmuir constants Freundlich constants

Qmax KL K

Adsorbents (mg/g)(L/mg) R2 (mg/g) n R2

FeOOH-coated brick

pellets 0.1790.3159 0.990 0.050 2.1700.950

(brick grains were

previously washed with

HCl 1 M at RT for

24 hours

0.2640.2297 0.993 0.036 1.5810.981

FeOOH-coated brick

pellets

(brick grains were

pre-activated with HCl

6 M at 90˚C for 6 hours)

0.6690.659 0.919 0.348 4.5310.881

tor or equilibrium parameter: RL = 1/(1 + KLCo), where

KL is the Langmuir isotherm constant, and Co represents

the initial Fe2+-ions concentration in the study aqueous

medium [21]. The values of RL calculated for Fe(II) ad-

sorption on to FeOOH coated-brick pellets were found to

be, in all cases, within 0 and 1, suggesting a highly fa-

vorable adsorption process, in addition, with a more ele-

vated adsorption efficiency when Fe(II) content in-

S. C. DEHOU ET AL.

470

creased in solution [raw brick pellets: 0.14 ≤ RL ≤ 0.68;

and pre-treated (activated) brick : 0.04 ≤ RL ≤ 0.08].

3.4. Effect of Temperature and Activation

Parameters for Fe(II) Adsorption onto

Modified Brick

The above kinetic investigations by considering either a

pseudo-first-order or a pseudo-second-order allowed us

to show that: 1) the adsorption was controlled by chemi-

cal sorption; and 2) an increase in temperature favored

Fe(II)-adsorption onto brick pellets, and consequently,

that this process was endothermic.

The linear form of Arrhenius equation as: Lnk =

–Ea/RT + Cst was used successfully in the temperature

range 17˚C - 45˚C in order to assess the activation energy

for Fe2+ ions adsorption onto brick grains: Lnk1 and Lnk2

were plotted against 1/T, and the corresponding curves

yielded straight lines with regression coefficient values

of = 0.95 and 2 = 0.97, respectively (see Figure

4). It should be noted that the curve Lnk1 vs 1/T con-

tained more experimental aligned dots and in addition its

regression coefficient was found to be better than that

determined from the curve Lnk2 vs 1/T, suggesting that

the pseudo-first-order model correlated better with Ar-

rhenius equation. The activation energies, Ea1 and Ea2,

were evaluated from the slope –Ea/RT: we found Ea1 =

16.20 ± 0.40 kJ/mole and Ea2 = 16.93 ± 0.85 kJ/mole.

These positive values of activation energy were found to

be closer each other and further revealed that the rise in

the solution temperature enhanced Fe(II) adsorption onto

treated brick according to an endothermic process. In

other words, a rise in temperature then led to help more

Fe2+ ions to overcome this energy barrier, Ea, and thereby

to get attached to brick surfaces/sites. On the other hand,

the activation energies for Fe(II) adsorption onto modi-

fied brick exceeded largely 4.2 kJ/mole. This Ea value is

usually considered as the maximum one measured for

physical adsorption since the forces involved in such a

process are low [22].

Accordingly, the involvement of a physical adsorption

process in our system should be excluded. Instead, Ea1

and Ea2 were found to be in the range of activation ener-

gies obtained for activated chemical adsorptions, i.e.: 8.4

–83.7 kJ/mole [22-24].

To provide a better understanding of the adsorption

thermodynamics of Fe2+ ions onto FeOOH-coated brick

pellets, the thermodynamic activation parameters of this

process were assessed using Eyring equation: Ln(k/T) =

Ln(kB/h) + ΔS/R – ΔH/RT, where kB is the Boltzmann

constant (1.3807 × 10–23 J·K–1); h is the planck constant

(6.6261 × 10–34 J·s); and ΔS and ΔH are the changes in

entropy and enthalpy of activation, respectively. Ln(k1/T)

and Ln(k2/T) were plotted against 1/T, see Figure 5, and

the slopes allowed us to determine –ΔH1/R and –ΔH2/R,

Figure 4. Activation energies measured for adsorption ki-

netics of soluble ions on to FeOOH-coated (activated) brick

pellets by plotting Ln(k) versus 1/T, where k represents the

pseudo-first order (k1) and pseudo-second order (k2) kine-

tics constant, and T is the reaction temperature.

Figure 5. Thermodynamic parameters obtained for adsorp-

tion kinetics of iron(II) on to FeOOH-coated (activated)

brick pellets by plotting Ln(k/T) against 1/T, where k

represents the pseudo-first order (k1) and pseudo-second

order (k2) kinetics constant, and T is the reaction tempera-

ture.

S. C. DEHOU ET AL. 471

and the intercepts [Ln(kB/h) + ΔS1/R] and [Ln(kB/h) +

ΔS2/R]; And from these results, the changes in entropy of

activation (ΔS1 and ΔS2) and in enthalpy of activation

(ΔH1 and ΔH2) were evaluated and reported in Table 4.

Also, the free energies of activation, ΔG1 and ΔG2

for

this process were calculated at different temperatures

ranging from 17˚C to 45˚C from the equation: ΔG = ΔH

– TΔS (see Table 4).

It should be noted that ΔS1 and ΔS2 were negative,

suggesting that Fe2+ ions at the solid-solution interface

were more organized than those located far in the bulk

solution phase, and consequently, their degree of free-

dom decreased [25-28]. The magnitude and sign of ΔS1

and ΔS2 were good indicators to know whether the study

reaction is an associative or dissociative mechanism

[29-31]. Our ΔS values did confirm that Fe(II) adsorption

onto this treated brick was an associative mechanism. As

for the changes in enthalpy of activation, ΔH, they were

found to be largely lower than –TΔS in all cases, which

meant that the reorientation step was mainly entropy

controlled at the activation state. The values of free en-

ergy indicated that the adsorption reaction was not a

spontaneous one and instead the system consumed en-

ergy from an entropic source that resulted from the stru-

ctural organization of Fe2+ ions at the brick surface; and

this free energy further increased with an increase in

temperature (see Table 4).

4. Conclusion

The research described here was devoted to examine the

Fe(II)-removal efficiency of FeOOH-brick composites in

aqueous solutions following a thermal acid pre-treatment

of the brick used as a support material. BET data re-

vealed larger surface areas on the modified brick, favor-

ing FeOOH coating. Column experiments indicated that

Fe(II)-adsorption characteristics of the new composite

were strongly improved when compared results to those

obtained for raw brick, FeOOH-coated (no-treated) brick

and iron oxides-coated sand. Fe(II) adsorption kinetics

was found to be better described by the pseudo-second-

order equation, showing that the chemical adsorption/

Table 4. Thermodynamic parameters for adsorption kine-

tics of iron(II) onto FeOOH-coated (pre-activated) brick.

Thermodynamic parameters First order Second order

∆H (kJ/mole) 15.07 16.55

∆S (J/mole/K) –173.69 –152.25

∆G (kJ/mole) at 20˚C 65.96 61.16

∆G (kJ/mole) at 25˚C 66.83 61.92

∆G (kJ/mole) at 30˚C 67.70 62.68

∆G (kJ/mole) at 35˚C 68.57 63.44

∆G (kJ/mole) at 39.5˚C 69.35 64.13

chemisorption was the rate-limiting step in the study

system. Also, the equilibrium data were better described

by the Langmuir equation, thus showing the involvement

of a monolayer coverage of iron(II) onto brick-grains

surfaces. The activation energy of this process was eva-

luated: Ea2 = 16.93 ± 0.85 kJ/mole. The magnitude of the

Ea value indicated clearly that the adsorption was con-

trolled by an activated chemical adsorption. Results from

this study did suggested that FeOOH—coated brick

could be used as an effective adsorbent for iron(II) re-

moval from contaminated waters in developing countries,

particularly when an acid pre-treatment of the support

material “brick” was initially performed.

5. Acknowledgements

This work is partly funded by the “Agence de l’Eau Artois-

Picardie”, the “Region Nord Pas-de-Calais”, the “Conseil

Général du Nord”, and the Town Hall of Villeneuve

d’Ascq. This study is part of the first-author (St C. De-

hou) Ph.D. thesis, and results from the cooperation be-

tween the University of Lille1 (France) and the Univer-

sity of Bangui (Central African Republic). This collabo-

ration and the Grant-in Aid to Mr. St C. Dehou for his

scientific research are financially supported by the Em-

bassy of France to Bangui.

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