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Separation and Purification Technology 51 (2006) 374378

Removal of nitrate from aqueous solution by using red mud

Yunus Cengeloglu a,, Ali Tor b, Mustafa Ersoz a, Gulsin Arslana

a Selcuk University, Department of Chemistry, Campus, 42031 Konya, Turkeyb Selcuk University, Department of Environmental Engineering, Campus, 42031 Konya, Turkey

Received 12 August 2005; received in revised form 7 February 2006; accepted 15 February 2006

Abstract

The objective of this study is to remove the nitrate from aqueous solution by using the original and activated red mud in batch adsorption

technique. The effects of pH, adsorbent dosage and contact time on the adsorption were investigated. The nitrate adsorption capacity of activated

red mud was found to be higher than that of the original form and decreased above pH 7. Adsorption process was expressed by using Langmuirand Freundlich isotherms. Langmuir isotherm curves was found to be significant compared to Freundlich isotherm. Adsorption capacity of the

original and activated red mud was found to be 1.859 and5.858 mmol nitrate/gred mud, respectively. It was found that sufficient time for adsorption

equilibrium of nitrate ions is 60 min. Themechanism fornitrate removal was explainedby considering of chemical natureof redmud and interaction

between metal oxides surface and nitrate ions.

2006 Elsevier B.V. All rights reserved.

Keywords: Red mud; Activated red mud; Adsorption; Removal of nitrate; Utilization

1. Introduction

Nitrate is mainly found in most of natural waters at moder-

ate concentrations but is often enriched to over the contaminantlevels from the excessive using of fertilizers and uncontrolled

discharge of raw [13]. Most important environmental problems

caused by nitrate are eutrophication in water supplies and infec-

tious disease [4]. Excessnitrate in drinking water maycauseblue

baby disease called as methemoglobinemia in newborn infants

as well as other illness [5,6]. In order to protect public health

from the adverse effects of high nitrate intake, World Health

Organisation (WHO) set the standard as 50 mg/L to regulate the

nitrate concentration in drinking water [7].

The conventional processes such as coagulation, filtration,

chlorination, etc. for water treatment are not useful with regard

to nitrate ion elimination from water [3]. Therefore, the tradi-

tional biological treatment [811], adsorption [5], ion exchange

[1215], Donnan dialysis [16], electrodialysis [1720] methods

have been applied to remove excessive nitrate from water. In

addition the different adsorbents such as activated carbon, sepi-

olite, slag, synthetic ion exchanger, etc. have been also used for

removing of nitrate. In recent years, considerable attention has

Corresponding author. Fax: +90 332 241 0106.

E-mail address: [email protected] (Y. Cengeloglu).

been devoted to the study of different types of low-cost materi-

als such as tree bark, wood charcoal, saw dust, alum sludge, red

mud and other waste materials for adsorption of some toxic sub-

stances [21]. Red mud (bauxite wastes of alumina manufacture)emerges as unwanted by-products during alkaline-leaching of

bauxite in Bayer process. About 500 000 m3 of strongly alka-

line (pH 1213) red mud-water pump is dumped annually

into specially constructed dams around Seydisehir Aluminum

Plant (Konya, Turkey). Since the plant began to process, red

mud has accumulated over years and causes a serious environ-

mental problem.

Therefore, in the present paper, the possibility of utilization

of the red mud in the original or activated form as an adsorbent

for removal of nitrate from drinking water was studied.

2. Experimental

NaNO3, NaCl, NaOH, HCl were of analytical grade obtained

from Merck Co. Darmstadt, Germany.

Red mud was supplied from the Etibank Seydisehir Alu-

minum Plant (Konya, Turkey). The grain size of red mud was

mostly (>94%) less than 10m and average composition of red

mud was given in Table 1.

Original red mud was prepared by suspending the red mud

in distilled water with a liquid to solid ratio of 2/1 on a weight

basis, stirring it until the equilibrium pH is around 8.08.5, than

1383-5866/$ see front matter 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.seppur.2006.02.020
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.seppur.2006.02.020http://localhost/var/www/apps/conversion/tmp/scratch_4/dx.doi.org/10.1016/j.seppur.2006.02.020mailto:[email protected]


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Y. Cengeloglu et al. / Separation and Purification Technology 51 (2006) 374378 375

Table 1

Average composition of red mud used (% by wt.)

Al2O3 18.7 0.6

Fe2O3 39.7 0.7

TiO2 4.9 0.5

Na2O 8.8 0.9

CaO 4.5 0.6

SiO2 14.5

0.4LOIa 8.1 0.4

a Loss on ignition.

it was died in an oven at 105 C. Later activation of red mud

was carried out as follows. The 10 g of water-washed and dried

red mud was boiled in 200mL of 20% wt. HCl for 20 min. The

acid slurry is then filtered and the residue washed with distilled

water to remove residual acid and soluble Fe and Al compounds.

Finally, theresidue is dried at 40 C, andusedfor theexperiments

without further treatment. The specific surface area of origi-

nal and activated red mud was 14.2 and 20.7 m2/g, respectively

[22].The nitrate solutions were prepared from stock solutions

(1000mg/L) prepared in laboratory to desired concentrations.

The nitrate concentration was determined with a specific ion-

electrode (Mettler Toledo) by use of ionicstrengthadjuster (ISA)

solution (2 M (NH4)2SO4) to eliminate the interference effect

of complexing ions. The ISA solution was added 100:2 into the

nitrate standard and other solutions [23]. Nitrate concentration

and pH were measured by an Orion EA940 ion meter.

The adsorption experiments were carried out with batch

method. All experiments were carried out at a constant ionic

strength of 0.1 M maintained with NaCl. A known amount of red

mud and nitrate solution were taken in a 100 mL stoppered coni-

cal flask. Sodium chloride was added to maintain ionic strength,

and pH was adjusted to the desired level with 0.1 M NaOH or

0.1 M HCl solutions. The final volume was adjusted to 50 mL

with distilled water and agitated at constant speed (500 rpm)

with magnetic stirrer in room temperature over a period of time

and then filtered. The concentration of nitrate in filtrate was

determined and the amount of nitrate removal was calculated

from the ratio of nitrate taken and that remaining in the solution.

Adsorbed nitrate was calculated from mass balance.

The experimental parameters studied are: amount of adsor-

bent (18 g/L), contact time (5200 min), initial nitrate concen-

tration (5250 mg/L), and the effect of pH (211).

3. Results and discussion

3.1. Effect of contact time

The removal of nitrate versus time was illustrated in Fig. 1,

and as can be seen that the removal of nitrate increases with time

up to reach a steady state value in 60 min.

The amount of nitrate ions adsorbed from water was

expressed by Eq. (1):

q =Q

wd

(1)

Fig. 1. Removal of nitrate as a function of equilibrium time. pH 6; initial nitrate

concentration, 1.61 mmol/L and activated red mud dose is 0.2g/50 mL.

where Q and wd are the amount (mmol) of nitrate ions adsorbed

on the red mud and the weight (g) of the dry red mud, respec-tively.

3.2. Adsorption isotherms

The adsorption isotherms of nitrate on red mud and activated

red mud were illustrated in Fig. 2, in which q values increased

with increasing of initial nitrate concentration (C) on both orig-

inal and activated red mud. As shown in Table 2, the adsorption

isotherm of nitrate was expressed both Langmuir andFreundlich

isotherms. The Langmuir isotherm equation is written as

C

q =

1

KbAs +

C

As (2)

where Cis the nitrate equilibrium concentration, the parameters

Kb and As are the adsorption binding constant (L/mmol) and

the saturation capacity (mmol nitrate/g dry wt. of red mud),

respectively. The Freundlich isotherm equation is written as

q = kC1/n (3)

where Cis equilibrium concentration, kis the saturation capac-

ity (mmol nitrate/g dry wt. of red mud) and n is an empirical

parameter. The experimental data were fitted to both Langmuir

and Freundlich isotherm equations. Nitrate adsorption constants

and correlation coefficients were calculated from Langmuir as

Table 2

Parameters of Langmuir and Freundlich isotherms for adsorption of nitrate on

activated and original red mud

Langmuir

isotherm model

Freundlich

isotherm model

As (mmol/g) Kb (L/mmol) R2 k(mmol/g) n R2

Activated

red mud

5.858 65.654 0.999 6.727 3.492 0.801

Original

red mud

1.859 1.674 0.945 0.874 1.599 0.962


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376 Y. Cengeloglu et al. / Separation and Purification Technology 51 (2006) 374378

Fig. 2. (a) The adsorption of nitrate on the activated and original red mud; (b) Langmuir isotherm for activated red mud; (c) Freundlich isotherm for activated red

mud; (d) Langmuir isotherm for original red mud; (e) Freundlich isotherm for original red mud. pH 6; contact time, 60 min; activated and original red mud dose is

0.2g/50mL.

well as Freundlich plots (Fig. 2(b)(e)) and are presented in

Table 2. The adsorption data in respect to nitrate ions provide

an excellent fit to Langmuir isotherms, giving correlation coef-

ficients of 0.999 and 0.945 for activated and original forms,

respectively. As seen in Fig. 2(a) and Table 2, the adsorption

capacity of activated red mud (5.858 mmol nitrate/g dry wt. of

red mud) is higher than that of original red mud (1.859 mmol

nitrate/g dry wt. of red mud). It canbe mentioned that monolayer

coverage does not occur on the heterogeneous surface of origi-

nal red mud [24]. This situation is attributed that various active

sites or heterogeneous mixture of several minerals on original

red mud has different affinities to nitrate anion [25].


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Y. Cengeloglu et al. / Separation and Purification Technology 51 (2006) 374378 377

For activated red mud, Langmuir isotherm exhibited a bet-

ter fit to experimental data compared to Freundlich isotherm. It

is very well-known that Langmuir type adsorption is resulted

with monolayer type adsorption, it means the adsorption is lim-

ited by surface site saturation and the sorption onto red mud

is saturated one sites of the surface layer because of heteroge-

neous mixture of several minerals in its structure [25]. In other

words, less heterogeneous mineral assemblage on the surface

of activated red mud results in a homogeneous surface. It was

pointed out by Genc-Fuhrman et al. [25] the adsorption capac-

ity of Bauxol (red mud) is hindered by the presence of surface

impurities (i.e. salts and basic properties), thus, applying of acti-

vation process improve the adsorption capacity. This statement

was also expressed that the acid treatment as well as heat treat-

ment significantly enhanced the adsorption capacity of the raw

material of Bauxsol.

On the contrary, adsorption of nitrate onto original red mud

can be better defined by Freundlich isotherm which is assumed

to be fit for exponential increasing in adsorption and thus it gave

lower saturation capacity compared to Langmuir isotherm. Inother words, the nitrate sorption in the original red mud obeys

the Freundlich isotherm model which is frequently encountered

when solutes interact with heterogeneous substrate. According

to the obtained results, the activation improves the adsorption

capacity by increasing binding sites. It can be found that similar

results in the literature [5,21,24,25], where removal efficiency of

different sorbates increased by following (the activation of red

mud and sepiolite with acid.) Red mud is high in sodalite com-

pounds ((Na, Ca)8(AlSiO4)6(SO4, OH, S, Cl)2) [26,27] which

are expected to hinder the adsorption by blocking the available

adsorption sites for nitrate. Therefore, the leaching out of the

sodalite compounds during acid treatment increases the adsorp-tion capacity [2830].

3.3. Effect of pH

The extent of adsorption of anions is strongly governed by

the pH of the solution. Since anion adsorption is coupled with

OH ions, the adsorption is favored in low or neutral pH values.

Red mud is a metal oxide adsorbent containing different metal

oxide in the structure. In a humid environment, hydroxylated

surfaces of these oxides developed charge on the surface. The

interaction between nitrate ion and metal oxide was modeled by

assuming ligand exchange reactions as follows [21,31]:

(4)

(5)

where M presents metal ions (Al, Fe or Si).

The pH of the aqueous solutions is an important variable,

and controls the adsorption between the adsorbent and aque-

Fig. 3. The effect of equilibrium pH on nitrate removal. Contact time is

60 min; activated red mud dose, 0.2 g/50mL and initial nitrate concentration,

1.61 mmol/L.

ous interface. The adsorption of nitrate on activated red mudwas studied at different pH values, ranging from 2 to 11. The

obtained results are given in Fig. 3. It is evident that that

removal of nitrate fluctuates very little in the pH range 27.

The obtained results for maximum adsorption are in agreement

with the nitrate removal study on the sepiolite and activated

sepiolite (about pH 6) [5]. It is seen from Fig. 3, the nitrate

removal decreases at a pH above 7, due to stronger competi-

tion with hydroxide ions on adsorbent surface. In other words

a high bias in the measured concentration could occur at pH

values above 7 due to presence of high concentration of OH

[27].

The solution pH relative to the point of zero charge (pHpzc)

for the red mud also needs to be considered. At pH values above

the pHpzc of the adsorber, the surface of adsorber particles is

negatively charged and as the pH rises above the pHpzc, anion

adsorption decreases. The pHpzc for red mud and activated red

mud has been reported as about 8.3 and 8.5, respectively, in the

literature [27,32], but the change from a strongly positive to a

strongly negative zeta potential takes place gradually over 1.52

pH units [33].

3.4. Effect of red mud dosage

The percentage of nitrate adsorption with varying amounts

of activated red mud is presented in Fig. 4. In general, theincrease in adsorbent dosage increased the percent removal of

adsorbate. This is consistent with the expectation that higher

adsorbent dosages will result in lower q values. The concentra-

tion of surface hydroxyl groups is related to red mud concen-

tration through surface site density [31]. Therefore percent of

adsorption increased with red mud dosage, whereas q decreased

(Fig. 4).

Various anions in drinking water or waste water have relative

binding ability on red mud surface [21]. Therefore, their effects

on the adsorption of nitrate should be considered. In the liter-

atures, it was reported that nitrate adsorption decreased from

41.4% to 31.4% in case of sulphate and 16% in phosphate on


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378 Y. Cengeloglu et al. / Separation and Purification Technology 51 (2006) 374378

Fig. 4. Thevariationof removal nitrate with activated redmud dose.pH 6;initial

nitrate concentration, 1.61 mmol/L and contact time is 60 min.

activated carbon, and from 33.4% to 5.6% and 0.5% in sulphate

and phosphate, respectively, on sepiolite.

4. Conclusion

In this study, the possibility of utilization of red mud as adsor-

bent for removal of nitrate from aqueous solution was studied.

The results are summarized as follows:

The nitrate saturation capacity of activated and original

red mud is 5.858 mmol nitrate/g dry wt. of red mud and

1.859 mmol nitrate/g dry wt. of red mud, respectively. In

other words, the removal of nitrate with activated red mudwas found as three times higher with regard to the original

form.

The effect of time for removal of nitrate was determined

within 60 min.

The removal of nitrate was decreased at a pH above 7.

Acknowledgement

The authors are grateful for kindly financial support provided

by Selcuk University Research Foundation (SUAF).

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