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Journal of Environmental Management 91 (2010) 1883e1891

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Physicochemical characterization of hydroxyapatite and its application towardsremoval of nitrate from water

Mahamudur Islam a,*, Prakash Chandra Mishra a, Rajkishore Patel b

aDepartment of Chemistry, Purushottam Institute of Engineering & Technology, Rourkela 770034, Orissa, IndiabDepartment of Chemistry, National Institute of Technology, Rourkela 769008, Orissa, India

a r t i c l e i n f o

Article history:Received 20 August 2009Received in revised form29 March 2010Accepted 20 April 2010Available online 23 May 2010

Keywords:HydroxyapatiteNitrateAdsorption isothermsThermodynamic parameter

* Corresponding author. Tel.: þ91 9437342098, þ91280625.

E-mail address: mahamudur.islam@gmail.com (M

0301-4797/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jenvman.2010.04.013

a b s t r a c t

A laboratory study was conducted to investigate the efficiency of hydroxyapatite (HAP) towards removalof nitrate from synthetic nitrate solution. In the present research HAP synthesized from egg-shell wascharacterized using SEM, XRD, FTIR and TGAeDSC. The removal of nitrate was 96% under neutralconditions, using 0.3 g of adsorbent in 100 mL of nitrate solution having an initial concentration of100 mg/L. An adsorption kinetic study revealed that the adsorption process followed first order kinetics.Adsorption data were fitted to a linearly transformed Langmuir isotherm with correlation coefficient(R2)> 0.98. Thermodynamic parameters were also calculated to study the effect of temperature on theremoval process. In order to understand the adsorption type, equilibrium data were tested with theDubinineRadushkevich isotherm. The process was rapid and equilibriumwas established within the first40 min.

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1. Introduction (Sundaram et al., 2006), surface activity, and thermal stability. The

Calcium phosphate compounds have generated a great deal ofinterestbecauseof itswidevarietyofmedicalapplications, especially inorthopedic, orthodontic and plastic surgery. Among these compoundshydroxyapatite (HAP), [Ca10(PO4)6(OH)2] has the chemical structure ofthe mineral component of bones and is widely used to repair, fill,extend and reconstruct damagedbone tissue. It can also be used in softtissue. This material can be obtained from mammal bones or coral. Inthe laboratory, it can be synthesized by reactions in solid state, co-precipitation (Silva et al., 2001), hydrothermal methods (Liu et al.,2003), or solegel process (Eshtiagh-Hosseini et al., 2007). Precipita-tion in aqueous solutions is the most employed experimental proce-dure because HAP is insoluble in water. The molar ratio of calcium tophosphorus Ca/P varies from1.2 to almost 2 inHAP. The stoichiometricmolar ratioofHAP is1.67;however, this isnot thevalueobserved in theorganism because small amounts of other materials such as carbon,nitrogen, iron and other elements are incorporated. The generalchemical formula of HAP is Ca10�x(HPO4)x(PO4)6�x(OH)2�x where0� x� 2 (Yamamuro et al., 2000).

HAP has been reported as an efficient adsorbent (Lin et al., 2008;Jang et al., 2008; Yang and Zhang, 2009). Many researchers haveassessed its dissolution (Queiroz et al., 2003), crystallizability

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. Islam).

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thermal stability of porous A-type carbonated HAP spheres wasfirst studied by He et al. (2008) via calcining in the wide temper-ature range from 393 K up to 1173 K, analyzed by FTIR, XRD andSEM. Cacciotti et al. (2009) reported that, pure HAP nanopowderdecomposed at around 1000 �C and Mg-substituted HAP wasthermally stable up to 660 �C. Tanaka et al. (2004) studied thesurface modification of calcium HAP with pyrophosphoric acid. Thesurface of synthetic colloidal calcium HAP was treated with pyro-phosphoric acid in acetone and thematerials were characterized byXRD, thermal analysis, N2 adsorption, TEM, and FTIR. Previousworks also mentioned that the synthetic HAP has higher efficiencyfor the removal of different toxic elements (Smi�ciklas et al., 2006;Corami et al., 2008; Jang et al., 2008; Corami et al., 2008; Zhuet al., 2008). So, in the present investigation, it was thoughtworthwhile to evaluate the removal efficiency of nitrate by HAP.

Nitrate is a stable and highly soluble anion with a low tendencyfor precipitation and adsorption. These properties make it difficultfor removal byconventionalwater treatment technologies. In recenttimes, the extensive use of chemical fertilizers and improper treat-ment of wastewater from the industrial sites and urban sites has ledto several environmental problems. Nitrate removal from waste-water, ground water and drinking water sources has become one ofthe major problems throughout India. Several nitrogenouscompounds, including ammonium compounds, nitrites and nitrateshave been frequently found in drinking water and various types ofagricultural, domestic and industrial wastewater (Peavy et al., 1985;Dhamoleet al., 2007).Nitrate contaminatedwater supplies havealso

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e18911884

been linked to outbreaks of infectious diseases (Barber and Stuckey,2000). Excess nitrate in drinking water may cause meth-emoglobinaemia (blue baby syndrome) in newborn infants (Felekeand Sakakıbara, 2002; Islam and Patel, 2009). Recently it has beenfound that nitrate ion also causes diabetes (Schober et al., 2003) andis a precursor of carcinogen (Yang et al., 2007).

Several nitrate removal methods such as, ion exchange, reverseosmosis (Schoeman and Steyn, 2003), electrodialysis (Midaouiet al., 2002), catalytic hydrogenation (Pintar et al., 2001), biolog-ical denitrification (Schmidt et al., 2003), adsorption etc. have beenused to remove nitrates from drinking water. Öztürk and Bektas(2004) studied the removal of nitrate from aqueous solution byadsorption onto sepiolite, sepiolite activated by HCl, slag andpowdered activated carbon. As reported in literatures, the use ofbiological reactor seems to be the most promising technique in thetreatment of water with high nitrate concentration. However,maintaining biological processes at their optimum conditions isvery difficult. In recent years chemical methods of nitrate removalusing zero-valent metals like iron (Fe0), aluminum (Al0) andmagnesium (Mg0) has been studied intensively (Huang and Zhang,2004; Huang and Zhang, 2002; Chen et al., 2005; Kielemoes et al.,2000; Fanning, 2000; Kumar and Chakraborty, 2006). In aqueoussystem at acidic pH, Fe0 is oxidized to ferrous ion (Fe2þ) and nitrateion is reduced to ammonia or nitrogen gas (Yang and Lee, 2005). AtpH above 10.5, Al0 also reduces nitrate to ammonium ion and itselfis oxidized to Al3þ ion (Huang and Zhang, 2002). In acidic medium,Mg0 is oxidized to Mg2þ ion and nitrate ion is reduced to nitrogengas or ammonium ion (Kumar and Chakraborty, 2006). However,challenges like high dose requirement, long reaction time and rapiddeactivation of metal surfaces due to hydroxide precipitation wereobserved. Low efficiency, ineffectiveness, and increase healthconcern due to residual Al3þ salt in water are problems withaluminum powder (Luk and Au-Yeung, 2002). On the other hand,adsorption is a very feasible process but there are not manyadsorbent available for removal of nitrate since nitrate is veryselective towards adsorption. Thus there is need to develop cheapand readily available adsorbent, which can remove nitrate effi-ciently under ambient temperature and pressure, within shortcontact time and at moderate dose.

Removal of nitrate using HAP is a novel method and has not beenreported so far. So, it was thought worthwhile to study the removalprocessofnitrateusingHAP. Theobjectiveof thepresent researchwasto investigate the feasibility of nitrate removal from water througha laboratory scale study under ambient temperature in open system.pH, dose, initial nitrate concentration, contact time and temperaturewere selected as variable parameters and attention was given toevaluate the kinetic constants and thermodynamic parameters.

2. Materials and methods

2.1. Reagents and chemicals

Different chemicals used in the present study were of analyticalgrade and were obtained from Merck. 1000 mg/L stock nitratesolutionwas prepared by dissolving 1.6305 g of KNO3 in 1 L distilledwater. The required concentration of nitrate solution was obtainedby serial dilution of 1000 mg/L nitrate solution.

2.2. Sample preparation

HAP samples were prepared from egg-shells. The shells werefirst washed and dried. The pretreated (washed and dried) egg shellwastes were ground and sieved to powder (mesh size 30). Then, theegg-shells were added to aqueous (1:1) H3PO4, kept at 303e313 Kfor 2e3 h, under pH 1e3. The following reaction took place:

CaCO3þH3PO4/ CaHPO4þ CO2 [þH2O.

Insoluble matter was separated and removed by filtration.Subsequently, Ca(OH)2 was added to the solution and was kept at323e333 K for 24 h, The pH was adjusted 9e12 by 0.1 M NaOH. Thereaction is represented by the following equation:

CaHPO4þ CO2þ Ca(OH)2/ Ca10[(PO4)6�x, (CO3$OH)x](OH)2

At last, the suspension was dried and ground to powder (meshsize 30) to get the sample.

2.3. Characterization of hydroxyapatite

2.3.1. FTIR studyFTIR of the sample was obtained using Perkin Elmer FTIR

spectrophotometer SPECTRUM RX-I. FTIR spectrum of the samplewas obtained by KBr pellet method. The ratio of the sample to KBrwas 1:50 and the pellet was prepared at a pressure of 5 Ton.

2.3.2. Thermal analysisThermogravimetric analysis and differential scanning calorim-

etry (TGA/DSC) analysis was carried out using NETZSCH STA 409C.A total of 30 mg of the sample was used and alumina was used asreference. TGA and DSC curve was obtained from 20 �C to 1000 �Cat a heating rate of 10 �C/min.

2.3.3. Scanning electron microscopyScanning electron micrograph of the sample was obtained by

JEOL JSM e 6480LV scanning electron microscope. The sample wascoated with platinum for 30 s at a current of 50 mA before the SEMmicrograph was obtained.

2.3.4. Surface area analysisBET surface area of the sample was measured using QUAN-

TACHROME Autosorb I. The sample was degassed at 100 �C invacuum. Helium was used as carrier gas and the surface area wasmeasured by nitrogen adsorption-desorption method at liquidnitrogen temperature.

2.3.5. Chemical compositionThe Ca/P molar ratio was determined by inductively coupled

plasma with optical emission spectroscopy (ICPOES). A PerkinElmer Optimal 4300DV instrument was used. A total of 0.1 g of thesample was digested in an acid sample made up of 2.5 mL of HNO3,1.5 mL of H2O2 and 0.3 mL of HCl in a 100 mL Erlenmeyer flask. Themeasurements of the concentration were made by triplicate onthree different days.

2.3.6. X-Ray diffraction studyPowder XRD of the material was obtained by using PHILLIPS

X’PERT X-Ray diffractometer with Cu Ka radiation (35 kV and30 mA) at a scan rate of 1�/min and was analyzed using standardsoftware provided with the instrument.

2.4. Batch experiments

The nitrate adsorption experiments from its aqueous solution byHAP were carried out using standard 100 mg/L, 150 mg/L and200 mg/L NO3

� solution. The adsorption experiments were carriedout in 250 mL glass conical flask provided with stopper, by adding0.1e0.8 g of HAP in 100 mL of synthetic nitrate solution. Stopperswere provided to avoid change in concentration due to evaporation.All the experiments were carried out at ambient temperature

4000 3500 3000 2500 2000 1500 1000 50030

35

40

45

50

55

60

607 cm-1

1052 cm-1

1462 cm-11637 cm-1

1739 cm-1

2923 cm-1

3467 cm-1

T %

cm-1

Fig. 1. FTIR pattern of hydroxyapatite.

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e1891 1885

(25� 2 �C). After continuous stirring over a magnetic stirrer atabout 400 rpm for a predetermined time interval, nitrate concen-tration was determined by nitrate ion selective electrode (OrionIonplus Sureflow Nitrate Electrode) and Orion 720 Aþ ion analyzer.Two standards were prepared that bracket the expected samplerange and which differ in concentration by a factor of ten.Measurements were done by taking 10 mL of each standard andsample into separate 50 mL beaker and 10 mL of nitrate interfer-ence suppressor (930710). All the sample and standards weremaintained at same temperature to avoid interference due todifference in temperature.

2.5. Desorption and regeneration studies

A total of 100 mL of 100 mg/L, 150 mg/L and 200 mg/L nitratesolution was treated with 0.3 g of HAP and was kept under stirring

Fig. 2. Thermogravimetric analysis and different

for 24 h. The content of the flask was filtered and separated. Thefiltered adsorbent was retreated with 100 mL neutral distilledwater and distilled water adjusted to different pH with the help of1 M HCl and 1 M NaOH. It was stirred for 24 h. The residual nitrateconcentration was measured. The study was carried out at roomtemperature (25� 2 �C).

3. Result and discussion

3.1. Characterization of hydroxyapatite

Fig. 1. shows the FTIR spectra of HAP. The spectra recorded ill-defined and low intensity absorption bands of PeO due to PO4

3�

groups in the 1052 cm�1 region, which are characteristic of theHAP, as well as of the HeO bands of adsorbed water (Prabakaranet al., 2005). A well distinguished peak at 607 cm�1 indicates theOeH vibrational mode (Prabakaran et al., 2005). Bands or shouldersat 3467 cm�1 is due to OeH stretching of water of crystallization(Zhai et al., 2004). The band at 2923 cm�1 is apparently due to CO2background of the measurement system (Zhai et al., 2004).

Fig. 2. corresponds to the thermogravimetric analysis (TGA) anddifferential thermal analysis (DSC) of the gel obtained by the sol-egel procedure. There are two stages of mass loss. The first one is of54% at 351 �C and the second, of 17.31% at 589 �C. There is an initialloss due to evaporation of water evaporation at less than 100 �C andthe decomposition and the formation of calcium carbonate. Overthe temperature interval 350 �C< T< 550 �C there is a very slightslope attributed to the release of gas inside the sample. Twoexothermic processes were observed, one registered at150 �C< T< 350 �C and the other registered at 550 �C< T< 590 �C.Both the exothermic processes were due to the crystallization ofHAP. The small peak located at 771 �C corresponds to the decom-position of CaCO3.

The SEM micrograph of the sample obtained is shown in Fig. 3.Clusters of HAP particles have a homogenous size. The average sizeof the clusters was 300 nm.

Fig. 4. shows X-ray diffraction pattern of the product after dryingfor 4 h at 200 �C and after calcination at 600 �C and 800 �C. Phases

ial scanning calorimetry of hydroxyapatite.

Fig. 3. SEM micrographs of hydroxyapatite (Magnification - 20,000X, 20 kV).

Table 1Effect of calcinations temperature on specific surface area and the removal of nitrateby HAP samples.

Calcination temperature(in �C)

Specific surfacearea of HAPsamples in m2/g

Percentage removal of nitratefrom 100 mg/L synthetic nitratesolution by HAP

200 187 85600 202 91800 212 95

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e18911886

of HAP were found in all the three samples. In case of calcined HAP,phases of CaO and CaCO3 were also formed. BET surface area andnitrate removal efficiency of samples obtained after calcinationwasexamined and is presented in Table 1. It is observed from the tablethat the percentage removal of nitrate and BET surface areaincreased with increase in temperature. Maximum percentageremoval of 95% andmaximum specific surface area of 212 m2/g wasobtained for HAP calcined at 800 �C. So, the HAP calcined at 800 �Cwas used for further studies. Chemical analysis was carried out andCa/P was calculated and was found to be 2.10.

Fig. 4. XRD pattern of hydroxyapatite calcined at different temperatures.

3.2. Removal study of nitrate by batch mode experiments

3.2.1. Effect of adsorbent doseThe effect of variation of adsorbent dose on percentage removal

of nitrate from aqueous solution with HAP used in this study isgraphically shown in Fig. 5. It is evident from the figure that theremoval of nitrate increased from 80% to 95%, 75% to 86% and 62% to79% for 0.1 to 0.8 g of HAP in 100 mL of synthetic nitrate solution ofinitial concentration, 100 mg/L, 150 mg/L and 200 mg/L respec-tively. However it is observed that after dosage of 0.3 g/100 mL,there was no significant change in percentage removal of nitrate.This may be due to the overlapping of active sites at higher dosage.So, there was not any appreciable increase in the effective surfacearea resulting due to the conglomeration of exchanger particles(Tahir, 2005). So, 0.3 g/100 mL was considered as optimum doseand was used for further study.

3.2.2. Effect of pHpH of the aqueous solution is one of the main variable param-

eters in the adsorption process. The pH may affect the ionizationdegree of the sorbate and the surface property of the sorbent (Linand Yang, 2002). Six different pH tests were carried out and theresults are shown in Fig. 6. The percentage removal of nitrate byHAP increased from 69% to 96%, 65% to 86% and 60% to 80% forincrease in pH from 2 to 12, for initial nitrate concentration of100 mg/L, 150 mg/L and 200 mg/L respectively. It is evident fromthe above data that there was increase in percentage removal forincrease in pH from 2 to 6 but for further increase in pH beyond 6,there wasn’t any increase in removal. It may due to the fact thatHAP is not stable under acidic condition and hence exhibited poorremoval efficiency. And the percentage removal stood almostconstant for pH 6e12. According to the results the pH does not

50

5560

6570

75

8085

9095

100

0 0.2 0.4 0.6 0.8 1Adsorbent dose in grams

Percen

tag

e rem

oval

100 mg/L

150 mg/L

200 mg/L

Fig. 5. Adsorbent dose versus percentage removal of nitrate by hydroxyapatite withinitial concentration of 100 mg/L, 150 mg/L and 200 mg/L.

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 10 20 30 40

Time in minutes

Lo

g (q

e - q

)

100 mg/L

150 mg/L

200 mg/L

Fig. 8. Linear plot of Lagergren rate equation using hydroxyapatite, time versus log(qe� q) with initial nitrate concentration of 100 mg/L, 150 mg/L and 200 mg/L.

50

55

60

65

70

75

80

85

90

95

100

0 5 10 15

pH

Percen

tag

e rem

oval

100 mg/L

150 mg/L

200 mg/L

Fig. 6. pH versus percentage removal of nitrate, hydroxyapatite, with initial concen-tration of 100 mg/L, 150 mg/L and 200 mg/L.

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e1891 1887

affect the percentage of nitrate removed for pH 6 and above.However in the pH below 6 it has a significant effect. Therefore caremust be taken with acidic solutions.

3.2.3. Effect of contact time and adsorption kineticsAdsorption of nitrate at different contact time was studied for

initial nitrate concentration of 100 mg/L, 150 mg/L and 200 mg/Lkeeping all other parameters constant. The result is presented inFig. 7. The percentage removal was found to increase from 47% to96%, 42% to 86% and 37.8% to 80% for a contact time of 5 min to40 min and initial nitrate concentration of 100 mg/L, 150 mg/L and200 mg/L, respectively. It is clear from the figure that about 40%removal took place within first 5 min and equilibrium was estab-lished after 40 min. The change in the rate of removal might be dueto the fact that initially all adsorbent sites contained replaceablehydroxide ion and the solute concentration gradient was also high.Later, the nitrate uptake rate by adsorbent was decreased signifi-cantly, due to the decrease in number of replaceable hydroxide ions

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50Time in minutes

Percen

tag

e rem

oval

100 mg/L

150 mg/L

200 mg/L

Fig. 7. Time versus percentage removal of nitrate, hydroxyapatite, with initialconcentration of 100 mg/L, 150 mg/L and 200 mg/L.

in adsorption sites as well as decrease in nitrate concentration.Decreased removal rate, particularly, towards the end of experi-ments, indicates the possible monolayer formation of nitrate ion onthe outer surface.

The rate constant Kad for sorption of nitrate was studied byLagergren rate equation (Islam and Patel, 2007, 2008) for initialnitrate concentration of 100 mg/L, 150 mg/L and 200 mg/L.

logðqe � qÞ ¼ log qe � Kad

�t

2:303

�(1)

where qe and q (both in mg/g) are the amounts of nitrate adsorbedat equilibrium and at time ‘t’, respectively. The plots of Log (qe� q)versus ‘t’ at different time interval was almost linear, indicating thevalidity of Lagergren rate equation of first order kinetics (Fig. 8). Theadsorption rate constant (Kad), calculated from the slope of theabove plot is presented in Table 2. It was observed from the datathat the adsorption rate constant values were almost similar forinitial nitrate concentrations of 100 mg/L, 150 mg/L and 200 mg/L.It was concluded that the adsorption rate constant was indepen-dent of initial nitrate concentration.

3.2.4. Intraparticle diffusion rate constantIn order to test the existence of intraparticle diffusion in the

adsorption process, the amount of nitrate sorbed per unit mass ofadsorbents, q at any time t, was plotted as a function of square rootof time, (t1/2). The rate constant for intraparticle diffusion wasobtained using WebereMorris equation given as follows (Yadavet al., 2006),

q ¼ Kpt1=2 þ C (2)

where ‘q’ is the amount of nitrate adsorbed in mg/g of adsorbent, Kpis the intraparticle diffusion rate constant and ‘t’ is the agitationtime in minutes. Due to stirring there is a possibility of transport of

Table 2Rate constants (Kad) obtained from the graph for HAP with different initialconcentrations of nitrate.

Initial concentration Slope Intercept Rate constant in min�1

(Kad)R2

100 mg/L �0.027 0.324 0.063 0.982150 mg/L �0.028 1.021 0.065 0.981200 mg/L �0.028 1.322 0.065 0.983

Table 3Intraparticle diffusion rate constants obtained from Weber-Morris equation fordifferent initial concentrations of nitrate.

Initial concentration Slope Intercept Rate of pore diffusion (kp) R2

100 mg/L 1.269 1.763 1.269 0.990150 mg/L 2.473 3.123 2.473 0.989200 mg/L 4.124 6.803 4.124 0.992

70

75

80

85

90

95

100

10 20 30 40 50 60Temperature in degree celcius

Pe

rc

en

ta

ge

re

mo

va

l

100 mg/L

150 mg/L

200 mg/L

Fig. 10. Temperature versus percentage removal of nitrate with hydroxyapatite, initialnitrate concentration of 100 mg/L, 150 mg/L and 200 mg/L.

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e18911888

nitrate species from the bulk into pores as well as adsorption atouter surface of HAP. The rate-limiting step may be either adsorp-tion or intraparticle diffusion. The results obtained are presented inthe Table 3 and graphically shown in the Fig. 9. The rate constants(Kp) for intraparticle diffusion for various initial concentrations ofnitrate solutionwere determined from the slope of respective plots.It is evident from the graph that, the plots were straight lines butthe y-intercept of the plots were nonzero (not passing through theorigin) thus indicating that intraparticle diffusion is not the solerate-limiting factor for the adsorption of nitrate onto HAP. In thepresent study, the intraparticle diffusion may be regarded asconcentration diffusion because the values of Kp in Table 3 showincrease in Kp with the increase in nitrate concentration (Kadirveluet al., 2000).

3.2.5. Effect of temperatureThe effect of temperature on the adsorption of nitrate with

initial concentration 100 mg/L, 150 mg/L and 200 mg/L onto HAPwas studied using optimum adsorbent dose and the results arepresented as percentage removal of nitrate versus temperature(Fig. 10). The percentage removal of nitrate with initial concentra-tion 100 mg/L, increased from 92% to 97%, the percentage removalof nitrate with initial concentration 150 mg/L, increased from 85%to 90% and the percentage removal of nitrate with initial concen-tration 200 mg/L, increased from 81% to 86% for 20 �C to 50 �Ctemperature. It is evident from the figure that, at the temperature of20 �C the removal was more than 80% and with increase intemperature, the percentage removal increased slowly and reachedalmost 97% (100 mg/L), which indicates the endothermic nature ofthe process.

This was further supported by calculating thermodynamicparameters. The change in free energy (DG), enthalpy (DH) andentropy (DS) of adsorption were calculated using the followingequations (Islam and Patel, 2007, 2008).

0

5

10

15

20

25

30

35

0 2 4 6

qe

100 mg/L

150 mg/L

200 mg/L

t

Fig. 9. Linear plot of Weber-Morris equation using hydroxyapatite, square root of timeversus qe with initial nitrate concentration of 100 mg/L, 150 mg/L and 200 mg/L.

log KC ¼ DS2:303R

� DH2:303RT

(3)

DG ¼ DH � TDS (4)

where DS and DH are the changes in entropy and enthalpy ofadsorption, respectively. A plot of log KC versus 1/T for initial nitrateconcentration of 100 mg/L, 150 mg/L and 200 mg/L was linear andis represented graphically in Fig. 11.

The Kc value was calculated using the following equation (Islamand Patel, 2008).

KC ¼ C1C2

(5)

where C1 is the amount of nitrate ion adsorbed per unit mass ofHAP and C2 is the concentration of nitrate in aqueous phase. Valuesof DH and DS were evaluated from the slope and intercept of Van’tHoff plots and represented in Table 4. The positive values of DSindicate some structural changes in the adsorbent and also reflectthe affinity of the adsorbent for nitrate species. The positive value ofentropy (DS) indicates the increase in randomness of the ongoing.Negative value of DG at each temperature indicates the spontaneityof ongoing adsorption. A decrease in values of DGwith the increasein temperature suggests more spontaneity of nitrate adsorption at

1

1.1

1.2

1.3

1.4

1.5

1.6

0.003 0.0031 0.0032 0.0033 0.0034 0.00351/T

lo

g K

c

100 mg/L

150 mg/L

200 mg/L

Fig. 11. Van’t Hoff plots, log Kc versus 1/T for hydroxyapatite, initial nitrate concen-tration of 100 mg/L, 150 mg/L and 200 mg/L.

Table 4Thermodynamic parameters using HAP, synthetic nitrate solution of 100 mg/L, 150 mg/L and 200 mg/L.

Initial Nitrate concentration (mg/L) DH (kJ/mol) DS (kJ/K/mol) DG (kJ/mol) R2

20 �C 25 �C 30 �C 35 �C 40 �C 45 �C 50 �C

100 13.162 0.072 �6.603 �8.413 �8.775 �9.137 �9.499 �9.861 �10.223 0.985150 11.668 0.063 �5.585 �7.165 �7.481 �7.797 �8.113 �8.429 �8.745 0.972200 10.982 0.059 �5.152 �6.629 �6.925 �7.221 �7.516 �7.811 �8.107 0.988

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e1891 1889

higher temperature. The endothermic nature of the process wasonce again confirmed by the positive value of enthalpy (DH).Positive value of enthalpy (DH) suggests that entropy is responsiblefor making the DG value negative. So, the adsorption process wasspontaneous, since the entropy contributionwas much higher thanthat of enthalpy.

3.2.6. Effect of initial concentration and adsorption isothermsTo investigate the effect of initial nitrate concentration on

nitrate adsorption onto HAP, batch mode experiments were per-formed and the result is presented in Fig. 12. The initial concen-tration of nitrate solution was varied from 100 mg/L to 200 mg/Lwith optimum adsorbent dose. It is evident from the result that thepercentage removal of nitrate decreased from 96% to 80%. Theresults indicate that there was a reduction in nitrate adsorption,owing to the lack of available active sites required for the highinitial concentration of nitrate. The higher uptake of nitrate at lowconcentration may be attributed to the availability of more activesites on the surface of the adsorbent for lesser number of adsorbatespecies.

The adsorption data were fitted to linearly transformed Lang-muir isotherm. The linearized Langmuir equation, which is valid formonolayer sorption onto a surface with finite number of identicalsites, is given by the following equation (Tahir, 2005).

1qe

¼ 1q0bCe

þ 1q0

(6)

where qo is the maximum amount of the nitrate ion per unit weightof HAP to form a complete monolayer on the surface, (adsorptioncapacity) Ce denotes equilibrium adsorbate concentration in solu-tion, qe is the amount adsorbed per unit mass of adsorbent, and b isthe binding energy constant. The linear plot of 1/Ce versus 1/qe(Fig. 13) with R2¼ 0.988 indicates the applicability of Langmuiradsorption isotherm. The values of Langmuir parameters, qo andb are 48.827 mg/g and 0.053 L/mg, respectively.

In order to predict the adsorption efficiency of the adsorptionprocess, the dimensionless equilibrium parameter was determinedby using the following equation (Islam and Patel, 2007, 2008).

7880828486889092949698

100 120 140 160 180 200Initial concentration in mg/L

Percen

tag

e rem

oval

Fig. 12. Percentage removal of nitrate by hydroxyapatite versus initial nitrateconcentration.

r ¼ 11þ bC0

(7)

where C0 is the initial nitrate concentration. Values of r< 1 repre-sent favorable adsorption. The r-value for initial concentration of100 mg/L, 150 mg/L and 200 mg/L was found to be 0.157, 0.110 and0.085, respectively. The values indicated a favorable system. If thefigure is observed carefully, two linear parts can be seen in thefigure. A higher slope is obtained below 0.05 L/mg followed bya second part with a lower slope. A high slope results in lowervalues of q0 and b indicating less adsorption affinity. The secondpart of the curve with low slope results higher values of q0 andb indicating higher adsorption affinity.

It is known that the Langmuir and Freundlich adsorptionisotherm constant do not give any idea about the adsorptionmechanism. In order to understand the adsorption type, equilib-rium data were tested with DubinineRadushkevich isotherm(Islam and Patel, 2007, 2008, 2009). The linearized D.R. equationcan be written as:

ln qe ¼ ln qm � Ke2 (8)

where e is Polanyi potential, and is equal to RT ln (1þ1/Ce), qe is theamount of nitrate adsorbed per unit mass of adsorbent, qm is thetheoretical adsorption capacity, Ce is the equilibrium concentrationof nitrate, K is the constant related to adsorption energy, R isuniversal gas constant and T is the temperature in Kelvin.

Fig.14. shows the plot of ln qe against e2, whichwas almost linearwith correlation coefficient, R2¼ 0.921. D.R. isotherm constants Kand qm were calculated from the slope and intercept of the plot,respectively. The value of K was found to be 0.003 mol2 kJ�2 andthat of qm was 0.021 g/g. The mean free energy of adsorption (E)was calculated from the constant K using the relation (Islam andPatel, 2007, 2008).

E ¼ ð2KÞ�1=2 (9)

It is defined as the free energy change when 1 mole of ion istransferred to the surface of the solid from infinity in solution. Thevalue of E was found to be 11.851 kJmol�1. The value of E is very

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0 0.05 0.1 0.15 0.2 0.251/Ce in L/mg

1/q

e in

g

/m

g

Fig. 13. Langmuir adsorption isotherm, 1/Ce versus 1/qe for hydroxyapatite.

-3.4

-3.35

-3.3

-3.25

-3.2

-3.15

-3.1

-3.050 5000 10000 15000 20000 25000 30000

ln

q

e

ε2

Fig. 14. DeR adsorption isotherm, ln qe versus 32 for hydroxyapatite.

M. Islam et al. / Journal of Environmental Management 91 (2010) 1883e18911890

useful in predicting the type of adsorption. If the value is less than8 kJmol�1, then the adsorption is physical in nature and if it is inbetween 8 and 16 kJmol�1, then the adsorption is due to exchangeof ions (Islam and Patel, 2008). The value found in the present studywas in between 8 and 16 kJmole�1. So, the adsorption can be bestexplained as exchange of ions. Probably OH� ions are exchanged byNO3

� ions.

3.2.7. Effect of competitive ionsThe adsorption results discussed above were obtained taking

nitrate ion only. In order to test effect of interfering ions onadsorption of nitrate, a mixture of known quantities commonlyoccurring anions in water, viz., phosphate, chloride, carbonate andsulphate were added to nitrate solutions. The initial concentrationof nitrate was fixed at 100 mg/L while the initial concentration ofother anions varied from 10 mg/L to 100 mg/L and the result ispresented in Fig. 15. The anions reduced the nitrate adsorption inthe order of chloride> carbonate> sulphate> phosphate. Thismaybe due to the fact that the affinity of HAP for different anions are inthe order of chloride> carbonate> sulphate> phosphate. So,chloride competes a lot with nitrate. As phosphate has least affinityfor HAP, it has least effect on the adsorption of nitrate. From above,it is evident that chloride has maximum and phosphate has leasteffect on the removal of nitrate by HAP. It was observed that therewas about 25% reduction in the percentage removal of nitrate bychloride.

0

10

20

30

40

50

60

70

80

90

0 20 40 60 80 100Initial anion concentration in mg/L

Percen

tag

e rem

oval o

f n

itrate

Phosphate

Sulphate

Carbonate

Chloride

Fig. 15. Percentage removal of nitrate with hydroxyapatite vs initial anion concen-tration of solution (initial nitrate concentration of 100 mg/L).

3.2.8. Desorption studiesDesorption study gives an idea about the nature of adsorption.

Desorption studies were carried out in order to know the reus-ability of the HAP. The results of the studies indicated that thedesorption of the adsorbed nitrate in neutral distilled waterresulted about 2.11%, 2.45% and 3.03% for initial concentration of100 mg/L, 150 mg/L and 200 mg/L, respectively. It is evident fromthe low desorption values, that the adsorption of nitrate onto HAPwas chemical in nature. So, once nitrate ions are adsorbed over thesurface of HAP, it is very difficult to regenerate. This probablyreduces its relevance for nitrate adsorption. But due to its extremelyhigh removal capacity (95%), it can be used in many places wherewater can be made completely free from nitrate.

4. Conclusion

HAP was synthesized from egg-shell by solegel method.Synthesis was ascertained by adopting various characterizingmethods. HAP exhibited much greater specific surface area. SEMmicrograph revealed that the average size of the HAP clusters were300 nm. Optimum dose of HAP was found to be 0.3 g/100 mL ofsynthetic solution. It is evident from the pH study that there wasincrease in percentage removal for increase in pH from2 to 6 but forfurther increase in pH beyond 6, there wasn’t any increase inremoval. It was found from the study that the equilibrium wasestablished after 40 min. Adsorption of nitrate was found to followfirst order kinetics. The effect of temperature on the adsorption ofnitrate onto HAPwas studied using optimum adsorbent dose. It wasobserved that there was increase in percentage removal of nitratewith increase in temperature. The effect of initial concentrationwasstudied and was found that the percentage removal of nitratedecreased from 96% to 80% for initial nitrate concentration of100 mg/L to 200 mg/L. The effect of other anions were also studiedandwas found that the anions reduced the nitrate adsorption in theorder of chloride> carbonate> sulphate> phosphate. Thepercentage removal was found to remain constant with increase inpH and the optimum pH was found to be 6. The removal of nitrateby HAP was very efficient and can bring down the nitrate concen-tration to its permissible limit. The process can be employed asa primary step for the removal of nitrate to meet permissible limitof nitrate in surface and ground water.

Acknowledgement

The authors are thankful to Prof. S.K. Sarangi (Director), Prof. K.M. Purohit, and staff members of Department of Chemistry,National Institute of Technology, Rourkela, for providing necessaryfacilities and necessary help in carrying out the research work. Theauthors are also thankful to Prof. U.C. Patra (Director), PurushottamInstitute of Engineering & Technology, Rourkela, and other staffs fortheir necessary help and cooperation.

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