Manganese dioxide improves the efficiency of earthenware in fluoride removal from drinking water

Desalination 272 (2011) 179–186

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Manganese dioxide improves the efficiency of earthenware in fluoride removal from drinking water Venkataraman Sivasankar a,⁎, Thiyagarajan Ramachandramoorthy b, André Darchen c a b c

Department of Chemistry, Thiagarajar College of Engineering (Autonomous), Madurai, 625 015, Tamil Nadu, India PG and Research Department of Chemistry, Bishop Heber College (Autonomous), Tiruchirappalli, 620 017, Tamil Nadu, India UMR CNRS No. 6226 Sciences Chimiques de Rennes, ENSCR, Avenue du Général Leclerc, CS 50837, 35708 Rennes Cedex 7, France

a r t i c l e

i n f o

Article history: Received 12 September 2010 Received in revised form 6 January 2011 Accepted 7 January 2011 Available online 5 February 2011 Keywords: Defluoridation Adsorption Low cost sorbent Regeneration Co-existing ions

a b s t r a c t The paper presents a detailed study of the effect of manganese dioxide on the defluoridation potential of disposed earthenware (DEW) of the particle size less than 300 μm. Manganese dioxide was added to DEW with weight content from 0.01 to 0.025%. The defluoridation was investigated in static experiments, at pH 5– 11 and with contact time of 35 min. The fluoride removal increased with the increasing content of manganese dioxide. In static sorption, the defluoridation with DEW dispersed with 0.025% of manganese dioxide increased from 1198 to 1888 mg-kg− 1 when the pH increased from 5 to 7. In the simulating equilibrium data, simple kinetic models namely, pseudo I and II order, particle and pore diffusion, Elovich and isothermal models of Langmuir and Freundlich were used. The fluoride removal was investigated in the presence of coexisting ions. It was found that the reduction in fluoride sorption was greater in the presence of SO2− 4 ion than − − in the presence of HCO− 3 , Cl and NO3 . DEW with dispersed manganese dioxide, showed an ability to lower the fluoride concentration to acceptable levels and improved the defluoridation efficiency of unmodified DEW. The spent sorbent was easily regenerated by NaOH solution. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Fluorine is well known as an important micronutrient for the production and maintenance of healthy bones and teeth, so its content as fluoride in drinking water is generally recommended (controlled) by health organizations. In India, the safe limit of fluoride in potable water is between 0.6 and 1.2 mg L− 1 [1]. In order to protect public health, World Health Organization (WHO) has set the guideline level for fluoride in drinking water at 1.5 mg L− 1 [2]. The presence of excess fluoride in water causes a lot of health disorders [3], decrease of growth and intelligence [4] and fluorosis [5]. Fluorosis is a kind of chronic disease of dental or skeletal problems such as mottling of teeth in mild cases, softening bones and neurological damage in severe cases [6–8]. In the world, around 200 million people have great health risks with high fluoride content in their available drinking water. India is dramatically concerned by fluorosis [9]. Various defluoridation techniques have been developed in order to decrease the fluoride content under the prescribed limits. The electrochemical methods like electrodialysis [10] or electrocoagulation [11] need some equipment. Adsorption, precipitation or ionexchange methods are easier to perform since they use only a few chemical compounds. Adsorption is the most investigated technique ⁎ Corresponding author. Tel.: +91 452 2482240 41; fax: +91 452 248342. E-mail addresses: [email protected] (V. Sivasankar), [email protected] (T. Ramachandramoorthy). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.01.021

and generally it works with low cost sorbents which have been reviewed in recent papers [12,13]. The fluoride removal studies using low cost materials have been of much importance and extensively attempted using flyash [14], waste residue [15], spent catalyst [16] and carbon amended clay [17]. In the recent years, the fluoride removal studies are entrusted using mixed-metal oxides [18–21]. Mohapatra et al. [22] have made a brief review on the removal of fluoride from drinking water using different adsorbents. The main phenomenon that might be efficient in the fluoride removal from water is a sorption/precipitation onto composite oxide materials. We can assume that poly oxides may be interesting materials in the defluoridation of water. As shown in our precedent work [23], we are interested in the use of manganese dioxide. Manganese dioxide is an efficient sorbent which is insoluble and not hazardous. It is not used directly because it may be required in higher quantities compared to the present work with dispersion over disposed earthenware (DEW). Several studies have shown interest in the use of manganese oxide in fluoride removal: MnO2 coated alumina [24–27], bentonite clay with incorporated MnO2 [28], MnO2 coated activated carbon [29], KMnO4modified activated carbon [30] and hydrated oxides of manganese ores [31]. In the present work we searched for an inorganic support of manganese dioxide in order to get an efficient and reusable low cost material. In this paper, we present the efficiency of earthenware as an inorganic support of manganese dioxide. This support contains

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Nomenclature qe qt t k1 k2 h Kp Ki C a b Qo B Co Ce Ci RL KF n DEWx

amount of fluoride adsorbed at equilibrium (mg g− 1) amount of fluoride adsorbed at time, t (mg g− 1) time (min) rate constant (min− 1) pseudo-second order rate constant (mg g− 1) initial sorption rate (mg g− 1 min− 1) particle diffusion rate constant (min− 1) intra-particle rate constant (mg g− 1 min− 1) a constant, which gives the thickness of boundary layer initial adsorption rate (mg g− 1 min− 1) desorption constant (g mg− 1) sorption capacity (mg g− 1) Langmuir isotherm constant (L mg− 1) initial fluoride concentration (mg L− 1) final fluoride concentration at equilibrium (mg L− 1) influent concentration of fluoride (mg L− 1) a dimensionless constant separation factor a measure of the adsorption capacity (mg1–(1/n)L1/n g− 1) adsorption intensity Disposed Earthenware with x% manganese dioxide

different metal oxides (Al, Si, Ca, Mg and Fe). Each oxide may be a fluoride sorbent. So far, no study has been attempted using DEW to remove fluoride from water. Hence, to study the defluoridation potential of DEW and also to improve its interest we investigated the doping effect of manganese dioxide. It is worth-stating that the dispersed manganese dioxide into the DEW micro-porous media can be recovered back and reused as a dispersive media with DEW or other stationary phase materials for the fluoride adsorption studies.

2. Materials and methods 2.1. Chemicals All reagents were of analytical reagent grade and were used as supplied. Manganese dioxide (MnO2) and sodium fluoride (NaF) were of analytical reagent grade and were supplied by E-Merck limited, India. The standard stock solutions of fluoride (100 mg L− 1) were prepared by dissolving 221 mg of sodium fluoride using double distilled water and stored under dark conditions at 4 °C.

2.2. Disposed earthenware and its characterization The DEW was collected from a local factory at Usilampatti, in Madurai District of Tamil Nadu, South India. The DEW was crushed and ground in a ball mill. The crushed DEW was sieved for a particle size less than 300 μm. The DEW particles of larger size were ground again. In order to eliminate the soluble components, the DEW powder was treated as follows: 250 g of dried DEW sample was shaken in 3 L of 0.1 M HNO3 for 1 h and the pH was recorded. This step was repeated five times until a constant pH was achieved. The DEW samples were then filtered, successively washed and dried at 150 °C for 24 h. The chemical composition of DEW was estimated by X-ray fluorescence (XRF) with an analyzer Philips Magix PW 2424. The surface area measurements were obtained using Micrometrics Gemini 2360 surface area analyzer. For a DEW of particle size b 300 μm, the surface area was 34.23 m2 g− 1.

2.3. Defluoridation experiments Batch-wise sorption study was carried out at 298 ± 1 K at a pH of 7.50 ± 0.03. 100 mL of a solution that contained the fluoride ion (2 mg L− 1) was treated with 1 g of DEW, with and without manganese dioxide. After 35 min, the sorbents were filtered and the remaining fluoride ion concentration was determined by the SPADNS photometric method [32], at 570 nm using the UV–Visible spectrophotometer (UV–Vis 8500, Techcomp Ltd, Hong Kong). The pH measurements were carried out with the help of the pH meter of LI 613 Elico model. Besides determining the defluoridation capacities, the effects of the other factors, namely initial fluoride concentration (2–6 mg L− 1), the pH of the medium and the presence of interfering ions which are normally present in drinking water, were also studied. The effect of solution pH on fluoride uptake was studied by maintaining the pH by the addition of 0.1 M HCl or 0.1 M NaOH. The pH was varied from 5 to 11. The effects of the various ions viz., bicarbonate, chloride, nitrate, sulfate and phosphate were studied under optimal experimental conditions. 2.4. Regeneration of spent sorbent 100 mL of a fluoride solution with 2 mg L− 1 was taken with 1.0 g of DEW with 0.025% manganese dioxide. Then, the fluoride desorption was followed using sodium hydroxide solutions from 0.5 to 1.5 M. After this desorption, the ability of the regenerated sorbent to remove fluoride was again tested in an adsorption experiment, as described in Section 2.3. 2.5. Instrumental studies The following instrument-based characterization of DEW was carried out before and after fluoride sorption. The surface morphological studies were characterized using Scanning Electron Microscope (SEM) with the JEOL JSM 5610 model. The Fourier Transformed Infrared (FTIR) spectral study was carried out using JASCO FTIR 460 plus model. The X-Ray Diffraction (XRD) was recorded using Phillips X'Pert PRO diffractometer operating with the CuKα radiation (λ = 1.54056 Å). The computations were done by using Microcal Origin (version 6.0) software. 3. Results and discussion 3.1. The chemical composition and structure of raw DEW The crushed DEW appeared as a red-brown powder which was analyzed by XRD, XRF and FTIR. The XRD pattern (Fig. 1) showed a lot of diffraction peaks which are attributed to the initial clay components of the earthenware. There were too many peaks for a reliable attribution to a specific compound involved in the defluoridation process. The observation of the DEW surface was performed with a SEM. It showed an irregular morphology with a lot of crevices capable of trapping the manganese dioxide particles. From the XRF analysis, we obtained the chemical composition which is shown in Table 1. The DEW contained Al, Mg, Ca and Fe. The oxides of these elements are well known as good sorbents of fluoride anions. So it is expected that DEW may be an interesting material for defluoridation. As a consequence of the presence of a lot of possible defluoridation sorbents, it is difficult to investigate the structural changes occurring after defluoridation. The FTIR spectra showed a broad peak around 3450 cm− 1 attributed to the stretching vibration of lattice water and hydroxide groups. The vibration of OH bending of water molecules was observed at 1635 cm− 1. Owing to the presence of alkaline compounds in the raw DEW, this material was washed in nitric acid solution in order to

181

DEW

120 110 100 90 80 70 60 50 40 30 20 10 0

DEW

Fads(mgg-1)

Intensity

V. Sivasankar et al. / Desalination 272 (2011) 179–186

5 10 15 20 25 30 35 40 45 50 55 60

0.025

1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 5

two theta

6

7

8

9

10

11

pH

Fig. 1. XRD spectrum of DEW.

Fig. 2. Effect of pH on the fluoride removal capacity of DEW0.025.

avoid a pH change during the defluoridation process. The broad band between 3550 cm− 1 and 3200 cm− 1 represents the OH stretching vibrations and a peak at ~1044 cm− 1 represents the characteristic stretching of Si–O or Al–O bonds. The absorption bands at wave number (υ) of value at 1620–1655 cm− 1, represent the bending modes of vibration for the OH groups. The peaks detected in the region within 400–500 cm− 1 may be the bending vibrations of Si–O or Al–O bonds. In the spectra, a peak due to stretching vibrations of Fe–O bond was observed at 460–500 cm− 1. It was reported that the peaks observed in the range of 1200–400 cm− 1 were the characteristic vibrations of mixed metals by Liu et al. [21]. The absorption band at 2360–2370 cm− 1 was common in the spectra which are presumed to be for the carbonate impurity [18].

3.2. Effect of pH on the sorption of fluoride ions The removal of the fluoride ions from the aqueous solution was highly dependent on the solution pH in many cases, as the pH altered the surface charge on the sorbents [33]. A comparative graph for the adsorption of fluoride onto DEW powder less than 300 μm added with 25 mg of manganese dioxide with plain DEW powder at various pH values is shown in Fig. 2. These results show that the best defluoridation capacity was obtained at a pH of 7.15. It is noteworthy that the presence of manganese dioxide in DEW increased the defluoridation capacity. The removal of fluoride was 70% for DEW and 77% for DEW with 0.025% MnO2. The adsorption of fluoride was decreased when the pH varied from 7.0 to 5.0 or from 7.0 to 11.0. The lower adsorption of fluoride in the acidic medium may be attributed to the formation of a weakly ionized hydrofluoric acid, which reduces the ability of the free fluoride for adsorption. In the alkaline medium, there is a competition between the OH− and F− ions for adsorption because of the similarity in F− and OH− in their charge and ionic radius. The results obtained are in good agreement with the studies reported for fluoride removal [34,35]. The pH-dependent interaction of fluoride on metal oxides of DEW0.025 can be represented schematically as follows: þ

þ

¼ MOH þ H → ¼ MOH2 þ

−

þ

−

ð1Þ

þ

¼ MOH2 þ F → ¼ MOH2 −F þ H2 O

ð2Þ

¼ MOH2 þ F → ¼ MF þ H2 O

ð3Þ

Table 1 Chemical composition of raw DEW. Components

SiO2

Al2O3

TiO2

K2O

MgO

CaO

Fe2O3

Others

wt.%

57.41

16.87

1.31

2.85

1.35

9.80

9.67

0.74

−

2½ = MOH + 2F → = MOF + H2 O j ¼ MF

ð4Þ

Eqs. (1), (2) and (3) represent the electrostatic interaction between the positively charged DEW0.025 surface and the negatively charged fluoride ions in the acidic pH range. Eq. (4) represents the ligand exchange interaction [27,36] between the fluoride and the hydroxyl groups that is found to occur at the neutral pH region, leading to an appreciable amount of adsorption in this region. This type of exchange occurs because F− and OH− are isoelectronic and with comparable ionic radius. In a study using laterite, the increase in the fluoride adsorption was contributed by the presence of iron oxide at pH 7.5 [37]. Yadav et al. [34] reported the maximum fluoride removal capacity using brick powder as an adsorbent between the pH of 7 and 8. 3.3. Sorption kinetics The kinetic data based on various models for the fluoride sorption onto DEW powder with manganese dioxide are gathered in Tables 2 and 3. 3.3.1. Pseudo-first-order kinetic model The pseudo-first-order equation is given by Eq. (3) where qe and q are the amounts of fluoride adsorbed at equilibrium and at time t in mg g− 1, respectively, and k1 is the pseudo-first order rate constant. lnðqe –qÞ = ln qe –k1 ðtÞ

ð5Þ

As such, the values of ln (qe − q) for fluoride were calculated and plotted against time. The plots as shown in Fig. 3 were found to be linear with good correlation coefficient R2, indicating that Lagergren's equation [38] is appropriate to the fluoride sorption onto DEW with varied manganese dioxide additions. A large adsorption rate constant k1 represents a fast adsorption. The quick removal of fluoride was observed from the increasing values of k1 and may indicate the availability of a large number of binding sites at the exterior surface of DEW, which increases with the addition of manganese dioxide [25]. The variation in rate should be proportional to the first power of concentration for strict surface adsorption. However, the relationship between the initial fluoride concentration and the rate of adsorption will not be linear, when pore diffusion limits the adsorption process. This is in agreement with a study conducted by Ghorai and Pant [39]. The pseudo-first-order rate constant (k1) increased with the increase in MnO2 addition with DEW from 5.312 × 10− 2 min− 1 to 7.392 × 10− 2 min− 1. The regression coefficient values (N0.95) have shown the applicability of the pseudo I order model with respect to the influence of MnO2 on DEW. The variation of pseudo I order rate constant from 0.057 min− 1 to 0.165 min− 1 for the initial fluoride concentrations (2–6 mgL− 1) is also shown with the R2 values.

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Table 2 Kinetic parameters: Effect of manganese dioxide on the defluoridation potential of DEW. Kinetic models

DEW%MnO2

Pseudo I order Rate ( × 10− 2) qe R2 Pseudo II order Rate H qe R2 Elovich A B R2 Diffusion-based models Pore diffusion (kp) Rate (×10-2) R2 Intra-particle diffusion Rate R2

DEW0

DEW0.01

DEW0.015

DEW0.025

4.941 1.141 0.957

5.312 1.072 0.983

5.583 1.011 0.979

7.392 1.434 0.986

0.012 0.066 2.397 0.987

0.055 0.194 1.885 0.974

0.064 0.242 1.947 0.982

0.065 0.279 2.243 0.988

0.235 2.831 0.947

0.173 2.803 0.919

0.027 2.754 0.941

0.271 2.305 0.965

4.991 0.869

5.963 0.973

6.544 0.945

7.102 0.977

0.192 0.932

0.193 0.977

0.199 0.995

0.228 0.992

DEW—Disposed Earthenware.

3.3.2. Pseudo-second-order kinetic model Though there are four types of linear pseudo-second-order kinetic models [40], the adsorption kinetics based on the adsorption capacity of the solid phase [41] is described by the pseudo-second-order equation. The differential equation and its integrated form are given by Eqs. (6) and (7), respectively, 2

dq = dt = k2 ðqe –qÞ

ð6Þ

2

t = qt = 1 = k2 qe + ð1=qe Þt

ð7Þ

where qt = q2e kt/(1 + qekt), the amount of the fluoride on the surface of the sorbent at any time, t (mg g − 1), k2 is the pseudosecond-order rate constant (g mg − 1 min− 1), qe is the amount of the fluoride ion adsorbed at equilibrium (mg g − 1) and the initial

sorption rate, h = kq2e (mg g− 1 min− 1). The value of qe (1/slope), k (slope2/intercept) and h (1/intercept) of the pseudo-secondorder equation can be found out experimentally by plotting t/qt against t. The fitness of the pseudo-second-order model (Eq. (7)) for the effect of manganese dioxide on the defluoridation potential of DEW was analyzed. The plot of t versus t/qt gives a straight line with higher R2 values indicating the applicability of the pseudosecond-order kinetic model (Fig. 4). The increase in the pseudo II order rate constant from 0.012 mg g− 1 min− 1 to 0.065 mg g− 1 min− 1 led to the inference that the performance of DEW on the removal of the fluoride may be enhanced with the addition of MnO2. The applicability of this model is evident from the regression values (R2) which are greater than 0.97. The pseudo II order rate constant for the effect of the initial fluoride concentration on the fluoride sorption onto DEW0.025 was in the range of 0.063–0.325 mg g− 1 min− 1. In

Table 3 Kinetic parameters for the defluoridation potential of DEW0.025 at various fluoride concentrations. Kinetic models

Pseudo I order Rate ((× 10− 2) qe R2 Pseudo II order Rate H qe R2 Elovich A B R2 Diffusion-based models Pore diffusion (kp) Rate (×10− 2) R2 Intra-particle diffusion Rate R2

[F]o 2

3

4

5

6

0.121 1.662 0.913

0.108 1.023 0.957

0.057 1.464 0.967

0.165 1.164 0.908

0.153 1.353 0.981

0.063 0.340 2.318 0.966

0.182 1.086 2.446 0.999

0.161 1.619 3.175 0.998

0.325 4.361 3.691 0.999

0.248 2.831 4.374 0.999

0.522 2.542 0.818

1.395 3.897 0.966

2.154 4.244 0.928

2.9935 5.197 0.947

3.219 3.716 0.936

0.052 0.875

0.018 0.958

0.058 0.968

0.165 0.938

0.053 0.981

0.209 0.906

0.127 0.946

0.115 0.969

0.106 0.935

0.132 0.973

[F]o Fluoride concentration is given in mg L− 1.

V. Sivasankar et al. / Desalination 272 (2011) 179–186

DEW 0 DEW 0.015 DEW 0.020 DEW 0.025

0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 -2.2 -2.4 -2.6

A

DEW 0 DEW 0.010 DEW 0.015 DEW 0.025

2.0 1.8 1.6

qt (mgg-1)

ln (qe - q)

A

183

1.4 1.2 1.0 0.8 0.6 0.4 0.2

5

10

15

20

25

2

30

3

2 mg/l 3 mg/l 4 mg/l 5 mg/l 6 mg/l

0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0 -4.5

B

5

6

7

2 mg/l 4.5

3 mg/l

4.0

4 mg/l 5 mg/l

qt (mg/g)

ln (qe - q)

B

4

time (min-0.5)

time (min)

3.5

6 mg/l

3.0 2.5 2.0

5

10

15

20

25

30

35

40

45

1.5

time (min)

2

3

4

Fig. 3. Pseudo I order kinetic model. (A) Effect of manganese dioxide on to DEW. (B) Effect of initial fluoride concentration on to DEW0.025.

this case, the applicability of this model is remarkable for the initial fluoride concentration from 3 to 6 mg L− 1 where the R2 values are closer to unity.

5

6

7

time0.5(min0.5) Fig. 5. Intra-particle diffusion model. (A) Effect of manganese dioxide on DEW. (B) Effect of initial fluoride concentration on DEW0.025.

A A

DEW 0 DEW 0.010 DEW 0.015 DEW 0.025

30 25

DEW DEW DEW DEW

2.0 1.8 1.6 1.4

20

qt

t/qt(g.mg-1.min)

35

15

0 0.010 0.015 0.025

1.2 1.0 0.8

10

0.6 5

0.4 0

5

10

15

20

25

30

35

40

0.2

45

1.5

time (min)

B

20

B

14 12

3.0

3.5

4.0

2 mg/l

4.5

3 mg/l 4 mg/l

4.0

5 mg/l

3.5

qt

t/qt(g.mg-1.min)

16

2.5

ln t

2 mg/l 3 mg/l 4 mg/l 5 mg/l 6 mg/l

18

2.0

10 8

6 mg/l

3.0 2.5

6 2.0

4 2

1.5

0 0

5

10

15

20

25

30

35

40

45

time (min) Fig. 4. Pseudo II order kinetic model. (A) Effect of manganese dioxide on to DEW. (B) Effect of initial fluoride concentration on DEW0.025.

1.5

2.0

2.5

3.0

3.5

4.0

ln t Fig. 6. Elovich model. (A) Effect of MnO2 on to DEW. (B) Effect of initial fluoride concentration on DEW0.025.

184

V. Sivasankar et al. / Desalination 272 (2011) 179–186

Table 4 Langmuir isotherm parameters for fluoride adsorption onto DEW.

Table 6 Freundlich parameters for fluoride adsorption onto DEW.

Parameters

Effect of manganese dioxide on DEW #

Effect of initial fluoride concentration$

Parameters

Effect of manganese dioxide on DEW

Effect of initial fluoride concentration

Qo b RL R2

1.110 11.574 0.041 0.979

8.70 2.93 0.146 0.955

KF N 1/n R2

1.195 1.048 0.954 0.867

3.121 0.474 2.110 0.997

#

#

$

$

Data obtained for the effect of MnO2 (0–0.025%) on DEW at 298 K. Data obtained from the effect of initial fluoride concentration using DEW0.025 at 298 K.

Therefore, the fitness of this model ensures that the rate limiting step may be chemisorptions, relating the valence forces through the sharing or exchange of the electrons between the adsorbent and the sorbate. A similar phenomenon was already observed [25,42]. 3.3.3. Diffusion-based models For the solid–liquid sorption process, the solute transfer is usually characterized either the by particle diffusion or the pore diffusion control. Normally, a process is diffusion controlled if its rate is dependent upon the rate at which the components diffuse towards one another. A simple equation for the particle diffusion controlled sorption process [43,44] is given by Eq. (8) where kp is the particle rate constant (min− 1). The value of the particle rate constant is obtained by the slope of ln(1 – Ct / Ce) against t. lnð1−Ct=Ce Þ = −kp t

ð8Þ

The pore diffusion model used here refers to the theory proposed by Weber and Moris [45,46]. The intra-particle diffusion equation is Eq. (9) where ki is the intra-particle rate constant (mg g− 1 min− 0.5). 1=2

qt ¼ k i t

ð9Þ

+C

The slope of the plot of qt against t0.5 will give the value of the intra-particle rate constant and C (mg g− 1) is a constant that gives an idea about the thickness of the boundary layer, i.e., the larger the value of C, the greater is the boundary layer effect. On plotting qt versus t0.5, an initial curve followed by a straight line is obtained (Fig. 5), which indicates that two types of mechanisms are involved in the adsorption process. The initial curve represents the boundary layer effect while the linear part corresponds to intra-particle diffusion. The high R2 values indicate in both the cases, the possibility of the sorption process being controlled by both the particle and the pore diffusion models [47,48]. The results are in agreement with the previous studies [49–51]. 3.3.4. Elovich model One of the useful models for describing chemisorption is the Elovich equation [52] Eq. (10) where A and B are constants during a specific experiment. dqt = A expð−Bqt Þ

Data obtained for the effect of MnO2 (0–0.025%) on DEW at 298 K. Data obtained from the effect of initial fluoride concentration using DEW0.025 at 298 K.

‘A’ is the initial adsorption rate (mg g− 1 min− 1) and ‘B’ is the constant of desorption (g mg− 1) during any one experiment. To simplify the Elovich equation, Chien and Clayton [53] assume ABt NN t, and by applying the boundary conditions qt = 0 at t = 0 and qt = qt at t = t, then Eq. (10) becomes Eq. (11). qt = ð1 = BÞlnðABÞ + 1 = BðlntÞ

ð11Þ

According to Eq. (11), the plot of qt vs ln t gives a slope. The 1/B value is indicative of the number of sites that are available for adsorption [52]. This equation predicts the behavior sorption over the whole range of variable studied. This fact strongly supports its validity and suggests that the adsorption is rate-determined by a chemisorption step [54]. The constants A and B of the Elovich equation, represent the intercept and the slope of the linear curves drawn between the adsorbed fluoride anions against time (Fig. 6). A high positive correlation may be an indication of adequate fluoride adsorption from the medium and the B values approve the ability of the adsorbent to hold the fluoride ions through chemisorption. The applicability of this model can be inferred from the high R2 values which substantiate the fitness of this equation as observed in a previous study [55]. From the data of the above kinetic models, in order to predict the treatment efficiency, it may be suggested that the best choice is the pseudo-second-order model for the effect of the initial fluoride concentration and the pseudo-first-order for the effect of MnO2 on DEW. 3.4. Sorption isotherms 3.4.1. Langmuir model The sorption isotherms express the specific relation between the concentration of the sorbate and its degree of accumulation onto the sorbent surface at constant temperature. The Langmuir isotherm models the monolayer coverage of the sorption surface and assumes that the absorption takes place on a structurally homogeneous surface. The isotherm is given by Eq. (13), where o

qe = Q bCe = 1 + bCe ……………………… ::

ð12Þ

ð10Þ

Table 5 Comparison of F sorption of DEW0.025 with other sorbents. Name of the sorbent

pH

Qo

Reference

MnO2 coated activated alumina MnO2 coated activated alumina MnO2 coated tamarind fruit shell Alumina Acid treated waste mud MnO2 coated activated alumina Aluminium titanate Original waste mud Granular red mud DEW with 0.025% manganese dioxide

7.0 4.0 6.5 7.0 5.0 4.7 7.0 5.0 4.7 7.15

0.16 0.17 0.22 1.08 2.80 2.85 3.01 4.20 8.92 9.02

[25] [24] [23] [25] [62] [25] [63] [62] [64] Present Study

Fluoride adsorbed (%)

ClSO4-2 NO3-

70 65 60 55 50 45 40 35 30 25 20 15 10

HCO3-

100

200

300

400

500

Co-ions (mgL-1) Fig. 7. Interference of co-ions in the fluoride removal process with DEW0.025.

V. Sivasankar et al. / Desalination 272 (2011) 179–186

100

Fdesorbed(%)

80 60 40 20 0 0.25

0.50

0.75

1.00

1.25

1.50

1.75

[NaOH] Fig. 8. Influence of NaOH concentration on fluoride desorption from spent DEW0.025.

o

o

Ce=qe = 1=Q b + Ce=Q ………………… :::

ð13Þ

The sorption capacity (Qo) is the amount of the sorbate at complete monolayer coverage (mg g− 1) and B (L mg− 1) is the Langmuir isotherm constant that relates to the energy of adsorption. The values of Langmuir parameters Qo and b were calculated from the slope and the intercept of the linear plots of Ce/qe versus Ce. The applicability of the Langmuir isotherm is evidenced from the computed R2 values (Table 4). A list of the sorbents with Qo values in comparison to our present study is given in Table 5. In order to find out the feasibility of the isotherm, the essential characteristics of the Langmuir isotherm can be expressed by Eq. (14) in terms of a dimensionless constant separation factor or equilibrium parameter, RL [56]. In this equation, B is the Langmuir isotherm constant and Co is the initial concentration of fluoride (mg L− 1). RL = 1 = ð1 + BCo Þ

ð14Þ

185

However, the fluoride uptake by DEW with 0.025% MnO2 was drastically reduced to 12.5% on the increase in SO2− ion concentra4 tion from 100 to 500 mg L− 1. The increase in the concentrations of − −1 HCO− and NO− decreases the fluoride 3 , Cl 3 from 100 to 500 mg L uptake of the sorbent to 47.3%, 34.7% and 47.3%, respectively. The decrease in the fluoride uptake (Fig. 7) may be due to the competition between fluoride and the other ions, for the active sites on the surface of the adsorbent. The activity of the competing anions towards sorption − − − against fluoride, is in the following order, SO2− 4 N HCO3 ~ Cl N NO3 . The reduction in the fluoride removal in the presence of the sulfate ion may be attributed to the facts that the sulfate ion may have competition with the fluoride ion for the same sorption sites, since the former forms a partial inner-sphere complex forming species [60] and the presence of the sulfate ion, being divalent, may have increased the coulombic repulsive forces leading to the reduced probability of fluoride interaction with the active sites. It is also reported that the multi-charge anions get adsorbed more easily than the monovalent anions [61]. 3.6. Recycling of spent sorbent Any sorbent is economically viable, if it can be regenerated and reused in many cycles of operation. One objective of the present work was to develop an adsorbent that can be reused, thereby making it cost effective. To study the reusability of the DEW0.025 adsorbent, the adsorption studies were conducted. After the complete saturation of the adsorbent, the fluoride loaded adsorbent was subjected for regeneration using NaOH solutions of concentrations ranging from 0.5 to 1.5 M. Fig. 8 shows the percentage of desorption of the fluoride obtained at various concentrations of the hydroxide solution. In 1.5 M NaOH, DEW with 0.025% manganese dioxide desorbed almost 95% of the adsorbed fluoride. 3.7. Elucidation of fluoride removal mechanisms

The RL values of 0.146 and 0.041 (Table 4) indicate that the adsorption was favorable for the present fluoride removal process and it tends to be more favorable towards a lower concentration of fluoride as witnessed from the increased RL value. The trend observed in our study is in agreement with results of Amin [51]. 3.4.2. Freundlich model The Freundlich [57] model is an empirical equation based on adsorption on a heterogeneous surface. It is given by Eq. (15). 1=n

qe = KF Ce

ð15Þ

qe is the amount of fluoride adsorbed per unit weight of the sorbent at equilibrium (mg g− 1), Ce is the equilibrium concentration of fluoride in solution (mg L− 1), KF is a measure of the adsorption capacity (mg1−(1/n)L 1/n g− 1) and 1/n is the adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero [58]. A value of 1/n below 1 indicates a normal Freundlich isotherm while 1/n above 1 is indicative of cooperative adsorption [59]. The Freundlich isotherm constants KF and n were calculated from the slope and the intercept of the plot of log qe versus log Ce and were presented in Table 6. The values of 1/n lie between 0.1 and 1.0 and the n values lying in the range of 1–10 confirm favorable conditions for adsorption [43]. 3.5. Effect of competing anions − − The individual effects of co-existing ions, including SO2− 4 , HCO3 , Cl − and NO3 , usually present in groundwater samples, have been investigated in the batch adsorption study. The initial concentration of 2.0 mg L− 1 was maintained at a pH and temperature of 7.15± 0.03 and 298 ± 1 K respectively. The results indicated that the sorption of fluoride was not adversely affected due to the presence of other ions.

Since the sorption of the fluoride onto DEW is relatively dependent on the pH, the mechanism of removal can be ascribed to outer-sphere complex formations. The desorption characteristics of DEW suggest the formation of outer-sphere complexes [65]. To quantify the process experimentally, and to understand the changes in the sorbent (if any) due to fluoride sorption, FTIR analysis was done before and after the sorption. The FTIR spectrum of the DEW with 0.025% manganese dioxide presents no spectroscopic change due to fluoride sorption. The role of manganese dioxide in the improvement of the efficiency of DEW in the fluoride removal may be attributed to the formation of mixed oxides at the interface between MnO2 particles and the DEW surface. Another hypothesis may be a modification due to MnO2 on the surface of the sheet structured DEW such that the negatively charged layers get altered with positive charges which caters to the scavenging of the fluoride ions from the fluoride solution. 4. Conclusion The present work showed that even a low addition of manganese dioxide to DEW increased the defluoridation potential of this material. The fluoride removal of DEW with 0.025% MnO2 was effective at a pH of 7.15. The regeneration of the spent sorbent was found to be efficient by washing it with 1.5 M NaOH which reveals the reversibility of the reaction used during the fluoride removal. The kinetic and isotherm data were found to fit well both with respect to the manganese dioxide addition and the initial fluoride concentration. A healthy competition between sulfate and fluoride ions was observed. The desorption characteristics of DEW suggest that the fluoride sorption is caused by outer-sphere complex formations. The fluoride removal process using DEW may be ascertained through an outer-sphere complex formation by strong bonds (adsorption). The mechanism of fluoride improvement in the presence of MnO2 is not fully understood and it will be investigated later.

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