defluoridation of water using phosphoric acid modified activated carbon obtained from...

8/3/2019 Defluoridation of Water Using Phosphoric Acid Modified Activated Carbon Obtained From Sugarcane-thrash

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Defluoridation of water using phosphoric acid modified activated carbon

obtained from sugarcane-thrash

Document by: Bharadwaj

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ABSTRACT

Carbon obtained from sugarcane thrash was chemically activated using different concentrations of phosphoric acid and tested for their capability for the adsorption of fluoride ions from drinking water. Physicochemical characterization of the raw and the activated carbons were performed using XRD,

microanalysis and surface area analyzer. The results revealed that the adsorption capacity of carbonincreased considerably on treatment with phosphoric acid. A comparison of the surface area of thecarbons did not reveal significant change due to activation. Adsorption isotherm and kinetic studies wereused to explain the fluoride adsorption characteristics of the adsorbents.

Keywords: Activated carbon, adsorption, fluoride, isotherm, kinetics

INTRODUCTION

Fluorine in the range of 0.5 - 1.5 mg/l is a micronutrient that contributes to calcification of dental enamel

and bone formation in human beings. It exists in water as fluoride ion (F - species). Higher concentration

of fluoride in drinking water can lead to fluorosis. The WHO recommended maximum contaminant level

(MCL) for fluoride in drinking water is 1.5 mg/l [1]. Fluoridation of drinking water is a world-wide

problem, occurring primarily due to natural weathering of the fluoride minerals present in the earth’scrust [2] and the discharge of fluoride containing wastewater coming from various industries. Increased

incidents of fluorosis among the people are being reported from all over the world [3].

The existing fluoride removal techniques include precipitation-coagulation, membrane filtration and ion

exchange or adsorption based processes. Among these, adsorption is considered to be the most efficient

and applicable technology for removal of fluoride from drinking water as the fluoride concentration in

ground water is very low (5 – 40 mg/l). A wide variety of adsorbents (e.g. activated and impregnated

alumina [4], natural and synthetic clays [5, 6], carbonaceous materials [7], bio-polymeric adsorbents [8],

industrial wastes like red mud and fly ash [9]) have been tested for defluoridation of water. The

carbonaceous materials obtained from agricultural wastes have attracted considerable attention to the

researchers, from economic and environmental point of view. Adsorbents like, wood charcoal, carbons

obtained from rice straw [10], coconut shell [11], have been explored for their fluoride removal

capability.

The present study reports adsorption of fluoride in water on activated carbon made from sugarcane thrash.

The carbon obtained from sugarcane thrash was chemically activated, characterized using different

techniques and tested for their fluoride adsorption capacity from drinking water. Adsorption isotherm and

kinetic studies were used to explain the fluoride adsorption characteristics of the adsorbent.

EXPERIMENTAL

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Reagents

Ortho-Phosphoric acid (H3PO4, 85 % pure, Merck) was used for chemical activation of the

carbon. Fluoride solutions for adsorption study were prepared from AR Grade Sodium Fluoride

(Merck). Distilled water was used for preparing the standard solutions.

Adsorbent activation

A fixed weight (10 g) of the pristine carbon obtained from sugarcane thrash was added to 100 ml of 0.25

N ortho-phosphoric acid, stirred well and kept overnight. The solid carbon was then separated by

filtration followed by washing with distilled water till the washed liquid was neutral to litmus. This

treated carbon was then dried overnight at 110 °C in an air oven to obtain the C-25AC. Two other

adsorbent samples, C-50AC and C-75AC were prepared by following a similar procedure using 0.50 N

and 0.75 N ortho-phosphoric acid, respectively. The pristine carbon obtained from sugarcane thrash is

denoted as C-NAC.

Characterization techniques

The chemically activated carbon adsorbents were characterized for their chemical composition,

crystallinity and BET surface area using different characterization techniques. Concentrations of N, H and

S present in the adsorbent samples were analyzed using C, H, N, S, analyzer (CARLO-ERBA, Italy). The

mineralogical phases and crystallinity of the raw- and activated- carbons were studied by X-ray

diffraction (XPERT-PRO from PANalytical Instruments, Netherlands) using Cu-K α radiation. BET

surface area was determined by N2 adsorption-desorption technique using Autosorb-1 from

Quantachrome Instruments, USA.

Batch adsorption experiments

Batch adsorption experiments were carried out under isothermal conditions at 25 °C in a thermostaticshaker (Julabo SW-21C). 50 cm3 of fluoride solution of known concentration (5-25 mg/l) was contacted

with 0.2 g of the adsorbent in polypropylene (PP) bottles for 24 h in thermostatic shaker. Residualfluoride concentration was analyzed at specific time intervals using fluoride ion selective electrode (ISE,

Metrohm 781 pH/Ion Meter) in presence of total ionic strength adjustment buffer (TISAB). The

adsorption capacity of the clay was estimated using the formula:

1000

)()/( 0

×

×−=

w

V C C g mg q e

e ……………………………………….(1)

where, C0 : initial fluoride concentration (mg/l); Ce : equilibrium fluoride concentration (mg/l); V :volume of solution (ml); w : weight of adsorbent (g); qe : amount of fluoride adsorbed per unit gram of the

adsorbent at equilibrium (mg/g).

RESULTS AND DISCUSSIONSCharacterization of the adsorbent

Concentrations of N, H and S present in the adsorbent samples and their specific surface area are

presented in Table 1.

Table 1. Physico-chemical characteristics of the pristine and activated carbons.

Adsorbent N (%) H(%) S(%) Specific surface

area (m2/g)

2



meme V C bV q

111+= ……………………………………..(2)

Here, b and Vm are Langmuir isotherm constants representing adsorption bond energy and monolayer

adsorption capacity (mg/g) respectively; Ce and qe are the equilibrium concentration and equilibrium

adsorption capacity.

Freundlich isotherm constant may be represented as

e F e C n

K q ln1

lnln += ……………………………..(3)

where, n and K F are Freundlich isotherm constants representing adsorption intensity and adsorption

capacity respectively.

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

Eq. Conc. (mg/l)

E q .

A d s .

C a p .

( m g / g )

C-NAC

C-25AC

C-50AC

C-75AC

Figure 2. Fluoride adsorption isotherm of the pristine and activated carbons at 25 °C (Vol. of solution 50

ml; contact time 24 h; adsorbent dose 0.2 g).

The values of isotherm constants calculated from the slopes and intercepts of the plots of Langmuir and

Freundlich isotherm equations are presented in Table 2. Although the values of correlation coefficients

for both the isotherm models represent good fittings of the adsorption data, the Langmuir model explains

the fluoride adsorption behavior of the pristine and activated carbons more precisely than the Freundlich

model. The adsorption bond energy (b from Langmuir model) clearly shows that the adsorption intensity

decreases with increase in phosphoric acid concentration beyond 0.5 N, during chemical activation of the

adsorbent.

Table 2. Langmuir and Freundlich isotherm constants for pristine and activated carbons

Adsorbents Langmuir constants Freundlich constants b Vm R² n K F R²

C-NAC 0.03 2.99 0.76 1.33 0.12 0.81

C-25AC 0.53 1.61 0.94 3.37 0.67 0.99

C-50AC 0.53 1.94 0.97 3.15 0.78 0.99

C-75AC 0.18 2.06 0.85 1.65 0.34 0.89

Adsorption Kinetics

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The fluoride adsorption kinetics of the pristine and activated carbons was studied up to 24 h. The rate of

adsorption for all the activated carbons (except the untreated adsorbent, C-NAC) was slow. The

equilibrium was attained within 24 h. The Lagergren first-order kinetic model was used to study the

fluoride adsorption kinetics of the pristine and activated carbons.

Lagergren first-order kinetic model is given below:

t K qqq et e .)ln()ln( 1−=− ………………………..……(4)

Here, K 1 represents the first-order rate constant; q t represents the adsorption capacity at time t and qe is the

equilibrium adsorption capacity obtained after 24 h of adsorption. The model was found to give a good fit

to the kinetic data. The values of K 1 and the corresponding equilibrium adsorption capacities (qe(cal))

calculated using the kinetic model are presented in Table 3 along with their experimental adsorption

capacities at different initial fluoride concentrations. The rate constants for all the activated carbons are

between 0.12 x10-2 to 0.45 x 10-2.

Table 3. First-order kinetic parameters for the pristine and activated carbons.

Adsorbent Initial F-

Conc.(mg/l)

qe(exp)

mg/gK 1 x 10

2

min-1qe(cal)

mg/gR

2

C-NAC 4.38 0.18 1.39 0.11 0.87

10 0.32 0.59 0.15 0.61

20.4 0.90 0.36 0.71 0.87

C-25AC 5.12 0.82 0.26 0.80 0.99

10 1.09 0.20 0.95 0.87

22.7 2.43 0.12 2.20 0.83

C-50AC 5.14 0.89 0.25 0.83 0.97

10 1.32 0.30 1.25 0.98

22.6 2.80 0.12 2.67 0.95

C-75AC 4.45 0.58 0.43 0.55 0.98

10 0.83 0.45 1.44 0.9122.7 2.25 0.21 2.11 0.95

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25

Initial Conc. (mg/l)

E q . a d s . c a p .

( m g / g )

C-NAC

C-25AC

C-50AC

C-75AC

5



Figure 3. Equilibrium adsorption capacity of pristine and activated carbons as a function of initial

fluoride concentration (Vol. of solution 50 ml; contact time 24 h; adsorbent dose 0.2 g).

Plots of equilibrium adsorption capacity of pristine and treated carbons presented in Fig. 3 shows that theequilibrium capacity increases with increase in initial concentration. This increase is higher in the case of

activated carbons. The increase in the equilibrium capacity is the highest for carbon activated with 0.5 NH3PO4. Activation with H3PO4 of higher concentration has a negative effect on the equilibrium adsorption

capacity of the carbon indicating that there is an optimum concentration of the acid that gives the highest

performance for the activated carbon.

CONCLUSION

Carbon obtained from sugarcane thrash can be used for defluoridation of drinking water after chemical

activation with phosphoric acid. XRD study of the pristine and active materials showed the phase of

carbon present was graphite. A comparison of the surface area of the carbons did not reveal significant

change due to activation. This indicates that the increase in adsorption capacity is mainly because of the

generation of new adsorption sites due to the acid treatment. Among the four carbon adsorbents, the

activated carbon adsorbent obtained by treatment with 0.5 N H3PO4 showed maximum fluoride adsorption

capacity. Adsorption data were explained using Langmuir and Freundlich isotherm models. The rate of fluoride adsorption was very slow and the adsorption kinetic followed a pseudo first-order kinetic model.

ACKNOWLEDGEMENT

Sujata Mandal wishes to thank Council of Scientific and Industrial Research (CSIR), New Delhi, for

financial support.

REFERENCES

[1] WHO (1984) Guidelines for drinking water quality, World Health Organization, Geneva, Vol. 2,

P. 249.

[2] CEPA, Canadian Environmental Protection Act. (1994) Priority substance list supportingdocument for inorganic fluorides prepared by eco-health branch and environment, Canada,

Ottawa (Ontario).

[3] Mella S., Mohira X. and Atalah E. (1994) Prevalence of endemic dental fluorosis and its relationwith fluoride content of public drinking water. Revista Medica Chile 122, 1263-1270.

[4] Maliyekkal, S.M., Shukla, S., Philip, L., Nambi, I. M., 2008. Enhanced fluoride removal from

drinking water by magnesia-amended activated alumina granules, Chemical Eng. J. 140, 183-192.

[5] Meenakshi, S., Sundaram, C.S., Sukumar, R.,2008. Enhanced fluoride sorption by

mechanochemically activated kaolinites, J. Hazard. Mater. 153, 164-172.

[6] Mandal, S., Mayadevi, S., 2008. Adsorption of fluoride ions by Zn-Al layered double

hydroxides, Appl. Clay Sci. 40, 54-62.

[7] Mohan, D., Singh, K.P., Singh, V.K., 2008. Wastewater treatment using low cost activated

carbons derived from agricultural byproducts—a case study, J. Hazard. Mater. 152, 1045-1053.

[8] Ma, W., Ya, F-Q., Han, M., Wang, R., 2007. Characteristics of equilibrium, kinetics studies for

adsorption of fluoride on magnetic-chitosan particle, J. Hazard. Mater. 143, 296-302.

[9] Cengeloglu, Y., Kir, E., Ersoz, M., 2002. Removal of fluoride from aqueous solution by using red

mud, Sep. Purif. Technol. 28, 81-86.

[10] Daifullah, A.A.M., Yakout, S.M., Elreefy, S.A., 2007. Adsorption of fluoride in aqueoussolutions using KMnO4-modified activated carbon derived from steam pyrolysis of rice straw. J.

Hazard. Mater. 147, 633–643.

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[11] Sai Sathish, R., Raju, N. S. R., Raju, G. S., Nageswara Rao, G., Anil Kumar, K., Janardhana, C.,

2007. Equilibrium and kinetic studies for fluoride adsorption from water on zirconium

impregnated coconut shell carbon. Sep. Sci. Technol. 42, 769–788.

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