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