(397k) Ultrasound Assisted Synthesis of Zirconium Impregnated Activated Carbon Nanocomposite and Its Effective Use for Defluorination of Water

Abstract

             The scarcity for potable water over
a wide region of Rajasthan, Gujarat and Andhra Pradesh in India and some other
countries like China, United States and Mexico is quite prominent owing to excessive
fluoride contamination in the ground water and surface water of those regions
[1]. Fluoride is a necessary element of human body helping to construct healthy
teeth and calcification of the dental enamel among infants, inhibiting dental
caries when present within a concentration range of 0.5-1 mg/L. But long exposure and intake of water with high
fluoride concentration leads to physiological, chronic and toxic effects. The
permissible limit of fluoride in drinking water ranges from 0.5–1.5 mg/L as per
the norms of World Health Organization (WHO) [2]. The well known water
defluorination techniques include precipitation, membrane separations, and
adsorption processes. Precipitation is the most frequently used technique for
fluoride decontamination, but it has several drawbacks such as formation of
fluoride precipitates as well as waste disposal issues.
Comparatively, membrane separation is a smarter and cleaner way of
fluoride decontamination by the use of ion exchange, reverse osmosis or
electrodialysis. On the other hand, adsorption is a method that has been under
the limelight for several decades as one of the most popular and economically
sound technologies having dynamic applications in pollution prevention and
water decontaminations. In this regard, research attention has been given to both
utilizing bioresources as the raw materials for synthesizing novel adsorbents
very inexpensively. Furthermore research effort is underway to improve the
efficiency of the commercially available adsorbents by physical or chemical functionalization.
Present study reports the feasibility of fluoride (F^-) adsorption by
use of zirconium (Zr(VI))  impregnated
powdered activated carbon (PAC) nanocomposite synthesized through the
assistance of ultrasound. Because of high electronegativity, fluoride has a
strong affinity for rare earth metals such as zirconium, lanthanum and
impregnating the zirconium metal ions on the activated carbon multiplies the
fluoride adsorption efficiency of activated carbon by 3 to 5 times [3]. During
synthesis, ultrasound was found to have significant impact on reducing the
particle size with high specific surface area as well as substantial reduction
in energy consumption. The nanocomposite was characterized by using scanning
electron microscopy (SEM), energy dispersive X-ray (EDX), X-ray diffraction
(XRD), N₂ adsorption
analysis (BET) and Fourier Transform Infrared Spectroscopy (FTIR) to study
the physical and chemical properties. The pH_pzc (Point of zero
charge) of the adsorbent was found to be 5.03; hence all experiments were
carried out at an optimum pH of 4 for adsorption of strong electronegative
fluoride ions with an optimum adsorbent dosage of 2 g/L. The initial fluoride
concentration was varied from 2.5 to 20 mg/L and the adsorption capacity was
obtained upto 5 mg/g with equilibrium reaching within 3 hours. The adsorption
kinetics was found to follow pseudo-second order kinetics and the isotherm to
follow Langmuir isotherm model. The
thermodynamic study showed the endothermic and spontaneous nature of the
process within the studied temperature range. The effects of various
co-existing ions (HCO₃^-, NO₃^-, SO₄^2-,
Cl^-) on the process performance were also studied and found to have no
significant effects on fluoride adsorption capacity. The adsorbent maintained high adsorption efficiency after
the regeneration studies carried out for five adsorption-desorption cycles
which signifies its potential to be a good defluorinating agent.

Keywords: Ultrasonication; Fluoride; Nanocomposite

Figure 1: Effect of solution pH on fluoride removal (F⁻: 10 mg/L)

Figure 2: Effect of adsorbent dosage on fluoride adsorption (F⁻: 10 mg/L)

Figure 3:
Effect of coexisting ions on fluoride adsorption

(F⁻: 5 mg/L, HCO₃^-, NO₃⁻, SO₄²⁻,
Cl^- :10 mg/L; pH 4)

Figure 4: Fluoride removal after regeneration of the adsorbent (F⁻: 10 mg/L)

Reference:

[1]  
P. Miretzky,
A.F. Cirelli, Fluoride removal from water by chitosan derivatives and composites:
a review, Journal of Fluorine Chemistry, 132(4) (2011) 231–240.

[2]  
X. Yu, S. Tong, M. Ge,
J. Zuo, Removal of fluoride from drinking water by cellulose@ hydroxyapatite
nanocomposites, Carbohydrate
polymers, 92(1)
(2013) 269-275.

[3]  
R.S. Sathish, N.S.R.
Raju, G.S. Raju, G. Nageswara Rao, K.A. Kumar, C. Janardhana, Equilibrium and
kinetic studies for fluoride adsorption from water on zirconium impregnated
coconut shell carbon, Separation
Science and Technology, 42(4)
(2007) 769-788.