Different adsorbents used in defluoridation of water

Adsorbent Preparation And Characterization

2.1 Different adsorbents used in defluoridation of water

In this study, four different adsorbents were chosen and used as defluoridating adsorbent. Pyrophyllite, acidic alumina, schwertmannite and nanomagnetite aggregated schwertmannite were chosen as adsorbents for treating fluoride contaminating water. A brief description of each adsorbent and its characterization was discussed below.

Pyrophyllite

Pyrophyllite is basically a non-swelling hydrous aluminum silicate with the chemical formula ofAlSi₂O₅OH . A schematic diagram of the structure of pyrophyllite is given in Figure 2.1 [1]. It belongs to the family of silicate minerals that are composed of three infinite layers formed by the sharing of oxygen ions at three corners of the silica tetrahedra. A layer of octahedrally coordinated AlOHions holds the two layers of tetrahedrally coordinated SiO ions together as a three - layer sheet [2]. Since this three-layer unit is electrically balanced as neutral on the basal plane due to the very small ionic substitution, the crystal is held together by Van der Waals forces, which are

comparatively weak with respect to the primary covalent or ionic forces that hold the atoms in the unit layers together [3]. Consequently, easy cleavage takes place along the plane of the layers.

Figure 2.1: Structure of pyrophyllite molecule [1]

Physically pyrophyllite is a white, micaceous and feels greasy compound.

Generally it is used as a raw material in ceramics and refractory industries due to its high refractive behaviour, low thermal and electrical conductivity, low expansion coefficient, low hot-load deformation, low reversible thermal expansion and low bulk density, excellent reheating stability and high resistance to corrosion by molten metals and basic slags [4]. In this study, pyrophyllite used was purchased from National chemicals, India and used directly as an adsorbent for fluoride removal.

Van der Waals bonds

Silicon

Oxygen

Hydroxyl Aluminum

Acidic alumina

In this study, acidic alumina was purchased from Merck, India and used directly as adsorbent for defluoridation of water. Generally alumina (aluminium oxide) is an amphoteric oxide with chemical formula Al₂O₃. In the present study, alumina used was acidic in nature. When neutral alumina is treated with acid e.g. HCl, acidic alumina is formed according to the following reaction [5]:

Alumina. ^HOH +HCl → Alumina. ^HCl+ HOH Reaction 1 In absence of hydroxyl ion (i.e., slightly acidic feed solution), if acidic alumina is in contact with fluoride (F^) ion, it displaces chloride (Cl^) ion as in reaction 2.

Alumina. HCl + NaF→ Alumina. ^HF + NaCl Reaction 2 To regenerate the fluoride containing adsorbent, a dilute solution of the most preferred hydroxide ion, in terms of NaOH is used. The regeneration occurs according to reaction 3.

Alumina. HF + 2NaOH → Alumina. ^NaOH + NaF + H₂O Reaction 3 Since alumina acts as both cation and anion exchanger, Na^ions are exchanged for hydrogen when Na^ion is in excess in the regenerating solution and H^is absent (high pH). To restore the fluoride removal capacity, the basic alumina is in contacted with an excess of dilute HCl(reaction 4).

Alumina. NaOH + 2HCl→ Alumina. ^HCl+NaCl + HOH Reaction 4 The acidic alumina (alumina. HCl), is now ready for another fluoride adsorption cycle.

Based on this mechanism, acidic alumina is considered as a better adsorbent for fluoride removal.

Schwertmannite

Schwertmannite (Sh) which is an iron oxyhydroxysulfate was selected as defluoridation adsorbent. It exists naturally as geo-material and can also be synthesized in laboratory. Synthetic Sh has a probable formula of^Fe₈^O₈

 

^OH ₆^SO₄. Presence of OH^ group in Sh considers it as a fluoride adsorbing adsorbent.

Preparation of Sh

Sh was prepared by wet chemical process where urea was used as a neutralizing agent [6]. The urea solution was prepared by dissolving 150 g of urea into 500 mL of distilled water, and Fe₂



SO₄



₃solution was made by dissolving about 25 g of



SO



H O

Fe_{2 4 3}.5 ₂ in 500 mL of distilled water. The Fe2



SO4



₃solution was preheated at 70^oC in a beaker and urea solution was added drop wise for about 2–4 hrs. Precipitates were formed and filtered with Whatmann filter paper. The residue obtained was dried at temperature less than 40 ^oC and the dried powder was considered as adsorbent for fluoride removal.

Nanomagnetite aggregated schwertmannite (NMSh) Preparation of NMSh

The procedure for the formation of nanomagnetite aggregated schwertmannite consists of two steps. Step 1 includes the formation of nanomagnetite particles by chemical precipitation method and step 2 covers the production of schwertmannite using wet chemical process where the nanomagnetite particles are embedded on it. Magnetite nanoparticles were prepared by co-precipitation from a mixture of FeCl₂ and FeCl₃ (1:

2 molar ratio) throughout the slow addition of diluted NH solution. Schwertmannite,

which is an iron oxide hydroxide adsorbent, was generated by homogenous hydrolysis ofFe₂



SO₄



₃.nH₂O using urea as a neutral agent. After dissolving about 12.5 g ofFe₂



SO₄



₃.5H₂O in 250 mL of distilled water, the solution was preheated at about 70°C and slowly stirred for 10 min. 5 mL of the slurry containing the fresh nanomagnetite particles was added to the solution and 250 mL of 5M urea solution was added drop-wise for about 2–4 hrs into the solution. When precipitating of crystals begun, the color of the solution was changed from red to brownish yellow and the reaction was continued until half of the initial solution was vaporized. After solid–liquid separation, solid part was rinsed for removal of impurities. Then the yielded material was dried below 40°C. The dried material is now ready for fluoride adsorption.