Bioadsorbents for defluoridation

Biosorptiou is a promising method for treatment of water having abundantly available biomaterials. Various bioadsorbents have been developed for fluoride removal, and among those chitin and chitosan-derivatives have gained wide attention as effective bioadsorbents due to their low cost and high contents of amino and hydroxyl functional groups, which show significant adsorption potential for the removal of various aquatic pollutants. Algal biomass pretreated with Ca²⁺ was also evaluated for the biosorptiou of fluoride from polluted waters. The phenomenon like fluoride sorption on fungal bio-adsorbent was called as chemical type of interaction. Collagen fiber, which is profuse natural biomass with abundant functional groups, reacts with various metal ions and can be utilized as a earner for metal ions (Jagtap et al. 2011). There are several materials that could be used as bioabsorbants such as:

6.1.1.1 Agricultural wastes as sorbents

Agricuhural waste materials are cost-effective and ecological significant due to their unique chemical composition, availability in large quantity and biodegradable nature. Low-cost adsorbents from different agricultural waste materials such as coconut shell, coconut shell fibers, and rice husk were developed and employed for the removal of various pollutants in industrial wastewater including fluoride (Alagumuthu and Rajan 2007).

6.1.1.2 Industrial waste as sorbents

Widespread industrial activities generate huge amount of solid waste materials as by-products and these byproducts such as fly ash, carbon slurry, original waste mud, red mud, acid-activated and precipitated waste mud, solid waste from edible oil processing industry, sludge and spent bleaching earth can be used as defluoridating agents with some treatment (Kemer et al. 2009).

6.1.1.3 Nauosorbents

In the past decade, nanotechnology has come out as a promising technology in various fields and use of nanoparticles as sorbents for water treatment is also gaming wide attention in recent years. Aligned carbon nanotubes, prepared by catalytic decomposition of xylene using ferrocene as catalyst, proved then good performance for fluoride removal from water. Both the surface and inner cavities of aligned carbon nauotubes were found to be readily accessible for fluoride sorption (Sarkar et al. 2007). A variety of nano-sized inorganic oxides prepared through thermolysis of a polymeric-based aqueous precursor were capable of giving the solution of the desired inorganic ions (Sarkar et al. 2007). Similarly, various efforts in this field have been made to take the advantage of nanoparticles (Ayoob and Gupta 2009).

6.1.1.4 Low-cost adsorbents for fluoride removal

Different low-cost adsorbent materials are available for effective removal of fluoride from water. The naturally available adsorbents, chalk powder, pineapple powder, orange peel powder, multani mitti (special type of clay soil), activated neem and babul leaves, rice husk, etc. are some of the different materials investigated as adsorptive agents for the removal of fluoride from water.

• a) Chalk Powder: Chalk powder, due to certain porosity, adsorbs fluoride from aqueous solution. The adsorption of fluoride by chalk powder is 86%. The main component of chalk is calcium carbonate (CaC0₃), a form of limestone.
• b) Pineapple peel powder: Pineapple (Ananascosmosus) is a tropical fruit which grows in countries which are situated in the tropical and subtropical regions. Hence, it is an agricultural waste which is cheap for modem communities and easily available. The fluoride adsorption efficiency of pineapple peel powder is 86%.
• c) Orange peel powder: Orange peel is rich in flavonoids with some trace elements, ascorbic acid, carotenoids, dietaiy fibers, and polyphenols, and the antiradical efficiency was assessed in the dried peels. Due to certain porosity, orange peel powder adsorbs fluoride from aqueous solution.
• d) Multani mitti: Multani mitti or fuller’s earth is clay material that is popularly used as a skin care ingredient. It is rich in magnesium chloride and has great surface area with excellent bonding and sealing properties. It contains fine sand particle with complex multicenter crystalline structures of oxides and hydroxide of aluminum, zinc, magnesium and silicon. It has the tendency to filter, decolorize, and clarifying properties without any chemical treatment. The efficient removal of fluoride can be achieved to 56% using multani mitti.
• e) Babul (Acacia nilotica) leaves: Babul is well known for its amazing use in treating gum problems and its leaf. bark, seeds and gum are used for medicinal purposes in India. Leaves bark, and flowers contain tannin and other polyphenolic compounds while fruits contain gallic acid. Babul tree has many medicinal properties with antiviral and antifungal characteristics. It also has antidianheal. antioxidant, antibacterial, antimalarial, anthelmintic and anti-inflammatory properties. Babul leaves are cheap adsorbents for fluoride removal with the removal efficiency of 59.2%.
• f) Rice husk: Rice husk is the by-product of rice milling. This husk contains about 75% organic volatile matter and remaining 25% of the weight of this husk is converted into ash during the firing process (gasification) known as rice husk ash (RHA). RHA is rich source of floristic fiber, protein, and some functional groups such ascarboxyl, hydroxyl and amidogeu, which makes adsorption processes possible. There is marginal variation in fluoride removal by rice husk over pH range of 2 to 10 from 83% to 84% removal efficiency.
• g) Neem (Azadiractaindica) leaves: Neem leaves can be used by rural communities as it is the most common and easily available tree. With the help of neem leaves, fluoride can be removed up to 58%.