Published on Nov 30, 2023
Fluoride, a naturally occurring inorganic element found in rock sediments, minerals and geological deposits, is rightly called as “double-edged sword” because inadequate ingestion is associated with dental caries, whereas excessive intake leads to dental, skeletal and soft tissue fluorosis. In fact, the discovery of the connection between excessive content of fluoride in water and endemic mottling of the enamel was one of the main starting points of research related to fluoride. The latest information shows that fluorosis is endemic in at least 25 countries across the globe. In 1993, 15 Indian states were identified as endemic for fluorosis by UNICEF. The fluorosis is irreversible, so providing the drinking water with optimum quantity of fluoride is the only way to prevent fluorosis. As per Indian standards, the permissible limit for fluoride concentration in drinking water is 1 mg/L to 1.5 mg/L. This stringent standard for fluoride urges for the development of more efficient methods to make water fit for drinking.
Currently, available methods for the removal of fluoride from water are broadly classified into three categories based on the fluoride removal mechanism like precipitation, adsorption and membrane based methods. The chemical and membrane based methods for defluoridation are costly and have the problems related to large sludge disposal and hence not economical.
Keywords: - Fluoride, fluorosis, adsorption, nanotechnology, activated sugarcane bagasse.
Adsorption is widely recognized as appropriate technique because of its simplicity and the availability of wide range of adsorbents for defluoridation. Researches throughout the world have tried various materials like activated alumina, bonechar, aluminium hydroxide, clay based composites etc. However, use of these materials is limited due to their low efficiency and high cost. Therefore, it is of paramount importance to identify and study the materials with high removal efficiency. Surface area is a key factor that influences the adsorption in larger extent. Higher the surface area of adsorbent, higher will be its adsorbing capacity. This makes nanosorbents suitable for the treatment of water.
Nanoscale materials due to their smaller size (<100 nm) can provide larger surface area required for efficient adsorption. Nanomaterials, when solely used for water treatment may be highly efficient but may not prove to be cost effective when used in large scale and the separation of nanomaterials from water may also be difficult. Researchers have been attempting to find some efficient ways to minimize cost. This can be done by combining the nanoparticles with low cost adsorbents. There is a need for technological innovation and research to develop some sustainable treatment methods. Currently, the world is focussing on tracing the technological advances to understand the physiochemical properties of nanomaterials which can be further used for sustainable water management.
The present work aims at exploring the possibility of utilizing the nanoparticles of Fe and Al oxides with activated sugarcane bagasse.In the present study “Adsorption of fluoride on the nanoparticles of Fe and Al oxides with Activated sugarcane bagasse” has been attempted in the laboratory. Adsorption technique was utilized to remove fluoride from water using nanoparticles of iron and aluminium metal oxide along with activated sugarcane bagasse. The effect of pH, contact time and the dosage of adsorbent were investigated. Tests were conducted in a batch mode at optimized conditions for the hybrid of nanoparticles of Fe and Al oxides and activated sugarcane bagasse. The column studies were conducted to evaluate the effect of bed heights and influent fluoride concentration on the adsorption characteristics at neutral pH.The studies indicate that with the application of nanotechnology and adsorption technique, the fluoride can be removed with an efficiency of 70-80% with a contact time of 45 minutes.
The present work is restricted to studies on adsorption of fluoride using nanoparticles of iron and aluminium metal oxides with activated sugarcane bagasse on laboratory scale.The following studies can be taken up for further for research and implementation into field.
The effect of temperature on adsorption can be studied.
Studies can be conducted for natural water samples and the effect of other parameters such as hardness, metal ions, etc. can be investigated.
Regeneration of used adsorbents can be attempted.
Column studies may be carried out by varying flow rate, fluoride concentration, etc.
Pilot plant studies for defluoridation by using low cost activated sugarcane bagasse and nanoparticles of iron and aluminium oxides can be conducted before recommending full scale use.
To enhance the adsorption efficacy of activated alumina, researchers have also modified alumina surface in the various forms as describe below:
Tripathy et al. (2006) investigated the fluoride adsorption by alum impregnated activated alumina that was capable enough to remove 92.6% of fluoride at pH 6.5 at the dose of 8 g/l and 3h contact time from water containing 25 mg/l and then regulated to decrease with a further increase in pH. In the acidic range, the fluoride removal decreases due to the formation of weak hydrofluoric acid or combined effect of both chemical and electrostatic interaction between the oxide surface and fluoride ion. At pH above 6.5, the fluoride removal decreases sharply due to strong competition with hydroxide ions on the adsorbent surface. The Langmuir sorption capacity of fluoride was found to be 40.68 mg/g at pH 6.5. Desorption of fluoride from the adsorbent was done by rinsing the fluoride-adsorbed AIAA with 0.1 M NaOH at pH 12.0 followed by the process of neutralization with 0.1 M HCl. However, the adsorbent can be used extensively for further removal of fluoride prior to impregnation with alum.
Nawlakhe et al. (1975) reported the Nalgonda technique for fluoride removal from water in which two chemicals alum in the form of aluminium sulphate and potassium aluminium sulphate and lime as calcium oxide was rapidly mixed with fluoride contaminated water to formed focus of aluminium hydroxide. After gentle, stirring it was allowed to settle down to remove the maximum amount of dissolved fluoride. Nalgonda technique has been introduced into various countries such as India, Kenya, Senegal and Tanzania.
Dahi et al. (1996) implemented Nalgonda technique in Tanzania village of Ngurdoto at the community and household level. According to house hold version, 12.8 g of alum and 6.4 gm of lime was mixed in a bucket of 20 litre and allowed to settle down the sludge for one hour. The treated water was withdrawn through a tap that was 5 cm above the bottom of first bucket and store for many days as drinking in the second bucket. This technique could reduce the 8.8 to 12.5 mg/l initial concentration of fluoride into 0.7 to 2.1 mg/l of fluoride, which was still notice to be above the WHO standard.35 Even if the Nalgonda process was recommended as an effective technique for fluoride removal, some critics highlighted its disadvantages as well. For example, Meenakshi and Maheshwari (2006) have listed the following drawbacks;36
(i) The process removes only a smaller portion of fluoride (18–33%) in the form of precipitates and converts a greater portion of ionic fluoride (67–82%) into soluble Al3þ–F_ complex ions which are themselves toxic in nature by Apparao and Kartikeyan, (1986).37
(ii) The concentration of the SO4 2- ion from the aluminium sulphate coagulant reaches to the high levels, and in a few cases it crosses, the maximum authorized limit of 400 mg/L.
(iii) The residual aluminium in excess of 200 ppb in treated water is believed to cause dementia, and it also affects musculoskeletal, respiratory and cardiovascular systems by Nayak, (2002).38
(iv) Many users do not like the taste of the treated water as it was appreciated earlier.
(v) Regular analysis of feed and treated water is required for calculating the correct dose of chemicals to be added, because the water matrix fluctuates enormously with time and season.
(vi) The maintenance cost of a community plant is high. On an average as experienced in the recent years, a plant of 10,000 L per day capacity requires Rs. 3000 every month which is quite expensive.
(vii) The process is not automatic as it requires a regular attendant during the treatment process for regulating.
(viii) Large space is required for drying of sludge. (ix) Silicates have adverse effect on defluoridation by Nalgonda technique. Temperature of an area also affects the defluoridation capacity.
Lunge et al. (2012) studied the alumina supported carbon composite prepared by waste of egg shell for removing fluoride from water. The Langmuir adsorption capacity of composite adsorbent was 37mg/g at 303 K for a wide range of pH between 3 and 9. The adsorbent capacity of 15.15 mg/g and 71.43 mg/g was observed in the tested field water and waste water, which generally had initial concentration of 8.5 and 46.4 mg/l respectively.
Bansiwal et al. (2010) investigated copper oxide coated alumina (COCA) for removal of fluoride. COCA was synthesized by saturating alumina with copper sulphate solution followed by a calcination process in presence of air at 450oC. The adsorption capacity of COCA for fluoride obtained as the basis of the Langmuir model was 7.22 mg/g, which was three times higher than that of unmodified AA where value obtained was 2.232 mg/g. The adsorption kinetics followed the pseudo-second-order model to the results of pH studies, it is revealed that the COCA can be used for defluoridation of water in broad pH range between 4 and 9. The significant increase in adsorption capacity of COCA was described, which could be due to the increase in zeta potential referred to as more positive values resulting in elevating fluoride sorption. A slight variation in the fluoride removal was seen with variation in pH. Marginal decreases in sorption capacities were reported at pH above 8 which might be due to the competition by OH− ions present in alkaline conditions.
Water is an important natural-resource for maintaining environmental and life that we have constantly regarded as around in wealth and free present of nature.60-62 Fluoride is a hard-to-eliminate leading groundwater impurity of global issue. 63-65 By the way a small concentration of fluoride in drinking-water is valuable for bone and tooth, its surplus has the undersirable. 66 This review has attempted to cover a wide range of aluminium based adsorbents which have been used so far for the removal of fluoride from the water and wastewater. Different experimental parameter and methods were applied for the synthesis of alumina based absorbents. Due to the synthesis of expected compound generally having high purity at comparatively low temperature, the sol-gel technique is prominent all over the world that are used extensively for defluoridation process. Another properties have been created due to the enhancement of new material which needs awareness for employing it in research studies and also for commercial uses. Therefore, alumina based absorbent is on the trench to become one of the beneficial absorbent for creating the surrounding free from pollution. The alumina based absorbent and its vast uniqueness opportunities in near future that have been set by the nanotechnology is regarded for advancement in the present world.
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