Studies on the Defluoridization Competency of a Mixture of Raw Vermiculite and Strychnos potatorum linn. Seeds

Published: 01-Jan-0001

Introduction

Water is an essential requirement for all the living creatures and it occupies a major part of the earth. The maintenance of good health of human being mainly depends on the quality of drinking water [1]. It has been estimated that only 0.6% of the globally available water is existing as fresh water sustainable for human consumption [2]. Unfortunately, this fresh water is increasingly contaminated by both lithogenic and anthropogenic pollutants. Among the various pollutants, the presence of fluoride in the drinking water in excess than the permissible limit is termed as fluorosis and is becoming a major contributory factor from the health point of view [3]. The incidence of fluorosis has been reported in many parts of the world especially India, China and Pakistan. Fluoride enters the aqueous environment mainly by weathering of fluoride-rich minerals and through anthropogenic activities [4]. Though the fluoride ingestion at low levels helps to reduce the solubility of apatite crystals in the bone and makes the structural system strong, excessive fluoride consumption is a detriment to bones and causes skeletal fluorosis which elicits severe pain and stiffness in joints [5]. Likewise, excessive chronic intake of fluoride, especially during tooth development, causes mild to severe dental fluorosis [6].

Fluoride exerts its toxicological effects even at the molecular level which ultimately results in increased apoptosis, impairment of glucose and protein metabolism and chronic oxidative stress [7]. Consumption of drinking water with permissible fluoride content (WHO 1.5 mg/l; BIS 1 mg/l) is the only option to prevent fluorosis and there is no established treatment for dental and skeletal fluorosis [8]. To overcome the hazardous wellbeing effects of fluorosis, several approaches for defluoridization have been proposed and their advantages and limitations were reported in detail. It is perceptible from a literature survey of more than 200 recent research articles that minimal efforts methods have exhibited extraordinary removal capacities for fluoride [9]. However, the major restrictions of the available defluoridization methods mainly include upkeep costs, the prerequisite of talented labor, issues related to sludge transfer, maintenance of pH and demineralization after treatment and non-attainable for rural regions.

The traditional system of removing fluoride from drinking water is liming and the attending precipitation of fluoride. Coagulation and precipitation method (Nalgonda technique) of defluoridization using lime and alum is suitable for 20 l of water for one-day utilization [10,11]. The membrane process involves reverse osmosis (RO), Nano-filtration (NF), Donnan- dialysis and electrodialysis is best suited for industrial waste water treatment particularly for the removal and recovery of fluoride from its effluents [12-14]. Reverse osmosis (RO) using the semi permeable membrane is capable of removing more than 90% of fluoride regardless of initial fluoride concentration [15]. Nano filtration (NF) is the later innovation of RO which essentially removes the larger dissolved solids at a relatively less pressure applied [16,17]. However, RO and NF techniques are highly expensive methods. Dialysis and electro dialysis is based on the removal of ionic components from aqueous solutions through ion-exchange membranes under the influence of an electric field [18-20]. Fluoride can be selectively eliminated from the water with the aid of costly anion exchange resins such as amberlite IRA-400, Indion FR10, Ceralite IRA-400, titanium oxohydroxide and aluminium loaded duolite C-467 [21-24].

Among the various defluoridization techniques, adsorption method is considered as more prominent because of the availability, accessibility and affordability of a variety of natural adsorbents. A few adsorbents which have been studied in the past include, activated alumina, activated charcoal, activated alumina coated with silica gel, activated saw dust, activated coconut shell powder, groundnut shell, bone charcoal and activated soil sorbent [25-28]. Earlier, we have extensively studied the defluoridization efficacy of vermiculite, a type of mica comprised of aluminium /iron/ magnesium/ silicate mixture. It is unearthed as a material composed of thin layers. We have reported that the raw vermiculite at a particle size of 80 mesh shows the maximum defluoridizing effect at acidic pH [29]. Several reports are available in the literature regarding the use of vermiculite, acidic and basic fly ashes, bauxite and xeolite for their defluoridizing efficacy [30-33].

In the recent past, several researchers have used plant materials as adsorbents for the removal of fluoride ions [34]. Strychnos potatorum Linn., commonly known as Nirmali, is a medium-sized, glabrous tree known for its seeds. The seeds are sub-spherical in shape and 10-13 mm. in diameter and approximately weigh about 1-1.5 g. The horizontally split seed shows that the endosperm is translucent white and has the same shape as the seed. Sanskrit writings mentioned in Sushruta Samhita from India reported that the seeds of S. potatorum were used to clarify turbid surface water over 4000 years ago [35,36]. In the traditional system of medicine, the nirmali seeds are used to clear the muddy water due to their coagulant action. This property was attributed to the presence of anionic polyelectrolytes having -COOH and free -OH surface groups that are present in the seed protein. On average, the seed binds 0.5 mg of the respective metals per 1.0 g of the seed [37]. A major polysaccharide consisting of a 1: 7 mixture of galactomannan and galactan was found to be responsible for its water purifying properties [38,39]. Further, the seed extract is non-toxic and widely used for the treatment of various ailments like jaundice, bronchitis, diabetes, conjunctivitis, chronic diarrhoea, dysentery etc. which in turn due to the presence of biologically active phytochemicals present in them.

In view of the potential credentials bestowed with vermiculite as well as the seeds of Strychnos potatorum, in the present study, an attempt has been made to combine these two natural adsorbents in the ratio of 70: 30 w/w and evaluate the defluoridizing capacity of the vermiculite and S. potatorum mixture.

Materials and Methods

Collection and preparation of adsorbent materials

Raw vermiculite samples with different particle sizes were obtained from Tamil Nadu Minerals Limited, Ambattur, Chennai and used for the present study (Figure 1). Based on our earlier studies with respect to particle size and the defluoridization capacity, the raw vermiculite was powdered to obtain the particle size of 80 mesh using agate motor [29]. S. potatorum seeds were collected from an organic shop in Mylapore, Chennai and authenticated by Prof. J. Muthumary, a taxonomist in Centre for Advanced Studies in Botany, University of Madras, Guindy Campus, Chennai (Figure 2). The seeds were dried in a hot air oven for 24 h at 50ºC. The seed coat was selectively removed and powdered to obtain a 100 mesh size [40]. After standardization by using different proportions, the raw vermiculite and S. potatorum seeds were mixed in the ratio of 70: 30 w/w using mortar and pestle and used as the adsorbing material for defluoridization studies.

Figure 1: Raw vermiculite

Figure 2: Strychnos potatorum linn. seeds

Column adsorption study

A column size of about 80 cm in height and 3 cm in diameter capable of accommodating 30 ml of water with an outlet fitted with a stopper at the bottom was selected to perform column adsorption study and the column was fixed to a stand [29]. A plug of cotton is placed to the bottom of the column and slurry of silica was prepared and poured into the column carefully so that silica will not pass through the column [41]. The columns were carefully packed with 15 g of either the raw vermiculite or the adsorbent mixture containing raw vermiculite and powdered S. potatorum seeds in the ratio of 70: 30 w/w. The water samples were allowed to stand in the column for 30 min.

Fluoride assay

A stock solution (100 mg/l) was prepared by dissolving 221 mg NaF (analytical grade) in 1 l of distilled water [42]. From the stock solution, three different concentrations of fluoride solutions namely 3 ppm, 4 ppm and 5 ppm were prepared for defluoridizing analysis. The flow rate was adjusted to 2 ml/min. The effluent fluoride solutions were collected and analyzed for fluoride content [43]. All the experiments were carried out in triplicate and the solutions for defluoridization as well as fluoride analysis were prepared by an appropriate dilution from the stock solution [44]. Fluoride content in the water samples before and after defluoridization was carried out with an Orion ion analyzer (Orion modal 720- pH-fluroimeter, USA) equipped with a fluorine selective electrode.

All reagents were of analytical-reagent grade and distilled. De-ionized water was used throughout the studies. Sodium fluoride (AR, BDH) was used as the source of fluoride. A stock solution of fluoride was prepared by dissolving 2.21 g of sodium fluoride and the solution was made up to 1 l in a standard flask [45]. By constructing the cell using the fluoride ion-selective electrode and the calomel reference electrode in a solution of fluoride at pH 5.35, adjusted with total ionic strength adjusting buffer (TISAB), the cell potential can be determined.

For the preparation of Total Ionic Strength Adjustment Buffer (TISAB), approximately 500 ml of water was added to 57 ml of analytical reagent grade Glacial acetic acid, 58 g of analytical reagent grade NaCl and 0.03 g of sodium citrate. The solution was titrated to pH 5.0-5.5 using analytical reagent grade 5 M sodium hydroxide and finally made up to 1 l. The TISAB buffer was transferred to a plastic container and stored in a refrigerator [46]. The fluoride standards were prepared by dilution from Orion Research 0.1 M Fluoride Activity Standard. Ten milliliters of a standard 1.0 ppm fluoride solution was pipette into a beaker and 10.0 ml of TISAB was added. The solution was stirred and the electrode potential determined after three minutes. The same 1: 1 mixing procedure was used with other standards and with unknowns [47].

Conclusion

Based on the results obtained, it is concluded that the mixture containing raw vermiculite and S. potatorum seeds seems to be a conventional, economical and efficient method for defluoridization of drinking water. Further detailed studies are in progress to determine the effect of different particle size, contact time, adsorbent dose, adsorption kinetics and the effect of co-existing anions present in the water samples and the regeneration of the adsorbents used.

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