(479e) Liquid Membrane Separation Process for the Removal of As (III) & As (V) from Drinking Water

For the past two decades arsenic contamination in drinking water around the globe has led to the research and development of several removal technologies due to its high toxicity. The liquid membrane separation technique being highly selective, simple design and requires low energy has been employed here for the removal of arsenic from its aqueous solution for the purification of drinking water. This work is intended to identify environmentally benign organic solvent, receiving phase and extractant for the extraction and recovery of arsenic ions from its aqueous solution through two-phase equilibrium study. Sodium arsenite and sodium arsenate salts have been used for feed phase solutions of As (III) and As (V), respectively. Amongst the conventional organic chemicals and environmentally benign solvents used as liquid membrane, sesame oil with Aliquat as the extractant increased the extraction of arsenic ions from 50% (without extractant) to 90% (with extractant).

The main aim of two phase study is to identify LM solvent and extractant to remove arsenic from water. Equal volume of aqueous solution of sodium arsenite and arsenate was taken separately and added to environmentally benign solvent in a conical flask. The mixture was continuously stirred by a mechanical shaker. Then it was kept undisturbed for some time until the two phase separated and the aqueous phase was carefully separated from organic phase. The concentration of As (III) and As (V) in the aqueous phase after extraction was measured by using AAS (at wave length 193.7 nm using VGA mode).

Two phase experiments were carried out with various green solvents. Maximum extraction for As (III) 53.3% and As (V) 64% was obtained using sesame oil. Sesame oil mainly consists of linoleic acid, oleic acid, palmitic acid and stearic acid. Saturated fatty acids like palmitic acid and stearic acid reacts with arsenic forming arsenolipid of the type (CH3)2As(O)-(CH2)nCOOH (n=14 and 16) with a dimethylarsinoyl group, (CH3)2As(O)-, replacing the methyl group in palmitic and stearic acids (Rumpler et al. (2008)). Arsenic reacts with unsaturated fatty acids like oleic acid forming C19H37AsO3 (Rumpler et al. (2008)).

Based on literature review, two phase experiments were conducted with four extractants, viz. trioctylamine (TOA), tributyl phosphate (TBP), di-2-ethylhexyl phosphoric acid (D2EHPA) and N- N-dimethyloctyl amine (Aliquat). In order to improve the percentage of extraction, extractants were added to the selected solvent (sesame oil) to identify the suitable extractant for arsenic ions. Though all four extractants showed a high percentage of extraction, aliquat being a basic extractant reacts with both dissociated and undissociated forms of arsenic (Pancharoen et al. (2009)). Aliquat in sesame oil has shown best extraction of 92%. (Figure 2).

The concentration of the selected extractant, Aliquat (NR4+Cl-) was varied from 1-10% (v/v). In neutral pH range of about 6-8 which is found in groundwater, As (III) exists mostly as H3AsO3 while As (V) occurs mostly as H2AsO4- and HAsO42- (Pancharoen et al. (2009)). As reported by Pancharoen et al. (2009) aliquat reacts with both undissociated H3AsO3 and dissociated forms H2AsO4- and HAsO42-, 6% (v/v) aliquat showed maximum extraction of As (III) (93.6%) while 10% (v/v) of aliquat was required for extraction of As (V) (95.2%) (Figure 3).

In subsurface environments, researchers have found that arsenic contamination might occur due to two reasons, one could be arsenic mobilization due to oxidation of pyrite minerals containing arsenic and the other could be reduction of iron oxy-hydroxides leading to the release and dissolution of arsenic in groundwater (Bose and Sharma (2002)). Based on this interaction of iron and arsenic, iron salts viz. ferric nitrate, ferric sulphate, ferric chloride, ferrous sulphate and ferrous chloride are selected as the receiving phase for recovery of As (III) and As (V). The idea is to implement the reverse mechanism that leads to contamination. Ferric nitrate for As (III) and ferrous chloride for As (V) were found to give a high recovery percentage.

The concentration of the selected receiving phases (ferric nitrate and ferrous chloride) was varied from 1.25-5ppm. It was observed that 5ppm of ferric nitrate and ferrous chloride showed maximum recovery of As (III) (69%) and As (V) (75%), respectively.

Based on the results of two phase studies, over 90% of As (III) and As (V) have been extracted by incorporating Aliquat in sesame oil. Ideal condition for maximum extraction of arsenic was found to be 200rpm at 25⁰C for a duration of 12 hours. Different extractant concentration was observed to give maximum extraction for As (III) and As (V). 6% and 10% (v/v) Aliquat showed maximum extraction of As (III) (93.6%) and As (V) (95.2%), respectively. Around 70% recovery was possible with 5ppm of ferric nitrate and ferrous chloride for As (III) and As (V), respectively. Further experiments for three phase studies is being carried based on the performance of two phase studies.

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