(149b) Arsenic Removal from Water By Flat Sheet Supported Liquid Membrane (FSSLM) Based Separation Technique

In many regions around the world, arsenic has been found to be a contaminant present in drinking water. Both geogenic and or anthropogenic causes such as burning of fossil fuels, mining, smelting plants, pesticide application and/or leaching out of arsenic into soil from pressure treated woods may lead to arsenic contamination causing arsenicosis and cancer. In the environment arsenic can occur in different oxidation states (-III, 0, +III and +V). In natural waters, arsenic mostly occurs in inorganic form as oxyanions of trivalent arsenite [As (III)] or pentavalent arsenate [As (V)]. Biological activity in surface waters significantly polluted by industrial wastes often leads to the production of organic As such as monomethylarsonous acid and dimethylarsinous acid [As (III)] or monomethylarsonic acid, dimethylarsinic acid and arsanilic acid [As (V)]. Trivalent arsenic has been reported to be more harmful and toxic than the pentavalent form due to its reactivity with sulfur containing compounds and generation of reactive oxygen species. Inorganic forms of arsenic are found to be potent toxins. The toxicological effects of arsenic has led to an exponential rise in scientific research for its removal in the past few decades. However, there are several separation techniques for removal of arsenic from water; liquid membrane separation technique is being studied for its selective and high separation factor.

In Liquid Membrane Separation Technique (LMST), there is an organic phase immiscible with water, containing extractant or complexing agent that is selective towards the target element. It enhances the extraction process by temporarily binding with the target element present in the source phase and releasing it to the receiving phase due to concentration gradient. Thus, the target element can be extracted and recovered simultaneously in a single step. Since the process is based on concentration difference, it does not require pressure or voltage for separation. Different configurations of liquid membrane based separation process include Bulk Liquid Membrane (BLM), Supported Liquid Membrane (SLM) and Emulsion Liquid Membrane (ELM). SLM can be further classified into Flat Sheet (FS) SLM and Hollow Fiber (HF) SLM.

The aim of this research work is removal of As (III) for purification of water using FSSLM based separation technique. This could be achieved through initial two-phase equilibrium studies through which environmentally benign solvent is identified that is easily available, non-toxic and low cost for extraction of As (III) from synthetic water. Vegetable oils including coconut oil, sesame oil, sunflower oil, soybean oil and mustard oil are environment friendly alternatives to the chemical solvents used. Sesame oil was selected for the study followed by identification of extractant suitable for enhancing the transport of solute (arsenic ions) from the feed phase (aqueous phase) to the receiving phase (aqueous phase) through the solvent-extractant pair acting as the intermediate organic membrane phase. Methyltrioctyl ammonium chloride (Aliquat 336) was selected amongst the extractants or carrier compounds used in this work. The optimum operating conditions were studied through variation in parameters such as extractant concentration, receiving phase concentration and pH on extraction and recovery of arsenic ions. Sodium arsenite salt was used to prepare the feed phase and ferric chloride salt for preparing the receiving phase solution. The feed and receiving phases were stirred carefully, without mixing with each other and disturbing the organic phase.

The extractant present in the liquid membrane binds with solute (arsenic ions) at feed-membrane interface, diffuses across the membrane and releases the solute at the membrane-receiving interface. The extractant then diffuses back to the feed-membrane interface to bind with another As (III) ion and this cycle continues. In this way simultaneous extraction of As (III) from feed phase to membrane phase and re-extraction or recovery of As (III) from membrane phase to receiving phase occurs. The arsenic was recovered as iron-arsenic complex in the receiving phase. Further, a mathematical model was developed for simulating the solute transport mechanism through liquid membrane that can be used for performing initial calculations for any scalability measures.