Removal of Hardness of Water by Ion Exchange Column

Introduction

The water containing dissolved calcium and magnesium salts more than the permissible limit i.e., more than 60 ppm than such water is called hard water. If the hardness is in larger quantity then the water has to be treated to remove the hardness causing salts from it. There are various methods available to remove the hardness from water such as soda-lime process, zeolite process, reverse osmosis process and ion-exchange method, etc. Ion exchange water treatment method involves the exchange of positively or negatively charged ions between solid resin and the come in contact with it with the water passing through it. Ion exchange resins are of two types namely cation exchange resin and anion exchange resins e.g., phenolic resins and resins consisting of polystyrene with sulphonic/ carboxylic acid attached to the polymeric backbone form cation exchange resins and with amine groups form anion exchange resins that are used commonly.

In case of softening of water ion exchange process targets the removal of calcium and magnesium ions that are responsible for causing hardness to the water. These ion-exchange resins are generally insoluble, cross linking polymers with porous spherical polymeric bed with specific functional groups. On passing the solution through column packed with the resin bed, the water sample come in intimate contact with the exchange sites and the hardness causing salts calcium/ magnesium ions get attached to the resin leaving the Na+/H+ ions in the solution. That are more soluble and do not cause sludge and scale. The basic principle is electrostatic attraction between resin and ions passed through and its exchange affinity depends on the size and charge of the cations i.e., ions with high charge and bigger size has more affinity to bind with the ion exchange resin.

Regeneration of Resin bed- Eventually, due to attachment of calcium/magnesium ions the resin bed get saturated which needs to be regenerated. Regeneration of exhausted beds can be done by passing the dilute acid or brine (NaCl solution) through the ion exchange column. In the present experiment of removal of hardness, hard water is passed through the cation exchange column and the hardness present in the water is being determined before and after passing the water sample through the column in this manner we come to know the hardness removed by the ion exchange resin. To determine the hardness complexometric (EDTA) method is used in this method disodium salt of EDTA (Ethylenediaminetetraaceticacid) forms colorless stable complexes with Ca2+ and Mg2+ ions present in the water sample. Eriochrome Black-T (EBT), a dye is used as an internal indicator, when 2-3 drops of EBT are added to the hard water it forms wine red colored unstable complex with Ca+2 and Mg+2 ions around pH-10 (see reaction scheme). Basic buffer solution (NH4Cl + NH4OH) is used to maintain the pH ~ 9-10 of the solution. This wine red-colored complex is titrated against standard EDTA solution the color of the complex changes wine red to original blue color at the endpoint .

Applications

1. Residential Water Softening: Ion exchange columns are widely used in homes to soften water for domestic purposes. Softened water reduces scale buildup in plumbing systems, water heaters, and appliances, improving their efficiency and extending their lifespan..

2. Laundry Facilities: Softened water is beneficial for laundry as it enhances the effectiveness of detergents, reduces soap consumption, and helps maintain the vibrancy and softness of fabrics. It also prevents mineral deposits on clothing.

3. Dishwashing: Water softening ensures spot-free and streak-free dishes, as it prevents the formation of scale on glassware and dishware. It also reduces the need for excessive detergent use.

4. Boiler Feedwater Treatment: In industrial settings, ion exchange is employed to remove hardness from boiler feedwater. This helps prevent scale formation on boiler surfaces, improving heat transfer efficiency and reducing maintenance costs.

5. Cooling Tower Water Treatment: Water used in cooling towers for industrial processes often contains hardness minerals. Ion exchange is used to soften this water, preventing scale formation in the cooling system and ensuring optimal heat exchange