Ion Exchange for the Recovery of Metals

Abstract

An experiment has been designed to determine the behavior of weak acid cation exchange resin and strong acid cation exchange resin for the removal of Cu+2 ions and Mn+2 ions during the resins service and regeneration steps. In this experiment, strong acid cation exchange resin, Lewatit S 100, and weak acid cation exchange resin, Ionac SR-5, have been used as an absorbent for Cu+2 ions and Mn+2 ions removal from the aqueous solutions.

It has been observed, during the experiment, that both Cu+2 ions and Mn+2 ions behave almost in a similar manner (during the service and regeneration steps). It is also found that initially the concentration of Cu+2 ions and Mn+2 ions in the solution, during the service step, is negligible as the resin is fully regenerated and it absorbs all the incoming cations. With the passage of time, however, concentration of these ions in the effluent solution increases. This is because the resin molecules are getting attached with the Cu+2 ions and Mn+2 ions and availability of regenerated resin is going to decrease for the attachment of cations. Finally the concentration of these ions in the effluent water becomes equal to the concentration of incoming solution. At this stage all the resin molecules are attached with the Cu+2 ions and Mn+2 ions and no resin molecules are now available for the exchange of incoming cations. The resin at this stage is called in exhausted and it needs regeneration.

On the other hand, during the regeneration step, the concentration of Cu+2 ions and Mn+2 ions ions in the effluent water is maximum at the start. This is because at this stage maximum numbers of cations are going to detach from the resin molecules. With the passage of time, however, concentration of these ions in the effluent solution decreases till it becomes negligible. At this stage the resin is fully regenerated.

Introduction

Industrial waste water contains heavy metals such as Cu+2, Fe+2, Mn+2 etc., which is a very serious threat to the ecosystem and the public health. It is therefore essential to remove the heavy metals before its disposal to outside the industries. There are various techniques to remove the heavy metals from the waste water; however, ion exchange technique on polymeric ion exchangers is most widely used due to high efficiency, cost effectiveness and easy handling. Strong acid cation exchange resins are styrene-divinylbenzene copoloymer compounds which are commonly used in all demineralization plants for the removal of cations (a necessary step for removal of hardness of water). This resin also finds its application in waste water treatment for the removal of heavy metals over the wide range of pH. Weak acid cation exchange resins are also used for the removal of cations from water, however, these resins work more effectively at a pH more than 5.

In our experiment we will use strong acid cation exchange resin for the removal of Cu+2 ions and Mn+2 ions at the appropriate pH. We will measure effectiveness of the resin by passing the solution, containing Cu+2 and Mn+2 ions, through the resin bed and collecting the effluent water from the resin bed (for the determination of cations present). The behavior of the resin will also be monitored during the regeneration step. In this case, acid solution will be passed through the resin bed and concentration of cations will be measured in the effluent water. The whole experiment will be repeated similarly for weak acid cation exchange resin and the observations will be made. Based on the results, we will select the type of resin (strong acid cation or weak acid cation) which will be more efficient for the removal of heavy metal ions (Cu+2, Mn+2) at the specified pH.

The materials, apparatus used and the method was:

CuCl2.2H2O, MnCl2.4H2O, 2M HCl, 2M NaOH, Strong Acid Cation exchange resin (Lewatit S100), Weak Acid Cation exchange resin (Ionac SR-5), Atomic absorption spectrometer, two 50ml burettes, Glass wool and Filling funnel for solids were used.

Method:

First of all soak the weak acid cation and strong acid cation with demineralized water for one hour. Then transfer strong acid cation exchange resin into the burette (to obtain height of 30 cm). Be sure that that the column is covered with demineralized water with no bubbles trapped in the column. After that prepare 1 litre solution with a Ph of 2. Pass 500 ml of this solution into the strong acid cation exchange column at the rate of 10 ml/min. Also prepare one litre solution containing 100 ppm of solution and of Cu+2 and Mn+2 each. Adjust the pH of solution at 2 and analyse the concentration of Cu+2 and Mn+2 with the atomic apsorption spectrometer. Pass 500 ml of this solution through Strong Acid Cation exchange colummn at the rate of 10ml/min. Collect the solution which has passed through the column initially once every minute for the first 20 minutes and then once every five minutes. Analyse the collected solution to determine the concentration of Mn+2 and Cu+2. Observe the color and change in volume of the column. Then pass 500 ml of 2M HCl solution through Strong Acid Cation exchange column at the rate of 5ml/min. Collect the solution which has passed through the column initially once every minute for the first 40 minutes and then once every five minutes. Analyze the collected solution to determine the concentration of Mn+2 and Cu+2. Observe the color and change in volume of the resin column. Next repeat step number 3 to 5 for weak acid cation exchange column and observe the physical changes.

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