Toxicity Testing of Ion Exchange Resins for Water Treatment

Abstract: The sources and characteristics of water-soluble leachates from ion exchange resins were discussed. The German methods and standards for toxicity testing of ion exchange resins were introduced, and five weakly basic anion exchange resins were tested for toxicity according to German standards. The results showed that 703 and D314 resins were qualified products in the toxicity test, while IRA96RF, 701, and WM1 were unqualified products.

Keywords: Ion exchange resin; Toxicity test; Dissolved organic carbon (DOC)
CLC Number: TU991.26; R123
Document Code: C
Article Number: 1000-4602(2001)02-0059-02

Currently, apart from certain restrictions on resin leachates in the food and nuclear industries, China lacks corresponding toxicity testing standards and regulations for ion exchange resins used in water treatment.
Ion exchange resins are generally insoluble, but the characteristics of their polymer structure, the complexity of polymer functional group reactions, and incomplete resin cross-linking often result in new resins containing small amounts of soluble impurities such as linear polymers. Furthermore, resins gradually degrade and deplete over prolonged use. Therefore, various countries have established corresponding toxicity testing regulations and standards for the use of ion exchange resins and adsorbent polymers in drinking water treatment and the production of food, beverages, and pharmaceuticals. For example, the United States has established two drinking water additive standards under the National Drinking Water Safety Act ^[1] , while Germany issued the testing standard DIN54411 for the application of ion exchange resins and adsorbent polymers in aqueous systems in April 1987. The following section introduces the methods and results of toxicity testing of several weakly basic anion exchange resins selected in the project "Removal of Heavy Metals and Organic Micropollutants from Chinese Drinking Water Sources Using Weakly Basic Anion Exchange Resins," conducted in cooperation with the Karlsruhe Research Center in Germany, according to German standards.

1. Test methods and apparatus

1.1 Testing Materials 

Five different types of weakly basic anion exchange resins from three manufacturers were selected for testing. Their performance indicators are shown in Table 1.

1.2 Test Method

According to the conditions specified in DIN 54411, a certain amount of ion exchange resin or adsorbent polymer is continuously circulated through deionized water in a specified apparatus for 168 hours. The content of leached organic matter (expressed as DOC) in the circulating water is then determined. This standard stipulates that when the concentration of leached organic matter in the circulating water, characterized by DOC, is <1 mg/L, the resin is considered a toxicity-qualified product; however, in actual operation, if the DOC in the effluent of the same continuously operating ion exchange resin is <3 mg/L, it can also be considered a toxicity-qualified product.

1.3 Test Apparatus
The toxicity test apparatus is shown in Figure 1. The pump used should be designed so that impurities contained in its material are minimized by deionized water leaching, and the moving mechanical parts should not contaminate the deionized water due to friction. The remaining components of the test apparatus are made of glass, and the glass tubes should be as compact as possible. Flexible hose connections should be made of materials that release as few impurities as possible into the water (such as polytetrafluoroethylene or silicone rubber). During assembly and testing of the apparatus, care should be taken to prevent any external contaminants from entering.

Test parameters: Ion exchange resin volume is 350 mL, recirculation water flow rate is 6 L/h, resin to circulating water volume ratio is 1:10, recirculation water temperature should be maintained at 20-25 ℃, and the recirculation test is completed after 168 h.

1.4 Chemical
reagents used for testing All chemical reagents used for testing are analytical grade, and the solutions are prepared with deionized water (all are mass fractions): NaCl 8%, NaNO3 2%, HCl 1%, NaOH 2%.

1.5 Pretreatment of resin
Before use, the resin must be pretreated to remove impurities introduced during preparation and storage. For example, before packing the column, weakly basic anion exchange resin needs to be washed with dilute sodium chloride and sodium nitrate solution to remove trace amounts of preservatives and residual monomer compounds. Similarly, the resin must be pretreated before toxicity testing.

1.5.1 Converting the resin to its operational form
Before being loaded into the toxicity testing device, the resin to be tested needs to be converted to the form specified for use. For weakly basic anion exchange resin, it should be converted to the free basic form. First, pass 2 L of 2% NaOH (by mass) through the resin column in a co-current manner for approximately 60 min, then rinse the resin with 3 L of deionized water for approximately 1.5 h.

1.5.2 Resin Loading:
Convert the resin to the appropriate loading form under actual conditions. For weakly basic anion exchange resins, pass 3 L of 1% HCl (by mass) through 400 mL of resin in a co-current manner for approximately 2 h, then rinse with 6 L of water for approximately 3 h. When testing ion exchange resins or adsorbent polymers used in water treatment plants, pretreatment as described in sections 1.5.1 and 1.5.2 should be followed by loading and regeneration 5 times.

1.5.3 Storage:
Place the regenerated resin in a suitable storage container (e.g., a glass bottle) and shake thoroughly.

1.6 Resin Toxicity Testing

1.6.1 Blank Value Determination:
Fill the toxicity testing device with a total volume of 3.5 L of deionized water, circulate at a flow rate of 6 L/h for 168 h, and then measure the DOC value in the water until a constant blank value similar to that obtained in routine determinations is obtained. Replace the deionized water to bring the apparatus into standby mode.

1.6.2 Resin Loading:
Load the resin to be tested into the filter column of the toxicity testing apparatus. First, fill the filter column with deionized water until it is close to the support plate, then load approximately 400 mL (usually 350 mL) of the resin to be tested. Backwash with deionized water from bottom to top until the deionized water at the top outlet is clear, all air bubbles are removed, and it is 1 cm above the resin surface.

1.6.3 Toxicity Testing
After circulating the replaced 3.5 L of deionized water through the resin column for 168 h, determine its DOC value (a blank value should be considered when evaluating the analytical results).

2. Test Results

To date, only five weakly basic anion exchange resins—IRA96RF, 701, 703, D314, and WM1—have undergone toxicity testing, and the results are shown in Table 2.

It is evident that 703 resin and D314 resin passed the toxicity test, while the other three resins failed.

References:

［1］Jay A，Miers Jr.Regulation of ion exchange resins for the food,wat er and beverage industries［J］.Reactive Polymers,1995,24(2):99-107.
［2］Cuvelier J M.Ion exchange resins used in the food industry and in the drinking water preparation in Europe［J］.Reactive Polymers,1995,24(2):109-1 16.