Uran i dricksvatten
litteraturstudie om reningsmetoder samt pilotförsök med jonbytesteknik
Uranium (U) is a naturally occurring component in bedrock and under the "right" conditions it can dissolve and move into the ground water. Since many people in Sweden depend on drilled wells as their main drinking water supply, they may be exposed to the uranium in the water.
Radioactive radiation is not the major concern associated with naturally occurring uranium. It is rather the chemical properties of uranium that make it a potential danger to human health. Uranium is proven to have toxic effects with respect to the functioning of the kidneys. In epidemiological studies and experiments performed on animals negative effects have been observed after consumption of drinking water with a uranium concentration of 200-300 micrograms U/l. By adding a safety margin a range of maximum contamination levels (MCL) have been defined for drinking water in a number of countries. In Sweden the intervention level for drinking water is 15 micrograms U/l.
To reduce the uranium level in drinking water a number of different techniques can be used, for example conventional coagulation and filtration, pressure-driven filtration, adsorption or ion exchange. How successful a method turns out to be mainly depends on the chemical constituents in the ground water. Factors such as pH influence the solubility of uranium-complexes as well as their charge.
In this experiment a small scale ion exchange pilot was constructed and tested at two locations. At first the pilot was located at the water laboratory in Uppsala for 13 months and after that at the waterworks in Björklinge for 10 months. Untreated water was pumped to a column containing an anion exchange resin material. After passing through the column samples of treated water were collected and the amount of water passing through the column was measured. During the experiment two anion exchange resins were tested in parallel to each other, Dowex 21 K and Amberlite 900 Cl. Both resins showed very good results, i.e. there was a U reduction of more than 99 %, at the start of the experiment and directly after each regeneration.
With respect to the concentration of uranium in the water the resins used in Uppsala should last longer than those in Björklinge since the water in Uppsala contains less uranium. In reality the opposite occurred despite that the same types of resins were used at the two locations. In Uppsala about 30 000 bed volumes could be treated before breakthrough while in Björklinge the U reduction was still 90 % after treatment of 120 000 bed volumes. Apart from uranium the greatest difference in water chemistry between the two locations was the amount of dissolved organic matter (DOM). There seemed to be an inverse relationship between the concentration of DOM and the capacity of the resin. This indicates that the chemistry of the untreated water is of great importance for the success of the technique.
The results indicate that ion exchange is an effective method to remove uranium from water. However one disadvantage is the lack of information on how to handle used resins. Another disadvantage is the build-up of high concentrations of uranium in the resin which may generate dangerously high loads of radiation.