Removal of Actinides and Other Radioactive Metal Ions From Water Systems

In previous work the adsorption of a number of radioactive ions from solution by a strongly-magnetic iron sulfide material has been studied. The material was produced by sulfate-reducing bacteria in a novel bioreactor. The uptake is rapid and the loading on the adsorbent is high due to the high surface area of the adsorbent and because many of the ions are chemisorbed. Following the success of the biologically-generated material, Lidzey at Bio Separation Ltd was able to produce an iron sulfide material; studies at the University of Southampton reveal that it has the tochilinite structure which has similar adsorption properties for cations, but not anions, as the biologically-generated material but the Lidzey material is considerably cheaper to produce. Tochilinite is a layered metal hydroxide/sulfide composite. The composition of tochilinite ranges around 2Fe_1-xS. {1.7Fe(OH)₂}. In nature, these materials also have Mg_0.7Al _0.3 replacing Fe in the hydroxide layer. By also precipitating magnetite, Fe₃O₄, together with the tochilinite Lidzey has made the adsorbent strongly magnetic. This paper presents individual results on the adsorption of ¹³⁷Cs, ⁵⁹Fe, ⁶⁰Co, ¹⁰⁶Ru, ^110mAg, ⁵¹Cr, ⁵⁴Mn, ²⁰³Hg, ¹⁰⁹Cd, ⁹⁰Sr, ²³⁹Pu and ²⁴¹Am. Using 0.5g.l^−l of the adsorbent the reduced the activity in solutions from 1190 Bq.ml⁻¹ to <530 Bq.ml⁻¹ after 1 h and to 340 Bq.ml⁻¹ after 24 h. with ⁵¹Cr and ²⁰³Hg being responsible for 270 Bq.ml⁻¹ of the total. For the decontamination of water streams containing dissolved radioactive isotopes the method proposed is to add the adsorbent and after a suitable residence time to extract the contaminated adsorbent using high gradient magnetic separation. In this method a slurry containing the particles to be separated are passed through a matrix of fine ferromagnetic wires magnetised to magnetic saturation by an externally applied magnetic field usually 5 Tesla. The particles to be separated are captured on the matrix, the decontaminated liquid passing through. The matrix is removed from the field and at the same time a clean matrix is inserted. The captured material recovered outside the magnetic field as a thick slurry and he cycle is repeated. These magnetic separation systems are in use throughout the world and will process up to 500m³.h⁻¹.