Pilot-scale demonstration of strontium removal from low-level radioactively contaminated process wastewater using continuous countercurrent ion exchange (CCIX)

A mobile pilot-scale Continuous Countercurrent Ion-exchange (CCIX) system was operated at the Oak Ridge National Laboratory (ORNL) for the treatment of wastewaters that contain predominantly calcium, sodium, and magnesium bicarbonates and are slightly contaminated with ⁹⁰Sr and ¹³⁷Cs radioisotopes. A demonstration study was conducted to evaluate the near-steady-state performance and feasibility of a pilot-scale continuous countercurrent ion-exchange column for the selective removal of strontium from the wastewater. Test results show that the process removes 99% of the strontium from the water while significantly reducing the volume of secondary waste generation. The technique entails recycling of the resin regeneration streams to minimize secondary waste. This study discusses (a) mathematical modeling of multicomponent equilibria using an iterative nonlinear parameter estimation technique (Levenberg-Marquardt) to determine equilibrium constants for predicting distribution coefficients, (b) applications of the Thomas model for predicting breakthrough curves, scale-up, and ion-exchange column behavior, and (c) a method predicting breakthrough behavior in a CCIX column using an extension of the Thomas model. Experimental results show that the breakthrough distribution for a multicomponent feed system can be reduced to a binary system and predicted (within 10%) using multicomponent equilibrium constants. Furthermore, the effect of changing feed concentrations on column performance are predicted using the Thomas model. Although the CCIX column effectively removes strontium from the wastewater, the height of a transfer unit was significantly greater than that observed in fixed-bed experiments, probably because of the simple fluid inlet distribution used and mixing during resin movement.