A Novel Electrochemically Regenerable Ion-Exchange System for Nitrate Removal

The objectives of the Phase II program are to optimize the nitrate extraction/recovery process and to develop a bench-scale redox-recyclable nitrate removal system to be evaluated in water treatment, residential, and industrial settings. All of the basic concepts were proven in the Phase I program, and feasibility was demonstrated. It was shown that a redox-active extractant can be dissolved in a water-immiscible, non-volatile, non-flammable alternative solvent, where it can be electrochemically oxidized to activate it for effective liquid-liquid ion-exchange extraction. The extractant is then electrochemically reduced, allowing recovery of the target ion in a small volume. There are issues remaining to be addressed for full-scale commercial application, including stability of the extractant phase, purification of used extractant phases, analysis of the simulants after extractions to determine if anything is added to them, selection of preferred anion- and cation-exchange membranes, extraction of other target ions, and use of more complex simulants and actual groundwater. In this program, a working prototype apparatus suitable for Phase III field-trial evaluations will be completed. Specific research objectives of the Phase II program include: 1) Synthesize alternative solvents. 2) Determine/improve stability of the extractant. Recycling studies will be done, and it will be determined how many extraction cycles an extractant phase can go through and remain effective. If instability of the extractants is a problem, a more stable extractant will be synthesized. 3) Develop a method to purify spent extractant phases so they can be reused. The extractants will eventually break down, and other contaminants may enter extractant phases from aqueous phases. Chilling, heating, extraction, and precipitation will be evaluated to remove contaminants. 4) Extract from more complex aqueous simulants and actual groundwater samples. Study effects of competing anions and other species present in groundwater. 5) Evaluate system for extraction of other anions. 6) Analyze aqueous phases after extraction to see if anything goes into them from the extractant phase. 7) Design/build a prototype apparatus consisting of two electrochemical cells, one for oxidation and one for reduction (with concomitant recovery of the target ion) of the extractant. For extraction, a commercially available centrifugal contactor will be used. Anion- and cation-exchange membranes will be evaluated during oxidation and reduction of the extractant, and recovery of the target ion. This approach to ion-exchange eliminates a major drawback to current ion-exchange technology. Ion-exchange materials, whether resins or dissolved extractants used in liquid-liquid extraction, are regenerated by contact with large volumes of a high-salt, acidic, or basic strip solution, creating a large secondary waste stream in which the target ion is contained in low concentration. Electrochemical regeneration of the extractant allows recovery of the target ion in a small volume without creation of a large, new waste stream.