Nitrates are considered an important pollutant and contaminant of drinking water worldwide. In the US, levels of nitrates in groundwater are controlled by Environmental Protection Agency (EPA). For example, methemoglobinemia, a blood disorder (a.k.a. “blue baby syndrome”), occurs in rural and private wells regions and appears to be the result of high nitrate levels. This condition depletes the blood cells of their ability to carry oxygen. Due to such detrimental biological effects, treatment and prevention methods must be considered to protect groundwater aquifers from nitrate leaching and high concentrations.
As a result, various methods have been used and proposed to remove nitrates from drinking water, including reverse osmosis and ion exchange. However, reverse osmosis is not optimized for selective removal specific ions in low salinity streams, and ionic exchange resins requires the disposal of high concentration brine waste.
TA group of United States researchers from Stanford University and the University of California, Merced departments of Mechanical Engineering as well as Lawrence Livermore National Laboratory recently presented a hybrid capacitive deionization method for removing nitrate from water. Prof. James Palko (UC Merced), Ph.D. candidate Diego Oyarzun, Ph.D. Ali Hemmatifar, Ph.D. candidate Byunghang Ha and Prof. Juan Santiago (all from Stanford) in collaboration with Dr. Michael Stadermann (LLNL) demonstrated a system to overcome the major drawback associated with existing methods for removing nitrates from water. Their work was published in the research journal, Chemical Engineering Journal.
Briefly, their demonstrated adsorbent consisted of a functionalized, activated carbon electrode with quaternary ammonium moieties. These moieties have a high affinity for adsorbing nitrate from an aqueous solution without voltage application. The adsorbent was then electrostatically regenerated by an applied voltage, thus eliminating brine disposal as for the case in ion exchange. An electric potential voltage was applied at the conductive carbon substrate to remove the adsorbed ions. The hybrid capacitive deionization cell was completed with a titanium current collector (cathode) supporting a Faraday reaction. The system was also designed to prevent readsorption of the nitrates rejected from the active electrode during the regeneration process.
The authors refined their approach and expanded the capability of the system in follow-up paper published in the journal Separation and Purification Technology. In the latter work, the device included a capacitive counter electrode. This design obviates the need for Faraday reactions to balance regeneration current. Hence, this newer system is a in fully functional so-called inverted capacitive deionization method (iCDI) configuration. iCDI refers to a CDI cell wherein ions are absorbed via chemical groups on a surface and then the cell is regenerated by applying voltage. To achieve this, the counter electrode was functionalized with sulfonate moieties. They demonstrated a nitrate removal iCDI cell with 10x less energy consumption than the Faradaic counter electrode design while maintaining a similar adsorption rate. Oyarzun described the system as follows: “Capacitive deionization has the potential to selectively remove ions with low waste generation and low energy consumption. CDI is not limited to nitrate, and has potential for extraction of rare metals and other contaminants from aqueous solutions”.
The system enables electric-potential-based “regeneration on demand” without the need for chemical treatments of the adsorbent. Palko added that: “A primary limitation for separations from solutions using traditional ion exchange methods is the requirement to supply and dispose of high concentration brines during regeneration. The approach presented in this work eliminates that difficulty, allowing separation of targeted species without generation of problematic wastes. The approach has broad applicability from water purification and softening to resource recovery.”
Palko, J.W., Oyarzun, D.I., Ha, B., Stadermann, M., & Santiago, J.G. (2018). Nitrate removal from water using electrostatic regeneration of functionalized adsorbent. Chemical Engineering Journal, 334, 1289-1296.Go To Chemical Engineering Journal
Oyarzun, D.I., Hemmatifar, A., Palko, J.W., Stadermann, M., & Santiago, J.G., (2018). Adsorption and capacitive regeneration of nitrate using inverted capacitive deionization with surfactant functionalized carbon electrodes. Separation and Purification Technology, 194, 410-415.Go To Separation and Purification Technology