Grantee Research Project Results
Final Report: Removal and Disposal of Perchlorate From Drinking Water by Novel Capacitive Deionization
EPA Contract Number: EPD04040Title: Removal and Disposal of Perchlorate From Drinking Water by Novel Capacitive Deionization
Investigators: Jaffe, Stephen M.
Small Business: Material Methods LLC
EPA Contact: Richards, April
Phase: I
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: Drinking Water , SBIR - Water and Wastewater , Small Business Innovation Research (SBIR)
Description:
Low concentrations of perchlorate ions existing in drinking water, particularly in the western regions of the United States, have caused great concerns for public health because of their toxicity. The leading treatments for perchlorate removal are ion exchange with catalytic destruction and bioreactors. These treatments require significant labor and material inputs. The associated operating costs prohibit scaling to small systems (less than 25 people or 15 service connections). Even in large systems, the leading treatments are expensive. Treatment costs range from $100 to $200 per acre-foot, or 20 to 40 percent of wholesale water costs. The American Water Works Association Research Foundation has investigated an effective alternative, electrochemical reduction of perchlorate. The findings of that study showed that perchlorate could be electrochemically converted into water and harmless chloride ions. The conversion rates, however, were generally very small, even in highly conductive aqueous solutions using a variety of catalytic electrodes. The study concluded that a practical device would require that the number of reactive sites on the electrode surface relative to the solution volume be increased by several orders of magnitude.
Over the past several years, Material Methods, LLC, has focused on water purification with the flow through capacitor (FTC). High surface area, nanostructured electrodes, and thin layer reactor designs have been developed. This Phase I research project was originally aimed to employ Material Methods’ nanostructured carbon electrodes in an FTC device as a platform to electrochemically decompose and remove perchlorate ions from contaminated drinking water. The goal of this research project was the in situ elimination of perchlorate in groundwater without adding any chemicals. Investigations were conducted using virgin and modified carbon electrodes of different compositions together with the incorporation of various electrocatalyts. Material Methods’ was able to nonselectively concentrate perchlorate. Unfortunately, it was found that the perchlorate ion, an extremely stable species, could not be decomposed by means of electrochemical reduction at a practical rate in tap water.
The research project goal then was refocused to the selective, chemical-free purification of perchlorate from groundwater. A purified output goes to the user; the concentrated waste stream can subsequently be eliminated by a biological or chemical reactor. The user does not depend on a biological reactor for purification. Material Methods’ technical approach still utilized nanostructured carbon electrodes in a heterogeneous reactor, but capacitive deionization was abandoned for electrochemically modulated ion exchange. No salt addition is required because the perchlorate exchanges with ambient chloride ions. The challenges included selective concentration of trace amounts of perchlorate in a high, ambient concentration of interfering ions; chemical-free operation; and maintenance-free operation.
Material Methods successfully synthesized an ion exchange compound and coated the proprietary carbon electrodes. Fundamental investigations on the coated carbon electrodes have been carried out using cyclic voltammetry and immersion tests. The selectivity coefficients between the perchlorate ion and the chloride ion, as well as the capacity of the modified carbon electrode with respect to perchlorate ion removal, was estimated at various concentrations. From immersion tests, the surface-modified carbon electrodes were found to exhibit excellent capability to remove low levels of perchlorate ion from tap water, even with a 1,000 times higher background concentration of chloride. The electrochemical regeneration of the carbon electrodes in tap water was conducted without adding any salts. Finally, the size of a practical device suitable for a small drinking water treatment system was calculated and estimated.
Summary/Accomplishments (Outputs/Outcomes):
Important findings obtained from this Phase I research project include the following:
- Capacitive deionization nonselectively concentrates perchlorate. The perchlorate ion is an extremely stable species, and no indication of electrochemical decomposition was found in tap water environments using the FTC with various modified carbon electrodes and electrocatalysts.
- An ion exchange compound was prepared in its reduced and oxidized form and coated on conductive carbon electrodes, with the oxidized form exhibiting the capability to selectively remove perchlorate ions from water.
- The selectivity coefficients (KClOs4/Cl) were calculated to be about 2,000 to 6,000. The higher the concentration of chloride ion, the more difficult it is to remove perchlorate.
- Treated (in oxidized form) carbon electrodes were found to exhibit selective removal of perchlorate from 22 ppb down to 13 ppb at a concentration ratio of 4,000 for [Cl-]/[ClO4-] by immersion test.
- Treated carbon electrodes can be electrochemically regenerated in tap water environments for reuse without adding any chemicals.
- The carbon usage rate for a practical device is estimated to be 50.0 kg carbon/day for the treatment of 10,000 gallons/day of drinking water by reducing 50 ppb of perchlorate down to 4 ppb. The packing volume of the device is calculated to be 0.76 m3 and the contact time is about 29 minutes using Material Methods’ carbon electrodes.
Conclusions:
Based on these findings, the technical feasibility of using ion exchange-treated carbon electrodes for the effective removal of perchlorate from tap water has been successfully proved in Phase I. Specifically, the ion exchange-modified carbon electrode has demonstrated that it efficiently functions at ppb levels against a ppm-level background of ambient ions and that it can be readily regenerated in tap water for continuous operation without introducing any chemicals into the water system. Therefore, the modified carbon electrodes have great potential to be fabricated into an effective drinking water treatment device for removal of perchlorate. Key issues concerning the engineering aspects on the design and fabrication of such a drinking water treatment device will be solved in Phase II.
The new device will be particularly suitable for use as a polishing unit to remove low levels of perchlorate in small drinking water systems at less than the 4 ppb level. The advantages of electrochemically regenerated ion exchange include:
- High purification less than 4 ppb perchlorate.
- Dependable, physical adsorption-based purification. Purification is independent of destruction.
- Low maintenance, suitable for small systems.
- Excellent perchlorate selectivity in groundwater environments.
- Chemical-free, electrochemical regeneration.
- High recovery. Efficient water use and high concentration ratio.
- Compatible with biological and chemical destruction systems. The low volume wastewater stream has neutral pH and low salinity.
- Life cycle costs are low.
Thus, it will have considerable commercial use potentials as an efficient, cost-effective, and environmentally benign device for the removal of perchlorate from drinking water in small systems. It also can be developed into point of entry drinking water purification devices for home and commercial applications. Journal papers and a U.S. patent application are in preparation.
Supplemental Keywords:
drinking water purification, capacitive deionization, perchlorate removal, perchlorate ion, nanostructured carbon electrodes, surface modification, ion exchange, electrochemical modulation, groundwater, tap water, ion exchange, SBIR,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Water, POLLUTANTS/TOXICS, Environmental Chemistry, Arsenic, Water Pollutants, Engineering, Chemistry, & Physics, Drinking Water, Environmental Engineering, electrochemical technology, alternative disinfection methods, monitoring, nanotechnology, perchlorate, capacitive deionization, groundwater remediation, electrochemical reduction, groundwater contamination, perchlorate removal, drinking water contaminants, drinking water treatmentThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.