Perchlorate salts are important chemicals for national defense. Large-scale production of rocket propellant, specifically ammonium perchlorate, began in the mid-1940s. The disposal and demilitarization of solid rocket motors from large propulsion systems has been a major task for the Department of Defense since the end of the Cold War. Additionally, perchlorate salts have been used in a wide variety of non-military applications such as stick matches, highway safety flares, fireworks, and other pyrotechnics. As a result, detection of perchlorate in the environment has become quite prevalent. Currently, there are two primary technologies for the treatment of large volumes of water containing perchlorate: ion exchange, biological processes, or a combination of the two. Other emerging technologies, such as membrane separation and electrochemical processes, are in developmental stages.
The overall objective of this project was to develop catalytic gas membrane systems for the removal of perchlorate from waters using hydrogen as the reducing agent. Specific objectives included: (1) prepare and characterize metallic catalysts; (2) study the effect of particle size, catalyst supports, and crystallinity of catalysts on the hydrogen reduction of perchlorate; (3) prepare catalytic membranes; and (4) assess the mid-term performance of catalytic gas membranes for the removal of perchlorate from waters.
Three reaction systems were investigated: dual-membrane, mono-membrane, and indirect electrochemical Ti-TiO2 reactors. The effect of major anions (nitrate, sulfate, bicarbonate, and chloride) and cations (calcium and magnesium) on perchlorate reduction was also investigated.
Of the three reaction systems tested, the catalytic dual membrane and the indirect electrochemical methods were found to be the most promising. For water containing perchlorate at ultra-low concentrations (i.e., ppb), the indirect electrochemical Ti-TiO2 method was best suited; it can remove perchlorate from water at a concentration of 245 ppb to less than 24.5 ppb in a reaction time of 6 to 8 hours under ambient conditions. The dual-membrane system was able to achieve a residual perchlorate concentration of around 100 ppb from an initial perchlorate concentration of 2 ppm. The catalytic dual-membrane process is most efficient in dealing with water containing perchlorate in the range of greater than 2-10 ppm.
In general, the effect of major common anions was insignificant if perchlorate was present at concentrations comparable to those of the anions. That is, if perchlorate is present at a concentration comparable to that of the anions, there is insignificant competition between the perchlorate and the anions. When perchlorate was present at concentrations much lower than that of the anions (e.g., 2 to 3 orders of magnitude less than that of the anions), there was competition by anions such as nitrate and sulfate. Bicarbonate did not exhibit any competition effect on perchlorate reduction, nor did the major cations such as Ca2+ and Mg2+. Chloride affected perchlorate reduction only when present in excessive concentrations. The presence of a small amount of chloride relative to perchlorate can benefit perchlorate reduction by providing sufficient current while lowering the voltage applied, which in turn minimizes the oxidation of chloride to perchlorate. Major cations such as calcium and magnesium had no effect on perchlorate reduction regardless of the concentration of perchlorate. Results also indicate that nitrate and sulfate do not inhibit perchlorate reduction; rather, they compete. Once reduced to concentrations comparable to that of perchlorate, perchlorate reduction took place readily.
An independent economic analysis of the technology was conducted based on the results to date, assuming the process would be operated on a semi-batch basis with a 2-hour turnover time. The scale of the reactor is assumed to be capable of processing 1,000 gallons of water per batch. The total treatment time per batch is assumed to be either 6 or 8 hours. With these assumptions, the total unit costs estimated for the catalytic dual-membrane system are $2.66 and $3.32 per 1,000 gallons for treatment times of 6 and 8 hours, respectively, when the constant current applied is 5 mA. The total unit cost is $4.26 and $5.33 per 1,000 gallons for treatment of 6 and 8 hours if the constant current applied is 10 mA. For the indirect electrochemical (Ti-TiO2) method, the total unit cost will be $7.36 and $9.04 per 1,000 gallons for treatment times of 6 and 8 hours, respectively, when a constant current of 20 mA is applied. The total unit costs will be $16.96 and $21.20 for treatment times of 6 and 8 hours, respectively, when a constant current of 50 mA is used. The cost of electricity dominates the total unit cost of both processes. These projected treatment costs are not favorable when compared against established perchlorate treatment technologies. Commercial ion exchange technologies for perchlorate removal from drinking water have treatment costs of less than $1 per 1,000 gallons. (Project Completed - 2007)