From Ground Water Currents, April 1995, Issue No. 11

Ultrasound Examined For In Situ Monitoring

By William H. Engelmann, EPA Environmental Monitoring Systems Laboratory, Las Vegas

Ultrasound is a new concept for field screening applicable to in situ ground water monitoring. EPA's Environmental Monitoring Systems Laboratory, Las Vegas (EMSL-LV) has been examining the potential of combining sonication (i.e., ultrasound) with existing measurement technologies for monitoring specific classes of organic pollutants in water. The research to date has addressed using ultrasound processors to decompose aqueous organochlorine compounds into ions as a method to screen organochlorine pollutants in water. The research demonstrated that sonication could produce anions specific to the inorganic components and that changes in ion concentrations before and after sonication could be used to monitor these pollutants. Success to date with compounds such as trichloroethylene (C2HCl3), chloroform (CHCl3) and carbon tetrachloride (CCl4) serve as proof-of-principle and form a rationale for expanding the research to other pollutant classes.

In the research design, the above compounds were tested in the range of 3-40 parts per million. The equipment used was anultrasonic system with either a cup-horn or a 1/2 inch diameter horn probe; commercially available probes such as ion selective electrodes (ISEs), conductivity cells and pH electrodes. The following parameters were investigated: sonication times (1-90 minutes); continuous vs. pulsed ultrasonic; sample temperature (constant 30 degrees Celsius); sample volumes (8-15 milliliters); and water sources (deionized, tap, well). The research on sonochemistry of organochlorine compounds in water gave much support for using sonication in combination with changes in chloride ion, conductivity and/or pH.

Common denominator in the aqueous sonochemistry is HCl, as it was the major ionic product. However, the mechanism and rate of the reaction may differ markedly depending on the conditions under which the sonication is performed.

Sufficient chloride ion was formed under the sonication conditions used to allow measurement using a commercially available chloride ISE. It was apparent that 5 minutes sonication with the cup horn at 60% pulse mode or one minute sonication with the 1/2 inch horn probe resulted in close to 3% or higher yields of chloride ion. This was sufficient to achieve detection with the commercial chloride ISE for 37-40 ppm of C2HCl3, CHCl3 and CCl4. Lower concentrations of these compounds should be detectable by increasing the chloride-ion yield.

It is believed that pH may be useful in driving the reaction toward HCl as the final product. Results from the present research confirmed the pH decreases. It also appears from the work that the sonolysis of organochlorine compounds was inhibited at higher pHs. Bicarbonate and carbonate may act as hydroxyl radical scavengers, thus inhibiting the organochlorine compound decomposition. However, more research is needed on real-world samples to better understand the implications of pH for monitoring methods development using ultrasound.

Overall, the potential of combining sonication with commercially available measurement technologies for monitoring specific pollutants in water is judged to be high. The approach in using sonication is applicable to other organic compounds, halides, phosphorus, nitrogen and sulfur.

For more information, call the principal researchers, Edward J.Poziomek (phone: 804-683-5643; FAX: 804-683-4628) and Grazyna E.Orzechowska (phone: 804-683-4105; FAX: 804-683-4628). A report on the research, "Potential Use of Ultrasound in Chemical Monitoring," (Order No. PB94-188190; cost: $17.50, subject to change) can be ordered only from National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161 (telephone:703-487-4650). The EPA Project Officer is Bill Engelmann at EMSL-LV at 702-798-2664 by phone or 702-798-2107 by FAX.