Timothy Mark Sivavec

Mineral precipitation and porosity losses in granular iron columns.

As permeable reactive barriers containing zero-valent iron are becoming more widely used to remediate contaminated groundwaters, there remains much uncertainty in predicting their long-term performance. This study focuses on two factors affecting performance and lifetime of the granular iron media: plugging at the treatment zone entrance and precipitation in the bulk iron media. Plugging at the system entrance is due principally to mineral precipitation promoted by dissolved oxygen in the influent groundwater and is an issue in aerobic aquifers or in above-ground canister tests. Designs to minimize plugging in field applications where the groundwater is oxygenated include the use of larger iron particles and admixing sand of comparable size with the iron particles. Beyond the entrance zone, the groundwater in anaerobic and mineral precipitation leads to porosity losses in the bulk iron media, potentially reducing flow through the treatment zone. The nature of the mineral precipitation and the factors that affect extent of mineral precipitation have been examined by a variety of tools, including tracer tests, aqueous inorganic profiles, and surface analytical techniques. At short treatment times, porosity losses as measured by tracer tests are due mainly to Fe(OH)(2) precipitates and possible entrapment of a film of hydrogen gas on the iron surfaces. Over longer treatment times, precipitation of Fe(OH)(2) and FeCO(3) in low carbonate waters and of Fe(OH)(2), FeCO(3) and CaCO(3) in higher carbonate waters begin to dominate porosity losses. The control of pH within the iron media by addition of ferrous sulfide was shown not to reduce significantly calcium and carbonate precipitates, indicating that mineral precipitation is controlled by more than simple carbonate equilibrium considerations.

Development of radio-frequency identification sensors based on organic electronic sensing materials for selective detection of toxic vapors

Selective vapor sensors are demonstrated that involve the combination of (1) organic electronic sensing materials with diverse response mechanisms to different vapors and (2) passive 13.56 MHz radio-frequency identification (RFID) sensors with multivariable signal transduction. Intrinsically conducting polymers such as poly(3,4-ethylenedioxythiophene) and polyaniline (PANI) were applied onto resonant antennas of RFID sensors. These sensing materials are attractive to facilitate the critical evaluation of our sensing concept because they exhibit only partial vapor selectivity and have well understood diverse vapor response mechanisms. The impedance spectra Z(f) of the RFID antennas were inductively acquired followed by spectral processing of their real Zre(f) and imaginary Zim(f) parts using principal components analysis. The typical measured 1σ noise levels in frequency and impedance magnitude measurements were 60 Hz and 0.025 Ω, respectively. These low noise levels and the high sensitivity of the resona...

Field evaluation of acoustic-wave chemical sensors for monitoring of organic solvents in groundwater

An instrument for in-situ monitoring of volatile organics in groundwater at pat-per-billion levels has been developed and field tested. The device is an acoustic wave thickness-shear mode sensor based on a 10-MHa AT-cut quartz resonator coated with a non-polar polymer film. The sensor demonstrates a detection limit of 8 and 12 parts per billion in water for trichloroethylene and toluene, respectively, and a rapid reversible response. This low detection limit is achieved by carefully minimizing the noise level in the electronic detection system, by optimizing the thickness of the sensing polymer film, and by performing measurements in the headspace. Preliminary field test demonstrated good correlation of sensor response with conventional laboratory purge-and-trap/GC analysis.

Recognition and quantitation of closely related chlorinated organic vapors with acoustic-wave chemical sensor arrays

An array of four acoustic wave chemical sensors has been developed and tested for recognition and quantitation of six closely related chlorinated organic vapors at low part-per- million concentrations. These vapors include PCE, TCE, VC, and DCE, cis-1,2-DCE, trans-1,2-DCE, and 1,1-DCE. The developed sensor array can detect as little as 0.2 ppm of PCE, 0.8 ppm of TCE, 1.4 ppm of cis-1,2-DCE, 1.4 ppm of trans-1,2-DCE, 3 ppm of 1,1-DCE, and 3.5 ppm of VC in air. Quantitation of TCE and cis-1,2-DCE vapor mixtures was achieved using multivariate calibration techniques. The locally weighted regression analysis provides more accurate results over the partial least squares regression.