The Impact of Liquids and Saturated Salt Solutions on Polymer-Coated Fiber Optic Sensors for Distributed Strain and Temperature Measurement.

The application of distributed fiber optic strain and temperature measurement can be utilized to address a multitude of measurement tasks across a diverse range of fields, particularly in the context of structural health monitoring in the domains of building construction, civil engineering, and special foundation engineering. However, a comprehensive understanding of the influences on the measurement method and the sensors is essential to prevent misinterpretations or measurement deviations. In this context, this study investigated the effects of moisture exposure, including various salt solutions and a high pH value, on a distributed strain measurement using Rayleigh backscattering. Three fiber optic sensors with different coating materials and one uncoated fiber were exposed to five different solutions for 24 h. The study revealed significant discrepancies (∼38%) in deformation between the three coating types depending on the surrounding solution. Furthermore, in contrast to the prevailing literature, which predominantly describes swelling effects, a negative deformation (∼-47 µÎµ) was observed in a magnesium chloride solution. The findings of this study indicate that corresponding effects can impact the precision of measurement, potentially leading to misinterpretations. Conversely, these effects could be used to conduct large-scale monitoring of chemical components using distributed fiber optic sensing.

Modeling of a thermo-chromatographic pulse (TCP) as radio-frequency (RF)-induced selective heating effect.

By utilizing the specific influence of water adsorption on the dielectric loss factor and, consequently, selective heating of the zeolite NaY in a radio-frequency (RF) electromagnetic field, a hot zone moving through the packed bed can be realized initiated by water injection. While the effect of water adsorption on RF heating and the phenomenon of the thermo-chromatographic pulse (TCP) itself were described in a previous paper, the present study presents a detailed model for the description and explanation of this effect. It involves the complex dependence of dielectric loss on temperature and moisture content, a diffusion model considering both hopping and Knudsen mechanisms and a power balance for a representative segment of the packed bed. The developed model was successfully applied to adequately describe various experimental situations observed for selective RF heating in a NaY zeolite bed.

Influence of low- and high-frequency electrical heating on biodegrading microorganisms in soil: soil respiration.

The influence of electrical heating on microbiological processes in soil has been studied to evaluate the potential for enhancing biodegradation of pollutants by controlling the temperature. A frequency of 50 Hz (power line frequency) was applied for resistive heating. Dielectric heating was realized using a frequency of 13.56 MHz (radio frequency). Both techniques were compared with conventional heating in a water bath. For experiments in laboratory and full scale, a model soil and a contaminated original soil were used. It was shown that under conditions capable for heating soil to 35 degrees C or even 60 degrees C, soil respiration as a measure for microbial activity was not hindered by electrical heating when temperature and moisture content were comparable with conventional heating. The variations of soil respiration were reversible upon temperature changes. Under certain conditions, periodical fluctuations of microbiological activity were observed. Several possible explanations including chronobiology are discussed without being able to provide an unambiguous interpretation for this effect.

Influence of low- and high-frequency heating on biodegrading microorganisms in soil: microbial degradation.

The influence of low-frequency (50 Hz) resistive and high-frequency (13.56 MHz, radio-frequency) dielectric heating in comparison to conventional heating on the microbial degradation of pollutants in soil was studied. The investigation of the biodegradation of model substances (benzoic acid, acetic acid, glucose, sodium acetate) added to a standard soil showed no significant influence of the electrical heating methods when compared with samples heated to the same temperature in a water bath. Therefore, a hindrance of the microbial degradation could be excluded as it was done for soil respiration in a previous study. This finding is especially relevant for the application of these electrical heating methods for thermally enhanced soil bioremediation as an option for making in situ or ex situ clean-up processes more efficient.

Migrating temperature "thermo-chromatographic" pulses (TCP) initiated by radio-frequency (RF) heating.

In the present study, the astonishing influence of water dosage on a purged dry packed bed of NaY zeolite in the presence of an electric field with a frequency of 13.56 MHz was investigated. The injection of a small amount of water to the inlet of the bed led to pronounced selective heating of the inlet zone by more than 150 K. Thus, water represented a very effective coupling medium for dielectric heating. The selectively heated zone then slowly moved through the whole packed bed and a water pulse finally left the zeolite. This effect correlated with a coupled water and heat flux was called thermo-chromatographic pulse (TCP) emphasizing its analogy to chromatography. The phenomenon could not be performed by using conventional (convective) or microwave heating. It was demonstrated under various conditions and explained by a new model based on own experimental results as well as data from literature. The model will be the objective of a forthcoming publication.

Guest diffusion in interpenetrating networks of micro- and mesopores.

Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.

Ambivalent role of water in thermodesorption of hydrocarbons from contaminated soil.

Thermodesorption studies with soil samples from a former filling station for light crude oil contaminated with mineral oil hydrocarbons (mainly benzene, toluene, ethylbenzene, xylenes, naphthalene, alkylnaphthalenes, and C(10) to C(14) alkanes) have revealed an ambivalent influence of water on desorption rates. Particularly, the influences of soil moisture content, humidity of the purge gas, temperature, and content of soil organic matter (SOM) were studied. At low temperature, purge gas humidity strongly affected the mobility of hydrocarbons in the soil organic matter (SOM) leading to an enhanced release of contaminants at higher moisture contents. Heating resulted in a decrease of thermodesorption when connected with desiccation of soil, in spite of the strong temperature impact on the vapor pressure of contaminants. At high water content of the SOM, the transfer of the pollutant molecules into the gas phase was found to be markedly hindered by the formation of water films or pore-filling by bulk water, both acting as diffusion barriers.

Mobile aliphatic domains in humic substances and their impact on contaminant mobility within the matrix.

Using a novel NMR option, magic angle spinning pulsed field gradient (MAS PFG) NMR, the mobility of aliphatic domains in humic substances in the presence of toluene (about 4.5 wt. %) has been monitored. Results show a strong correlation between the diffusivities of the mobile aliphatic chains and those of the adsorbed toluene molecules in the matrix as a function of temperature. Particularly, a strong influence of structural relaxation of the humic matrix on the diffusivity of toluene is observed. Our findings confirm that the aliphatic domains in humic substances play an important role in the mobility of sorbed contaminants within this matrix. These findings further confirm the potential of MAS PFG NMR method in monitoring diffusion processes in particulate humic substances.

Paramagnetic relaxation enhancement (PRE) as a tool for probing diffusion in environmentally relevant porous media.

The transport diffusivity of the paramagnetic molecule 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was measured by monitoring its influence on the NMR transverse relaxation time (T2) on surrounding water protons - also known as paramagnetic relaxation enhancement (PRE). Due to the nature of the PRE effect, few paramagnetic molecules are able to simultaneously reduce the T2 of many NMR active nuclei, which represents a significant gain in sensitivity. In an aqueous solution, the minimal detectable TEMPO concentration was around 70 ppm. The value of the diffusivity was estimated by fitting the relaxation data, collected as a function of time, with the appropriate solutions of the second Fick's law in respect to the corresponding sample geometry and dimensions. Considering the experimentally determined TEMPO relaxivity in water ("TEMPO-water relaxivity"; R(TEMPO) = (1.05 ± 0.12) × 10⁻³ ppm⁻¹ s⁻¹), the obtained diffusion coefficients (D) of TEMPO in homogeneous solution and in a water saturated sand column (D(bulk) = (6.7 ± 0.4) × 10⁻¹° m² s⁻¹ and D(sand) = (1.4 ± 0.5) × 10⁻¹° m² s⁻¹, respectively) are in good agreement with the expected values (literature values: D(bulk) = 6.6 × 10⁻¹° m² s⁻¹, 1.3 × 10⁻¹° m² s⁻¹ < D(sand) < 2.3 × 10⁻¹° m² s⁻¹). This new approach enables one to determine the diffusivity of paramagnetic molecules in homogeneous (aqueous solution) and porous media with basic NMR equipment, at low concentrations and in a noninvasive manner.

Influence of moisture content and temperature on the dielectric permittivity of zeolite NaY.

The influence of moisture content and temperature on the dielectric permittivity (complex resistance) of the zeolite NaY was investigated for a fixed radio frequency (RF) of 13.56 MHz. Sealed glass tubes containing zeolite with defined moisture contents were simultaneously heated in a homogeneous high-frequency electromagnetic field. The dielectric loss factor, i.e. the imaginary part of the permittivity ε(r)″, was calculated from the obtained heating rates. On the basis of the resulting values for various moisture contents and temperatures and utilizing the knowledge of elementary cation hopping processes occurring at low and high frequencies (LF and HF) from the literature, a new model was introduced for the description of dielectric radio-frequency heating of moist zeolites. Since adsorption of water is correlated with an enhancement of the activation energy of the cations on SII sites, cations in the zeolite NaY are moving from SII sites to unoccupied SIII sites when the water content is increasing. Thus, four different transfer processes for the cations have to be considered in total. On the basis of these assumptions, the resulting dielectric loss factor ε(r)″ as a function of water content and temperature for a fixed frequency of 13.56 MHz was modelled. The experimental data are in good agreement with the values obtained from the model. Especially, the measured pronounced maximum of dielectric loss at temperatures below 300 °C and water contents below 4 wt.-% can be explained by the new model.

Influence of in situ steam formation by radio frequency heating on thermodesorption of hydrocarbons from contaminated soil.

Thermal desorption of a wide spectrum of organic contaminants, initiated by radio frequency (RF) heating, was studied at laboratory and pilot-plant scales for an artificially contaminated soil and for an originally contaminated soil from an industrial site. Up to 100 °C, moderate desorption rates were observed for light aromatics such as toluene, chlorobenzene, and ethylbenzene. Desorption of the less volatile contaminants was greatly enhanced above 100 °C, when fast evaporation of soil-water produced steam for hydrocarbon stripping (steam-distillation, desorption rates increased by more than 1 order of magnitude). For hydrocarbons with low water solubility (e.g., aliphatic hydrocarbons), the temperature increase above 100 °C after desiccation of soil again led to a significant increase of the removal rates, thus showing the impact of hydrocarbon partial pressure. RF heating was shown to be an appropriate option for thermally enhanced soil vapor extraction, leading to efficient cleaning of contaminated soils.

Influence of salt impregnation on the initiation of thermo-chromatographic pulses by dielectric heating.

Selective dielectric heating using the influence of a coupling medium such as water can lead to the formation of combined temperature-mass transport pulses moving through a packed bed. Depending on the dielectric and sorption properties of the material, these so-called thermo-chromatographic pulses (TCP) can cause large temperature differences between the starting temperature and the peak temperature in the pulse. The impregnation of porous substances with various salts was shown to be a simple method for improving material properties leading to the formation of stable TCP. This procedure was proven to only slightly reduce the specific surface area and the sorption capacity towards organic compounds of the investigated sorbents. Consequently, a wide spectrum of silica gels, zeolites and porous alumina can be applied as matrices for TCP-based processes in chemical engineering and environmental technology.

Selectivity of dielectric heating: temperature-programmed desorption (TPD) experiments and initiation of thermo-chromatographic pulses.

The occurrence and extent of selective dielectric heating with microwaves (MW) and radio waves (RW) was studied with a variety of model systems using temperature-programmed desorption (TPD). Over a wide pressure and temperature range, selectivity effects were neither found for polar adsorbates (compared to non-polar compounds) nor expressed by an overheating of metal clusters supported on a nearly MW- and RW-transparent support. In contrast, significant temperature gradients between particles consisting of materials with various dielectric losses could be established under certain conditions. The utilization of adsorbates significantly modifying the dielectric properties of a material was investigated as a further approach to initiate selective dielectric heating. Applying water as a coupling medium, a combined heat and mass transport, which we call a thermo-chromatographic pulse, can be created in a packed-bed column consisting of various zeolites. In this case, selective heating of bed zones by more than 100 K was observed. The suitability of a material for the creation of thermo-chromatographic pulses depends on its dielectric properties as well as on its sorption properties with respect to water or other coupling media. The study provided further insight into not only the potential but also the limitations of selective dielectric heating of solid materials relevant to chemical engineering, environmental technology and sorption processing.

Isotope fractionation in phase-transfer processes under thermodynamic and kinetic control - Implications for diffusive fractionation in aqueous solution.

Diffusive isotope fractionation of organic compounds in aqueous solution was investigated by means of liquid-liquid and liquid-gas partitioning experiments under kinetic control. The two-film model was used to describe phase-transfer kinetics. It assumes the diffusion of solutes across a stagnant water boundary layer as the rate-controlling step. For all investigated solutes (benzene-D0 and -D6, toluene-D0, -D5, and -D8, cyclohexane-D0 and -D12), there was no significant observable fractionation effect between nondeuterated and perdeuterated isotopologues, resulting in a ratio of diffusion coefficients Dlight: Dheavy=1.00±0.01. In addition, isotope fractionation due to equilibrium partitioning of solutes between water and n-octane or gas phase was measured. The deuterated compounds are more hydrophilic than their light isotopologues in all cases, giving rise to fractionation coefficients αHpart=Koctane/water,H: Koctane/water,D=1.085 to 1.15. Thus, thermodynamic fractionation effects are much larger than diffusion fractionation effects. Methodical and environmental implications of these findings are discussed.

New option for characterizing the mobility of organic compounds in humic acids.

A new NMR option for monitoring the mobility of organic contaminants in SOM in the solid state has been successfully applied for the first time. This recently available noninvasive technique, magic angle spinning pulsed-field gradient (MAS PFG) NMR, combines both NMR spectroscopy and diffusometry to selectively monitor the diffusion of compounds sorbed in porous media or polymer matrices. Using this technique, the diffusion of toluene in humic acid particles has been studied. Measurements were performed under varying temperatures from 25 to 80 degrees C. The obtained diffusion coefficients were found to be in good agreement with those obtained from computer simulations reported elsewhere. Our results show a strong influence of the interaction of toluene with humic acid on its diffusion in the matrix even at elevated temperatures of up to 80 degrees C. The Arrhenius plot of the diffusivities shows a decrease in the activation energy of diffusion above 50 degrees C by a factor of 3. This change of activation energy is attributed to a structural change in the humic acid matrix that influences the mobility of toluene.