Effect of relative humidity on the adsorption of selected water-miscible organic vapors by activated carbon.

The adsorptive capacity of activated charcoal was determined experimentally for the vapors of 2-ethoxyethanol, pyridine, acetic acid, and piperidine from dry air and from air saturated with water vapor. Vapor concentrations ranged from 100 mg/m3 to at least 1000 mg/m3; the temperature was kept constant at 25 degrees C. The reduction in the adsorptive capacity of the activated charcoal by the relative humidity over the entire range of experimental conditions was accounted for by the Hansen-Fackler modification of the Dubinin-Radushkevich equation. This procedure allows the use of the activity coefficients, which are basic thermodynamic factors often available in the literature, to estimate the effect of adsorbed moisture on the adsorption of these organic compounds from a humidified atmosphere.

The adsorption of argon, krypton, and xenon on activated charcoal.

Charcoal adsorption beds are commonly used to remove radioactive noble gases from contaminated gas streams. The design of such beds requires the adsorption coefficient for the noble gas. Here an extension of the Dubinin-Radushkevich theory of adsorption is developed to correlate the effects of temperature, pressure, concentration, and carrier gas on the adsorption coefficients of krypton, xenon, and argon on activated carbon. This model is validated with previously published adsorption measurements. It accurately predicts the equilibrium adsorption coefficient at any temperature and pressure if the potential energies of adsorption, the micropore volume, and the van der Waals constants of the gases are known.

Correlation of mass transfer rates for a diffusive sampler with air speed and incidence angle.

An accurate measurement of a gas concentration in air by diffusive sampling requires knowing the sampling rate. Both the boundary layer between turbulent ambient air and the sampler and the stagnant air layer inside the sampler impose resistance to the transport of analyte into the sampler. As the boundary layer mass transfer resistance is a function of the air speed and direction of the air movement, the sampling rate also depends on these variables. By the procedure developed here, the boundary layer mass transfer resistance was accurately measured as a function of wind speed and direction, and from these data a basic correlation with dimensionless parameters describing mass transfer was obtained. Deviation of air incidence angle and speed during sampling from the calibration conditions may produce a small positive bias, probably not in excess of 10%. Random variation of incidence angle and air speed while the sampler is in use may also contribute to the variability of this sampling method.

Basic theory for the diffusive sampling of radon.

From the closed-form solution of the Fickian equation describing the uptake of radon by a diffusive sampler, the following are calculated: 1) the optimal estimate of the time-weighted average radon concentration; 2) the effect of the geometry of the diffusive sampler on performance; 3) the maximum sampling time consistent with a predetermined maximum error in the estimated time-weighted average concentration of radon; and 4) the effects of temperature and pressure on the performance of the sampler. It is shown that the maximum sampling error can be greatly reduced by dividing the adsorbent bed into two layers placed in series and by using a weighted average of the uptakes on the two layers.

Convective transport in diffusive samplers.

Convective transport in diffusive samplers was determined by the loss of a dilute dye solution from these samplers while held in a water bath. The water flow in the bath was adjusted to give the same Reynolds number had the diffusive sampler been exposed to an airborne analyte at a predetermined flow velocity. By numerical analysis, estimates were made of the degree of interference of convection on sampler performance. The results indicate an enormous difference between commercial diffusive samplers with respect to the effect of convection on the transport of analyte into the sampler.

Optimal design of diffusive samplers.

Some commercially available diffusive samplers use two layers of adsorbent placed in series. After sampling is completed, the time weighted average concentration of analyte is estimated from the weighted sum of the uptake of analyte on these two layers. It is known that such a division into layers can increase the permissible sampling time. Here the principles underlying this sampling procedure are analyzed through a fundamental application of the theory of diffusion. Using a trial and error procedure, the optimal division of adsorbent was calculated, and the increase in sampling time that such a division can give was confirmed theoretically. Also, should the uptake in the backup layer exceed a predetermined fraction of the total uptake, this will indicate misuse of the diffusive sampler.

Error bounds in diffusive sampling with reversible adsorption.

Diffusive samplers are commonly used in the work place for compliance monitoring of gases and vapors. Because the workplace concentrations of analyte are not constant, the usual procedure of calibration of a diffusive monitor by exposure to a known constant concentration of analyte may lead to an unacceptable error. By considering the maximum possible error in the time weighted average (TWA) concentration, with no restrictions regarding the concentration as a function of time, a permissible sampling time is defined that is consistent with any possible concentration fluctuation.

Calculation of the performance of activated carbon at high relative humidities.

The Dubinin-Radushkevich potential theory was extended to include a term giving the effect of relative humidity on the uptake of adsorbate. This extended equation permits the adsorptive capacity of the activated charcoal in a respirator cartridge to be estimated for any combination of temperature, relative humidity, and concentration of contaminant. Application of this theory to previously published data of Werner showed a good correlation between theory and experiment. This equation is consistent with the experimental observations that 1) below a certain value, the relative humidity has little effect on the uptake of adsorbate, and 2) the effect of relative humidity, if observed, is more severe for lower than for higher concentrations of contaminant.

Efficiency of passive sampling by adsorbents.

The efficiency of passive samplers can be influenced strongly by the adsorption isotherm. In this study calculations of sampling efficiency were made in terms of dimensionless variables for adsorption controlled by the Langmuir, Freundlich, and Dubinin-Radushkevich isotherms, respectively. Of particular importance is that for the latter isotherm, which is often followed by activated charcoal for the adsorption of solvent vapors, passive sampling may remain highly efficient until a significant fraction of the capacity of the adsorbent has been utilized.

A critique of the U.S. standard for industrial exposure to sodium hydroxide aerosols.

Published studies of the toxicity of sodium hydroxide aerosols are few. These studies were generally marred by inadequate characterization of particle size and chemical compositon and of the ambient humidity. Because NaOH aerosols can readily undergo reaction with carbon dioxide to form sodium carbonate, a much less alkaline (and less hygroscopic) compound, these shortcomings may warrant a reconsideration of the NaOH standard and the consideration of a Na2CO3 standard.

Respiratory response of guinea pigs to ozone alone and with sulfur dioxide.

This study was designed to utilize our guinea pig bioassay method for irritant response to address the question of whether or not ozone and sulfur dioxide appeared to react to form sulfuric acid in the respiratory tract. Animals were exposed to 0.2, 0.4, and 0.8 ppm of each gas alone and to the combination at concentrations of 0.4 and 0.8 ppm. In these experiments sulfur dioxide alone produced no statistically significant alterations in respiration. All concentrations of ozone produced an increase in respiratory frequency. At the two higher concentrations the increase in frequency was accompanied by a decrease in compliance. The response to the combinations was the same as the response to those levels of ozone alone. No sulfuric acid was detected in the chamber atmosphere. The biological data suggest that none was formed in the lung.