Measurement of Capacity and Breakthrough Curves for Selected Pairs of Pollutants and Adsorbents
The focus of this part of the project was to quantify measurement of pollutant capacity and breakthrough curves for two acids at three different relative humidities with 18 adsorbents.
Selecting the proper adsorbent should be based on matching the adsorbent's capacity with measured or theorized gas species known to be - or strongly suspected to be - present in a given microenvironment. In the event that more than one pollutant is expected, a balanced compromise is needed.
The two most important concepts are breakthrough and capacity. The breakthrough is the point at which a downstream contaminant concentration is measurable at the outlet of the adsorbent and begins to rise rapidly. This identifies the point where the mass transfer zone (MTZ) has passed through the entire adsorbent bed.
The capacity is a measure of the quantity of material adsorbed at the exposure concentration. For chemical adsorbents, the capacity tends to be, but is not always, a fixed quantity. It represents an irreversible process. For physical adsorbents, the amount of materials picked up is reversible and related to the exposure concentration. Physically adsorbed gases and vapors can, and usually do, desorb when the exposure concentration drops or other similar gases exert a displacement force.
In this project, one zeolite demonstrated a good capacity to mitigate organic acids in display cases, but no direct comparison of it with activated carbon or potassium permanganate exists in the conservation literature. A great deal of interest has been generated for zeolites, and some clays, because they are clean, inert, inconspicuous, and can be incorporated into paper and paperboard products. One may be more familiar with clays such as sepiolite, attapulgite, and montmorillionite, as they have been adapted as self-clumping commercial pet litters. This makes them feasible, low-cost, gas-phase adsorbents available any place in the world. The project has clarified the strengths and weaknesses of zeolites and clay materials.
Calcium carbonate was included in this research as a mini-study within the larger framework. As a gas scavenger, calcite is highly inefficient at low and moderate relative humidity. Yet it can be modified to give it a specific surface area similar to that of a true microporous chemisorbent. By increasing calcium carbonate's surface area by a factor of 100, even at low humidity (30 percent RH) acid gas scavenging is effected and strongly correlated to surface area for acetic acid. Formic acid is so much more reactive; the benefits of microporosity are not as clear as for acetic acid.
One of the advantages of using TG/DTA/MS to evaluate adsorbent performance is the ability to gain a sense of the heats of adsorption and relative contribution to overall adsorption contributed by chemisorption versus physisorption processes?and, by changing the carrier gas used in the analysis, to perform active chemical experiments with pollutants, in situ. In this way, it is possible to generate direct evidence for the conversion of nitrogen dioxide, sulfur dioxide, and carboxylic acids to nitrates, sulfates, and carboxylates, which in turn, are indicators of reduced off-gassing. Off-gassing is an issue when a source for these acids declines in output thereby setting up a condition in which the adsorbent may readmit what has been adsorbed. In this situation a lesser long-term benefit would have been accrued. All adsorbents that primarily utilize weaker atomic and molecular forces to physically hold gases will desorb them to some extent when placed in cleaner air or flushed with water vapor. The zeolite, SPZ, however, had better retention than two other typical zeolite structures and all five clay mixtures. Activated carbon retained acetic acid better and its capacity was 2 to 3 times greater. However, activated carbon adsorbs a larger range of pollutant chemical structures while many non-polar, larger molecules are excluded from SPZ.