Scavenger

Description

Any substance that actively absorbs or consumes unwanted impurities and pollutants in a system. For example, vapor phase inhibitors are used to scavenge hydrogen sulfide before it can tarnish silver.

See also Hydrogen sulfide scavenger, Oxygen scavenger and Desiccant.

Table for selected sorbents.

Sorbent	Form	Uses	Drawbacks
Activated carbon (granulated activated carbon, GAC)	Black pellets	Most commonly used; a catchall sorbent that removes hydrocarbons, chlorine, nitrogen oxides, and VOCs. Also removes organic carbonyl pollutants. Used in HVAC systems to filter gaseous pollutants.	Difficult to determine when exhausted and hence when to regenerate (by baking in high-temperature oven) or replace. In some countries, used carbon is classified as a hazardous waste material. In high humidity situations such as rain or fog, water vapor can replace organic compounds adsorbed within the pores of the carbon. These low-risk VOCs are released into the environment, which accounts for the odor frequently associated with rain events in facilities with carbon gas filtration media. Small molecules not as effectively adsorbed as large molecules. Hence, effectiveness increases with molecular weight of pollutant.
Activated carbon and Activated alumina impregnated with Potassium hydroxide	Irregular pellets	Used to remove hydrocarbons, VOCs, and NO2.	Potassium permanganate is caustic. Can only be regenerated as activated carbon without impregnated chemical.
Activated charcoal cloth or carbon cloth (Zorflex®)	Cloth or paper impregnated with activated carbon	Useful in microenvironments with low-level emissions starting with low concentrations of pollutants; eliminates same gases as activated charcoal; Good mitigation treatment for display cases and storage cabinets; Easy to use; Apply between base and object, e.g., under fabric liner	Small reserve of carbon to adsorb pollutants; Cannot expect longevity; Should be replaced frequently; Cannot be regenerated; Fire risk
Activated alumina impregnated with Potassium permanganate (Purafil, Carusorb, etc)	Pink/purple spherical pellets	Highly reactive, strong oxidant. Removes nitrogen monoxide, aldehydes, sulfur oxides, and other odorous gases such as hydrogen sulfide. Also targets amines, mercaptans, and organic acids. Fresh material is purple; spent (exhausted) pellets are brown. This is a useful indicator that the sorbent should be replaced.	Potassium permanganate is caustic. Use precautions when handling and avoid contact with objects. Quickly exhausted. Must be replaced frequently. Water vapor will decrease reactivity. Cannot be regenerated; must dispose of as hazardous waste.
Silica spheres	Porous silica spheres, size approximately 2 microns;	High affinity for polar carbonyls, aldehydes, and aromatic VOCs	A combination of small pore and large pore size silica is needed to captures both VOC as well as sulfur and nitrogen compounds
Zeolites	Form	Uses	Drawbacks
Zinc oxide catalyst G 72-D	cylindrical white pellets	Removes reduced sulfur gases, e.g., hydrogen sulfide. Higher affinity for hydrogen sulfide than either activated carbon or alumina impregnated with potassium permanganate. Easy to use. Place a few pellets in a Petri dish in a microenvironment. Used by the British Museum to protect silver and other susceptible objects.	Cannot be regenerated.
Modified zinc oxide (Miracle Sac)	solid granules in porous bags similar to tea bags	Removes hydrogen sulfide, carbonyl sulfide, and mercaptans, which cause tarnishing.	Cannot be regenerated.
Treated silver cloth – similar to bags sold commercially to protect silver (Pacific Silvercloth®)	Cotton or polyester felted acrylic cloth impregnated with finely ground silver	Acts as a sacrificial surface to rapidly remove reduced sulfides before they can reach silver objects Cloth can be glued to drawers or cabinet walls or ordered as bags to hold silver objects.	Limited lifetime, although can be effective for several decades, depending on the environment where the silver is stored. Cannot be regenerated.
Activated carbon impregnated with Potassium iodide	Irregular pellets	Used to remove hydrogen sulfide and prevent generation of sulfuric acid in microenvironments	Can only be regenerated as activated carbon without impregnated chemical.
Activated carbon impregnated with iron oxide	Irregular pellets	Used to trap and oxidize gaseous pollutants such as hydrogen sulfide.	Elemental sulfur is a byproduct and can react in air to form sulfuric acid, which is damaging. Can only be regenerated as activated carbon without impregnated chemical.

Note: Portions of this table were extracted from: Cecily Grzywacz, "Monitoring for Gaseous Pollutants in Museums", Getty Publications, 2006.

Synonyms and Related Terms

getter; pollutant absorber; vapor phase corrosion inhibitor; activated charcoal; activated carbon; charcoal cloth;

Brand names: Purafilter; Pacific silvercloth; Ageless®; Scavengel [Art Preservation Services]

Resources and Citations

• Cecily Grzywacz, "Monitoring for Gaseous Pollutants in Museums", Getty Publications, 2006.
• AIC Conservation Wiki: Exhibit Technical Notes: Environmental Control
• Exhibit Guidelines Technical Note: Using Pollutant Absorbers Inside an Exhibit Case
• Delaney P, Healy RM, Hanrahan JP, Gibson LT, Wenger JC, Morris MA, Holmes JD. "Porous silica spheres as indoor air pollutant scavengers", J Environ Monit. 2010 Dec; 12(12):2244-51. Link
• Richard S. Lewis, Hawley's Condensed Chemical Dictionary, Van Nostrand Reinhold, New York, 10th ed., 1993
• Pam Hatchfield, Pollutants in the Museum Environment, Archetype Press, London, 2002
• Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
• The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998