scavenger - chemical additive that removes or inactivates impurities or undesired materials in a mixture or process. (1) When tetraethyl or tetramethyl lead are added to gasoline as anti-knock additives, lead scavengers must be used to ensure that lead oxide is not deposited in combustion chambers. Mixtures of dibromoethane and dichloroethane are added to the lead alkyls in order to produce volatile lead halides which are removed with the exhaust gases.