Method and compositions useful for removal of mercury from a flue gas stream with relatively high concentrations of acid gas precursors and/or acid gases. The method includes contacting the flue gas stream with a multi-functional agent, where the multi-functional agent includes a salt having a cation of valency 3 or higher. A composition comprises a (1) sorbent material such as powdered activated carbon which will ultimately capture and sequester oxidized mercury, (2) a multi-functional agent that protects the sorbent core from unfavorable reaction conditions or inhibitors, oxidizes or helps catalyze the oxidation of elemental mercury, and/or amalgamates with elemental mercury to make it easier to capture, among other benefits, and optionally (3) a halogen such as in the form of a halide salt that helps facilitate the oxidation of elemental mercury into its oxidized form.