an improvement in the method for preventing re - emissions of mercury from a wet flue gas desulfurization system by addition of an additive to the fgd scrubber liquor which interacts in the system scrubber with mercury present in the flue gas to curtail the mercury re - emissions ; the mercury re - emissions are reduced to substantially zero by use of an additive selected from one or more members of the group consisting of a dithiol , a dithiolane , and a thiol having a single thiol group and either an oxygen or a hydroxyl group .