Controlled Re-emission of Mercury in Waste Water Treatment

Abstract

This paper introduces a novel process for the controlled re-emission and immobilization of Mercury (Hg) in waste water treatment plants of wet flue gas desulfurization (FGD). The aim is to create a highly concentrated sink for Hg for further processing. Experiments are carried out with a continuously operated lab-scale wet FGD system and a batch-wise operated waste water treatment unit. By aeration of the liquid phase, the controlled desorption of Hg during the alkalization process of the waste water treatment is enhanced. It is demonstrated, that Hg is emitted in its elemental form (Hg0). Thus, a chemical reduction of Hg2+ takes place prior to its desorption. Linear dependency of Hg0 formation on Sulfite- (SO32-) and Hydroxide-ion (OH-) concentration indicates first order dependency of reaction kinetics. Decreasing concentration of Hg0 in the exhaust air for increasing Cl--concentration is observed. The results show exponential dependence of Hg0 mass transfer on stirring speed and almost constant re-emissions for the aeration flow rates investigated. Thus, the application of low air flux is beneficial in terms of energy demand of the fan and also for the purpose of creating a highly concentrated sink for Hg in the process. The concentration decrease of Hg2+ in the waste water is proportional to the savings in terms of precipitating agent consumption of the further processing steps.