MEC 2014

ORAL - Overcoming the Challenges of Mercury, Arsenic and Selenium Determination in FGD Wastewater

Authors: Dr Warren T Corns (PSA)

In June 2013, the EPA proposed a new rule (40 CFR, part 423) in an attempt to reduce the amount of toxic metals and other pollutants discharged to surface water from power plants. Previous regulations do not consider the additional burden of water discharge pollutants from air pollution control systems such as flue gas de-sulfurization, selective catalytic reduction and mercury control. Pollutants have essentially been shifted from flue gas to wastewater and this has created an urgent need for abatement and new measurement technologies for wastewater. The main pollutants of concern include metals (mercury, arsenic and selenium), nitrogen and total dissolved solids (TDS). Selenium is currently a major issue as current water treatment technologies exhibit poor removal efficiencies due to complex chemical forms present. In this paper we present newly developed online and offline measurement systems for Hg, As and Se for power plant wastewater discharges. The instrument is based on vapour/hydride generation with atomic fluorescence spectrometry (AFS). The coupling of this approach to ion chromatography will also be described for Arsenic and Selenium speciation in FGD wastewater samples. The current and proposed method of analysis for these elements is ICP-MS equipped with a collision cell to overcome isobaric interferences. The ICP-MS approach has numerous limitations and does not offer the method detection limits because large sample dilutions are required to overcome the issues due to total dissolved solids found in this sample type. The AFS system however overcomes these issues providing detection limits in the part per trillion range with linearity over 7 orders of magnitude. Waste water samples are treated online using a UV photolysis system which provides a determination for total metal content. The system offers an online measurement solution to ensure that abatement systems are effectively achieving permitted levels. Data from recent studies will be presented.