Flue gas desulfurization (FGD) wastewaters present a considerable analytical challenge for inductively coupled plasma–mass spectrometry (ICP-MS) because of high and variable concentrations of potentially interfering elements. For this reason, the United States (US) Environmental Protection Agency (EPA) commissioned the development of a new procedure, based on methods 200.8 and 1638 with requirements and quality control specific to the matrix. This paper discusses the development and performance of that procedure.
Sulfur emissions from coal combustion have been a focus of concern for some time because of their contribution to the formation of acid rain, accelerated soil acidification, and forest degradation. Air quality regulations established in the United States require SO2 scrubbing for most coal fired plants, with the resulting formation of flue gas desulfurization (FGD) wastewaters. Steam electric power plants are responsible for a significant amount of the toxic pollutants discharged to surface waters by point sources and FGD systems are the source of many of these pollutants. Based on the findings from the Environmental Protection Agency's (EPA) multiyear study of the steam electric power generating industry, a plan to revise the current effluent guidelines is in the works. The revised guidelines will apply to plants "primarily engaged in the generation of electricity for distribution and sale which results primarily from a process utilizing fossil-type fuel (coal, oil, or gas) or nuclear fuel in conjunction with a thermal cycle employing the steam water system as the thermodynamic medium." This includes most large-scale power plants in the United States. This decision is largely driven by the high level of toxic-weighted pollutant discharges from coal-fired power plants and the expectation that these discharges will increase significantly in the next few years as new air pollution controls are installed. The discharge of coal combustion wastewater from coal-fired power plants has caused a wide range of environmental effects to local aquatic life. Coal combustion wastewater also has been linked to human health concerns because of elevated pollutant concentrations in surface and ground water (1). Effluents from these plants, especially coal-fired plants, can contain several hundred to several thousand parts per million of the "matrix" elements: calcium, magnesium, manganese, sodium, boron, chloride, nitrate, and sulfate.
ICP-MS Is the Preferred MethodEnvironmental matrices, which may contain low concentrations of toxic contaminants and high concentrations of interfering matrix elements, have historically presented challenges for sample analysis. Inductively coupled plasma–mass spectrometry (ICP-MS) was developed in the 1980s and has been used increasingly in the environmental field because of its multielemental isotopic capability, high sensitivity, and wide linear dynamic range. Provided that some care is taken during sample preparation and that appropriate calibration strategies are used to circumvent nonspectroscopic interferences, the technique is readily applicable to the analysis of a wide variety of environmental samples (natural waters, soils, rocks, sediments, vegetation, and so forth) (2). Two EPA ICP-MS methods are commonly used for regulatory compliance in environmental testing: Method 200.8 for water and wastewater and Method 6020A for solid waste. Both have recently been updated or are in the process of being updated.
However, analysis of FGD wastewater matrix for low parts-per-billion levels of toxic trace metals (for example, As, Cd, Cr, Cu, Pb, Se, Tl, V, and Zn) by ICP-MS presents a significant challenge because of high levels of dissolved solids and potential interferences from molecular species. FGD wastewater can vary significantly from plant to plant depending on the type and capacity of the boiler and scrubber, the type of FGD process used, and the composition of the coal, limestone, and makeup water. As a result, FGD wastewaters represent one of the most challenging sets of samples for ICP-MS. That is, they are both very high in matrix elements (for example, calcium, magnesium, and chloride), known to cause interferences, and are highly variable.