Analysis of Mercury in Wastewater by Microwave Digestion Followed by Inductively Coupled Plasma Mass Spectrometry: Comparison with U.S. Environmental Protection Agency Methods Approved in 40 CFR 136.3

Abstract

As a cost-saving measure, the use of an existing microwave digestion system and inductively coupled plasma mass spectrometry instrument was investigated for the analysis of mercury in domestic wastewater, industry effluent, stormwater, and other aqueous matrices analyzed under the City of Portland's (Oregon) various National Pollutant Discharge Elimination System (NPDES) permits. A formal alternate test procedure application (ATP) study was undertaken to add total mercury to the analyte list for the CEM microwave digestion method (CEM Corporation, 1992) and for U.S. Environmental Protection Agency (Washington, D.C.) (U.S. EPA) method 200.8, both approved for NPDES work in 40 CFR 136.3 (U.S. EPA, 2001a). The ATP was submitted in March 2005, and final approval was received in November 2006. During the study period and while waiting for approval, all NPDES mercury samples were sent to a contract laboratory for analysis by U.S. EPA method 1631E (U.S. EPA, 2002). Splits were analyzed in-house by the ATP protocol. For treatment plant wastewater and industrial effluents, the 1631E results averaged approximately one-half of those obtained by the ATP method, with a 1% difference slightly above 1000% (order of magnitude) and several in the 200% to 300% range. Preliminary results were confirmed by further studies that compared the ATP method with the room temperature and the heated, closed-vessel digestion option of U.S. EPA methods 245.7 and 1631E (U.S. EPA, 2001b, 2002). This research suggests that the room temperature bromine chloride (BrCl) digestion is ill-suited for the determination of total mercury in wastewater and industrial effluents. Because of the probability of committing type II errors--that is, assuming mercury is present in low concentrations when, in fact, it is not--results for these matrices found using the room temperature BrCl digestion should be interpreted with caution. It is suggested that such results might best be considered "bromine chloride extractable" and, as such, should not be used in studies that require accurate estimates of total mercury.