Facilities & Laboratories
Sulfide Analysis Laboratory
Acid Volatile Sulfide (AVS) and Simultaneously Extracted Metals (SEM) in Sediments and Dissolved Sulfide in Aqueous Samples by a Colorimetric Method
Introduction
Determination of acid volatile sulfide (AVS) and simultaneously extracted metals (SEM) in sediments is a common practice to assess the bioavailability of metals to biota. Many metals interact strongly with dissolved sulfide in reducing environments to form highly insoluble sulfide minerals. Sequestering the metal in sediments in this form can significantly reduce the availability of the metal to biota. When the total molar abundance of metals in sediments which form strong sulfide precipitates (i.e. SEM) exceeds that of the reduced sulfide present (as determined by an AVS measurement), the metals are potentially bioavailable.
Overview of ProcedureThe Marine Sciences Laboratory conducts AVS measurements based on the procedure described by Allen et al. 1991 (Draft EPA Method EPA-821-R-91-100) and Lasorsa and Casas (1996). For sediment and aqueous samples, sulfide is first volatilized after the addition of acid by converting it to gaseous hydrogen sulfide (H2S) at room temperature. The H2S is purged from the sample by an inert gas and trapped in a sodium hydroxide (NaOH) solution. The abundance of reduced sulfur is quantified using a colorimetric method with the reagent MDR (mixed-diamine reagent). The sulfide binds with the reagent forming a methylene blue color and the intensity of the colored complex is determined using a spectrometer.
The acid treatment to recover reduced sulfur also liberates metals associated with the sulfide minerals into the acid solution. The metals liberated into the acid are then analyzed for simultaneously extractable metals (SEM) using Inductively Coupled Plasma - Mass Spectrometry (ICP-MS). Common metals usually determined include: cadmium (Cd), copper (Cu), nickel (Ni), lead (Pb), silver (Ag) and zinc (Zn). Mercury (Hg) can also be determined using cold vapor atomic absorption (CVAAS) or fluorescence (CVAFS) spectroscopy.
SensitivityEPA 6010B = Determination of trace metals in solution using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
EPA 6020 (modified) = Determination of sub-μg/L concentrations of a large number of elements in water samples and in waste extracts or digests using inductively coupled plasma-mass spectrometry (ICP-MS).
EPA 245.5 = Determination of total mercury (organic & inorganic) in soils, sediments, bottom deposits and sludge type materials using atomic absorption spectrometry.
ReferencesAllen, H.E., G. Fu, W. Boothman, D. M. DiToro, and J. D. Mahoney (1991). Determination of Acid Volatile Sulfides (AVS) and Simultaneously Extracted Metals in Sediment. Draft Analytical Method for Determination of Acid Volatile Sulfide in Sediment. USEPA Office of Science and Technology, Washington, DC., December, 1991. EPA-821-R-91-100.
Lasorsa, Brenda and Atina Casas (1996). A comparison of sample handling and analytical methods for determination of acid volatile sulfides in sediment. Marine Chemistry, 52: 211-220.