EVALUATING ACID-VOLATILE SULFIDE (AVS) PARAMETERS IN THE FIELD
Abstract:A growing body of literature exists supporting the theory that toxicity to benthic organisms due to sediment-associated metals can be mitigated by the presence of acid-volatile sulfides (AVS). The current laboratory method for determining sediment AVS and simultaneously extracted metals (SEM) involves a cold/dilute acid extraction, purge-and-trap procedure that converts labile solid-phase metal sulfides to hydrogen sulfide (H2S). Evolved H2S is transferred via inert carrier-gas to a stabilizing solution for detection and quantification. The method, however, includes steps that can introduce systematic bias to the analytical results. For instance, the prominent component of AVS in anoxic sediment (i.e., amorphous iron (II) sulfide; FeS) is readily oxidized by aqueous dissolved or atmospheric oxygen. Sample manipulations that expose sediment samples to aqueous dissolved or atmospheric oxygen, therefore, can change ambient sediment AVS concentrations.
Sediment toxicity and SEM-AVS data are useful for evaluating sediment quality as related to metal concentrations. Obtaining accurate SEM-AVS data is imperative to accurate application of this investigative tool. One approach to improving SEM-AVS data is to reduce the likelihood of sediment AVS oxidation. To this end, a field analytical method has been developed to extract and stabilize sediment AVS (i.e., cold/dilute acid extractable sulfides) in the field with negligible exposure to sample-altering (e.g., oxidizing) conditions. The method can be applied as a hybrid field/lab method, combining characteristics of both a field method and the traditional laboratory method, or as a pure field method.
Two types of analyses were performed: water-only sulfide spikes and AVS-spiked formulated sediment samples. Time course measurements indicated that the time for completion of the sulfide extraction was approximately 15 to 25 minutes. The time to completion was likely limited by the sample/acid-reagent mixing efficiency achieved in the reaction chamber. Recovery efficiencies were less consistent, ranging from 83.9 to 168 percent for water-only trials and from 62.8 to 123 percent for AVS-spiked formulated sediment. The recovery efficiency was affected by several factors, including the stability of the ORP electrode response to sulfide in the alkaline sulfide trap solution. Better recovery efficiencies are expected by using sulfide ion-specific electrodes, sulfide anti-oxidation buffer solution, and stabilized sulfide stock solutions.
Analytical results from this method appear to be comparable to laboratory analysis of sediment AVS (as indicated by comparison of recovery efficiencies of water-only sulfide spikes). Future work on this new field AVS analytical method will compare results from field and laboratory analyses of formulated and natural sediments. This new method can provide investigators with a convenient alternative for screening a large number of sediment samples for AVS and SEM.
Document Type: Research Article
Publication date: January 1, 2001
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