Evaluation of Arsenic and Selenium Quantitation in Marcellus Shales and Flowback Saline Waters using Thiol Cotton Fibre-Graphite Furnace Atomic Absorption Spectroscopy and Diffusive Gradient in Thin Film Techniques.

摘要

Trace levels of arsenic and selenium can be toxic to living organisms yet their quantitation in high ionic strength or high salinity aqueous media is difficult due to the matrix interferences which can either suppress or enhance the analyte signal. The objective of this proposed study has been to apply and/or modify inexpensive, simple, and interference-free analytical methods for the sub-ppb to ppb quantification of both inorganic and bioavailable arsenic and selenium fractions in highly saline/salty waters such as flowback wastewater produced from natural gas well drilling sites in the Marcellus shale and/or simulated or artificial high-salinity prepared from deionized water and spikes of different ions, selenium and arsenic concentrations.;A modified thiol cotton fiber (TCF) method employing lower flow rates and centrifugation has been developed and used to remove the analyte from complex aqueous media and minimize the matrix interferences. This method has been tested using a USGS (SGR-1b) certified reference shale. It has been used to analyze Marcellus shale samples following microwave digestion as well as spiked samples of high salinity water (HSW) and flowback wastewater (WRF6) obtained from an actual gas well drilling operation. Quantitation of arsenic and selenium was carried out by graphite furnace atomic spectroscopy (GFAAS). Extraction of arsenic and selenium from Marcellus shale exposed to HSW and WRF6 for varying lengths of time is also reported.;In addition, the role of hydrofluoric acid in microwave-assisted digestion and in the elimination of spectral interferences from the aluminum matrix at 189 nm for arsenic quantitation by GFAAS with deuterium lamp back-ground correction has been investigated. When sufficient amounts of hydrofluoric acid are added to the sample for microwave-assisted digestion, the excess or residual hydrofluoric acid serves an additional role of matrix modification to inhibit the formation of aluminum oxide that has been reported to cause the spectral interference. The presence of sufficient fluoride in the sample enables formation of aluminum fluoride which volatilizes at 1291° C, and this significantly reduces the spectral interference. The use of 0.5 mL of concentrated hydrofluoric acid and 4 mL of concentrated trace metal grade nitric acid and 0.25 g of sample enabled accurate and precise determination of arsenic in saline matrices containing aluminum up to 0.053 M Al 3+ with LOD and LOQ varying with amount of hydrofluoric acid used.;Furthermore, since mobility and toxicity of arsenic and selenium in natural waters are related to the aqueous species distribution, a Diffusive Gradients in Thin Film technique (DGT) employing a polyacrylamide diffusive gel or 3-mecarptopropyl-functionalized silica and high-capacity nanocrystalline titanium (IV) oxide adsorbent (Metsorb) has been modified and used to isolate the bioavailable analytes species and thereby minimize the matrix effects observed in quantitation by graphite furnace atomic absorption spectroscopy (GFAAS). The DGT-GFAAS procedure has been used to determine the bioavailable arsenic and selenium isolated from flowback water from an actual gas well drilling operation (WRF6), as well as spiked samples of WRF6 and flowback wastewater (FS1). In addition, Marcellus shale samples were exposed to WRF6 for varying lengths of time and DGT-GFAAS was used to determine the bioavailable arsenic and selenium in these solutions. Speciation analysis was also carried out. The elution efficiencies using 1 M sodium hydroxide for arsenic and selenium were between 80--93% and the detection limit for arsenic and selenium for 3 day deployments based on the standard deviation of the blank were 0.064 and 0.10 microg/L, respectively.