Coupling of oxygen and pH requirements for effective microwave-assisted digestion of soils for tungsten analysis.

Abstract

Fully understanding the environmental fate of tungsten is an ongoing challenge from an analytical perspective, due to the lack of effective laboratory methods for quantifying tungsten in soil. Developing the methods necessary to accurately and consistently monitor tungsten in soil matrices is essential for understanding the behavior of tungsten in the environment. Traditional digestion procedures such as SW-846 Method 3051 that are usually employed for the analysis of heavy metals are not consistently useful for the complete digestion of tungsten. The chemistry of tungsten presents unique challenges due to the metal’s ability to polymerize under acidic conditions. Method modifications to the digestion procedure outlined in SW-846 Method 3051 to enhance tungsten recovery from soils are described in this paper. These method modifications were evaluated for determining tungsten concentration in five soil types. A modified acid composition of 8 mL of concentrated nitric acid and 2 mL of concentrated phosphoric acid as the digestion solution, coupled with 6 mL of 50% (w/w) hydrogen peroxide, increased the tungsten per cent recovery range in five different soil types. The modified digestion yielded an average recovery of 88.2% compared to 10.2% obtained using Method 3051 for the five soil types evaluated. Method modifications are presented, with the goal of improved extraction efficiency, and greater precision in the analysis of tungsten.