This paper examines the stability of Hg in a six-step sequential extraction scheme, focusing in particular on the first leach designed to extract ‘soluble’ Hg in sediment and soil samples. The recovery of Hg as a spike (Hg2+, equivalent to 15 ng g−1 in a sample), alone and mixed with a background granite ‘GG’, was found to be low and dependent on the time of extraction (15–60 min) in the following reagents: Ca(NO3)2, NaNO3, KNO3, CaCl2 and MgCl2. While the complexing action of thiourea stabilized Hg in solution (0.1M Ca(NO3)2), it also acted as an extractant itself and therefore could not be used to minimize resorption of Hg onto the sample during leaching.
The minerals Hg2Cl2, HgCl2, HgO (red and yellow), HgS (synthetic and cinnabar) and Hg (0) were used alone and mixed with GG to evaluate the selectivity of the scheme. The reagents used in sequence are as follows: 0.01M Ca(NO3)2 (categorized as ‘easily soluble’, F1); 0.1M Na4P2O7 (labile organic component, F2); 0.25M NH2OH.HCl/0.25M HCl (amorphous Fe and Al oxides, F3); 1M NH2OH.HCl/25% CH3COOH (crystalline Fe and Al, F4); 40% HNO3 (non-labile organic component and elemental Hg, F5); and aqua regia (residual phase, containing sulphides (HgS), oxides, some silicates, refractory organics etc., F6). Alone, each mineral dissolved fully in the appropriate reagent, for example: HgCl2 in F1 (soluble), HgO in F3 (with amorphous Fe/Al oxide phase), Hg (0) in F5, and HgS in the final aqua regia digestion. However, when mixed with GG, these minerals (except HgS) also dissolved in subsequent stronger leaches. For example, Hg in the sample HgCl2 mixed with GG (total Hg concentration of 513 ng g−1) dissolved in F1, F2, F4 and F5. Thus, resorption in the initial leaches of a sequential extraction can confound the results for Hg and lead to incorrect results and interpretation of leaches purported to define the bioavailable component of Hg in environmental materials. New methods should be designed to minimize this phenomenon, using, for example, a flow-through system to achieve short contact times.