Mercury determination in soil and sludge samples by HR CS GFAAS: comparison of sample preparation procedures and chemical modifiers

Abstract

Mercury (Hg) is an admittedly toxic element. From an analytical viewpoint, Hg volatility makes preparation of samples containing Hg as well as Hg detection therein a challenging task. The quality of the results is mainly associated with sample pretreatment. Moreover, there are difficulties inherent to thermal stabilization in the graphite furnace during atomic absorption. Here, Hg was determined in different samples (clay soil, sewage sludge, and light sandy soil) by high-resolution continuum source graphite furnace atomic absorption spectroscopy (HR CS GFAAS). Three different sample preparation methods were investigated: slurry, microwave acid digestion with HF and HNO₃ (which resulted in total sample decomposition), and acid leaching with aqua regia. Different chemical modifiers (palladium nanoparticles (PdNPs), Pd(II) solution, gold nanoparticles (AuNPs), and Au(III) solution) were tested, and their efficiencies were compared. Both the slurry method and acid leaching with aqua regia prevented Hg loss and yielded satisfactory results as compared to the certified, indicative or spiked concentrations of all the samples (confidence level 95%), using AuNPs as a chemical modifier. The Au–Hg amalgam resulting from a Hg(II) reduction in the AuNP surface leads to diffusion and alloying. Hg determination in the sewage sludge sample agreed with the certified value for all studied modifiers, when the sample is prepared by aqua regia leaching and slurry. Pd(II) solution and PdNPs also provided good results, but under specific conditions. Pd(II) binds to the analyte, thereby avoiding Hg loss during the pyrolysis step. As for AuNPs, they are an effective modifier because they enhance the interaction between the clusters formed after pyrolysis and prevent Hg loss, confirming the potential for application of these modifiers.