Speciation of mercury in South African coals

Abstract

Total mercury (Hg_TOT) concentrations were determined by inductively coupled plasma mass spectrometry (ICP MS) for South African Highveld coals. The distribution of Hg in coals was investigated using a four-stage sequential leaching protocol and isotope dilution/gas chromatography coupled to ICP MS (ID-GC-ICP MS). The results show that Hg_TOT ranged from 144 to 303 µg kg⁻¹ with a mean of 199 ± 26 µg kg⁻¹, while Hg_TOT leached from coals using different solvents ranged between 103 and 310 µg kg⁻¹ (mean: 218 ± 60 µg kg⁻¹). Hg leaching rates of 53–78% were achieved in crushed coals. Hg⁰, Hg²⁺, and CH₃Hg⁺ were identified in all coals. CH₃Hg⁺ in studied coals ranged between 0.1 and 0.4 (mean: 0.2) µg kg⁻¹. GC ICP MS chromatograms also showed unknown Hg peaks which were identified as other organomercury species such as ethylmercury. Modes of occurrence of Hg in coals were variable with the organic-bound (37–40%) and the sulfide-bound (37–39%) being the dominant mercury forms. Increasing the HCl concentration in the used protocol increased the amount of Hg leached (16%) during this step.

Keywords:

• mercury speciation
• organomercury
• coal
• GC ICP MS
• leaching

Acknowledgments

The authors thank CNRS-LCABIE-IPREM (Pau, France) for allowing to carry out part of this study in their laboratory, The National Research Foundation (NRF, South Africa), the South African National Energy Research Institute (SANERI) for funding of this study, and Eskom (South Africa) for providing part of the samples.