Quantification and thermo-desorption studies of mercury in sediments from Mokolo River, Limpopo Province, South Africa

ABSTRACT

Mercury can travel over long distances, and bioaccumulate in the trophic chain thereby posing a threat to not only humans but also animals and plants. Mercury also causes neurological problems such as the Minamata diseases. Pollution and toxicity of mercury have become a global problem that requires immediate attention. In this study, a thermal-desorption technique using a Zeeman direct mercury (Hg) analyser (DMA) was employed for the quantification of Hg in river sediments as well as for Hg thermo-scanning to investigate the behaviour of Hg release from sediments. Thermal speciation was based on comparison of the temperatures at which Hg species in sediments were thermally released to the temperatures at which Hg in known standards were released. Moreover, total Hg concentrations in river sediments were measured using a DMA. The accuracy of the method for total Hg quantification was confirmed by the analysis of several standard reference materials of sediment (SRM2709a, NCS DC 73325, NCS DC 73374, NCS DC 73316 and NCS DC 73322) and quantitative percentage recoveries ranging from 84 to 99% were obtained. The total Hg concentrations in sediments ranged from 7.4 to 35.2 ng/g and 2.1 to 21.2 ng/g in the low and high flow seasons, respectively. Different forms of Hg associated with river sediments were successfully identified by thermo-desorption technique. The concentrations of Hg in the sediment samples were all below 200 ng/g which is the sediment quality guideline set by the United States Environmental Protection Agency (US EPA). Different species of Hg were present in the sediment samples, with humic acid and/or iron oxide being the most predominant forms of Hg associated with the sediments.

KEYWORDS:

• Mercury
• Mokolo River
• sediment
• speciation
• thermal-desorption

Acknowledgments

Dipuo P. Kgabi is thankful to the National Research Foundation (NRF) under the Department of Science and Technology (DST)-NRF Innovation Master’s Scholarship Grant number 114500 for financial support. This study was supported by the NRF of South Africa under the Thuthuka Programme Grant Number 117673 and the Water Research Commission (WRC) of South Africa Project Number K5/2515//1. The authors would also like to acknowledge the University of Johannesburg Research Centre for Synthesis and Catalysis for financial support. We also acknowledge Prof N. Panichev from the Department of Chemistry at the Tshwane University of Technology for allowing us to use their facility and for his assistance with the thermal analysis of sediment samples.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Water Research Commission (WRC) [K5/2515//1]; National Research Foundation (NRF) [117673].