The development of preservation methods for mercury and its species in water samples

Some of the most toxic compounds in the environment are mercury (Hg) and its species. Monitoring the pollution of Hg is of paramount performance in all spheres of the environment, and in particular in vulnerable areas such as artisanal and smallscale gold mines (ASGM). These areas produce the majority of anthropogenic Hg emissions, yet currently, recommended sampling and preservation methodologies in the scientific literature are not appropriate for the determination of Hg and its species from water sources in these and other such challenging environments. These waters are fully integrated into the local communities, providing drinking water, domestic, agricultural, aquacultural and industrial uses, among others. In water samples, Hg species are known to be highly unstable, with total loss of Hg within a few days if unpreserved. Recommended procedures for preserving Hg in water samples involve the acidification of waters, which can pose significant risk to field workers in uncontrolled environments. Therefore, it is recommended to collect at least 500 mL of water in glass or PTFE containers, which are fragile and expensive, respectively, and require immediate shipment of samples to a laboratory for analysis. This is inadequate for use in challenging environments, such as remote locations or ASGM sites, due to the difficulty in sample transport, particularly when collecting multiple samples for total Hg and different Hg species, as well as the lack of accessible laboratories with appropriate facilities to preserve and analyse Hg, often found in environmental and polluted waters at μg L-1 to ng L-1 ranges. Therefore, the aim of this thesis was to develop a sampling method for Hg and its species in water samples, that can preserve Hg speciation for an adequate time from field-to-laboratory and is applicable to supporting the study of ASGM activities and their impact on environmental and human health. To achieve this, a literature survey of Hg preservation methods and solid-phase extraction (SPE) methods was conducted and thus a dithizone-functionalised SPE methodology was developed as a sampling technique for the preservation of Hg for up to 4-weeks from waters associated with ASGM activities. This timescale was chosen as a practical time for transport of samples from field to laboratory, including where international transportation is required. Total Hg concentrations are used for guideline concentrations and therefore it is vital to ensure the method can reliably produce representative data from the field, and so the functionalised cartridge demonstrated 85 ± 10% recovery of total Hg from water samples after 4-weeks of storage. The method was also applied in ASGM sites to demonstrate the robustness of the method in an appropriate environment, showing total Hg concentrations below guideline limits (6 μg L-1 in drinking water and 0.77 μg L-1 in environmental waters), between 0.01 to 0.35 μg L-1 Hg across river, mineshaft, and spring water, and ore washing ponds. The functionalised cartridge was further assessed for Hg species sampling and preservation (inorganic mercury, Hg2+, and methylmercury, MeHg) from waters associated with ASGM activities. The functionalised cartridge showed recovery of 115 ± 8% (4°C, absence of light) and 109 ± 13% % (16°C, absence of light) MeHg and 100 ± 14% % (4°C, absence of light) and 94 ± 12% % (16°C, absence of light) Hg2+ over 4 weeks of storage. The cartridges were then used in ASGM sites in western Kenya alongside collection of river sediment samples, to obtain Hg speciation data and assess the relationship between waters and sediments, in the context of human health exposure. All sampled water sources provided no detectable MeHg and 0.06 to 0.67 μg L-1 Hg2+, below environmental guideline limits (0.77 μg L-1 Hg). Drinking water sources showed Hg2+ concentrations making up approximately 30% of the total weekly Hg intake. At the same drinking water sources, sediments measured up to 2 mg kg-1 total Hg and 64.8 μg kg-1 MeHg, showing contamination from the mining activities and significant methylation of the Hg species. Total Hg concentrations in sediments were between 0.09 to 1.72 mg kg-1 total Hg, with the most elevated concentrations from sampling points near to active alluvial work and ponds previously used for alluvial activities. The case study highlighted relationships between sediment and river water Hg2+ and MeHg concentrations, where areas of stagnation show greater concentrations of both Hg species. The results of this work contribute to the knowledge of Hg sampling and preservation, demonstrating the application of SPE as an in-field sampling and preservation method for Hg and its species. The project outcome will enable the robust monitoring of Hg species and consequences of their exposure to environment and human health, enabling a more holistic approach to environmental Hg pollution studies. Thus, this can pave the way for both local communities and policymakers to be better informed of the impact from Hg pollution originating from ASGM activities, building the case to empower change to Hg-free alternatives.

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