New insights on the biogeochemical cycle of mercury in freshwater environments. Development and application of the DGT technique for bioavailability assessment and studies of methylmercury photodegradation

Resumen

Mercury (Hg) is naturally present in the Earth’s crust, however the historical use of this metal by the human being has spread and increased its presence in the environment. Both inorganic and organic species can be found in aquatic ecosystems, which are very sensitive to Hg pollution. In anoxic waters and sediments, sulphate-reducing bacteria can convert inorganic Hg into methylmercury (MeHg), a very toxic organic form which has the ability to bioaccumulate and biomagnify throughout trophic chains. Thus, to assess the risk associated to Hg pollution, the development of techniques to monitor Hg levels in water is necessary, as well as investigating MeHg elimination pathways. In this doctoral thesis, two aspects related to the biogeochemical cycle of Hg in freshwater were addressed. On one hand, a passive sampling technique –Diffusive Gradients in Thin films (DGT)– was developed both for determination of total Hg (THg) and MeHg in freshwater. Apart from providing a time-average Hg concentration, these samplers are able to measure the labile fraction of Hg, since they supposedly mimic biological membranes. Thus, the DGT technique is considered a good monitoring tool to estimate the bioavailability of Hg in water. On the other hand, MeHg photodegradation in freswhwaters dominated by dissolved organic matter (DOM) was studied. Regarding the DGT technique, several preliminary tests were carried out both at the laboratory and in the field to test the validity of a commercial type of samplers to measure the labile dissolved Hg in continental waters. Later, the performance of this commercial type was compared to that of two in-house manufactured samplers; both of them with the same receiving gel consisting of a resin with 3-mercaptopropyl groups embedded in a polyacrylamide gel, but one with an agarose gel (A-DGT) and the other with polyacrylamide gel (P-DGT) as the diffusive layer. The uptake kinetics of Hg(II) and MeHg, both in the absence and in the presence of DOM, were studied. The diffusion coefficient, D, of Hg in the DGT diffusive layer varied among Hg species and also depended on the absence/presence of DOM in the solution. This confirms the need to use the Hg species of interest and simulate the characteristics of the water to be sampled when performing the DGT calibration if the aim is to accurately measure the labile Hg fraction. The P-DGT was chosen as the most appropriate to determine both THg and MeHg in natural waters. Thereby, this type of sampler was used in a case study to assess the removal, methylation and lability of Hg(II) in a experimental scale plant for wastewater treatment consisting in several constructed wetlands with different conformation in series. With respect to the MeHg photodegradation issue, this phenomenon was studied in waters from a lake-wetland gradient in the boreal Sweden. The influence of DOM on this process by attenuating light, forming reactive oxygen species and binding MeHg to its thiol groups to form complexes, was examined. It was observed that the desmethylation rate constant (kpd Full Spectrum) varied significantly among the three studied waters, but the wavelength-specific rate constants (kpdPAR, kpdUVA and kpdUVB) were indistinguishable. Therefore, kpd PAR, kpd UVA and kpd UVB can be considered universal, at least in waters dominated by DOM and in which MeHg is complexed by organic thiols, if the photon fluxes of PAR, UVA and UVB radiation are separately determined and the wavelength-specific light attenuation by DOM is corrected for. Furthermore, the relationship between the photodegradation and bioavailability of MeHg was examined in an experiment involving different types of natural and artificial freshwater, but no apparent connection was observed between them.