VLIZ Spatial and Temporal Assessment of high Resolution Depth profiles Using novel Sampling Technologies Brussels, September 24, 2004 Mercury analysis (speciation) in porewaters using the DGT technique

Importance of Hg in porewaters Hg extremely toxic element Hg extremely toxic element Toxicity related to the presence of methylated forms of Hg: Toxicity related to the presence of methylated forms of Hg: –Monomethylmercury –Dimethylmercury Sediments are the principal sites for the methylation of Hg Sediments are the principal sites for the methylation of Hg –Sulfate reducing bacteria are responsible for the methylation –In porewaters methylmercury may account for 90% of the Hg present

Hg analysis: difficulties Routine procedures used for other trace metals can often not be applied Routine procedures used for other trace metals can often not be applied –Adsorption on plastic containers –Low concentrations: samples easily contaminated –Volatile element –ICPMS: problems with memory effects  Generally specific analytical instrumentation (CVAFS, CVAAS) and specific procedures

Principle of DGT: Principle of DGT: Diffusive Gradient in Thin film gg Concentration of ion rr Resin in receiving gel distance Diffusive gel CbCb solution Diffusive Boundary Layer Diffusive gel: polyacrilamide Resin gel: Chelex Classical method Mass of metal accumulated on the gel is dependent on the exposure time and diffusive thickness (∆ g) Diffusive coefficient (D) for the metal must be determined

DGT for Hg analysis Method developped in University of Brno, Czech Republic Method developped in University of Brno, Czech Republic –Polyacrylamide gels accumulate Hg! Inadequate as diffusive gel –Better results obtained with agarose gel als diffusive gel –Diffusive coefficient (D) for Hg in agarose gel determined –Direct analysis of the gels in the solid phase with the Advanced Mercury Analyser (AMA) –Comparaison of ion exchange resin (Chelex) and resins with thiol groeps (Spherontiol, Sumichelate, Dualite, Lewatit): only Spherontiol has small particle size) Application in field: Upper Scheldt, Leie Application in field: Upper Scheldt, Leie –Erasmus PhD student: Pavel Divis (University of Brno)

Sampling by diving

Deployment of the DGT probes in the laboratory

Removing filter and diffusive gel and sectioning of the resin gel

Measurements with Advanced Hg Analyser (Lille+ Brno)

Leie: Menen Agarose diffusive gel Resin gels: Chelex : binds labile Hg forms Spherontiol : binds strongly bound Hg (complexes with humic acids, etc and organomercury forms Good agreement with total Hg obtained by centrifugation of porewater on separate core

North Sea, Oostende, station 130 In seawater: larger fraction of Hg present as labile Hg due to Cl-complexation In porewaters: large difference between Chelex and Spherontiol resins Good agreement between Spherontiol and centrifugation sample Large fraction of Hg probably present als methylmercury Muddy coastal sediments are important sources of methylmercury to the marine environment

Conclusions DGT Hg Due to preconcentration of Hg on the resins it is possible to analyse ng/L levels of Hg in porewater using the Advanced Hg Analyser (AMA). No further sample pretreatment is required Due to preconcentration of Hg on the resins it is possible to analyse ng/L levels of Hg in porewater using the Advanced Hg Analyser (AMA). No further sample pretreatment is required Using different resins (Chelex, Spherontiol) a speciation between the labile and strongly bound Hg forms may be made Using different resins (Chelex, Spherontiol) a speciation between the labile and strongly bound Hg forms may be made Further development is required to determine methylmercury with the DGT technique Further development is required to determine methylmercury with the DGT technique