Application of diffusive gel-type probes for assessing redox zonation and mercury methylation in the Mekong Delta sediment

Abstract

The vertical profiles of PO4(3-), Mn, Fe, S(2-), Hg, and CH3Hg(+) in sediment pore water were investigated using DGT and DET probes in the Tien River, the northern branch of Vietnam's Mekong Delta. Although some of the DGT measurements could be lower than the actual pore water concentrations due to the depletion of the species, the measurements provided information for understanding redox zonation and Hg methylation. The gradual increases in the measured species concentrations with the sediment depth were observed and the diffusive fluxes of the species to overlying water were expected. The vertical profiles suggested that (1) SO4(2-) seemed to be reduced before Fe(3+), or the two electron acceptors were reduced simultaneously; (2) the release of PO4(3-) was more closely related to S(2-) than Fe release; and (3) Hg methylation was active in the micro-niche between the aerobic and anaerobic transition zones. The maximum pore water CH3Hg(+) concentrations were observed at depths just above where the maximum S(2-) concentrations were detected. Hence, the maximum CH3Hg(+) concentration was observed near surficial sediments (less than 1 cm from the surface) in brackish water, and at a depth of 3 cm in fresh water. The different vertical profiles led to a CH3Hg(+) diffusive flux eight-times greater in brackish than in fresh water. The present study showed that the in situ application of DGT and DET probes was helpful to understand coupled biogeochemical reactions and mercury methylation by measuring pore water redox species.

Quantification of Mercury Bioavailability for Methylation Using Diffusive Gradient in Thin-Film Samplers

“…Rates of Hg uptake and accumulation onto the chelating resin are quantified and typically used to infer the “truly dissolved” concentration of Hg based on steady state diffusive uptake equations. 23 − 27 , 30 , 31 …”

Application of diffusive gradients in thin films and core centrifugation methods to determine inorganic mercury and monomethylmercury profiles in sediment porewater

“…An arsenic/iron speciation and mobilization study using combined DGT (arsenic) and DET (iron) confirmed that arsenic mobilization was associated with the reductive dissolution of ferric (hydr)oxide in the sediment suboxic zone of 1 mm to 3 mm depth . The vertical profiles of Hg, MMHg, phosphate, sulfide, manganese, and iron were investigated using codeployment of DGT and DET probes in Mekong River sediments, and the maximum MMHg production was observed at a depth just above the maximum sulfide concentration . On the basis of the need for fine vertical resolution and high metal concentration to be collected, we conclude that DGT is one of the most reliable methods for measuring metal depth profiles in sediment porewater.…”

“…As with Hg, a flat probe was used to deploy the dissolved sulfide DGT in the sediment cores . The window on the probe provided a 1.8 × 24 cm 2 area for the diffusion of sulfide.…”

“…where C is the labile metal species concentration in the water (mg cm À3 ); M is the mass of the species accumulated in the resin (mg); Dg is the total thickness of the diffusion layer, including the filter membrane and diffusive gel (cm); D is the apparent diffusion coefficient of the species in the hydrogel (cm 2 s À1 ); t is the deployment time (s); and A is the exposed interfacial area (cm 2 ). As with Hg, a flat probe was used to deploy the dissolved sulfide DGT in the sediment cores [24]. The window on the probe provided a 1.8 Â 24 cm 2 area for the diffusion of sulfide.…”

Methodological approaches for fractionation and speciation to estimate trace element bioavailability in engineered anaerobic digestion ecosystems: An overview

“…In another study, As speciation was investigated in anoxic sediment columns, using both DGT and DET (Diffusive Equilibrium in Thin films) (Bennett et al 2012). The DGT technique has not been applied yet to AD; nevertheless, it can be assumed that such applications would be similar to applications on anoxic sediments or soils (Hong et al 2014;Williams et al 2014).…”