RESUMEN
In this study, the vertical distribution of mercury (Hg) in estuarine and marine sediment porewaters and solid phases was assessed by conventional and passive sampling techniques in the historically polluted Scheldt Estuary and Belgian Coastal Zone (BCZ). The Diffusive Gradients in Thin-films (DGT) measured labile Hg concentrations (HgLA) were mostly lower than the porewater Hg concentrations (HgPW), and they also presented different vertical distribution patterns. Still high Hg concentrations in the sediment solid phases, comparable to the historical ones, were observed. Even though pH, redox potential and dissolved sulfide concentration could influence the Hg biogeochemical behaviour, organic matter (OM) played a key role in governing Hg mobilization from sediment solid phase to porewater and in its partitioning between porewater and solid phase over depth. In the marine sediments, where OM had a marine signature, higher labile Hg concentrations in the porewater and faster resupply from the solid phase were observed. The DGT technique showed significant potential not only for the measurement of bioavailable Hg fractions in porewater, but also for the assessment of kinetic parameters governing the release of labile Hg species from the solid phase with the assistance of the DGT Induced Fluxes in Sediments (DIFS) model.
RESUMEN
The geochemical behavior of metals in water and sediment was investigated in the tidal section of the Zenne River in Belgium. Twelve-hour sampling campaigns were performed in October 2013 and March 2021 at the mouth of the Zenne River, under dry and rainy weather conditions respectively. Water samples were collected every hour while the passive samplers of Diffusive Gradients in Thin-films (DGT) were deployed continuously during a tidal cycle. In addition, bottom sediments were sampled at the tidal station and water samples were taken upstream and downstream of that station to identify the metal sources. The highest concentrations of Fe, Mn, Pb, Cr, Ni and Zn appear at low tide, indicating the Zenner River as a main source. However, for Co, Cd and Cu, other sources including upstream transport may explain their behavior during a tidal cycle. Fe, Pb and Cr are essentially transported in the particulate phase (<10 % dissolved) while the other metals in the dissolved phase (20 to 90 %). Rainfall and wind gust events also play an important role in trace metal distribution, increasing sediment resuspension and metal desorption. A good agreement was found between the time-averaged dissolved and DGT-labile metal concentrations with the exception of Cu and Fe, which form strong organic Cu complexes and Fe colloids respectively. The sediments of the tidal Zenne are contaminated by trace metals, thus acting as a secondary pollution source to the river. The reductive dissolution of Mn and Fe oxyhydroxides and the release of associated trace metals are the main mobilization mechanisms. Knowledge of the upstream and downstream levels in the water column, the benthic fluxes, which are based on turbulent diffusion, and the partitioning between dissolved and particulate phases allow to explain the metal concentration variations during the tidal cycle.
RESUMEN
This study aimed to expand the knowledge on the application of the most common industrial byproduct, i.e., fly ash, as a supplementary cementitious material. The characteristics of cement-based composites containing fly ash as supplementary cementitious material were discussed. This research evaluated the mechanical, durability, and microstructural properties of FA-based concrete. Additionally, the various factors affecting the aforementioned properties are discussed, as well as the limitations associated with the use of FA in concrete. The addition of fly ash as supplementary cementitious material has a favorable impact on the material characteristics along with the environmental benefits; however, there is an optimum level of its inclusion (up to 20%) beyond which FA has a deleterious influence on the composite's performance. The evaluation of the literature identified potential solutions to the constraints and directed future research toward the application of FA in higher amounts. The delayed early strength development is one of the key downsides of FA use in cementitious composites. This can be overcome by chemical activation (alkali/sulphate) and the addition of nanomaterials, allowing for high-volume use of FA. By utilizing FA as an SCM, sustainable development may promote by lowering CO2 emissions, conserving natural resources, managing waste effectively, reducing environmental pollution, and low hydration heat.
