Sediment characteristics affecting bioavailability of heavy metals in flooding areas and intertidal zones along the river Scheldt

Mobility and bio-availability of heavy metals in intertidal zones and floodplains is currently studied at the Laboratory of Analytical Chemistry and Applied Ecochemistry (Ghent University). The relation between sediment properties and metal uptake by reed plants (Phragmites australis (Cav.) Steudel) was investigated based on field observations at selected locations along the estuary of the river Scheldt (Flanders, Belgium). The sites represented a varying degree of metal contamination and salinity. At each site, reed plants were sampled and analysed for heavy metals. Sediments were also sampled and characterised for metal contents and various physico-chemical properties. Cd contents of Phragmites australis (Cav.) Steudel were higher on sites with lower total Cd contents in the upper 20 cm of the sediments. Contents of leaves, stems and rhizomes were however positively and significantly correlated with the chloride content of the sediments. Zn contents were also higher on sites with lower total Zn contents in the sediments, but, in contrast to Cd, were negatively correlated with chloride contents. When the sites with the lowest salinity were considered separately, the Zn content of leaves and stems on the one hand and the ratio of exchangeable Zn to the total cation exchange capacity on the other hand were significantly and positively correlated. This can suggest that reed plants do not take up Zn selectively, and that the uptake and/or transfer mechanism is adapted at the highest salinity. Cu contents of the plants were negatively correlated with chloride contents in the sediments. However, Cu contents in leaves and stems were significantly and positively correlated with Cu contents in the reducible fraction of the sediments, according to the BCR sequential extraction procedure. At the sites with the largest reducible Cu fractions, the mean water level was also higher and the upper sediment layer was reduced at sampling time

Dynamics of heavy metals in reedbeds along the banks of the river Scheldt

The pollution status of recent sediment deposits in the intertidal reedbeds along the banks of the river Scheldt was assessed, to be able to predict potential metal accumulation in controlled flooding areas and new wetlands. Moreover, the most important factors that affect metal accumulation and mobility in these marshes were identified. The superficial intertidal sediment layer was found to be significantly contaminated with trace metals. The metal concentrations could be predicted from clay or organic matter concentrations. They were somewhat higher then predicted at sites within a range of a few km from specific point-sources, whereas they were lower than predicted at sites which are regularly subjected to flooding by high salinity water. In the deeper sediment layers, sulphides also seem to play an important role in the metal accumulation. An increased salinity increases the metal mobility in oxidised sediment layers in the brackish part of the estuary. This was mainly observed for Cd. It was reflected in increasing metal concentrations in the reed plants, which were however primarily related to the clay and organic matter concentrations in the sediments. The decomposition of stems and leaves of reed and willow can both increase and decrease the metal mobility in the upper sediment layer. The concentrations of most metals in the reed litter itself increased strongly during decomposition under field conditions. The hydrological regime to which metal-polluted sediments are subjected, affects the metal concentrations in the pore water to an important extent. Flooding of a calcareous sediment led to an increased mobility of Fe, Mn, Ni and Cr and a decreased mobility of Cd, Cu and Zn. Alternating hydrological conditions resulted in fluctuating metal concentrations in the pore water. Reduction and oxidation of Fe and Mn, carbonate breakdown and the formation and re-oxidation of sulphides were the main controlling processes

Arsenic in soils and waters around the Kori Kollo gold mine on the Bolivian Altiplano: redox-induced speciation and mobilization

Mining activities in the Bolivian Altiplano have caused considerable negative environmental consequences on the water, soils, vegetation resources, biodiversity, and the atmosphere over the years. In this study, samples of soils, sediments, river water, water from drinking pools, and groundwater were collected from the area around the Kori Kollo gold mine, located near the city of Oruro on the Bolivian Altiplano to investigate the concentrations of arsenic (As) as an important contaminant associated with the mining activities in this area. Moreover, the redox-induced speciation, mobilization, and release dynamics, of As in soil/sediment samples was studied under controlled reducing and oxidizing conditions using an automated biogeochemical microcosm apparatus. The total As concentrations in the soils ranged between 10 and 81 mg kg-1 and exceeded the international trigger action values (10-65 mg kg-1) of As in agricultural soils. Arsenic concentrations (µg L-1) reached values up to 2,688, 952, and 300 in the groundwater, drinking pools, and surface water, respectively and exceeded the current WHO provisional guideline value of 10 µg L-1. The total dissolved concentrations of As varied from 368 to 3,130 µg L-1. The dissolved concentrations of As increased under oxidizing conditions and decreased under reducing conditions. Data of As speciation showed that the As (III) accounted from 0.0 to 79% of the total dissolved As and increased under reducing conditions, while the As (V) accounted from 21-100% of the total dissolved As and increased under oxidizing conditions. The results conclude that i) although the total concentrations of As in the soils around the mine are not very high, the concentrations of As in the waters were very high, 2) the concentrations of total dissolved As were very high which might indicate the high mobilization of As and support the anthropogenic source of As in theses soils, 3) the release and mobilization of As increased under oxidizing conditions as compared to the reducing conditions, and 4) the As (III) accounted values up to 79% of total dissolved As, which might increase the toxicity and risk of As in the soils and waters especially under reducing conditions. These results highlight the environment risk of As which might be a main reason for the gradual death of goats and cows, the biodiversity and the decline of fishing and agricultural sources in this area

Factors affecting metal mobilisation during oxidation of sulphidic, sandy wetland substrates

Most metals accumulate as sulphides under anoxic conditions in wetland substrates, reducing their bioavailability due to the solubility of metal sulphides. However, upon oxidation of these sulphides when the substrate is occasionally oxidised, metals can be released from the solid phase to the pore water or overlaying surface water. This release can be affected by the presence of carbonates, organic matter and clay. We compared changes of Cd, Cu and Zn mobility (CaCl2 extraction) during oxidation of a carbonate-rich and a carbonate-poor sulphidic, sandy wetland substrate. In addition, we studied how clay with low and high cation sorption capacity (bentonite and kaolinite, respectively) and organic matter (peat) can counteract Cd, Cu and Zn release during oxidation of both carbonate-rich and carbonate-poor sulphidic sediments. CaCl2-extractability of Cu, a measure for its availability, is low in both carbonate-poor and carbonate-rich substrates, whereas its variability is high. The availability of Cd and Zn is much higher and increases when peat is supplied to carbonate-poor substrates. A strong reduction of Cd and Zn extractability is observed when clay is added to carbonate-poor substrates. This reduction depends on the clay type. Most observations could be explained taking into account pH differences between treatments, with kaolinite resulting in a lower pH in comparison to bentonite. These pH differences affect the presence and characteristics of dissolved organic carbon and the metal speciation, which in turns affects the interaction of metals with the solid soil phase. In carbonate-rich substrates, Cd and Zn availability is lower and the effects of peat and clay amendment are less clear. The latter can also be attributed to the high pH and lack of pH differences between treatments

CNT microtubes with entrapped Fe3O4 nanoparticles remove micropollutants through a heterogeneous electro-fenton process at neutral pH

Catalyst-coated carbon electrodes require two preparation stages: electrode assembly using carbon and polymeric binders and subsequent catalyst immobilization on the porous carbons electrode. Such conventional coating methods require several steps, which is time-, chemical-, and energy-consuming. Also, polymeric binders can impair the porosity and block catalytic sites of final electrodes. This study introduces a novel one-pot synthesis method in which Fe3O4 nanoparticles are entrapped within a multi-walled carbon nanotube network, the latter being templated with microtubular geometry. Such carbon microtubes (CMT) represent a standalone geometry, serving as a binder-free electrode for an energy-efficient heterogeneous electro-Fenton (HEF) process. Fe3O4-containing CMTs remove carbamazepine (CBZ), a frequently detected pharmaceutical micropollutant in water bodies. While almost all literature reports degradation at acidic conditions requiring the use of acid, this material system functions at pH 7 +/- 0.3 with lengthy reusability. The remarkable mineralization of the total oxidized CBZ emerges from the confinement of oxidation by-products in CMTs' 3D framework, where unselective radicals are formed once the electro-generated H2O2 reacts with embedded Fe3O4. Additionally, the 3D network prevents the entrapped catalysts from leaching in acidic environments because of the increased local pH during electrolysis

Presence and mobility of arsenic in estuarine wetland soils of the Scheldt estuary (Belgium)

We aimed to assess the presence and availability of arsenic (As) in intertidal marshes of the Scheldt estuary. Arsenic content was determined in soils sampled at 4 sampling depths in 11 marshes, together with other physicochemical characteristics. Subsequently, a greenhouse experiment was set up in which pore water arsenic (As) concentrations were measured 4 times in a 298-day period in 4 marsh soils at different sampling depths (10, 30, 60 and 90 cm) upon adjusting the water table level to 0, 40 and 80 cm below the surface of these soils. The As content in the soil varied significantly with sampling depth and location. Clay and organic matter seem to promote As accumulation in the upper soil layer (0–20 cm below the surface), whereas sulfide precipitation plays a significant role at higher sampling depths (20– 100 cm below the surface). The As concentrations in the pore water of the greenhouse experiment often significantly exceeded the Flemish soil sanitation thresholds for groundwater. There were indications that As release is not only affected by the reductive dissolution of Fe/Mn oxides, but also by e.g. a direct reduction of As(V) to As(III). Below the water table, sulfide precipitation seems to lower As mobility when reducing conditions have been sufficiently established. Above the water table, sulfates and bicarbonates induce As release from the solid soil phase to the pore water

Selenium digestibility and bioactivity in dogs : what the can can, the kibble can't

There is a growing concern for the long-term health effects of selenium (Se) over-or underfeeding. The efficiency of utilization of dietary Se is subject to many factors. Our study in dogs evaluated the effect of diet type (canned versus kibble) and dietary protein concentration on Se digestibility and bioactivity. Canned and kibble diets are commonly used formats of dog food, widely ranging in protein concentration. Twenty-four Labrador retrievers were used and four canned and four kibble diets were selected with crude protein concentrations ranging from 10.1 to 27.5 g/MJ. Crude protein concentration had no influence on the digestibility of Se in either canned or kibble diets, but a lower Se digestibility was observed in canned compared to kibble diets. However, the biological activity of Se, as measured by whole blood glutathione peroxidase, was higher in dogs fed the canned diets than in dogs fed the kibble diets and decreased with increasing crude protein intake. These results indicate that selenium recommendations in dog foods need to take diet type into account