Sand as a relevant fraction in geochemical studies in intertidal environments

Abstract

Soil and sediment samples from several intertidal environment exposed to different types of contamination were studied to investigate the importance of grain size in relation to the capacity of the substrates to retain trace metals. The unfractionated samples (referred to as bulk samples) were separated into the following grain/size fractions: fine-coarse sand (2-0.100 mm), very fine sand (0.100-0.050 mm), silt (0.050-0.002 mm), and clay (0.002 mm). The sample into its fractions was carried out was in a glove box under high-purity N2 atmosphere in order to minimize any alterations to the samples. The bulk samples were characterized in terms of physicochemical properties such as pH, redox potential, and grain size. The total organic carbon (TOC), total sulfur (S), iron (Fe) pyrite, Fe, and manganese (Mn), and trace metals lead (Pb), mercury (Hg), chromium (Cr), and nickel (Ni) were analyzed in the bulk samples and in each fraction. The sand fractions were also examined by scanning electron microscopy (SEM). Comparisons of the above parameters were made between fractions and between each fraction and the corresponding bulk sample. The fine-coarse sand fraction contained high levels of the primary elements of the geochemical processes that occur in marine sedimentary environments such as TOC, total Fe, Mn, and S. The net concentrations of these four elements were higher in the fine-coarse sand fraction than in the very fine sand fraction and were similar to the net concentrations in the silt and clay fractions. Detailed SEM analysis of the sand coarse fraction revealed the presence of Fe and aluminum oxyhydroxide coatings in the oxic layers, whereas the framboidal pyrites and coatings observed in the anoxic layers were Fe sulfides. The presence of the various coatings explains why the trace metal concentrations in the sand fine-coarse fraction were similar to those in the clay fraction and higher than those in the very fine sand fraction. The present results highlight the importance of the sand fraction, which is generally disregarded in geochemical and environmental studies of sedimentary layers.

How Do Plants and Climatic Conditions Control Soil Properties in Hypersaline Tidal Flats?

“…Each sampling site was georeferenced using a portable GPS and the coordinates of each site were used for the measurement of the distance to the nearby river using geoprocessing techniques. At the laboratory, the soil adhered to the roots were removed for analysis by gently shaking and brushing the soil-rhizosphere system [37][38][39][40]. Subsequently, the samples of bulk and rhizospheric soil were air-dried, crushed, and 2-mm sieved, whereas subsamples were frozen for posterior determination of iron forms by a sequential extraction procedure.…”

Soil memory in mineral surface coatings: Environmental processes recorded at the nanoscale

“…Furthermore, micro-laminations in rock varnish, a coating composed of mainly clays, silica, and Mn-oxides, can provide information on the sequence of wet and dry climatic conditions as the Mn content in the laminations correlates with the precipitation rate (Liu and Broecker, 2008;Krinsley et al, 2013) In many soils, records of atmospheric processes are not preserved in the upper horizons where a strong interaction of minerals or chemical constituents with atmospheric and biological components rapidly renews the soil memory (Fedoroff et al, 2010). This resetting of the soil memory may not be the case in mineral surface coatings as their nano-to micrometersize confined pore and interface spaces are known to (1) limit the percolation rate of solutions and thus slow down equilibration between coating constituents and the environment (Singer et al, 2013;Wang et al, 2003, Bockheim andHartemink, 2013), and (2) sequester and retain metal(loid)s in contaminated soils (e.g., Hiller et al, 2013;Otero et al, 2013).…”

Human bioaccessibility and absorption by intestinal cells of potentially harmful elements from urban environmental matrices

“…Elements normally are concentrated in the clay and silt fractions, although depending on the type of material they can also be found at high concentrations in the sand fraction (Otero et al, 2013). The median concentration for Pb was higher in the 2 mm (405 mg kg -1 ) than in 250 µm (257 mg kg -1 ).…”