Research on Heavy Metals in Soils and Sediments

This special edition of the journal Soil Systems contains 14 articles dealing with current research on cationic trace elements and semi-metals. Almost all continents have been included. Whereas As, Cd, Co, Cu, Pb and Zn have been examined quite often, the elements Hg, Tl, Mo Se and Sb and the noble metals have attracted much less attention. The everlasting questions of this topic are concerned with soil to plant transfer, accumulations in the food web, anthropogenic influences, mobilizations by weathering and fixations by remediation processes. The term “available fraction” is still controversial, as well as the sampling depth of the soils, which might lead to misinterpretations when the obtained numbers are directly compared. Unfortunately, many authors are not aware that the soil to plant transfer depends on species, timescale and climatic conditions, even on nutrient levels, and not just on the soil chemistry itself. Thus, fundamental field studies, which establish correlations between mobile fractions and plants, i.e., soil–biota uptake, should be considered. Metal levels also influence bacterial, fungal and enzymatic activities in soils.

Therefore, it is necessary to characterize test substrates using fundamental parameters, like pH, grain size, carbon and nitrogen contents, etc., to enable reasonable comparisons of data.

Whereas analytical techniques of multi-element determinations for total contents and aqua regia extracts have been established, developments to determine speciation and mobilities are still in progress. Though current research in analytical chemistry is largely focused on large biomolecules, pesticides and metabolites, some progress in the detection of surface coatings, secondary minerals and weathering products can still be seen. ICP-OES and ICP-MS are widely used techniques to determine various elements as solutions, whereas X-ray fluorescence is favored to directly target solids for total contents. Discrepancies between total contents and aqua regia mobiles still remain but are marginal for cationic trace elements like Cd, Cu, Co, Hg, Pb and Zn in many cases.

The selective and sequential leaching of soils, sediments and minerals are somehow complementary to microlocal analyses like µ-XRF (micro-X-ray fluorescence) and SEM-EDA (scanning electron microscopy electron diffraction analysis). Whereas leaching targets the mobilities of rather than the representative amounts of solids in a changing chemical environment, microlocal techniques investigate mineral phases and coatings at a small scale. Examples in this volume are details of pyrite weathering or alterations of microplastic surfaces.

A review of remediation techniques using bacterial biomass or phosphogypsum reveals that similar remediation techniques could be employed both for metal and radioactive contaminations, which emanate from the U-238 and the Th-232 decay series.

Although many sewage treatment plants and purification facilities for industrial effluents have been built, soils and sediments are still polluted by industrial effluents, mining waste, agricultural residues and urban inputs. The control and development of remediation techniques are necessary on a permanently basis. The remediation of metal-polluted sites will be surely a future challenge, through either fixation for un-soluble components, through biosorption for bacterial biomass, or through transfer to hyperaccumulator plants, which was discussed in two reviews.