Understanding the role of clay in the uptake dynamics of hazardous cadmium by beneficial soil organisms such as earthworms

Guidelines have been developed using earthworms as indicators for the assessment of risk emerging from hazardous substances, such as cadmium. Cadmium is one of the most hazardous substances released by human activities into soils. These guidelines, e.g. the OECD guideline 222, are usually based on the exposure of the earthworm E.fetida under laboratory conditions to a selected toxicant in a soil medium made up artificially. In contrast to natural soils being extremely heterogenic, artificial soils consist of a defined mixture of usually sand, peat moss and clay. In principle, the data obtained from exposures to artificial soils should be able to serve as a benchmark for the interpretation of data obtained under field conditions. However, biological availability and consequently toxicity in nature are influenced by a number of different abiotic factors. Amongst them, the proportion and characteristics of clay have a large impact on the absorbance behaviour of metals. Clay is defined by particle size and only to a lesser degree by its composition. In the case of kaolinite clay, the amount of kaolinite (Al₂Si₂O₅OH₄), what is defined as pure clay, may range from 60 to 100%. For that reason, the aim of the proposed study is to understand the role of clay containing different clay constituents (being within the range as recommended by the OECD guideline 222) on the environmental and biological availability of cadmium. This provides a deeper insight into the uncertainties linked to the extrapolation between field and laboratory data. In a broader frame, it will contribute to the identification of hazardous conditions in soils and subsequently to the protection of soil ecosystems.