A new assessment of mobility of elements in sediments and wastes

Abstract

ABSTRACTIn soil science, mobility is defined as the capacity for elements, ions, and molecules to move from one compartment of the soil (atmosphere, water and solid phases biota) to another. In agronomy, mobility is defined as the capacity for elements and compounds to move from any compartment of the soil to water and finally to living organisms, especially plant roots. It is considered as a rough estimate of bioavailability. In environmental science, mobility could be defined as the capacity for potentially toxic elements or compounds to move from polluted materials spread in soils to any compartment of the soil, especially the edible part of plants and water of water tables. So, from the environmental standpoint, the feeding and protection of plants—like the control of potentially toxic elements or substances in ecosystems—involve a knowledge of the mobility of these elements, ions or molecules in soils. In the last decade, a tremendous effort has been devoted to assessing the amount of these elements or compounds which is mobile and/or bioavailable. For example, with respect to fertilization, agronomists have developed analytical techniques for determining the fraction of soil element which can be absorbed by plants. In their view mobility is defined by the technique used to evaluate the amount of an element which may move from the solid phase to the soil solution and be taken up by plants. More recently, soil scientists have used sequentially specific chemical reagents to obtain information about the state and location of elements or ions present in soil materials. The sequential use of specific chemical reagents to determine the potential mobility of elements has its own limitations related to the order and nature of the reagents used. Another means of obtaining information on the mobility of elements and substances is to perform the kinetics of extraction. In this respect, a new approach to extraction can be to leach material with water in a modified soxhlet apparatus. This allows: (1) use of water as an extractant, and temperature control; (2) concentration of leachates making analysis of trace elements easier; and (3) determination of the equivalent release time if it occurs in the field. The analysis of such kinetics of extraction plotting, for example, the amount released versus t12 shows that diffusion processes are involved only at the beginning of the release processes. Surface reactions become rapidly predominant and control the release of elements. Such data may be used to characterize contaminated soils or materials by calculating the flux of potentially toxic elements or compounds released per surface and time unit. Due to the role of surface reactions the specific surface area determined by the so-called BET method is taken into account. The kinetics of release may also be used to calculate versus time—per ha and per year (equivalent time) the flux of pollutants from contaminated soils to ground water.