Potentially toxic elements (PTEs) in soils are mainly associated with the solid phase, bound to the surface of solid components, or precipitated as minerals. For most PTEs, only a small portion is dissolved in the soil solution. However, there is an interest in following the fate of mobile PTEs in the environment, for a growing amount of evidence indicates that downward movement of PTEs may occur in biosolids amended soils, leading to groundwater contamination. Therefore, it is crucial to understand the factors that control the release of these elements after land application of biosolids, in order to overcome problems related to downward movement of PTEs in the soil profile. 1. Introduction The treatment of municipal wastewater produces huge amounts of sludge. This material consists of the solids that were originally present in the wastewater and/or new suspended materials originated as the result of wastewater treatment processes [1]. The term biosolid was officially recognized in 1991 by the Water Environment Federation (WEF) and refers to the organic solids that have received a biological stabilization treatment at the municipal wastewater treatment plant, to make a distinction from other types of sludges that cannot be beneficially recycled as a soil amendment. In recent years, the quantity of biosolids generated all over the world has increased dramatically, and this trend is expected to increase many folds in the years to come. The safe disposal of biosolids is a major environmental concern. Since biosolids contain significant amounts of macro- and micronutrients [2–4], land application of this waste is an economically attractive management strategy. Biosolids contain a high concentration of organic matter, which can ameliorate soil quality by improving soil structure, water-holding capacity, air, and water transport [5]. Nonetheless, the presence of biosolids-borne potentially toxic elements (PTEs) such as cadmium (Cd), copper (Cu), chromium (Cr), lead (Pb), nickel (Ni), and zinc (Zn) is the most critical long-term hazard when this amendment is land applied. Elevated levels of PTEs in agricultural soils may adversely affect soil’s quality and may represent an ecological and human health risk if they enter the food chain or leach into ground waters, ultimately causing metabolic disorder and chronic diseases in humans. Potentially toxic elements accumulation in soils and, in some cases, in crops has been reported when biosolids have been applied for a long time [6–8]. When PTEs are introduced into the soil, they may be subjected to a series of
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