## 1 Introduction  
Some of the most detrimental soil pollutants are heavy metals. The sorption of heavy metals in soil is an important process governing their availability for uptake by organisms and mobilization out of soil [1]. Soil contains mineral and organic constituents that carry hydroxyl and carboxylic surface functional groups. The acid-base characteristics of these functional groups contribute to the formation of a surface charge that plays an important role in the retention of heavy metals [2]. For example, in the case of organic constituents of the soil represented by humic substances, high pH values can cause the deprotonation of humic-carboxylic-COOH groups, becoming negatively charged moieties and thus attracting the positively charged heavy metal ions and protons [3]. The interaction of heavy metals with soil particles is controlled by reactions on the surfaces of soil minerals and organic matter, i.e., adsorption as outer- and inner-sphere complexes and surface precipitation [1], which depend on soil pH, soil sorption capacity, organic matter content, the particle size distribution of the soil, the age of contamination, and the presence of different inorganic contaminants in soil [4]. There has been increasing concern about the potential toxic effects of heavy metals in the soil. Heavy metals affect soil fertility and thus crop productivity and exert toxic effects on soil microorganism, hence resulting in the change of the soil structure [5], diversity, population size, and overall activity of the soil microbial communities [6, 7]. For example, elevated concentrations of Pb in soils may decrease soil productivity, and a very low Pb concentration can inhibit some vital plant processes like photosynthesis, mitosis, and water absorption, producing toxic symptoms as dark green leaves, wilting of older leaves, and brown, short leaves or roots [8]. Therefore, depending on the concentration, heavy metals can be valuable and beneficial to crops. Crop productivity is linked to the aggregation of the soil. Low crop productivity was observed in low-aggregated soils while higher crop production was noted in well-aggregated soils. Soil aggregation is influenced by the level of heavy metal concentrations present in the soil. It is well known that heavy metals cations make strong bonds with fine clay and soil organic particles. Consequently, the presence of heavy metals that interacted with clay particles affects the aggregation and soil structure, leading to a modification of the hydraulic parameters of the soil [9]. On the other hand, heavy metals are persistent, difficult to transform, and cannot be biodegradation [6; 10], leading to their accumulation in living organisms and posing an important threat to human health [11; 12; 13; 14]. The surge of industrial activity has intensified soil pollution with heavy metals. Elevated levels of heavy metals were reported in many industrial areas. For example, 231 mgkg\({}^{-1}\) Pb, 0.80 mgkg\({}^{-1}\) Cd, 335 mgkg\({}^{-1}\) Zn, 24 mgkg\({}^{-1}\) Cu, 16 mgkg\({}^{-1}\) As, 1.1 mgkg\({}^{-1}\) Fe, and 669 mgkg\({}^{-1}\) Mn were found in soil near the Cabezo Rajao abandoned mine (Spain) [15]. Extremely elevated contents of metals were also found in Greece near Lavrion Technology and Cultural Park mining and metallurgy complex: 64,195 mgkg\({}^{-1}\) Pb, 7540 mgkg\({}^{-1}\) As, 4100 mgkg\({}^{-1}\) Cu, 55,900 mgkg\({}^{-1}\) Zn, and 6500 mgkg\({}^{-1}\) Mn [4]. For this reasons, various technologies have been developed, tested, and investigated in past decades for the remediation of heavy metal-contaminated soils, e.g., solidification/stabilization [16], soil washing [17], soil amendment [18], phytoremediation [3; 19], and electrokinetic methods [20]. Among these, soil washing, which involves the extraction of heavy metals by chemical solutions, is one of the few effective, non-time-consuming, and relatively inexpensive treatment technologies for the decontamination of severely heavy metal-polluted soils [21; 22; 23].