## 1 Introduction  
The growth of the human population and activities, like industrialization, mining, and the intensification of agriculture, have made soil pollution one of the world's most serious environmental problems. It could negatively influence agricultural production, threaten food safety, and endanger the health of natural ecosystems and lifeforms. There are over 10 million major soil-contaminated areas around the world, of which 50% are contaminated by heavy metals and metallolds [1, 2]. The Food and Agriculture Organization of the United Nations (FAO) classified the prevention of soil pollution as a top priority worldwide [3]. Heavy metals and metalloids are of great concern due to their potential toxicity, persistency and since they cannot be degraded. These elements are frequently released into terrestrial environmental compartments with anthropogenic emissions in the form of easily soluble, highly mobile chemical species. These are bioavailable and can contaminate both surface and ground waters [4, 5]. Significant efforts are being made to remediatesoil-contaminated sites in Europe [6] and around the world [1]. The crucial role of the remediation of contaminated soils is to reduce the mobility of metals and metalloids in the soil using different immobilizing agents. The process of immobilization occurs through adsorption, complexation, and precipitation reaction [7; 8; 9] and redistributes the heavy metals from the soil solution to the less-soluble soil fractions, while their total concentrations remain unchanged. The most commonly used amendments for in situ immobilization include cement [10], clay minerals [11], zeolites [12], biochar [13], phosphates [14], humic substances [15], and various industrial by-products like fly ash [16], paper-ash [17; 18] and red mud from aluminum production [18]. Among the immobilization processes, a carbonation treatment under alkaline conditions (pH 10-10.5) was shown to effectively reduce the mobility of lead (Pb), zinc (Zn), chromium (Cr), copper (Cu), and molybdenum (Mo) in residues from the combustion of municipal solid waste [19]. When immobilization techniques are applied for remediation, the stability of the immobilized metals must be evaluated. For this purpose, it is necessary to know to which mineral phases the contaminants were redistributed after the remediation. To evaluate the immobilization efficiency, leaching tests using demineralized water as the extracting solution (liquid-to-solid ratio of 10 L/kg) [20] or single-extraction procedures were often applied [21]. Sequential extraction procedures in combination with element-specific detectors, e.g., inductively coupled plasma mass spectrometry (ICP-MS), provide useful information about the partitioning of metals in soils, sediments, and waste materials [22; 23]. They involve different extractants, which are applied successively to the solid material to selectively leach the particular chemical forms of the metals from the samples analyzed. Each extractant is chemically more rigorous than the previous one [21]. Many of these extraction procedures are based on the method developed by [PERSON] et al., which includes the five-step sequential extraction [24]. With slight modifications, [PERSON]'s scheme was used in the evaluation of the environmental impacts of waste materials and contaminated sediments and soils [25; 26]. In addition, frequently used were the BCR three-step sequential extraction procedures [27; 28]. The BCR sequential extraction scheme, in which 0.11 M acetic acid is used to extract the exchangeable fraction, is not appropriate for highly alkaline sample matrices. After the addition of acetic acid, the pH of the sample is reduced, leading to apparently greater mobility of most metals, which does not reflect their mobility in stabilized chemical forms in highly alkaline conditions [29]. Sequential extraction procedures can also provide essential information on a metal's mobility after remediation. [PERSON] et al.