**1. Introduction**

Illegal disposal of industrial waste containing different pollutants, both organic and inorganic, is responsible for severe agricultural soil pollution [1]. Although such contamination can be related to the presence of so-called emerging pollutants, such as liquid-ionic pollutants or pesticides [2], in most cases it is related to the presence of potentially toxic elements (PTEs) [3,4]. Among others, the presence of Cu and Zn is quite common, because these metals have a wide application in numerous industrial processes as well as in the production of many pesticides and herbicides [5]. The remediation of PTEs-contaminated soils can be carried out using different techniques. Among them, the soil washing process proved to be one of the few permanent contaminated soil remediation treatments [6]. Therefore, this process can achieve good remediation performances if the operative conditions are carefully selected and optimized [7–9]. However, in the case of soil washing treatment, the excavation of the contaminated soil is required, and this operation is time-consuming and expensive. This step is then followed by an in-site or on-site treatment by mixing the solid phase with a washing solution able to solubilize the pollutants in the liquid phase. The two phases are successively separated. The remediated soil can be returned to its original place [10] or used for different purposes. The contaminated washing solution, on the other hand, must be appropriately treated [8,11,12] before its final disposal. Unfortunately, this process cannot be applied if the contamination is extended to large or deep areas, because of the excessive costs related to the soil excavation. In these cases, an interesting alternative to soil washing is soil flushing [13,14]. Soil flushing consists of the direct injection in the soil of a leaching solution, avoiding the necessity of contaminated soil excavation.

Several chemical reagents have been positively tested in the past as leaching solutions for soil remediation in the flushing process (i.e., HCl, EDTA, CaCl2) [15,16]. However, part of the flushing solution can be retained in the soil and/or leaches into the groundwater, representing a further source of environmental contamination. Therefore the use of a biodegradable agent, such as Ethylenediamine-*N*,*N*-disuccinic acid (EDDS) [17,18], may be more advisable in order to decrease the potential risk of negative effects on the environmental quality. While the application of EDDS as a washing agen<sup>t</sup> has been widely studied in several scientific research papers [9,19,20], its use as flushing agen<sup>t</sup> has been tested in limited cases [21,22], and thus requires further investigation in more detail.

The present paper aims to provide a deeper understanding of the applicability of EDDS-enhanced flushing for the remediation of Cu- and Zn-contaminated agricultural soil. The study was conducted at the lab scale to keep all process parameters under control, as well as to analyze the removal efficiency and the extraction kinetic trend. Metals distribution at different depths was also investigated to understand the mechanisms of metal desorption and adsorption during transportation, according to their initial distribution in the different contaminated soil fractions. Moreover, the release of PTEs in the leachate after the soil flushing treatment was evaluated. Finally, results from EDDS-enhanced washing tests conducted at the lab scale on the same soil were reported in order to compare the pollutants removal efficiency and the treatment environmental suitability of the two processes (i.e., soil flushing and soil washing). All of these aspects have been rarely considered in a single experimental work, thus reflecting the novelty of this study.
