## 1 Introduction  
Currently, about half of the world's population lives in cities, often in overcrowded conditions [1; 2]. This, in addition to the growing industrialization, has led to massive overbuilding and to the reduction of available green areas. For this reason, the interest in the restoration of urban and peripheral areas, assigning their management to associations or private citizens for the cultivation of flowers, fruits and vegetables, has grown in the last few years [3]. However, the position of these gardens within the urban environment exposes the soil to widespread pollution coming from different sources, such as atmospheric deposition as a result of industrial emissions, vehicular traffic, energy production and waste treatment plants [4; 5]. The nature of pollutants present in these soils ranges from inorganic compounds (such as potentially toxic elements (PTEs)) to recalcitrant organic compounds (e.g., polycyclic aromatic hydrocarbons) [6] at concentrations that, when exceeding legal limits, entail the application of soil reclamation strategies. Various chemical-physical techniques for the remediation of contaminated soils are currently available, such as leaching, vitrification, isolation of the polluted soil and its removal and electrokinetic techniques [7]. Immobilization or extraction of metals from the contaminated site by physical techniques are generally chosen for areas where rapid or complete decontamination is required. Many of these methods, besides having high costs, can cause secondary pollution or negative effects on the biological activities, on the structure and on the fertility of the soil [8]. Furthermore, these reclamation processes often involve expensive environmental and civil engineering practices and are unattractive to citizens, making them inappropriate for urban spaces. Therefore, phytoremediation, among the less expensive and impactful remediation techniques, is an effective approach to achieve the abatement of potentially toxic elements (PTEs). It basically involves the use of green plants and associated soil microbes to reduce the concentrations or toxic effects of contaminants in the environment by exploiting the great ability of plants to take up pollutants from soils and to detoxify them through various mechanisms [9; 10]. Phytoremediation techniques can be classified as follows:  
* Phytoextraction, that uses pollutant-accumulating plants to absorb contaminants from soil by concentrating them in the harvestable biomass;
* Phytoegradation, that is related to the use of plants and associated microorganisms to degrade organic pollutants;
* Rhiziltration, that utilizes plant roots to absorb pollutants, mainly metals, from water and aqueous waste streams;
* Phytostabilization, that refers to the use of plants to reduce the bioavailability of pollutants in the environment; and
* Phyto volatilization, that uses plants to volatilize pollutants [11; 12]. The choice of the kind of phytoremediation depends on the matrix to be treated, on the type and bioavailability of pollutants and on the characteristics of the plants used [13]. In particular, plants for phytoextraction should ideally respect some characteristics, namely rapid growth, large biomass production, huge accumulation of metals (or other pollutants of interest) in the parts of the plant developed above the soil surface (which can subsequently be used for non-food purposes or ashed, followed by disposal in a landfill or, in the case of valuable metals, recycling of the accumulated element [13]) and good tolerance to high concentrations of pollutants [14; 15]. The aim of this study was to evaluate the efficiency of a phytoremediation approach for the recovery of contaminated soil located in Turin by applying a green and low-cost process. For this purpose, the capability of PTE accumulation of four plant species (_Brassica juncea_, _Helianthus annuus_, _Zea mays_ and _Pteris vittata_) cultivated in two Turin urban soils characterized by different PTE contents was studied.