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1. Introduction
Groundwater represent about 96% of the freshwater reserves of the planet [1] and its pollution can constitute a serious risk to human health [2]. In this framework, groundwater remediation is a fundamental tool to ensure environmental safeguards and acceptable levels of quality. Useful technologies for remediation of contaminated aquifers are, for instance, pump and treat or permeable reactive barriers (PRB) [3,4,5,6,7,8,9]. Although widespread, the pump and treat methodology is often inadequate to achieve target concentrations since its effectiveness depends strongly on the hydrogeology of the site. On the other hand, PRB allows in situ degradation of contaminants forcing the polluted plume to pass through a reactive medium placed in a dedicated trench. The proper choice of the reactive material is fundamental for the achievement of the remediation goals [10]. For the treatment of heavy metals, the reactivity of many substances was investigated in the past considering different processes such as precipitation, adsorption, ion exchange, and reverse osmosis [11,12,13,14,15,16,17,18]. Nevertheless, these methods present several disadvantages, such as the high cost and the limited availability of reactive materials. For instance, good reactants usually employed are the activated carbons which are well known for their effectiveness in removing both organic and inorganic substances, such as heavy metals [11]. Chen and Wang [12] obtained important result for Cu, Zn and Pb removal of from waste water using a fixed bed of granular activated carbon pretreated by deionized water at a given pH. The appropriate choice of the reactive material for a PRB depends mainly on the contaminant type and other factors such as the hydrogeological conditions of the aquifer or the environmental impact associated with the pollutant to be removed. Obiri-Nyarko et al. [19] reported an overview of the most used materials for PRB and treated contaminants. Currently, with particular reference to heavy metals contamination, the reactive medium usually employed for real-scale applications in PRB is zero valent iron (ZVI), which is able to reduce oxidized organic species, chlorinated solvents, heavy metals and radionuclides [20,21,22,23,24]. However, the effectiveness of ZVI is often questionable. In fact, the percolation of pollutant solutions in soil was considered, for a long time, to be a potential cause of groundwater quality deterioration. Moreover, the reactivity of the iron as a reducing agent...