## 1 Introduction  
In many parts of Europe--especially where surface water is of limited supply or poor quality--groundwater is a fundamental resource. It is used as the main source of drinking water by many European cities and towns, and it is a major source for irrigation in most European countries. Unfortunately, a variety of toxic and bioaccumulating contaminants have been detected in groundwater, including a wide spectrum of chlorinated hydrocarbons, halogenated aliphatic hydrocarbons, pesticides, pharmaceuticals, and metals [1]. The contamination of groundwater by persistent organic contaminants can have long-term adverse effects on both human health and the environment [2]. In accordance with the European Union (EU) Groundwater Directive (2006/118/EC) on the protection of groundwater against chemical pollution, the Member States will have to prevent or limit the input of emerging contaminants to groundwater, and tailor their regulation to the specific circumstances [3]. One of the most largely applied approaches in groundwater remediation is based on the so called \"Pump and Treat\" (P & T) technology, in which the whole contaminated plume is intercepted by a hydraulic barrier, and the extracted water is treated on-site before being discharged, typically in a superficial body [4]. Depending on the spectrum and complexity of the contamination, a groundwater treatment may consist of a single cleanup method, but often requires several treatment units for the removal of different types of contaminants or high concentrations of a single contaminant [5; 6]. In both cases, the treatment plant often includes units based on columns packed with sorbent (usually activated carbon) for the removal of dissolved concentrations of hydrophobic organic contaminants. One of the main drawbacks of this approach is the cost associated with the treatment combined with the length of the intervention, ranging from a few years to several decades, depending on the contaminant concentration, plume size, type of contaminant source, and hydrogeological setting. This often makes P&T unsustainable from an economical point of view. During the last two decades, permeable reactive barriers (PRBs) have been established as robust alternatives to traditional pump and treat approaches for groundwater remediation [7; 8; 9]. PRBs--a passive in situ treatment of groundwater contaminated by organic and inorganic contaminants--are trenches or gates excavated in the subsoil to intercept the contaminated plume, filled with a granular (permeable) reactive material (e.g., zero-valent iron, activated carbon, zeolites, compost, etc., or their combination) chosen to treat the contaminated groundwater passing through it under a natural hydraulic gradient [10; 11]. Depending on the specific case, pollutants may be thus trapped in situ by sorption or converted into non-hazardous compounds by chemical or biological routes. Granular zero-valent iron (ZVI) is the most commonly used filling material, thanks to its capability to degrade a wide range of organic contaminants, and reducing and immobilizing harmful metal ions [12]. Usually, ZVI is successfully used for the remediation of groundwater contaminated by chlorinated aliphatic compounds through a series of dechlorination reactions until the formation of nontoxic species, such as ethene and ethane [13; 14]. Available iron-based materials that are also used in large-scale applications may have various shapes, sizes, and surface properties, depending on the manufacturing sources. As a consequence, their reductive powers vary in a wide spectrum. For example, many commercial sources of ZVI currently used to construct PRBs have ferric oxides on their particle surfaces. As water penetrates the iron particle surface oxides, anaerobic corrosion begins with a consequent hydrogen evolution that is of considerable importance in developing PRB remediation technology [15].