## 1 Introduction
Different methods have been proposed to provide a good reference for the quantitative assessment of the reactivity of metallic reducing agents of environmental interest, including ZVIs. The most relevant examples include the use of concentrated Fe(III) solution [16], tri-iodide [17], and methylene blue [18]. To date, ZVI has been applied in the construction of hundreds of PRBs worldwide, particularly for the degradation of chlorinated solvents [19; 20]. However, PRB-ZVI technology is still affected by some operational problems, the main one most likely being the long-term decrease in performance due to loss of ZVI reactivity and and/or porosity [21]. In fact, the alkaline pH (8-9) and the presence of ferrous and ferric ions within the barrier (both due to ZVI corrosion) lead to the precipitation of minerals which--depending on the groundwater chemical composition--can form a passive layer on the surface of the iron that hampers the electron transfer processes and reduces iron reactivity. Additionally, iron precipitation could cause pore occlusion, resulting in a greater seepage velocity, thus reducing the treatment effectiveness (due to a residence time decrease). Moreover, some compounds (i.e., chloromethane 1,2-dichloroethane, dichloromethane) are not chemically reduced in the presence of ZVI [22]. Most of these compounds can, however, be degraded via biological reductive dechlorination, provided that specific microorganisms and available electron donors are present [23]. In this regard, the most common strategy to stimulate natural biological activity is to inject essential nutrients and electron donors, thereby maximizing the metabolism of the existing microbiome. The commercial products usually employed are soluble molasses or easily degradable organic compounds (i.e., lactate, polylactate, etc. ), with the major drawback being the need for their continuous delivery in the groundwater for a lengthy period (long periods are usually required to achieve the remediation goals by using this approach). To overcome these limitations, in recent years, several researchers have shown a growing interest in developing composite materials obtained by the combination of consolidated chemically reactive media with a slow-releasing carbon source (i.e., guar gum, cellulose, starch) to extend treatment to recalcitrant compounds and/or to enhance microbial activity in a synergistic reaction [24; 25; 26; 27]. In this context, polyhydroxybutyrate (PHB) could be a valuable carbon source. PHB can be efficiently fermented by different microorganisms under anaerobic conditions into well-defined and predictable fermentation products, such as volatile fatty acids and molecular hydrogen, which may be used as an electron donor by dechlorinating bacteria. Moreover, PHB can be produced by bacteria starting from mixed substrates or even wastewaters [28]. In a recent study, we developed nano-ZVI/PHB composites that showed interesting properties in preliminary reactivity studies towards perchloroethene (PCE) [29; 30]. The aim of this work was to synthesize and accurately characterize the chemical/physical properties of core-shell materials of micrometric dimensions based on the interaction of iron particles (the core) and biopolymers such as polyhydroxyalkanoate (the surrounding shell) for the removal of chlorinated pollutants from contaminated groundwater. In particular, fermentable polymers such as PHB were used to couple with ZVI. In this work, we used commercially available ZVI (whose cost is significantly lower than that of nanoscale ZVI--approximately 0.7 $/kg vs. 120 $/kg), and this would be very advantageous for large-scale applications. This work was entirely dedicated to the development and characterization of innovative ZVI-PHB composites. In the future, such materials have the potential to enhance the performance of PRBs towards chlorinated compounds through the combined action of ZVI and dechlorinating microorganisms, mediated by the use of PHB as a slow-releasing electron donor.