Zero-valent iron coupled with lactic acid bacteria enhances the degradation of p-Nitrophenol in an anoxic–oxic alternated system

Zero-valent iron (ZVI) has been widely used in Fenton-like reactions but is easily passivated during pollutant degradation. Lactobacillus acidophilus (a type of lactic acid bacteria (LAB)) is capable of producing lactic acid (LA) and H 2 O 2 under anoxic and oxic conditions, respectively. Thus, it inspires us to build a green recycling system that couples LAB with ZVI (LAB-ZVI) to degrade p-Nitrophenol (p-NP) in an alternated anoxic–oxic system. LAB significantly improve the reactivity of ZVI and achieves long-term stable removal of p-NP. In the anoxic environment, p-NP is reduced to p-Aminophenol (p-AP) by LAB-ZVI, and p-AP is subsequently degraded into smaller molecular structures. The complexation between LA and Fe(Ⅱ/Ⅲ) not only creates a potential difference (ψ) to facilitate the formation of reactive oxygen species (ROS) but also reduces intracellular ROS in LAB. ZVI promotes extracellular production of •OH but extracellular ROS attack does not necessarily lead to cell death. ZVI enhances the secretion of supernatant fluid, thereby improving the •OH scavenging ability of LAB. Additionally, ZVI positively affects the activity of several antioxidant enzymes through the release of Fe(Ⅱ/Ⅲ). Moreover, extracellular polymeric substances (EPS) can slow down the oxidation process by regulating antioxidant enzymes. Overall, this study provides an environmentally friendly scientific guidance for the application of ZVI.