Regulatory effects of nano-zero-valent iron on microalgae: Biological properties and auto-flocculation process

Microalgae biotechnology presents a promising and sustainable strategy for resource recovery from wastewater. However, its application is often constrained by poor biological activity, inefficient nutrient removal, and inadequate auto-flocculation properties. This study investigated the effects of nano-zero-valent iron on microalgae. A systematic investigation was conducted into the time and concentration-dependent effects of adding nano-zero-valent iron at different growth phases. The findings demonstrated a dual role of low-dose nano-zero-valent iron (0.05-0.10 mg/L) for microalgae growth. While transient growth inhibition was observed initially, nano-zero-valent iron subsequently acted as a potent signaling stimulus, significantly enhancing lipid accumulation by 46.95 % and carbohydrate production to 22.97 mg/L, respectively. During the exponential growth phase, the addition of 0.10 mg/L nano-zero-valent iron significantly enhanced wastewater treatment efficiency, achieving removal rates of 70.69 % for chemical oxygen demand and 95 % for phosphate. Moreover, nano-zero-valent iron enhanced auto-flocculation efficiency to 93.84 % by promoting exopolysaccharide secretion and effective charge neutralization, while simultaneously reducing the release of inhibitory proteins. This study provided novel insights into the effects of iron on microalgae and proposes a strategic approach for optimizing microalgae-based wastewater treatment systems through precisely timed, low-concentration nano-zero-valent iron supplementation.