Synergistic effect of extracellular polymeric substances and carbon layer on electron utilization of Fe@C during anaerobic treatment of refractory wastewater

Nano zero-valent iron (NZVI) has been widely used to improve refractory wastewater treatment. However, the rapid dissolution of NZVI causes a waste of resources and an unstable bioaugmentation. Herein, to verify the essential role of slow release of NZVI on biological systems, a core-shell structured Fe@C composite was developed to demonstrate the long-term feasibility of Fe@C for enhancing azo dye biodegradation in comparison to a mixture of NZVI and carbon powder (Fe+C). The 150 days of long-term reactor operation showed that, although both Fe@C and Fe+C enhanced azo dye degradation, the former achieved a better performance than the latter. The strengthening effect of Fe@C was also more durable and stable than Fe+C. It may be due to the fact that the carbon layer of Fe@C could interact with extracellular polymeric substances (EPS) through physical adsorption and chemical bonding to form a stable buffer to regulate NZVI dissolution. The buffer layer could not only regulate the attack of H+ on NZVI to reduce its dissolution rate but also complex released Fe2+ and neutralize OH- to alleviate the passivation layer formed on the NZVI surface. Moreover, microbial community analysis indicated that both Fe@C and Fe+C increased the abundance of fermentative bacteria (e.g., Bacter-oidetes_vadinHA17, Propionicicella) and methanogens (e.g., Methanobacterium), but only Fe@C promoted the growth of azo dye degraders (e.g., Clostridium, Geobacter). Metatranscriptomic analysis further revealed that only Fe@C could substantially stimulate the expression of azoreductase and redox mediator (e.g., riboflavin, ubi-quinone) biosynthesis involved in the extracellular degradation of azo dye. This work provides novel insights into the bioaugmentation of Fe@C for refractory wastewater treatment.

Automated summary

This study developed a core-shell structured Fe@C composite to demonstrate its long-term feasibility for enhancing azo dye biodegradation, compared to a mixture of NZVI and carbon powder (Fe+C). The results showed that Fe@C had better performance, durability, and stability than Fe+C, possibly due to the carbon layer's ability to form a stable buffer to regulate NZVI dissolution. The microbial community analysis revealed that Fe@C promoted the growth of azo dye degraders, and metatranscriptomic analysis further revealed the substantial stimulation of genes involved in the extracellular degradation of azo dye by Fe@C. This study provides novel insights into the bioaugmentation of Fe@C for refractory wastewater treatment.