DOI: 10.14457/tu.the.2020.1026
title: Characteristics and catalytic activity of zero-valent iron nanoprticles embedded on chitosan for fermentative hydrogen production

description: Nano zero-valent iron was prepared using chemical reduction method in a chitosan solution. The derived nano zero-valent iron/chitosan composite (nZVI/CS) was characterized by transmission electron microscope and x-ray diffraction in order to determine the size, state of agglomeration and to detect whether iron was available in the form of ZVI. The catalytic activity of nZVI/CS on dark fermentative biohydrogen production was tested using glucose as the substrate. The fermentation was initially carried out by an Enterobacter aerogenes strain (ATC1148). Since E. aerogenes is a pH sensitive facultative anaerobe, the optimum initial pH for biohydrogen production in the presence of nZVI/CS was identified. With 0 (control), 50, 100, 150, 200 and 600 mg/L nZVI/CS supplementation, the optimum initial pH was identified as 6.0. The impact of phosphate buffer concentration was tested incorporating 0.2M, 0.4M and 0.6M phosphate buffer concentrations in the fermentation media. nZVI/CS concentrations were provided as 0, 100, 200, 300, 400 and 600 mg/L. It was revealed that the H2 yield was experiencing a collaborative impact from both nZVI/CS and phosphate buffer. The highest H2 yield was identified at 0.6M phosphate buffer concentration revealing 300 mg/L as the optimum nZVI/CS concentration (0.85 mol H2/molglucose, a 65% increment from control). Continuous supplementation of Fe2+ from nZVI/CS to enhance Ni-Fe hydrogenase in E. aerogenes along with maintaining the pH level with the use of phosphate buffer were observable from Fe leaching and final pH, respectively. Further analysis revealed apart from regulating the pH, phosphate buffer increase ATP synthesis and facilitate Acetyl-coA formation in pyruvate metabolism.Investigations on pH and VFA concentration variations were conducted for optimum phosphate buffer and nZVI/CS concentration (0.6 M phosphate buffer and 300 mg/L nZVI/CS) and control in order to further analyze how H2 production occurred throughout the fermentation period. Acetic and butyric acids availability indicated both acetic and butyric pathways had produced H2 via pyruvate metabolism. The distribution occurred in line with the pH variation demonstrating a shift in pathways from acetic to butyric at lower extracellular pH in order to control intercellular pH levels. Cell viability of the optimum condition revealed a 28% reduction compared to control at the end of fermentation. Since no significant reduction on cell growth was visible, it was identified nZVI/CS composite could be used effectively with variety of biomasses. Catalytic activity of nZVI/CS was tested while glucose was fermented by anaerobic sludge. At 5.5 initial pH, 200 mg/L nZVI/CS resulted in 1.66 mol H2/molglucose with a 65% increment from control. No phosphate buffer was added for the fermentation media and yet the H2 production was carried out around 96h. Similar to E. aerogenes, anaerobic sludge also undertook both acetic and butyric pathway to produce H2. The increased lifespan of nZVI/CS can be attributable to chitosan support which enhanced the durability and stability of ZVI. While being a support material, chitosan prevented reaching toxic concentrations extracellular Fe3+ concentration by forming complexes via in-situ bonds with –O- and –NH2 sites so that H2 production continued up to 96h.