Iron speciation control on mineral-biochar composites for environment decontamination

Abstract

Biochar, a carbon-rich material produced from the thermal treatment of biomass wastes with limited oxygen, was highlighted as a negative emission technology. Engineered biochar with a fit-for-purpose production can solve multiple environmental problems with the achievement of the circular economy. Among different modification technologies, impregnating iron (Fe) with the formation of Fe-biochar has been widely investigated owing to its high reactivity, natural abundance, facile synthesis, and environmental benignancy, while the precise control of Fe species on Fe-biochar for target pollutants immobilization is still unclear. This thesis focused on the precise control of Fe-species on Fe-biochar for the toxic metals immobilization process, and the key findings include: (i) Pyrolysis temperature affected the redox activity of the produced Fe-biochar. Increasing pyrolysis temperature altered the Fe-biochar from reductive to oxidative for the pollutants immobilization; (ii) The content of different carbon species in biochar led to distinct reductive-Fe and amorphous-Fe contents in the resulting Fe-biochar composites, and thus affected its immobilization mechanisms for arsenic; (iii) The crystallinity of Fe mineral affected the electron transfer routine with biochar during the Cr(VI) reduction process; (iv) Reductive transformation of Fe within the Fe-Mn binary oxide on Fe-Mn biochar was found during the pyrolysis process, forming embedded Fe(0) clusters within FeMnO2 with a high Fe efficiency for the Cr(VI) immobilization; and (v) The hydrothermal pretreatment affects the mineral species on the Fe-Mn biochar. Higher hydrothermal temperature with co-­existing urea led to the formation of nano-sized Fe(0) surrounded by the lattice-expanded MnO and cotton-like graphitic carbon, providing a high immobilization capacity for the frequently-appeared toxic metals in electroplating water. Results from this study could serve as the guideline for efficient Fe-biochar production for toxic metal immobilization. Based on the current finding, future studies about particle size control, metal-carbon framework design, production standard of Fe-biochar, carbon emission of modified biochar, and overall economic concern are suggested to promote the engineered biochar for industrial-level application.