Redox properties of nano-sized biochar derived from wheat straw biochar

Nano-sized biochar (NBC) has received increasing attention due to its unique physicochemical characteristics and environmental behaviour, but an understanding of its redox properties is limited. Herein, the redox properties of NBC derived from wheat straw were investigated at two pyrolysis temperatures (400 and 700 °C). These NBC materials were prepared from bulk-biochar by grinding, ultrasonication and separation treatments. The resulting NBC had average particle sizes of 78.8 ± 1.9 and 122.0 ± 2.1 nm after 400 and 700 °C treatments, respectively. The physicochemical measurements demonstrated that both the NBC prepared at 400 °C (NBC-400) and the NBC prepared at 700 °C (NBC-700) were enriched in carboxyl and phenolic oxygen-content groups. Electrochemical analyses showed that both NBC-400 and NBC-700 were redox active and had an electron transfer capacity (ETC) of 196.57 μmol^-1 g_C ^-1 and 363.47 μmol^-1 g_C ^-1, respectively. On the basis of its redox activity of NBC, the NBC was capable of mediating the reduction of iron and manganese minerals as well as the degradation of methyl orange (MO) by sulfide. The NBC-700 could stimulate these reactions better than the NBC-400 due to its higher redox activity. Meanwhile, the NBC was more active in stimulating these reactions than bulk-biochar. Our results highlight the importance of size in evaluating the redox reactivity of biochar and related environmental processes and improve our understanding of the redox properties of biochar.