Preparation and Characterization of Biochar Sorbents Produced from Malt Spent Rootlets

The present study aims at the preparation and characterization of biochars produced from malt spent rootlets (MSR) under different pyrolysis temperatures. The biochars were characterized for their surface area, microporosity, suspension pH, acid–base behavior, and functional groups on their surface. The highest surface area (340 m2/g) and porosity (0.21 cm3/g) were observed for MSR pyrolized at 800 °C. For the same biochar, 67% of the pore volume corresponds to micropores (<2 nm). The high-temperature biochars resulted in highly alkaline suspensions (pH 9.9) compared to MSR suspensions (pH 5.3). The functional groups (e.g., hydroxyl, phosphonic, amine, or carbon-bonded sulfhydryl) found on the MSR surface disappear as the pyrolysis temperature increases without new ones forming. Thus, the surface charge of high-temperature biochars is low. For phenanthrene, the sorption capacity of biochars pyrolyzed at or above 750 °C increases by almost 1 order of magnitude compared to the raw material. For mercury, sorption capacity of biochars increases by a maximum factor of 4 and a factor of 6 for low- (300–500 °C) and high-temperature (750–900 °C) biochars, respectively.