Rapid formation of pyrogenic char (biochar) with high and low sorption capacity towards organic chemicals.

Abstract

Pyrogenic char (biochar) with a high sorption capacity (B-HSC) can sequester hazardous chemicals (e.g., phenanthrene). However, when sorption inhibits bioavailability of some functional chemicals (e.g., the herbicidal efficacy of diuron in soil), biochar with a low sorption capacity (B-LSC) is required to prevent sorption effects. The pyrolytic B-HSC generation has been reported, but information on B-LSC formation is scarce. How fast B-HSC and B-LSC could be generated is unknown until now. Here, biochars were rapidly prepared (the shortest heating time reached 5min and the cooling time reached<30min) by a direct-pyrolysis method by directly exposing packaged rice straw and pine wood to 350°C, 500°C and 700°C and out-of-furnace cooling at room temperature. The sorption of diuron, phenanthrene, and twelve other chemicals was investigated. B-HSCs were obtained within 30min of rice straw pyrolysis, and the biochar Kd values quickly increased to 7-730-fold that of the raw biomass as -OH and C-O-C in (hemi)cellulose of rice straw rapidly degraded, increasing hydrophobic interactions between the char and chemicals (solubility≤82.8g/L). In contrast, B-LSCs were generated within 30min of PW pyrolysis, and the Kd values of the biochars were 0.2-3.0-fold that of the raw biomass, as the surface area development and hydrophobicity-driven sorption were probably delayed by the late degradation of lignin aromatic C-O and phenolic -OH. Biochar amendment revealed an enhancement effect of B-HSC but not of B-LSC on soil sorption. The fast formation of B-LSC and B-HSC provides a guide to develop time- and cost-effective technique in pyrolytically producing weakly or strongly sorbing biochars for organic chemical management.