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Finely divided carbon char products offer a cost-effective method for the safening of herbicides in soil. In this investigation, two contrasting carbon char sources: Virgin Activated Charcoal (VAC) and Recovered Carbon Black (RCB) products were evaluated for safening of metsulfuron, sulfentrazone and pyroxasulfone herbicides in soil using sugar beet bioassay, and of rimsulfuron using perennial ryegrass bioassay. Plants were grown for seven days in WhirlPack® bags in soil with added carbon char product and added herbicide, and the root or shoot lengths were measured. The VAC was more efficacious in herbicide safening than the RCB. For the VAC, concentrations in soil (wt/wt) as low as 0.375% fully deactivated metsulfuron, sulfentrazone and pyroxasulfone, while for the RCB product a gradual degree of safening was observed, except for pyroxasulfone that was completely deactivated by both VAC and RCB. The VAC at concentrations of 3% and above fully deactivated rimsulfuron, while RCB did not. The two carbon char products examined in this study had different physical properties, of which the specific surface area (91 and 515 m2 g−1 for the RCB and VAC, respectively) and specific surface area of pores (75 and 247 m2 g−1 for RCB and VAC, respectively) showed the greatest difference. Higher surface area promoting greater sorption may explain more effective deactivation of herbicides by the VAC. To obtain the same degree of herbicide safening, more RCB would be required. [ABSTRACT FROM AUTHOR]

This study investigated the effect of two willow (Salix spp.) biochars, produced using either fast- or slow-pyrolysis, on the bioavailability of metsulfuron and sulfentrazone herbicides in soil. Five rates (0%, 1%, 2%, 3%, and 4%; w/w) of each biochar were used, along with varying rates of metsulfuron (0–3.2 µg ai kg−1) and sulfentrazone (0–200 µg ai kg−1), followed by a sugar beet bioassay. The fast-pyrolysis biochar had minimal effect, while the slow-pyrolysis biochar decreased the bioavailability of both herbicides. Despite using the same feedstock, the two biochars had different physical and chemical properties, of which specific surface area was most contrasting (3.0 and 175 m2 g−1 for fast- and slow-pyrolysis biochar, respectively). Increased anionic herbicide adsorption associated with greater surface area of the slow-pyrolysis biochar is considered to be the primary mechanism responsible for reducing herbicide bioavailability with this biochar. [ABSTRACT FROM AUTHOR]