Improved screening of biochar compounds for potential toxic activity with microbial biosensors

Biochar is a carbon rich product destined for agricultural use, which can be produced using an array of feedstock and pyrolysis conditions. As such, the resultant biochar product can exhibit characteristics that result in either beneficial or detrimental environmental effects. We set out to establish what the environmental hazards might be for a range of softwood biochars. To facilitate this we conducted biochar headspace analysis, plant germination and bacterial biosensor assays. Our headspace analysis indicated the presence of aldehyde and ester based compounds, which were affected by pyrolysis conditions and in some cases contained compounds that were hazardous to human and environmental health. Germination assays, utilising barley, buckwheat, white clover and oil seed rape, showed that the plants responded differently to the same biochar samples and that in some instances, where germination was unaffected, there were visible physiological effects on seedlings. Finally, we screened water extracts of the biochars under buffered pH, for the presence of potentially toxic elements and compounds using bioluminescence-based bacterial biosensors. Bioluminescence inhibiting compounds were present in extracts prepared from biochar samples pyrolysed at 500 °C under conditions with inadequate control of biochar-volatile interaction. The findings were taken as an indication that bacterial biosensors could be used as a rapid method to screen anomalous biochar products for potential ecotoxicity hazard assessment.