Spectral induced polarization for the characterisation of biochar in sand

The use of biochar as a soil amendment attracts increasing research interest. However, the lack of methods to detect and monitor biochar in situ limits the validation of the field-scale application of biochar. Spectral induced polarization is a potential tool to characterise biochar in soil. The aim of this study is to investigate the sensitivity of spectral induced polarization to biochar in sand and to understand how the physicochemical properties of both the biochar and the surrounding matrix influence the spectral induced polarization response. To this end, spectral induced polarization measurements were conducted on four types of biochar with different mass fractions disseminated in saturated sand as a host media with changing electrical conductivity. In addition, it was investigated how the spectral induced polarization response depends on the particle size of biochar. The measured SIP data were interpreted by Debye decomposition to obtain values for the peak relaxation time, tau(peak); total chargeability, M; and normalised total chargeability, M-n. Spectral induced polarization showed a clear and specifically differentiated response to the presence of all four types of biochars. M was found to be proportional to the mass fraction of biochars, although relationships varied for each type of biochars. tpeak of biochars increased with increasing particle size. Increased electrolyte concentration enhanced M-n for all biochars, although again, the specific response was different for each biochar. In addition, higher electrolyte concentrations decreased tau(peak) for biochars derived from wood through pyrolysis but did not affect tau(peak) of biochar derived from miscanthus through hydrothermal carbonisation. It was concluded that the spectral induced polarization response of pyrolytic biochars resembled that of conductors or semiconductors, whereas the spectral induced polarization response of hydrothermal carbonisation biochar more closely resembled that of clay. Overall, the findings in this study suggest that spectral induced polarization is a promising method for the detection and characterisation of biochar in soil.