Spectral induced polarization for the characterisation of biochar in sand

Abstract

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, τ 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. τ peak 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 τ peak for biochars derived from wood through pyrolysis but did not affect τ 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.

Preliminary assessment on the application of biochar and spectral‐induced polarization for wastewater treatment

“…This behaviour is expected and consistent with recent literature (Gao et al . ), strengthening our hypothesis that the SIP method is suitable for characterization of biochar containing soils. Interestingly enough the real component does not show the same trend (Fig.…”

“…This observation is in direct agreement with previous research suggesting that biochars behave as conductors (Gao et al . ).…”

“…(a)–(d) Distribution differences based on biochar concentration (a control, b 5%, c 10%, 25%); after a certain concentration, biochar could form continuous ‘vein’‐like structures; because biochar behaves as a conductor (Gao et al . ) such structures should increase the real component of complex conductivity. (e) Biochar has a stronger EDL polarization, compared to sand grains, with direct implications on the SIP signature; the IP signature can be the result of EDL polarization and/or membrane polarization mechanisms.…”

“…However, for biochar‐related studies, it is worth mentioning IP contributions in the presence of metals, or at least grains that behave like metals, because biochar has been shown to resemble the electrical properties of conductors or semi‐conductors (Gao et al . ). The presence of disseminated metallic particles in electrolytically conductive porous media will impact both the real and imaginary components of complex conductivity.…”

“…Gao et al . () demonstrated one application of the proposed SIP method in the detection and characterization of biochar in sand sample. Recent research explored the SIP method as a monitoring tool in biochar‐based remediation processes with very promising results (Kirmizakis ; Kirmizakis et al .…”