Impact of two contrasting biochars on the bioaccessibility of 14C-naphthalene in soil

• Biochar made from differing feedstock and production condition exhibit obvious contrasting properties.
• Biochar exhibiting higher micropore distribution strongly inhibited losses of naphthalene in soils following increase in contact time.
• Indigenous microorganisms expressed the capacity to readily mineralise naphthalene, but as the concentration of biochar increased, the extent of mineralisation decreased due to surface area and microporosity.
• HPCD solution was shown to be a rapid tool to predict bioaccessibility of naphthalene.
• Biochar showed to be a cheap tool to mitigate risk of exposure to PAHs.

This study investigated the impact of two different wood biochars (BioC1 and BioC2) on the extractability and biodegradation of 14C-naphthalene in soil. Both biochars had contrasting properties due to difference in feedstocks and pyrolytic conditions (450–500 °C and 900–1000 °C, designated as BioC1 and BioC2, respectively). This study investigated effects of biochar on the relationship between 14C-naphthalene mineralisation and calcium chloride (CaCl2), hydroxypropyl- ββ-cyclodextrin (HPCD) or methanol extraction in soil amended with 0%, 0.1%, 0.5% and 1% BioC1 and BioC2 after 1, 18, 36 and 72 d contact times. Total extents of 14C-naphthalene mineralisation and extraction were reduced with increasing concentrations of biochar; however, BioC2 showed greater sorptive capacity. Good linear correlation existed between total extents of 14C-naphthalene mineralisation and HPCD extractions in BioC1 (slope=0.86, r2=0.92r2=0.92) and BioC2 (slope=0.86, r2=0.94r2=0.94) amended soils. However CaCl2 and methanol extractions underestimated and overestimated extents of mineralisation, respectively. These results indicate that biochar can reduce the bioaccessibility of PAHs and the corresponding risk of exposure to biota, whilst HPCD extraction estimated the bioaccessible fraction of PAHs in soil. Bioaccessibility assessment is vital in evaluation of biodegradation potential and suitability of bioremediation as a remediation option.