Evaluation of sorbed PAH compounds on biochar

Biochar is the name given to the chemical and/or thermal transformation of biomass feed stocks into a more stable carbon form for purposes of carbon sequestration. Soil has been the focused, but not exclusive, application target for biochar. Biochar additions have resulted in both positive and negative alterations in plant growth/yield and microbial soil processes, with some of these factors hypothesized linked to sorbed organic compounds. In the present work, we have determined the sorbed polycyclic aromatic hydrocarbon (PAH) content of over 75 unique biochars created from various suppliers, techniques, and post-production processing. From this analysis, napthalene was the most commonly identified PAH sorbed to biochar. Similar to the sorbed volatile compounds, sorbed PAH concentrations are highly variable across different pyrolysis units; despite similarity in production temperature, feedstock and residency times. In general, higher pyrolysis temperatures lead to lower amounts of total PAH compounds being sorbed to the biochar. A couple of other trends are noteworthy. First, longer residency times typically result in lower total PAH concentrations. Second, there is no clear relationship across different pyrolysis units, but analogous to wood ash research, the initial moisture content and oxygen content during pyrolysis and/or cooling appear to be two of the controlling factors in the quantity of sorbed PAH compounds. Lastly, these sorbed PAH compounds on biochar do decompose (through abiotic mechanisms) on the biochar surface, so the time since production is also an important factor. Some biochars have total sorbed PAH concentrations that exceed regulatory guidance for soil amendments in some countries. However, in addition to post-processing, there appears to be the suggestion that sorbed PAH content can be minimized through process control alterations and feedstock selection/processing.
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Kurt Spokas
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