2 research outputs found
Contaminant issues in production and application of biochar
For widespread use of biochar in agriculture and horticulture, it must be ensured that
application will neither adversely affect soil and plants, nor exceed legislated
contaminant concentrations. The most relevant groups of contaminants in biochar are
potentially toxic elements (PTEs), polycyclic aromatic hydrocarbons (PAHs) and
volatile organic compounds (VOC). In this thesis, the concentrations of these groups
of contaminants were analysed in 90 different biochars produced by slow pyrolysis.
Subsequently, the concentrations were compared to legislation/guideline threshold
values and linked to production conditions. The risk these contaminants pose to plant
growth was also assessed, to give recommendations on production of safe biochar.
PTEs can neither be formed nor destroyed, which means their presence in biochar is
predominantly determined by feedstock type. However, significant levels of Cr, Fe
and Ni were introduced into biochar from the furnace steel, whilst PTEs with low
boiling points, such as As, Cd and Zn, partially evaporated during pyrolysis. PTEs
were not responsible for phytotoxic effects observed for PTE-rich biochars despite
biochar’s exceedance of available and total PTE threshold values for soil and soil
amendments. Although initial tests were promising, the risk that PTE-rich biochars
as amendment for soil and growing media pose, needs further investigation.
The PAH concentration in biochar was markedly reduced by increasing carrier gas
flow rate, and the type of feedstock also influenced the PAH content. However, there
was no clear dependence of pyrolysis temperature on PAH concentrations, which
was attributed to PAHs being increasingly formed and evaporated at higher pyrolysis
temperatures. Ultimately, condensation of pyrolysis vapours and deposition on
biochar was identified as the main risk for biochar contamination with PAHs, as this
resulted in elevated concentrations of high-risk, higher molecular weight PAHs.
Weaknesses in the pyrolysis unit design, such as cold zones, resulted in elevated
concentrations of VOCs, as well as PAHs, in biochar. Comparing concentrations and
phytotoxic potential of both compound groups, it was concluded that observed toxic
effects were much more likely caused by VOCs in biochars containing both
contaminants. Overall, formation of VOCs and PAHs cannot be prevented, but their
presence in biochar resulting from retention and deposition can be minimised