Pyrolysis of contaminated wheat straw to stabilize toxic metals in biochar but recycle the extract for agricultural use

Minimizing potential environmental risks of toxic metals is a prerequisite for recycling crop straw from contaminated farmlands. In this study, feedstocks of wheat straw were collected from paddy fields with low and high level of metal contamination and pyrolysed at temperature of 350 °C and of 550 °C, respectively. The produced biochars were subsequently extracted with hot water and potassium hydroxide solution. Contents of potentially toxic metals (PTEs), polycyclic aromatic hydrocarbons (PAHs) and mineral nutrients were analyzed both of the biochars and their extracts. To test the effect of the biochar extracts on plant growth and quality, a pot experiment was carried out with foliar amendment to Chinese cabbage. The results showed that contents of nutrients, PTEs and PAHs of both the biochars and their extracts differed significantly both between feedstocks and between pyrolysis temperatures. Total concentrations of PTEs and PAHs of the biochars were well under the recommended quality limits with their negligible presence in the biochar extracts. The pot experiment revealed a very significant promotion for cabbage yield and quality by the extracts of biochar pyrolysed at 350 °C, regardless of the feedstock. Thus, pyrolysing crop straw from metal contaminated fields at 350 °C could be an option to stabilize toxic metals in biochar as soil amendment but to recycle the biochar extract as foliar fertilizer to off-site crop production. Therefore, a viable mechanism is urged to develop in China where direct crop residue return has been state-funded under a policy of strict ban of straw burning.