Pollutant contents in biochar and their potential environmental risks for field application

Biochar is widely used in carbon sequestration, pollution restoration, improvement of acid soils, and so on. However, there are few studies on examining the pollutant contents in biochar from different raw materials and preparation conditions, and the environmental risk of biochar application is unclear. In this study, the biochars with pine needles and wheat straw as raw materials were prepared under different conditions. The contents of heavy metals and polycyclic aromatic hydrocarbons (PAHs) in biochar were determined, and the environmental risk of biochar for field application was assessed. The results showed that different kinds of biochar contained a certain amount of heavy metals (0.301-128 mg/kg) and PAHs (1.48-5.48 mg/kg). The contents of heavy metals in the biochar from pine needles were higher than those from wheat straw, but opposite for the contents of PAHs. The Cu, Zn and Cd contents in the biochar by high temperature preparation were higher than those by low temperature preparation, and high temperature preparation reduced the PAHs contents in biochar. With high application rate of biochars, the PAHs in biochars tend to make soil PAHs contents reach moderate or severe pollution level. The environmental risk of heavy metals in biochars is little regardless of low or high application rate in field.