Characteristics of different types of biochar and effects on the toxicity of heavy metals to germinating sorghum seeds

Soils contaminated with heavy metals are often notably nutrient poor and unsuitable for plant growth. The addition of biochar can significantly improve soil properties. In this study, the contents of organic compounds and inorganic elements in biochar and the effect of biochar on plant germination were examined. The PAH content of biochar from four different sources was examined, and naphthalene, phenanthrene, fluoranthene and pyrene were identified as primary compounds. The most abundant inorganic elements were potassium, calcium, magnesium, sodium, aluminium, iron and manganese, with strontium and barium also being significantly elevated. The pH of biochar from all sources was strongly alkaline. The sorption characteristics for heavy metals (Cd, Cu and Pb) were also tested for the different types of biochar. Adsorption data were well-described by a Langmuir isotherm with maximum Cd (II), Cu (II) and Pb (II) adsorption capacities of 20.16, 7.83 and 70.92 mg/g for bamboo-derived biochar; 18.80, 13.85 and 200 mg/g for rice husk-derived biochar; 11.63, 20.08 and 123.46 mg/g for ash tree-derived biochar; and 15.11, 10.86 and 196.08 mg/g for beech tree-derived biochar, respectively. The effect of biochar on the toxicity of heavy metals was measured by the inhibition of sorghum seed germination. With biochar, the toxicity of cadmium, copper and lead was reduced. Bamboo-derived biochar was less efficient in reducing the toxicity of cadmium and copper compared with the other types of biochar. For lead, the rice husk-derived biochar was the least efficient in reducing the toxicity.