Fe oxides-biochar composites produced by hydrothermal carbonization and pyrolysis of biomass waste

The synthesis of biochar-metal oxide composites for removing heavy metals from aqueous solutions or remediating polluted soils has attracted increased attention. Amongst them, Fe oxide composites stand out due to possible magnetic properties and low price. Two types of Fe oxide-biochar composites were prepared by impregnation with ferric sulfate salts of biomass waste or corresponding hydrochar, followed by pyrolysis at 550 °C for 5 h. During pyrolysis, Fe increased carbonization, leading to biochar with low H/C values. Properties of Fe oxide-biochar composite greatly depend on the feedstock. The Fe content in the final Fe oxide-biochar was higher than in biochar obtained by direct pyrolysis of biomass waste. Some of Fe was in the form of FeS. Impregnation of hydrochar followed by pyrolysis lead to a Fe oxide-biochar composite with lower ash content and higher micro- and macro-porosity. In this case, the Fe was in the form of Fe oxides.