Composite biochar: a potential approach for comprehensively utilizing various solid wastes

Pyrolysis of solid waste to produce biochar is a scientifically supported waste disposal and resource recycling method. Pyrolying sewage sludge to give sludge biochar is an attractive way of utilizing sewage sludge. However, sewage sludge has a low carbon content, low specific surface area, and underdeveloped pore structure and poses environmental risks because of the heavy metal content. Co-pyrolysis of sludge with other solid wastes to give composite sludge biochar (CSBC) can optimize the physicochemical properties and decrease the environmental risks. The excellent physical and chemical properties of CSBC make it suitable for use as an adsorbent to remove pollutants from wastewater. Here, progress in preparing and using CSBC is reviewed. CSBC prepared at high temperatures has a larger specific surface area, which is favourable for physical adsorption. Low-temperature CSBC is more likely to perform chemical adsorption because of more functional groups produced. In addition, adding biomass waste improves the specific surface area, porosity and oxygen functional group content in CSBC, and that enhances the removal of both organic and inorganic pollutants in wastewater. CSBC prepared by adding non-biomass waste containing certain elements (such as Ca) has excellent adsorption performance for target pollutants (such as heavy metals). The conclusion is that CSBC is a promising material that can be used to remove pollutants, manage wastewater, and utilize solid waste. Preparing CSBC using multiple solid wastes is an environmentally friendly treatment solution for sludge.