Recent advances in metal oxide-biochar composites for water and soil remediation: A review

Abstract

During the last decades, biochar (BC) has proven to be an innovative candidate in various environmental applications. Nevertheless, the use of biochar in improving soil fertility, immobilizing contaminants and treating wastewater has presented many limitations. Nanomaterial technology has been used to improve the physicochemical properties of biochars, including emerging methods such as chemical precipitation, direct pyrolysis and impregnation of biochars with metals (Fe, Zn, Ce, Cu, Zr, Ti etc) or their oxides. This review discusses recent advancements on the potential, limitations, and prospects of using metal-biochar composites for wastewater treatments and soil remediation. Functionalization of biochar with metal oxides can exacerbate the specific surface area up to 98 %, and diminish the porosity structure up to 89 %. The performance of these metal-biochar composites, which depends on the raw material/precursor of the biochar, the identity of the metal/guest ion, the preparation conditions and the activation method, has shown that the removal rates for inorganic and organic contaminants in the aqueous phase vary from 80.5 to 100 % and from 52.90 to 100 % respectively and that the immobilization rates for heavy metals in soils vary from 16 to 100 %. Remediation mechanisms for various adsorbates in aqueous media and soils generally include electrostatic attraction, oxidation/reduction, complexation and precipitation. Life cycle assessment (LCA), pilot-scale, cost analysis, potential environmental risks, and machine learning modelling studies are found to be lacking for metal-biochar composites and provide areas for future research.

Keywords

• Environmental applications
• Functionalization
• Physicochemical properties
• Metal/biochar composites
• Metal oxides