review · Hybrid Advances
Metal oxide-biochar composites offer an advanced approach to environmental decontamination, addressing conventional biochar limitations in wastewater treatment and soil remediation. Biochars functionalised with metals such as iron, zinc, cerium, copper, zirconium, and titanium through chemical precipitation, direct pyrolysis, or impregnation exhibit altered material properties. These modifications can expand specific surface area by up to 98 percent while reducing porosity by up to 89 percent. Across different preparation conditions, precursors, and metal ions, the resulting composites achieve removal rates of 80.5 to 100 percent for inorganic pollutants and 52.9 to 100 percent for organic contaminants in water. In soils, heavy metal immobilisation efficiency spans 16 to 100 percent. Pollutant uptake operates through mechanisms including electrostatic attraction, complexation, precipitation, and redox reactions. Widespread deployment still requires addressing gaps in environmental risk, life cycle impacts, and pilot-scale performance.
Pollution from industrial effluent and heavy metals in agricultural land presents persistent threats to water security and food safety. While standard biochar has operational constraints, modifying it with metal oxides dramatically boosts its capacity to extract harmful compounds from water and lock contaminants in soil. Improving these materials is vital for designing more effective, scalable remediation interventions.
These composites could benefit wastewater treatment operators, environmental engineering firms, and contaminated land managers targeting organic and heavy metal pollutants. However, the technology remains in early-stage research. Transitioning from laboratory evidence to market-ready products requires overcoming notable developmental hurdles, specifically the current lack of pilot-scale demonstrations, comprehensive cost analyses, environmental risk evaluations, and life cycle assessments.
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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.
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DOI: 10.1016/j.hybadv.2024.100292
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