Recent advances in nanoscale-metal assisted biochar derived from waste biomass used for heavy metals removal

•Novel synthesis methods of nanoscale-metal assisted BC are described.•Nanoscale metals can improve the surface properties and active sites of BC.•The effect of environmental factors on HMs removal with BC composites is discussed.•Evaluation of operation strategies and scale-up system is presented.•Future prospects of the nanoscale-metal assisted BC are proposed. Pollution of heavy metals (HMs) is a detrimental treat to human health and need to be cleaned up in a proper way. Biochar (BC), a low-cost and “green” adsorbent, has attracted significant attention due to its considerable HMs removal capacity. In particular, nano-metals have recently been used to assist BC in improving its reactivity, surface texture and magnetism. Synthesis methods and metal precursors greatly influence the properties and structures of the nanocomposites, thereby affecting their HMs removal performance. This review presents advances in synthesis methods, formation mechanisms and surface characteristics of BC nanocomposites, along with the discussions on HMs removal mechanisms and the effects of environmental factors on HMs removal efficiency. Performance of using BC nanocomposites to remediate real HMs-containing wastewater and issues associated with its process scale-up are also discussed. This review aims to provide useful information to facilitate the development of HMs removal by nanoscale-metal assisted BC.