As a low-cost adsorbent, biochar can be used as a low-cost adsorbent for wastewater treatment, particularly with respect to treating heavy metals in wastewater. A number of studies have demonstrated effective removal of heavy metals from aqueous solutions by biochar and, in some cases, proven the superiority of biochars to activated carbons. Among several factors affecting the sorption ability of biochars, feedstock materials play a significant role. This review incorporates existing literature to understand the overall sorption behavior of heavy metals on biochar adsorbents. Depending on the biochar type, heavy metal can be removed by different mechanisms such as complexation, physical sorption, precipitation and electrostatic interactions. Mathematical sorption models can be used to understand the efficiency of biochar at removing heavy metals, and promote the application of biochar technology in water treatment.

Biochar (BC) has gained attention for removal of toxic elements (TEs) from aqueous media; however, pristine biochar often exhibits low adsorption capability. Thus, various modification strategies in BC have been developed to improve its removal capability against TEs. Nanoscale zero-valent iron (nZVI) and iron oxides (FeOx) have been used as sorbents for TE removal. However, these materials are prone to agglomeration and also expensive, which make their usage limited for large-scale applications. The nZVI technical demerits could be resolved by the development of BC-based composite sorbents through the loading of nZVI or FeOx onto BC surface. Nano zero-valent iron modified BC (nZVIBC), FeOx-modified BC (FeOxBC) have attracted attention for their capability in removing pollutants from the aqueous phases. Nonetheless, a potential use of nZVIBC and FeOxBC for TE removal from aqueous environments has not been well-realized or reviewed. As such, this article reviews: (i) the preparation and characterization of nZVIBC and FeOxBC; (ii) the capacity of nZVIBC and FeOxBC for TE retention in line with their physicochemical properties, and (iii) TE removal mechanisms by nZVIBC and FeOxBC. Adopting nZVI and FeOx in BC increases its sporptive capability of TEs due to surface modifications in morphology, functional groups, and elemental composition. The combined effects of BC and nZVI, FeOx or Fe salts on the sorption of TEs are complex because they are very specific to TEs. This review identified significant opportunities for research and technology advancement of nZVIBC and FeOxBC as novel and effective sorbents for the remediation of TEs contaminated water.