Modified-biochar adsorbents (MBAs) for heavy-metal ions adsorption: A critical review

With the rapid growth of new-energy batteries and metallurgical industry, water pollution caused by toxic heavy-metals from these sectors has increased to a new level, which is seriously threating the eco-system and human health. Thus, the effective disposal of metal-bearing water has become a vital requirement. To resolve such a problem, developing eco-friendly biochar-based adsorbents has become urgent. As an eco-friendly material, biochar has attracted increasingly attention due to its low-cost, high surface area and abundant functional groups. However, pristine biochar also shows low adsorption efficiency for heavy-metals. Consequently, improving its adsorption efficiency via modification is considered as one of most effective techniques. In the last 15 years, various methods were proposed to modify biochar. However, review articles on the usage of modified-biochars for heavy-metals adsorption are unavailable. To summarize these newly-developed modification methods including organic-modified, nonmetal-modified, metal-modified, metal/organic-modified, and metal/mineral-modified, this review will focus on the preparation, characterizations and applications of modified-biochar adsorbents (MBAs) for heavy-metals. More emphasis will be on the adsorption of MBAs for heavy-metals in a single-metal, mixed multi-metal, and mixed metal/organic matter solution systems. Meanwhile, the adsorption mechanism of MBAs for Cr(VI), Cd (II), Pb(II) and Cu(II) will be highlighted. It is found that the adsorption mechanism of MBAs in different mixed multi-metal solution systems is complicated and competitive adsorption is inevitable. Typically, the effect of functional groups from MBAs on heavy-metals adsorption is diverse, more studies are required in the future. Finally, the present state and future perspective will be discussed.