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.