Sludge-derived biochar has been considered as a promising sorbent for As(V) due to its high content of Fe. However, role of different endogenous iron minerals and the bulk carbon in this process remained unclear. Here, biochar with different Fe content and species were prepared from two types of sludge (SSA and SSB) at 350 degrees C or 700 degrees C for As(V) immobilization. Although SSB biochar had considerable Fe (3.6%-8.6%), they showed limited sorption capacity for As(V) due to the electrostatic repulsion related to the negatively charged surface at the experimental pH. By contrast, SSA biochar demonstrated superior sorption abilities for As(V) with the maximum capacities of 60.2-90.2 mg g(-1). Amorphous iron determined the removal of As(V) by SSA350 due to the adsorption and the co-precipitation of released Fe2+ with H2AsO4-, which accounted for 77.6% and 22.4%, respectively. For SSA700, iron minerals were converted to more crystalline species, including gamma-FeOOH, Fe-4(O, OH,SO4)(9), and Fe-2(SO3)(3)center dot xH(2)O, and the co-precipitation with As(V) caused 74.6% of As(V) removal by SSA700. The bulk carbon of biochar was proved to serve as heterogeneous nucleation sites for the co-precipitation, showing its synergistic effect with iron on As(V) removal, which raised the removal efficiency from 0.9-2.9% by Fe2+ alone to 13.9-25.5% by Fe2+ co-existing with bulk carbon. Findings of this study not only demonstrate the importance of iron species in biochar to As(V) removal, but also provide new insights on the synergistic effect of iron interaction with bulk carbon.
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