Simultaneous removal of As(III) and Cu(II) by biochar modified siderite: The key roles of ROS and mineral conversion

Mine drainage frequently contained arsenite (As(III)) and copper (Cu(II)), posing significant threats to human health. Herein, we developed a novel composite, crab shell biochar modified siderite (SID@BC) for the simultaneous removal of As(III) and Cu(II). The results revealed that the incorporation of BC into the siderite (SID) significantly increased the adsorption sites and improved the electrochemical properties of SID@BC, thereby enhancing the adsorption capacity of SID@BC for As(III) and Cu(II), which was 1.3 and 4.2 times higher than that of SID, respectively. Notably, in the process of removing As(III) and Cu(II), co-existed Cu(II) enhanced the transformation of iron phases from siderite to goethite and facilitated the Fe(II)/Fe(III) cycling. During this cycling process, Fe(III) was reduced to Fe(II), which subsequently reacted with oxygen to generate additional reactive oxygen species (ROS). Quenching experiments confirmed that O₂^•- and H₂O₂ were the key ROS responsible for the oxidation of As(III) to As(V), which was more easily electrostatically attracted and complexed. Besides, BC enhanced the Cu(II) adsorption performance of SID@BC through mineral precipitation and complexation, which further facilitated the participation of Cu(II) in the Fe(II)/Fe(III) cycle, thereby promoting the ROS generation. These findings provided insight into developing an effective strategy for the simultaneous removal of As(III) and Cu(II) from wastewater.