Development of biochars from pyrolysis of lotus stalks for Ni(II) sorption: Using zinc borate as flame retardant

•Zinc borate (ZB) was used as flame retardant for biochar (BC) production.•ZB kept the structures of the produced BCs as its starting material (lotus stalk).•Adding ZB dramatically enhanced the yields and the oxygen content of the BCs.•Sorption of Ni(II) was enhanced about 3–10 times on the BCs by adding ZB.

In this work, zinc borate (ZB) was employed as flame retardant for preparation of biochar (BC). A series of BC samples were generated by varying the ZB to lotus stalks (LS) ratio (0.25, 0.5 and 1.0) under different temperatures (300, 350 and 400 °C) for 1 h. The BCs were analyzed for their surface morphologies, surface areas, surface elemental compositions and yields. The results indicated that, after charring, ZB kept the structures of the produced BCs as its starting material (LS), dramatically enhancing their yields, and promoting their surface oxygen content. The BCs were used as adsorbent for removal of Ni(II) from aqueous solutions. Sorption of Ni(II) on the BCs was enhanced about 3–10 times compared with that of BCs derived from pyrolysis of LS without adding ZB.