Fig. 1

Scanning electron microscope (SEM) images of palm frond-biochar (PF) without acid treatment (a), PF-treated with 0.25 M acetic acid (b), PF-treated with 0.25 M phosphoric acid (c), and guava branches-biochar (GB) without acid (d), GB-treated with 0.25 M acetic acid (e), GB-treated with 0.25 M phosphoric acid (f).