Fig. 1

(a,b) FTIR of the biochars in the wave number range of 400–2000 cm^− 1. Notes: R, rice husk biochar; RA, rice husk biochar modified by HNO₃; RB, rice husk biochar modified by NaOH; S, sheep manure biochar; SA, sheep manure biochar modified by HNO₃; SB, sheep manure biochar modified by NaOH.