<p>The presence of Fe(III) or oxygen can affect the biochar-mediated degradation of pollutants. However, the effects of changes in the form and species of Fe(III) on the degradation of pollutants have not been systematically investigated. Therefore, this study investigated biochar-mediated <i>p</i>-nitrophenol (PNP) degradation, which is influenced by species of Fe(III) at pH 2.5, 5.7, and 8.0. At pH 2.5, the anoxic degradation of PNP by biochar was 9.30&#xa0;mg&#xa0;g⁻¹. Fe(III) could compete for electrons from biochar; therefore, PNP degradation was reduced 29%. Comparison with O₂, the higher redox potential of Fe(III) prevents the occurrence of a Fenton-like reaction. Consequently, the degradation of PNP decreased by 77%. At pH 5.7, the degradation of PNP (9.62&#xa0;mg&#xa0;g⁻¹) by biochar was greatest, but after Fe(III) was introduced into the system, most of the Fe(III) precipitated, which significantly inhibited PNP degradation by 18% and 66% under anaerobic and aerobic conditions, respectively. Oxygen can take electrons from biochar to form ⋅O₂⁻ to reduce PNP; therefore, PNP degradation is the same under anaerobic and aerobic conditions. Interestingly, at pH 8.0, the addition of Fe(III) significantly increased the apparent degradation of PNP by 41% and 15% under anaerobic and aerobic conditions, respectively. Acidification experiment revealed that the increase in PNP degradation was due to the co-precipitates Fe(III) with PNP on the biochar surface. This study demonstrated that additional caution should be taken when estimating the biochar-mediated degradation performance of organic pollutants in practical environmental applications, especially when Fe(III) coexist in the system.</p> Graphical Abstract <p></p>