Equal Surface Oxygen, Different Adsorbent: Ammonium Persulfate Demineralises NaOH-Activated Plastic Pyrolytic Char While Hydrogen Peroxide Does Not

Oxidative post-treatment of alkali-activated char is normally assessed by how much oxygen it adds to the carbon surface, so the inorganic fraction, which in a high-ash char makes up most of the solid, is rarely tracked separately. We treated NaOH-activated plastic pyrolytic char (PPC) with either H2O2 or ammonium persulfate (AP) and characterised the carbon and the mineral matrix independently. By the two measures normally used to describe an oxidised surface, the two oxidants are indistinguishable: the atomic O/C ratio reaches 0.457 and 0.455, and the zeta potential reaches −35.0 ± 3.0 and −36.7 ± 3.4 mV (n = 7), equivalent within 5 mV by two one-sided tests. The mineral matrix separates them. Thermogravimetry in air places the ash content of the activated char at 60.7 wt%, which falls to 52.1 wt% after H2O2 treatment and to 16.8 wt% after AP treatment. X-ray diffraction shows that the H2O2-treated char retains its crystalline assemblage, whereas AP treatment removes the calcite that survives H2O2 and reduces the assemblage to traces of quartz and rutile over a turbostratic carbon hump at d002 = 3.58 ± 0.07 Å. Scaled by ash content, the AP route depletes every major mineral element by 67–88%, compared with 0–56% for H2O2. The apparent methylene blue capacity at 120 min rises from 44.7 to 57.6 mg g−1 after H2O2 treatment but only from 113.7 to 120.2 mg g−1 on an ash-free basis, and the two oxidised chars respond differently to solution pH. The AP route is carried out in 2 M H2SO4, so its effects are attributed to the route as practised rather than to persulfate alone. Surface oxygen content and surface charge therefore do not predict what an oxidant has done to a high-ash char, and reporting either alone leaves the larger structural change unrecorded.