Activation of periodate with pinewood biochar-CuO composite for the removal of recalcitrant organic pollutants – Mechanisms and degradation products

The degradation kinetics and mechanisms of various types of recalcitrant organic pollutants (ROPs) by a novel biochar-CuO (B-CuO)/periodate (PI) system have been deeply investigated in the present study. More precisely, the study aimed to explore the significant effects of pH on the formation of various oxidative species using this treatment system. According to the results achieved, iodate (IO₃∙) and periodate radicals (IO₄∙), ∙OH and O(³P) were mostly generated at pH ≈ 7, resulting in the efficient removal of methylene blue (MB) (90% after 30 min, k_obs = 0.09 min⁻¹), while the system favoured the formation of O(³P) under highly alkaline pH values (pH = 11), leading to lower degradation kinetics of the pollutant (51% after 30 min, k_obs = 0.028 min⁻¹). Additionally, no oxidative species were found at highly acidic pH values (pH = 3), and no significant MB removal was observed (21% after 30 min, k_obs = 0.0081 min⁻¹). Elevating the temperature (from 25 °C to 80 °C) and bubbling with nitrogen or oxygen gas also did not have any significant effects on the degradation of ROPs. Additionally, the system maintained its efficiency in various water matrices, including real wastewater, confirming the applicability of the biochar-CuO/PI system for real water treatment processes. The system also exhibited a high degradation efficiency for other types of ROPs, such as ciprofloxacin. Experiments using ultra-high performance liquid chromatography in combination with quadrupole time-of-flight mass spectrometry were performed to identify the degradation products and pathways of MB, revealing the contribution of two different degradation pathways and the formation of five degradation products in the B-CuO/PI system. The ECOSAR program was employed to evaluate the predicted toxicity of the degradation products. The results showed that the system could efficiently detoxify MB.