Oxygen-containing functional groups of biochars enhancing the oxygen release of CaO2 and the generation of free radicals

The low dissolved oxygen (DO) concentration in groundwater is one of the key challenges limiting the efficiency of aerobic microbial remediation, leading to the development of slow-release oxygen agents such as CaO 2 . Our previous studies have demonstrated that the oxygen content in biochars plays a role in the oxygen release reaction of CaO 2 . This study aims to further investigate the relationship between the oxygen-containing functional groups on biochar surface and oxygen release performance of CaO 2 . Surface oxidation and reduction modifications were employed to regulate the oxygen-containing functional groups on bamboo biochars while minimizing alterations to its physical structure. Results showed that the oxidized and reduced biochars were similar in oxygen content (∼20–22 %) and pore structure (with specific surface area of 8–16 m 2 ·g −1 and average pore size of 10–13 nm). On the contrary, the difference in contents between C-O and O C-O functional groups reached ∼10 %, indicating the successful regulation of the oxygen-containing functional groups. The loading capacity of CaO 2 increases with the increase in C-OH groups in biochars, while the O C-O group is identified as the oxygen-containing functional group that contributes most significantly to accelerating the oxygen release of CaO 2 . The dissolution product hydrogen peroxide (H 2 O 2 ) potentially interacted with biochars to activate free radical synthesis. The yield of hydroxyl radicals (•OH) shows a positive correlation with the oxygen content in biochars, while the production of superoxide radicals (•O - 2 ) is most significantly influenced by the presence of O C-O groups. As a result, the yield of •O - 2 was 216–317 times higher than that of •OH.